View
218
Download
0
Category
Preview:
Citation preview
N° d’ordre : 4227
THÈSE
Présentée à
L’UNIVERSITÉ DE BORDEAUX 1 ÉCOLE DOCTORALE DES SCIENCES CHIMIQUES
Par HARIKRISHNA EROTHU
Pour obtenir le grade de
DOCTEUR SPÉCIALITÉ : CHIMIE POLYMÉRE
SYNTHESIS AND PHOTOVOLTAIC APPLICATIONS
OF NOVEL COPOLYMERS BASED ON POLY(3-HEXYLTHIOPHENE)
Date de soutenance : 25 Février 2011
Devant la commission d’examen formée de :
M. M. L. TURNER Professeur, Université de Manchester, UK Rapporteur Mme L. LUTSEN Directrice de recherche, IMEC, Belgique Rapporteur M. P. HUDHOMME Professeur, Université de Angers, France Examinateur Mme L. VIGNAU Maitre de conférences, IPB, Bordeaux Examinatrice M. E. CLOUTET Chargé de Recherche CNRS, LCPO Directeur de thése M. H. CRAMAIL Professeur, Université de Bordeaux 1 Directeur de thése M. R. C. HIORNS Chargé de Recherche, CNRS, PAU Invité
-2011-
1
Abstract
Synthesis and photovoltaic applications of novel copolymers based on poly(3-hexylthiophene)
Abstract : The performance of organic photovoltaic cells mainly depends on the active layer nano-morphology. Rod-coil block copolymers (BCPs) are well known in their ability to self-assemble into well-ordered nanoscopic morphologies. BCPs containing electron-donor and acceptor segments are of particular interest for use in photovoltaic cells because electronic light-excited states exist over distances similar to the typical size of block copolymer domains (~10 nm). Therefore, we designed novel donor-acceptor BCPs to exploit this coincidence in dimensions. This thesis is focused on BCPs based on regioregular poly(3-hexylthiophene) (rr-P3HT) due to its high hole mobility and good processibility from various solvents. Simplified and versatile syntheses of donor-acceptor rod-coil di- and tri- BCPs consisting of the donor block P3HT (rod) and polystyrene or poly(4-vinylpyridine) (coil) blocks to carry the acceptor C60 in different ways were developed. These materials were used as additives to stabilize the nano-morphology of reference P3HT: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) based devices. Photovoltaic characterizations were then tied to copolymer structural data with the help of AFM and a range of complementary characterization techniques.
Keywords : Organic photovoltaic cells, rod-coil block copolymers, regioregular poly(3-hexylthiophene) (P3HT), GRIM polymerization, functional poly(3-hexylthiophene), controlled radical polymerization, anionic polymerization, C60, nano-structuration. -------------------------------------------------------------------------------------------------------
Synthèse et application en cellules solaires organiques de nouveaux copolymères à base de poly(3-hexylthiophène)
Résumé : Dans cette étude, des copolymères à blocs rigide-flexible comprenant des segments donneur [poly(3-hexylthiophène) régiorégulier, (rr-P3HT)] et accepteurs d’électrons (C60) ont été synthétisés. L’auto-assemblage en masse de ces copolymères à blocs avait pour objectif d’atteindre des morphologies dont la taille des domaines coïncide avec la distance idéale de transport de l’exciton (~10 nm) en vue d’utiliser ces systèmes comme matériaux de couche active dans les cellules photovoltaïques organiques de type P3HT-PCBM. La maîtrise et l'optimisation des conditions de synthèse de rr-P3HT de fonctionnalité terminale bien définie nous ont permis d'accéder à différentes architectures de copolymères linéaires di- et triblocs, constitués de P3HT comme bloc rigide et de polystyrène ou poly(4-vinylpyridine) comme bloc ‘flexible’. La fonctionnalisation du bloc flexible avec des dérivés du fullerène (C60 ou PCBM) a ensuite été réalisée et ces copolymères utilisés comme additifs pour stabiliser la morphologie de la couche active des cellules solaires organiques de type P3HT/PCBM. Les caractéristiques photovoltaïques des matériaux ainsi préparés ont été déterminées et corrélées aux analyses morphologiques de la couche active. Mots-clés : cellules photovoltaïques organiques, copolymères à blocs rigide-flexible, poly(3-hexylthiophène) régiorégulier (rr-P3HT), polymérisation GRIM, poly(3-hexylthiophène) fonctionnel, morphologie, compatibilisation.
2
3
Acknowledgements
I would like to express my sincere thanks to Prof. Yves GNANOU and Prof. Henri CRAMAIL to permit me as a PhD student in LCPO. I am very grateful to Prof. Henri CRAMAIL and Dr. Eric CLOUTET for giving a chance to do my PhD under their very kind supervision in LCPO and also for their encouragement and valuable suggestions during my research, writing publications and thesis. It is my great pleasure to express my sincere thanks to Dr. Roger C Hiorns, my research advisor for his excellent guidance, cooperation and encouragement throughout my research work. He has nurtured me with excellent scientific input to carry out my research work and also with his valuable discussions and insights, which helped me in the successful completion of my thesis. I am very grateful for his great advices for my personal life also.
I would like to thankful to my collaborators Dr. Laurence VIGNAU, Habiba and Mafoudh from IMS, Bordeaux, for their excellent help to characterize my polymers and also for giving the best results. I express my sincere thanks to external Jury members, Prof. Michael Turner, Prof. L. Lutsen, Prof. P. HUDHOMME and Dr. Laurence VIGNAU for accepting my thesis evaluation.
Its very great pleasure to express my sincere thanks to my previous PhD supervisor (late) Prof. G. Sundararajan (IIT Madras, India) for giving me an opportunity to work in his excellent group and also for introducing me to this great field of conducting polymers. Its very pleasure as well as great honour to thank Prof. Pierre Dixneuf (PHD) for his kind support at my difficult times and I am very grateful for his immense encouragement, inspiration, and hospitality during my stay in France. My sincere thanks to Prof. A. K. Mishra (IIT Madras, India) for all his kind help as well as for his valuable research guidance and also my heartful thanks to Prof. U. V. Varadaraju for his kind personal help during my stay at IIT Madras, India. I wish to thank all my teachers and professors for their guidance and inspiration. I am indeed thankful to my other colleagues in LCPO particularly Mathieu Urien, Bertrand, Jean, Maryliine and Mumtaz for their initial support to handle the instruments etc at the beginning of my PhD work and also my office-mates; Cedric,
4
Chantal, Vincent, Jennyfer and also Autumn for their kind personal help. I am extremely grateful to Emmanuel IBARBOURE for his excellent help in AFM and DSC, Nicolas GUIDOLIN for his kind help in GPC, Michele Schappacher for his good teaching in NMR, Christelle Absalon for her wonderful job for MALDI-TOF mass results. I am very thankful to Catherine ROULINAT (especially for her kind personal help during all my stay at Bordeaux), Corine GONCALVES de CARVALHO, Mimi, Bernadette GUILLABERT and Nicole GABRIEL for their excellent help in the administrative work. My sincere thanks to many great people from LCPO especially Cyril Brochon (Expert in anionic polymerization) who helped me a lot, Vijayakrishna (Expert in RAFT) and also Feng, Jerome, Flu, Stephane, Stephanie, Julie, Gabriel, Celia, Dargie, Samira, Katerina.... from LCPO who helped me during my research work. It was very good discussions with my Indian friends: Dakshina, Dynesh, Arvind, Anil when we used to go for coffee every evening. I am equally thankful to all permanent and non-permanent members of LCPO and IMS, Bordeaux for their kind help and moral support during my research work. I am really very grateful to all Indians in Bordeaux, which I passed excellent time during my PhD. I cannot forget the time we passed together with my Indian friends during trips, dinner, lunch parties, Indian festivals and other occasions. I am very thankful to all Indian friends who helped me necessary times particularly Srikumar, Veena, Ujwala, Vidya, Ramana, Aroun, Laxmireddy, Srinivas, Gowda, kamal, Arka, Amol, Mythili, Basabdatta and also all my friends at IIT Madras. Finally, my heartful thanks to my dear mother (Lakshmiswarajyam), father (Brahmaiah), brother (Nani), sister (Vani) and all my in-laws, relatives for their love and moral supports. I am extremely thankful to my best friend cum wife, Dr. Anitha who stood always with me at difficult times in my life and gave me endless love, encouragment and positive support to finish my doctorate successfully. Last but not least, I am really happy to thank my sweet daughter, Haritha for being with us during my doctorate time and also for giving more happiness to finish my thesis soon. My sincere thanks to many other people (space doesn’t allow me to mention names) who helped me in my life to reach this position.
5
Dedication
I feel very pleasure to dedicate this thesis to my dear parents, grandparents,
my dear wife (Anitha), my dear sweet daughter (Haritha) for their love, moral
support and also to my dear teachers, professors for their inspiration,
encouragement.
HARIKRISHNA EROTHU
6
7
Abbreviations
A Electron acceptor
AFM Atomic Force Microscopy
AIBN α,α'-azobisisobutyronitrile
AlCl3 Aluminum chloride
ATRA Atom Transfer Radical Addition
ATRP Atom Transfer Radical Polymerisation
AM Air Mass number
a.u. Arbitrary unit
Br2 Bromine
n-BuLi n-butyl lithium
s-BuLi sec-butyl lithium
t-BuLi tert-butyl lithium
Bu4NI Tetrabutylammonium iodide
CaH2 Calcium hydride
CB Conduction Band
CdS Cadmium(II) sulfide
CdTe Cadmium telluride
CMC Critical Micelle Concentration
CRP Controlled Radical Polymerisation
CuBr2 Copper(II) bromide
CuCl2 Copper(II) chloride
CuI Copper(I) iodide
D Electron donor
Đ Dispersity (Mw/Mn)
DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene
o-DCB ortho-Dichlorobenzene
DEH-PPV Poly(diethylhexyloxy-p-phenylenevinylene)
DIEA or DIPEA N,N-Diisopropylethylamine
DMSO Dimethyl sulfoxide
DMF N,N’-dimethylformamide
DSC Differential Scanning Calorimetry
EA Electronic Affinity
EQE External Quantum Efficiency
FeCl3 Ferric chloride
FF Fill Factor
8
GPC Gel Permeation Chromatography (also known as
Size Exclusion Chromatography or SEC)
GRIM Grignard Metathesis
HBr Hydrobromic acid
HCl Hydrochloric acid
HH Head-to-Head
HT Head-to-Tail
H2O2 Hydrogen peroxide
H2SO4 Sulfuric acid
HOMO Highest Occupied Molecular Orbital
I Current
I2 Iodine
IP Ionization potential
IPCE Incident Photon to Current Efficiency
Isc short-circuit current
i-PrMgCl iso-propylmagnesium chloride
IR Infra Red
ITO Indium tin oxide
LDA Lithium Diisopropylamide
LUMO Lowest Unoccupied Molecular Orbital
Mn Number average molecular weight
Mw Weight average molecular weight
MALDI-TOF Matrix Assisted Laser Desorption Ionisation - Time Of Flight
NBS N-bromosuccinimide
NMP N-methyl pyrrolidone
NMR Nuclear Magnetic Resonance
Ni(dppp)Cl2 1,3-bis(diphenylphosphino)propane Nickel(II) chloride
OPV Organic Photovoltaic
OSC Organic Solar Cell
PA Polyacetylene
PPA Poly(phenylacetylene)
PAT Poly(alkylthiophene)
PCBM [6,6]-Phenyl-C61-Butyric acid Methyl ester
PCE Photo Conversion Efficiency
PEO Poly(ethylene oxide)
PI Polyisoprene
PEDOT:PSS Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)
9
PLED Polymer Light Emitting Diode
PMA Poly(methylacrylate)
PMDETA N,N,N’,N’’,N’’-pentamethyldiethylenetriamine
PPP Poly(para-phenylene)
PPV Poly(p-phenylenevinylene)
PPy Polypyrrole
PS Polystyrene
PT Polythiophene
PTSA p-toluenesulfonic acid
PVA Poly(vinyl alcohol)
P3AT Poly(3-alkylthiophene)
P3HT Poly(3-hexylthiophene)
P2VP Poly(2-vinylpyridine)
P4VP Poly(4-vinylpyridine)
RAFT Reversible Addition-Fragmentation Chain Transfer
Polymerization
RR Regioregularity
T Temperature
Tc Crystallization temperature
Tg Glass transition temperature
Tm Melting temperature
TBAF.3H2O Tetra-n-butylammonium fluoride trihydrate
t-BuMgCl tert-butylmagnesium chloride
TEA Triethylamine
TEM Transmission Electron Microscopy
TGA Thermo Gravimetric Analysis
THF Tetrahydrofuran
TMS Tetramethylsilane
TT Tail-to-Tail
UV Ultra Violet
VB Valence Band
V Voltage
Voc Open-circuit voltage
δ Chemical shift
λ Wavelength
η Conversion efficiency
10
11
Table of contents
Section Title Page
Abstract........................................................................................ 1 Acknowledgements..................................................................... 3 Dedication.................................................................................... 5 Abbreviations............................................................................... 7 Table of Contents........................................................................ 11 General Introduction................................................................... 17
Chapter 1 Literature Review................................................................. 25
1.1 Polymers as semiconductors................................................................ 27 1.1.1 Electrical conductivities of conjugated polymers........................... 27 1.1.2 Archetypal polymeric semiconductors........................................... 30 1.1.3 Electronic structures of conjugated polymers................................ 31 1.1.3.1 Undoped polymers......................................................... 31 1.1.3.2 Doped polymers............................................................. 33 1.1.4 Improving the processibilities of conjugated polymers.................. 34
1.2 Organic photovoltaics............................................................................ 35 1.2.1 Solar energy.................................................................................. 35 1.2.2 Definition, history and development of photovoltaic cells.............. 37 1.2.2 Operating principle of organic photovoltaic cell............................. 42 1.2.4 Efficiency characteristics of organic photovoltaic cells.................. 43 1.2.5 Organic photovoltaic active layer architectures............................. 46 1.2.6 Novel low band-gap polymers....................................................... 49
1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexyl thiophene) (P3HT) ............................................................ 53
1.3.1 Regioregularity.............................................................................. 53 1.3.2 McCullough and Rieke methods.................................................... 54 1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT..... 55 1.3.4 Chain-growth condensation polymerisations leading to P3HT...... 56 1.3.5 Chain-end capping of P3HT using GRIM...................................... 57
1.4 Organic photovoltaic cells and block copolymers.............................. 59 1.4.1 Importance of morphology of active layer...................................... 59 1.4.2 Controlling morphology of active layer in blends........................... 60
12
1.4.3 Self-assembly behaviour of rod-coil block copolymers.................. 63 1.4.4 Why block copolymers in photovoltaic cells? ............................... 65
1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers.................................................................................... 67
1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s....... 68 1.4.5.2 Copolymers based on polythiophenes........................... 71
1.5 References............................................................................................... 77
Chapter 2 Towards comb copolymers based on P3HT via ω-acetylene-P3HT and ω-vinyl-P3HT macromonomers................................................................ 87
2.1 Introduction............................................................................................. 89 2.2 Syntheses of monomers......................................................................... 91
2.2.1 Synthesis of 3-hexylthiophene....................................................... 91 2.2.2 Synthesis of 2,5-dibromo-3-hexylthiophene.................................. 92
2.2.3 Synthesis of 2-bromo-3-hexyl-5-iodo-thiophene............................ 93 2.3 Synthesis and characterization of regioregular P3HT......................... 95
2.3.1 Regioregular α,ω-diH-P3HTs........................................................ 95 2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs.................... 97
2.4 Synthesis of ω- or α,ω-alkynyl-P3HT by the GRIM method................ 98 2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT(P3) .. 99
2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method......... 103
2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator....................................................................................... 105
2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) ..................... 106 2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni-
initiator [(Ph)Ni(PPh3)2-Br] ............................................................ 107 2.7 Syntheses and characterizations of polyacetylene-graft-P3HT.......... 112
2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst.............. 113 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst......... 115 2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl
acetylene....................................................................................... 118 2.7.4 Attempted polymerisation of ω-vinyl-P3HTs.................................. 119
2.8 Conclusions............................................................................................. 120 2.9 References............................................................................................... 121
13
Chapter 3 Block copolymers based on P3HT and PS or P4VP 123
3.1 Introduction............................................................................................. 125 3.2 Synthesis of azide-terminated polystyrene.......................................... 127
3.2.1 Principle of atom transfer radical polymerisation (ATRP) ............. 127 3.2.2 Synthesis of azide initiator............................................................. 128 3.2.3 Synthesis of α-azido polystyrenes................................................. 131
3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry.................................................... 133
3.3.1 History and principle of “click” chemistry....................................... 133 3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS......... 134 3.3.3.1 Triblock copolymers PS-b-P3HT-b-PS........................... 134 3.3.3.2 Diblock copolymers P3HT-b-PS..................................... 138
3.4 Synthesis of donor-acceptor and acceptor-donor-acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block -PS-C60............................................................................................................. 143
3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) ..... 143 3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60......... 143
3.5 Synthesis and characterization of block copolymers P4VP-block-P3HT-block- P4VP.................................................................................... 147
3.5.1 Synthesis of α,ω-difunctionalised-P3HT by GRIM polymerisation 148 3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block-P4VP
by anionic polymerisation.............................................................. 154 3.5.2.1 Introduction to anionic polymerisation............................ 154 3.5.2.2 A short history of anionic polymerisation........................ 155 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP............................... 156
3.6 Physical characterisation di- and triblock copolymers....................... 158 3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and
without C60 chain-ends.................................................................. 158 3.6.2 P4VP-b-P3HT-b-P4VP block copolymers..................................... 163
3.7 Conclusions............................................................................................. 166 3.8 References............................................................................................... 167
14
Chapter 4 Photovoltaic performances and morphological characterizations of block copolymers........................ 169
4.1 Introduction............................................................................................. 171 4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b
and Plextronics P3HT) ........................................................................... 173 4.3 Photovoltaic performances of block copolymers................................ 177
4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM....................................................... 178
4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM...................... 182
4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block-P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM.................................................................................. 186
4.3.4 Triblock copolymer P4VP-block-P3HT-block-P4VP as a compatibilizer in the mixture of P3HT-blend-PCBM...................... 188
4.4 Conclusions............................................................................................. 190 4.5 References............................................................................................... 192
Chapter 5 Experimental Section ......................................................... 193
1 Materials................................................................................................... 197 1.1 Purification of Solvents.................................................................... 197 1.2 Purification of Monomers................................................................. 197 1.3 Chemicals........................................................................................ 197
2 Synthesis................................................................................................. 199 2.1 Monomers...................................................................................... 199 2.1.1 3-Hexylthiophene................................................................ 199 2.1.2 2,5-Bibromo-3-hexylthiophene............................................ 199 2.1.3 2-Bromo-3-hexylthiophene.................................................. 200 2.1.4 2-Bromo-3-hexyl-5-iodo-thiophene..................................... 200 2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis
(GRIM) ............................................................................................ 201 2.2.1 α,ω-DiH-P3HTs (P1, P1a, P1b and P1c) ............................ 201 2.2.2 Chain-end functionalised w-P3HTs or ω-P3HTs................. 202 2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω-pentynyl-
P3HTs................................................................... 202 2.2.2.2 α,ω-Diformyl and α,ω-dihydroxy-P3HTs................ 203
15
2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni-catalyst initiator............................................................................. 205
2.3.1 Ni-initiator: [(Ph)Ni(PPh3)2-Br] ............................................ 205 2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator. 205 2.4 Azide-terminated Polystyrene...................................................... 206 2.4.1 Azide initiator for ATRP....................................................... 206 2.4.1.1 3-Azido-1-propanol................................................ 206 2.4.1.2 3-Azidopropyl-2-bromoisobutyrate........................ 206 2.4.2 α-Azido-polystyrenes (PS1-PS6) ....................................... 207 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-
PS by Click Chemistry.................................................................. 208 2.5.1 Triblock copolymers PS-b-P3HT-b-PS................................ 208 2.5.2 Diblock copolymers P3HT-b-PS.......................................... 209 2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA....... 210 2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by
anionic polymerisation................................................................. 211 2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) ................................ 212 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst... 212 2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2
catalyst ............................................................................... 212 2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with
phenyl acetylene................................................................. 213 3 Characterization...................................................................................... 213 4 Photovoltaic device fabrication and characterization......................... 215
General conclusions........................................................................................ 217 Appendix........................................................................................................... 223 Publications and Conferences........................................................................ 227
16
17
General Introduction
18
19
General Introduction
The continuous use of fossil fuels (coal, oil, gas) that produces CO2, which
increases global warming, is drastically damaging our environment. At current
rates of consumption, CO2 levels are projected to reach considerably higher
levels than the “moderately stringent” limit of 550 ppm set by
Intergovernmental Panel on Climate Change (IPCC).1 The growing global
energy demand and the depletion of conventional fossil fuels means finding
an alternative to non-renewable resources. It is essential to develop research
into the materials that will enable renewable energy sources (such as nuclear,
wind, hydropower, biomass and solar) that will improve the quality of life.
From the considerations of energy sustainability and environmental
protection, solar energy is the largest carbon-neutral energy source to be
explored and can be utilized much more extensively.2 It is known that the
earth receives more energy from the sun in one hour than is required for all
human needs in a year. Therefore, if harvested economically, solar power is
clearly the most rational energy source to produce the step-change in energy
provision required to shape our world for the future environmental, economical
and technological demands. This energy source has also the advantage of
being available everywhere on the planet and enjoy a huge energy potential.
Today, solar cells based on silicon are about 99% of global production of
photovoltaic cells. But the present-day silicon technology has some
disadvantages: the silicon purification processes are extremely expensive and
the silicon availability is limited due to its extensive use in the field of
microelectronics. Consequently, the production cost of silicon solar panels is
too high to be economically viable. This is the major motivation for the
development of organic photovoltaic materials and devices (organic solar cells
or OSCs), which are envisioned to exhibit advantages such as low cost, high
device flexibility, and cheap fabrication from highly abundant materials.3,4
This new generation of photovoltaic cell is based on the discovery of
semiconducting polymers in 1977 by A. McDiarmid, H. Shirakawa and A.
Heeger, who won the Nobel Prize in chemistry, 2000. In 1986, C. W. Tang
introduced the first efficient bilayer OSC with 1% efficiency5 that was a
20
monumental shift from inorganic photovoltaics, which are thick, rigid and
fragile. OSCs have very interesting advantages, related to the nature of these
materials, such as the possibility of flexible modules, their low cost, but also
their shape, which can be controlled by roll-to-roll printing on a large scale.6
The simplest and to date, most successful technique is based on
solution-processed bulk heterojunction (BHJ) OSCs composed of electron-
donating semiconducting polymers and electron-withdrawing fullerides as
active layers.7 In the past 20 years, many modifications to OSCs have been
performed and introduced new photoactive materials, deposition techniques,
device architectures and electrode materials.8,9 These changes brought to
certified power conversion efficiencies of nearly 8% as reported recently10
which is an impressive milestone. However, the efficiency of OPVs is still
significantly lower than their inorganic counterparts, such as silicon, CdTe and
copper indium gallium selenide (CIGS), which prevents practical applications
in large scale. However, the scientific community as a whole accepts that
OSCs must overcome the 10% efficiency benchmark to become commercially
attractive. If the field continues to develop at the same dynamic rate,
expectations that the 10% efficiency milestone will be reached by 2011.
There are many factors limiting the performance of the BHJ solar
cells.11 Several physical processes occur successively in OSCs; absorption of
photons, creation of excitons, exciton diffusion exciton dissociation, and finally
the transport of charge carriers to the electrodes. However, the morphology of
the films in OSCs plays a very important role and affects both the process of
dissociation of the exciton but also the charge transport to electrodes. The
majority of OSC research carried out over the past 17 years have focused on
the donor/acceptor bulk heterojunction approach using conjugated
semiconducting polymer as the donor, and a fullerene derivative as the
acceptor. The most successful system consists of a physical mixing of poly(3-
hexylthiophene) (P3HT) as donor and a fullerene derivative, [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM), as acceptor. However, it is very difficult in
this case to control the morphology of the active layer, the two compounds
(donor and acceptor) behave independently of one another, leading to
21
completely random structures. Several solutions are proposed to overcome
this problem, the use of double-cable polymers or the use of block copolymers
(BCPs), which is the main subject of this thesis since BCPs have the ability to
self-organize to form nanoscale structures to optimize various parameters of
the photovoltaic process and also the exciton distance exactly coincides well
with the typical size of block copolymer domains. Since the early 2000s, BCPs
for photovoltaic applications has generated a lot of research and multiple
strategies, which depend on the nature of each block that has been
developed. Though these BCPs provide access to morphologies whose
structure and size are favorable for photovoltaic process, the synthesis of
these new materials suffer, in general, from complex methods, and
performances that are still rather low.
Thesis overview The motivation of the research work described here was to develop a
simplified and versatile synthesis of BCPs, to understand the microstructure of
functional BCPs and to explore the use of these materials as active layer or
compatibilizer in OSCs. This work is mainly focused on BCPs based on P3HT
due to their high hole mobility, their chemically tunable electronic properties
and their processibility from various solvents.12-14 Chain-end functionalised
P3HTs were used as the building blocks for the syntheses of BCPs containing
other blocks.
Chapter 1 begins with an introduction to the main families of
conjugated polymers, their electronic structures and mechanism of electrical
conduction. The second part of Chapter 1 gives brief update on organic
photovoltaics; history, development of OSCs, the operating principle of an
OSC, various device architectures currently used in OSCs and focused review
on low band-gap polymers. Then a review of the archetypal conjugated
polymer, P3HT, which is the main focus in this thesis, looks at the various
synthetic methods and the mechanism leading to regioregular P3HT. Finally,
a detailed review on the interest of BCPs in the field of OSCs and the
literature on rod-coil BCPs are explained.
22
Chapter 2 explores the synthesis of regioregular P3HTs, and
regioregular end-functionalised ω- and α,ω-alkynyl, and alkenyl P3HTs. It
explores the synthesis of a small molecule Ni-catalyst initiator based on
phenyl bromide for an attempt to prepare purely monofunctionalised P3HT.
Attempts were then made to synthesize PA-graft-P3HT using synthesized
macromonomers of P3HTs.
Chapter 3 describes the synthesis of donor-acceptor rod-coil block
copolymers in which rod block is P3HT and the coil block polystyrene (PS) or
poly(4-vinylpyridine) (P4VP) for their application in photovoltaics. It explains
the two different synthetic approaches to obtain donor-acceptor block
copolymers. In one case, the acceptor fullerene (C60) is covalently attached to
the insulating block polystyrene (PS), and in the other case, weak
supramolecular interactions produced by complex formation between
insulating block poly(4-vinylpyridine) (P4VP) and a C60 derivative (PCBM) is
explored. The di- and tri-block copolymers P3HT-b-PS and PS-b-P3HT-b-PS
were synthesized by 1,3-dipolar Huisgen addition, known as "click" chemistry
from P3HT functionalized alkyne and azide functionalized PS. Then fullerene
(C60) was then attached to these block copolymers by atom transfer radical
addition (ATRA) to obtain the donor-acceptor copolymers. The other tri block
copolymers of ABA coil-rod-coil, P4VP-b-P3HT-b-P4VP in which rod block is
P3HT and the coil block is P4VP were synthesized by anionic polymerisation
from quenching of living P4VP chains with P3HT di-functionalized aldehyde.
Finally, it describes the physical characterization of all the synthesized
copolymers.
Chapter 4 explores the photovoltaic characterization of some of the
synthesized materials in this thesis. First, it describes the photovoltaic
performances of synthesized P3HTs of different molecular weights, P1 (25
kg/mol), P1a (50 kg/mol), P1b (100 kg/mol) and compared the performances
with the commercially available P3HT (Plextronics, 50 kg/mol). It was
therefore necessary to characterize and optimize the P3HTs performance in
mixture with PCBM for a reference and compare its power and photovoltaic
characteristics with the addition of block copolymers as compatibilizers to the
23
reference P3HT-blend-PCBM. Finally some of the copolymers were examined
as active layer or compatibilizers in the OPV devices.
Chapter 5 shows all the detailed experimental methods for the
synthesis and characterization of monomers, regioregular P3HTs, chain-end
regioregular end-functionalised P3HTs by the GRIM method, “small molecule”
Ni-catalyst initiator, monofunctionalised P3HTs, graft copolymers PA-graft-
P3HT, di- and tri-block copolymers P3HT-b-PS, P3HT-b-PS-C60 and PS-b-
P3HT-b-PS, C60-PS-b-P3HT-b-PS-C60, P4VP-b-P3HT-b-P4VP.
References:
1. http://www.ipcc.ch/ipccreports/tar/wg3/pdf/2.pdf.
2. Lewis, N. S. Global Energy Prospective. Solar Energy Workshop; US
Department of Energy: Washington, DC, 2005.
3. Lewis, N. S. Science 2007, 315, 798.
4. Brabec, C. J. Organic Photovoltaics: Technology and Market. Sol. Energy
Mater. Sol. Cells 2004, 83, 273–292.
5. Tang, C. W. Two-layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48,
183–185.
6. Brabec, C. J.; Durrant, J. R. MRS Bull. 2008, 33, 670–675.
7. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,
270,1789.
8. Sun, S.; Sariciftci, N. S.; Eds. Organic Photovoltaics: Mechanisms, Materials,
and Devices; Taylor & Francis: Boca Raton, FL, 2005.
9. Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58–77.
10. Liang, Y.; Xu, Z.; Xia, J.; Tsai S-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater.
2010, 22, E135–E138.
11. Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323.
12. Nalwa, H. S.; Ed. Handbook of Organic Conductive Molecules and Polymers;
J. Wiley & Sons: New York, 1996.
13. McCullough, R. D. Adv. Mater. 1998, 10, 93–116.
14. Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202–1214.
24
25
Chapter 1: Literature Review
26
Contents
1.1 Polymers as semiconductors................................................................ 27 1.1.1 Electrical conductivities of conjugated polymers........................... 27 1.1.2 Archetypal polymeric semiconductors........................................... 30 1.1.3 Electronic structures of conjugated polymers................................ 31 1.1.3.1 Undoped polymers......................................................... 31 1.1.3.2 Doped polymers............................................................. 33 1.1.4 Improving the processibilities of conjugated polymers.................. 34
1.2 Organic photovoltaics............................................................................ 35 1.2.1 Solar energy.................................................................................. 35 1.2.2 Definition, history and development of photovoltaic cells.............. 37 1.2.2 Operating principle of organic photovoltaic cell............................. 42 1.2.4 Efficiency characteristics of organic photovoltaic cells.................. 43 1.2.5 Organic photovoltaic active layer architectures............................. 46 1.2.6 Novel low band-gap polymers....................................................... 49
1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexyl thiophene) (P3HT) ............................................................ 53
1.3.1 Regioregularity.............................................................................. 53 1.3.2 McCullough and Rieke methods.................................................... 54 1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT..... 55 1.3.4 Chain-growth condensation polymerisations leading to P3HT...... 56 1.3.5 Chain-end capping of P3HT using GRIM...................................... 57
1.4 Organic photovoltaic cells and block copolymers.............................. 59 1.4.1 Importance of morphology of active layer...................................... 59 1.4.2 Controlling morphology of active layer in blends........................... 60 1.4.3 Self-assembly behaviour of rod-coil block copolymers.................. 63 1.4.4 Why block copolymers in photovoltaic cells? ............................... 65
1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers.................................................................................... 67
1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s....... 68 1.4.5.2 Copolymers based on polythiophenes........................... 71
1.5 References............................................................................................... 77
27
1.1 Polymers as semiconductors 1.1.1 Electrical conductivities of conjugated polymers Conductive polymers are organic polymers that possess both metallic
conductivity and processibility. Generally, current is defined as the net flow of
charge through a material in a definite direction for a given time and the
charge carriers can be free electrons or holes. A hole may be described as a
vacancy previously occupied by an electron. Hence, a hole is oppositely
charged to an electron and in the presence of applied electric field, hole
moves in the opposite direction of an electron.1 The conduction mechanism
varies depending on the material in which the three most common categories
are metals, semiconductors and insulators. Metals readily conduct current and
normally “electron sea” model is used to explain the flow of current. In metals,
the delocalization of the valence electrons due to the nature of the metallic
bond, the electrons can move easily in the “sea” under an applied electric
field.2 In semiconductors, the electrons are more strongly bound to the nuclei
of their associated atoms.
The best understood of semiconductors are probably those based on
inorganic materials. Indeed, much of the theory for organic materials arose
from that previously developed in this area. Generally charge transport in
inorganic semiconductors is demonstrated by band theory.1,3,4 The underlying
concept is that regular covalent bonding creates a crystal structure which
allows to form bands where charge is transported. The valence electrons,
which are bound to the nuclei of the semiconducting atoms, create the
valence band. A sufficient energy is given to an electron to overcome its
attraction to the nuclei, and then the electron can enter to the conduction band
where it can move freely in the semiconductor by creating a hole in the
valence band. The energy difference between the valence and conduction
bands is called as the band gap energy (Eg). Semiconductors are having
small band gap energies (Eg < 4 eV) and therefore, a considerable amount of
electrons can be transferred from the valence band (VB) to the conduction
band (CB).1-4 Inorganic insulators are also having same type of band structure
as semiconductors but Eg of an insulator is very high. Therefore, electrons are
28
not promoted to the CB and then no flow of current observed in insulators.
Most of the organic small molecules and polymers are insulating materials
due to their very large band gaps. In organic solid, individual molecules are
prepared by covalent bonds. Generally intermolecular interactions (van der
Waals interactions) are much weaker in organic solid than in inorganic solid
and prohibit the formation of band-like transport.5 But, organic semiconductors
have characteristic bonding patterns in which alternate arrangement of
carbon-carbon bonds between single and double bonds (“conjugation”) in
such a way that only three atoms covalently bound to each carbon nucleus
(sp2-hybridization).6 Thus this hybridization permits valence p-orbital electrons
to become delocalized and contribute to the current.
The conductivities of representative conjugated polymers both in the
neutral and doped states are shown in Figure 1.1.
Figure 1.1 Electronic conductivities of conjugated polymers with different degrees of doping. [Reference: Skotheim TA, Elsenbaumer RL and Reynolds JR (eds), Handbook of Conducting Polymers, 2nd edition, Marcel Dekker Inc, New York (1998)].
29
While conductivity in conjugated polymers is typically at least one order
of magnitude lower than that of metals, the ability to control conjugated
polymers from insulating to conducting and their ease of processing has led to
some unique applications.
The electrical conductivity (σ) of a conjugated polymer is equal to the
inverse of its specific resistivity (ρ), which is a measure of the ability of the
conjugated polymer to conduct an electrical charge. Electrical conductivity is
determined by measuring the resistance (R) to charge transport through a
known volume, where L is the length over which resistance is being measured
and A is the cross-sectional area through which the current passes:
σ =1/ρ = L/(R·A) Eq. 1.1 From the above discussion it is clear that conducting polymers are
alternative source of semiconducting materials that can be used to replace
relatively expensive and environmentally dangerous inorganic semiconducting
materials. They are excellent candidates for electroluminescent devices,
rechargeable batteries, sensors, electrochromic windows, photovoltaic
devices, photodiodes etc. due to their interesting opto-electronic properties.
The great discovery, high conductivities of doped polyacetylene (PA) in
1970s7,8 by MacDiarmid, Heeger and Shirakawa who received chemistry
Nobel prize in 2000, encouraged researchers for optimizing their electronic
properties.9 Especially, π-Conjugated organic compounds are very important
in the electro-optics field because of their non-linear optical (NLO) behaviour
and photoconductivity.10-13 Hence the development of these conjugated
polymers brought their use in various applications such as electroluminescent
diodes (PLED),14,15 photovoltaic cells,16-20 stable electronic memories,21
polymer field effect transistors (PFET)22-25 and so on.26-28
30
1.1.2 Archetypal polymeric semiconductors Since the discovery in 19778 that PA demonstrates conductivity about
103 S cm-1 by doping with Br2, I2 or AsF5, it triggered a real interest in the field
of conducting polymers. But the application of PA is limited because of its
poor solubility and low thermal stability. To avoid these problems, researchers
have developed many aromatic conjugated polymers such as poly(para-
phenylene) (PPP),29 polythiophene (PT),30 polypyrrole,31 and others shown in
Figure 1.2. Recently, many conjugated polymers resulting from these parental
structures have been synthesized such as poly(3,4-ethylenedioxythiophene)
(PEDOT), one of the most widely explored.32-35 This polymer shows a high
conductivity (ca 300 S cm-1), quasi-transparency in the form of film and very
high stability in the oxidized state.33-36 Another great discovery by Friend and
colleagues in 1990 was the green electroluminescence of undoped poly(p-
phenylenevinylene) (PPV)37 and it was followed in 1991 by the manufacture of
the first blue polymer light emitting diode (PLED) made up of poly(9,9'-di-n-
hexylfluorene).38
n S nn N
H
n
trans-polyacetylene poly(para-phenylene) polythiophenepolypyrrole
n n S n
poly(p-phenylene vinylene) poly(p-phenylene ethynylene) poly(thienylene vinylene)
S
OO
nn
polyfluorene poly[3,4-(ethylenedioxy)thiophene]
Nn
polycarbazoleH
SHN
HN
n nn
poly(phenylenesulfide) poly(diphenylamine) polyaniline Figure 1.2 Chemical structures of principal families of conjugated polymers.
31
1.1.3 Electronic structures of conjugated polymers 1.1.3.1 Undoped polymers As I mentioned earlier, the majority of conjugated polymers consist of a
regular alternate single and double bonds that allow the π-delocalization of
single 2pz valence electrons at each carbon atom along the polymer skeleton.
The π-delocalization, is influenced by the geometry of the system, is
maximum if π-conjugated system is planar and any deviation from planarity
results the reduction in conjugation.39 The electronic structure of these
systems depends on their different levels of molecular orbitals and
predominantly the value of their HOMO (Highest Occupied Molecular Orbital)
and LUMO (lowest unoccupied Molecular Orbital). The HOMO represents
together the highest occupied energy levels and the LUMO represents the
lowest unoccupied energy levels. This energy difference corresponds to a π-
π* transition in simple molecules and band gap in the polymers.
The HOMO and LUMO levels of a conjugated polymer depend on the
degree of conjugation, i. e. the number of monomer units (Figure 1.3). If the
repeating units become very high, it passes a series of discrete levels where
the energy levels are grouped into two bands, the valence band (VB) and the
conduction band (CB). All HOMO group together to form the VB and all LUMO
combine to form the CB, the energy difference between these two levels is
called as band gap or forbidden band (Eg) as shown in Figure 1.3. This value
Eg determines the electronic properties of the conjugated polymers and limits
the polymers as semi-conductors instead of metallic conductors. Therefore,
the electrons from VB must overcome this band gap (Eg) to move. This band
gap can also be defined as the difference between the energy to pull an
electron from the highest point of the CB (i.e. the ionization potential, or IP)
and the energy required to inject an electron into the lowest point of the VB
(i.e. the electron affinity, or EA). Eg is usually in between 0.8-4.0 eV that
coincides with the energy of visible light. So the electrons can interact with
light and therefore this property is explored in various opto-electronic
applications. Many parameters such as chain planarity, in-chain defects and
also impurities can further change the energy levels of the bands.40-47
32
Figure 1.3 Molecular orbital diagram (π-levels) as a function of the number of monomer units. (Reference: A.J. Attias, Techniques de l'Ingénieur, E1862, 2002).
According to this model, it is thus possible to categorize materials with
respect to their band gap size. Intrinsic semiconductors have a band gap
values between 0 and 3 eV. The insulators have a similar band structure as
that of the semiconductors but their bandgap is too high (>4 eV). The majority
of conjugated polymers are semiconductors and band gaps of some
conjugated polymers are given here; trans-polyacetylene (PA) (1.4 - 1.5
eV),48,49 polythiophene (PT) (2.0 - 2.1 eV),50,51 poly(p-phenylene) (PPP) (2.7
eV),52 poly(p-phenylene vinylene) (PPV) (2.5 eV),53 polypyrrole (PPy) (3.2
eV),54 poly(3,4-ethylenedioxythiophene) (PEDOT) (1.6 eV ).55,56
The Peierls effect means that the conjugation of conductive polymers
permits them to have two electronic resonance structures. If they are
equivalent in energy, then the system is called “degenerate” as in the case of
trans-PA shown in Figure 1.4. But if the two resonance structures are not
energetically equal, then the energy levels of the system are called “non-
degenerate” which is the case for PPP, PT and PPV. The majority of the
conjugated polymers exist in two resonance forms: aromatic and quinoid
forms which are not equal in energy. There is a ground state aromatic form
and a more excited quinoid state in PT shown in Figure 1.5.42
33
a.
n n b.
Figure 1.4 (a) Mesomeric structures of trans-PA and (b) Potential energy curve for PA showing two energetically equivalent structures (degenerate).
Figure 1.5 Total energy curve for PT showing two energetically inequivalent structures (non-degenerate). 1.1.3.2 Doped polymers To combine the mechanical properties of polymers with the conducting
properties of metals, one can introduce a load into semiconductor polymer by
a process known as “doping”. This process involves a charge-transfer redox
reaction in which the introduction of electron-withdrawing (p-type doping) or
electron-donating (n-type doping) impurities into the polymer and is mainly
carried out by chemical or electrochemical ways. However, semiconducting
polymers do not undergo easily reversible and controllable doping. In the case
Ene
rgy
Deformation coordinate
34
of chemical doping with a chemical oxidant (p-type doping) or reductant (n-
type doping), a neutral conjugated polymer is transformed into a poly(cation)
or poly(anion) respectively. At the same time, a counter-ion is associated to it
inorder to maintain the overall electro-neutrality of the system, as shown in the
following examples (Eqs 1.2 and 1.3).57
• p-type doping:
(π-polymer)n + 3/2 ny(I2) → [(π-polymer)+y (I3-)y]n Eq 1.2
• n-type doping:
(π-polymer)n + [Na+ (C10H8)-•]y → [(Na+)y (π-polymer)-y]n + (C10H8)0 Eq 1.3
1.1.4 Improving the processibilities of conjugated polymers The precessibility of many conjugated polymers is often suffered by their poor
solubility and mechanical properties. With the introduction of some side
groups, such as alkyl58-60 and poly(ethylene oxide) (PEO)61 chains, or some
polar groups such as quaternary sulfonates62,63 or ammoniums,64 these
problems can be rectified (Figure 1.6).
S n
C6H13
S n
S n
H3C OO m
SO3- Na+
S n
OO
O
SO3- Na+
n
C6H13 C6H13poly(3-hexylthiophene) (P3HT)poly(9,9'-di-n-hexylfluorene)
poly[3-oligo(ethylene oxide)-4-methylthiophene]
sodium poly[2-methoxybutylenesulfonate-(3,4-ethylenedioxythiophene)]
sodium poly(3-butylenesulfonate-thiophene)
Figure 1.6 Conjugated polymers with pendant groups to improve the handling properties.
The conjugated polymers combined with flexible coil-like polymers
such as polystyrene,65-67 polyisoprene,68-70 poly(methyl methacrylate),67 and
poly(ethylene oxide)66,70 brought a radical change in the properties of these
35
polymers. As well as improving mechanical properties and solubilities, the
resulting rod-coil copolymers based on the structure shown in Figure 1.7
produced an extraordinary range of supra-macromolecular structures. This
self-assembly of diblock and triblock conjugated copolymers will be detailed
later in the Section 1.4.3.
Figure 1.7 Schematic representation of rod-coil di- and triblock copolymers.
1.2 Organic photovoltaics 1.2.1 Solar energy Due to continuous industrialization and growth of the human
population, the energy consumption in 2050 is expected to be 28‐35 TW
which cannot be met with the energy sources currently in hand. Most of our
present energy is derived from fossil fuels (coal, oil, gas) but the supply is
finite and the energy derived from combustion of fossil fuels produces CO2
which is supposed to be responsible for the acceleration of global warming
(Figure 1.8).71 This limited supply of fossil fuel sources and the negative
long‐term effects of CO2 call for the development of renewable energy
resources. Providing energy from non‐CO2‐emissive sources is required to
prevent global warming that might induce irreversible climate changes.72
Figure 1.8 (a) World market energy consumption in exajoule (EJ = 1018 J); and (b) World carbon dioxide emission in billion metric ton (BMT).71
36
Extensive studies have been done in exploring various renewable
energy sources to respond the growing energy demand. The expansion is
limited for hydropower because most sites are already utilized whereas in the
case of wind power, the ideal location to install wind turbines depends
critically on geographic and climate conditions since the generated power is
relative to the cube of the wind speed. And also, most of these places are far
away from population and industry. Sunlight strikes everywhere at the surface
of the earth with 165 000 TW of power that corresponds to 1000 W/m2.73 In
other words, the earth receives approximately 430 EJ per hour from the sun
which is equivalent to all human present needs in one year. Hence,
comparing the other energy sources and global consumption, solar energy is
the most attractive and abundant (Table 1.1).
Energy source EJ/year
Solar energy 430 (per hour)
Hydropower 1.9
Wind power 0.4
Geothermal 0.04
Global consumption 480
Table 1.1 Comparison of renewable energy sources and global consumption.71, 74
Thus harvesting energy directly from sunlight and converting into
electrical energy using photovoltaic (PV) technology is increasingly
recognized as part of the solution to the growing energy challenge and a
fundamental factor of the future global renewable energy production.75
The intensity of sunlight that reaches Earth’s outer surface of the
atmosphere is referred as air mass zero (AM0) and is equal to 1353 W/m2.76
After passing through the Earth’s atmosphere, the intensity of light decreases
due to absorption and scattering of light by dust particles. Obviously, the
amount of solar radiation that reaches a terrestrial observer depends on the
person’s exact location on the Earth. The commonly used standard for
obtaining power conversion efficiencies of photovoltaics is air mass 1.5
(AM1.5) because it is representative of the sunlight available in most of United
States and Europe. Specifically, it represents the average sunlight incident on
a south-facing position at 37° N during a year.77 The solar spectrum at AM0
37
and AM1.5 are shown in Figure 1.9. It shows that most of the spectral
irradiance is at wavelengths of less than 2000 nm. There is a large amount of
sunlight present at wavelengths 750 nm < λ < 2000 nm (equivalent to 1.65 to
0.62 eV, respectively) that cannot be absorbed by many of the prototypical
semiconductors due to their relatively wide band gaps. Therefore, an active
area of current research is finding low band gap organic semiconductors (Eg <
1.5 eV) so that more incident radiation can be harvested.78 The AM1.5
spectrum can be obtained by the use of lamps and filters in the laboratory and
solar simulators are commercially available.79
Figure 1.9 Solar spectra for AM0 and AM1.5 air mass conditions.77
1.2.2 Definition, history and development of photovoltaic cells The “photovoltaic effect” is the conversion of absorbed solar photons directly
into electrical energy and was first discovered in 1839 by the French physicist
A. E. Becquerel. He found that a photocurrent emerged when platinum
electrodes, covered with silver bromide or silver chloride, was illuminated in
aqueous solution.80 The term "photovoltaic" (PV) comes from the Greek word
“phōs” meaning "light", and "voltaic", meaning electric, from the name of the
Italian physicist Volta, after whom a unit of electro-motive force, the volt, is
named. The term "photo-voltaic" has been in use in English since 1849.81
38
Smith and Adams’ work on the photoconductivity of selenium in 187382
and 187683 respectively, lead to further understanding PV effect. However, it
was not until 1883 that the first solar cell was built, by Charles Fritts, who
coated selenium with an extremely thin layer of gold to form junctions and the
device was around 1% efficient. Subsequently Russian physicist Aleksandr
Stoletov built the first solar cell based on the outer photoelectric effect
(discovered by Heinrich Hertz earlier in 1887). Albert Einstein explained the
photoelectric effect in 1905 for which he received the Nobel Prize in Physics
in 1921. Russell Ohl patented the modern junction semiconductor solar cell in
194684, which was discovered while working on the series of advances that
would lead to the transistor. The first inorganic silicon-based solar cell with an
efficiency of 6% was discovered by Pearson, Fuller and Chapin at Bell
Laboratories in 1954.85 During the 1960s and 1970s, the terrestrial installation
of PV cells opened the initial market of daily utilization.86 Over the years, the
efficiency of the crystalline silicon cell has recently attained 25 %.87
The field of photovoltaics is at the moment dominated by silicon-based
solar cells. Due to the large availability of the used material and extensive
knowledge from the microelectronics industry, crystalline silicon solar cells
currently have a 90% market share.88 The main drawback of this type of
devices is the high purity needed for proper device operation. The energy,
and thus costs, needed in the fabrication process limits its usefulness as an
alternative energy source. Second generation photovoltaics are under active
investigation in order to further reduce the cost of produced electricity. This is
so-called thin film photovoltaic technology that includes cadmium sulphide
(CdS), cadmium telluride (CdTe), chalcogenides such as copper indium
diselenide (CIS) or copper indium gallium selenide (CIGS), amorphous and
nanocrystalline silicon. Such inorganic semiconductor materials are more
absorbing than crystalline silicon and can be processed into thin film directly
onto large area substrates using techniques such as sputtering, physical
vapour deposition, and plasma‐enhanced chemical vapour deposition. The
fabrication of low cost inorganic thin film solar cells with efficiencies ranging
from 10‐19% have been demonstrated in the laboratory89 but the controlled
manufacturing still remains a challenge. The best laboratory efficiencies of
39
solar cells obtained for various materials and technologies are shown below
(Figure 1.10).
Figure 1.10 The best laboratory efficiencies of solar cells obtained for various materials and technologies. [Source: NREL] In the meantime, the research based on the photo-electronic properties
of organic molecules and devices accelerated after the photoconductivity was
discovered in anthracene by Pochettino in 190690 and Volmer in 1913.91
During 1950-60s, the development of organic materials as photoreceptors
extended the possibility of organic molecules as electronic materials.92 The
first organic heterojunction solar cell based on a copper phthalocyanine and a
perylene tetracarboxylic acid derivative was reported by Tang in 1986 (Figure
1.11).93
Figure 1.11 Chemical structures of archetypal organic molecules.
40
After the discovery of the ultrafast charge transfer between poly[2-
methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and
buckmisterfullerene (C60) by Sariciftci et al. in 1992,94 there has been a rapid
increase in efficiencies from less than 0.1% to greater than 7% in 2010 for
laboratory scale devices.95,96a Recently, National Energy Renewable
Laboratory (NREL) has announced that Konarka’s latest organic based
photovoltaic (OPV) solar cells have demonstrated a world record efficiency of
8.3%.96b It is therefore hoped that the research will deliver higher efficiencies
and stabilities appropriate to long-term industrialisation in the near future
(Table 1.2).
The pace of development of organic photovoltaic devices (OPV) makes
them the expected low-cost alternative to their more expensive inorganic
counterparts. OPVs are particularly attractive because of their ease of
processing, mechanical flexibility and potential for low cost fabrication of large
area devices. In addition, their material properties can be substantially
adapted by modifying their chemical structure, resulting in greater
customization compared to traditional inorganic solar cells.
The field of organic photovoltaics can be divided into three classes
spanning small molecules,97,98 dye‐sensitized99‐101 and polymer based solar
cells. π‐conjugated polymers in OPVs are an especially attractive alternative
to traditional silicon‐based solar cells because they are strong absorbers of
visible light, in even <100 nm thin film devices, and can be deposited onto
flexible substrates over large areas using wet‐processing techniques such as
spin‐coating, printing or roll‐to‐roll coating.102‐115
41
Year Author Research contribution Ref.
1986 C.W. Tang First organic heterojunction PV cell. 93
1991 Hiramoto et al. First dye/dye bulk heterojunction PV cell. 116
1993 Sariciftci et al. First polymer/C60 heterojunction PV cell. 117
1994 Yu et al. First bulk polymer/C60 heterojunction PV cell. 118
1995 Yu et al./Halls et al. First bulk polymer/polymer heterojunction PV cell. 119,
120
2000 Peeters et al. Oligomer-C60 dyads/triads PV cells. 121
2001 Schmidt-Mende et al. Self-organized liquid crystalline PV cell. 122
2001 Ramos et al. Double cable polymer PV cells. 123
2002 Brabec et al. First low-band gap polymer/PCBM PV cell with η = 1% 124
2003 Wienk et al. First bulk heterojunction polymer/PC70BM PV cell 125
2009 Sung et al. Bulk heterojunction polymer solar cell η = 6.1% 126
2010 Yongye Liang et al. Bulk Heterojunction Polymer Solar Cell η = 7.4% 95
2010 Solarmer OPV Record efficiency η = 8.13 % 96a
2010 Konarka OPV World record efficiency η = 8.3 % 96b
Table 1.2 Milestones of Organic PV cells.
The dye-sensitized solar cell (DSC) was introduced by O’Reagan and
Grätzel in 1991 and consists of a nanoporous titanium oxide (TiO2) layer.101
The main disadvantage of DSC’s is the use of the liquid electrolyte, which
causes stability problems.127
Small molecule solar cells are fabricated by thermal evaporation of a
donor and acceptor material in either a double layer structure93 or a bulk
heterojunction similar to polymer solar cells.128 The advantage of small
molecule cells is the large control of the deposition enabling for instance
combinations of bilayer and bulk heterojunctions. On the downside the
vacuum based deposition does not comply with the concept of a low cost and
high throughput fabrication techniques.
Polymer solar cells are based on π -conjugated polymers as electron
donors. Modification of the molecular structure allows to modify chemical and
physical properties and have resulted in a number of well performing
materials with different band gaps and energy levels such as poly(3-
hexylthiophene) (P3HT),129 poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta‐
42
[2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (PCPDTBT),130
poly[9,9-didecanefluorene-alt-(bis-thienylene)benzothiadiazole] (PF10TBT)131
and poly[3,6-bis-(4’-dodecyl-[2,2’]bithiophenyl-5-yl)-2,5-bis-(2-ethyl-hexyl)-2,5-
dihydropyrrolo[3,4-]pyrrole-1,4-dione] (PBBTDPP2).132 As an acceptor either
another semiconducting polymers,133,134 inorganic materials135 or fullerenes136
can be used.
1.2.3 Operating principle of organic photovoltaic cells When the HOMO‐LUMO levels are appropriately matched between proximate
electron donor and acceptor material, absorption of light by either of the
materials can lead to photoinduced charge transfer between the materials.
For example, upon light absorption in the donor material an electron is excited
from the HOMO into the LUMO to obtain an exciton. From this excited state,
the electron may be transferred into the LUMO of the acceptor resulting in
free charge carriers. The driving force for this photoinduced charge transfer is
the difference in ionization potential (ID) of the excited donor and the electron
affinity (EA) of the acceptor, minus Coulombic correlations.18 After the
photoinduced charge transfer, the positively charged hole remains on the
donor material whereas the electron is located on the acceptor material.
Finally the free charge carriers need to be transported to the respective
electrodes to create a photovoltaic effect. At this point the donor material
serves to transport the holes whereas the electrons travel within the acceptor
material. The charge carrier transport is driven by internal electric fields
across the photoactive layer caused by the different work function electrodes
for holes and electrons.
43
The complete process starting from an absorbed photon to charges
collected at the electrodes is mainly divided into four steps (Figure 1.12).137
1.2.4 Efficiency characteristics of organic photovoltaic cells The solar cell performance and electrical characteristics are determined by
measuring the current density to voltage (J-V) characteristics, both in dark
and under illumination. In the dark, there is almost no current flowing until
external voltages larger than the open circuit is applied. Figure 1.13 shows J-
V characteristics of an organic solar cell under illumination. From the J-V
curve, four parameters can be obtained. The current density under
illumination at zero applied bias is called the short circuit current density (Jsc),
Figure 1.12 Operative process in an OPV (From G. Chidichimo and L. Filippelli International Journal of Photoenergy 2010, 123534)
An exciton after a photon absorbed by the donor material (1). This exciton diffuses
towards a donor/acceptor interface where the electron is transferred to the acceptor
material (2). Even though the hole and electron are now on different materials they are
still strongly bound by Coulombic interactions and need to be dissociated into free
carriers (3). Then finally they are transported through the two respective phases and are
collected at the electrodes (4). During each of the above-mentioned processes energy
can be lost due to various loss mechanisms: all photons are not absorbed by the active
layer, not only due to limitations of the bandgap but also due to the often limited thickness
of the active layer; excitons decay when created too far from the D-A interface; geminate
recombination of the bound electron hole pair can occur; and bimolecular recombination
of free charge carriers during transport to the electrodes.
44
when the current density under illumination is zero the cell is at the open
circuit voltage (Voc). The Voc is limited by the energy difference between the
HOMO of the donor material and the LUMO of the acceptor.
Figure 1.13 Current-voltage (I-V) characteristics and the corresponding power-voltage curve for a solar cell under illumination.138 The essential parameters determining the photovoltaic performance are shown: Jsc is the short-circuit current, Voc is the open-circuit voltage, Jmp and Vmp are the current and voltage, respectively, at which a given device’s electrical power output is the maximum, Pmax, the fill factor (FF) is a graphic measure of the “squareness” of the I-V curve, and the power conversion efficiency (PCE) is defined as the ratio of maximum power output (Pmax) to power input (Pin).
The Voc of a conjugated polymer/PCBM solar cell can be estimated by:
Voc = [−ELUMO (A) − EHOMO (D)] − 0.4V Eq 1.4
where EHOMO(D) is the oxidation potential of the polymer (donor), ELUMO(A) is
the reduction potential of PCBM and the value 0.4 V is the approximate
voltage loss at the interfaces.139,140
The maximum power the device can produce is characterized by the
maximum power point (MPP). The MPP is determined by:
Eq 1.5
where Vmax and Jmax are the voltage and current at the MPP.
The fill factor (FF) is the ratio between the MPP and the maximum theoretical
power output:
45
Eq 1.6
With the value of FF, the power conversion efficiency (η) can be written as
Eq 1.7
where Pin is the incident light power.
The power conversion efficiency has to be determined under standard
test conditions which includes the temperature of the solar cell (25 ºC), an
illumination intensity of 1000 W/m2 and a spectral distribution of the
illumination source (AM1.5).141 Since the spectrum of the used illumination
source is in general not the same as the AM1.5 solar spectrum, the mismatch
factor (M) for the measurement has to be determined using the equation142
Eq 1.8
where ER(λ) and ES(λ) are the AM1.5 solar spectrum and spectrum of the
used illumination source and SR(λ) and ST(λ) are the spectral responses of a
reference cell and the tested cell, respectively.
To determine the spectral response of the tested cell, Incident photon-to-
current efficiency (IPCE) measurements can be done. Figure 1.14 shows an
example of an IPCE characterisation, also known as External Quantum
Efficiency (EQE) measurement, which besides determining the mismatch
factor of the measurement is also very useful for determining loss
mechanisms in solar cells.
46
Figure 1.14 External Quantum Efficiency (EQE), also known as Incident Photon-to-Current Efficiency (IPCE) of a polymer : fullerene solar cell, measured at short circuit conditions.
The incident photon to current efficiency (IPCE) is the ratio of the
number of charge carriers collected at short circuit per incoming photon of a
given energy shining on the device. The IPCE can be calculated by:
Eq 1.9
Where e is the elementary charge (1.602 × 10‐19 C) and PPhotons is the number
of photons.
1.2.5 Organic photovoltaic active layer architectures There are three predominant architectures that have been used in the
fabrication of organic solar cells (OSCs). The first generation was based on a
single organic layer sandwiched between two different metal electrodes
(Figure 1.15a).143 The current was generated due to the potential difference
induced by the asymmetric work functions of the electrodes under light
irradiation. Because of the large exciton binding energy in organic
semiconductors,144 the difference in the work functions is usually not high
enough to produce sufficient photoinduced charge generation. And also, the
exciton diffusion distance is low (~5-20 nm),145-148 and only excitons
47
generated in the region close to the electrodes can convert into separate
charges that can be collected.
(a) Single layer (b) Bilayer (c) Bulk-heterojunction
Figure 1.15 Different structures employed in OSCs.
In order to improve the efficiency of OSCs, the bilayer heterojunction
(Figure 1.15b) structure, where two separate layers for the electron and hole
transporting organic materials are stacked between the electrodes was
introduced. Electrostatic forces result at the interface between the electron
and the hole transporting materials due to the difference in the electron affinity
of the electron transporting material and the ionization potential of the hole
transporting material. When this local electric field is strong enough to exceed
the exciton binding energy, the excitons are dissociated into electrons and
holes. The first bilayer heterojunction solar cell produced 1%, reported by
Tang in 198693 is also limited by the exciton diffusion length as excitons
formed at positions further away from the donor-acceptor interface than the
exciton diffusion length have a lower probability of generating free charge
carriers.
The limitation of the bilayer approach was overcome with the
development of the bulk heterojunction (BHJ),136,149 where the photoactive
layer consists of an intimately mixed blend of the donor and acceptor material
(Figure 1.15c and Figure 1.16b), which indeed led to a major increase in
generated free charge carriers upon light absorption. Ideally, a nanoscale
interpenetrating bicontinous network of donor and acceptor materials are
created within the entire photoactive layer, ensuring that every generated
exciton can reach the donor‐acceptor interface. At the same time, the
constructed BHJ should ensure a direct or percolating pathway of the charge
carriers to the respective electrodes in order to effectively transport and
48
collect the charges. In this approach, efficient charge separation can be
achieved within the exciton lifetime, and geminate recombination is greatly
reduced. For an optimal morphology, the electron D and A materials must be
interpenetrated with a domain size close to the typical exciton diffusion length
in the material.
The first report of photoinduced charge transfer from a conjugated
polymer, poly[2‐methoxy‐5‐(2‐ethylhexyloxy)phenylene vinylene] (MEH‐PPV),
to a buckminsterfullerene (C60) in1992 by Sariciftci et al.,18 has guided the
development of the field of polymer–fullerene BHJ solar cells. Now most of
the polymer solar cells are based on the BHJ concept as proposed by Yu et
al.136 The most widely studied system is that based on P3HT (shown in Figure
1.18a) as the electron donor. The acceptor molecule is generally a modified
fullerene (C60), the archetypal product being [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM, Figure 1.18a),150 in order to increase its solubility but
retain its electronic behaviour. The bulk heterojunction solar cells based on
P3HT (Figure 1.18b) and the fullerenes [60]PCBM and [70]PCBM where
efficiencies reported generally are in the range of 4‐5%.151‐153
The typical device architecture of a BHJ solar cell based on P3HT and
PCBM is shown in Figure 1.16(b). First a layer of hole conducting
poly(3,4‐ethylenedioxythiophene)-blend-poly(styrenesulfonate) (PEDOT-
blend-PSS) is spin coated on a glass substrate coated with the transparent
electrode indium‐tin oxide (ITO). The PEDOT:PSS layer improves the surface
roughness of the substrate and improves and stabilizes the electrical contact
between ITO and the active layer. Subsequently, a mixture of the donor and
acceptor material is spin-coated from a suitable organic solvent. During
evaporation of the solvent a phase separation of the donor and acceptor
material take place with the formation of an interpenetrating network within the
photoactive layer. Finally a thin hole blocking layer of lithium fluoride (LiF) and
a layer of aluminium (Al) is evaporated on top as the back electrode.
49
O
OMe
S
C6H13
n
P3HT PCBM (a)
(b)
Figure 1.16 (a) Chemical structures of P3HT and PCBM; (b) Representation of a typical BHJ device based on P3HT (red) and PCBM (blue). 1.2.6 Novel low band-gap polymers To improve efficiencies further towards 10% new materials are needed
because the P3HT:PCBM system is approaching optimal device performance.
The main disadvantage of P3HT is the poor matching of its absorption
spectrum with the solar emission spectrum. The band gap of P3HT is around
1.9 eV, limiting the absorbance to wavelengths below 650 nm. Since the
photon flux reaching the surface of the earth from the sun has a maximum of
approximately 1.8 eV (700 nm) P3HT is only able to harvest up to 22.4%
(Figure 1.17) of the available solar photons.155,156 So by decreasing the band
gap of the active material, it is possible to harvest a larger amount of the solar
photons and thereby increase the power conversion efficiency.
50
Figure 1.17 Photon flux from the sun (AM1.5) as a function of the wavelength. The percentage of the total photon flux and the corresponding maximum obtainable current density is displayed on the right y‐axis.155,156 Novel promising polymer materials are shown in Figure 1.18. The
highest reported photovoltaic performances in blends with PCBM are listed in
Table 1.3. One of the most promising low band gap polymers to date is
PCPDTBT based on a benzothiadiazole acceptor unit and the planar
cyclopentadithiophene (CPDT) as the donor unit. Zhu et al. have reported
power conversion efficiencies up to 3.5% for BHJ solar cells based on
PCPDTBT and [70]PCBM with a maximum EQE of 38% around 700 nm and
over 25% in the range 400 to 800 nm.157 Further optimizing of the processing
conditions, by incorporating a few volume percent of alkanedithiol in the
solution used to process the films of PCPDTBT:[70]PCBM, improved the PCE
up to 5.5% through improving the BHJ morphology.158 Upon optimization, the
short circuit current was enhanced up to 16.2 mA/cm2, which is among the
highest reported to date. Silole derivatives of CPDT (PSBTBT, Figure 1.18)
showed a hole mobility of 3 × 10‐3 cm2/(V s), 3 times higher than that for
PCPDTBT.159 Efficiencies up to 5.1% have been reported for solar cells based
on PSBTBT:[70]PCBM blends.
51
b)
Figure 1.18 Novel donor materials used in polymer solar cells: (a) PTB7 and PC71BM95; and (b) recently discovered low band gap polymers (Table 1.3).
Polymer Acceptor η (%) Ref. PTB7 [71]PCBM 7.4 95
PCDTBT [70]PCBM 6.1 160
PTPTBT [70]PCBM 4.3 162
PTB4 [60]PCBM 6.1 161
PBBTDPP2 [70]PCBM 4.0 163
PCPDTBT [70]PCBM 5.5 158
PSBTBT [70]PCBM 4.7 159
Table 1.3 Efficiencies of some low bandgap polymers in blends with PCBM.
Recently, power conversion efficiency of 6.1% was reported for a BHJ
solar cell based on a blend of the polymer
poly[N‐9’’‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4’,7’‐di‐2‐thienyl‐2’,1’,3’‐benzot
hiadiazole) (PCDTBT, Figure 1.20) and [70]PCBM.160 The
PCDTBT:[70]PCBM solar cell demonstrated the best performance of any
single junction polymer solar cell studied to date. PCDTBT (Figure 1.18) is
52
based on a 4,7‐dithienylbenzothiadiazole unit and a soluble carbazole unit
that gives it an optical band gap around 1.88 eV. It should be pointed out that
the high performance is not reached via reduction of the band gap, but
through the deep HOMO level of the polymer, mainly fixed by the carbazole
moiety, which leads to higher values for the open circuit voltage. The latest
report of highly efficient polymer solar cells involve PTB4161 (Figure 1.18) that
is based on thieno[3,4‐b]thiophene and benzodithiophene units resulting in a
optical band gap around 1.63 eV. Fine tuning of the structure and electronic
properties has been done by introducing electron‐withdrawing fluorine to the
thieno[3,4‐b]thiophene unit, which reduce the HOMO energy level of the
polymer. A power conversion efficiency of over 6% was achieved in solar cells
based on fluorinated PTB4:[60]PCBM blends. After an extensive structural
optimization, Yongye Liang et al., further developed a new polymer from the
PTB family, PTB7, which exhibited an excellent photovoltaic effect. The
structure of PTB7 is shown in Figure 1.18 (a). The branched side chains in
ester and benzodithiophene render the polymer good solubility in organic
solvents. The weight average molecular weight (Mw) of PTB7 is 97500 g mol-1
and a dispersity (Đ = Mw/Mn) of 2.1. A PCE of about 7.4% has been achieved
from PTB7/PC71BM [Figure 1.18 (a) PC71BM1/4phenyl-C71-butyric acid
methyl ester] solar cell devices, which is the first polymer solar cell showing a
PCE over 7% to date.95
Although the performance of polymer solar cells has increased steadily
as indicated in Table 1.3, further improvements in efficiency are required for
large-scale commercialization. Aside from the power conversion efficiency,
processing and stability are two other important aspects that have to be
addressed with equal intensity for the success of polymer solar cells. With the
knowledge of low band gap materials that have been demonstrated, it is clear
that for the long-term stability of devices (required for large scale, i.e. greater
than 1 m2 installations), other routes to develop more stable morphologies will
be required. If the device stabilities can be improved then this will permit
OPVs to be more widely used in the market.
53
1.3 Synthesis of the archetypal conjugated polymer, poly(3-hexylthiophene) (P3HT) The most widely studied substituted polythiophenes are poly(3-
alkythiophene)s (P3AT)s. Poly(3-alkylthiophene) represents the most
important conjugated polymer in recent years, which was used in various
applications like light-emitting diodes, field-effect transistors and plastic solar
cells because of its excellent optical and electrical properties. Early reports for
the polymerization of unsubstituted thiophene were metal-catalyzed
condensation reactions.164,165 Another frequently used method by iron (III)
chloride was oxidative polymerization of thiophene monomers.166 Although
these methods are successful for polymerizing unsubstituted thiophene,
polythiophenes are not soluble in common organic solvents. Then the
researchers have focused by developing new synthetic techniques for
substituted polythiophenes.
This Section will consider the chemistry of P3HT, an important part of
this thesis’s work. P3HT is widely considered a “standard” for photovoltaic
devices, morphological studies and the manipulation of materials within OPV
active layers. This is because it can be prepared with predetermined
molecular weights,167 chemically modified,168 has high solubility in common
organic solvents, has a well understood electronic behaviour,169 and exhibits a
semi-crystallinity which both enhances interfacial interactions with the electron
acceptor molecule and facilitates charge transfer through crystalline
domains.170
1.3.1 Regioregularity The optoelectronic properties of P3ATs are mainly dependent on the
regiochemical couplings along the polymer chain. So it is necessary to define
the concept of regioregularity of P3ATs. As 3-alkylthiophene is an
asymmetrical molecule, there are three possible couplings between the
thiophene repeat units during polymerisation. These sequences are head-tail
(HT or 2-5’), head-head (HH or 2-2’) or tail-tail (TT or 5-5’) as in the case of
poly (3-hexylthiophene) (P3HT) shown in Figure 1.19. If the thiophene rings
54
are coupled in a HH manner Figure 1.19 (b) with twisted out of conjugation
due to steric repulsion between alkyl chains, it leads to regio-irregular P3HT
that reduces the electrical conductivity of the polymer. Otherwise, the
thiophene rings are coupled in a consecutive HT manner Figure 1.19 (a)
during polymerization leads to regio-regular P3HT that adopts coplanar
conformation resulting in a lower energy. This arrangment gives a highly
conjugated low bandgap polymer. The regioregularity can be defined as the
percentage of head-tail sequences of 3-hexylthiophene units and brought a
certain arrangement of polymer chains. Regioregularity plays a crucial role on
the electronic properties of P3ATs. In order to achieve good crystal packing
and electrical transport properties, the P3ATs must be regioregular; this
means that almost all of the linkages along the polymer chain are of the same
type (usually head-to-tail).
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
S
C6H13
a b c
1 1 1
2 2 2
3 3 34 4 4
555
1' 1' 1'
2'2'2'
3' 3' 3'4' 4'4'
5' 5' 5'
Head-to-tail (HT) Coupling Head-to-Head (HH) Coupling Tail-to-tail (TT) Coupling
Figure 1.19 Regiochemical couplings of P3HTs: (a) head-to-tail (HT or 2-5’) (b) head-to-head (HH or 2-2’) (c) tail-to-tail (TT or 5-5’).
1.3.2 McCullough and Rieke methods In 1992, two methods the McCullough method171 and the Rieke method172
were introduced for the synthesis of regioregular P3ATs which is shown in
Scheme 1.1. In the McCullough method, 2-bromo-5-bromomagnesio-3-
alkylthiophene [Scheme 1.1(a)] monomers are cross-coupled together with
1,3-bis(diphenylphosphino)propane nickel (II) chloride [Ni(dppp)Cl2]. This was
the first report for the synthesis of regioregular P3ATs with 90% head-to-tail
regioselectivity whereas the Rieke method uses activated (Rieke) zinc to
generate the substituted thiophene monomer, which was then coupled with a
nickel catalyst [Scheme 1.1(b)]. Both methods generated regioregular P3ATs
with tunable molecular weights and low PDIs, but they required cryogenic
temperatures and highly reactive metals limit the large-scale production of
55
P3ATs. Several methods for the synthesis of poly(3-hexylthiophene) were
developed after the interest of this polymer in the field of organic
electronics.173
S
R
Br
1. LDA
2. MgBr2 S
R
BrBrMg
Ni(dppp)Cl2
S
R
n
S
R
Br
Rieke Zn
S
R
BrBrZn
Ni(dppe)Cl2
Br
a
b
S
R
n
Scheme 1.1 Synthesis of poly(3-alkylthiophene)s (P3ATs) by (a) McCullough method171 and (b) Rieke method.172
1.3.3 Grignard metathesis (GRIM) polymerisations leading to P3HT Later on, McCullough and coworkers in 1999 again retooled his method and
discovered another method for the synthesis of regioregular P3ATs by
Grignard metathesis (GRIM) (Scheme 1.2).174 This is the most commonly
employed method for synthesizing well-controlled, highly regioregular, and
economical poly(3-alkylthiophenes). In this method, reaction of 2,5-dibromo-3-
hexylthiophene with alkyl Grignard reagents gives two metallated
regioisomers (A:B) in 85:15 or 75:25 ratio via a magnesium exchange reaction
(Scheme 1.2). Then addition of Ni(dppp)Cl2 to this reaction mixture produces
P3HT with more than 95% regioregularity. [GC-MS analysis after addition of
Ni(dppp)Cl2 showed that only the A isomer is incorporated within the polymer
while B is not consumed.]
S
C6H13
Br Br
RMgX
S
C6H13
XMg Br S
C6H13
Br MgX
+
85% 15%
Ni(dppp)Cl2
S
C6H13
n
A B Scheme 1.2 Synthesis of poly(3-hexylthiophene)s (P3HTs) by Grignard metathesis (GRIM) McCullough method.174
56
1.3.4 Chain-growth condensation polymerisations leading to P3HT Yokozowa and his coworkers found that Mn values of P3HT were controlled
by feed ratio of [monomer]/[Ni catalyst] when the polymerization was carried
out at room temperature and used the exact amount of isopropylmagnesium
chloride for the formation of 2-bromo-5-chloromagnesio-3-hexylthiophene
from the corresponding bromoiodothiophene175 (Scheme 1.3) leading to P3HT
with dispersity around 1.1 even upto Mn of 28700 g mol-1 when the
polymerization was quenched with hydrochloric acid.176
S
C6H13
I Br
i-PrMgCl THF
S
C6H13
ClMg Br
1. Ni(dppp)Cl2, THF
2. 5M HClS
C6H13
n
Scheme 1.3 Synthesis of poly(3-hexylthiophene)s (P3HTs) by Yokozawa et al.175,176
McCullough and Yokozawa independently demonstrated the GRIM
polymerization of 3-alkylthiophenes follows in a living chain growth
mechanism instead of the traditionally accepted step growth
polycondensation. As a result, low dispersities (1.1-1.3) and well-defined
molecular weights can be prepared by controlling the feed ratio of monomer to
the Ni catalyst.177-179 After detailed investigation of the polymerization of M2a
(Scheme 1.4), Yokozowa et al., proposed a mechanism called as “Catalyst
transfer condensation polymerization” which is shown in Scheme 1.4.180
According to this, first Ni(dppp)Cl2 reacts with two equivalents of M2a and
forms a dimer of M2a in situ which is chain initiator with the zero-valent Ni(0)
complex. The Ni(0)-complex without diffusing into reaction mixture is inserted
into intramolecular C-Br bond by reductive elimination involving C-C bond
formation. Again another M2a reacts with this Ni, then coupling reaction and
transfer of the Ni catalyst to the next C-Br bond. In this way, growth will
continue with the Ni catalyst moves to the polymer end group.178 So this
reaction can be done both at room temperature and on a large scale, the
Grignard metathesis/Kumada-Corriu coupling has become the most broadly
used method for the synthesis of predetermined high molecular weight
P3ATs.
57
SBrClMg
C6H13
Ni(dppp)Cl2
M2a
SBr
C6H13
SBr
C6H13
NiL2
SBr
C6H13
SBr
C6H13
NiL2
SBr
C6H13
S
C6H13
NiL2 Br M2a
SBr
C6H13
S
C6H13
NiL2
SBr
C6H13
SBr
C6H13
S
C6H13
NiL2SBr
C6H13
SBr
C6H13
S
C6H13
NiL2
S
C6H13
BrM2a
SBr
C6H13
S
C6H13
NiL2
S
C6H13
Br 5M HCl
SBr
C6H13
S
C6H13
n-1H
(L2 = dppp)
Scheme 1.4 Mechanism of Catalyst Transfer Condensation Polymerization of P3HT proposed by Yokozawa et al.180
Regioregular poly(3-hexylthiophene) (P3HT) is widely studied for
electronic and photovoltaic devices because of its high hole mobility, high
solubility in common solvents and good chemical stability.181 Hence this GRIM
synthetic route was chosen for its ease of implementation and also it leads to
highly regioregular P3HT.
One of the main benefits of this chemistry is that once the
polymerization is finished, the chain-ends remain active and can be used to
perform end-capping reactions with Grignard reagents. This technique will be
widely exploited in this thesis.
1.3.5 Chain-end capping of P3HT using GRIM To synthesize copolymers based on P3HT, it is essential to functionalize.
Jeffries-El et al.177d showed that it was possible to do so by adding a second
Grignard reagent at the end of polymerization. This provides access to a wide
variety of different terminal functional groups (vinyl, ethynyl, aryl, aldehyde
and amine...). Scheme 1.5 describes the reaction mechanism of
functionalization of P3HT. The addition of a second Grignard reagent on a
58
living P3HT allows, firstly, to stop the chain growth, but also to functionalize
the P3HT.
SBrMg Br
C6H13
+ Ni ClCl
L
L
THF/R.T.
After severalcatalytic cycles
SBr
C6H13
S
C6H13
S
C6H13 n
Ni RLL
SBr
C6H13
S
C6H13
S
C6H13 n
Ni BrL
L
RMgX
SBr
C6H13
S
C6H13
n
Ni(0)L
L S
C6H13
R+
Associated pair
Monocapped Polymer
reductive elimination
SNi
C6H13
S
C6H13
nS
C6H13
R L
LBr
oxidative addition
SNi
C6H13
S
C6H13
nS
C6H13
R L
LR SR
C6H13
S
C6H13
nS
C6H13
RNi(0)L
L+
reductive elimination
n
M1a4
5
6 7Dicapped Polymer
RMgX
Scheme 1.5 Proposed Mechanism for the end-capping reaction of P3HTs.177d
The authors have shown to synthesize a mono or difunctionalization of
polymers depending on the nature of Grignard reagent and not depending on
its concentration in the medium. Jeffries-El et al177d observed that species
containing a double or triple bond led to mono-adducts P3HT whereas others
particularly aromatics give polymers with diadducts which is shown in Scheme
1.5. This is because the alkenyl and alkynyl may react with Ni(0) to form a
stable π -complex, which prevents any further reaction with terminal bromine
of P3HT. However, it should not be generalized because many functionalised
Grignard reagents remain to be tested in this scheme to access other types of
functional groups at the end of P3HT chains. In order to ultimately prepare
block copolymers from functional P3HT, two types of Grignard reagents were
used177d to obtain mono and di-functionalised P3HT in this thesis work.
59
1.4 Organic photovoltaic cells and block copolymers 1.4.1 Importance of morphology of active layer in OPVs In organic solar cells, the morphology of the active layer is critical and affects
several physical parameters of the photovoltaic process.182 First, it enables
the transformation of light-formed excitons into separate positive and negative
charges by providing interfaces between donor and acceptor domains.
Second, well-organised domains of the respective materials enhance
percolation of charges to the electrodes. Third, it allows control over the
mechanical properties of the materials that are destined for use in roll-to-roll
processing.5 Good organization of donor and acceptor materials allows them
to limit electron-hole recombination by generating a phase separation whose
characteristic size is equivalent to the diffusion length of excitons, but also
optimizes carrying loads by creating channels conduction to the electrodes.
An idealized morphology is shown in Figure 1.20. The presence of a thin layer
of acceptor material in contact with the cathode material and contact the
donor to the anode may minimize charge recombination at the electrodes.
Figure 1.20 Ideal morphology of BHJ structure for organic solar cells.16
Various parameters influence the morphology of the active layer. They
include the structure of the materials, the concentration of donor and
acceptor, the solvent used to make films, the concentration of the solutions,
the deposition temperatures and subsequent heat treatments. These
parameters can be classified into two broad classes that are the
thermodynamic parameters and kinetic parameters. The thermodynamic
parameters correspond to the nature and properties of the initial solution
60
(materials, ratio between donor and acceptor materials-solvent interaction).
The kinetic parameters for their work mainly during the film formation
(evaporation time of solvent, crystallization of materials, thermal annealing).
1.4.2 Controlling morphology of active layer in blends In the case of donor-acceptor blends, the morphology of the films is very
difficult to control because it depends on many parameters. The two
components of the mixture are very different, behave independently of each
other, and/or cooperatively for example in the formation of eutectic mixtures
leading to a wide variety of structures.183 It is however possible to improve the
morphology by optimizing the deposition conditions, such as concentrations of
donor and acceptor in the solvent, the rate of deposition, and the use of
thermal and solvent annealing.
P3HT tends to be organized in fibrillar form, but this organization is
highly dependent on the deposition techniques and solvents used.184-188
Indeed, spin-coating, which allows a very fast evaporation of the solvent,
leads to less ordered structures than the dip-coating and drop-casting, which
allows a slow evaporation of the solvent and thus lead to more orderly
arrangements (Figure 1.21).
Dip-coating Drop-casting Spin-coating
Figure 1.21 AFM phase images of thin layers obtained from P3HT (Mn=1.9 kDa) using different processing techniques in chloroform.185
Ideally, there should be an optimzation of the evaporation time so that
it is short enough to limit phase separation and long enough to facilitate the
crystallization of materials.189 The choice of solvent also determines the
morphology of the films. Firstly, the constituents of the mixture will have
61
varying affinities with respect to the organic solvents chosen which effects the
formation of films. Secondly, the solvent will have a particular boiling point that
will lead to a particular evaporation time resulting in different structures that
are more or less organized. Thus, for the system P3HT:PCBM, the films
deposited from solvents of high boiling points (tetrahydronaphthalene,
dichlorobenzene) have, in the absence of heat treatment, a better structure
and therefore better mobility of charges that those made from solvents with
low boiling temperatures (chloroform, chlorobenzene).188
Another way to improve the morphology is by thermal annealing of
devices before or after the addition of the final electrode interface. The
purpose of this annealing is both to nano-structure the films, playing on the
ability of species to reptate and crystallize, but also to assess the stability of
the active layers. In the case of the P3HT:PCBM system, numerous studies
have shown the beneficial effect of annealing on the morphology and the
concomitant increase in efficiency.184,190-196 However, the temperature and the
annealing time varies greatly from one study to another. This is partly due to
the nature of P3HT used (molecular weight, dispersity, regioregularity,...) and
the variations in equipment and processes in each laboratory. Neverthless, a
paper by Yang et al.196 revealed by transmission electron microscopy (TEM),
an improvement of the morphology after annealing with better crystallization
of the two components and the formation of a network of interconnected fibrils
P3HT (Figure 1.22).
Figure 1.22 TEM images of films P3HT: PCBM (a) general view, (b) zoom and (c) schematic representation of the morphology of the active layer before and after annealing.196
62
This trend was also confirmed by X-ray diffraction measurements, which show
a marked increase in the crystallinity of P3HT after annealing.193,195 This nano-
structuring results in a significant increase in the short circuit current and thus
the cell efficiency. Indeed in some cases, the energy conversion efficiency is
increased ten-fold after thermal treatment.193 The increase of short-circuit is
due to both the mobility of charge carriers increased when the materials are in
a crystalline state, but also to a decrease in recombination of excitons by
optimizing the size of crystalline domains (≈ 15 nm).
Annealing enhances the charge carrier mobility197,198 (see Figure 1.23) in
combination with changing the recombination behavior from a Langevin type
into a non-Langevin type recombination mechanism.199 X-ray, AFM and TEM
investigations enabled a microscopic picture of the annealing process to be
developed,196,200 which is considered to take place in three subsequent steps:
(i) annealing softens the P3HT matrix, which (ii) allows PCBM molecules to
diffuse out of disordered P3HT clusters and form larger fullerene aggregates,
before (iii) the now fullerene-free P3HT matrix recrystallizes into larger fibrillar
type crystals, which are embedded in a matrix considered to consist of PCBM
nano-crystals and amorphous P3HT.
Figure 1.23 I–V curves of P3HT/PCBM solar cells under illumination with white light of 800 Wm−2 : as produced solar cells (filled squares), annealed solar cells (open circles) and cells simultaneously treated by annealing and applying an external electric field (open triangles).197
63
Despite these improvements, the ideal morphology (shown in Section 1.4.1) is
far from being reached which severely limits the conversion efficiencies.
Several solutions are being proposed to overcome this problem by curing of
the active layer,201-203 the use of double cable polymers,204-207 or the use of
block copolymers,208 which is one of the main subjects of this thesis.
1.4.3 Self-assembly of rod-coil block copolymers Block copolymer consists of different adjacent blocks that are derived
from different monomers or from the same monomer with different
composition. These different A and B blocks in a block copolymer tend to
minimize their contact surface that cannot separate in a macroscopic scale
unlike simple mixtures and therefore forced to self-organise into domains (10-
50 nm) which are close in size to the length of each block.209,210 The Flory-
Huggins parameter (χAB), characterizes the incompatibility between the two
blocks, positive value of this term represents repulsion between the chains
and negative value represents compatibility between the blocks.211
The research on “microphase separation” has been widely described
both theoretically and experimentally over 30 years.212,213 Most of the
theoretical research, initiated by Meier in 1969,214 related linear coil-coil
diblock copolymers which are now generally well understood. In the 1990s,
Matsen and Bates215 proposed a theoretical phase diagram (Figure 1.24a)
depending on the volume fraction (f) of each component and the product, χN
(N = total degree of polymerization). Various thermodynamically stable
microstructures have been predicted: they are lamellae (L), cylinders
organized in a hexagonal arrangement (H), body-centered cubic (QIm3m),
closed-packed spheres (CPS) and bicontinuous cubic (gyroid) phase of an
Ia3d symmetry (QIa3d), as shown in Figure 1.24b.
64
(a)
(b)
Figure 1.24 (a) Phase diagram of coil-coil diblock copolymer 215; and (b) schematic representation of block copolymer morphologies.211b
In the 1980s, Semenov and Valencino216 researched theoretical
studies on the bulk behaviour of rod-coil diblock copolymers and proposed
two phases: a nematic phase and a smectic A phase and they also introduced
the smectic C phase after subsequent developments217,218 as shown in Figure
1.25. Later on, Williams and Fredrickson in 1992 reported on non-lamellar
structures of rigid segments with high coil block volume fractions (fcoil > 0.9),
which are called as "hockey pucks" (Figure 1.25e).219
Figure 1.25 Rod-coil copolymers self-assembly (a) nematic phase, (b) bilayer smectic-A phase, (c) monolayer smectic-A phase, (d) monolayer smectic-C phase and (e) "hockey pucks".219
65
In 1993, Williams and Halperin220 reported lamellar, cylindrical and
spherical micro-segregated structures and later, Matsen221 described the
influence of the rigidity on the lamellar phases. Different research groups222-224
reported that only morphologies where the presence of flexible coil blocks on
the outer side of an interface are thermodynamically stable and the suitable
introduction of compatible homopolymers, the domain sizes of rod-coil block
copolymers can be controlled.225 Stupp and coworkers226 introduced the
interesting concept of “mushroom” shaped nano-aggregates which is good in
various applications, for example in photovoltaics where the crystallisation of
the rod domain is very important for charge transfer.
1.4.4 Why block copolymers in photovoltaic cells? Block copolymers (BCPs) are currently exploited in photovoltaic cells for two
reasons. The first reason was because the excited electronic state termed
exciton227 (see Section 1.2.3) for a typical polymer can exist over a distance of
around 5-20 nm228,229 and this distance coincides extremely well with the
typical size of block copolymer domains.230 As the domain width can often be
tailored simply by varying the length of the polymer, it should be relatively
simple to design block copolymers that can have dimensions correct for
exciton formation and conversion into charges as per the ideal structures
shown in Figure 1.20 of Section 1.4.1. The increasing interest in BCPs is their
ability to obtain a tunable nano scale self-assembly.231 With the help of
modern synthetic chemistry; one can design BCPs with specific lengths and
geometries to obtain variety of ideal nanostructures which may enhance the
device efficiencies and also the introduction of rod blocks in BCPs bring a
competition between crstallization and microphase separation which can
further affects the morphology. An example of a target system, donor-
acceptor rod-coil BCP connected by a linker is shown in Figure 1.26.
The second main reason for the use of block copolymers has been as
stabilisers or compatibilizers in solar cells. Attempts to increase efficiencies,
while making active-layers more stable and less sensitive to environmental
perturbations during their preparation, have used the self-organising
66
properties of copolymers containing opposing segments of donors and
acceptors.232,233 In these systems, like-polymer blocks self-assemble into
excitonic-scale domains that can provide pathways through which charges
may pass to the electrodes as previously mentioned.
Figure 1.26 A target device based on conjugated rod-coil block copolymers where domains are sized to be around 10 nm to enhance exciton capture and charge percolation to electrodes.
Notable examples reported by Hadziioannou234 and Fréchet235
incorporating fullerene, and Thelakkat236 and Emrik237 using perylene, have
shown how meso-scale separation of different moieties have been feasible. In
general, however, these systems have not been able to show high
efficiencies, and often suffer from complex multi-step syntheses, an important
point when considering industrialisation. The synthesis of block copolymers of
P3HT and polynorbornenes carrying high concentrations of C60 reported by
Fréchet is of particular relevance (Figure 1.27).235 Even though the
copolymers did not enhance efficiencies when mixed as an optimised portion
with P3HT-blend-PCBM, it was demonstrated that through compatibilization
the physical stability of the active layer could be dramatically improved. This is
important as devices exposed to sunlight over long periods of time can easily
reach temperatures above the first glass transition temperature (Tg) of P3HT
(at ca 50 ˚C).238-240
67
Figure 1.27 Chemical structure of P3HT-based diblock copolymers incorporating fullerene by Fréchet et al.235 It is apparent that there is a necessity to find a facile route to rod-coil donor-
acceptor copolymers that can both compatibilize and enhance organisation of
the P3HT-blend-PCBM layer, which is main object of this thesis. Examples of
specific copolymers for both these types are detailed in the following sections. 1.4.5 Synthesis and self-assembly of exampled rod-coil block copolymers Conjugated rod-coil BCPs have good immiscibility between rod and coil
segments which allows the formation of domains even with quite low molar
masses unlike coil-coil BCPs.241 Because of the lower solubilities of the rod
blocks, the synthetic procedures are very complex compared to coil-coil
systems. Therefore first, rod block is prepared and then the coil block is
attached by condensation reactions, or rod macro-initiators can be used for
the polymerisation of coil blocks.
The use of BCPs can optimize several parameters of organic
photovoltaic process, based on the ability of these species to self-organizing.
In recent years, many reseach groups have addressed the block copolymers
consisting of rod-coil coupled rigid block, because of their ability to self-
organize and their better formatting patterns (thanks to the presence of a
flexible block which increases their solubility). Examples are copolymers
based on poly(p-phenylene)242-244, poly(p-phenylene ethynylene)245-247,
polyfluorene248-251, poly(p-phenylene vinylene)(PPV)252-257 and poly(3-
hexylthiophene) (P3HT)258-264, which is the main subject of this thesis.
68
1.4.5.1 Copolymers based on poly(p-phenylene vinylene)s
Chemists have prepared BCPs made of p-type block such as PPV or its
soluble derivatives and for the n-type, preparing polymers that carry electron
acceptors as pendant, grafted groups attached to coil polymers such as PS.
Hadziioannou and coworkers contributed considerably in the synthesis and
self-assembly of block copolymers based on rod-coil derivatives of PPV for
photovoltaic application. Their work was based on an alkoxy-subsituted PPV
(MEH-PPV) coupled to a fullerene substituted PS (Figure 1.28a).265 Alkoxy-
subsituted PPV was used as macro-initiator for obtaining copolymers of PS
and poly(4-chloromethylstyrene) with predetermined molecular weights by
controlled radical polymerisations. The chloromethyl groups were then
attached to C60 by atom transfer radical addition (ATRA). This copolymer as
active layer showed better photovoltaic performance and improvement in the
short circuit current (0.15 to 5.8 µA/cm2) than the mixture with the same donor
and acceptor (Figure 1.28b).
Figure 1.28 (a) Chemical structure of D-A block copolymer PPV-b-P(S-stat-C60MS); (b) Photovoltaic performance of copolymer PPV-b-P(S-stat-C60MS) (B) compared with a blend of donor homopolymer and acceptor polymer (A) under illumination.265
Macromonomer for the polymerisation of acrylates266 was achieved by
the preparation of an alkyloxylated PPV with an alkoxyamine chain-end.
Another facile route was found that the addition of the alkoxyamine to the
alkyloxylated PPV using Grignard chemistry for the synthesis dialkyloxylated
PPV macro-initiator carrying chlorostyrene groups.267 To reduce crosslinking
reactions with C60, the chloromethyl groups were first converted to azido
69
groups before addition to C60. This method again produced rod-coil BCPs
carrying pendant C60s on the coil block. They have performed the effect of C60
grafting density of the PS on the material’s electronic properties and then, an
increase in electron charge mobilities with density was found.268
The morphology of the dialkyloxylated PPV-PS based rod-coil BCPs
was further investigated and it was found that the π -stacking of the PPV
resulted a thermodynamically stable lamellar structure of the conjugated
blocks within PS matrix.269 N. Sary et al.270 have synthesized and studied the
self-organization properties of rod-coil BCPs based on PPV-b-P4VP [poly(2,5-
di(2'-ethylhexyloxy)-1,4-phenylenevinylene)-block-poly(4-vinylpyridine)] shown
in Figure 1.29 (a). These copolymers were synthesized by anionic radical
method in which the chains of PPV-aldehyde were used to deactivate “living”
P4VP chains. The living polymerization of 4-vinylpyridine can perfectly control
the size of chains and the authors were able to synthesize copolymers PPV-b-
P4VP with different sizes of P4VP (copolymers Px, with x the volume fraction
of P4VP). After heat treatment in several steps to promote self-assembly of
copolymers, the morphologies were determined by TEM [Figure 1.29 (b)]. The
resulting structures are very different and depend strongly on the proportion of
each block in the copolymer. Indeed, for P55 (55% of coil), the observed
morphology corresponds to a lamellar phase of 20 nm. This phase is
organized in grains, in the order of micrometers. However, the interfaces of
these areas, the PPV channels appear distorted [Figure 1.29 (b)]. By
increasing the proportion of P4VP, the structure is changed to hexagonal
phase formed by sticks PPV [white on Figure 1.29 (b) P80]. However, for 88%
of coil (P88), the structure is disorganized and has a nodular phase. This work
therefore shows the great variety of morphologies that can be obtained by
choosing P4VP as a coil block.
From solid-state phase diagram for PPV-b-P4VP, It was found that the
lamellar phase dominated over a proportion of PPV to P4VP blocks as
expected because of the liquid-crystalline nature of the rod-blocks. But
hexagonal and spherical microphase separated morphologies are also
possible at high volume ratios of P4VP due to dominated force towards
macrophase separation.
70
Figure 1.29 (a) Chemical structure of copolymers PPV-b-P4VP; (b) TEM images (annealed) of various morphologies obtained for different proportions of coil block.270
Another report regarding rod-coil copolymers, an alkyloxylated PPV rod
connected with the coil block, which is a combination of azido-styrene units
and butyl acrylate groups, attached with C60 as shown in Figure 1.30.271 The
solid-state morphology of this copolymer showed that the fullerene still
directed to aggregate even it was bonded to the flexible coil blocks. The
utilization of high crystalline PTs instead of PPVs recommended as one of the
best solutions to overcome this problem.
Figure 1.30 Chemical structure of the PPV based rod coil copolymer with C60s attached to the coil block via a tertiary amine bridge.271 The efficiency of BHJ based on composite of dialkyloxy-substituted PPV
(MEH-PPV) as donor and cyano-substituted PPV as an electron acceptor was
around 2 %272,273 which is very high compared to present block copolymer
based systems.
71
1.4.5.2 Copolymers based on polythiophenes Since PPV copolymers have limitations in the device efficiency; the research
on PT copolymers, especially P3HT has been widely explored in organic
electronics. McCullough and colleagues were synthesized for the first time
P3HT based ATRP macro-initiators which was then used for the synthesis of
tri BCPs, PS-b-P3HT-b-PS and PMA-b-P3HT-b-PMA with coil block PS,
poly(methyl acrylate) (PMA). They were also successful in synthesizing
polyurethane elastomer based on P3HT using Vilsmeier-Hack formylation of
P3HT chain ends274 as shown in Scheme 1.6.274
Scheme 1.6 Synthetic routes for P3HT-based polyurethane elastomers and also triblock copolymers via Vilsmeier-Hack formylation reaction.274
The GRIM method via chain-growth condensation polymerisation
(shown in Scheme 1.4 of Section 1.3.4) has been widely explored in the
synthesis of rod-coil block copolymers because it has many advantages; (i) by
varying the ratio of Ni-catalyst to monomer one can predetermine the length of
the rod block (ii) since the width of the domains is directly associated to the
length of the rod polymers, it helps to design the target domain size and also
(iii) the polymerisation can be terminated with functionalized Grignard
reagents which lead to block copolymer chemistry (Scheme 1.5). Some of the
important examples are given below.
The synthesis of rod-coil BCPs P3HT-b-PMA were successful with
narrow dispersities (Đ < 1.3) by GRIM method as shown in Scheme 1.7.275
Another recent report by Dante et al. also used GRIM method for synthesizing
72
P3HT-macroinitiator for reversible addition-fragmentation chain transfer
(RAFT) chemistry to obtain copolymer, P3HT-b-PS-C60 as shown in Figure
1.31a.276 AFM images showed a fibrillar structure and also localised
conductivity measurements revealed that the lighter, darker areas on the
surface corresponding to P3HT, PS and C60 respectively as shown in Figure
1.31b.
Scheme 1.7 Synthesis of the ATRP macro-initiator based on P3HT and the synthesis of copolymers P3HT-b-PMA.275a
(a) (b)
Figure 1.31 (a) The chemical structure of rod-coil copolymer based on P3HT carrying C60, P3HT-b-PS-C60 (b) AFM image (topographic) of the same copolymer.276
73
Sary et al.,277 recently reported rod-coil BCPs based on P3HT which is
good for organic solar cell devices. They have synthesized diblock
copolymers P3HT-b-P4VP varying molecular weights of P4VP to achieve
domains suitable for excitons capture as well as charge transfer. They
anticipated that P4VP domains could interact with PCBM due to
supramolecular interactions retaining the self-assembly of BCP when
copolymer P3HT-b-P4VP with mixed PCBM (Figure 1.32a). But they observed
very low efficiency around 0.03% and it was found that the P4VP has a
tendency to wet the PEDOT-blend-PSS substrate and thus disturbing the
vertical profile of the device. This problem was resolved using an inverse
structure (Figure 1.32b) and they obtained an efficiency of around 1.2%.
Thus, it was found that casting conditions and environment effect on the self-
assembly of block copolymers.
(a) (b)
Figure 1.32 (a) Supramolecular interactions between P3HT-b-P4VP and PCBM; and (b) Inverted photovoltaic structure.277
P3HT macro-initiator was again used for the synthesis of rod-coil BCPs
consisting of poly(perylene bisimide acrylate)278 as electron acceptors (Figure
1.33) with suitable chain lengths for domain formation. The device efficiency
increased after annealing in the order of ca 0.5% because of ordered micro-
phase separation.278c It was an Important observation that the BCPs could
work as compatibilisers in the blend of P3HT and perylene bisimide active
layer. The OSC device efficiency was increased around 50% by restricting the
excessive crystallisation of perylene.278d Similar systems have also been
74
prepared using P3HT macro-initiator by either ATRP279a or RAFT279b
methods, shown in Figure 1.33. It was observed that the quenching of P3HT
photoluminescence is better in the block copolymers as active layers in OSCs
than in the simple blends of P3HT and PCBM.279a
Figure 1.33 Double-crystalline block copolymers based on poly(3-hexylthiophene) P3HT and perylene bisimide by Emrick et al. (a), Frechet et al. (b), Segalman et al. (c) and Thelakkat et al. (d).278f Our group used chain-end chemistry via GRIM method to synthesize
alkynyl-P3HT and then by “click chemistry” approach between α,ω-dipentynyl-
P3HT and azide-terminated-PS (obtained by ATRP), we could synthesize rod-
coil P3HT-b-PS diblock and PS-b-P3HT-b-PS triblock copolymers
successfully as shown in Scheme 1.8.168g This work represents the first
example of "click chemistry" on conjugated polymers based on P3HT.
75
S
C6H13
m
N NN O Br
nNN
NOBrn
O O
S
C6H13
m
N NN O Br
n
O
P3HT-b-PS
PS-b-P3HT-b-PS
S m
C6H13
N3 O
O
Brn
CuI, DBU
THF
Scheme 1.8 Synthetic route to rod-coil di- and triblock copolymers based on P3HT and PS by “Click chemistry”.168g
Recently, we again explored the chain-end chemistry of P3HT to
prepare the multi-block rod-coil copolymers incorporating fullerene as a repeat
unit, which is shown in Figure 1.34(a).280 These copolymers, poly{(1,4-
fullerene)-alt-[1,4-dimethylene-2,5-bis(cyclohexylmethylether)phenylene]}-
block-poly(3-hexylthiophene) (PFDP-b-P3HT), were obtained using
condensation reactions between functionalised-P3HT and novel fullerene
polymer. AFM images revealed that fibrillated wires like structures are formed
[Figure 1.34(b)]. The sizes of the resulting domains are around 20 nm, which
is expected in which the P3HT chain length could be predestined and around
9 C60 repeat units are presented in the C60 coiled block. The photovoltaic
performances of these multi-block copolymers for OSCs are in progress.
(a)
NiBr (dppp)BrMg O
Oi)
ii) HCl, THFiii) methanol
O
O
BrBr+
CuBr, pyridine
toluene, 115 ˚C
K2CO3, toluene, 18-crown-6,
85 ˚C
O
O
Br
m
O
O Br
S
C6H13
O O
n
O
O
Br
m
H
p
O
O
S
C6H13
O O
n
HHS
C6H13
n
Br
76
(b)
Figure 1.34 (a) Chemical structure of multi-block rod-coil copolymers, PFDP-b-P3HT and (b) AFM phase image of PFDP-b-P3HT annealed at 220 °C (500 x 500 nm).280 Rod-coil BCPs have not reached high efficiencies sofar due to several
reasons; processing and optimisation, especially correlating the absorption
spectrum to that of the solar spectrum, the effective interfacial structures
between the block copolymer and the electrodes, and perfecting the size,
position and type of insulating, covalent link between the two blocks that will
optimise charge transfer through space between the blocks.230e This indicates
that it is essential to develop efficient routes for cost-effective organic
photovoltaic devices based on block copolymers.
Hence, it is very important to find a simplified and versatile synthesis of rod-coil donor-acceptor copolymers that can both stabilize and enhance the organisation of the P3HT-blend-PCBM layer, which may result high efficiency OSC devices , which is the main objective of this thesis .
77
1.5 References 1 Kittel, C. Introduction to Solid State Physics, 8th Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. 2 Callister, W. D. Materials Science and Engineering: An Introduction, 7th Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. 3 Pierret, R. F. Advanced Semiconductor Fundamentals, 2nd Ed.; Prentice Hall: Upper Saddle River, NJ, 2002. 4 Sze, S. M. Physics of Semiconductor Devices, 2nd Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 1981. 5 Sun, S.; Sariciftci, N. S.; Eds. Organic Photovoltaics: Mechanisms, Materials,
and Devices; Taylor & Francis: Boca Raton, FL, 2005. 6 Hagen, K.; Ed. Organic Electronics: Materials, Manufacturing, and
Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006.
7 Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci. Pt A Polym. Chem. Ed. 1974, 12, 11.
8 Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. Chem. Commun. 1977, 578. 9 Skotheim, T. A.; Reynolds, J. R.; editors. Handbook of Conducting Polymers,
3rd Edition, vols 1 and 2, New York, CRC Press, 2007. 10 Yesodha, S. K.; Pillai, C. K. S.; Tsutsumi, N. Prog. Polym. Sci. 2004, 29, 45. 11 Cho, M. J.; Choi, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton, L. R. Prog.
Polym. Sci. 2008, 33, 1013. 12 Moliton, A. Optoelectronics of molecules and polymers, New York, Springer,
2005. 13 McCulloch, I.; Yoon, H. Encyclopedia of Materials: Science and Technology, 2008, 8476. 14 Choi, M-C.; Kim, Y.; Ha, C-S. Prog. Polym. Sci. 2008, 33, 581. 15 Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875. 16 Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. 17 Thompson, B. C.; Fréchet, J. M. J. Angew. Chem. Int. Ed. 2008, 47, 58. 18 Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. 19 Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.;
Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Nat. Mater. 2008, 7, 158.
20 Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A, J. Nature Photonics 2009, 3, 297.
21 Ling, Q-D.; Liaw, D-J.; Zhu, C.; Chan, D. S-H.; Kang, E-T.; Neoh, K-G. Prog. Polym. Sci. 2008, 33, 917. 22 Singh, Th. B.; Sariciftci, N. S.; Jaiswal, M.; Menon, R. ; Handbook of Organic Electronics and Photonics. Nalwa, H. S. editor 2008, 3, 153. 23 Singh, Th. B.; Sariciftci, N. S. Annu. Rev. Mater. Res. 2006, 36,199. 24 Dimitrakopoulos, C. D.; Mascaro, D. J.; IBM J. Res & Dev. 2001, 45,11. 25 Fréchet, J. M. J. Prog. Polym. Sci. 2005, 30, 844. 26 McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537. 27 Adhikari, B.; Majumdar, S. Prog. Polym. Sci. 2004, 29, 699. 28 Guimard, N. K.; Gomez, N.; Schmidt, C. E. Prog. Polym. Sci. 2007, 32, 876. 29 Ivory, D. M.; Miller, G. G.; Sowa, J. M.; Shacklette, L. W.; Chance, R. R.;
Baughman, R. H. Chem. Phys. 1979, 71, 1506. 30 Tourillon, G.; Garnier, F.; Electroanal. Chem. Interfacial Electrochem.
1982, 135, 173. 31 Diaz, A. F.; Kanazawa, K. K. J. Chem. Soc. Chem. Commun. 1979, 635.
78
32 (a) Bayer, A. G. Eur. Patent No. 339 340, 1988; (b) Kirchmeyer, S.; Reuter, K. Journal of Material Chemistry 2005, 15, 2077.
33 Jonas, F.; Schrader, L. Synth. Metal. 1991, 41, 831. 34 Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116. 35 Winter, C.; Reece, C.; Hormes, J.; Heywang, G.; Jonas, F. Chem. Phys. 1995, 194, 207. 36 Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. Phys. 1994, 369, 87. 37 Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539. 38 Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, 30(12B), L1941. 37 Kippelen, B.; Marder, S. R.; Hendrickx, E.; Maldonado, J. L.; Guillemet, G.;
Volodin, B. L.; Steele, D. D.; Enami, Y.; Sandalphon; Yao, Y. J.; Wang, J. F.; Rockel, H.; Erskine, L.; Peyghambarian, N. Science 1998, 279, 54.
38 Ballantyne, A. M.; Chen, L.; Dane, J.; Hammant, T.; Braun, F. M.; Heeney, M.; Duffy, W.; McCulloch, I.; Bradley, D. D. C.; Nelson, J. Adv. Funct. Mater. 2008, 18, 2373.
39 Babel, A.; Zhu, Y.; Cheng, K-F.; Chen, W-C.; Jenekhe, S. A. Adv. Funct. Mater. 2007, 17, 2542. 40 Jaiswal, M.; Menon, R. Polym. Int. 2006, 55, 1371. 41 Kirova, N. Polym. Int. 2008, 57, 678. 42 Moliton, A.; Hiorns, R. C. Polym. Int. 2004, 53, 1397. 43 Roncali, J. Chem. Rev. 1992, 92, 711. 44 Kertesz, M.; Choi, C. H.; Yang, S. Chem. Rev. 2005, 105, 3448. 45 Zade, S. S.; Bendikov, M. Chem. Eur. J. 2007, 13, 3688. 46 Hiorns, R. C.; Iratçabal, P.; Bégué, D.; Khoukh, A.; de Bettignies, R.; Leroy, J.; Firon, M.; Sentein, C.; Martinez, H.; Preud’homme, H.; Dagron-Lartigau, C. J. Polym. Sci. Pt. A Polym. Chem. 2009, 47, 2304. 47 Ouhib, F.; Dkhissi, A.; Iratçabal, P.; Hiorns, R. C.; Khoukh, A.; Desbrières, J.;
Pouchain, C.; Dagron-Lartigau, C. J. Polym. Sci. Pt. A Polym. Chem. 2008, 46, 7505.
48 Burroughes, J. H.; Friend, R. H.; Silbey, R.; editor. Conjugated Polym.: Kluwer Academic Press, Dordrecht, 1991, 555.
49 Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628.
50 Chung, T. C.; Kaufman, J. H.; Heeger, A. J. Physical Review B Condensed Matter. 1984, 30, 702. 51 Sato, M.; Tanaka, S.; Kaeriyama, K. Synth. Met. 1986, 14, 279. 52 Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Synth. Met. 1992, 51, 383. 53 Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Markes, R. N.;
Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Brédas, J. L.; Löglund, M.; Salaneck, W. R. Nature 1999, 397, 121.
54 Brédas, J. L.; Dekker, M.; editor. Handbook of Conducting Polymers. New York, 1986, 2, 859. 55 Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganäs, O. Polymer 1994, 35, 1347. 56 Akoudad, S.; Roncali, J. Synth. Met. 1998, 93, 111. 57 Heeger, A. J. Angew. Chem. Int. Ed. 2001, 40, 2591. 58 Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules
1987, 20, 212. 59 Elsenbaumer, R. L.; Jen, K. Y.; Oboodi, R. Synth. Met. 1986, 15, 169. 60 Fukuda, M.; Sawada, K.; Yoshino, K. J. Polym. Sci. Pt. A Polym. Chem. 1993, 31, 2465. 61 Levesque, I.; Leclerc, M. Chem. Mater. 1996, 8, 2843.
79
62 Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858. 63 Stéphan, O.; Schottland, P.; Le Gall, P-Y.; Chevrot, C. J. Chem. Phys. 1998,
95, 1168. 64 Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970. 65 François, B.; Zhong, X. F. Synth. Met. 1991, 41, 955. 66 Marsitzky, D.; Brand, T.; Geerts, Y.; Klapper, M.; Müllen, K. Macromol. Rapid Commun. 1998, 19, 385. 67 François, B.; Widawski, G.; Rawiso, M.; Cesar, B. Synth. Met. 1995, 69, 463. 68 Radzilowski, L. H.; Stupp, S. I. Macromolecules 1994, 27, 7747. 69 Hiorns, R. C.; Martinez, H. Synth. Met. 2003, 139, 463. 70 Hiorns, R. C.; Holder, S. J.; Schué, F.; Jones, R. G. Polym. Int. 2001, 50, 1016. 71 International Energy Outlook 2009, Energy Information Administration, 2009, http://www.eia.doe.gov/oiaf/ieo/ 72 Solomon, S.; Plattner, G. K.; Knutti, R.; Friedlingstein, P. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (6), 1704‐1709. 73 Smalley, R. E. MRS Bull. 2005, 30 (6), 412‐417. 74 Renée M. Nault, Basic energy sciences workshop on solar energy utilization,
Argonne National Laboratory, 2005. 75 Lewis, N. S. Science 2007, 315 (5813), 798‐801. 76 Thekaekara, M. P. Suppl. Proc. 20th Annu. Meet. Inst. Environ. Sci. 1974. 77 Solar Spectral Irradiance:Air Mass 1.5. http://rredc.nrel.gov/solar/spectra/am1.5/ (June 2009), National Renewable Energy Laboratory (NREL) website. 78 Roncali, J. Chem. Rev. 1997, 97, 173–205. 79 Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct.
Mater. 2006, 16, 2016–2023. 80 Becquerel, A.E. Compt. Rend. Acad. Sci. 1839, 9, 145. 81 Alfred Smee 1849. Elements of Electro-Biology, or The Voltaic Mechanism of
Man; of Electro-Pathology, Especially of the Nervous System.... London: Longman, Brown, Green, and Longmans.
82 Smith, W. Nature, 1873, 7, 303. 83 Adams, W.G.; Day, R.E. Proc. R. Soc. Lond. 1876, 25, 113. 84 "Light sensitive device" U.S. Patent 2,402,662 85 Chapin, M.; Fuller, C.S.; Pearson, G.L. J. Appl. Phys. 1954, 25, 676. 86 Perlin, J. From space to earth: The story of solar electricity. Havard University
Press, Cambridge, MA, 2002. 87 University of New South Wales (2008, October 24). Highest Silicon Solar Cell
Efficiency Ever Reached. Science Daily. Retrieved September 22, 2010, from http://www.sciencedaily.com/releases/2008/10/081023100536.htm
88 Slaoui, A.R.; Collins, T. MRS Bull. 2007, 32, 211. 89 Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovolt: Res. Appl. 2009, 17 (5), 320‐326. 90 Pochettino, A. Acad. Lincei Rend. 1906, 15, 355. 91 Volmer, M. Ann. Physik 1913, 40, 775. 92 Borsenberger, P.M.; Weiss, D.S. Organic photoreceptors for imaging
systems, Marcel Dekker, New York, 1993. 93 Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. 94 Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science, 1992, 258,
1474. 95 Liang, Y.; Xu, Z.; Xia, J.; Tsai S-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater.
2010, 22, E135–E138. 96 (a) http://www.pv-
tech.org/news/_a/new_polymers_push_solarmers_opv_efficiency_to_record_8.
80
13/ [accessed 13th September 2010]; (b) http://www.gizmag.com/world-record-efficiency-for-organic-photovoltaic-solar-cells/17186/ [accessed 5th December 2010].
97 Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93 (7), 3693‐3723. 98 Peumans, P.; Forrest, S. R. Appl. Phys. Lett. 2001, 79 (1), 126‐128. 99 Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.;
Gratzel, M. Nat. Mater. 2008, 7 (8), 626‐630. 100 Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.;
Grätzel, M. Nat. Mater. 2003, 2 (6), 402‐407. 101 O'Regan, B.; Gratzel, M. Nature 1991, 353 (6346), 737‐740. 102 Blankenburg, L.; Schultheis, K.; Schache, H.; Sensfuss, S.; Schrodner, M.
Sol. Energy. Mater. Sol. Cells 2009, 93 (4), 476‐483. 103 Dennler, G.; Lungenschmied, C.; Neugebauer, H.; Sariciftci, N. S.; Labouret,
A. J. Mater. Res. 2005, 20 (12), 3224‐3233. 104 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 394‐412. 105 Krebs, F. C.; Jørgensen, M.; Norrman, K.; Hagemann, O.; Alstrup, J.;
Nielsen, T. D.; Fyenbo, J.; Larsen, K.; Kristensen, J. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 422‐441.
106 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 465‐475. 107 Krebs, F. C.; Alstrup, J.; Spanggaard, H.; Larsen, K.; Kold, E. Sol. Energy
Mater. Sol. Cells 2004, 83 (2‐3), 293‐300. 108 Krebs, F. C.; Spanggaard, H.; Kjaer, T.; Biancardo, M.; Alstrup, J. Mater. Sci.
Eng. B 2007, 138 (2), 106‐111. 109 Krebs, F. C. Sol. Energy Mater. Sol. Cells 2009, 93, 1636‐1641. 110 Krebs, F. C. Org. Electron. 2009, 10, 761‐768. 111 Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem. 2009, 19, 5442‐5451. 112 Lungenschmied, C.; Dennler, G.; Neugebauer, H.; Sariciftci, S. N.; Glatthaar,
M.; Meyer, T.; Meyer, A. Sol. Energy Mater. Sol. Cells 2007, 91 (5), 379‐384. 113 Niggemann, M.; Zimmermann, B.; Haschke, J.; Glatthaar, M.; Gombert, A. Thin
Solid Films 2008, 516 (20), 7181‐7187. 114 Tipnis, R.; Bernkopf, J.; Jia, S.; Krieg, J.; Li, S.; Storch, M.; Laird, D. Sol. Energy Mater. Sol. Cells 2009, 93 (4), 442‐446. 115 Zimmermann, B.; Glatthaar, M.; Niggemann, M.; Riede, M. K.; Hinsch, A.;
Gombert, A. Sol. Energy Mater. Sol. Cells 2007, 91 (5), 374‐378. 116 Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Appl. Phys. Lett., 1991, 58,
1062. 117 Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.;
Wudl, F. Appl. Phys. Lett. 1993, 62, 585. 118 Yu, G.; Pakbaz, K.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 3422. 119 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,
270, 1789. 120 Halls, J. J. M.; Walsh, C. A.; Greenham N. C. Nature, 1995, 376, 498. 121 Peeters, E.; Van Hal, P.A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen,
J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000, 104, 10174. 122 Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.;
MacKenzie, J. D. Science 2001, 293, 1119. 123 Ramos, A. M.; Rispens, M. T.; Hummelen, J. C.; Janssen, R. A. J. Syn.
Metals 2001, 119, 171. 124 Brabec, C. J.; Winder, C.; Sariciftci, N. S. Adv. Funct. Mater. 2002, 12,
709.
81
125 Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H. Angew. Chem. Int. Ed. 2003, 42, 3371.
126 Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nature Photonics 2009, 3, 297.
127 Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissörtel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Nature 1998, 395, 583. 128 Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. Adv. Mater. 2005, 17, 66. 129 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 85. 130 Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.;
Brabec, C. Adv. Mater. 2006, 18, 2884. 131 Slooff, L. H.; Veenstra, S. C.; Kroon, J. M.; Moet, D. J. D.; Sweelssen, J. S.;
Koetse, M. M. Appl. Phys. Lett. 2007, 90, 143506. 132 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008 20, 2556. 133 Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;
Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. 134 McNeill, C. R.; Abrusci, A.; Zaumseil, J.; Wilson, R.; McKiernan, M. J.; Burroughes, J. H.; Halls, J. J. M.; Greenham, N. C.; Friend, R. H. Appl. Phys. Lett. 2007, 90, 193506. 135 Moet, D. J. D.; Koster, L. J. A.; de Boer, B.; Blom, P. W. M. Chemistry of Materials, 2007, 19, 5856. 136 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,
270, 1789. 137 Cheng, Y-J.; Yang, S-H.; Hsu, C-S. Chem. Rev. 2009, 109, 5868–5923. 138 Martijn Lenes, PhD thesis 2009, University of Groningen, The Netherlands. 139 Gadisa, A.; Svensson, M.; Andersson, M. R.; Inganas, O. Appl. Phys. Lett. 2004, 84 (9), 1609‐1611. 140 Mihailetchi, V. D.; Blom, P. W. M.; Hummelen, J. C.; Rispens, M. T. J. Appl. Phys. 2003, 94 (10), 6849‐6854. 141 IEC-904-3, IEC Standard 142 Kroon, J. M.; Wienk, M. M.; Verhees, W. J. H.; Hummelen, J. C. Thin Solid Films 2002, 403, 223. 143 Whrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129. 144 Knupfer, M. Appl. Phys. A 2003, 77, 623. 145 Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. 146 Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Appl. Phys. Lett. 1996, 68 (22), 3120‐3122. 147 Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldmann,
J.; Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Physical Review B 1999, 59 (23), 15346‐15351.
148 Pettersson, L. A. A.; Roman, L. S.; Inganas, O. J. Appl. Phys. 1999, 86 (1), 487‐496.
149 Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376 (6540), 498‐500.
150 Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15. 151 Ko, C. J.; Lin, Y. K.; Chen, F. C.; Chu, C. W. Appl. Phys. Lett. 2007, 90(6). 152 Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.
Nat. Mater. 2005, 4 (11), 864‐868. 153 Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15 (10), 1617‐1622. 154 (a) Kim Y, Ballantyne AM, Nelson J, Bradley DDC, Organic Electronics
10:205 (2009); (b) de Cuendias, A.; Hiorns, R. C.; Cloutet, E.; Vignau L.;
82
Cramail, H. Polym. Int. 2010, 11, 1-25. 155 Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91 (11), 954‐985. 156 Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; de Boer, B. Polym. Rev. 2008, 48 (3), 531‐582. 157 Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher, D.; Scharber, M.;
Brabec, C. Macromolecules 2007, 40 (6), 1981‐1986. 158 Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6 (7), 497‐500. 159 Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130 (48),16144‐16145. 160 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3 (5), 297‐302. 161 Liang, Y. Y.; Feng, D. Q.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu, L. P. J. Am. Chem. Soc. 2009, 131 (22), 7792‐7799. 162 Yu, C. Y.; Chen, C. P.; Chan, S. H.; Hwang, G. W.; Ting, C. Chem. Mater.
2009, 21 (14), 3262‐3269. 163 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008, 20
(13), 2556‐2560. 164 Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym. Sci., Polym. Lett. Ed.
1980,18, 9-12. 165 Lin, J. W. P.; Dudek, L. P. J. Polym. Sci., Poly. Chem. 1980, 18, 2869- 2873. 166 Leclerc, M.; Diaz, F. M.; Wegner, G. Makromol. Chem. 1989, 190, 3105. 167 (a) Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules 2004, 37, 1169; (b) Sheina, E. E.; Liu, J.; Iovu, M. C.; Darin, W.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526; (c) Yokozawa, T.; Yokoyama, A. Chem. Rev. 2009, 109 (11), 5595; (d) Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, C.; Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. Adv. Funct. Mater. 2006, 16, 2263. 168 (a) Jeffries-El, M.; Sauvé, G.; McCullough, R. D. Macromolecules 2005, 38,10346; (b) Liu, J.; McCullough, R. D. Macromolecules 2002, 35, 9882; (c) Liu, J.; Loewe, R. S.; McCullough, R. D. Macromolecules 1999, 32, 5777; (d) Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131, 12894; (e) Kaul, E.; Senkovskyy, V.; Tkachov, R.; Bocharova, V.; Komber, H.; Stamm, M.; Kiriy, A. Macromolecules 2010, 43, 77; (f) Hiorns, R. C.; Khoukh, A.; Gourdet, B.; Dagron-Lartigau, C. Polym. Int. 2006, 55, 608; (g) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. Macromolecules 2008, 41, 7033. 169 Lan, Y-K.; Yang, C. H.; Yang, H. C. Polym. Int. 2010, 59, 16. 170 (a) Brinkmann, M.; Rannou, P. Macromolecules 2009, 42, 1125; (b) Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G. Macromolecules 2006, 39, 2162; (c) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312; (d) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Nanotechnology 2004, 15, 1317; (e) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nano Letters 2005, 5(4), 579. 171 McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992, 1, 70–
72. 172 Chen, T. A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114, 10087-10088. 173 McCullough, R.D. Adv. Mater. 1998, 10, 93.
83
174 Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater. 1999, 11, 250–253.
175 Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules 2004, 37, 1169–1171.
176 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004, 25, 1663.
177 (a) Jeffries-EL, M.; Sauvé, G.; McCullough, R. D. Adv. Mater. 2004, 16, 1017– 1019; (b) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526–3528; (c) Iovu, M.C.; Sheina, E.E.; Gil, R.R.; McCullough, R.D. Macromolecules 2005, 38, 8649; (d) Jeffries-EL, M.; Sauvé, G.; McCullough, R. D. Macromolecules 2005, 38, 10346–10352. 178 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542. 179 Yokozawa, T.; Yokoyama, A. Prog. Polym. Sci. 2007, 32, 147. 180 Yokozawa, T.; Yokoyama, A. Chem. Rev. 2009, 109, 5595-5619. 181 Osaka, I.; McCullough, R. D. Acc. Chem. Res., 2008, 41, 1202-1214. 182 Lindner, S. M.; Hüttner, S.; Chiche, A.; Thelakkat, M.; Krausch, G. Angewandte Chemie International Edition 2006, 45 (20), 3364. 183 Muller, C.; Toby, A. M.; Ferenczi, M. C-Quiles.; Jarvist, M. F.; Bradley, D. D. C.; Smith, Paul.; Stingelin-Stutzmann, N.; Nelson, J Adv. Mater. 2008, 20, 3510– 3515. 184 Al-Ibrahim, M.; Ambacher, O.; Sensfuss, S.; Gobsch, G. Applied Physics
Letters 2005, 86, 201120. 185 Verilhac, J. M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.;
Travers, J. P.; Pron, A. Synthetic Metals 2006, 156, 815. 186 Mihailetchi, V.D.; Xie, H.; de Boer, B.; Popescu, L.M.; Hummelen, J.C.; Blom,
P.W.M.; Koster, L.J.A. Applied Physics Letters 2006, 89, 012107. 187 Zhao, Y.; Xie, Z.; Qu, Y.; Geng, Y.; Wang, L. Applied Physics Letters 2007, 90,
043504. 188 Janssen, G.; Aguirre, A.; Goovaerts, E.; Vanlaeke, P.; Poortmans, J.; Manca, J.
The European Physical Journal Applied Physics 2007, 37, 287. 189 Berson, S.Thesis 2007, University of Grenoble. 190 Huang, J.; Li G.; Yang, Y. Applied Physics Letters 2005, 87, 112105. 191 Li, G.; Shrotriya, V.; Yao Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704. 192 Kim, Y.; Choulis, S.A.; Nelson, J.; Bradley, D.D.C.; Cook, S.; Durrant, J.R.
Applied Physics Letters 2005, 86, 063502. 193 Ma, W.; Yang, C.; Gong, X.; Li, K.; Heeger, A.J. Advanced Functional Materials
2005, 15, 1617. 194 Padinger, F.; Rittberger, R.S.; Sariciftci, N.S. Advanced Functional Materials
2003, 13, 85. 195 Zhokhavets, U.; Erb, T.; Hoppe, H.; Gobsch, G.; Sariciftci, N.S. Thin Solid
Films 2006, 496, 679. 196 Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J.
M.; Michels, M. A. J.; Janssen, R. A. J. Nano Lett. 2005, 5, 579. 197 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 1. 198 Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Adv.
Funct. Mater. 2005, 15, 1260. 199 Pivrikas, A.; Juska, G.; Mozer, A. J.; Scharber, M.; Arlauskas, K.; Sariciftci, N.
S.; Stubb, H.; Österbacka, R. Phys. Rev. Lett. 2005, 94, 176806. 200 Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stuhn, B.; Schilinsky, P.;
Waldauf, C.; Brabec, C. J. Adv. Funct. Mat. 2005,1,1193. 201 Drees, M.; Hoppe, H.; Winder, C.; Neugebauer, H.; Sariciftci, N.S.; Schwinger,
W.; Schäffler, F.; Topf, C.; Scharber, M.C.; Zhu, Z.; Gaudiana, R. J. Mater. Chem. 2005, 15, 5158.
202 Nierengarten, J.F.; Setayesh, S. New J. Chem. 2006, 30, 313.
84
203 Zhou, E.; Tan, Z.; Yang, Y.; Huo, L.; Zou, Y.; Yang, C.; Li, Y. Macromolecules 2007, 40, 1831.
204 Cravino A.; Sariciftci, N.S. J. Mater. Chem. 2002, 12, 1931. 205 Cravino, A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson,
M.R.; Neugebauer, H.; Brabec, C.J.; Sariciftci, N.S. Monatshefte für Chemie 2003, 134, 519.
206 Cravino, A. Polymer International 2007, 56, 943. 207 Tan, Z.; Hou, J.; He, Y.; Zhou, E.; Yang C.; Li, Y. Macromolecules 2007, 40,
1868. 208 Hadziioannou, G. MRS Bulletin 2002, 27, 456. 209 (a) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Chem. 1990, 41, 525; (b)
Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. 210 Hadziioannou, G.; Picot, C.; Skoulios, A.; Ionescu, L. M.; Mathis, A.; Duplessix,
R.; Gallot, Y.; Lingelser, J. P. Macromolecules 1982, 15, 2560. 211 (a) Segalman, R. A. Materials Science and Engineering R 2005, 48, 191; (b)
Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152. 212 Klok, H-A.; Lecommandoux, S. Adv. Mater. 2001, 13, 1217. 213 Noshay, A.; McGrath, J. E. Block Copolymers. Academic Press, New York
1977. 214 Meier, D. J. J. Polym. Sci. B 1969, 34,1821. 215 Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. 216 Semenov, A. N.; Vasilenko, S. V. Sov. Phys. JETP A 1986, 63, 70. 217 Semenov, A. N. Cryst. Liq. Cryst. 1991, 209, 191. 218 Semenov, A. N.; Subbotin, A. V. Sov. Phys. JETP A 1992, 74, 690. 219 Williams, D. R. M.; Fredrickson, G. H. Macromolecules 1992, 25, 3561. 220 Williams, D. R. M.; Halperin, A. Phys. Rev. Lett. 1993, 71, 1557. 221 Matsen, M. W. J. Chem. Phys. 1996, 104, 7758. 222 Borsali, R.; Lecommandoux, S.; Pecora, R.; Benoît, H. Macromolecules 2001,
34, 4229-4234. 223 Müller, M.; Schick, M. Macromolecules 1996, 29, 8900. 224 Semenov, A. N. Sov. Phys. JETP A 1985, 61, 733. 225 Tao, Y.; Olsen, B. D.; Ganesan, V.; Segalman, R.A. Macromolecules 2007, 40,
3320. 226 Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.;
Amstutz, A. Science 1997, 276, 384. 227 Kirova, N. Polym. Int. 2008, 57, 678. 228 Nunzi, J-M. C. R. Physique 2002, 3, 523. 229 Watkins, P. K.; Walker, A. B.; Verschoor, G. L. B. Nano Letters 2005,
5(9),1814. 230 (a) Darling, S. B. Energy Environ. Sci. 2009, 2, 1266; (b) Sun, S.; Fan, Z.;
Wang, Y.; Haliburton, J. Journal of Materials Science 2005, 40, 1429; (c) Gratt, J. A.; Cohen, R. E. J. Appl. Polym. Sci. 2004, 91, 3362; (d) Sun, S-S. Solar Energy Materials & Solar Cells 2003, 79, 257; (e) Shah, M.; Ganesan, V.; Macromolecules 2010, 43, 543.
231 Botiz, I.; Darling, S. B. Materials Today 2010,13 (5), 42-51. 232 Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. 233 Sun, S.-S.; Zhang, C.; Ledbetter, A.; Choi, S.; Seo, K.; Bonner, J. C. E.; Drees,
M.; Sariciftci, N. S. Appl. Phys. Lett. 2007, 90, 043117. 234 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; van de Wetering, K.;
Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701; (b) Adamopoulos, G.; Heiser, T.; Giovanella, U.; Ould-Saad, S.; van de Wetering, K.; Brochon, C.; Zorba, T.; Paraskevopoulos, K. M.; Hadziioannou, G. Thin Solid Films, 2006, 371, 511-512; (c) Heiser, T.; Adamopoulos, G.; Brinkmann, M.; Giovanella, U.; Ould-Saad, S.; Brochon, C.; van de Wetering, K.; Hadziioannou, G. Thin Solid Films, 2006, 219, 511-512.
85
235 a) Sivula, K.; Ball, Z. T.; Watanabe, N.; Fréchet, J. M. J. Adv. Mater. 2006, 18, 206; b) Ball, Z. T.; Sivula, K.; Fréchet, J. M. J. Macromolecules 2006, 39, 70.
236 (a) Lindner, S. M.; Hüttner, S.; Chiche, A.; Thelakkat, M.; Krausch, G. Angew. Chem. Int. Ed. 2006, 45, 3364; (b) Sommer, M.; Lang, A. S.; Thelakkat, M. Angew. Chem. Int. Ed. 2008, 47, 7901; (c) Sommer, M.; Hüttner, S.; Wunder, S.; Thelakkat, M. Adv. Mater. 2008, 20, 2523.
237 Zhang, C.; Cirpan, A.; Russell, T. P.; Emrick, T. Macromolecules, 2009, 42(4), 1079.
238 Katz, E. A.; Faiman, D.; Tuladhar, S. M.; Kroon, J. M.; Wienk, M. M.; Fromherz, T.; Padinger, F.; Brabec, C. J.; Sariciftci, N. S. J. Appl. Phys. 2001, 90, 5343.
239 Liu, C.; Oshima, K.; Shimomura, M.; Miyauchi, S.; Synth. Met. 2006, 156, 1362. 240 Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, C.;
Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. Adv. Funct. Mater. 2006, 16, 2263.
241 Leclère, P.; Hennebicq, E.; Calderone, A.; Brocorens, P.; Grimsdale, A. C.; Müllen, K.; Brédas, J. L.; Lazzaroni, R. Prog. Polym. Sci. 2003, 28, 55.
242 Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387. 243 Tsitsilianis, C.; Voyiatzis, G. A.; Kallitsis, J. K. Macromolecular Rapid
Communications 2000, 21, 1130. 244 Leclère, P.; Calderone, A.; Marsitzky, D.; Francke, V.; Geerts, Y.; Müllen, K.;
Brédas, J. L.; Lazzaroni, R. Advanced Materials 2000, 12, 1042. 245 Francke, V.; Räder, H.J.; Geerts, Y.; Müllen, K. Macromolecular Rapid
Communications 1998, 19, 275. 246 Tsolakis, P.K.; Kallitsis, J.K.; Godt, A. Macromolecules 2002, 35, 5758. 247 Leclère, P.; Surin, M.; Jonkheijm, P.; Henze, O.; Schenning, A.P.H.J.; Biscarini,
F.; Grimsdale, A.C.; Feast, W.J.; Meijer, E. W.; Müllen, K.; Brédas, J. L.; Lazzaroni, R. European Polymer Journal 2004, 40, 885.
248 Marsitzky, D.; Klapper, M.; Mullen, K. Macromolecules 1999, 32, 8685. 249 Kong, X.; Jenekhe, S.A. Macromolecules 2004, 37, 8180. 250 Lin, H. C.; Lee, K. W.; Tsai C. M.; Wei, K. H. Macromolecules 2006, 39, 3808. 251 Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J.
Am. Chem. Soc. 2000, 122, 5464. 252 Heiser, T.; Adamopoulos, G.; Brinkmann, M.; Giovanella, U.; Ould-Saada, S.;
Brochon, C.; Wetering, K. v. d.; Hadziioannou, G. Thin Solid Films 2006, 219, 511-512.
253 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2007, 40, 6990.
254 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; Wetering, K. v. d.; Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701.
255 Van der Veen, M. H.; Boer, B. d.; Stalmach, U.; Wetering, K. v. d.; Hadziioannou, G. Macromolecules 2004, 37, 3673.
256 Boer, B. d.; Stalmach, U.; vanHutten, P.F.; Melzer, C.; Kranikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097.
257 Iovu, M. C.; Jeffries-El, M.; Sheina, E. E.; Cooper, J. R.; McCullough, R. D. Polymer 2005, 46, 8582.
258 Sauvé, G.; McCullough, R. D. Adv. Mater. 2007, 19, 1822. 259 Iovu, M. C.; Zhang, R.; Cooper, J. R.; Smilgies, D. M.; Javier, A. E.; Sheina, E.
E.; Kowalewski, T.; McCullough, R. D. Macromol. Rapid Commun. 2007, 28, 1816.
260 Iovu, M. C.; Jeffries-El, M.; Zhang, R.; Kowalewski, T.; McCullough, R. D. J. Macromol. Sci. Part A 2006, 43, 1991.
261 Radano, C. P.; Scherman, O. A.; Stingelin-Stutzmann, N.; Muller, C.; Breiby, D. W.; Smith, P.; Janssen, R. A. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 12502.
86
262 Muller, C.; Goffri, S.; Breiby, D. W.; Andreasen, J. W.; Chanzy, H. D.; Janssen, R. A. J.; Nielsen, M. M.; Radano, C. P.; Sirringhaus, H.; Smith, P.; Stingelin-Stutzmann, N. Advanced Functional Materials 2007, 17, 2674.
263 Dai, C. A.; Yen, W. C.; Lee, Y. H.; Ho, C.C.; Su, W. F. J. Am. Chem. Soc. 2007, 129, 11036.
264 Moliton, A. Optoelectronics of molecules and polymers, New York, Springer, 2005.
265 (a) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097; (b) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 5464.
266 Stalmach, U.; de Boer, B.; Post, A. D.; van Hutten, P. F.; Hadziioannou, G. Angew. Chem. Int. Ed. 2001, 40(2), 428.
267 van der Veen, M. H.; de Boer, B.; Stalmach, U.; van de Wetering, K. I.; Hadziioannou, G. Macromolecules 2004, 37, 3673.
268 Adamopoulos, G.; Heiser, T.; Giovanella, U.; Ould-Saad, S.; van de Wetering, K. I.; Brochon, C.; Zorba, T.; Paraskevopoulos, K. M.; Hadziioannou, G. Thin Solid Films 2006, 371, 511-512.
269 Sary, N.; Mezzenga, R.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J. Macromolecules 2007, 40, 3277.
270 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2007, 40, 6990.
271 Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; van de Wetering, K.; Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701.
272 (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498; (b) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510.
273 Friend, R. H. Pure. Appl. Chem. 2001, 73(3), 425. 274 Liu, J.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Angew. Chem. Int. Ed.
2002, 41(2), 329. 275 (a) Iovu, M. C.; Jeffries-EL, M.; Sheina, E. E.; Cooper, J. R.; McCullough, R. D.
Polymer 2005, 46, 8582; (b) Sauvé, G.; McCullough, R. D. Adv. Mater. 2007, 19, 1822.
276 Dante, M.; Yang, C.; Walker, B.; Wudl, F.; Nguyen, T-Q. Adv. Mater. 2010, 22, 1835.
277 Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Lévêque, P.; Audinot, J-N.; Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. Adv. Mater. 2010, 22, 763.
278 (a) Sommer, M.; Lang, A. S.; Thelakkat, M. Angew. Chem. Int. Ed. 2008, 47, 7901; (b) Sommer, M.; Hüttner, S.; Steiner, U.; Thelakkat, M. Appl. Phys. Lett. 2009, 95, 183308; (c) Zhang, Q.; Cirpan, A.; Russell, T. P.; Emrick, T. Macromolecules 2009, 42(4), 1079; (d) Rajaram, S.; Armstrong, P. B.; Kim, B. J.; Fréchet, J. M. J. Chem. Mater. 2009, 21(9), 1775; (e) Tao, Y.; McCulloch, B.; Kim, S.; Segalman, R. A. Soft Matter. 2009, 5, 4219; (f) Sommer, M.; Hüttner, S.; Thelakkat, M. J. Mater. Chem. 2010, 20, 10788-10797.
279 (a) Lee, J. U.; Cirpan, A.; Emrick, T.; Russell, T. P.; Jo, W. H. J. Mater. Chem. 2009, 19, 1483; (b) Yang, C.; Lee, J. K.; Heeger, A. J.; Wudl, F. J. Mater. Chem. 2009, 19, 5416.
280 Hiorns, R. C.; Cloutet, E.; Ibarboure, E.; Khoukh, A.; Bejbouji, H.; Vignau, L.; Cramail, H. Macromolecules 2010, 43, 6033.
87
Chapter 2: Towards comb copolymers based on P3HT
via ω-acetylene-P3HT and ω-vinyl-P3HT
macromonomers
88
Contents
2.1 Introduction............................................................................................. 89 2.2 Syntheses of monomers......................................................................... 91
2.2.1 Synthesis of 3-hexylthiophene....................................................... 91 2.2.2 Synthesis of 2,5-dibromo-3-hexylthiophene.................................. 92
2.2.3 Synthesis of 2-bromo-3-hexyl-5-iodo-thiophene............................ 93 2.3 Synthesis and characterization of regioregular P3HT......................... 95
2.3.1 Regioregular α,ω-diH-P3HTs........................................................ 95 2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs.................... 97
2.4 Synthesis of ω- or α,ω-alkynyl-P3HT by the GRIM method................ 98 2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT(P3) .. 99
2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method......... 103
2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator....................................................................................... 105
2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) ..................... 106 2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni-
initiator [(Ph)Ni(PPh3)2-Br] ............................................................ 107 2.7 Syntheses and characterizations of polyacetylene-graft-P3HT.......... 112
2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst.............. 113 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst......... 115 2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl
acetylene....................................................................................... 118 2.7.4 Attempted polymerisation of ω-vinyl-P3HTs.................................. 119
2.8 Conclusions............................................................................................. 120 2.9 References............................................................................................... 121
89
2.1 Introduction
Much research effort has been devoted to the design and synthesis of
polymers with extended π-conjugation due to their potential applications in
non-linear optics and opto-electronic applications. Polyacetylene (PA) is
perhaps the best-known conjugated polymer and exhibits metallic conductivity
upon doping.1 But its intractability and instability have greatly limited its scope
for practical applications. The PA’s with appropriate backbone-pendant
combinations show various functional properties such as liquid crystallinity,
photoconductivity, light emission, photoresistance, helical chirality and optical
nonlinearity.2
Our research group (late Prof. G. Sundararajan’s group) at IIT madras,
India was interested in synthesizing novel linear and tribranched polymers
spanned by polyphenylacetylenes (polyPAs), with redox-active ferrocene
and/or (arene)Cr(CO)3 as end-groups through a simple metathesis route and
studying the existence of an electronic communication between the metal
centers through electrochemical studies.3,4 Polycyclic aromatic compounds
such as naphthalene and anthracene are well-known emitters and their
photophysical properties are well-established and exploited for various
applications. In this connection, we were interested to synthesize
polyphenylacetylenes (PPA) with one end anthracene and other end
naphthalene or substituted phenyl groups as donor/acceptors by metathesis
route using the classical metathesis polymerization catalyst W(CO)6 under
photolytic condition (Scheme 2.1) and also to study their photophysical
properties with respect to their intramolecular electron-transfer interactions.
The chain lengths of these conjugated linear polymers can be varied by
changing the number of equivalents of PA.
ArX1. W(CO)6
UV
2. n PA
UV
3. End capping with
Anthracene
ArX
X, Y = CN, NO2, NMe2, OMe, Me
Ar = Phenyl, Naphthyl
PA = Phenylacetylelne
CPh
Yn
Scheme 2.1 Synthesis of Donor-acceptor conjugated polymers spanned by polyPAs.
90
Given this past experience in PA chemistry in India that it seemed an
opportune moment to exploit the well-defined P3HT chemistry, discussed in
the Introduction, with that of PA. P3HT is known for its excellent optical and
electrical properties and its use in various applications such as light-emitting
diodes, field-effect transistors and plastic solar cells (again, see Section 1.3
for more information). The reason was to see if their combined use could lead
to new structures and architectures in the solid state that might be appropriate
to photovoltaic applications. We expected that the incorporation and grafting
of P3HT chains to a PA backbone might lead to conjugated polymers with
new properties that would result in microphase separated morphologies
(Figure 2.1b). We anticipated that the PA-graft-P3HT chains might have better
interactions with PCBM in the solar cells while retaining crystalline zones and
thus increase the efficiencies through improved exciton formation and charge
collection (Figure 2.1).
Figure 2.1 Schematic representation of (a) P3HT-blend-PCBM and (b) (PA-graft-P3HT)-blend-PCBM.
In this Chapter, we therefore describe the synthesis of two thiophene
monomers required for the preparation of regioregular P3HTs, the synthesis
of regioregular P3HTs, and regioregular end-functionalised ω- and α,ω-
alkynyl, and alkenyl P3HTs by the GRIM method. In an attempt to prepare
monofunctionalised P3HT, we also prepared a small molecule Ni-catalyst
initiator based on phenyl bromide. Attempts were then made to synthesize
PA-graft-P3HT using synthesized macromonomers of P3HTs.
91
2.2 Syntheses of monomers
For the synthesis of regioregular P3HT, two types of thiophene monomers
were prepared. They were 2,5-dibromo-3-hexylthiophene (M1) and 2-bromo-
3-hexyl-5-iodothiophene (M2), as shown in Scheme 2.2. In both cases, the
first step was to prepare 3-hexylthiophene (3).
C6H13BrDiethylether
MgC6H13MgBr
S
Br
Ni(dppp)Cl2Diethylether S
C6H13
1 2 3
S
C6H13
3
NBS, acetic acid
Dichloromethane
1. NBS, THF
2. PhI(OAc)2, I2Dichloromethane
S
C6H13
Br Br
S
C6H13
I Br
2,5-dibromo-3-hexylthiophene(M1)
2-bromo-3-hexyl-5-iodothiophene(M2)
Scheme 2.2 Syntheses of thiophene monomers, 2,5-dibromo-3-hexylthiophene (M1) and 2-bromo-3-hexyl-5-iodothiophene (M2). 2.2.1 3-Hexylthiophene (3) The synthesis of 3-hexylthiophene was inspired from the synthesis of 3-(2-
ethylhexyl) thiophene proposed by Somanathan et al.5 as shown in Scheme
2.2. Hexylmagnesium bromide (2) was prepared by the Grignard reaction of
Mg with hexylbromide (1). This active Grignard species (2) was added in a
flask containing bromothiophene and 1,3-bis(diphenylphosphino)propane
nickel(II) [Ni(dppp)Cl2] in diethyl ether to give 3-hexylthiophene (3). Very high
yields, around 90% were obtained. These Grignard reactions are very
sensitive and must be performed under anaerobic conditions to prevent their
92
degradation. Figure 2.2 shows the 1H NMR spectrum of 3, which matches the
structure of hexylthiophene. All peaks are attributed.
Figure 2.2 1H NMR (400 MHz, CDCl3) spectrum of 3-hexylthiophene (3).
2.2.2 2,5-Dibromo-3-hexylthiophene (M1) The synthesis of first monomer, 2,5-dibromo-3-hexylthiophene (M1) was
performed by mixing 3-hexylthiophene (3) and N-bromosuccinimide (NBS)
(2.2 equivalents) in a mixture of acetic acid and dichloromethane. This
reaction leads to the formation of three species by bromination on positions 2,
4 and 5, to yield the mono-, di- and tribrominated products. Only the
dibromothiophene (M1) is useful for the polymerization; to separate the
various reaction products, then distillation seems to be the most appropriate
method. However, M1 is a compound with a high boiling point (around 300 °
C), and it was therefore distilled at ca 170 ˚C under high vacuum to avoid
subjection to excessive heat. The 1H NMR spectrum of M1 is shown in Figure
2.3 and corresponds to what was expected. The thiophene unit no longer
bears aromatic protons, except that in position 4 corresponding to a peak at
6.80 ppm in 1H NMR spectrum, confirming the dibromination of
hexylthiophene. The relative integration of this peak with the peak
93
corresponding to the proton in position 2 located at 7.1 ppm (a witness to the
monobromothiophene impurity) indicated a purity of 99.5%. It is very
important to obtain a very pure product in order to avoid irregularities in the
chain polymerization. That is why this product was distilled 2-3 times. This
reaction leads to molar yields of ca 70%.
Figure 2.3 1H NMR (400 MHz, CDCl3) spectrum of 2,5-dibromo-3-hexylthiophene
(M1).
2.2.3 2-Bromo-3-hexyl-5-iodo-thiophene (M2) The second monomer, 2-bromo-3-hexyl-5-iodothiophene (M2) was prepared
from the 3-hexylthiophene (3) in two steps.9 The first step was to synthesize
2-bromo-3-hexylthiophene by the reaction of NBS with 3-hexylthiophene, the
second is to add an iodine atom at position 5 of the thiophene ring via a
reaction with iodine (I2) in the presence of iodobenzene diacetate. The
resulting product is distilled twice to remove iodobenzene and also passed
through silica gel column by cyclohexane as eluent to obtain the purest
possible monomer. The 1H NMR spectrum of M2 is shown in Figure 2.4. This
spectrum is very similar to that of M1. Only the position 4-thiophene proton is
slightly offset, moving from 6.8 ppm (M1) to 7.0 ppm (M2), due to the slightly
different environment in the 5 position of thiophene, which has a bromine
94
atom in the case of M1 and an iodine atom in the case of M2. The purity of M2
is around 97%.
Figure 2.4 1H NMR (400 MHz, CDCl3) spectra of 2-bromo-3-hexylthiophene and 2-bromo-3-hexyl-5-iodo-thiophene (M2).
95
2.3 Synthesis and characterization of regioregular P3HTs 2.3.1 Regioregular α,ω-diH-P3HTs
Here is the general procedure for the preparation of α,ω-diH-P3HTs (P1, P1a,
P1b, P1c), which is shown in Scheme 2.3. The reaction conditions for all the
principal P3HT polymers are shown in Table 2.1. In a flask 2,5-dibromo-3-
hexylthiophene M1 was dissolved in THF and stirred under nitrogen. Tert-
butylmagnesium chloride was added and the mixture was stirred at room
temperature for 3 h. The catalyst Ni(dppp)Cl2 was added in one-shot and the
mixture was allowed to stir for 24 h at room temperature. Termination and
removal of bromine chain ends was accomplished by slow addition of LiAlH4.
The reaction was quenched by addition of HCl and the polymer was
recovered by precipitation in ethanol and filtered into a soxhlet extraction
thimble. Following extensive Soxhlet washing with methanol, hexanes and
chloroform, α,ω-diH-P3HTs were recovered from the Soxhlet filter with
chloroform. The reaction conditions for all synthesized α,ω-diH-P3HTs are
summarized in Table 2.1. (See experimental section for complete detailed
procedures and purifications).
S
C6H13
Br Br
1. tBuMgCl2. Ni(dppp)Cl2
3. LiAlH44. HCl (aq) THF, R.T.M1
S
C6H13
H Hn
P1, P1a, P1b, P1c
Scheme 2.3 Synthesis of regioregular α,ω-diH-P3HTs.
Figure 2.5 shows the 1H NMR spectrum of P1 after purification, which
is representative NMR spectrum of all samples. This spectrum is in perfect
agreement with the expected structure and all peaks are attributed to P3HT.
The peak at 6.98 ppm corresponds to aromatic proton of thiophene 4 position)
for regioregular P3HT (HT-HT). The protons corresponding to regioirregular
sequences are 7.00 ppm (TT-HT), 7.02 ppm (HT-HH) and 7.05 ppm (TT-HH)
respectively.5 Therefore it is possible to determine the degree of
regioregularity of a P3HT by integration of these peaks. Similarly, the peak at
2.80 ppm corresponds to the proton bound to α-carbon of 3-hexyl chain in an
96
HT-HT regioregular arrangement whereas the peak at 2.60 ppm corresponds
to the same proton in a regio-irregular arrangement. The relative integration of
these two peaks can also give percentage of regioregularity (HT-HT coupling).
Table 2.1 collects the values of different regioregularity P3HT synthesized,
which are between 92% and 98%. The normalized SECs of all the samples
are overlayed in Figure 2.6.
Debrominated P3HT (α,ω-diH-P3HTs)
P3HT
Monomer M1 (g)
Grignard reagent
tBuMgCl (mL)
Ni(dppp)Cl2 (mol %)
Polymeri-ation time
(h)
LiAlH4 (1 M sol.
THF)
Mn (g/mol)
Đ
RR %
P1 2.83 8.6 0.21 24 4.3 30 000 1.6 96
P1a 3.01 9.2 0.10 24 5.0 50 000 1.7 98
P1b 6.0 18.4 0.06 24 10.0 117 000 1.7 97
P1c 3.14 9.5 1.70 2 10.0 7 000 1.1 92
Table 2.1 Reaction conditions and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of α,ω-diH-P3HTs. [Note: Dispersity, Đ = Mw/Mn]
Figure 2.5 Representative 1H NMR (400 MHz, CDCl3) spectrum of α,ω-diH-P3HT.
97
Figure 2.6 Normalised SECs (UV detection) of α,ω-diH-P3HTs in THF against PS standards.
It should be noted that the values of dispersity are relatively high. This
is most likely due to polymers aggregating as their molecular weight increases
during their formation, thus disrupting the chain-growth polymerization and
leading to varying chain-lengths.
2.3.2 Regioregular, end-functionalised ω- and α,ω-P3HTs
Here is the general procedure for the typical end-capping reaction (GRIM
method). In a flask, 2,5-dibromo-3-hexylthiophene (M1) was dissolved in THF
and stirred under nitrogen. tert-Butylmagnesium chloride (1 M in THF) was
added via syringe and the mixture stirred at room temperature for 2.5 h. Then
Ni(dppp)Cl2 was added in one portion and the mixture stirred for 30-60 min at
room temperature. The Grignard functionalization reagent (50-60 mol % of
monomer) was added via syringe to the reaction mixture and stirred for
additional 30-60 min at room temperature. Finally the reaction was quenched
by adding conc. HCl (5 M) and then poured into methanol to precipitate the
polymer. The polymer was filtered into an extraction thimble and then washed
by Soxhlet extraction with methanol, pentane and chloroform. The polymer
was isolated from the chloroform extraction and concentrated, dried under
reduced pressure.
98
2.4 Synthesis of ω- or α ,ω-alkynyl-P3HT by the GRIM method
Alkynyl functionalized P3HTs were prepared using alkynyl-
magnesiumbromide as end-capping Grignard reagent in the polymerization.
All the synthesized alkynyl-P3HTs from P2 to P3a are shown in Table 2.2.
Two different Grignard reagents (ethynyl-MgBr and pentynyl-MgBr) were
tested to evaluate the effect of distance between the triple bond and π -
conjugated backbone of P3HT. In addition, both groups should lead to P3HT
near (majority) mono-adducts, according to the work performed by Jeffries-El
et al.6 The Grignard reagents used were: (i) ethynylmagnesium bromide to
synthesize ω-ethynyl-P3HT; and (ii) (5-chloromagnesio-1-pentynyl)
trimethylsilane to synthesize, once deprotected, pentynyl-P3HT (see Scheme
2.4). In the latter case, the Grignard reagent, not available commercially, was
obtained by reacting (5-chloro-1-pentynyl)trimethylsilane with magnesium in
THF and we have observed dipentynyl-P3HT as the major product (Scheme
2.4).
Following purification, the P3HT was deprotected by the action of basic
tetrabutylammonium trihydrate (TBAF.3H2O). The trimethylsilane group
protects the alkyne function, most noticeable in the case of pentynyl
functionalization. In both cases (ethynyl and pentynyl), the Grignard reagent
was added at the end of polymerization and the reaction was left for 15-30
min before precipitation in methanol.
S
C6H13
Br Br
1. tBuMgCl2. Ni(dppp)Cl2
THF, R.T.
3. BrMg
3. ClMg SiMe3
4. TBAF.3H2O, THF
S
C6H13
H/Br n
S
C6H13
n
M1
P2
P3
Scheme 2.4 Synthesis of P3HT-terminated with acetylene [ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT (P3)] by GRIM method.
99
2.4.1 Synthesis of ω-ethynyl-P3HT (P2) and α,ω-pentynyl-P3HT (P3)
In a typical experiment, M1 was dissolved in THF and stirred under
nitrogen. Tert-butylmagnesium chloride was added, and the mixture was
stirred at room temperature for 2.5 h. The mixture was then diluted with THF,
Ni(dppp)Cl2 was added, and the mixture stirred for 30 min at room
temperature. The termination of the polymers with the respective Grignard
functionalization agent was carried out in a one-shot addition using 50-60 mol
% with respect to the monomer. As mentioned above, ω-ethynyl-P3HT (P2),
the Grignard reagent was ethynylmagnesium bromide. For α,ω-pentynyl-
P3HT (P3), the reagent was (5-chloromagnesium-1-pentynyl) trimethylsilane
which was prepared separately by the reaction of (5-chloro-1-
pentynyl)trimethylsilane and magnesium in THF (30 mL) for 1 day at room
temperature. P3HTs were purified by a series of precipitation from methanol
solution and analyzed by SEC, 1H NMR, DSC and MALDI-TOF.
Alkynyl Functionalised P3HT
P3HT
Monomer
M1 (g)
Grignard
reagent
tBuMgCl
(mL)
Ni(dppp)Cl2
(mol %)
Polymerization time
(min)
Grignard reagent
used for
functionalisation
Mn
(g/mol)
Đ
RR
(%)
P2 4.50 13.7 0.98 40 Ethynyl-MgBr 14 000 1.1 97
P2a 4.56 14.0 1.80 60 Ethynyl-MgBr 9 000 1.2 95
P2b 3.33 10.0 1.71 30 Ethynyl-MgBr 7 700 1.1 95
P2c 3.0 9.2 1.80 30 Ethynyl-MgBr 8 500 1.1 94
P2d 1.43 4.3 1.68 60 Ethynyl-MgBr 3 500 1.1 90
P2e 3.19 9.5 1.69 60 Ethynyl-MgBr 2 500 1.1 90
P3 5.68 17.4 1.69 60 ClMg(C5H6)Si(Me)3
TBAF.3H20
8 000 1.1 98
P3a 1.6 4.8 1.69 30 ClMg(C5H6)Si(Me)3
TBAF.3H20
6 200 1.1 97
Table 2.2 Reaction conditions, and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of alkynyl-functionalised-P3HTs.
100
Representative 1H NMR spectra of P2 and P3 for alkynylP3HTs, which
correspond to ethynyl and pentynyl functionalized P3HT are shown in Figure
2.7. These spectra characterize the chain ends of polymers, P2 and P3.
Figure 2.7 (a), which shows the 1H NMR spectrum of P2, confirms the
expected structure. Indeed, this is a typical spectrum of regioregular P3HT
has an additional peak around 3.52 ppm that corresponds to the ethylinic
proton. This is also confirmed by the 13C NMR of P2, the peak at 67.98 ppm
assigned alkynyl carbon (-C ≡ CH), which usually appears in a range between
60 and 90 ppm.16,17
The 1H NMR spectrum P3 (Figure 2.7 (b)) shows the expected peaks.
The alkynyl proton (-C ≡ CH) appeared around 3.49 ppm and the protons of
the pentynyl group carbons α, β and γ are to 2.5 ppm, 1.9 ppm and 2.3 ppm
respectively and also the peak at 68.94 ppm in 13C NMR, which can be
attributed to pentynylic carbon (-C ≡ CH). The difference between the alkyne
protons of P2 and P3 is certainly due to the delocalization of electrons by
lower insertion of an alkyl chain between the thiophene and the triple bond in
the case of P3.
The NMRs were also used, as decribed in Section 2.3.1, to determine
regioregularities of the samples. It is noticeable that while most samples
display high regioregularities, P2d and P2e are rather lower. This is probably
due to their considerably lower molecular weight. It is known (see Section 1.3)
that the mechanism for the formation of P3HT results in a regio-irregularity at
the chain-ends and, at lower molecular weights, the concentration of this
irregularity becomes relatively high.
In addition, Figure 2.8 shows the MALDI-TOF mass spectra (Matrix
Assisted Laser Desorption Ionization - Time Of Flight) of P2b and P3. In the
case of ω-ethynyl-P3HT P2b [Figure 2.8 (a)], the spectrum shows a major
population (85%), which corresponds to the mono functionalized P3HT. It was noted that the presence of difunctionalised α,ω-diethynyl-P3HTs might be of
concern when attempting the graft copolymers discussed in Section 2.7.2 as they could lead to crosslinking reactions. α,ω-Di-pentynyl-P3HT P3 [Figure
101
2.8 (b)] was found to be, by far, the major population (85%), in contrast to that
found in the literature where lower concentrations of difunctionalised P3HT
was found, due to possible triple bond complexation with the Ni.6 This would
effectively exclude the use of α,ω-dipentynyl-P3HT from the planned graft
copolymer formation reactions.
(a)
(b)
Figure 2.7 1H NMR (400 MHz, CDCl3) spectra of (a) ω-ethynyl-P3HT (P2) and (b) α,ω-pentynyl-P3HT (P3) [Note that the peaks at ca 2.6 ppm are due to chain-end alkyl α-Hs on the P3HT (see Section 2.3.1)].
102
Figure 2.8 shows two examples of calculation of molar masses that agree well
with molecular weights of the corresponding molecular peaks.
(a)
(b)
Figure 2.8 MALDI-TOF mass spectra of: (a) ω-ethynyl-P3HT (P2b); and (b) α,ω-pentynyl-P3HT (P3).
103
2.5 Synthesis of regioregular ω-vinyl-P3HTs by the GRIM method Chain-end vinyl functionalized P3HT was considered a valid target to
investigate the possibility for obtaining mono-functionalised P3HT
macromonomer for graft copolymers. Functionalization of vinyl P3HT can
achieve the copolymerizations to obtain novel copolymers. For instance, it has
been shown to be possible to synthesize a macro-initiator of ATRP-P3HT
bromoester from ω-vinyl-P3HT.7 The Grignard reagent we used in this case
was vinyl-magnesium bromide. It was added to the growing chains of P3HT
for 15 min before being precipitated in methanol (Scheme 2.5). Figure 2.9
shows the 1H NMR spectrum of P6. The vinyl protons appear at around 5.1
ppm, 5.5 ppm and 6.8 ppm. In addition, MALDI-TOF mass characterizations
(Figure 2.10) confirmed that this polymer resulted in a 100% mono-addition to
the P3HT as first reported by Jeffries-El et al.6
S
C6H13
Br Br
1. tBuMgCl2. Ni(dppp)Cl2
3. THF, R.T.
M1S
C6H13
Br/H n
P6, P6a, P6b
BrMg
Scheme 2.5 Synthesis of ω-vinyl-P3HTs (P6, P6a, P6b) by GRIM method.6 ω-Vinyl-P3HTs of different molecular weights and regioregularities by
varying the amount of Ni-catalyst used in the polymerization were synthesized
as detailed in Table 2.3. The normalised SECs of the samples are shown in
Figure 2.11. We observed a small hump for all the functionalised P3HTs at
high molecular weights probably due to coupling between growing chains and
Ni disproportionation when quenching the polymerisation.8
ω-Vinyl-P3HT
Ni(dppp)Cl2
(mol %)
Mn
(g/mol)
Đ
Regioregularity
(RR %)
P6 2.5 5 500 1.2 90
P6a 1.8 7 400 1.1 94
P6b 1.7 9 000 1.2 96
Table 2.3 Macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards and NMR (RR) of ω-vinylP3HTs.
104
Figure 2.9 1H NMR (400 MHz, CDCl3) spectrum of ω-vinyl-P3HT (P6).
Figure 2.10 MALDI-TOF mass spectrum of ω-vinyl-P3HT (P6).
105
Figure 2.11 Normalised SECs (THF, UV-254 nm) of ω-vinyl-P3HTs (P6, P6a, P6b) against PS standards. To conclude, this section has presented the synthesis of regioregular
P3HTs and showed that it was difficult to precisely control the parameters of
the synthesized polymers. For the functionalization, the results are different
from those published by other groups and showed that the mono-or di-
functionalization is not only dependent on the chemical nature of the groups
but also on the reactivity of functional groups.
In addition, alkyne terminated P3HTs have been synthesized which
made possible subsequent coupling reactions using "click" chemistry to form
rod-coil block copolymers (as detailed in Chapter 3) and also attempts to
synthesize polyacetylene-graft-polythiophene using ethynyl-P3HT and vinyl-
P3HT (see Section 2.7).
2.6 Synthesis of mono-functionalised-P3HT by externally added Ni-catalyst initiator Using the well-known GRIM method, we were able to synthesize
regioreglar P3HTs and also regioregular end-functionalized P3HTs
successfully. However, as mentioned above, the drawback of this method is
that it was not completely possible to ensure the preparation of totally mono-
ethynyl functionalised P3HTs. And this is very important for the synthesis of
106
novel polymer architectures such as brushes, starlike and block copolymers.
Recently, reports in which “external” initiators for the synthesis of
conjugated polymer brushes of regioregular P3HT via surface-initiated
catalyst-transfer polycondensations have been demonstrated.9,10 Senkovskyy
et al., in particular, demonstrated the chain-growth polymerization of P3HT by
both small-molecule and surface initiators as shown in Scheme 2.6.9 This
method, which appeared to allow the formation of completely mono-
functionalised P3HTs, inspired us to prepare regioregular mono-ethynylated
P3HTs of controlled molecular weights and use these macromonomers to
synthesize graft copolymers, which was one of our major aims. Therefore we
attacked this route.
Scheme 2.6 Catalyst-transfer polycondensation of 1a initiated by small molecules or macroinitiators according to Senkovskyy et al.9
2.6.1 Synthesis of the Ni-initiator: [(Ph)Ni(PPh3)2-Br] (4) The extremely reactive complex [Ni(PPh3)4] reacts easily with various
arylhalides to produce the desired adducts [(Ar)Ni(PPh3)2-Br].11 In a flame-
dried Schlenk flask; to a solution of [Ni(PPh3)4] in dry toluene, bromobenzene
was added at room temperature under argon atmosphere. Then the
homogeneous mixture was allowed to stir for about 30 min and allowed to
107
stand for overnight. The original deep red colour of the reaction mixture
gradually changed to brownish yellow colour with the precipitation of
[(Ph)Ni(PPh3)2-Br] (4) as yellow crystals that were, rather tediously, filtered
under argon atmosphere and washed with dry pentane (Scheme 2.7).
Br [Ni(PPh3)4]-2PPh3, RT, Ar
Ni[PPh3]2-Br+Dry toluene
4 Scheme 2.7 Synthesis of the “small molecule” Ni-initiator.
2.6.2 Synthesis of mono-functionalised P3HT by “small molecule” Ni- initiator [(Ph)Ni(PPh3)2-Br] We adopted the synthetic procedure of Senkovskyy et al.,9 for the
synthesis of mono-functionalized P3HTs, using “small molecule” Ni-initiator
[(Ph)Ni(PPh3)2-Br]. In a typical polymerization, the Ni-catalyst initiator (4)
solution in dry toluene was added to Grignard regio-isomer of M2a at 0 °C
under argon and the reaction mixture was stirred at 0 °C for about 6 h under
argon. At the end of the polymerization, the reaction was quenched with 5 M
HCl or a functional Grignard reagent to result in chain-end capping. We used
the protected Grignard reagent (5-chloromagnesio-1-pentynyl) trimethylsilane
with the aim of preparing α-Ph-ω-pentynyl-P3HT (P7, P7a). The former case,
with HCl led to α-Ph-ω-H-P3HT (P8, P8a, P8b) (Scheme 2.8). The reaction
conditions and the details of characterised mono-functionalised P3HTs from
P7 to P8b are shown in Table 2.4. The polymers’ regioregularities are quite
low while the dispersities are high when compared with those prepared by the
GRIM method. When the polymerization temperature was raised from 0 °C to
RT (Table 2.4), the regioregularity decreased and the dispersities increased,
in agreement with the observations of Senkovskyy et al., which means that
the polymerization for high molecular weight P3HTs was not well-controlled by
this method. The detailed experimental procedures are given for all polymers
in the experimental section. The polymers were characterized by 1H NMR,
SEC and MALDI-TOF.
108
S
C6H13
I Br
iPrMgCl
S
C6H13
ClMg Br S
C6H13
R
Ni[PPh3]2-Br1.
2. R-MgBr3. 5M HCl n
M2 M2a
THF, Ar, 0 oC
P7, P7a (R = Pentynyl)P8, P8a, P8b (R = H)
Scheme 2.8 Synthesis of mono-functionalised P3HT by “small molecule” Ni-initiator [(Ph)Ni(PPh3)2-Br] according to Senkovskyy et al.9
Mono-functionalised P3HT by [(Ph)Ni(PPh3)2-Br]
P3HT Monomer
M2 (g)
Grignard reagent tPrMgCl 2 M (mL)
Ni-initiator (4) (mg)
Polyme-rization
time (h)
Grignard reagent for endcapping
Target structure
Mn (g/mol) Đ
RR
(%)
P7 1.28 1.74 100 6
(0 ˚C)
ClMg(C5H6)Si(Me)3
TBAF.3H20
α-Ph-ω-ethynyl-
P3HT 6 500 1.7 92
P7a 0.58 0.8 40 6
(0 ˚C)
ClMg(C5H6)Si(Me)3
TBAF.3H20
α-Ph-ω-ethynyl-
P3HT 6 200 1.6 88
P8 0.50 0.7 35 6
(0 ˚C) 5 M HCl α-Ph-ω-H-P3HT 8 600 1.4 90
P8a 0.60 0.8 40 3 (RT) 5 M HCl α-Ph-ω-H-P3HT 5 300 1.7 88
P8b 0.50 0.7 35 3 (RT) 5 M HCl α-Ph-ω-H-P3HT 5 900 1.6 89
Table 2.4 Reaction conditions, molecular weights (Mn) and regioregularity (RR) of mono-functionalised-P3HTs prepared using the “small molecule” Ni-initiator. From 1H NMR and MALDI-TOF techniques, we have investigated the
initiation efficiency for the polymerization and also obtained the information
about both the starting groups as well as end-groups of resulting P3HT
products. Representative 1H NMR (Figure 2.12) and MALDI-TOF (Figure 2.13)
mass spectra of P7 and P8 for P3HTs, which correspond to α-Ph-ω-pentnyl-
P3HT and α-Ph-ω-H-P3HT, respectively. Figure 2.14 shows the normalised
overlay SECs of P3HTs P7 and P8. Figure 2.12(a) shows the 1H NMR
spectrum of P8, confirms the expected structure. From the integration of
signals in the aromatic region and a detailed assignment of the 1H NMR of the
product showed that the obtained P3HT had around 70% α-phenyl groups
along with Ph/H, Ph/Br and a small amount of H/H end-groups, thus agreeing
109
with Senkovskyy’s et al.8 results. However, the MALDI-TOF mass spectra
characterisations of the obtained P3HTs indicate results which are in
disagreement with those obtained by Senkovskyy et al.9 Figure 2.13 (a)
MALDI-TOF spectrum of P8 shows that the Ph-initiated P3HT (thus giving
Ph/H or Ph/Br chain-ends) is not a major portion and in fact the majority
population is α-H-ω-bromo-P3HT, probably due to chain-transfer reactions.
This contradicts the results of Senkovskyy et al.9 and the reason for this is
probably the poor resolution of their MALDI-TOF spectra which made
impossible a correct assignment of chain-ends. A considerable amount of
work was performed by our service (CESAMO, Bordeaux) in order to
distinguish between these chain-ends. It should be noted that there is a
collusion in the molar masses of phenyl and bromo chain-end groups (Ph is
2571 g mol-1, Br is 2574 g mol-1).
Figure 2.12 (b) shows the 1H NMR spectrum of P7 (α-Ph-ω-pentynyl-
P3HT) that confirms the expected structure. The alkynyl proton (-C ≡ CH)
appears at around 3.49 ppm and the protons of the pentynyl group carbons α,
β and γ are at 2.5 ppm, 1.9 ppm and 2.3 ppm, respectively. Integration of the
peaks in the aromatic region show that the majority of the portion is due to
H/H end-groups and that this functionalized P3HT has Ph-initiating groups on
only around 50% of the P3HT chains. This is confirmed in the NMR spectrum,
as the intensity of peak (f) at 6.9 ppm is high in the case of P7, which is not
the case in P8. This further confirmed from MALDI-TOF mass spectrum
results of P7 [Figure 2.13 (b)]. From this mass spectrum, we have observed
that the product α-Ph-ω-pentynyl-P3HT (P7) was a mixture, which
corresponds to end groups H/H (major population), Ph/Pentynyl, H/Pentynyl
and Ph/H (minor population). The chain-end functionalisation was not very
efficient here. We did not separate the fractions from this mixture. However,
we nevertheless attempted to use this chain-end mono functionalized P3HT to
prepare graft copolymers (see Section 2.7).
110
(a)
(b)
Figure 2.12 1H NMR (400 MHz, CDCl3) spectrum of: (a) α-Ph-ω-H-P3HT (P8); and (b) α-Ph-ω-pentynyl-P3HT (P7).
111
(a)
(b)
Figure 2.13 MALDI-TOF mass spectrum of: (a) α-Ph-ω-H-P3HT (P8); and (b) α-Ph-ω-pentynyl-P3HT (P7).
112
Figure 2.14 Normalised SECs (UV detection) of P3HTs, P7 and P8 in THF against PS standards. In this section, we have synthesized mono-functionalised P3HT by
external “small molecule” Ni-initiator. We could not reproduce the results
reported by Senkovskyy et al.9 even though considerable time and care was
expended to do so. It is proposed that the results were essentially the same
as Senkovskyy et al.’s but that the interpretations differ. The synthesis of
regioregular P3HT by this “external” initiation method was not as efficient as
GRIM method. The low regioregularities and high dispersities of P3HT were
obtained using Senkovskyy et al.’s procedure. More recently Luscombe’s
research group also investigated methods for the external initiation of P3HT
including the effect of varying substituents on the intiating aryl halide12 and
they were very successful in synthesizing P3HTs with high regioregularity and
narrow dispersity by an improved “external” initiation method.13
2.7 Syntheses and characterizations of polyacetylene-graft-P3HT
Graft copolymers derived from poly(macromonomers) and branched polymers
generally exhibit unique properties in terms of their organisation and assembly
both in solution and in bulk as compared to their linear counterparts.14,15
Generally graft copolymers can be achieved mainly by three methods:
grafting-onto, in which side chains are first synthesized and then attached to a
multifunctional linear backbone;16 grafting-from, in which grafting of monomer
113
from a linear macroinitiator;17 grafting-through (macromonomer method), in
which macromonomers are copolymerized with low molecular weight
comonomers.18 In a particular case, homopolymerisation of macromonomers
produces comb polymers or polymer brushes.19 Substituted PAs have attracted considerable interest due to their unique
properties. PA-based grafted copolymers are more stable due to the
protection of the main chain by side chains as unsubstituted PAs decompose
gradually in solution. Here we use macromonomer alkynyl-P3HTs synthesized
earlier for the synthesis of polyacetylene-graft-P3HT (PA-g-P3HT). These
graft copolymers may be interesting candidates for applications in organic
electronics.
2.7.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst It is well known that substituted acetylenes polymerize with transition
metal catalysts.20-23 Among various catalysts used, Rh based catalysts attract
particular interest as they efficiently polymerize mono-substituted acetylenes,
especially phenylacetylene derivatives.23-31 Particularly Rh complex catalyst,
[Rh(norbornadiene)Cl]2 can stereoregularly polymerize monosubstituted
acetylenes to produce corresponding polyacetylenes with cis-transoid
structure in high yields under mild conditions. 23,26,27
In this section, the use of an Rh catalyst to polymerize
macromonomers alkynyl-P3HTs (P7 and P2b) is described with the reaction
conditions reported elsewhere.23
Before doing this, we optimized reaction conditions by polymerizing the
monomer phenylacetylene to produce poly(phenylacetylenes) (PPA) with the
Rh-based catalyst and triethylamine (TEA) as a co-catalyst in THF at room
temperature for about 24 h (Scheme 2.9). The polymerization in THF
proceeded smoothly to obtain PPAs in moderate yields after precipitation in
methanol. The obtained PPAs with different molecular weights and
dispersities are detailed in Table 2.5 and are soluble in all organic solvents.
The PPAs are characterized by 1H NMR and SEC. Figure 2.15 (a) shows the
114
representative 1H NMR spectrum of PPA2 in CDCl3 at room temperature. This
spectrum features a typical cis-transoid structure because it exhibits very
sharp lines indicating highly regular structure.26 This was clearly evidenced
that the ratio of an integrated area due to =C-H proton at 5.85 ppm and five
phenyl protons observed at 6.5-7.0 ppm estimated as 1:5. Figure 2.15 (b)
shows the normalised overlay SECs of PPA1 and PPA2. The optimized
reaction conditions were then used for the polymerizations of alkynyl-P3HTs.
[Rh(nbd)Cl]2/TEA
THF, RT, 24 hrs
Hn
PA PPA Scheme 2.9 Synthesis of poly(phenylacetylene) (PPA) by Rh-based catalyst.23
Monomer [Rh(nbd)Cl]2
(mg)
Polymer Mn(g mol-1) Đ
Phenylacetylene 25 PPA1 52 000 2.5
Phenylacetylene 20 PPA2 58 000 2.1
Table 2.5 Molecular weight characteristics (SEC, UV-254 nm) of PPAs by Rh catalyst.
(a)
115
(b)
Figure 2.15 (a) Representative 1H NMR (400 MHz, CDCl3) spectrum of PPA2 and (b) Normalised SECs (UV detection) of PPAs (PPA1 and PPA2) in THF and against PS standards. 2.7.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst
Here two types of macromonomers are utlised: namely ω-ethynyl-
P3HT (P2b) prepared by the GRIM method and α-phenyl-ω-pentynyl-P3HT
(P7) synthesized by the “external” small molecule Ni-initiator. Each is
subjected to homo-polymerization by the Rh-based catalyst in a highly original
manner to obtain polyacetylene-graft-P3HT (PA-g-P3HT). The
polymerizations were performed at room temperature in THF under the
optimized reaction conditions used for the synthesis of PPAs and SEC was
used to monitor the reaction performance. The macromonomer α-phenyl-ω-
pentynyl-P3HT (P7) in which the ethynyl group is far from the conjugated
P3HT backbone did not undergo polymerization even after 24 h and there
was no polymerisation though the reaction was forced by increasing the
temperature. But the macromonomer ω-ethynyl-P3HT (P2b) in which the
ethynyl group is directly attached to thiophene ring was able to undergo
polymerization in the presense of the Rh-based catalyst and produced the
graft copolymer, PA-g-P3HT shown in Scheme 2.10. 1H NMR and SEC were
used to confirm the graft copolymers’ structures. The molecular weights and
dispersities of the graft copolymers are given in Table 2.6.
116
S
C6H13 n [Rh(nbd)Cl]2/TEA
THF, RT S
C6H13 n
m
P2b = Ethynyl-P3HTP7 = Pentynyl-P3HT
PA-graft-P3HT for P2bNo reaction for P7
Scheme 2.10 Synthesis of PA-g-P3HT by Rh-based catalyst.23
Experiment
SEC, Mn (g mol-1) Graft copolymer PA-g-P2b
(Macromonomer, P2b)
Đ
1 34 800 (7 700) 1.4
2 32 100 (7 700) 1.4
3 23 400 (7 700) 1.3
Table 2.6 Molecular weights (Mn) and dispersities (Đ) of PA-g-P3HTs by Rh catalyst. The SECs shown in Figure 2.16 (c) confirm that SECs of α-phenyl-ω-
pentynyl-P3HT (P7) and reaction mixture of P7 with Rh catalyst shows that
the curves are exactly overlayed indicating that P7 was not able to polymerize
in the presence of the Rh-based catalyst.
However, Figure 2.16 (a) showing the representative 1H NMR
spectrum of PA-g-P3HT and confirms the expected structure following the
polymerisation of ω-ethynyl-P3HT (P2b). The disappearance of ethynylic-
proton of ω-ethynyl-P3HT (P2b) at 3.49 ppm indicates that the
macromonomer P2b participated in the polymerization by Rh catalyst. But due
to the low concentration of =C-H proton compared to polymer, it was not able
to identify in the 1H NMR spectrum even at longer scans and high relaxation
time. The formation of graft copolymer further confirmed by the SEC. Figure
2.16 (b) represents the normalised overlay SECs of macromonomer P2b and
PA-g-P3HT in which graft copolymer peak shifted towards high molecular
weight along with small amount of unreacted macromonomer, ω-ethynyl-
P3HT (P2b).
117
(a)
(b) (c)
Figure 2.16 (a) Representative 1H NMR (400 MHz, CDCl3) spectrum of PA-g-P3HT; (b) Normalised overlay SECs of ω-ethynyl-P3HT (P2b) and PA-g-P2b demonstrating copolymerisation reaction; and (c) Normalised overlay SECs of α-phenyl-ω-pentynyl-P3HT (P7) and reaction mixture of P7 by Rh catalyst showing non-reaction. All SECs (UV detection) in THF and against PS standards.
To achieve complete homopolymerization of the macromonomer ω-
ethynyl-P3HT (P2b), reaction conditions were varied by changing parameters
such as the reaction temperature, reaction times and dilution. It should be
noted that it was found that despite the high molecular weight of the products,
the solubilities of the graft copolymers were high in common organic solvents.
118
2.7.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl acetylene
In general, polymerization of macromonomer with low molecular weight
monomer helps to achieve better yields.32 Since we were not successful to
achieve complete homopolymerization of macromonomer ω-ethynyl-P3HT,
this idea prompted us to give a try for copolymeriztion of ω-ethynyl-P3HT with
low molecular weight monomer phenylacetylene. So we have attempted the
copolymerization of the macromonomer, ω-ethynyl-P3HT (P2c) with
phenylacetylene (PA) by varying the amount of PA in the reaction mixture
from 10% to 50% to produce graft copolymers, poly(P2c-co-PA) which is
shown in Scheme 2.11 and monitored the efficiency of the reaction by SEC.
But unfortunately, we were again unsuccessful to obtain expected graft
copolymers by complete copolymerization of ω-ethynyl-P3HT (P2c) with
phenylacetylene.
Figure 2.17 (b) showing the overlayed SECs of macromonomer, ω-
ethynyl-P3HT (P2c) with reaction mixture of P2c and PA (10%) by Rh catalyst
clearly indicates that obtained copolymer is a mixture of products with
remaining unreacted macromonomer, ω-ethynyl-P3HT (P2c) that is significant
amount. To obtain complete copolymerization; we varied the amount of PA
(from 10% to 50%) in the reaction mixture for the copolymerization of P2c, but
the efficiency of copolymerization was decreased by retaining the significant
macromonomer in the reaction mixture which was shown in Figure 2.17 (a).
S
C6H13 n
PA(10% - 50%)
[Rh(nbd)Cl]2/TEA
THF, RT
Expected graft copolymerPoly(P2c-co-PA)
S
C6H13 n
x y
P2c = Ethynyl-P3HT
Scheme 2.11 Copolymerization of macromonomer, ω-ethynyl-P3HT (P2c) with phenylacetylene (PA) by [Rh(nbd)Cl2] catalyst.32
119
(a) (b)
Figure 2.17 (a) Normalised overlay SECs of ω-ethynyl-P3HT (P2c) and reaction mixture of P2c and PA (10%-50%) by Rh catalyst demonstrating copolymerisation reaction; and (b) Normalised overlay SECs of ω-ethynyl-P3HT (P2c) and reaction mixture of P2c and PA (10%) by Rh catalyst showing uncomplete reaction (for clarity). All SECs (UV detection) in THF and against PS standards.
2.7.4 Attempted polymerisation of ω-vinyl-P3HTs
Finally we have made attempts to prepare graft copolymers by other
methods namely RAFT method and olefin polymerization by Ni-metallocene
catalysts from ω-vinyl-P3HT (Scheme 2.12) and monitored the reaction
efficiency by SEC. But even after 2 days also, we have not observed
formation of graft copolymers by SEC. Figure 2.18, the SECs of
macromonomers, ω-vinyl-P3HTs (P6 and P6a) and their reaction mixtures
shows that the curves are exactly overlayed indicating that ω-vinyl-P3HTs
were not able to polymerize by RAFT and olefin polymerization methods. The
reason for this was probably the conjugation of the chain-end and the steric
encoumbrance of the vinyl moiety. Therefore other routes are envisaged.
S
C6H13
Br/H n
1. RAFT method CTA, AIBN, THF
2. Olefin polymerization Ni-catalyst, MAO, Toluene
No graft copolymerP3HT
nXP6 or P6a
Scheme 2.12 Synthesis of P3HT grafted copolymers from ω-vinyl-P3HT by RAFT and Olefin polymerization methods.
120
(a) (b)
Figure 2.18 (a) Normalised overlay SECs of ω-vinyl-P3HT (P6) and reaction mixture of P6 by RAFT polymerisation demonstrating no-reaction; and (b) Normalised overlay SECs of ω-vinyl-P3HT (P6a) and reaction mixture of P6a by olefin polymerisation demonstrating no-reaction. All SECs (UV detection) in THF and against PS standards. 2.8 Conclusions In this chapter we have synthesized regioregular P3HTs and also chain
end-functionalised P3HTs with narrow dispersities by the GRIM method. A
small molecule Ni-initiator was also synthesized and utilized to prepare
completely mono-functionalised P3HTs. But we could not reproduce
Senkovskyy et al.’s results. We obtained a mixture of products when we used
the “external” initiator whereas the GRIM method produced better results. We
were somewhat more successful for our attempts to prepare P3HT grafted
copolymers by alkynyl-P3HTs. Meanwhile we found that conjugation and
steric hindrance play a key role for the polymerization of alkynyl-P3HTs by Rh
catalyst and also in the polymerization of ω-vinyl-P3HTs by RAFT and olefin
polymerization methods.
For the efficient polymerization of P3HT-substituted acetylenes, the
acetylene group should be directly attached to the aromatic group; however, it
is probable that a spacer is necessary to separate the bulk of the aromatic
acetylene group from P3HT due to conjugation and steric hindrance.
Therefore further investigations are required.
121
2.9 References 1 Shirakawa, H. Angew. Chem. Int. Ed. 2001, 40, 2575. 2 Jacky, W. Y. L.; Tang, B. Z. Acc. Chem. Res. 2005, 38, 745. 3 (a) Dhanalakshmi, K.; Sundararajan, G. J. Organomet. Chem. 2002, 645, 27; (b) Dhanalakshmi, K.; Sundararajan, G. Polym. Bull. 1999, 42, 683. 4 Santhosh, N. S.; Sundararajan, G. Org. Lett. 2006, 8 (4), 605–608. 5 Somanathan, N.; Radhakrishnan, S.; Mukundan, T.; Schmidt, H. W.
Macromolecular Materials and Engineering 2002, 287, 236. 6 Jeffries-EL, M.; Sauve, G.; McCullough, R. D. Macromolecules 2005, 38,
10346–10352. 7 Iovu, M.C.; Jeffries-EL, M.; Sheina, E.E.; Cooper, J.R.; McCullough, R.D.
Polymer 2005, 46, 8582. 8 Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004,
25, 1663. 9 Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling,
D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626. 10 Sontag, S. K.; Marshall, N.; Locklin, J. Chem. Commun. 2009, 3354. 11 Hiday, M.; Kashiwagi, T.; Ikeuchi, T.; Uchida, Y. J. Organomet. Chem. 1971,
30, 279. 12 Doubina, N.; Ho, A.; Jen, A. K.-Y.; Luscombe, C. K. Macromolecules 2009, 42,
7670–7677. 13 Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009, 131, 12894-12895. 14 (a) Zhang, M.; Mueller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461-3481; (b) Hadjichristidis, N.; Pitsikalis, M.; Iatrou, H.; Pispas, S. Macromol. Rapid Commun. 2003, 24, 979-1013; (c) Ito, K.; Kawaguchi, S. Adv. Polym. Sci. 1999, 142, 129-178.
15 a) Desvergne, S.; Heroguez, V.; Gnanou, Y.; Borsali, R. Macromolecules 2005, 38, 2400-2409; (b) Zhang, B.; Zhang, S.; Okrasa, L.; Pakula, T.; Stephan, T.; Schmidt, M. Polymer 2004, 45, 4009-4015; (c) Viville, P.; Leclere, P.; Deffieux, A.; Schappacher, M.; Bernard, J.; Borsali, R.; Bredas, J.-L.; Lazzaroni, R. Polymer 2004, 45, 1833-1843; (d) Liu, Y.; Abetz, V.; Mueller, A. H. E. Macromolecules 2003, 36, 7894-7898; (e) Qin, S.; Matyjaszewski, K.; Xu, H.; Sheiko, S. S. Macromolecules 2003, 36, 605-612.
16 (a) Gacal, B.; Durmaz, H.; Tasdelen, M. A.; Hizal, G.; Tunca, U.; Yagci, Y.; Demirel, A. L. Macromolecules 2006, 39, 5330-5336; (b) Li, A.; Lu, Z.; Zhou, Q.; Qiu, F.; Yang, Y. Polymer 2006, 47, 1774-1777; (c) Ryu, S. W.; Hirao, A. Macromolecules 2000, 33, 4765-4771; (d) Schappacher, M.; Deffieux, A. Macromolecules 2000, 33, 7371-7377.
17 (a) Lee, H.-i.; Jakubowski, W.; Matyjaszewski, K.; Yu, S.; Sheiko, S. S. Macromolecules 2006, 39, 4983-4989; (b) Muthukrishnan, S.; Zhang, M.; Burkhardt, M.; Drechsler, M.; Mori, H.; Mueller, A. H. E. Macromolecules 2005, 38, 7926-7934; (c) Cheng, G.; Boeker, A.; Zhang, M.; Krausch, G.; Mueller, A. H. E. Macromolecules 2001, 34, 6883-6888; (d) Borner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.; Moller, M. Macromolecules 2001, 34, 4375-4383.
18 (a) Nguyen, S.; Marchessault, R. H. Macromolecules 2005, 38, 290-296; (b) Cai, Y.; Hartenstein, M.; Mueller, A. H. E. Macromolecules 2004, 37, 7484-7490; (c) Nagai, A.; Ochiai, B.; Endo, T. Macromolecules 2004, 37, 4417-4421; (d) Batis, C.; Karanikolopoulos, G.; Pitsikalis, M.; Hadjichristidis, N. Macromolecules 2003, 36, 9763-9774; (e) Schulze, U.; Fonagy, T.; Komber, H.; Pompe, G.; Pionteck, J.; Ivan, B. Macromolecules 2003, 36, 4719-4726; (f) Breitenkamp, K.; Simeone, J.; Jin, E.; Emrick, T. Macromolecules 2002, 35, 9249-9252.
122
19 Morandi, G.; Montembault, V.; Pascual, S.; Legoupy, S.; Fontaine, L. Macromolecules 2006, 39, 2732-2735.
20 Masuda, T.; Sanda, F. Polymerization of substituted acetylenes. In Handbook of metathesis; Grubbs, R.H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, Chapter 11, 375 p.
21 Sedlacek, J; Vohlidal, J. Collect. Czech. Chem. Commun. 2003, 68, 1745-1790.
22 Choi, S.-K.; Gal, Y.-S.; Jin, S.-H.; Kim, H. K. Chem. Rev. 2000, 100, 1645-1682.
23 Tabata, M.; Sone, T.; Sadahiro, Y. Macromol. Chem. Phys. 1999, 200, 265-282.
24 Furlani, A.; Napoletano, C.; Russo, M.V.; Camus, A.; Marsich, N. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 75-86.
25 Furlani, A.; Napoletano, C.; Russo, M.V.; Feast, W. J. Polym. Bull. 1986, 16, 311-317.
26 Tabata, M.; Yang, W.; Yokota, K. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1113-1120.
27 Tabata, M.; Yang, W.; Yokota, K. Polym. J. 1990, 22, 1105-1107. 28 Mastrorilli, P.; Nobile, C. F.; Gallo, V.; Suranna, G. P.; Farinola, G. J. Mol.
Catal. A: Chem. 2002, 184, 73-78. 29 Tang, B. Z.; Poon, W. H.; Leung, S. M.; Leung W. H.; Peng, H.
Macromolecules 1997, 30, 2209-2212. 30 Kishimoto, Y.; Itou, M.; Miyatake, Y.; Ikariya, T.; Noyori, R. Macromolecules
1995, 28, 6662-6666. 31 Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett. 1993, 22, 2009. 32 Zhang, W.; Shiotsuki, M.; Masuda, T. Macromolecules 2007, 40, 1421-1428.
123
Chapter 3: Block copolymers based on
poly(3-hexylthiophene) (P3HT) and polystyrene (PS) or
poly(4-vinylpyridine) (P4VP)
124
Contents
3.1 Introduction............................................................................................. 125 3.2 Synthesis of azide-terminated polystyrene.......................................... 127
3.2.1 Principle of atom transfer radical polymerisation (ATRP) ............. 127 3.2.2 Synthesis of azide initiator............................................................. 128 3.2.3 Synthesis of α-azido polystyrenes................................................. 131
3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry.................................................... 133
3.3.1 History and principle of “click” chemistry....................................... 133 3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS......... 134 3.3.3.1 Triblock copolymers PS-b-P3HT-b-PS........................... 134 3.3.3.2 Diblock copolymers P3HT-b-PS..................................... 138
3.4 Synthesis of donor-acceptor and acceptor-donor-acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block -PS-C60............................................................................................................. 143
3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) ..... 143 3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60......... 143
3.5 Synthesis and characterization of block copolymers P4VP-block-P3HT-block- P4VP.................................................................................... 147
3.5.1 Synthesis of α,ω-difunctionalised-P3HT by GRIM polymerisation 148 3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block-P4VP
by anionic polymerisation.............................................................. 154 3.5.2.1 Introduction to anionic polymerisation............................ 154 3.5.2.2 A short history of anionic polymerisation........................ 155 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP............................... 156
3.6 Physical characterisation di- and triblock copolymers....................... 158 3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and
without C60 chain-ends.................................................................. 158 3.6.2 P4VP-b-P3HT-b-P4VP block copolymers..................................... 163
3.7 Conclusions............................................................................................. 166 3.8 References............................................................................................... 167
125
3.1 Introduction Rod-coil block copolymers are well-known in their ability to self-assemble
into well-ordered nanoscopic morphologies that can be tuned in size and
shape by varying the molecular weight and size of the individual blocks.
Block copolymers (BCPs) containing electron-donor and electron-acceptor
are of particular interest for their application in photovoltaic cells because
the exciton distance exactly coincides well with the typical size of block
copolymer domains (see Section 1.4.4). The solar cells efficiency does not
only depend on the charge conduction but also on the efficiency of exciton
dissociation that is mainly related to the amount of electron donor-electron
acceptor interfaces. The synthesis of block copolymers containing both
electron donor-electron acceptor moieties is therefore necessary and also
to retain their self-assemble behaviour of BCPs by incorporating fullerenes
into their insulating part of BCPs which is challenging now. So our aim is
to design donor-acceptor block copolymers to exploit the coincidence in
dimensions between the formation of domains and exciton mean pathways
in polymer photovoltaic cells and also to use these block copolymers as
compatibilizers and stabilizers in the active layer of photovoltaic cells.
Here we have used two different approaches to obtain donor-
acceptor block copolymers in which acceptor domain fullerene (C60) is
covalently attached by grafting to the insulating block polystyrene (PS)
and/or weak supramolecular interactions produced by complex formation
between insulating block poly(4-vinylpyridine) (P4VP) and fullerene
derivative (PCBM).
This section describes the synthesis of donor-acceptor rod-coil
block copolymers in which rod block is poly(3-hexylthiophene) (P3HT) and
the coil block polystyrene (PS) or poly(4-vinylpyridine) (P4VP) for their
application in photovoltaics. The di- and tri-block copolymers P3HT-b-PS
and PS-b-P3HT-b-PS were synthesized by 1,3-dipolar Huisgen addition,
known as "click" chemistry from alkyne functionalized P3HT and azide
functionalized PS which was described by our group in 2008.1 This
method, developed since the early 2000s, has many advantages and is an
126
innovation in the field of organic electronics. The fullerene (C60) was then
grafted onto these block copolymers by atom transfer radical addition
(ATRA) to obtain the donor-acceptor copolymers. The other tri block
copolymers of ABA coil-rod-coil, P4VP-b-P3HT-b-P4VP in which rod block
is poly(3-hexylthiophene) (P3HT) and the coil block is poly(4-vinylpyridine)
(P4VP) were synthesized by anionic polymerization from quenching of
living P4VP chains with P3HT di-functionalized aldehyde. All these BCPs
are schematically represented in Figure 3.1.
C60
a
b
c
d
P3HTPS
P3HT
P3HT
P3HT
PS or P4VP PS or P4VP
PS
PSPS
C60
C60
Figure 3.1. Schematic representation of rod-coil block copolymers (a) P3HT-b-PS (b) PS-b-P3HT-b-PS or P4VP-b-P3HT-b-P4VP (c) P3HT-b-PS-C60 (d) C60-PS-b-P3HT-b-PS-C60.
These copolymers may have a great interest in the field of organic
photovoltaics by their ability to organize themselves. This structuring of the
active layer can achieve favorable morphologies at different physical
processes taking place within the organic solar cell (OSC), and thus
increase the solar cell performances. To our credit, we have obtained good
improvement in the photoconversion efficiency (PCE) as these block
copolymers used as compatibilizers instead of using as donor materials in
the blends of solar cell devices.
127
3.2 Synthesis of azide terminated-polystyrene 3.2.1 Principle of atom transfer radical polymerisation (ATRP) Azide terminated polystyrenes were synthesized by atom transfer radical
polymerization (ATRP) technique described in this paragraph. The name
atom transfer radical polymerization (ATRP) comes from the atom transfer
step, which is the key elementary reaction responsible for the uniform
growth of the polymeric chains. In a conventional radical polymerization,
the stages of initiation, propagation and chain termination occur at the
same time to prevent the growth of chains. Increasing the lifetime of a
propagating radical, however, reduced the likelihood of irreversible
termination to obtain well-defined polymers. The principle of controlled
radical polymerization (CRP) is based on a temporal deactivation of
macro-radical growth, so as to form dormant species in equilibrium with
active chains. In the case of a conventional radical polymerization, the
lifetime of a growing chain may be less than a second, it can reach several
hours in CRP.2
Atom transfer radical polymerization (ATRP) based on the
mechanism of Karasch addition3 was proposed simultaneously by
Matyjaszewski4 and Sawamoto5 in 1995. This method proceeds via
transfer of a halogen (eg Br or Cl) carried by a halogenated initiator (RX) to
a transition metal (eg copper) complexed by a ligand typically polyamine.
This complex will alternately capture and release the halogen atom leading
to redox equilibrium between the metal species (eg CuI and CuII). During
this exchange, the radical R• formed, reacts with the monomer M to give
the active species •RM radical which momentarily returns its "dormant"
halogenated (RMX). Growth intervenes between each cycle of reduction /
oxidation occured by the chain ends. The ligand of the metal / ligand
complex plays an essential role because it enables the solubilization of the
complex and makes the metal more easily oxidized by its donor character
(Figure 3.2).6 The polymers synthesized under these conditions can
achieve 100% conversion and molar masses well defined (DPn = [M]0 / [I]0)
with narrow distributions (1< Đ <1.3). Extensive studies have shown that
128
the various components of the system like the monomer, initiator, metal,
ligand, solvent and additives are the parameters that influence the control
of the polymerization.
R-Mm-X CuI / Lkact
kdeact
R-Mm X-CuII / L
kp
monomer
termination
kt
Mm+n
L - ligand X - halogen atom (Br or Cl)kact << kdeact
Figure 3.2 Mechanism of Transition-Metal-Catalyzed ATRP.6
Our objective here was to synthesize different molar masses of
polystyrene functionalized with azide (PS-N3) by ATRP through the use of
a specific initiator having an azide moiety. These polymers would then be
“clicked” with P3HTs to give the BCPs shown in Figure 3.1. The first step
was to obtain a polymerization initiator bearing the azide group, and
second to perform the polymerization of styrene using this initiator under
standard ATRP conditions.
3.2.2 Synthesis of azide initiator The synthesis of the azide initiator (6) involves in two steps, as shown in
Scheme 3.1.
Br OH
NaN3, Bu4NI18 Crown-6
2-ButanoneReflux, 24h
N3 OHBr
O
Br
THF,TEA25 οC, 3h
N3 O Br
O
4 5 6
Scheme 3.1 Synthesis of ATRP initiator bearing an azido group.
The first step was to synthesize 3-azido-1-propanol (5) from 3-
bromo-propanol (4) by nucleophilic substitution. Several test reactions
were conducted by treating 4 with sodium azide (NaN3) and
tetrabutylammonium iodide (Bu4NI) in acetone, following a literature
procedure.7 However, these reactions leading to the expected product (5)
with very low yields of around 15%. Fernandez-Santana et al.8 have
129
reported by adding reagents a crown ether, the dicyclohexano-18-crown-6
in the presence of butanone gives better yields. Crown ethers are
compounds with specific properties of complexation of cations, which give
rise to the catalysis of nucleophilic substitution. They can make extremely
nucleophilic azide ion by trapping the cation Na+ in their cavity.9 This
procedure works very effectively and leads to 3-azido-1-propanol (5) with a
yield of around 83%. Figure 3.3 shows the 1H NMR spectrum of 5 that
corresponds perfectly to the expected structure.
Figure 3.3 1H NMR (400MHz, CDCl3) spectrum of 3-azido-1-propanol (5). The second step is the synthesis of ATRP initiator, 3-azidopropyl-2-
bromoisobutyrate (6) from 3-azido-1-propanol (5) (Scheme 3.1). This
reaction proceeds by esterification of an alcohol (5) with an acyl bromide,
(α-bromoisobutyryl bromide) in the presence of triethylamine (TEA),
according to a literature procedure.6 However, in this study, the authors
used the acyl bromide in excess of alcohol (1.5 equivalents). The first tests
carried out by following strictly the procedure led to a mixture of products 6
and acyl bromide, which are impossible to separate by column
chromatography due to the close nature of the two components. However,
these two compounds must be separated because the acyl bromide can
130
also initiate the polymerization of styrene, which would lead to non-
functionalized polystyrenes. Several eluents were tested, such as
dichloromethane or mixtures of toluene / ethyl acetate / hexane / ethyl
acetate at different ratios with little success, these methods leading to the
production of a few milligrams of the expected product.
However, by introducing 1.1 equivalents of acyl bromide compared
to 5, the reaction leads exclusively to the expected product 6, as shown by
the 1H NMR spectrum in Figure 3.4. This spectrum perfectly matches to
the expected structure both in chemical shift and peak integration. This
spectrum confirms the unique presence of 6, which overcomes the step of
separation by column chromatography leading to very long time and
obtaining little product. In addition, this synthesis occurs with the yield of
81% that is quite good to do several polymerization reactions of styrene by
varying its molar mass.
Figure 3.4 1H NMR (400MHz, CDCl3) spectrum of 3-azidopropyl-2-bromoisobutyrate (6).
131
3.2.3 Synthesis of α-azido-polystyrenes
Polystyrenes of different molecular weights were synthesized, in order to
study subsequently the influence of the length of coil insulating block on
the photovoltaic performance. This reaction involves the polymerization of
styrene using the initiator 6 (bearing the azide function) in the presence of
catalytic system CuBr/Bipyridine at 130 ºC (Scheme 3.2). The reaction was
stopped abruptly by lowering the temperature of the reaction at 0 ºC.
N3 O Br
O
6
Styrene
CuBr/2,2-Bipyridyl130 οC
N3 O
O
Brn
α-azido-PS
Scheme 3.2 Synthesis of Polystyrenes terminated with azide function.
Various chain length PSs were obtained by varying the
polymerization time. The polymerization times and the obtained molecular
weights (PS1 to PS6) are summarized in Table 3.1. The SEC calibration
was performed against polystyrene standards, and therefore in contrast to
P3HT, require no coefficient to obtain “real” values.
Polystyrene-N3
(PS) Polymerization
Time, min Mn (SEC, g mol-1)
Đ
PS1 12 2600 1.08
PS2 25 3800 1.17
PS3 10 1900 1.21
PS4 30 4500 1.30
PS5 11 2000 1.11
PS6 30 5200 1.29
Table 3.1 Characteristics of the synthesized α-N3-ω-bromo-polystyrenes (SEC in THF, UV-254 nm).
As expected, the polymerization time is used to vary the molecular
weight of polystyrene (Table 3.1). In this study, short reaction times were
chosen at low conversion rates and to avoid side reactions, but also to
generate low molecular weight PS (between 2000 g/mol and 5200 g/mol).
132
Indeed, given that the target rod-coil copolymers are intended for
use in electronics, the proportion of insulating PS in these macromolecules
should not be too high. It should be noted that the state of purification of
the copper bromide (CuBr) played an important role on the characteristics
of polymers. We have observed PS with increased dispersities when using
unpurified CuBr for the polymerizations (eg. PS1, PS3 and PS5 in Table
3.1).
Figure 3.5 shows the 1H NMR spectrum of PS2, characteristic of all
samples. The spectrum perfectly matches the expected structure. The
polymers were purified on alumina column and by precipitation in
methanol. This procedure allowed us to obtain relatively pure PS as shown
in the NMR spectrum of PS2. In addition, the presence of the chain-end,
as evidenced by the peak at 4.5 ppm (proton in Figure 3.5), can be
considered for subsequent fullerene grafting.
Infrared spectroscopy further confirmed the presence of the azide
function (N3), required for coupling reactions. It has a characteristic intense
signal (νN3) at 2100 cm-1. The IR spectrum of PS2 is shown in Figure 3.6,
which is also characteristic of all the synthesized PS.
Figure 3.5 1H NMR (400MHz, CDCl3) spectrum of PS2.
133
Figure 3.6 Representative IR spectrum of polystyrene PS2.
To conclude, this Section described the synthesis of azide-
terminated polystyrenes of varying molecular weights. The most difficult
step was involved in the synthesis of the ATRP initiator.
3.3 Synthesis of block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by “Click” chemistry 3.3.1 History and principle of “click” chemistry Since Kolb, Finn, and Sharpless introduced “click” chemistry in
200110, there has been an extensive growth in this area of chemistry. The
term relates to the chemical reactions generating substances quickly and
simply by linking two different units.
Among these reactions, the most popular is the Huisgen cycloaddition,
which is a 1,3-dipolar addition between the azide function (N3: 1,3-dipole)
and alkyne function (triple bond: dipolarophile), leading to the formation of
a triazole ring. Copper catalysis can also exclusively obtain the 1,4-
disubstituted regioisomer as shown in Figure 3.7. This click reaction has
many advantages. In particular, it leads to pure products, requires very
simple reaction conditions, gives high yields, generates no toxic
byproducts and can be applied to many domains10, which are very
interesting points for a potential future industrialization of OSCs.
134
H
N NR'
N
R
+1 2
3
45
AlkyneDipolarphile
Azide1,3-Dipole
1
2
3NN
N
R
R'
H 45
1,4-Triazole
[Cu]
Figure 3.7 Mechanism of copper catalyzed Huisgen 1,3-dipolar cycloaddition. Since 2005, "click" chemistry has been extended to polymer science
and numerous studies have shown an interest in the synthesis of block
copolymers.7,11-13 However, very few reports in the synthesis of block
copolymers concern conjugated polymers.1a,14,15 For all the above reasons,
this click chemistry path was chosen for the synthesis of block copolymers
P3HT-b-PS and PS-b-P3HT-b-PS from a P3HT end functionalized alkyne
and an azide-terminated polystyrene respectively.
3.3.2 Synthesis of copolymers P3HT-b-PS and PS-b-P3HT-b-PS 3.3.2.1 Triblock copolymers PS-b-P3HT-b-PS We synthesized di- and triblock copolymers using "click" chemistry
between polystyrenes terminated with azides and P3HT-alkynes. For the
synthesis of triblock copolymers PS-b-P3HT-b-PS, we chose α,ω−
pentynylP3HT (P3 and P3a) reacted with different molecular weight
polystyrenes (PS1, PS2, PS3 and PS4, Table 3.2). Scheme 3.3 shows the
coupling reactions for P3 and P3a which were performed in THF at 40 °C,
using the catalytic system CuI/DBU. This work describes the first example
of "click" chemistry on conjugated polymers1a but the main difficulty was in
choosing the solvent system and catalyst/ligand. Most studies reported so
far for obtaining block copolymers by "click" chemistry used
dimethylformamide (DMF) as a solvent of choice for these cycloaddition
reactions.13 However, P3HT being not soluble in DMF, so the relatively
polar solvent THF was chosen as the reaction solvent to dissolve both the
P3HT and PS.1
135
S m
C6H13
P3 or P3a
+N3 O
O
Brn
CuI, DBU
THF, 40 oC
PS (2 eq.)
S
C6H13
m
N NN O Br
nNN
NOBrn
O O
PS1-b-P3-b-PS1
PS2-b-P3-b-PS2
PS3-b-P3a-b-PS3
PS4-b-P3a-b-PS4 Scheme 3.3 Synthesis of Triblock copolymers PS-b-P3HT-b-PS by Click chemistry.1a
α,ω-Dipentynyl-P3HT (P3 and P3a) was reacted with stoichiometric
amounts (2eq.) of PS1, PS2 and PS3, PS4 respectively according to the
synthesis shown schematically in Scheme 3.3, in order to obtain triblock
copolymers PS-b-P3HT-b-PS. The products obtained were characterized
by SEC, 1H NMR and infrared spectroscopy. Indeed, Figure 3.8 shows the
typical SEC chromatograms of homopolymers P3, PS1 and the triblock
copolymer PS1-b-P3-b-PS1. These chromatograms show an increase in
molecular weight for the final product, with a shift towards lower elution
time, confirmed by the Mn values in many of these species, collected in
Table 3.2. In addition, the chromatogram of the copolymer is monomodal
with small shoulder at high molecular weight region may be due to the
aggregation of P3HT and has a low dispersity of 1.21, demonstrating the
formation of a unique population of copolymer. In addition, similar curves
were obtained in the case of copolymerizations of P3 with the copolymers
PS2 and also P3a with the copolymers PS3 and PS4, confirming the
formation of triblock copolymers (PS2-b-P3-b-PS2, PS3-b-P3a-b-PS3 and
PS4-b-P3a-b-PS4) of varying both P3HT and PS-block lengths (Table 3.2).
The efficiency of "click" chemistry coupling reaction was also
confirmed by 1H NMR; the spectrum of copolymer PS1-b-P3-b-PS1 is
shown in Figure 3.9. This spectrum is typical of the four copolymers
obtained, and demonstrates the structure of expected triblock copolymer.
Indeed, all peaks corresponding to PS and P3HT are present in the
136
spectrum. In addition, the peak at 7.51 ppm can be attributed to the proton
of triazole ring (f), formed during the cycloaddition. The disappearance of
alkynyl proton at 3.49 ppm and peaks at 1.9 ppm, 2.3 ppm and 2.5 ppm,
corresponding to the protons of the pentynyl group, also confirmed the
formation of triblock copolymers.
α-Azido-PS α ,ω -PentynylP3HT PS-b-P3HT-b-PS PS Mn, SEC
(g/mol)
Đ P3HT Mn, SEC
(g/mol)
Đ Copolymer Mn, SEC
(g mol-1)
Đ
PS1 2 600 1.08 P3 8 000 1.1 PS1-b-P3-b-PS1 12 800 1.21
PS2 3 800 1.17 ‘’ “ ‘’ PS2-b-P3-b-PS2 13 200 1.37
PS3 1 900 1.21 P3a 6 200 1.1 PS3-b-P3a-b-PS3 9 800 1.20
PS4 4 500 1.30 ‘’ “ ‘’ PS4-b-P3a-b-PS4 10 900 1.30
Table 3.2 Molecular weight characteristics of synthesized homopolymers azido-PS, α,ω-pentynylP3HT and triblock copolymers PS-b-P3HT-b-PS (SEC in THF, PS as standards). Infrared spectroscopy further confirmed the formation of copolymers
by click chemistry, from P3 and P3a. Indeed, the signal at 2100 cm-1,
corresponding to the azide function present on the spectra of PS1 and the
mixture of P3+PS1 (Figure 3.10), disappears completely in the spectrum
for the click reaction product between P3 and PS1, indicating that the
efficiency of the reaction high. This feature is also well verified for the
products of reactions between P3, P3a and polystyrenes PS1, PS2, PS3,
and PS4, respectively.
Figure 3.8 Normalised overlay SECs (THF, UV-254 nm) of homopolymers azido-PS1, α,ω-pentynylP3HT (P3) and triblock copolymer PS1-b-P3-b-PS1.
137
Figure 3.9 Representative 1H NMR (400 MHz, CDCl3) spectrum of triblock copolymer PS1-b-P3-b-PS1.
Figure 3.10 Overlayed IR spectra of homopolymers azido-PS1; mixture of azido-PS1+α,ω-PentynylP3HT (P3) and click product of triblock copolymer PS1-b-P3-b-PS1.
138
3.3.2.2 Diblock copolymers P3HT-b-PS We are also interested to study the influence of the chain ends of P3HT, in
this instance the alkyne function to see the effect of conjugation between
thiophene unit and ethynyl functional group. In the case of ethynyl-P3HT
(P2), the proximity of the P3HT to the ethynyl group reduced its reactivity
towards the azide group, because of its involvement in the conjugation of
the P3HT chain. In the case of α,ω-PentynylP3HT (P3), the alkyne function
is not influenced by the electronic conjugation of the P3HT because of the
alkyl spacer between P3HT and alkynyl functional groups, and should be
able to react freely with the azide terminated polystyrenes.
Hence cycloaddition reactions from ω-ethynyl-P3HT (P2) did not
work, as evidenced by the SEC and infrared spectroscopy. Figure 3.11 (a)
shows the typical chromatograms SEC of P2, PS2 and the product of the
reaction between these two homopolymers under initial “click” reaction
conditions. The chromatogram of the final compound is a combination of
the two chromatograms starting homopolymers (P2 and PS1); it is strictly
identical to that of the P2, with the same shoulder due to the aggregation
of P3HT. These test reactions therefore showed that the SEC product is a
mixture of two homopolymers, and therefore that the copolymerization has
not worked successfully by initial "click" reaction conditions.
This result was also further confirmed by infrared spectroscopy. The
IR spectrum of reaction product between P2 and PS2 (Figure 3.12) has a
signal at 2100 cm-1, indicating the presence of the azide function of the
homopolymer PS2, which confirms the failure of the reaction. For these
reactions, two different catalyst systems/ligand were used namely
CuI/DBU, identical reactions which were successful by "click" chemistry
from P3, but also CuBr/PMDETA, very commonly used in such coupling
reactions.13
139
Figure 3.11 Normalised overlay SECs (THF, UV-254 nm) of (a) homopolymers azido-PS2, ω-EthynylP3HT (P2) and click reaction product of P2 and PS2; (b) homopolymers azido-PS2, ω-ethynyl-P3HT (P2) and diblock copolymer P2-b-PS2; and (c) close view for overlay of ω-ethynyl-P3HT (P2) and diblock copolymer P2-b-PS2.
Figure 3.12 Overlayed IR spectra of homopolymers azido-PS2; mixture of azido-PS2 and ω-ethynyl-P3HT (P2) and click product of diblock copolymer P2-b-PS2.
140
Furthermore, P2 and P3 with similar characteristics (regioregularity
and dispersity) are very similar, only the chain ends are responsible for the
success or failure of the reaction. In case of P3, alkyne function, separated
from the conjugated polymer by a short alkyl chain, is not constrained and
reacts easily with the azide of polystyrene. Regarding P2, the alkyne
function is linked directly to the conjugated chain in position 2 and
necessarily involved the conjugation of P3HT, and therefore the
delocalization of electrons from the alkyne on the P3HT chain reaction
seems to prevent "click" reactions to take place.
More recently, Benanti et al.,14 and Tao et al.,15 have shown that it
is possible to obtain copolymers from ethynylP3HT by varying “click”
chemistry reaction conditions i.e., reaction temperature, Cu catalyst/ligand
with sonication to enhance the solubility of ethynylP3HT and Jatsch et
al.,16 showed that it was also possible to perform "click" chemistry
reactions on ethynyl-oligothiophenes.
The inspiration from the literature mentioned above gave a way to
synthesize diblock copolymers P3HT-b-PS successfully from ethynylP3HT
by varying the click reaction conditions. We have finally optimized reaction
conditions using CuI and Hunig’s base diisopropylethylamine [(i-pr)2NEt,
DIEA or DIPEA] in THF at 50 °C and also with the help of sonication to aid
ethynyl-P3HT dissolution in THF. So we have chosen ω-ethynyl-P3HT of
different molecular weights, P2 and P2a reacted with polystyrene PS2
(Table 3.3). Finally, two different rod-coil diblock copolymers have been
synthesized from P2 and P2a, namely P2-b-PS2 and P2a-b-PS2, as in
Scheme 3.4. This study represents the modification of our literature
procedure1, which is the first example of synthesis of exclusively rod-coil
diblock copolymers, by "click" chemistry and shows the effectiveness of
this type of reaction.
141
SBr/H m
C6H13
+N3 O
O
Brn
PS2P2 or P2a
CuI, DIEA
THF, 50 oCSonication
SBr/H
C6H13
m
N NN
O
O
Brn
P2-b-PS2
P2a-b-PS2
Scheme 3.4 Synthesis of Diblock copolymers P3HT-b-PS by Click chemistry.
The diblock copolymers obtained were characterized by SEC, 1H
NMR and infrared spectroscopy. Indeed, Figure 3.11 (b) and (c) shows the
SEC chromatograms of homo polymers P2, PS2 and the diblock
copolymer P2-b-PS2. These chromatograms showed little increase of
molar mass for the final product, with a slight shift towards lower elution
time, confirmed by the values of molar masses collected in Table 3.3. In
addition, the chromatogram of the copolymer is monomodal and has a low
dispersity of 1.27, which would tend to indicate the formation of a unique
population of copolymer. In addition, similar curves were obtained in the
case of copolymerizations of P2a with the copolymers PS2, confirming the
formation of diblock copolymer (P2a-b-PS2) whose length of P3HT block
varied (Table 3.3).
The reaction efficiency of coupling "click" chemistry can also be
confirmed by 1H NMR, the spectrum of copolymer P2-b-PS2 is shown in
Figure 3.13. This typical spectrum demonstrates the expected structure of
diblock copolymer. Indeed, all peaks corresponding to PS and P3HT are
present in the spectrum. In addition, the formation of diblock copolymers
was further confirmed by the disappearance of alkynyl proton at 3.52 ppm
and appearance of the new peak at 7.51 ppm that corresponds to the
proton of triazole ring (f) formed during the cycloaddition.
α-Azido-PS ω-EthynylP3HT P3HT-b-PS PS Mn, SEC
(g/mol)
Đ P3HT Mn, SEC
(g/mol)
Đ Copolymer Mn, SEC
(g/mol)
Đ
PS2 3 800 1.17 P2 14 000 1.1 P2-b-PS2 14 900 1.27
PS2 3 800 1.17 P2a 9 000 1.2 P2a-b-PS2 10 200 1.36
Table 3.3 Characteristics of synthesized homopolymers azido-PS, ω-ethynyl-P3HT and diblock copolymers P3HT-b-PS (SEC in THF, PS as standards).
142
Figure 3.13 Representative 1H NMR (400 MHz, CDCl3) spectrum of diblock copolymer P2-b-PS2.
Infrared spectroscopy further confirmed the formation of diblock
copolymers by click chemistry, from P2 and P2a. Indeed, the signal at
2100 cm-1, corresponding to the azide function and present on the spectra
of PS2 and the mixture P2/PS2 (Figure 3.12), disappears completely in the
spectrum of click reaction product between P2 and PS2, which means the
complete disappearance of the azide function and therefore “click”
chemistry worked very good by varying the reaction conditions. This
feature is also well verified for the products of reactions between P2a and
polystyrene PS2.
All detailed experimental procedures for the synthesis of di-and tri-block
copolymers are described in the experimental section (see chapter-5).
143
3.4 Synthesis of donor-acceptor and acceptor-donor- acceptor block copolymers P3HT-block-PS-C60 and C60-PS-block-P3HT-block-PS-C60
In this part, our objective was to take the advantage of a bromine end
group in the previously described block copolymers (P3HT-b-PS-Br and
Br-PS-b-P3HT-b-PS-Br) to attach C60 moiety by ATRA for synthesizing
P3HT donor based block copolymers attached with C60 acceptor moieties.
3.4.1 Grafting of fullerene by atom transfer radical addition (ATRA) Many studies have reported the grafting of polymer chains on the fullerene,
by different synthetic routes.17 Only few reports concerned the
incorporation of C60 at the chain end of conjugated polymers. In particular,
Gu et al.18 have synthesized oligomers terminated by C60 through
cycloaddition reactions with N-methylglycine targeting photovoltaic
applications.
In the early 2000s, the research groups of Li19,20 and Mathis21,22
simultaneously proposed the grafting polymer radicals onto C60. Indeed
chains of PS-Br, converted into macro-radicals by the reaction of the
system CuBr/bipyridine, are added to the fullerene by the atom transfer
radical addition mechanism (ATRA) (similar to ATRP, but involving the
intra-C60 transfer of a radical site). So this synthetic route was chosen for
the grafting of synthesized di-and tri-block copolymers to the fullerene by
the reaction of macro-radicals (via the PS block) on the C60.
3.4.2 Synthesis of P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60
The ATRA of C60 was achieved onto the diblock copolymers P2-b-PS2,
P2a-b-PS2 and triblock copolymers PS1-b-P3-b-PS1, PS2-b-P3-b-PS2
respectively, syntheses described in the prior section. The C60 attached
copolymers are synthesized according to a procedure reported by Zhou et
al.19, shown in Scheme 3.5. The block copolymers P3HT-b-PS and PS-b-
P3HT-b-PS are treated with C60 and the system CuBr/bipyridine in
chlorobenzene used as solvent. According to the reported procedure, it
144
enables access to mono-addition of C60 on copolymers. In addition, the C60
is used in all these reactions with large excess compared to the copolymer
to promote this mono addition. After mono- and di-addition of C60 to block
copolymers, there was slight increment in the molecular weights of these
block copolymers with retained good dispersity (Table 3.4) which indicated
the efficiency of the reaction. The synthesized C60-attached di-and tri-
block copolymers were characterized by 1H NMR, SEC and DSC.
S
C6H13
mN N
NO
O
n
S
C6H13
m
N NN O
nNN
NOn
O O
P2-b-PS2-C60
P2a-b-PS2-C60
C60-PS1-b-P3-b-PS1-C60
C60-PS2-b-P3-b-PS2-C60
S
C6H13
m
N NN
O
O
Brn
P2-b-PS2-Br
P2a-b-PS2-Br
S
C6H13
m
N NN O Br
nNN
NOBrn
O O
Br-PS1-b-P3-b-PS1-Br
Br-PS2-b-P3-b-PS2-Br
CuBr/Bipyridine
Chlorobenzene, 110 oC
CuBr/Bipyridine
Chlorobenzene, 110 oC
Scheme 3.5 Synthesis of C60-attached di-and tri-block copolymers by ATRA.
Block copolymer Mn, SEC
(g/mol)
Đ C60-attached block
copolymer
Mn, SEC
(g/mol)
Đ
P2-b-PS2-Br 14 900 1.27 P2-b-PS2-C60 15 400 1.43
P2a-b-PS2-Br 10 200 1.36 P2a-b-PS2-C60 11 000 1.38
Br-PS1-b-P3-b-PS1-Br 12 800 1.21 C60-PS1-b-P3-b-PS1-C60 13 000 1.26
Br-PS2-b-P3-b-PS2-Br 13 200 1.37 C60-PS2-b-P3-b-PS2-C60 14 000 1.37
Table 3.4 Characteristics of di- and tri- block copolymers, C60-attached di and tri block copolymer (SEC in THF, UV-254 nm, PS as standards). Figure 3.14 shows the UV-visible absorption spectrum of product of
the ATRA reaction between PS2-b-P3-b-PS2 and C60. This spectrum
consists of a large contribution with maximum absorption around 480 nm,
which corresponds to the P3HT segment, and two peaks at 210 nm and
330 nm, due to fullerene, and absorptions below 220 nm and at 258 nm,
145
corresponding to the PS block. The presence of the characteristic peaks of
fullerene confirms the grafting of the polymer as residual C60 was removed
by precipitation in THF, and then filtered off by repeated passing through
an alumina column (C60 is not soluble in THF). The addition of fullerene to
the triblock copolymer PS2-b-P3-b-PS2 was also confirmed on the
absorption spectrum by the contribution at 330 nm due to reacted C60.19
The reaction was carried out in chlorobenzene with a large excess of C60
compared to the copolymers produced expected mono-C60 diblock
copolymers and di-C60 triblock copolymers. The representative SECs of
PS2-b-P3-b-PS2 and its ATRA product, show unimodal distributions with a
single population in the reaction of C60 with PS2-b-P3-b-PS2 (Figure 3.15).
The representative 1H NMR spectra of mono-C60 diblock copolymers and
di-C60 triblock copolymers are shown in Figure 3.16.
Figure 3.14 UV-visible absorption spectrum (film) of C60-PS2-b-P3-b-PS2-C60 (the product of ATRA reaction between triblock copolymer PS2-b-P3-b-PS2 and C60).
Figure 3.15 Overlayed SECs of C60-PS2-b-P3-b-PS2-C60 and triblock copolymer PS2-b-P3-b-PS2 (THF, UV-254 nm, PS as standards).
146
(a)
(b)
Figure 3.16 Representative 1H NMR (400 MHz, CDCl3) spectra of (a) P2-b-PS2-C60 and (b) C60-PS2-b-P3-b-PS2-C60.
147
Two types of C60-functionalized block copolymers have thus been
obtained as shown in Figure 3.17. These block copolymers containing a
rod-coil and coil-rod-coil both with an acceptor (C60) and an electron donor
(P3HT) were examined for organic photovoltaic devices as compatibilizers
in P3HT/PCBM blends (see Chapter 4).
C60
P3HT
P3HT
PS
PSPS
C60
C60
Figure 3.17 Shematic representation of donor-acceptor copolymer P3HT-b-PS-C60 and acceptor-donor-acceptor copolymer C60-PS-b-P3HT-b-PS-C60.
3.5 Synthesis and characterization of block copolymers
P4VP-block-P3HT-block-P4VP This section describes facile and efficient synthesis of well-defined ABA
triblock coil-rod-coil copolymers, P4VP-b-P3HT-b-P4VP in which the rod
block is P3HT and the coil block is P4VP. These copolymers are
schematically represented in Figure 3.18. These copolymers were
synthesized by anionic polymerisation from quenching of living P4VP
chains with P3HT di-functionalized aldehyde and the coupling was very
effective due to the higher electrophilicity of the carbon in aldehyde group
of P3HT. This route has been adapted from reported literature for the
synthesis of a PPV-based block copolymer.23 Since poly(4-vinylpyridine)
(P4VP) is capable of complexing with a C60 charge transfer between the
nitrogen of the pyridine and the electrophilic fullerene,24 these copolymers
are of particular interest to test their potentiality in photovoltaics application
due to supramolecular interactions between P4VP and PCBM (Figure
3.18) in the active layer of solar cell devices. We have obtained flexible
electron acceptor sequence novel copolymers P4VP-b-P3HT-b-P4VP.
148
Figure 3.18 Schematic representation of coil-rod-coil block copolymers, P4VP-b-P3HT-b-P4VP having supramolecular interaction with PCBM in a solar cell device.
3.5.1 Synthesis of α ,ω -difunctionalised-P3HT by GRIM polymerisation Here we describe the synthesis of three different α,ω-difunctionalised-
P3HTs by the GRIM method (Scheme 3.6).25 To synthesize α,ω-
difunctionalised P3HTs, monomer (M1) was dissolved in THF and stirred 5
min under nitrogen. tert-Butylmagnesium chloride was added, and the
mixture was stirred at room temperature for 2.5 h. The mixture was then
diluted with THF, Ni(dppp)Cl2 was added, and the mixture allowed to stir
for 30-120 min at room temperature depending upon the functional group
(Table 3.5). The termination of the polymers with the respective Grignard
functionalization agent was carried out in a one-shot addition using 70-90
mol % with respect to the monomer. In all the cases, the mixture was
stirred for an additional 30-60 min and then poured into methanol to
precipitate the polymer. After precipitating in methanol, P3HTs were
purified by soxhlet extraction with methanol, acetone, pentane respectively
and finally pure polymers were extracted with THF. All the functionalised
polymers (Table 3.5) were characterized by SEC and 1H NMR.
The deprotection of polymers was performed as follows. In the case
of α,ω-diphenylformyl-P3HT (P4 and P4a), the Grignard reagent was 4-
(1,3-Dioxan-2-ylphenyl) magnesium bromide and the deprotection was
done by overnight refluxing the polymer in THF with pyridinium p-
toluenesulfonate (PTS). In the case of α,ω-diethylformyl-P3HT (P4b), the
Grignard reagent was 4-(1,3-dioxan-2-ylethyl) magnesium bromide and the
deprotection was not successful with PTS under same conditions used for
149
P4. It was successful, however, with concentrated HCl and overnight
refluxing in THF. For the synthesis of α,ω-diphenylhydroxy-P3HT (P5),
the Grignard reagent was 4-(2-tetrahydro-2H-pyranoxy)-phenyl
magnesium bromide and the complete deprotection was observed by
refluxing the polymer with concentrated HCl in THF for 18 h. All the
deprotected α,ω-difunctionalised P3HTs were again purified by soxhlet
with methanol and extracted with chloroform and characterised by SEC, 1H
NMR, DSC and MALDI-TOF techniques.
S
C6H13
Br Br
1. tBuMgCl2. Ni(dppp)Cl2
THF, R.T.
S
C6H13
n
M1
O
OBrMg
OBrMg O
3.
4. PTS, THF, reflux CHOOHC
S
C6H13
n OHHO
S
C6H13
nCHOOHC
3.
4. 5M HCl, THF, reflux
O
OBrMg3.
4. 5M HCl, THF, reflux
P4, P4a
P4b
P5 Scheme 3.6 Synthesis of α,ω-difunctionalised P3HTs [α ,ω-diphenylformyl-P3HT (P4 and P4a), α ,ω-diethylformyl-P3HT (P4b) and α ,ω-diphenylhydroxy-P3HT (P5)] by GRIM method.25
α ,ω -Difunctionalised P3HT
P3HT
Monomer 2,5-dibromo-3-
hexylthiophene (M1 g)
Grignard
reagent tBuMgCl
(mL) 1M sol.
Ni(dppp)Cl2
mol %
Poly time
min
Grignard reagent used for
functionalisation
Mn (g mol-1)
Đ
RR
%
P4 3.31 10.1 1.70 40 4-(1,3-Dioxan-2-
ylphenyl)-MgBr/PTS 7 000 1.1 98
P4a 3.22 9.8 1.15 120 4-(1,3-Dioxan-2-
ylphenyl)-MgBr/PTS 14 000 1.1 98
P4b 3.22 9.9 2.20 30 4-(1,3-Dioxan-2-
ylethyl)-MgBr/HCl 5 800 1.1 98
P5 3.0 9.2 1.35 120 4-(2-tetrahydro-2H-
pyranoxy)-phenyl
MgBr/HCl
11 000 1.2 95
Table 3.5 Reaction conditions, and molecular weight characteristics (GPC, THF, UV-254 nm) and regiorgularities (NMR) of α ,ω-difunctionalised-P3HTs.
150
In all the cases, 1H NMR and MALDI-TOF techniques were used to
find out the chain end-functionalisation of all the polymers after the
deprotection step. Representative 1H NMR spectra of P4 and P4b, which
correspond to ethyl- and phenyl aldehyde-difunctionalized-P3HT
respectively are shown in Figure 3.19. Figure 3.19 (a), shows the 1H NMR
spectrum of P4 and confirms the expected structure. Indeed, this is a
typical spectrum of regioregular P3HT with three additional peaks at 7.69
ppm, 7.93 ppm and 10.05 ppm corresponding to end-groups phenyl and
aldehyde protons respectively. This was also further confirmed by MALDI-
TOF mass characterisation [Figure 3.20 (a)] that showed the major
population corresponding to aldehyde di-functionalised P3HT. 1H NMR
spectrum of polymer P4b [Figure 3.19 (b)] showed all the peaks
correponding to P3HT and also three additional peaks around 2.5 ppm, 3.1
ppm and 9.85 ppm which corresponds to ethyl-CHO chain end functional
group, again confirming the expected structure. According to MALDI-TOF
mass spectrum in the case of P4b, the polymer product was a mixture of
mono, di-protected aldehyde-P3HT and deprotected aldehyde-P3HT.
Figure 3.21 shows the 1H NMR spectrum of regioregular α,ω-
diphenylhydroxy-P3HT (P5) which is a typical spectrum of regioregular
P3HT with three additional peaks at 6.89 ppm, 7.35 ppm and 7.52 ppm
corresponding to the chain-end groups with phenyl and hydroxy protons (-
OH) respectively and confirming the difunctionalised polymer. The MALDI-
TOF mass spectrum of this polymer, P5 was not clear due to its high
molecular weight. However, the deprotection step was effective in all the
above cases as confirmed by the 1H NMR spectra, which showed the
disappearance of peaks correponding to protective groups.
151
(a)
(b)
Figure 3.19 1H NMR (400 MHz, CDCl3) spectra of: (a) α,ω-diphenylformyl-P3HT (P4); and (b) α,ω-diethylformyl-P3HT (P4b).
152
(a)
(b)
Figure 3.20 MALDI-TOF mass spectra of: ((a) α,ω-diphenylformyl-P3HT (P4); and (b) α,ω-diethylformyl-P3HT (P4b).
153
Figure 3.21 1H NMR (400MHz, CDCl3) spectrum of α,ω-diphenylhydroxy-P3HT (P5).
We thus successfully synthesized three regioregular P3HTs with
different functional groups by GRIM method. We have chosen α,ω-
diphenylformyl-P3HT (P4) instead of α,ω-diethylformyl-P3HT (P4b) to
synthesize the copolymers P4VP-block-P3HT-block-P4VP by anionic
polymerization since P3HT (P4b) was not completely functionalized
according to MALD-TOF mass analyses; and the also phenylaldehyde of
P4 is more electrophilic towards ‘living” anionic centres than the ethyl-
aldehyde of P4b. Another polymer, α,ω-diphenylhydroxy-P3HT (P5) was
used to synthesize donor-acceptor multiblock copolymers incorporating
fullerene backbone repeat units in collaboration with my colleague Dr.
Roger C Hiorns in our lab. The photovoltaic characterizations of these
multiblock copolymers as compatibilizers in the active layer of P3HT-blend-
PCBM based devices are in progress.
154
3.5.2 Synthesis of triblock copolymer P4VP-block-P3HT-block- P4VP by anionic polymerisation 3.5.2.1 Introduction to Anionic polymerisation Anionic polymerization is one of the most common controlled
polymerization techniques for coil-like polymers, and is generally termed
”living”. The definition of “living” is "polymers that retain their ability to
propagate for a long time and grow to a desired maximum size while their
degree of termination or chain transfer is still negligible".26 In order to use
living anionic polymerizations, an unsaturated bond (such as a vinyl group
or cyclic structure) must be present in the monomer. These
polymerizations proceed via organometallic sites i.e. carbanions with
metallic counterions.27 The C-M+ ion pair (where C- is the propagating
carbanion and M+ is the metal gegen ion) is a reactive centre (Scheme
3.7). The carbanion is extremely sensitive and contact with water or air or
impurities will arrest the polymerisation. Polymerisations are normally
conducted in a dry solvent under a high vacuum or a dry inert gas. The
“living” polymer is coloured, varying from a deep blood red for those of α-
methylstyrene to a light orange for those of dienes.
CR
R'CH2 M C
R
R'CH2 C
R
R'CH2 C
R
R'CH2 M
Scheme 3.7 A typical propagation of the anionic polymerisation.
Polymers generated in this manner have finely tuned molecular
weights, very narrow molecular weight distributions, and can be used to
produce block copolymers and end-functionalized polymers. These
reactions are defined such that irreversible termination and chain transfer
are not present. Flory advanced the idea of the probability of growth of
polymer chains, and stipulated that without termination, the molecular
weight of a polymer should approach that of the 'Poisson' distribution,
shown in equation 3.1.28
Mw/Mn = Đ ≅ 1+1/N Eq 3.1
155
where Mw is the weight-average molecular weight, Mn is the number-
average molecular weight, and N is the number of repeat units in the
polymer. Therefore, the molecular weight distribution approaches unity as
the molecular weight of the polymer increases. For example, a modest
molecular weight polymer with N = 1000 has Đ = 1.001.
3.5.2.2 A short history of anionic polymerisation The interest in anionic polymerization started in early 1910, when Matthew
and Strange polymerised isoprene using an alkali metal in a
heterogeneous reaction and observed a viscous product. However, the
mechanism of the reaction was not understood at that time.29 Later on,
Ziegler et al. in their series of publications suggested that the addition of
two sodium atoms to the unsaturated double bonds of diene producing two
carbon–sodium linkages and in 1934 Ziegler proposed a mechanism which
is now accepted through propagation via insertion of monomer into the
carbon–sodium linkage.30 The nature of the linkage between carbon and
metal was not understood clearly at that time, though it was believed that it
is a covalent bond.
Michael Szwarc who first demonstrated the anionic polymerization
of styrene using sodium naphthalenide in tetrahydrofuran (THF).31,32 He
suggested that the initiation occurs via electron transfer from the sodium
naphthalenide radical anion to styrene monomer. A new styryl radical
anion forms upon addition of an electron from the sodium naphthalenide
and subsequent rapid dimerization yielding dimeric-dicarbanion (Scheme
3.8), which starts the propagation of styrene. Michael Szwarc also
characterized the living behavior of the polymerization as ‘‘living
polymerization’’ and called the polymers as ‘‘living polymers’’.32 Here, the
term ‘living’ refers to the ability of the chain-ends of these polymers
retaining their reactivity for a sufficient time enabling continued propagation
without termination and transfer reactions. Although several reports of
anionic polymerization of vinyl monomers were available in the literature,
Szwarc’s first report of living anionic polymerization of styrene free from
156
termination and transfer reactions in THF marks the beginning of lively
research activities in this field.31,32
Na +CH CH2 THF +
HC CH2
Na
CH-CH2-CH2-CH NaNa
CH-CH2-CH-CH2-CH2-CH-CH2-CHn-2 n-2
NaNaCH CH2n
THF/-78 oC
Scheme 3.8 Anionic polymerization of styrene using sodium naphthalenide as initiator in THF.33 3.5.2.3 Synthesis of P4VP-b-P3HT-b-P4VP We have synthesized block copolymers P4VP-b-P3HT-b-P4VP by an
anionic convergent route by termination with di-functionalized P3HT
(Section 3.5.1). P4VP was synthesized by anionic polymerization of 4-
vinylpyridine using an initiator (1) obtained by reaction of sec-butyllithium
on α-methylstyrene (Scheme 3.9). This initiator (1) was chosen over sec-
butyllithium for two reasons - its steric hindrance associated with the
ternary character of the carbanion formed to ensure that the anionic
polymerization takes place on the vinyl groups of vinyl-pyridine without
secondary reaction on the aromatic rings and - its color can show the
presence of the anions in the reactive medium.
The anionic polymerization of 4-vinylpyridine was stopped by the
addition of well-defined aldehyde di-functionalised-P3HT (P4), synthesized
by GRIM method as a quencher in the reaction medium (Scheme 3.9).
Thus the anions of the growing P4VP reacted by nucleophilic substitution
on the terminal aldehydes to form the corresponding copolymer with the
alcohol functions on both sides. To obtain pure copolymers instead of
undesired chain deactivation during quenching, we used an excess of
P4VP living chains (10 eq.) with respect to the P3HT “quencher” (P4). The
excess of P4VP homopolymer was then removed by washing the organic
phase several times (at least 3 times) with acidic water (pH = 4) in which
the P4VP was protonated and soluble whereas the block copolymer was
157
not. The efficiency of quenching was easily determined by 1H NMR since
the aldehyde peak of homo P3HT (P4) at 10.05 ppm disappears
completely in the final purified copolymers. We synthesized three
copolymers of varying P4VP molecular weights. The molecular weights of
copolymers were not determined by SEC in THF and DMF also due to the
strong aggregations of P4VP in solvents used in SEC. The copolymers
were characterized by NMR only. With the integrating ratio of P3HT H1
(6.98 ppm) and P4VP H2 (6.42 ppm) in 1H NMR, we estimated the
molecular weights of P4VP in the copolymers. The results of these
copolymers are summarized in Table 3.6 and the representative 1H NMR
spectrum of copolymers, P4VP-b-P3HT-b-P4VP (3) was shown in Figure
3.22.
Sec-BuLi
-78 oC, THF
C- Li+ N C-
N N
m
HLi+
-78 oC, THF(+HMTP)
S PhCHOOHCPh
C6H13
n
N
m
OH
N
m
OHS
C6H13
n
-78 oC to RT overnight
P4
P4VP-b-P4-b-P4VP
1 2
Scheme 3.9 Synthesis of copolymers P4VP-b-P3HT-b-P4VP by anionic polymerization.
Polymer Mn
(g/mol)*
Mn (g/mol)*
P3HT
Mn (g/mol)*
P4VP P3HT (P4) 4 200 4 200 0
Copolymer
P4VP-b-P4-b-P4VP (1) 6 800 4 200 2 600
P4VP-b-P4-b-P4VP (2) 7 200 4 200 3 000
P4VP-b-P4-b-P4VP (3) 9 400 4 200 5 200 Table 3.6 Characteristics of copolymers P4VP-b-P3HT-b-P4VP (*molecular weights determined by 1H NMR).
158
We have synthesized three different molecular weights copolymers,
P4VP-b-P3HT-b-P4VP by varying the P4VP chain length using convergent
anionic polymerization. The photovoltaic studies of these copolymers will
be described in the chapter-4 in which we used these copolymers as
donors and also as surfactants in the P3HT/PCBM blends.
Figure 3.22 Representative 1H NMR (400 MHz, CDCl3) spectrum of copolymer, P4VP-b-P3HT-b-P4VP (3). 3.6 Physical characterisation di- and triblock copolymers This section describes the characterization of synthesized copolymers by
UV-visible absorption spectroscopy and differential scanning calorimetry
(DSC). These techniques help to determine the physical properties of
these synthesized materials and thus attempt to assess their potential
application in organic photovoltaics.
3.6.1 P3HT-b-PS and PS-b-P3HT-b-PS block copolymers with and without C60 chain-ends Figure 3.23 shows the overlayed UV-visible absorption spectra of rod-coil
di-and coil-rod-coil tri-block copolymers P2-b-PS2, PS2-b-P3-b-PS2 and
and their corresponding C60-attached block copolymers P2-b-PS2-C60, C60-
PS2-b-P3-b-PS2-C60 whose syntheses is described in previous section.
159
The absorption spectra of di-and tri-block copolymers, identical to each
other, consist of two regions of characteristic absorptions, that of P3HT,
between 330 nm and 660 nm, and that of PS below 230 nm. The
absorption of P3HT block copolymers does not change significantly by
addition of the PS block, but a slight blue shift of absorption maximum with
increasing the number of the coil blocks in the case triblock copolymer; the
spectrum shape remains the same marked with two shoulders at 550 nm
and 600 nm. The peak at 600 nm is also attributed to inter-chain
interactions, and indicates a stacking of P3HT chains. The attachment of
the PS block does not alter the self-organization of P3HT, which retains its
electronic properties.
Figure 3.23 Overlayed UV-visible absorption spectrum (films made by spin-coating using o-DCB as solvent) of (a) P2-b-PS2 and P2-b-PS2-C60 (b) PS2-b-P3-b-PS2 and C60-PS2-b-P3-b-PS2-C60.
For copolymers carrying C60 chain-ends, there are two new peaks
around 250 nm and 320 nm confirming the attachment of fullerene to block
copolymers. Moreover, the absorption of P3HT block is blue shifted with a
shift of 35 nm and maximum at 490 nm for C60-PS2-b-P3-b-PS2-C60 vs
525 nm for PS2-b-P3-b-PS2, which means a significant loss of absorption
of radiation to high wavelengths whereas in the case of P2-b-PS2, a slight
blue shift is observed which is only due to one C60. In addition, the
shoulders, structuring of P3HT, disappear completely in the case of the
C60-PS2-b-P3-b-PS2-C60 with the addition of C60 that is thus indicated to
disrupt the self-organization of P3HT chains.
160
The synthesized di- and tri-block copolymers and their C60-attached
copolymers were characterized by differential scanning calorimetry (DSC)
to determine their characteristics and thus their thermal properties.
Measurements were performed from 0 °C to 300 °C at the heating rate of
10 °C/min. The samples underwent two heating and cooling cycles. Figure
3.24 shows the representative overlayed DSC thermograms of copolymer
P2-b-PS2-C60 with their corresponding diblock copolymer P2-b-PS2 and
homopolymer P2 (cooling and second heating cycle). The melting
temperature (Tm) and crystallization temperature (Tc) measured for their
copolymers (P2-b-PS2-C60 and P2-b-PS2) are lower than for the original
polymer (P2), as shown in Figure 3.24. All values of characteristic
temperatures and enthalpies of homopolymers, di- and triblock copolymers
and their C60-attached copolymers are summarized in Table 3.7.
The block copolymers P2-b-PS2, P2a-b-PS2, PS1-b-P3-b-PS1and
PS2-b-P3-b-PS2 thermograms reveal relatively similar with both
amorphous behavior with a glass transition temperature due to the PS
block, but also showed semi crystal-melting peak and crystallization peak,
due to the block P3HT (Table 3.7). The presence of these two phases
indicates a thermodynamic incompatibility between the two blocks (P3HT
and PS) and therefore the self-assembly of the synthesized copolymers.
The crystallization and melting temperature are decreased for the diblock
P2a-b-PS2 due to decrease in molecular weight of P3HT compared to P2-
b-PS2, which agreed with literature. The length of PS present in the
copolymer PS2-b-P3-b-PS2 thus significantly alters the behavior of
semicrystalline P3HT block compared to PS1-b-P3-b-PS1 (Table 3.7).
161
Figure 3.24 Overlayed DSC curves of donor-acceptor copolymer P2-b-PS2-C60 with their corresponding diblock copolymer P2-b-PS2 and homopolymer P2.
Crystallization temperature
Melting temperature Homopolymer / Block
Copolymer / C60-attached
Block Copolymer Tc
(°C)
Enthalphy
(J/g)
Tm
(°C)
Enthalphy
(J/g)
P2 203 12.5 231 16.5
P2a 192 15.4 215 14.8
P3 188 17.0 208 16.1
P2-b-PS2 194 14.9 225 12.2
P2a-b-PS2 183 14.9 210 12.6
PS1-b-P3-b-PS1 171 7.5 200 5.5
PS2-b-P3-b-PS2 164 6.6 203 3.7
P2-b-PS2-C60 192 13.0 222 6.5
P2a-b-PS2-C60 181 11.0 208 9.2
C60-PS1-b-P3-b-PS1-C60 175 7.2 202 6.7
C60-PS2-b-P3-b-PS2-C60 163 7.9 199 3.9
Table 3.7 Crystallization, melting temperature and enthalphy values of homopolymers, di-and tri-block copolymers and their C60-attached copolymers.
162
The thermograms of C60-attached copolymers are different from
their corresponding copolymers (eg. Figure 3.25). As expected, the
addition of fullerene alters the crystallization properties of the copolymer.
The melting temperature (Tm) and crystallization temperature (Tc)
measured are lower than for the original copolymer, as shown in Table 3.7
(eg. Tm = 222 °C, Tc = 192 °C for P2-b-PS2-C60; Tm = 225 °C, Tc = 194 °C
for P2-b-PS2). In the case of C60-PS2-b-P3-b-PS2-C60 (Tm = 199 °C, Tc =
162 °C), there was significant decrease in the melting and crystallization
temperatures compared to P2-b-PS2-C60 (Tm = 222 °C, Tc = 192 °C). This
temperature decrease in the case of the P2-b-PS2-C60 indicates a lower
macromolecular order due to the attaching of C60 and thus the loss of good
crystalline properties of P3HT block. But it clearly indicates that the
number of C60 increases in the copolymers, the crystallinity of the
copolymers decreasing dramatically as it is shown in Figure 3.25 (Tc = 194
°C for P2-b-PS2, Tc = 192 °C for P2-b-PS2-C60, Tc = 163 °C for C60-PS2-b-
P3-b-PS2-C60).
Figure 3.25 Overlayed DSC curves of diblock copolymer P2-b-PS2, donor-acceptor copolymer P2-b-PS2-C60, triiblock copolymer PS2-b-P3-b-PS2 and acceptor-donor-acceptor copolymer C60-PS2-b-P3-b-PS2-C60 showing crystallization temperatures.
163
3.6.2 P4VP-b-P3HT-b-P4VP block copolymers Figure 3.26 shows the representative overlayed UV-visible
absorption spectra of coil-rod-coil tri-block copolymers P4VP-b-P3HT-b-
P4VP (3) and homopolymer P3HT (P4) at room temperature and annealed
at 180 °C, respectively. The absorption spectrum of P3HT in block
copolymers changed significantly by the addition of P4VP block, a slight
blue shift of absorption maximum (shifted 560 nm - 500 nm) in the case of
tri block copolymer was observed. Unfortunately the shoulder around 600
nm, which is related to vibronic absorption decreased in the case of the
copolymer at room temperature and also for samples annealed at 180 ºC
indicating disruption of the P3HT crystalline order. The grafting of P4VP
block to P3HT significantly disturbs the self-assembly of P3HT and hence
affects its electronic properties.
Figure 3.26 Overlayed UV-visible absorption spectrum (film) of triblock copolymer with their corresponding homopolymer (a) P4VP-b-P3HT-b-P4VP (3) and P3HT (P4) at RT (b) P4VP-b-P3HT-b-P4VP (3) and P3HT (P4) at 180 °C.
The thermal properties of synthesized tri-block copolymers, P4VP-
b-P3HT-b-P4VP were characterized by differential scanning calorimetry
(DSC). Measurements were performed from 0 °C to 300 °C at the heating
rate of 10 °C/min. The samples underwent two cycles of heating and
cooling cycle. Figure 3.27 shows the DSC thermograms of the cooling and
heating (second cycle). All values of characteristic temperatures and
enthalpies of reactions are summarized in Table 3.8.
164
The DSC thermograms (Figure 3.27) of copolymers P4VP-b-P3HT-
b-P4VP (1) and P4VP-b-P3HT-b-P4VP (3) are relatively similar, displaying
both amorphous behavior with a glass transition temperatures due to the
P4VP block, but also showing semi crystal-melting peak and crystallization
peak, due to the P3HT block (Table 3.8). The presence of these two
phases indicates a thermodynamic incompatibility between the two blocks
(P3HT and P4VP) and therefore the synthesized copolymers probably self-
organize into discrete structures. The crystallization and melting
temperature of copolymers are decreased compared to homopolymer
P3HT (P4) due to introduction of P4VP coil block and also the length of
P4VP present in the copolymers significantly alters the melting
temperature of P3HT (P4) compared to P4VP-b-P3HT-b-P4VP (Table 3.8).
The representative DSC overlay (Figure 3.27) of copolymers P4VP-
b-P4-b-P4VP (3), P4VP-b-P4-b-P4VP (1) with their corresponding homo
polymer P3HT (P4) clearly shows that the attachment of P4VP coil block
significantly changes the melting and crystallization temperatures of P3HT
present in the copolymer [eg. Tm = 194 °C, Tc = 171 °C for P4VP-b-P3HT-
b-P4VP (3); Tm = 206 °C, Tc = 186 °C for P3HT (P4)].
Crystallization temperature
Melting temperature Homopolymer /
Copolymer Tc (°C) Enthalphy (J/g) Tm (°C) Enthalphy (J/g) P3HT (P4) 186 16.9 206 13.6
P4VP-b-P4-b-P4VP (1) 169 2.7 199 1.8
P4VP-b-P4-b-P4VP (2) 183 3.7 205 3.1
P4VP-b-P4-b-P4VP (3) 171 4.0 194 2.9
Table 3.8 Crystallization, melting temperature and enthalphy values of tri-block copolymers P4VP-b-P3HT-b-P4VP (1), (2), (3) with homo polymer P3HT (P4).
The length of coil block P4VP present in the copolymers also played
a significant role in their melting and crystallization temperatures of
copolymers. As P4VP chain length increases, especially the melting
temperatures of copolymers are significantly decreased [eg. Tm = 194 °C
165
for P4VP-b-P4-b-P4VP (3), Tm = 199 °C for P4VP-b-P3HT-b-P4VP (1) and
Tm = 206 °C, Tc = 186 °C for P3HT (P4)]. There was a dramatic change in
the crystallization temperatures of copolymers with the introduction of
P4VP coil blocks to P3HT (P4) [eg. Tc = 171 °C for P4VP-b-P3HT-b-P4VP
(3); Tc = 183 °C for P4VP-b-P3HT-b-P4VP (2) and Tc = 186 °C for P3HT
(P4)] which is shown in Table 3.8.
Figure 3.27 Overlayed DSC curves of triblock copolymers, P4VP-b-P4-b-P4VP (1), P4VP-b-P4-b-P4VP (3) with their corresponding homopolymer P3HT (P4).
166
3.7 Conclusions This chapter describes the synthesis of donor-acceptor block copolymers
based on P3HT, PS and P4VP by two different approaches. In the first
approach, we synthesized the di- and tri-block copolymers, P3HT-b-PS
and PS-b-P3HT-b-PS by "click" chemistry between polystyrene terminated
azide and P3HT alkyne in the presence of copper catalysts. This study
represents the modification of the reported literature1a by our group, which
is the first example of synthesis of exclusively rod-coil block copolymers,
by "click" chemistry. However, the influence of the conjugated chain of
P3HT on the alkyne function is very important and necessary to introduce
a separation between the two entities to achieve efficient coupling reaction
or by varying the “click” chemistry conditions with the help of sonication,
one can achieve the expected copolymers.
The C60-attached copolymers (P3HT-b-PS-C60 and C60-PS-b-P3HT-
b-PS-C60) were obtained by ATRA of bromine terminated PSs. This
reaction was performed by reacting the copolymers P3HT-b-PS and PS-b-
P3HT-b-PS with C60 in the presence of CuBr/bipyridine in chlorobenzene.
In the second approach; the triblock copolymers, P4VP-b-P3HT-b-
P4VP that contained the donor P3HT blocks and acceptor domains P4VP
coil blocks were synthesized via anionic polymerization. All these
copolymers were then characterized by UV-visible absorption
spectroscopy and differential scanning calorimetry, to assess their physical
properties. These measures have enabled to determine their characteristic
temperatures (glass transition, melting, crystallization), very important
elements with respect to their potential application into organic
photovoltaics, which will be further discussed in Chapter-4.
167
3.8 References 1 (a) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H.
Macromolecules 2008, 41, 7033-7040; (b) Urien, M. PhD thesis, 2008,
IMS, University of Bordeaux 1.
2 (a) Nicolay, V. T.; Matyjaszewski, K. Chem. Rev. 2007, 107, 2270-2299;
(b) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921-2990.
3 Kharash, M. S.; Jensen, E. V.; Urry, W. H. Science 1945, 102, 128.
4 Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614.
5 Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules
1995, 28, 1721.
6 Xia, J.; Zhang, X.; Matyjaszewski, K. ACS Symp. Ser. 2000, 760, 207.
7 Quémener, D.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Chem.
Comm. 2006, 5051.
8 Fernandez-Santana, V.; Gonzalez-Lio, R.; Sarracent-Perez, J.; Verez-
Bencomo, V. Glycoconjugate Journal 1998, 15, 549.
9 Carey, F.A.; Sundberg, R.J. Chimie Organique Avancée Tome 2:
Réactions et Synthèses, Ed. De Boeck Université, 1997.
10 Kolb, H. C., Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001, 40,
2004.
11 Agut, W.; Taton, D.; Lecommandoux, S. Macromolecules 2007, 40, 5653.
12 Opsteen, J. A.; Hest, J. C. M. v. Chem. Comm. 2005, 57.
13 Binder, W. H.; Sachsenhofer, R. Macromol. Rapid Commun. 2007, 28, 15.
14 Benanti, T. L.; Kalaydjian, A.; Venkataraman, D. Macromolecules 2008,
41, 8312-8315.
15 Tao, Y.; Mcculloch, B.; Kim, S.; Segalman, R. A. Soft Matter, 2009, 5,
4219–4230.
16 Jatsch, A.; Kopyshev, A.; Mena-Osteritz, E.; Bauerle, P. Organic Letters,
2008, 10, 961-964.
17 Wang, C.; Guo, Z.-X.; Fu, S.; Wu, W.; Zhu, D. Progress in Polymer
Science 2004, 29, 1079.
18 Gu, T.; Tsamouras, D.; Melzer, C.; Krasnikov, V.; Gisselbrecht, J.-P.;
Gross, M.; Hadziioannou, G.; Nierengarten, J.-F. Chem. Phys. Chem.
2002, 3, 124.
19 Zhou, P.; Chen, G.-Q.; Li, C.-Z.; Du, F.-S.; Li, Z.-C.; Li, F.-M. Chem.
Commun. 2000, 9, 1948.
168
20 Zhou, P.; Chen, G.-Q.; Li, C.-Z.; Du, F.-S.; Li, Z.-C.; Li, F.-M.
Macromolecules 2001, 33, 1948.
21 Audoin, F.; Nunige, S.; Nuffer, R.; Mathis, C. Synthetic Metals 2001, 121,
1149.
22 Audoin, F.; Nuffer, R.; Mathis, C. Journal of Polymer Science: Part A:
Polymer Chemistry 2004, 42, 3456.
23 Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Ruokolainen, J.;
Mezzenga, R. Macromolecules 2007, 40, 6990.
24 Laiho, A.; Ras, R. H. A.; Valkama, S.; Ruokolainen, J.; Osterbacka, R.;
Ikkala, O. Macromolecules 2006, 39, 7648-7653.
25 Jeffries-EL, M.; Sauve, G.; McCullough, R. D. Macromolecules 2005, 38,
10346–10352.
26 Szwarc, M. J. Polym. Sci. Polym. Chem. ix, 1998, 36.
27 Hsieh, H. L.; Quirk, R. P. Anionic Polymerization, Principles and Practical
Applications; Marcel Dekker, Inc.: New York, NY, 1996.
28 Flory, P. J. J. Am. Chem. Soc. 1940, 62, 1561.
29 Matthews, F. E.; Strange, E. H. British Patent, 1910, 24, 790.
30 Ziegler, K. Angew. Chem. 1936, 49, 499.
31 Szwarc, M.; Levy, M.; Milkovich, R. J. Am. Chem. Soc. 1956, 78, 2656.
32 Szwarc, M. ‘Living’ polymers. Nature (London) 1956, 178, 1168.
33 Baskaran, D.; Muller, A.H.E. Prog. Polym. Sci. 2007, 32, 173–219.
169
Chapter 4: Photovoltaic performances and
morphological characterizations of block
copolymers
170
Contents
4.1 Introduction............................................................................................. 171 4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b
and Plextronics P3HT) ........................................................................... 173 4.3 Photovoltaic performances of block copolymers................................ 177
4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM....................................................... 178
4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM...................... 182
4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block-P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM.................................................................................. 186
4.3.4 Triblock copolymer P4VP-block-P3HT-block-P4VP as a compatibilizer in the mixture of P3HT-blend-PCBM...................... 188
4.4 Conclusions............................................................................................. 190 4.5 References............................................................................................... 192
171
4.1 Introduction
The efficiencies of bulk heterojunction solar cells decrease with exposure
to heat and light.1,2 This is in part due to introduction of thermal and
radiative energy into the film causing changes in the morphology of the
film. Researchers have explored the use of additives to stabilize the
polymer-fullerene microstructures and observed some improvements in the
life-time of the devices.1-3 This is very important because devices are
exposed to sunlight for long periods of time. Generally, block copolymers
(BCPs) have been used as stabilizers or compatibilizers to avoid excessive
macrophase separation of polymer blends and also to produce the nano
and microstructured materials.4,5 Keeping this knowledge in mind, we have
used some of the synthesized block copolymers as additives that can both
compatibilize and enhance the organisation of the P3HT-blend-PCBM
active layer.
The synthesized materials were examined for photovoltaic
characterization using the structure shown schematically in Figure 4.1.
Indium-doped tin oxide (ITO) coated glass substrates were sonicated
sequentially in water, acetone, ethanol and isopropanol for 10 min, and
then dried with compressed nitrogen. The substrates were then exposed to
a UV-ozone treatment for 5 min. Immediately following this procedure, an
aqueous poly(ethylene dioxythiophene)-blend-poly(styrene sulfonate)
(PEDOT-blend-PSS) solution was spin-coated onto the ITO substrates
with a speed rate of 1000 rpm for 1 min and then annealed at 110 ºC for 10
min under vacuum. The active layer solution containing P3HT-blend-
PCBM (1:1 ratio, 20 mg in 1 mL) or P3HT-blend-PCBM-blend-
(%)copolymer in o-dichlorobenzene (ODCB) was stirred overnight to form
an homogeneous solution. Then these solutions were filtered (PTFE
membrane, 20 µm pore size) and spin-coated onto the PEDOT:PSS
coated substrates at a rotation rate of 1000 rpm for 1 min. Finally, the
device was completed by a thermal evaporation of aluminium cathode
under a secondary vacuum (10-6 mbar) through a shadow mask. The
active surface of the device for all the solar cells was 10 mm2. The
172
annealing process was carried out under an inert atmosphere by keeping
the cells directly onto a temperature-controlled hot plate. Cell
performances were evaluated following free cooling to ambient
temperature. Current-tension curves were recorded using a Keithley 4200
SCS, under an illumination of 100 mW cm-2 from a KHS Solar Celltest 575
solar simulator with an AM1.5 G filter. The luminous intensity was verified
against an IL1400 radiometer.
Figure 4.1 Schematic representation of organic solar cell device.
In all the studies we presented below, the thickness of the active
layer was not varied and was adjusted to 100 nm (± 10 nm) for the sake of
comparison. The heat treatments were applied after the deposition of the
cathode. They were performed in all cases for 5-10 min at the desired
temperature. It should be noted that all the series of manipulations were
repeated at least twice to ensure reproducibility of results.
173
4.2 Photovoltaic performances of synthesized P3HTs (P1, P1a, P1b and Plextronics P3HT) First, we have examined the photovoltaic performances of synthesized
P3HTs of different molecular weights, P1 (25 kg/mol), P1a (50 kg/mol),
P1b (100 kg/mol) and compared the performances with the commercially
available P3HT (Plextronics, 50 kg/mol). It was therefore necessary to
characterize and optimize the P3HTs performance in mixture with PCBM
for a reference and compare its power and photovoltaic characteristics with
the addition of block copolymers as compatibilizers to P3HT-blend-PCBM.
Regarding the weight ratio of P3HT: PCBM, the best results were obtained
for the ratio of P3HT: PCBM equivalent to 1:1. We have maintained this
ratio in all the photovoltaic studies.
It is necessary to optimize all parameters (ratio, annealing
temperature,..) of materials used and it is not necessary to apply a pre-
established formula, because all P3HTs are different in their chemical and
physical characteristics (molecular weight, dispersity, the transition
temperature,...) and therefore give various electronic properties. It is also
known that thermal treatment applied to the components can have a great
influence on the morphology of the active layer and therefore on device
performance. Figure 4.2 shows the photovoltaic parameters (Voc, Jsc, FF
and photo conversion efficiency) of all the P3HTs (P1, P1a, P1b and
Plextronics) used in the mixture P3HT:PCBM as a function of annealing
temperature. This Figure 4.2 shows first that the annealing improves the
cell performance. Indeed, maximum photo conversion efficiency around
2.8 % is achieved for P3HT (P1a) at an annealing temperature of 180 °C,
which is good, compared to the efficiency of 0.75 % of the unannealed
device. This value is greater than those of the similar molecular weights of
P3HT (Plextronics). All the values Voc, Jsc, FF and efficiency of P3HT (P1a)
were better than all the P3HTs especially P3HT (Plextronics) of similar
molecular weight which is shown in Table 4.1. Though the Voc of P3HT
(P1b) is slightly higher than P3HT (P1a), we have observed the maximum
current, 7.7 mA/cm2 and fill factor, 0.62 for P3HT (P1a). The increase in
174
these parameters (Jsc and FF) with the annealing is well-known, and is
mainly due to better structuring of the active layer.
Figure 4.2 Photovoltaic characteristics; open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and photo conversion efficiency (η) of the P3HTs (P1, P1a, P1b, Plextronics) used in the mixture P3HT-blend-PCBM at different annealing temperatures.
Voc (V) Jsc (mA/cm2) FF η (%) P3HT
(Mn, kg/mol) RT 180 ºC RT 180 ºC RT 180 ºC RT 180 ºC
P1 (25) 0.39 0.49 4.02 6.86 0.50 0.60 0.80 2.02
P1a (50) 0.40 0.56 3.61 7.73 0.52 0.62 0.75 2.76
P1b (100) 0.45 0.58 2.58 7.32 0.46 0.50 0.59 2.12
Plextronics (50) 0.47 0.55 3.18 7.50 0.49 0.59 0.73 2.48
Table 4.1 Photovoltaic characteristic values of Voc, Jsc), FF and η of the P3HTs in the mixture P3HT-blend-PCBM for unannealed (RT) and annealed (180 ºC) devices.
175
Figure 4.3 shows the UV-visible absorption spectra of P3HT/PCBM
blends [where the P3HTs used are P1, P1a, P1b and P3HT (Plextronics)]
for unannealed and annealed (180 ºC) devices. The absorption bands at
270 nm and 330 nm are characteristic absorption peaks of PCBM. The
absorption peaks between 450 nm and 650 nm correspond to the
absorption of P3HT. The spectrum of the annealed device exhibits better-
defined peaks due to P3HT with contributions around 516 nm (maximum),
550 nm (shoulder) and 600 nm (shoulder). Its absorption maximum is
slightly red shifted relative to P3HT absorption spectrum of the unannealed
device. In addition, the peak at 600 nm behaves independently of the other
peaks without any wavelength shift. Brown et al.6 have shown that this
contribution at 600 nm could be attributed to interchain interactions and
therefore was is due to intrachain 0-0 vibronic transition. This peak helps to
quantify the stacking ("packing") of P3HT chains. The absorption spectra
of the mixture P3HT:PCBM (Figure 4.3) after annealing show peaks at 550
nm and 600 nm, much better resolved and intense than at room
temperature, indicating better organization of polymer chains and a better
"π-stacking". In addition, after annealing, the absorption spectrum is more
intense and wider, which means an increase in the number of charge
carriers in the active layer and thus a better Jsc (Table 4.1).
Figure 4.3 Normalised UV-Visible absorption spectra of P3HT:PCBM blends (P3HT = P1, P1a, P1b and Plextronics) for unannealed and annealed (180 ºC) devices.
176
This strong structuring of P3HT chains is further illustrated in Figure
4.4 which presents the AFM image (phase and height) of a device based
on P3HT (P1a) obtained by spin coating in o-DCB annealed at 180 ºC. This
image, which provides information on the self-organization of P3HT chains,
shows a good morphology in which P3HT chains are organized in a fibrilar
structure. The literature commonly reports two types of possible structures
for P3HT/PCBM mixtures, namely the formation of "nano-rods' for low
molecular weight P3HT and fibrillar structures for P3HT of higher
molecular weights (higher than 20 000 g/mol).6,7 The structure obtained
here is composed of small fibrils stacked at a small micron scale.
Figure 4.4 AFM images (tapping mode) of: (a) phase; and (b) height of the fibrillar structure of the film P3HT(P1a):PCBM made by spin coating in o-DCB and annealed at 180 ºC. The photovoltaic performance of the in-house prepared P3HT (P1a)
was slightly better than that of P3HT (Plextronics) even though their
molecular weights are directly comparable. Figure 4.5 shows I-V
characteristics of solar cells based on the synthesized P3HTs and the
P3HT from Plextronics. Indeed, it was shown that as the molecular weight
of P3HT increased, the better the photovoltaic results, especially with
respect to FF and Jsc values.9,10 This is partly due to the crystallization
properties and hence self-organization of P3HT, which very dependent on
the chain length. However, P1b (100 kg/mol) was probably of too high a
molecular weight and therefore unable to self-organise easily. Hence
P3HT (P1a) was used as the reference for all the following photovoltaic
177
studies where block copolymers are used as additives to the P3HT-blend-
PCBM mixture.
Figure 4.5 I-V characteristics of solar cells based on the P3HTs (P1, P1a, P1b, and Plextronics) in the mixture of P3HT-blend-PCBM used as active layer in the dark and under illuminations. 4.3 Photovoltaic performances of block copolymers This section describes some of the use of the synthesized di- and triblock
copolymers (P3HT-b-PS, P3HT-b-PS-C60, C60-PS-b-P3HT-b-PS-C60 and
P4VP-b-P3HT-b-P4VP) as stabilizers or compatibilizers in a mixture of
P3HT-blend-PCBM active layer for organic photovoltaic cells. P1a (P3HT-
50 kg/mol) was used as the matrix donor in all the photovoltaic studies of
block copolymers. The PCBM used as the acceptor in this study had a
purity of 99.5 % (Solaris). Our aim in this study was to force the active
layer to self-organize with the addition of these block copolymers at
different proportions (0-5 %) into the mixtures of P3HT-blend-PCBM. In
that case a more favorable morphology can be achieved for efficient
charges to electrodes in organic photovoltaic process and also to see the
effect of these block copolymers as additives on device efficiencies.
178
4.3.1 Diblock copolymer P3HT-block-PS as compatibilizer in the mixture of P3HT-blend-PCBM Initially, the diblock copolymer, P2-b-PS2, shown in Figure 4.6, was used
as an additive at different proportions (0.5%, 1.0%, 1.5%, 2.5% and 5.0%)
in a mixture of P1a-blend-PCBM (1:1). The solar cell devices were tested
at different annealing temperatures, from room temperature up to 195 ºC.
The best performance for the mixture P1a-blend-PCBM-blend-(P2-b-PS2)
(0-5 %), both the P1a-based devices alone and also those based on P2-b-
PS2 was observed following annealing at 167 ºC.
S
C6H13
60
N NN
O
O
Br36
P2-b-PS2
P3HTPS
Figure 4.6 Chemical structure of diblock copolymer, P2-b-PS2.
Figure 4.7 shows the photovoltaic characteristics of devices based
on the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) based on the
amount of copolymer added. Indeed, the mixture for P1a-blend-PCBM
(0%) alone has an efficiency of 2.90%, it increases with the addition of
copolymer and reaches a significant maximum efficiency of 3.7% for P1a-
blend-PCBM-blend-(P2-b-PS2) (1.0%) following annealing at 167 ºC. The
efficiency on further additions of copolymer decreased to 2.6% for the
addition of 5.0% copolymer. While the FF had a maximum value of 0.61
with 1.0% copolymer addition, the Voc remained relatively constant at
between 0.54 V and 0.56 V. Only the short-circuit current (Jsc) increased
dramatically with the addition of copolymer (P2-b-PS2) and reached a
maximum value of 11 mA/cm2 at 1.0% copolymer blend (the best in our
studies), but decreased on further addition of copolymer. Figure 4.8
represents the I-V characteristics of solar cells based on the mixture P1a-
blend-PCBM-blend-(P2-b-PS2) (0-5 %). It was shown that the addition of
179
copolymer (1.0%) to P1a-blend-PCBM (0%) significantly changes the
photovoltaic parameters involved, especially photo conversion efficiency,
(from 2.9-3.7%) and short-circuit current, (from 8.7-11.0 mA/cm2).
Figure 4.7 Photovoltaic characteristics: Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) of unannealed and annealed (167 ºC) devices.
Figure 4.8 I-V characteristics of solar cells based on the the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) annealed at 167 ºC in the dark and under illumination.
180
Figure 4.9 shows the UV-Visible absorption spectra (film) of the
mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) at room temperature
(left side) and at annealing temperature 175 ºC (right side). The absorption
bands at 270 nm and 330 nm, the characteristic absorption peaks of
PCBM, did not change on addition of copolymer, whereas the absorption
peaks between 450 nm and 650 nm, corresponding to absorptions by
P3HT slightly red shifted with addition of copolymer compared to the P1a-
blend-PCBM alone at room temperature. The spectra of annealed (175 ºC)
samples are much better defined than that unannealed and the peaks at
550 nm (maximum) and 600 nm (shoulder) corresponding to P3HT are
better resolved, indicating that the addition of copolymer helps to organize
P3HT.
Figure 4.9 Normalised UV-Visible absorption spectra (film) of the mixture P1a-blend-PCBM-blend-(P2-b-PS2) (0-5 %) unannealed (left) and annealed (175 ºC) (right). The addition of copolymer structuring and its effect on P3HT chains
is further illustrated in Figure 4.10 which presents the AFM images (phase
and height) of a device based on P1a-blend-PCBM-blend-(P2-b-PS2) (0, 1
and 5 %) obtained by spin coating in o-DCB annealed at 180 ºC. Indeed,
the addition of 1% copolymer leads to an improvement of the structure,
with fibrils perfectly stacked together and in which areas of disorder
present in the mixture at 0% disappear. This structure is extremely regular,
and whose areas are well defined, promoting the mobility of charges and
limiting the exciton recombination. This contributes to the increase in FF
and of the Jsc and explains that the maximum energy conversion efficiency
(3.7%) was obtained for the mixture of 1% copolymer. At 5%, the structure
181
is completely disorganized with randomly tangled fibrils. Such structures
lead to a substantial decrease in the Jsc, and explain the drop in
performance for samples with 1.5%, 2.0% and 5.0% copolymers.
Figure 4.10 AFM images (tapping mode) of phase (left) and height (right) of the fibrillar structure of the film P1a-blend-PCBM-blend-(P2-b-PS2) (0, 1 and 5 %) made by spin coating from o-DCB and annealed at 180 ºC.
182
4.3.2 Donor-acceptor diblock copolymer P3HT-block-PS-C60 as compatibilizer in the mixture of P3HT-blend-PCBM Here the donor-acceptor diblock copolymer P2-b-PS2-C60 which is shown
in Figure 4.11 has been used as an additive at different proportions (0.5%,
1.0%, 1.5%, 2.5% and 5.0%) in a mixture of P1a-blend-PCBM (1:1). The
best performance for the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60)
(0-5 %), both the P1a-based devices and also those based on P2-b-PS2-
C60 was observed after annealing at 180 ºC.
S
C6H13
N NN
O
O
P2-b-PS2-C60
60
36
P3HTPS
C60
Figure 4.11 Chemical structure of donor-acceptor diblock copolymer, P2-b-PS2-
C60.
Figure 4.12 shows the photovoltaic characteristics of devices based
on the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) based on
the amount of copolymer added for unannealed and annealed devices.
The efficiency for the mixture P1a-blend-PCBM (0%) alone was 3.0%,
which significantly increased to maximum efficiency of 4.0% with addition
0.5 % of copolymer at 180 °C and started decreasing the efficiency of the
device on further addition of copolymer to reach a lower efficiency of 2.9%
for 5.0% copolymer. Here surprisingly the FF reached the maximum value
of 0.65 with 0.5% copolymer addition and the Vocs are in turn relatively
constant at between 0.53 V and 0.56 V. But the Jsc increases dramatically
with the addition of copolymer, P2-b-PS2-C60 and reaches a maximum
value of 12.0 mA/cm2 for 0.5% copolymer blend to decrease upon further
addition of copolymer. Figure 4.13 represents the I-V characteristics of
solar cells based on the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60)
183
(0-5 %). It was shown that the addition of copolymer (0.5%) to P1a-blend-
PCBM (0%) significantly changes the photovoltaic parameters involved,
especially the photo conversion efficiency, (from 3.0-4.0%), the fill factor
with a maximum value of 0.65 and the maximum current Jsc, 12.0 mA/cm2
which are the best results in our PV studies.
Figure 4.12 Photovoltaic characteristics; Voc, Jsc, FF, and η of the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of unannealed and annealed (180 ºC) samples.
Figure 4.13 I-V characteristics of solar cells based on the the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of annealed (180 ºC) device in the dark and under illumination.
184
Figure 4.14 shows the UV-Visible absorption spectra (film) of the
mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) in unannealed
and annealed (180 ˚C) devices. For the former, the intensity of the
absorption bands at 270 nm and 330 nm, corresponding to PCBM, and the
absorption peaks between 450 nm and 650 nm, corresponding to the
absorption of P3HT, increase with addition of copolymer compared to P1a-
blend-PCBM alone at room temperature. The absorption spectra of the
annealed device with (0-5 %) copolymer is much better resolved and the
intensity of peaks at 550 nm (maximum) and 600 nm (shoulder)
correponding to P3HT is increased and slightly red shifted compared to
room temperature, whereas the intensity of peaks corresponding to C60
decrease. This indicates that the addition of copolymer helps to improve
the organization of P3HT and hence lead to higher efficiences.
Figure 4.14 Normalised UV-Visible absorption spectra (film) of the mixture P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0-5 %) of unannealed an annealed (180 ˚C) devices. The morphology of the devices was investigated by AFM images (0-
5 % copolymer) as shown in Figure 4.15. The addition of 0.5% copolymer
to P1a-blend-PCBM (annealed, 180 ºC) achieved a better fibrilar
morphology which resulted in the high FF and the best observed efficiency
whereas further addition of copolymer (5%) disrupted the P3HT fibrillar
structure chains (height images of Figure 4.15) leading to a reduced FF
and lower energy conversion efficiencies.
185
Figure 4.15 AFM images (tapping mode) of phase (left side) and height (right side) of the film P1a-blend-PCBM-blend-(P2-b-PS2-C60) (0, 0.5, 1 and 5 %) made by spin coating from o-DCB and annealed at 180 ºC.
186
4.3.3 Acceptor-donor-acceptor triblock copolymer C60-PS-block- P3HT- block-PS-C60 as compatibilizer in the mixture of P3HT- blend-PCBM The acceptor-donor-acceptor triblock copolymer, C60-PS2-b-P3-b-PS2-C60,
shown in Figure 4.16, was used as a compatibilizer at different proportions
(0%, 0.5%, 1.0%, 1.5%, 2.0%, 4.0%, 5.0% and 7.0%) in a mixture of P1a-
blend-PCBM (1:1).
S
C6H13
30
N NN ONN
NO
O O
C60-PS2-b-P3-b-PS2-C60
3636
C60P3HT
PSPS
C60
Figure 4.16 Chemical structure of the acceptor-donor-acceptor triblock copolymer, C60-PS2-b-P3-b-PS2-C60. The photovoltaic characteristics of the devices as a function of the
amount of compatibilizer in the film P1a-blend-PCBM-blend-(C60-PS2-b-
P3-b-PS2-C60) (0-7 %) both unannealed and annealed (180 ºC) are shown
in Figure 4.17. There is a dramatic decrease in the Jsc with addition of
copolymer for annealed devices. In fact, the addition of 0.5% copolymer
reduces the Jsc value from 9.5 to 7.3 mA/cm2 (Table 4.2) and this value
continues to decrease with the further addition of copolymer. However, for
unannealed devices, the Jsc value significantly increased with 0.5%
addition of copolymer from 3.1 to 6.3 mA/cm2 but started decreasing on
further addition of copolymer to reach a low value of 1.0 mA/cm2 with 7%
weight copolymer. On the other hand, the Voc of annealed devices remains
approximately constant (Voc ≈ 0.50 V at 180 ºC) whereas at room
temperature it varies 0.28 to 0.42 V. The FF of the devices both for
unannealed devices and after annealing also gradually decreased with the
increasing amount of the copolymer added.
Because Voc and FF were approximately constant, the efficiency
values followed the trend of the Jsc and decreased with increasing the
amount of copolymer (0-7 %). But unannealed devices, with 0.5% addition
of copolymer increases the efficiency which nearly doubles from 0.67 to
187
1.27 %, although this decreased on further addition of copolymer to reach
η = 0.11% at 7% copolymer. This is an interesting result as 1.27 %
efficiency can be reached without annealing.
Figure 4.17 Photovoltaic characteristics; Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-5 %) for room temperature (black line) and annealed (180 ºC) devices (red line).
Voc (V) Jsc (mA/cm2) FF η (%) Copolymer
(%) RT 180 ºC RT 180 ºC RT 180 ºC RT 180 ºC
0 0.42 0.51 3.13 9.50 0.53 0.62 0.67 3.05
0.5 0.40 0.52 6.36 7.33 0.43 0.56 1.27 2.13
1.0 0.40 0.53 4.48 7.48 0.38 0.56 0.68 2.26
1.5 0.37 0.53 4.36 6.77 0.37 0.54 0.61 1.95
2.0 0.39 0.53 4.79 7.72 0.43 0.51 0.81 2.12
4.0 0.33 0.51 1.40 7.25 0.27 0.34 0.13 1.29
5.0 0.28 0.51 2.10 6.01 0.26 0.41 0.15 1.29
7.0 0.38 0.54 1.06 7.52 0.28 0.29 0.11 1.19
Table 4.2 Photovoltaic characteristic values of Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) for unannealed (RT) and annealed (180 ºC) devices.
188
The decreasing short-circuit current on increasing weights of C60-
PS2-b-P3-b-PS2-C60 suggested that the compatibilizer directly affected the
charge transport in the devices. The reason for this may be due to the low
molecular weight of the copolymer used in these blends as it has been
shown that polythiophene-C60 bulk heterojunctions composed of higher
molecular weight P3HT have better device performances10 and also it has
been indicated in our case that the donor-acceptor diblock copolymer, P2-
b-PS2-C60 of high molecular weight P3HT has improved the Jsc when at
0.5% copolymer (Section 4.3.2.).
4.3.4 Coil-rod-coil triblock copolymer P4VP-block-P3HT-block-P4VP as compatibilizer in the mixture of P3HT-blend-PCBM The coil-rod-coil triblock copolymer, P4VP-b-P4-b-P4VP (1), which is
shown in Figure 4.18, is of particular interest in which it can have
supramolecular interactions from its P4VP block with PCBM in the blended
device. It was tested both as donor (P4VP-b-P4-b-P4VP):PCBM (1:1) and
also as a compatibilizer at varying proportions (0.5% and 1.0%) in a
mixture of P1a-blend-PCBM (1:1).
Figure 4.18 Chemical structure of the coil-rod-coil triblock copolymer, P4VP-b-P4-b-P4VP. Initially, we used the copolymer as a compatibilizer and followed
photovoltaic characteristics of the devices as a function of the amount of
compatibilizer in the film P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP) (0,
0.5% and 1.0%) both for unannealed and annealed (175 ºC) devices, as
shown in Table 4.3. There is a decrease in the Jsc on addition of copolymer
189
for annealed devices. In fact, the addition of 0.5% copolymer reduced the
Jsc value from 6.8 to 6.6 mA/cm2 (Table 4.3) and this value decreased
dramtically with the 1.0% addition of copolymer to 1.8 mA/cm2 whereas
unannealed devices, the Jsc value significantly increased on 0.5% addition
of copolymer from 3.6 to 4.9 mA/cm2 but decreased on further additions to
a very low value of 0.76 mA/cm2 at 1% weight copolymer. The Voc and the
FF of the devices both unannealed and annealed (175 ºC) also gradually
decreased with the increasing amount of the copolymer added. Finally the
efficiency decreased with increasing copolymer (0-1 %) reaching the low
value of 0.11% with 1% copolymer.
We also attempted using the copolymer as donor material in
unannealed and annealed (167 ºC and 175 ºC) devices (P4VP-b-P4-b-
P4VP):PCBM (1:1). The photovoltaic characteristic values of unannealed
devices were Voc = 0.47, Jsc = 3.19 mA/cm2, FF = 0.49 and η = 0.74 and
annealing the devices at 167 ºC showed a slight increment in the Jsc to
3.43 mA/cm2 (Table 4.3). There was dramatic decrease of these values
(Voc = 0.07, Jsc = 0.12 mA/cm2, FF = 0.24 and η = 0.002) when the device
annealed at 175 °C.
Voc (V) Jsc (mA/cm2) FF η (%) Copolymer
(%) RT 175 ºC RT 175 ºC RT 175 ºC RT 175 ºC
0 0.40 0.56 3.61 6.87 0.52 0.62 0.75 2.36
0.5 0.29 0.38 4.92 6.67 0.30 0.38 0.45 0.98
1.0 0.15 0.17 0.76 1.81 0.23 0.34 0.03 0.11
Copolymer:PCBM RT 167 ºC RT 167 ºC RT 167 ºC RT 167 ºC
1:1 0.47 0.45 3.19 3.43 0.49 0.50 0.74 0.74
at 175 ºC ‘’ 0.07 ‘’ 0.12 ‘’ 0.24 ‘’ 0.002
Table 4.3 Photovoltaic characteristic values of Voc, Jsc, FF and η of the mixture P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP) (0, 0.5 and 1 %) at room temperature (RT) and at annealing temperature (175 ºC) and device based on (P4VP-b-P4-b-P4VP)-blend-PCBM (1:1) at room temperature, annealing temperaures 167 ºC and 175 ºC.
190
The annealing of devices (P4VP-b-P4-b-P4VP)-blend-PCBM (1:1)
at high temperatures did not improve the efficiencies. The low values found
are similar to those reported with a near-similar polymer (η =0.017-
0.026%) using a standard device.11 The low efficiency for this device may
be due to the preferential wetting of one of the copolymer blocks (P4VP) at
the PEDOT:PSS interface during the film formation. P4VP tends to
preferentially wet oxides or charged surfaces.12 The efficiency of the device
using this type of copolymer (P4VP-b-P4-b-P4VP) may be enhanced by an
inverted PV device, as already observed in the literature.11
4.4 Conclusions In this section, we have described the photovoltaic characterization of
synthesized di- and triblock copolymers P2-b-PS2, P2-b-PS2-C60, C60-PS2-
b-P3-b-PS2-C60 and P4VP-b-P3HT-b-P4VP used as compatibilizer in a
mixture of P1a-blend-PCBM active layer for organic photovoltaic cells. The
first study involves the optimization of P3HT mixed with PCBM, for which
maximum yield is obtained for P1a among all the synthesized P3HTs and
also commercial P3HT of similar molecular weight from Plextronics
annealing at 180 ºC. The block copolymers were then tested as
compatibilizers (0-5 %) in combination with P1a and PCBM (1:1) based
devices. The device based on P1a-blend-PCBM-blend-(P2-b-PS2) using
1% addition of copolymer has achieved the highest short-circuit current 11
mA/cm2 and also highest photoconversion efficiency, 3.7% at annealing
temperature of 167 ºC in our solar cell studies. At 5% addition of this
copolymer P2-b-PS2, the structure is completely disorganized and tangled
fibrils random structures lead to a substantial decrease in the Jsc, and
explains the drop in performance for the mixtures of 1.5%, 2.0% and 5.0%.
This is explained by taking into account the degree of crystallinity of this
copolymer, which is superior to others, reflecting its ability to facilitate
better mobility of charge carriers in the active layer. The donor-acceptor
diblock copolymer, P2-b-PS2-C60, has been characterized in organic solar
cell as compatibilizer by adding 0-5% amounts to P1a-blend-PCBM (1:1)
based devices. Surprisingly it was shown that the addition of copolymer
191
(0.5%) to P1a-blend-PCBM (0%) at 180 ºC annealing significantly changes
the photovoltaic parameters involved especially photoconversion
efficiency, (from 3.0-4.0%), fill factor with maximum value of 0.65, but the
maximum short-circuit current, 12.0 mA/cm2 was observed at 0.5%
addition of copolymer. In this case, we have observed an excellent fibrilar
morphology. The 5% weight addition of copolymer disrupts the fibrillar
structure of the P3HT chains (height images of Figure 4.15) which lead to
reduced fill factor and reduced energy conversion efficiencies.
In the case of triblock copolymers, the devices based on P1a-blend-
PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) (annealed, 180 ºC)
showed dramatic decreases in the Jsc with addition of copolymers whereas
unannealed devices showed Jsc values significantly increased with 0.5%
copolymer, but started decreasing on further addition of copolymer to
reach a low value of 1.0% at 7% copolymer. Neverthless, with 0.5%
copolymer the efficiency nearly doubled from 0.67 to 1.27%. The devices
based on P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP), in which the
copolymer was used as a compatibilizer (0, 0.5% and 1.0%) showed
dramatic decreases in Jsc with the addition of copolymer for annealed
devices. The Voc and the FF of the devices both unannealed and annealed
also gradually decreased with the increasing copolymer. We have also
used the copolymer, P4VP-b-P4-b-P4VP as donor material in the device,
(P4VP-b-P4-b-P4VP):PCBM (1:1) at room temperatures and at annealed
temperatures 167 ºC and 175 ºC, but only low photovoltaic efficiences were
found.
We have observed that the addition of triblock copolymers as
compatibilizers disrupts the molecular structure of P3HT chains and also
the disorganization of the active layer, resulting in low efficiencies. But in
the case of diblock copolymers as compatiblizers, we have observed the
enhancement of Jscs and efficiencies with respect to P3HT-blend-PCBM
device alone. This might be due to the nano-domain constraints placed
upon such systems by tri-block copolymers.
192
4.5 References 1 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324-
1338.
2 Kim, B. J.; Miyamoto, Y.; Ma, B.; Fréchet, J. M. J. Adv. Mater. 2009, 19,
1-9.
3 Ball, Z. T.; Sivula, K.; Fréchet, J. M. J. Macromolecules 2006, 39, 70–72.
4 Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry; CRC Press: Boca Raton,
FL, 2007.
5 Sperling, L. H. Introduction to Polymer Science, 3rd Ed.; John Wiley &
Sons, Inc.: Hoboken, NJ, 2001.
6 Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale,
C.
M.; Sirringhaus, H.; Friend, R. H. Physical Review B 2003, 67, 064203.
7 Verilhac, J.-M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.;
Travers, J.- P.; Pron, A. Synthetic Metals 2006, 156, 815.
8 Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet, J. M. J.;
Toney, M. F. Macromolecules 2005, 38, 3312.
9 Schilinsky, P.; Asawapirom, U.; Scherf, U.; Biele, M.; Brabec, C. J. Chem.
Mater. 2005, 17, 2175.
10 Ma, W.; Kim, J. Y.; Lee, K.; Heeger, A. J. Macromol. Rapid Commun. 2007,
28,
1776–1780.
11 Sary, N.; Richard, F.; Brochon, C.; Leclerc, N.; Lévêque, P.; Audinot, J. N.;
Berson, S.; Heiser, T.; Hadziioannou, G.; Mezzenga, R. Adv. Mater. 2010,
22, 763–768.
12 Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280.
193
Chapter 5: Experimental Section
194
195
Contents
1 Materials................................................................................................... 197 1.1 Purification of Solvents.................................................................... 197 1.2 Purification of Monomers................................................................. 197 1.3 Chemicals........................................................................................ 197
2 Synthesis................................................................................................. 199 2.1 Monomers...................................................................................... 199 2.1.1 3-Hexylthiophene................................................................ 199 2.1.2 2,5-Bibromo-3-hexylthiophene............................................ 199 2.1.3 2-Bromo-3-hexylthiophene.................................................. 200 2.1.4 2-Bromo-3-hexyl-5-iodo-thiophene..................................... 200 2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis
(GRIM) ............................................................................................ 201 2.2.1 α,ω-DiH-P3HTs (P1, P1a, P1b and P1c) ............................ 201 2.2.2 Chain-end functionalised w-P3HTs or ω-P3HTs................. 202 2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω-pentynyl-
P3HTs................................................................... 202 2.2.2.2 α,ω-Diformyl and α,ω-dihydroxy-P3HTs................ 203 2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni-
catalyst initiator............................................................................. 205 2.3.1 Ni-initiator: [(Ph)Ni(PPh3)2-Br] ............................................ 205 2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator. 205 2.4 Azide-terminated Polystyrene...................................................... 206 2.4.1 Azide initiator for ATRP....................................................... 206 2.4.1.1 3-Azido-1-propanol................................................ 206 2.4.1.2 3-Azidopropyl-2-bromoisobutyrate........................ 206 2.4.2 α-Azido-polystyrenes (PS1-PS6) ....................................... 207 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-
PS by Click Chemistry.................................................................. 208 2.5.1 Triblock copolymers PS-b-P3HT-b-PS................................ 208 2.5.2 Diblock copolymers P3HT-b-PS.......................................... 209 2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA....... 210 2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by
anionic polymerisation................................................................. 211 2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) ................................ 212 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst... 212
196
2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2
catalyst ............................................................................... 212 2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with
phenyl acetylene................................................................. 213 3 Characterization...................................................................................... 213 4 Photovoltaic device fabrication and characterization......................... 215
General conclusions........................................................................................ 217 Appendix........................................................................................................... 223 Publications and conferences........................................................................ 227
197
1. Materials 1.1 Purification of Solvents Solvents were distilled over their respective drying agents under reduced
pressures and stored under inert atmosphere. Tetrahydrofuran (THF, JT
Baker, 99%) was first distilled over calcium hydride (CaH2) and then cryo-
distilled over sodium benzophenone just before use. Dichloromethane
(DCM, Xilab, 99%) was cryo-distilled after refluxing 1 h over CaH2.
Diethylether (JT Baker, 99%) and toluene (JT Baker, 99%) were first
distilled over calcium hydride (CaH2) and then cryo-distilled over polystyryl-
lithium just before use. Chlorobenzene (Aldrich, 99%) was distilled after
stirring with CaH2 overnight. Methanol (Xilab, 99%) was cryo-distilled after
refluxing overnight over Mg turnings. Triethylamine (TEA, Aldrich, 99%)
was cryo-distilled following overnight stirring over KOH pellets. Hexane (JT
Baker, 95%), acetic acid (Aldrich, 99%), ethylacetate (JT Baker, 99%) and
diisopropylethylamine (DIEA, Aldrich, 99%) were used as received without
purification.
1.2. Purification of Monomers Styrene (Aldrich, 99%), α-methylstyrene (Aldrich, 99%) and 4-vinylpyridine
(Aldrich, 99%) were cryo-distilled over CaH2 just prior to polymerisation.
2,5-Dibromo-3-hexylthiophene and 2-bromo-3-hexyl-5-iodothiophene were
distilled 2-3 times to obtain 100% purity (1H NMR) for polymerisation and
stored at 4 ºC.
1.3. Chemicals Bromohexane (98%), 3-bromothiophene (97%), 1,3-
bis(diphenylphosphino)propane nickel(II) chloride [Ni(dppp)Cl2], N-
bromosuccinimide (NBS, 99%), iodine, iodobenzene diacetate (98%),
sodium bicarbonate (NaHCO3), sodium sulfate (Na2SO4), sodium
thiosulfate (Na2S2O3), 3-bromo-1-propanol (97%), sodium azide (NaN3, ≥
99.5%), tetrabutylammonium iodide (Bu4NI, 98%), dicyclohexano-18-
crown-6 (18-crown-6, 98%), 2-butanone (≥ 99.9%), α-bromoisobutyryl
198
bromide (98%), bipyridine (≥ 99.9%), copper(I) iodide (CuI, ≥ 99.9%), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU, 98%), N,N,N',N",N"-
pentamethyldiethylenetriamine (PMDETA, 99%), C60 (99.9%), styrene (≥
99.9%), triethylamine (TEA, 99.5%), ammonium chloride (NH4Cl), and
potassium hydroxide (KOH), were used as received from Aldrich without
purification. Mg turnings (98%, Aldrich) were dried overnight in an oven at
150 ºC before use.
Tert-butylmagnesium chloride (tBuMgCl, 1 M solution in THF), iso-
propylmagnesium chloride (i-PrMgCl, 2 M solution in THF),
vinylmagnesium bromide (2 M solution in THF), ethynylmagnesium
bromide (0.5 M solution in THF), (5-chloro-1-pentynyl)trimethylsilane (97
%), 4-(2-tetrahydro-2H-pyranoxy)phenylmagnesium bromide were used as
received from Aldrich without purification. 4-(1,3-Dioxan-2-
ylphenyl)magnesium bromide was purchased from Rieke Metals Inc.
Copper(I) bromide (CuBr, Aldrich, 98%) was purified by stirring
overnight in a mixture of acetic acid and water (1:1), then filtered, rinsed
sequentially with ethanol and ether and dried in vacuum oven for about 24
h at 40 ˚C. It was kept under an inert atmosphere until use. Typically, CuBr
(2 g) was washed with a mix of acetic acid (50 mL) and water (50 mL).
199
2. Synthesis All reactions were carried out under a dry argon atmosphere, using flame-
dried glassware.
2.1 Monomers 2.1.1 3-Hexylthiophene: In a 500 mL flask equipped with condenser
magnesium (15.13 g, 0.62 mol) and dry diethyl ether (210 mL) were
introduced into the flask and cooled to 0 ºC. A solution of bromohexane
(82.25 g, 70 mL, 0.5 mol) was added slowly by dropping funnel. The
resulting mixture was stirred under nitrogen for 3 h and transferred to a
dropping funnel (250 mL) fitted to another 1 L flask equipped with
condenser containing 3-bromothiophene (50 g, 28 mL, 0.306 mol) and
Ni(dppp)Cl2 (0.75 g, 1.38 mmol) in dry diethyl ether (100 mL). After cooling
with an ice bath, the Grignard reagent was added dropwise and the
resulting adduct was allowed to warm to room temperature and stirred for
3 days under nitrogen. To terminate the reaction, slightly acidified (HCl)
water was added (100 mL) slowly. To recover the monomer, chloroform
(200 mL) was added, and the organic layer washed 3 times with water and
dried over Na2SO4. The crude product was distilled under reduced
pressure to obtain in the pure form as a colorless oil (45 g, 87%).
Characterization of 3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH
0.90 (t, 3H, (CH2)5-CH3), 1.32 (m, 6H, CH2-CH2-(CH2)3-CH3), 1.63 (q, 2H,
CH2-CH2-(CH2)3-CH3), 2.63 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.92 (m, 1H, CH
Ar), 6.95 (m, 1H, CH Ar), 7.23 (m, 1H, CH Ar).
2.1.2 2,5-dibromo-3-hexylthiophene: NBS (80.65 g, 0.453 mol) was
added to a stirred solution of 3-hexylthiophene (42.15 g, 0.25 mol) in acetic
acid (320 mL) and CH2Cl2 (320 mL). The mixture was stirred at room
temperature for 24 h under nitrogen. The organic layer was washed five
times with water, five times with a saturated aqueous NaHCO3 solution,
dried over Na2SO4, filtered and concentrated. The crude product was
recovered as pale yellow oil by two successive secondary vacuum
distillations (5 x 10-4 mbar, 100 ºC) (58 g, 70%).
200
Characterization of 2,5-dibromo-3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.30 (m, 6H, CH2-CH2-(CH2)3-CH3),
1.55 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.51 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.78
(s, 1H, CH aromatic). δ C 14.10 ((CH2)5-CH3), 22.60 ((CH2)4-CH2-CH3)),
26.96 (-CH2-(CH2)4-CH3), 28.81 ((CH2)2-CH2-(CH2)2-CH3), 29.51 ((CH2)3-
CH2-CH2-CH3), 31.60 (CH2-CH2-(CH2)3-CH3), 107.95 (C2 Ar), 110.32 (C5
Ar.),130.98 (C4 Ar), 143.02 (C3 Ar).
2.1.3 2-Bromo-3-hexylthiophene: NBS (20.54 g, 115.4 mmol) was
added to the stirred solution of 3-hexylthiophene (19.43 g, 115.4 mmol) in
THF (200 mL) at 0 ºC, and the mixture was stirred at 0 ºC for 1 h. After
addition of water, the mixture was extracted with Et2O. The organic layer
was washed successively with 10% aqueous Na2S2O3, 10% aqueous
KOH, and water, and dried over anhydrous Na2SO4. Distillation (5 x 10-4
mbar, 100 ºC) and filtering through cotton gave the pure product as
colorless oil (28 g, 95%).
Characterization of 2-bromo-3-hexylthiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.31 (m, 6H, CH2-CH2-(CH2)3-CH3),
1.57 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.56 (t, 2H, CH2-CH2-(CH2)3-CH3), 6.78
(d, 1H, CH5 Ar), 7.18 (d, 1H, CH4 Ar). 2.1.4 2-Bromo-3-hexyl-5-iodothiophene: Iodine (12.51 g, 49.2 mmol)
and iodobenzene diacetate (17.33 g, 53.8 mmol) were added successively
to a stirred solution of 2-bromo-3-hexylthiophene 3 (22.17 g, 89.68 mmol)
in CH2Cl2 (200 mL) at 0 ºC, and the mixture stirred at room temperature for
4 h. 10% Aqueous Na2S2O3 was added, and the mixture extracted with
Et2O. The organic layer was washed with 10% aqueous Na2S2O3 and dried
over anhydrous Na2SO4. After filtration, the solvent and iodobenzene were
removed by evaporation under reduced pressure. The residue was distilled
to remove traces of iodobenzene and then purified by silica gel column
chromatography (eluent:cyclohexane) to give the pure product as pale
yellow oil (32.26 g, 96%).
Characterization of 2-bromo-3-hexyl-5-iodothiophene: 1H NMR (400 MHz, CDCl3): δH 0.89 (t, 3H, (CH2)5-CH3), 1.30 (m, 6H, CH2-CH2-(CH2)3-
201
CH3), 1.55 (q, 2H, CH2-CH2-(CH2)3-CH3), 2.52 (t, 2H, CH2-CH2-(CH2)3-
CH3), 6.96 (s, 1H, CH4 Ar).
2.2 Regioregular P3HTs (P1-P6) by the Grignard metathesis (GRIM) 2.2.1 α ,ω -diH-P3HTs (P1, P1a, P1b and P1c) (representative method): To a three-necked round bottom flask 2,5-dibromo-3-hexylthiophene M1 (3
g, 9.2 mmol) was dissolved in THF (20 mL) and stirred under nitrogen. Tert-butylmagnesium chloride (9.3 mL, 9.2 mmol, 1 M in THF) was added
and the mixture was stirred at room temperature for 3 h. The mixture was
then diluted to 80 mL with THF, Ni(dppp)Cl2 (0.0054 g, 0.01 mmol) was
added at once and the mixture was allowed to stir for 24 h at room
temperature. Termination and removal of bromine chain ends was
accomplished by the slow addition of LiAlH4 (4.6 mL, 4.6 mmol, 1 M in
THF). After another 16 h, excess LiAlH4 was quenched by slow dropwise
addition of HCl (10 mL, 1 M) (caution: this step may evolve rapid H2), the
polymer was recovered by precipitation in ethanol (1 L) and filtered into a
Soxhlet extraction thimble. Following extensive Soxhlet washing with
methanol, hexane and chloroform, α,ω-diH-P3HT was recovered from the
Soxhlet filter with chloroform. Following precipitation in ethanol and filtered
with G4 crucible, drying under reduced pressure overnight at 70 ˚C, the
polymer was stored under an inert atmosphere and protected from light.
Debrominated P3HT (α ,ω -diH-P3HTs)
P3HT
Monomer M1
[g, (mmol)]
Grignard reagent t-BuMgCl
(1 M in THF) [mL, (mmol)]
Ni(dppp)Cl2
[g, (mmol)] Polymerization
time
(h)
LiAlH4 (1M in THF)
[mL, (mmol)]
Mn (g mol-1,
GPC)
Đ
P1 2.83
(8.6)
8.6
(8.6)
0.01
(0.02) 24 4.3 (4.3) 30 000 1.6
P1a 3.01
(9.2)
9.2
(9.2)
0.0054
(0.01) 24 5.0 (5.0) 50 000 1.7
P1b 6.0
(18.4)
18.4
(18.4)
0.006
(0.01) 24 10.0 (10.0) 117 000 1.7
P1c 3.14
(9.62)
9.5
(9.5)
0.090
(0.166) 2 10.0 (10.0) 7 000 1.1
Table 1: Reaction conditions and macromolecular characteristics determined by SEC (THF, UV 254 nm) against polystyrene standards of α,ω-diH-P3HTs (Note: Dispersity, Đ = Mw/Mn).
202
2.2.2 Chain-end functionalized ω-P3HTs or α,ω-P3HTs (P2-P6)
(typical end-capping reaction): In a three-necked round bottom flask; 2,5-
dibromo-3-hexylthiophene (M1, 3 g, 9.2 mmol) was dissolved in THF (20
mL) and stirred under nitrogen. tert-Butylmagnesium chloride (9.3 mL, 9.2
mmol, 1 M in THF) was added via syringe and the mixture stirred at room
temperature for 2.5 h. Following dilution to 80 mL with THF, Ni(dppp)Cl2
(0.068 g, 0.125 mmol) was added in one portion and the mixture stirred for
30-60 min at room temperature. The Grignard reagent (50-60 mol % of
monomer) with respect to the functionalisation was added via syringe to
the reaction mixture and stirred for an additional 30-60 min at room
temperature. Finally the reaction was quenched by adding conc. HCl (5 M)
and then poured into methanol (800 mL) to precipitate the polymer. The
polymer was filtered into an extraction thimble and then washed by Soxhlet
extraction with methanol, hexane and chloroform. The polymer was
isolated from the chloroform extraction and concentrated under reduced
pressure, precipitated into methanol, filtered with G4 crucible, dried
overnight under vacuum and finally stored under inert atmosphere
protecting from light.
2.2.2.1 ω-Ethynyl, ω-vinyl-P3HTs and α,ω -pentynyl-P3HTs: The deprotection of polymers was performed as follows. In the case of P2,
P2a, P2b, P2c, P2d and P2e; the Grignard reagent used for
functionalization was ethynyl-magnesium bromide, leading to the ω-
ethynylP3HTs, whereas for P6, P6a and P6b; vinyl-magnesium bromide is
used to obtain ω-vinylP3HTs (Table 2). For these two functional groups,
the Grignard reagent is commercially available which is not the case of
α,ω-pentynylP3HTs (P3 and P3a), in which the terminating Grignard agent,
5-chloromagnesio-1-pentynyl)trimethylsilane, was synthesized by the
following procedure. In a 50 mL two-necked flask were introduced (5-
chloro-1-pentynyl)trimethylsilane (4.97 g, 28.4 mmol), magnesium turnings
(0.972 g, 40 mmol) and THF (30 mL). The mixture was stirred for 24 h at
25 ºC and then added to the end of the P3HT polymerization . The
polymers were then deprotected to obtain α,ω-pentynyl-P3HTs after the
203
purification steps. The polymers were dissolved in THF and then cooled to
-20 ºC. TBAF.3H2O solution (0.20 M) was added slowly to the medium at
this temperature, then the mixture is stirred 4 h at room temperature. The
solution is then passed through a short silica column to remove excess
TBAF. α,ω-pentynylP3HTs were then recovered by precipitation in
methanol and dried under reduced pressure.
2.2.2.2 α ,ω -Diformyl and α ,ω -dihydroxy-P3HTs: In the case of α,ω-diphenylformyl-P3HT (P4 and P4a), the Grignard
reagent was 4-(1,3-dioxan-2-ylphenyl) magnesium bromide and the
deprotection was done by overnight refluxing the polymer (0.70 g, 0.166
mmol) in THF with pyridinium p-toluenesulfonate (PTS) (0.160 g, 0.637
mmol). In the case of α,ω-diethylformyl-P3HT (P4b), the Grignard reagent
was 4-(1,3-dioxan-2-ylethyl) magnesium bromide and the deprotection was
performed successfully by refluxing the polymer (0.05 g) overnight in THF
(20 mL) with concentrated HCl (0.6 mL). For the synthesis of α,ω-
diphenylhydroxy-P3HT (P5), the Grignard reagent was 4-(2-tetrahydro-
2H-pyranoxy)-phenyl magnesium bromide and the complete deprotection
was observed by refluxing the polymer (0.75 g) with conc. HCl (2.0 mL) in
THF (150 mL) for 18 h. All the deprotected α,ω-difunctionalised P3HTs
(Table 2) were again purified by Soxhlet washing with methanol and
extraction with chloroform.
204
Chain-end Functionalised P3HT (ω- or α ,ω-P3HTs)
P3HT
Monomer
M1 [g, (mmol)]
Grignard reagent
t-BuMgCl (1 M in THF)
[mL, (mmol)]
Ni(dppp)Cl2
[g, (mmol)]
Polym.
time (min)
Grignard reagent
used for functionalisation
Mn
(g mol-1, GPC)
Đ
P2 4.50
(13.7)
13.7
(13.7)
0.074
(0.136) 40 Ethynyl-MgBr 14 000 1.1
P2a 4.56
(14.0)
14.0
(14.0)
0.137
(0.252) 60 Ethynyl-MgBr 9 000 1.2
P2b 3.33
(10.2)
10.0
(10.0)
0.095
(0.175) 30 Ethynyl-MgBr 7 700 1.1
P2c 3.0
(9.2)
9.2
(9.2)
0.090
(0.166) 30 Ethynyl-MgBr 8 500 1.1
P2d 1.43
(4.38)
4.3
(4.3)
0.04
(0.073) 60 Ethynyl-MgBr 3 500 1.1
P2e 3.19
(9.78)
9.5
(9.5)
0.09
(0.166) 60 Ethynyl-MgBr 2 500 1.1
P3 5.68
(17.4)
17.4
(17.4)
0.160
(0.295) 60
ClMg(C5H6)Si(Me)3/
TBAF.3H20 8 000 1.1
P3a 1.6
(4.9)
4.8
(4.8)
0.045
(0.083) 30
ClMg(C5H6)Si(Me)3/
TBAF.3H20 6 200 1.1
P4 3.31
(10.15)
10.15
(10.15)
0.0976
(0.18) 40
1,3-dioxan-2-yl)
phenyl-MgBr/PTS 7 000 1.1
P4a 3.22
(9.8)
9.8
(9.8)
0.06
(0.11) 120
(1,3-dioxan-2-yl)
phenyl-MgBr/PTS 14 000 1.1
P4b 3.22
(9.8)
9.8
(9.8)
0.117
(0.215) 30
(1,3-dioxan-2-yl)
ethyl-MgBr/HCl 5 800 1.1
P5 3.0
(9.19)
9.2
(9.2)
0.068
(0.125) 120
4-(2-tetrahydro-2H-
pyranoxy)phenyl-
MgBr/HCl
11 000 1.2
P6 2.09
(6.4)
6.4
(6.4)
0.09
(0.166) 30 Vinyl-MgBr 5 500 1.2
P6a 2.78
(8.52)
8.5
(8.5)
0.078
(0.144) 45 Vinyl-MgBr 7 400 1.1
P6b 3.25
(9.96)
9.9
(9.9)
0.09
(0.166) 120 Vinyl-MgBr 9 000 1.2
Table 2: Reaction conditions, and macromolecular characteristics determined by SEC (THF, UV-254 nm) against polystyrene standards of chain-end functionalized ω-P3HTs or α,ω-P3HTs (P2-P6).
205
2.3 Mono-functionalised P3HTs (P7-P8) by externally added Ni- catalyst initiator 2.3.1 Ni-initiator [(Ph)Ni(PPh3)2-Br]: In a flame-dried Schlenk flask, to a solution of [Ni(PPh3)4] (0.2 g, 0.18
mmol) in dry toluene (3 mL), bromobenzene (0.1 mL, 0.95 mmol) was
added at room temperature under argon atmosphere. Then the
homogeneous mixture was allowed to stir for about 30 min and allowed to
stand for unperturbed overnight. The original deep red colour of the
reaction mixture gradually changed to brownish yellow colour with the
precipitation of [(Ph)Ni(PPh3)2-Br] (4), the yellow crystals of which were
filtered under argon atmosphere and washed with dry pentane (yield:
0.055 g).
2.3.2 Mono-functionalised P3HTs by small molecule Ni-initiator: In a typical polymerization; 2-Bromo-3-hexyl-5-iodothiophene (M2) (0.586
g, 1.57 mmol) was placed in a round-bottomed flask equipped with a
magnet stirrer bar, and the atmosphere was replaced with argon. Dry THF
(30 mL) was added via a syringe, and the mixture was cooled to 0 ºC.
Afterwards, isopropylmagnesium chloride (2.0 M solution in THF, 0.80 mL,
1.57 mmol) was added via a syringe, and the mixture was stirred at 0 ºC for
1 h. A solution of Ni-catalyst initiator (4) in dry toluene (40 mg in 2 mL, 3.50
mol %) was added via a syringe at 0 ºC, and then the mixture was stirred
for 6 h at 0 °C. At the end of the polymerization, the reaction was
quenched with 5 M HCl or a functional Grignard reagent to result in chain-
end capping. We used the protected Grignard reagent (5-chloromagnesio-
1-pentynyl) trimethylsilane with the aim of preparing α-Ph-ω-pentynyl-
P3HT (P7, P7a). The former case, with HCl led to α-Ph-ω-H-P3HT (P8,
P8a, P8b) (Table 3). The reaction conditions leading to the characterised
mono-functionalised P3HTs from P7 to P8b are shown in Table 3.
206
P3HT Monomer
M2
[g, (mmol)]
Grignard reagent tPrMgCl
(2 M in THF) [mL, (mmol)]
Ni-initiator [(Ph)Ni(PPh3)2Br]
[g, (mmol)]
Polymerization time
(h)
Grignard reagent for endcapping
Target structure
P7 1.28
(3.43)
1.74
(3.43)
0.1
(0.135)
6
(0 ˚C)
ClMg(C5H6)Si(Me)3/
TBAF.3H20 α-Ph-ω-
ethynyl-P3HT
P7a 0.58
(1.57)
0.8
(1.57)
0.04
(0.054)
6
(0 ˚C)
ClMg(C5H6)Si(Me)3/
TBAF.3H20 α-Ph-ω-
ethynyl-P3HT
P8 0.50
(1.34)
0.7
(1.34)
0.035
(0.047)
6
(0 ˚C)
5 M HCl
(10 drops) α-Ph-ω-H-
P3HT
P8a 0.60
(1.60)
0.8
(1.60)
0.04
(0.054) 3 (RT)
5 M HCl
(10 drops) α-Ph-ω-H-
P3HT
P8b 0.50
(1.34)
0.7
(1.34)
0.035
(0.047) 3 (RT)
5 M HCl
(10 drops) α-Ph-ω-H-
P3HT Table: 3 Reaction conditions of mono-functionalised-P3HTs (P7-P8) prepared using the small molecule Ni-initiator. 2.4 Azide-terminated Polystyrene 2.4.1 Azide initiator for ATRP: 2.4.1.1 3-Azido-1-propanol: 3-bromo-1-propanol (4) (10 g, 72 mmol),
NaN3 (7 g, 108 mmol), Bu4NI (4 g, 11 mmol) and dicyclohexano-18-crown-
6 (20 mg, 0.07 mmol) were dissolved in 2-butanone (50 mL) and the
mixture was stirred under reflux for 24 h. The mixture was then filtered, the
solid rinsed with acetone and the combined solutions were concentrated.
After distillation, the pure product (5) was obtained as a colorless oil (6 g,
83 %).
Characterization of 3-azido-1-propanol: 1H NMR (400 MHz, CDCl3): δH1.81 (q, 2H, CH2-CH2-CH2), 2.02 (s, 1H, CH2-OH), 3.43 (t, 2H, CH2-N3),
3.72 (t, 2H, CH2-OH); δ C 31.44 (CH2-CH2-CH2), 48.47 (CH2-N3), 59.84
(CH2-OH).
2.4.1.2 3-Azidopropyl-2-bromoisobutyrate: A solution of α-
bromoisobutyryl bromide (11.95 g, 6.43 mL, 52 mmol, 1.05 eq.) in THF (50
mL) was added dropwise to a solution of 5 (5 g, 49.5 mmol) and
triethylamine (6.5 g, 9 mL, 64.4 mmol) in THF (50 mL) at 0 ºC. After
complete addition, the reaction mixture was stirred for 2 h at 25 ºC. Excess
acid bromide was quenched by addition of degassed methanol (50 mL).
The formed triethylammonium bromide salt was filtered off and the solution
207
was concentrated. The crude product was dissolved in CH2Cl2, washed 3
times with a saturated ammonium chloride solution and 3 times with
distillated water. The organic layer was dried over sodium sulfate
(Na2SO4), filtered and concentrated, yielding pale yellow oil, which was
dried under vacuum (9 g, 81%).
Characterization of 3-azidopropyl 2-bromoisobutyrate: 1H NMR (400 MHz, CDCl3): δH1.92 (s, 6H, (CH3)2C), 1.96 (q, 2H, CH2-CH2-CH2), 3.44 (t,
2H, CH2-N3), 4.27 (t, 2H, CH2-O-C(=O)); δC 27.97 (CH2-N3), 30.70 ((CH3)2-
C), 48.03 (CH2-CH2-CH2), 55.66 (C-Br), 62.74 (CH2-O), 171.53 (C=O).
2.4.2 α-Azido-polystyrenes (PS1-PS6): Here is the general procedure for the synthesis of polystyrenes terminated
with azide functional group (PS1-PS6). Experiments were carried out
varying the polymerization time to achieve different molar mass of
polystyrenes, which was showed in the Table 4 below. In a typical
experiment; 3-azidopropyl-2-bromoisobutyrate (6) (0.25 g, 1 mmol), freshly
purified CuBr (0.143 g, 1 mmol), 2,2’-bipyridyl (0.468 g, 3 mmol) and
styrene (5 g, 5.5 mL, 48 mmol) were added to a Schlenk flask. The mixture
was stirred for 5 min and degassed three times by freeze-pump-thaw
cycles to remove residual oxygen. The polymerization reaction was
performed at 130 ºC. The reaction was stopped by dropping the
temperature of the Schlenk flask to 0 ºC. The solution was then dissolved
in THF and passed through a basic alumina column. After being
concentrated, the solution was precipitated in methanol and the polymer
was dried overnight under vacuum and characterized by FTIR and 1H
NMR, SEC and DSC.
208
PS
Initiator
[mg, (mmol)]
CuBr
[mg, (mmol)]
Bipyridine
[mg, (mmol)]
Styrene
[g, (mmol)]
Polymerisation
time (min) Mn
(SEC, g/mol) Đ
PS1 260
(1.04)
148
(1.03)
487
(3.12)
5
(48) 12 2600 1.08
PS2 250
(1.0)
143
(1.0)
468
(3.0)
5
(48) 25 3800 1.17
PS3 250
(1.0)
143
(1.0)
468
(3.0)
5
(48) 10 1900 1.21
PS4 250
(1.0)
143
(1.0)
468
(3.0)
5
(48) 30 4500 1.30
PS5 500
(2.0)
286
(2.0)
936
(6.0)
10
(96) 11 2000 1.11
PS6 260
(1.04)
148
(1.03)
487
(3.12)
5
(48) 30 5200 1.29
Table 4: Reaction conditions and characteristics of the synthesized α-N3-ω-bromo-polystyrenes (PS1-PS6, SEC in THF, UV-254 nm). 2.5 Block copolymers P3HT-block-PS and PS-block-P3HT-block-PS by Click Chemistry 2.5.1 Triblock copolymers PS-b-P3HT-b-PS For the synthesis of triblock copolymers PS-b-P3HT-b-PS, α,ω-
pentynylP3HT of different molecular weights (P3 and P3a) was reacted
with different molecular weight α-azido-polystyrenes (PS1, PS2, PS3 and
PS4, Table 5). In a typical experiment for the synthesis of PS1-b-P3-b-
PS1, α,ω-pentynylP3HT (P3, 250 mg, 0.052 mmol), PS1 (405 mg, 0.156
mmol) and CuI (37 mg, 0.259 mmol) were introduced to a 50 mL round-
bottom flask, evacuated for 10 min and backfilled with nitrogen (3 cycles).
A solution of degassed DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene, 316 mg,
2.08 mmol) in THF (25 mL) was added and the flask was placed in a
constant temperature oil bath at 50 ºC for 5 days. The solution was passed
through a neutral alumina column in order to remove copper salt. After
concentration, the product was recovered by precipitation in methanol,
dried under reduced pressure, and then three times dissolved in a
minimum of THF and precipitated in acetone to remove unreacted PS and
low molar mass P3HT. Further drying under reduced pressure yielded pure
copolymers characterized by SEC, FTIR and 1H NMR.
209
PS-b-P3HT-b-PS P3 or P3a
[mg, (mmol)] Azido-PS
[mg, (mmol)] CuI
[mg, (mmol)]
DBU [mg, (mmol)]
PS1-b-P3-b-PS1 250 (0.052)
405 (0.156)
37 (0.259)
316 (2.08)
PS2-b-P3-b-PS2 250 (0.052)
600 (0.157)
37 (0.259)
316 (2.08)
PS3-b-P3a-b-PS3 125 (0.034)
275 (0.068)
20 (0.105)
210 (1.38)
PS4-b-P3a-b-PS4 70 (0.02)
104 (0.054)
40 (0.210)
210 (1.38)
Table 5: Reaction conditions for the synthesized triblock copolymers, PS-b-P3HT-b-PS. 2.5.2 Diblock copolymers P3HT-b-PS For the synthesis of diblock copolymers P3HT-b-PS; ω-ethynyl-P3HT of
different molecular weights (P2 and P2a) reacted with α-azido-polystyrene
(PS2, Table 6). In a typical experiment for the synthesis of P2-b-PS2;
ω-ethynyl-P3HT (P2, 219 mg, 0.022 mmol), PS2 (280 mg, 0.073 mmol),
CuI (40 mg, 0.210 mmol), DIPEA (diisopropylethylamine, 565 mg, 4.38
mmol) and THF (30 mL) were introduced to a 50 mL round-bottom flask,
evacuated for 10 min and backfilled with nitrogen (3 cycles). Then the
reaction mixture was subjected for sonication (2 h) to aid ethynyl-P3HT
dissolution in THF (clear orange solution) and the flask was placed in a
constant temperature oil bath at 50 ºC for 5 days. The solution was passed
through a neutral alumina column in order to remove copper salt. After
concentration, the product was recovered by precipitation in methanol,
dried under reduced pressure, and then three times dissolved in a
minimum of THF and precipitated in acetone to remove unreacted PS and
low molar mass P3HT. Further overnight drying under reduced pressure
yielded pure copolymers characterized by SEC, FTIR and 1H NMR.
P3HT-b-PS P2 or P2a [mg, (mmol)]
Azido-PS2 [mg, (mmol)]
CuI [mg, (mmol)]
DIPEA [mg, (mmol)]
P2-b-PS2 219 (0.022)
280 (0.073)
40 (0.210)
565 (4.38)
P2a-b-PS2 425 (0.077)
500 (0.131)
100 (0.526)
742 (5.75)
Table 6: Reaction conditions for the synthesized diblock copolymers, P3HT-b-PS.
210
2.6 P3HT-block-PS-C60 and C60-PS-b-P3HT-b-PS-C60 by ATRA The reaction conditions for all the synthesized C60-attached di-and tri-block
copolymers by ATRA were given in the Table 7. In a typical experiment for
the synthesis of P2-b-PS2-C60; C60 (28 mg, 0.039 mmol), CuBr (12 mg,
0.084 mmol), 2,2’-bipyridine (26 mg, 0.017 mmol) and P2-b-PS2-Br (91
mg, 0.0065 mmol) were introduced to a 50 mL round-bottom flask and
dissolved in 20 mL of chlorobenzene (freshly distilled over CaH2). The
mixture was stirred for 5 min and degassed three times by freeze-pump-
thaw cycles to remove residual oxygen. Then the reaction was performed
at 110 ºC for about 24 h. After 24 h, the mixture was dropped into THF
(200 mL) to precipitate unreacted fullerene that was then removed along
with copper salts by passing the solution through a neutral alumina
column. Once precipitated from THF in excess methanol, the polymer was
recovered by filtration, redissolved in THF and again passed through a
fresh column. This procedure was then repeated for three times, following
precipitation in methanol and dried under reduced pressure at 40 ºC for 3
days yielded pure C60-attached copolymers.
C60-attached block copolymer
Block copolymer [mg, (mmol)]
C60 [mg,
(mmol)]
CuBr [mg,
(mmol)]
Bipyridine [mg,
(mmol)]
P2-b-PS2-C60 P2-b-PS2-Br
[91, (0.0065)]
28
(0.039)
12
(0.084)
26
(0.017)
P2a-b-PS2-C60 P2a-b-PS2-Br
[113, (0.012)]
51
(0.071)
22
(0.015)
48
(0.031)
C60-PS1-b-P3-b-PS1-
C60
Br-PS1-b-P3-b-PS1-Br
[150, (0.015)]
65
(0.09)
22
(0.015)
58
(0.037)
C60-PS2-b-P3-b-PS2-
C60
Br-PS2-b-P3-b-PS2-Br
[150, (0.012)]
52
(0.072)
21
(0.014)
47
(0.030)
Table 7: Reaction conditions for the synthesized C60-attached di-and tri-block copolymers by ATRA.
211
2.7 Triblock copolymers P4VP-block-P3HT-block-P4VP by anionic polymerisation The reaction conditions for all the synthesized triblock copolymers P4VP-b-
P3HT-b-P4VP by anionic polymerisation were given in the Table 8.
Preparation of initiator (we prepared three times the desired amount of
initiator): In a 50 mL flame dried Schlenk flask, 10 mL of distilled THF was
cooled at -78 ºC under argon and then freshly distilled α-methylstyrene (0.5
mL, 3.8 mmol) was added. A few drops of sec-butyllithium (1.4 M in
cyclohexane) were added until the persistence of light red color and the
desired amount of sec-butyllithium (2.2 mL, 3.0 mmol) was rapidly injected
to form the initiator. The reaction mixture was stirred for about 15 min and
then kept at -78 ºC which is stable.
Polymerization of 4-vinylpyridine (4-VP) and synthesis of triblock copolymer, P4VP-b-P4-b-P4VP (3): In a three-neck 500 mL flame dried
flask, 200 mL of freshly distilled THF (1 mL HMTP) was cooled at -78 ºC
under argon and 4-vinylpyridine previously purified by two distillations (3.9
mL, 36.2 mmol) was then added. A few drops of initiator were added to the
stirred solution until persistent yellow coloration obtained and then the
required amount of initiator (4 mL of the initiator solution) was immediately
injected. The polymerization was left at -78 ºC for about 30 min. Aldehyde
end-functionalized P3HT (P4) quencher was dried by azeotropic
distillation: P3HT (P4) was dissolved in distilled toluene and evaporated
under reduced pressure three times before it actually dissolved in 10 mL of
distilled toluene. Finally, polymerization of 4-vinylpyridine was quenched by
rapid addition of aldehyde-end functionalized P3HT (P4, 0.1 g, 0.024
mmol) into the reactive medium. The reaction mixture is slowly allowed to
return to room temperature and left it for overnight. The solvents are then
evaporated under vacuum and the obtained polymer was redissolved in 50
mL of chloroform. According to the reported procedure, a large excess of
living anionic P4VP chains were used. To separate the copolymer P4VP-b-
P3HT-b-P4VP (3) from the P4VP homopolymer, the P4VP was protonated
by washing several times (at least 3 times) the organic phase with 100 mL
of HCl/H2O (pH=4). The organic phase was washed three times with
212
distilled water, dried over Na2SO4, filtered and the solvent was evaporated
under reduced pressure to yield pure copolymer P4VP-b-P3HT-b-P4VP
(3).
Copolymer Initiator
[mL, (mmol)] 4-VP
[mL, (mmol)] P3HT (P4)
[mg, (mmol)]
P4VP-b-P4-b-P4VP (1) 10 (4.82)
7.80 (72.38)
200 (0.048)
P4VP-b-P4-b-P4VP (2) 4 (0.95)
1.95 (18.09)
100 (0.024)
P4VP-b-P4-b-P4VP (3) 4 (0.95)
3.90 (36.19)
100 (0.024)
Table 8: Reaction conditions for the synthesized triblock copolymers, P4VP-b-P3HT-b-P4VP by anionic polymerisation.
2.8 Polyacetylene-graft-P3HT (PA-graft-P3HT) 2.8.1 Phenylacetylene polymerisation by [Rh(nbd)Cl]2 catalyst Polymerization was carried out under argon atmosphere in a Schlenk tube
equipped with a three-way stopcock. A typical polymerization procedure is
as follows: A distilled THF solution (8.0 mL) of phenylacetylene (0.465g,
4.56 mmol) was added to a triethylamine (TEA) solution (2.0 mL) of
[Rh[(nbd)Cl]2 (0.02 g, 0.043 mmol) and then polymerization was carried
out at 30 ºC for 24 h. The polymer was precipitated in methanol, filtered
and dried under vacuum to obtain pure polymer.
2.8.2 Polymerisation of ω-alkynyl-P3HTs by [Rh(nbd)Cl]2 catalyst In a typical polymerization of ω-ethynyl-P3HT (P2b); to a 50 mL flame dried
Schlenk flask contained P2b (0.2 g, 0.049 mmol) in a distilled THF solution
(25 mL) was added a triethylamine (TEA) solution (5.0 mL) of [Rh(nbd)Cl]2
(0.01 g, 0.021 mmol) and then polymerization was carried out at 30 ºC for
24 h. The polymer was precipitated in methanol, filtered and dried under
vacuum to obtain pure graft copolymer, PA-graft-P3HT.
213
2.8.3 Attempted copolymerisation of ω-acetylene-P3HT with phenyl acetylene This is a typical copolymerization of ω-ethynyl-P3HT (P2c) with
phenylacetylene (50 %). To a 50 mL flame dried Schlenk flask; P2c (0.4 g,
0.08 mmol), phenylacetylene (0.008 g, 0.08 mmol) and 30 mL distilled THF
were introduced. The reaction mixture was stirred and sonicated for 30
min. Now the catalyst solution [Rh[(nbd)Cl]2 (0.0062 g, 0.0134 mmol) in
TEA (5.0 mL) was injected rapidly to the reaction mixture and then the
polymerization was carried out at 30 ºC for 48 h. The polymer was
precipitated in methanol, filtered and dried under vacuum.
3. Characterization
♦ 1H, 13C and 2D-HMQC NMR spectra were recorded using a Bruker
AC-400 NMR spectrometer (400 MHz). All the samples are analyzed in
CDCl3 solution.
♦ Fourier Transform InfraRed measurements (FTIR) spectra were
performed on a Bruker Tensor 27 spectrometer having a beam
diameter of 0.6 mm, a resolution of 4 cm-1 and a spectral range
between 4000 cm-1 and 400 cm-1. The different samples were
analyzed qualitatively after evaporation of a drop of solution containing
1 g/mL on an ATR cell. The spectra were all corrected by the reference
spectrum.
♦ The relative molecular weights of polymers and copolymers
synthesized were determined by size exclusion chromatography (SEC)
at room temperature in THF. These tests were performed on a system
equipped with a Waters pump type 880-PU. It includes a set of three
columns Tosohaas TSK-gel (styrene-divinylbenzene) in series, a
differential refractometric detector (Varian RI-4) and a UV absorption
detector (Jasco 875, λ = 254 nm). The values of molecular weights
were evaluated against a series of well-defined polystyrenes.
214
♦ MALDI-TOF mass spectra were performed at the Center for Study and
Structural Analysis of Organic Molecules by C. Absalon (CESAMO,
University of Bordeaux 1) on a Voyager mass spectrometer (Applied
Biosystems). The unit is equipped with a pulsed nitrogen laser (λ = 337
nm). The spectra were made in positive ionization mode with
reflectron, and an acceleration voltage of 20 kV. The samples were
dissolved in THF at 5 mg/mL. Dithranol matrix was prepared by
dissolving 10 mg of product in 1 mL of dichloromethane. A cationizing
agent solution (NaI) in methanol (10 mg/mL) was also prepared. These
different solutions were combined in a 10:1:1 ratio
(matrix:sample:cationizing agent). µL of this solution was deposited on
the target and dried under vacuum.
♦ Differential scanning calorimetry (DSC) analyses were performed
using the DSC Q100 (TA Instruments). The samples were subjected to
two heating cycles (50 ºC to 250 ºC) and a cooling cycle. All
measurements were made at a constant speed of 10 ºC/min.
♦ The UV-visible absorption spectra of films were obtained on the
spectral range from 200 nm to 900 nm on two spectrometers: a Varian
Cary 3E and SAFAS UVmc2. The spectra were all corrected by the
reference spectrum. The films were prepared by deposition of
solutions of polymers and copolymers by spin-coating with a SCS
P6700 device on quartz plates. In each series of experiments, the
thickness is kept constant and verified by profilometry (KLA Tencor,
Alpha Step IQ).
♦ Atomic force microscopy (AFM) images were recorded in air with a
Nanoscope IIIa microscope operating in tapping mode (TM). The
probes were commercially available as silicon tips with a spring
constant of 42 N m-1, resonance frequency of 285 kHz, and a typical
radius of curvature in the 10-12 nm range. Both the topography and
the phase signal images were recorded at Centre de Recherche Paul
Pascal (CRPP) by E. Ibarboure (LCPO).
♦ TGA measurements were taken using PERKIN ELMER
Thermogravimetric Analyzer (TGA 7).
215
4. Photovoltaic device fabrication and characterization Organic solar cells were fabricated on indium tin oxide (ITO)
substrates on glass (Merck Display) cleaned by successive ultrasonic
baths in water, acetone, ethanol and isopropanol and then treated with UV-
ozone. The substrates were then covered by a 40 nm thick layer of
PEDOT-blend-PSS deposited by spin-coating at 4000 rpm for 1 min and
then annealed at 110 ºC under rotary pump vacuum for 1 h. The P3HT-
blend-PCBM-blend- (%copolymer) was deposited by spin coating at 1200
rpm for 90 s from anhydrous chlorobenzene solution. A 1:1 P3HT-blend-
PCBM weight ratio was used throughout all experiments and (0-5 %) of
copolymer added to P3HT-blend-PCBM. Film thicknesses were measured
using an Alpha-step IQ profilometer and all found to be ca 100 nm. The
aluminium cathode was thermally evaporated under a secondary vacuum
(10-6 mbar) through a shadow mask. The active surface area of the device
was 10 mm2. The annealing process was carried out under an inert
atmosphere by placing the cells directly onto a controlled hot plate.
The Current-voltage curves and conversion efficiencies of the solar
cells were recorded using a Keithley 4200 SCS, under an illumination of
100 mW/cm2 from a KHS Solar Celltest 575 solar simulator with an AM1.5
G filter. The luminance intensity was checked against an IL1400
radiometer.
216
217
General Conclusions
218
219
General Conclusions The objective of this research work was to develop a simplified and
versatile synthesis of novel block copolymers (BCPs) based on poly(3-
hexylthiophene) (P3HT), to understand the microstructure of the resulting
functional BCPs, and to examine the use of these materials as active
layers or compatibilizer in organic solar cells (OSCs). These materials
were chosen for their ability to self-organize and thus to form structures
that can optimize certain physical parameters in the photovoltaic process,
such as the dissociation of excitons or transportation of charges to the
electrodes.
We have prepared two types of BCPs based on P3HT: (i)
polyacetylene-graft-P3HT (PA-g-P3HT) graft copolymers using
macromonomer alkynyl-P3HTs; and (ii) donor-acceptor rod-coil di- and tri-
BCPs in which P3HT was chosen as the donor block (rod), and
polystyrene (PS) or poly(4-vinylpyridine) (P4VP) were chosen as coil
blocks to carry the acceptor C60 in different ways. The first stage of the
thesis was to develop a simple and versatile synthesis of such copolymers,
which could meet the requirements of a potential industrialization. The
second stage of the thesis was to utilize these BCPs in different
proportions as compatibilizers in P3HT-blend-PCBM devices to enhance
the photo conversion efficiency (PCE).
In the first stage, we synthesized high molecular weight
regioregular P3HTs and also highly regioregular (above 95%) chain end-
functionalised P3HTs with narrow dispersities by the GRIM method. A
“small molecule” Ni-initiator was also synthesized and utilized to prepare
completely mono-functionalised P3HTs. But we could not reproduce
Senkovskyy et al.’s results. We obtained a mixture of products when we
used the so-called “external” initiator, whereas the GRIM method produced
better results. We were somewhat more successful in our attempts to
prepare P3HT grafted copolymers by alkynyl-P3HTs. We found that
conjugation and steric hindrance play a key role for the polymerization of
alkynyl-P3HTs by Rhodium based catalyst and also in the polymerization
220
of ω-vinyl-P3HTs by RAFT and olefin polymerization methods. For the
efficient polymerization of P3HT-substituted acetylenes, the acetylene
group should be directly attached to the aromatic group; however, it is
probable that a spacer is necessary to separate the bulk of the aromatic
acetylene group from P3HT due to conjugation and steric hindrance.
Therefore further investigations are required.
The synthesis of donor-acceptor block copolymers based on P3HT,
PS and P4VP by two different approaches was successful. Azide
terminated polystyrenes of different molecular weights were successfully
synthesized by atom transfer radical polymerization (ATRP). In the first
approach, we synthesized the di- and tri-block copolymers, P3HT-b-PS
and PS-b-P3HT-b-PS by "click" chemistry of polystyrene terminated azide
and P3HT alkyne in the presence of copper catalysts. This study
represents the modification of the reported literature by our group, which is
the first example of synthesis of exclusively rod-coil block copolymers, by
"click" chemistry. However, the influence of the conjugated chain of P3HT
on the alkyne function is very important and necessary to introduce a
separation between the two entities to achieve efficient coupling reaction
or by varying the “click” chemistry conditions with the help of sonication,
one can achieve the expected copolymers. The C60-attached copolymers
(P3HT-b-PS-C60 and C60-PS-b-P3HT-b-PS-C60) were obtained by atom
transfer radical addition (ATRA) of bromine terminated PSs. This reaction
was performed by reacting the copolymers P3HT-b-PS and PS-b-P3HT-b-
PS with C60 in the presence of CuBr/bipyridine in chlorobenzene.
In the second approach; the triblock copolymers, P4VP-b-P3HT-b-
P4VP which contained the donor P3HT blocks and acceptor domains
P4VP coil blocks were successfully synthesized via anionic polymerization.
The molecular weight of these polymers was identified by 1H NMR only
since the solubility of these triblock copolymers was not high in THF and
DMF (the solvents used in our SECs). All these copolymers were then
characterized by UV-visible absorption spectroscopy and differential
scanning calorimetry, to assess their physical properties. These measures
221
have enabled to determine their characteristic temperatures (glass
transition, melting, crystallization), very important elements with respect to
their potential application in OSCs. Finally the synthesized di- and tri-BCPs P2-b-PS2, P2-b-PS2-C60,
C60-PS2-b-P3-b-PS2-C60 and P4VP-b-P3HT-b-P4VP were used as
compatibilizers in a mixture of P1a-blend-PCBM active layer for OSCs.
The first study involved the optimization of P3HT mixed with PCBM, for
which a maximum efficiency was obtained for P1a, i.e. a higher PCE was
found that with the commercial sample (Plextronics) of a similar molecular
weight, and when annealing at 180 ºC. The BCPs were then tested as
compatibilizers (0-5 %) in combination with P1a and PCBM (1:1) based
devices. The device based on P1a-blend-PCBM-blend-(P2-b-PS2) using
1% addition of copolymer achieved the highest short-circuit current (11
mA/cm2) and also highest PCE (3.7%) at an annealing temperature of 167 ºC in our solar cell studies. At 5% addition of this copolymer P2-b-PS2, the
structure was completely disorganized and tangled fibrils random
structures lead to a substantial decrease in the Jsc, and explains the drop
in performance for the mixtures of 1.5%, 2.0% and 5.0%. This is explained
by taking into account the degree of crystallinity of this copolymer, which is
superior to others, reflecting its ability to facilitate better mobility of charge
carriers in the active layer.
The donor-acceptor diblock copolymer, P2-b-PS2-C60, was
characterized in OSC as compatibilizers by adding 0-5% amounts to P1a-
blend-PCBM (1:1) based devices. Surprisingly it was shown that the
addition of copolymer (0.5%) to P1a-blend-PCBM (0%) at 180 ºC annealing
significantly changes the photovoltaic parameters involved especially PCE,
(from 3.0-4.0%), fill factor with maximum value of 0.65, but the maximum
short-circuit current, 12.0 mA/cm2 was observed at 0.5% addition of
copolymer. In this case, we have observed an excellent fibrilar
morphology. The 5% weight addition of copolymer disrupts the fibrillar
structure of the P3HT chains (height images of Figure 4.15) which lead to
reduced fill factor and reduced energy conversion efficiencies.
222
In the case of triblock copolymers, the devices based on P1a-blend-
PCBM-blend-(C60-PS2-b-P3-b-PS2-C60) (0-7 %) (annealed, 180 ºC)
showed decreases in the Jsc with addition of copolymers whereas
unannealed devices showed Jsc values significantly increased with 0.5%
copolymer, but started decreasing on further addition of copolymer to
reach a low value of 1.0% at 7% copolymer. Nevertheless, with 0.5%
copolymer the efficiency nearly doubled from 0.67 to 1.27%. The devices
based on P1a-blend-PCBM-blend-(P4VP-b-P4-b-P4VP), in which the
copolymer was used as a compatibilizer (0, 0.5% and 1.0%) showed
decreases in Jsc with the addition of copolymer for annealed devices. We
have also used the copolymer, P4VP-b-P4-b-P4VP as donor material in
the device, (P4VP-b-P4-b-P4VP):PCBM (1:1) at room temperatures and at
annealed temperatures 167 ºC and 175 ºC, but only low PCEs were found.
Hence we have observed that the addition of triblock copolymers as
compatibilizer disrupts the molecular structure of P3HT chains resulting in
low efficiencies. But in the case of diblock copolymers as compatiblizers,
we have observed the enhancement of Jscs and efficiencies with respect to
P3HT-blend-PCBM device alone. This might be due to the nano-domain
constraints placed upon such systems by tri-block copolymers, and would
tend to indicate that di-block copolymers, under certain circumstances,
would be better for use in OSCs than tri-block materials.
This research work of thesis has shown the potential application of
rod-coil and donor-acceptor BCPs as a compatibilizer in the field of OSCs
and opens broad prospects for the future. First, concerning the synthetic
chemistry, a simplified and versatile synthesis of rod-coil block copolymers
based on P3HT conjugated block was developed by the well-known GRIM
method. This method can be applied to obtain BCPs with various coil
blocks. Moreover, the concept of compatibilizing blends donor-acceptor
seems to be very promising, especially with diblock copolymers. To
improve the best current performance in triblock copolymers, it seems that
we should synthesize a copolymer of high molecular weight P3HT in order
to structure the active layer blend P3HT:PCBM.
223
Appendix
224
225
Table 1: Glass transition temperature of all synthesized polystyrenes (PS1-PS6).
Polystyrene-N3
(PS)
Mn (SEC,
g mol-1)
Glass transition
Temperature, Tg (ºC)
PS1 2600 87
PS2 3800 88
PS3 1900 84
PS4 4500 89
PS5 2000 84
PS6 5200 93 (a)
(b)
Figure1: Representative DSC curves of Polystyrene-N3 (PS-N3) showing glass transition temperature, Tg (ºC) (a) PS1 and (b) PS2.
226
227
Publications and Conferences
Publications :
1. Poly(3-hexylthiophene) Based Block Copolymers Prepared by “Click” Chemistry Urien, M.; Erothu H; Cloutet, E.; Hiorns, R. C.;
Vignau, L.; Cramail, H. Macromolecules 2008, 41, 7033-7040. 2. Poly(3-hexylthiophene) Based Donor-acceptor Block Copolymers
for Photovoltaics. Erothu H; Urien, M; Mafoudh, R; Cloutet, E.; Hiorns,
R. C.; Vignau, L.; Cramail, H. (manuscript in preparation) 3. Photovoltaic characterization of multiblock copolymers containing
polythiophenes and polyfullerenes. Hiorns, R. C.; Erothu H; Habiba,
B.; Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. (manuscript in
preparation)
4. Block Copolymers Based on Poly(3-hexylthiophene) and Poly(4-vinylpyridine) by anionic polymerisation for Photovoltaics.
Erothu H; Mafoudh, R; Brochon, C; Cloutet, E.; Hiorns, R. C.; Vignau,
L.; Cramail, H. (manuscript in preparation)
5. Photovoltaic characterization of novel Polyacetylene-grafted-Poly(3-hexylthiophene) Based Block Copolymers. Erothu H; Urien,
M, Cloutet, E.; Hiorns, R. C.; Vignau, L.; Cramail, H. (manuscript in
preparation)
Conferences (oral and poster presentations) : 1. Synthesis and Photovoltaic application of Block copolymers Based
on Poly(3-hexylthiophene) and Polystyrene (poster presentation)
Harikrishna Erothu, M. Urien, R. Mafoudh, Roger C. Hiorns, Eric
Cloutet, Laurence vigneau and Henri Cramail, MACRO 2010, 43rd
IUPAC World Polymer Congress, 11-16 July 2010, SECC, Glasgow,
UK.
2. Synthesis of Poly(3-hexylthiophene) grafted Polyacetylene and their Photovoltaic Characteristics (poster presentation)
Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail
MNPC 2009, 19-23 October 2009, Arcachon, France.
228
3. Synthesis of Poly(3-hexylthiophene) Based Copolymers for Organic Electronics (oral presentation)
Harikrishna Erothu, M. Urien, Roger C. Hiorns, Eric Cloutet and Henri
Cramail, Journées GFP Sud-Ouest, 5-6 February 2009, Eauze, Gers,
France.
4. Synthesis of polyacetylene-grafted-poly(3-hexylthiophene) for organic photovoltaic cells (poster presentation)
Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail
GFP Polymeres & Photovoltaics,14-15 octobre 2008, ENSCPB, Pessac,
France.
5. Synthesis of polyacetylene-grafted-poly(3-hexylthiophene) for organic photovoltaic cells (poster presentation)
Harikrishna Erothu, Roger C. Hiorns, Eric Cloutet and Henri Cramail,
XXIII International Conference on Organometallic Chemistry ICOMC
2008, July 13-18, Rennes, France.
Recommended