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Synthetic Metals 161 (2011) 856–863
Contents lists available at ScienceDirect
Synthetic Metals
journa l homepage: www.e lsev ier .com/ locate /synmet
and-gap tuning of pendant polymers for organic light-emitting devices andhotovoltaic applications
khil Guptaa,∗, Scott E. Watkinsa, Andrew D. Scullya, Th. Birendra Singha, Gerard J. Wilsona,ynn J. Rozanskib, Richard A. Evansa
CSIRO Materials Science and Engineering, CSIRO Future Manufacturing Flagship, Bag 10, Clayton South, Victoria 3169, AustraliaCSIRO Energy Technology, P.O. Box 330, Newcastle, NSW 2300, Australia
r t i c l e i n f o
rticle history:eceived 23 September 2010eceived in revised form 20 January 2011ccepted 11 February 2011vailable online 8 March 2011
eywords:
a b s t r a c t
The preparation of a series of novel polymers comprising pendant electro-active “push–pull” chro-mophores and their performance in solution-processed organic electronic devices is described. The designof the electro-active pendant chromophores was based on the well-known motif of cyano-substitutedpoly(p-phenylenevinylene). Optical band-gap engineering within this series of polymers was achievedby varying the conjugation length and the electron donor/acceptor functionalities of the pendant chro-mophores. The introduction of a cyanoimine group into the electro-active pendant module resulted
rganic electronic devicesolymer solar cellsendant polymersulk heterojunctionyanoimine
in a marked narrowing of the optical band-gap compared with the other electro-active pendant chro-mophores investigated in this work. Bulk heterojunction solar cell devices comprising these polymerswere prepared by solution processing blends of each polymer with [6,6]-phenyl-C61-butyric acid methylester, and their performance was evaluated by measuring power conversion efficiencies. The best-performing solar cell in this series exhibited a power conversion efficiency of 0.29% and a maximum
nt cophor
rganic light emitting diodeslectroactive
incident photon-to-currethe electro-active chromo
. Introduction
Organic photovoltaic (OPV) solar cells have attracted a greateal of attention as a promising alternative to silicon-based solarells in view of their potential for fabrication of low-cost andexible devices [1–3]. Much effort has been expended in the syn-hesis of novel materials and their application in OPV devices [4–6],ut energy conversion efficiencies are not yet sufficiently high forommercial application. Bulk heterojunction (BHJ) solar cells caneliver a maximum power conversion efficiency (PCE) from solar
ight into electrical energy of up to about 7.5% [7–14], and solution-rocessed tandem solar cells, which are complex device structures
n which two individual sub-cells are stacked and usually connectedn series, having higher PCE values than their component sub-cells,ave also been reported [15].
Currently, the favoured materials for BHJ solar cells are blendsf n-type semi-conducting fullerenes such as [6,6]-phenyl-C61-utyric acid methyl ester (PCBM) with p-type semi-conductingolymers based on poly(3-hexylthiophene) (P3HT). However,
∗ Corresponding author at: CSIRO/Monsash University, Department of Chemistry,aterials Science and Engineering, Private Bag 10, Bayview Avenue, Melbourne,
ictoria, Australia. Tel.: +61 3 9545 2196.E-mail address: [email protected] (A. Gupta).
379-6779/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.synthmet.2011.02.013
nversion efficiency of 22% and was produced using the polymer in whiche comprised the cyanoimine group.
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
polymers based on 2,7-carbazole, fluorene, thiophene, cyclopen-tadithiophene, and dithienylbenzothiadiazole electro-active units,as well as metallated polyynes, have also been considered asalternative p-type electron-donor materials for construction ofhigh performance polymer solar cells [16]. These p-type semi-conducting polymeric materials have been shown [16] to becapable of producing solar cells having efficiencies of comparablemagnitude with that of amorphous silicon solar cells (normally6–8%). These composites display good charge-carrier mobility,but improvements in band gap and absorption coefficients arehighly desirable. The promising performance of these materials isstimulating research efforts in the development of new electron-donating/hole-transporting polymeric materials that can deliverimproved efficiencies, open-circuit voltages, and short-circuit cur-rents. Indeed, it has been shown [17,18] that, in principle, up to 10%and 15% efficiencies can be achieved for single-junction and tan-dem solar cells, respectively, through judicious molecular designof materials to optimise electronic properties.
Polymers that contain pendant electro-active units [19] andthat can be prepared via the use of living radical polymerization
methodologies [20,21] are an attractive alternative to conjugatedpolymers because they offer the potential benefits of ease of synthe-sis, solution processability, and access to exotic architectures (suchas block copolymers). In addition, the use of block co-polymersraises the possibility of preparing single component (hole andghts reserved.
A. Gupta et al. / Synthetic Metals 161 (2011) 856–863 857
ally p
ewofc(sCp
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Asldeatrf6aMiTo
2
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Scheme 1. Design origin of radic
lectron transporting) materials for use in OPVs. This approachould circumvent problems currently seen with phase separation
f two component blends having a marked influence on device per-ormance [22]. Poly(p-phenylenevinylene)s (PPVs) and analoguesomprising a cyano group added to a dialkoxy derivative of PPVCN-PPV) are good charge-transporting materials and have beentudied widely [2]. We have utilised structural elements from theN-PPV motif and transformed them into a radically polymerizableendant form, as illustrated in Scheme 1.
Herein, we report the synthesis, reactivity, and effectiveness ofvariety of CN-PPV inspired pendant polymers (Fig. 1) in BHJ OPV
ells and organic light-emitting devices (OLED).
. Experimental
.1. Materials and chemical analysis
All chemicals used in reactions were used as received fromldrich. The solvents used for reactions were obtained from Merckpeciality chemicals (Sydney, Australia) and were used after distil-ation. Benzene used for polymer syntheses was distilled twice andried over activated molecular sieves (4 A) overnight. Unless oth-rwise specified, all 1H and 13C NMR spectra were recorded usingBruker Av400 spectrometer at 400 MHz and 100.6 MHz, respec-
ively, or a Bruker Av200 spectrometer at 200 MHz and 50 MHz,espectively. Chemical shifts (ı) are measured in ppm. TLC was per-ormed using 0.25 mm thick plates pre-coated with Merck Kieselgel0 F254 silica gel, and visualized using ultraviolet (UV) light (254 nmnd 366 nm). Melting points were measured using a GallenkampPD350 digital melting point apparatus and are uncorrected. Pos-
tive ion electron impact (EI) mass spectra were measured using ahermoQuest MAT95XL mass spectrometer using ionisation energy
f 70 eV..1.1. Synthesis of monomers and polymersThe corresponding monomers for polymers 1, 2 and 3
ere prepared by condensing the appropriate benzaldehyde
Fig. 1. Chemical structures of the pendant
olymerizable CN-PPV fragments.
derivatives with p-cyanomethylstyrene (CNMS), as shown inFig. 2.
Monomers comprising less active electron-donating groups(e.g. single methoxy group or no substituent at all), or with apara-substituted electron-withdrawing group (such as cyano ortrifluoromethyl), were also prepared but they did not undergofree radical polymerization. The monomer for polymer 4 was pre-pared by condensing CNMS with p-nitrosobromobenzene and thenreacting the product with di-p-tolylamine (Fig. 2). The monomerswere polymerized via uncontrolled free-radical polymerization.The detailed synthetic procedures and characterisation data ofintermediates, monomers, and pendant polymers are describedbelow.
2.1.2. Synthesis of cyanomethylstyrene (CNMS)A mixture of chloromethylstyrene (ClMS) (5.00 g, 32.79 mmol)
in acetonitrile (10 ml) was added to the mixture of potassiumcyanide (3.19 g, 49.18 mmol), 18-crown-6 (0.53 g, 2.00 mmol), andacetonitrile (20 ml). The resulting final reaction mixture was stirredovernight at room temperature (RT). Thin layer chromatography(TLC) (hexane: ethyl acetate: 8:2) indicated that the reaction wascomplete. The product was extracted with ethyl acetate. Organiclayer was separated, washed with water twice and finally withbrine solution. The organic layer was dried on anhydrous sodiumsulphate and concentrated in vacuo to obtain 4.20 g (29.37 mmol,89.50%) of the light brown CNMS oil which was used without fur-ther purification.
1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.41 (m, 2H); 7.27 (m, 2H);6.70 (m, 1H); 5.74 (m, 1H); 5.28 (m, 1H); 3.73 (s, 2H); 13C NMR(200 MHz, CDCl3, 25 ◦C): ı 23.22, 114.62, 117.99, 126.84, 128.15,129.48, 135.97, 137.33.
2.1.3. Synthesis of 5A mixture of CNMS (2.00 g, 13.99 mmol) and 2,5-
dimethoxybenzaldehyde 10 (2.32 g, 13.99 mmol) was taken in20 ml methanol under stirring. Pyrrolidine (1.75 ml, 21.00 mmol)was added to the mixture at RT and the resulting mixture was
polymers investigated in this work.
858 A. Gupta et al. / Synthetic Metals 161 (2011) 856–863
ClMS
KCN/ACN
18-crown-6
CNMS
+
CHO
R1
R2
R3
MeOH/Reflux
Pyrrolidine NC
R3
R1
R2
AIBN (2% molar)
BenzeneNC
R3
R1
R2
**
CNMS
+
NO
Br
DBU
Toluene
4-BromonitrosobenzeneNC
NNC
Br8
+ NH
Di-p-tolylamine
[Pd2(dba)3]
Sodium t-butoxide
(t-Bu)3PBF4-
NNC
N
AIBN (2% molar)
Benzene
NNC
N
**
R1 R2 R3 Aldehyde Monomer Polymer
-OCH3 -OCH3 -H 10 5 11112
10-12Cl NC
94
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hwbes
71C11f
2
(2whrmm(fl
7411[
2
(t2mt
-H -H -N(C2H5)2-H -H -N(C6H5)2
Fig. 2. Reaction schemes for sy
eated to reflux overnight. After being cooled to RT, the solventas removed under vacuum and the crude product was purified
y silica-gel chromatography with hexane/ethyl acetate (8:2) asluent to obtain 1.80 g (6.18 mmol, 44.20%) of 5 as a cream-yellowolid which was fluorescent when irradiated with 365 nm UV light.
1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.96 (s, 1H); 7.78 (m, 1H);.65 (m, 2H); 7.47 (m, 2H); 6.96 (m, 1H); 6.87 (m, 1H); 6.71 (m,H); 5.80 (m, 1H); 5.31 (m, 1H); 3.85 (s, 6H)); 13C NMR (400 MHz,DCl3, 25 ◦C): ı 55.54, 55.67, 98.32, 105.03, 108.19, 114.68, 116.21,18.78, 125.93, 126.68, 129.53, 134.54, 136.05, 136.38, 137.76,59.49, 163.04; MS (EI) calcd for C19H17NO2 [M]+ (m/z) 291.34,ound 291.10.
.1.4. Synthesis of 6A mixture of CNMS (1.50 g, 10.49 mmol) and 4-
diethylamino)benzaldehyde 11 (1.86 g, 10.49 mmol) was taken in0 ml methanol under stirring. Pyrrolidine (1.75 ml, 15.73 mmol)as added to the mixture at RT and the resulting mixture waseated to reflux for 5 h. After being cooled to RT, the solvent wasemoved under vacuum and crude solid was crystallized withethanol, separated by vacuum filtration and washed with coldethanol. The solid was dried under vacuum to obtain 2.00 g
6.62 mmol, 62.10%) of 6 as a golden-coloured material which wasuorescent when irradiated with 365 nm UV light.
1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.83 (m, 2H); 7.58 (m, 2H);.39 (m, 3H); 6.76–6.68 (m, 3H); 5.76 (m, 1H); 5.27 (m, 1H); 3.40 (q,H); 1.20 (t, 6H); 13C NMR (200 MHz, CDCl3, 25 ◦C): ı 12.62, 44.54,03.28, 111.14, 114.30, 119.55, 120.85, 125.42, 126.69, 131.68,35.04, 136.12, 137.09, 142.05, 149.37; MS (EI) calcd for C21H22N2M]+ (m/z) 302.41, found 302.20.
.1.5. Synthesis of 7A mixture of CNMS (2.20 g, 15.38 mmol) and 4-
diphenylamino)benzaldehyde 12 (4.20 g, 15.38 mmol) wasaken in 20 ml methanol under stirring. Pyrrolidine (1.90 ml,3.07 mmol) was added to the mixture at RT and the resultingixture was heated at reflux overnight. After being cooled to RT,
he solvent was removed under vacuum and the crude product
6 27 3
is of monomers and polymers.
was purified by silica-gel chromatography with hexane/ethylacetate (8:2) as eluent. The clean fractions containing the requiredproduct were combined and evaporated under vacuum to obtain2.00 g (5.02 mmol, 32.60%) of 7 as a dark yellow solid which wasbrightly fluorescent when irradiated with 365 nm UV light.
1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.77 (m, 2H); 7.60 (m, 2H);7.40 (m, 3H); 7.30 (m, 4H); 7.17–7.04 (m, 8H); 6.70 (m, 1H); 5.78(m, 1H); 5.30 (m, 1H); 13C NMR (400 MHz, CDCl3, 25 ◦C): ı 107.39,114.81, 118.67, 120.90, 124.38, 125.70, 125.79, 126.45, 126.76,129.56, 130.66, 134.34, 135.97, 137.86, 141.19, 146.61, 149.95; MS(EI) calcd for C29H22N2 [M]+ (m/z) 398.50, found 398.20.
2.1.6. Synthesis of 8A mixture of CNMS (6.20 g, 43.14 mmol) and freshly sub-
limed 4-bromonitrosobenzene (6.98 g, 37.52 mmol) was taken in50 ml toluene under stirring. 1,8-diazabicycloundec-7-ene (DBU)(7.30 ml, 48.77 mmol) was added slowly to the mixture at RT andthe resulting mixture was stirred at RT overnight. The solvent wasremoved under vacuum and the crude product was purified bysilica-gel chromatography with petroleum ether (60–80 ◦C)/ethylacetate (9:1) as eluent. The clean fractions containing the requiredproduct were combined and evaporated under vacuum to obtain4.05 g (13.02 mmol, 34.70%) of 8 as a shiny dark yellow powder.
1H NMR (200 MHz, CDCl3, 25 ◦C): ı = 8.08 (m, 2H); 7.55 (m, 4H);7.07 (m, 2H); 6.72 (m, 1H); 5.88 (m, 1H); 5.42 (m, 1H).
2.1.7. Synthesis of 9Di-p-tolylamine (2.56 g, 13.02 mmol) was taken in 30 ml toluene
under stirring and tris(dibenzylideneacetone)dipalladium(0)[Pd2(dba)3] (0.19 g, 0.21 mmol) was added followed bythe addition of sodium-t-butoxide (2.18 g, 19.53 mmol)and the resulting mixture was stirred for 30 min. N-(4-bromophenyl)-4-vinylbenzimidoyl cyanide 8 (4.05 g, 13.02 mmol)
was added to the mixture followed by addition of tri-t-butylphosphoniumtetrafluoroborate (0.13 g, 0.45 mmol) andthe resulting mixture was stirred overnight at RT. The solvent wasremoved under vacuum and the crude product was purified bysilica-gel chromatography with hexane/dichloromethane (8:2)
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A. Gupta et al. / Syntheti
s eluent. The clean fractions containing the required productere combined and evaporated under vacuum to obtain 2.30 g
5.39 mmol, 41.40%) of 9 as a brick red powder.1H NMR (400 MHz, acetone-d6, 25 ◦C): ı = 8.07 (m, 2H); 7.69 (m,
H); 7.33 (m, 2H); 7.17 (m, 4H); 7.01 (m, 6H); 6.84 (m, 1H); 5.99 (m,H); 5.41 (m, 1H); 2.31 (s, 6H)); 13C NMR (400 MHz, CDCl3, 25 ◦C):20.89, 112.14, 116.42, 121.15, 123.26, 125.37, 126.65, 127.99,
30.09, 133.57, 133.93, 134.19, 135.92, 141.12, 141.19, 144.56,48.54; MS (APCI) calcd for C30H25N3 (m/z) 428.50, found 428.40M+H]+.
.1.8. Synthesis of polymer 1A mixture of monomer 5 (3.00 g, 10.31 mmol) and azobisisobu-
yronitrile (AIBN) (0.03 g, 0.21 mmol) was taken in a polymerizationask in 10 ml benzene under stirring and the resulting solution was
reezed-thaw-degassed under vacuum 4 times. The mixture waseated at 68–72 ◦C (oil bath temperature) with overnight stirring.he solvent was removed under vacuum and the soluble impu-ities were removed by silica-gel chromatography of the crudeolymeric product with hexane/ethyl acetate (7:3) as eluent. Theure polymer was then extracted from the silica-gel support usingichloromethane and the solvent was evaporated under vacuum tobtain 0.38 g (12.67%) of 1 as a dark yellow solid.
1H NMR (200 MHz, CD2Cl2, 25 ◦C): ı = 5.60–8.50 (bd, 8H);.10–4.20 (bd, 6H); 0.50–2.10 (bd, 3H).
.1.9. Synthesis of polymer 2A mixture of monomer 6 (0.75 g, 2.48 mmol) and azobisisobuty-
onitrile (AIBN) (0.01 g, 0.05 mmol) was taken in a polymerizationask in 10 ml benzene under stirring and the resulting solution was
reezed-thaw-degassed under vacuum 4 times. The mixture waseated at 68–72 ◦C (oil bath temperature) with overnight stirring.he solvent was removed under vacuum and the soluble impu-ities were removed by silica-gel chromatography of the crudeolymeric product with hexane/ethyl acetate (7:3) as eluent. Theure polymer was then extracted from the silica-gel support usingichloromethane and the solvent was evaporated under vacuum tobtain 0.15 g (20.00%) of 2 as a yellowish brown solid.
1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.00–8.00 (bd, 5H);.00–7.00 (bd, 4H); 3.08–3.56 (bd, 4H); 1.35–2.20 (bd, 3H);.90–1.30 (bd, 6H).
.1.10. Synthesis of polymer 3Polymer 3 was prepared from its monomer 7 using the method
escribed for 2. Golden yellow fluorescent solid. Yield: 23.30%1H NMR (200 MHz, CD2Cl2, 25 ◦C): ı = 6.50–8.00 (bd, 19H);
.50–2.20 (bd, 3H).
.1.11. Synthesis of polymer 4Polymer 4 was prepared from its monomer 9 using the method
escribed for 2. Brick red solid. Yield: 31.80%1H NMR (400 MHz, CDCl3, 25 ◦C): ı = 7.50–8.10 (bd, 2H);
.25–7.25 (bd, 16H); 2.16–2.38 (bd, 6H); 1.30–2.10 (bd, 3H).
.2. Characterisation of polymers
.2.1. Molecular weightMolecular weights of polymers were characterised by gel per-
eation chromatography (GPC) performed in tetrahydrofuranTHF, 1.0 ml/min) at 30 ◦C using a waters GPC instrument, with a
aters 2414 refractive index detector, a series of four polymer lab-ratories PLGel columns (3 �m × 5 �m mixed-C and 1 �m × 3 �mixed-E), and empower pro software. The GPC was calibratedith narrow polydispersity polystyrene standards (polymer lab-
ratories EasiCal PS-1, MW from 264 to 7,500,000—Batch No.
ls 161 (2011) 856–863 859
PS-1-43), and molecular weights are reported as polystyrene equiv-alents.
2.2.2. Spectroscopic and thermal analysisUltraviolet–Visible (UV–Vis) absorption spectra were recorded
using a Hewlett Packard HP 8453 diode array spectrometer. Flu-orescence spectra were measured using a Perkin Elmer LS50Bfluorimeter. Thermal gravimetric analysis (TGA) was carried outusing a Mettler Toledo TGA/SDTA851, and differential scanningcalorimetry (DSC) was performed using a Mettler Toledo DSC821.Photoelectron Spectroscopy in Air (PESA) measurements wererecorded using a Riken Keiki AC-2 PESA spectrometer with a powersetting of 5 nW and a power number of 0.5. Samples for PESA wereprepared on glass substrates.
2.2.3. Electrochemical characterisationElectrochemical measurements were carried out using a power
lab ML160 potentiostat interfaced via a power lab 4/20 controllerto a PC running e-chem for windows Ver. 1.5.2. The measurementswere run in argon-purged dichloromethane with tetrabutylammo-nium hexafluorophosphate (0.2 M) as the supporting electrolyte.The cyclic voltammograms were recorded using a standard 3 elec-trode configuration with a glassy carbon (2 mm diameter) workingelectrode, a platinum wire counter electrode and a silver wirepseudo reference electrode. The silver wire was cleaned in con-centrated nitric acid and then in concentrated hydrochloric acidto generate the Ag/Ag+ reference. Voltammograms were recordedwith a sweep rate of 50–200 mV/s. Polymer samples were coatedonto the working electrode by drop casting solutions of the poly-mer in chloroform. All the potentials were referenced to E1/2 of theferrocene/ferrocenium couple.
2.3. Device fabrication and characterization
UV/ozone cleaning of glass substrates was performed using aNovascan PDS-UVT, UV/ozone cleaner with the platform set tomaximum height. The intensity of the lamp was greater than36 mW/cm2 at a distance of 100 cm. At ambient conditions theozone output of the UV cleaner is greater than 50 ppm. Indium tinoxide (ITO)-coated glass (Kintek, 15 �/�) was cleaned by stand-ing in a stirred solution of 5% (v/v) deconex 12PA detergent at90 ◦C for 20 min. The ITO-coated glass was then successively soni-cated for 10 min each in distilled water, acetone and isopropanol.The substrates were then exposed to a UV–ozone clean at roomtemperature for 10 min.
Aqueous solutions of PEDOT/PSS (HC Starck, Baytron P AI 4083)were filtered (0.2 �m RC filter) and deposited onto glass substratesin air by spin coating (Laurell WS-400B-6NPP lite single wafer spinprocessor) at 5000 rpm for 60 s to give a layer having a thickness of38 nm. The PEDOT/PSS layer was then annealed on a hotplate in aglovebox at 145 ◦C for 10 min.
For OPV devices, 5 mg of the polymers and 20 mg of PCBM(Nano-C) were dissolved in 1 ml of chlorobenzene (Aldrich, anhy-drous) in individual vials with stirring. The solutions were thencombined, filtered (0.2 �m RC filter) and deposited by spin coating(SCS G3P spin coater) onto the ITO-coated glass substrates inside aglovebox. Thin layers of the active materials used for OLED deviceswere prepared in a similar manner by dissolving 10 mg of the elec-troactive material in 1 ml of chloroform. Film thicknesses weredetermined using a Dektak 6 M Profilometer.
The coated substrates were then transferred (without exposure
to air) to a vacuum evaporator (Edwards 501) inside an adjacentargon-filled glovebox (H2O and O2 levels both <1 ppm). Sampleswere placed on a shadow mask in a tray with a source-to-substratedistance of approximately 25 cm. The area defined by the shadowmask gave device areas of exactly 0.2 cm2. Deposition rates and
8 c Metals 161 (2011) 856–863
fin(ifr
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Table 1Properties of the pendant polymers.
Polymer Mwa (×104) �max
b (nm) EHOMOc (eV) ELUMO
d (eV)
1 1.30 (2.80) 373 −5.6 −2.82 0.60 (1.80) 410 −5.1 −2.53 0.96 (1.74) 413 −5.1 −2.54 2.30 (1.90) 487 −5.4 −3.2
a Determined by gel permeation chromatography (GPC). Polydispersity is shownin parentheses.
b Longest wavelength UV–Visible absorption maxima of spin-cast films.c Determined by cyclic voltammetry calibrated against the ferrocene standard
60 A. Gupta et al. / Syntheti
lm thicknesses were monitored using a calibrated quartz thick-ess monitor inside the vacuum chamber. Layers of calcium (Ca)Aldrich) and aluminium (Al) (3 pellets of 99.999%, KJ Lesker) hav-ng thicknesses of 20 nm and 100 nm, respectively, were evaporatedrom open tungsten boats onto the active layer by thermal evapo-ation at pressures less than 2 × 10−6 mbar.
A connection point for the ITO electrode was made by manuallycratching off a small area of the active layers. A small amount of sil-er paint (silver print II, GC electronics, part no.: 22-023) was theneposited onto all of the connection points, both ITO and Al. Theompleted devices were then encapsulated with glass and a UV-ured epoxy (Summers Optical, Lens Bond type J-91) by exposing to65 nm UV light inside a glovebox (H2O and O2 levels both <1 ppm)or 10 min. The encapsulated devices were then removed from thelovebox and tested in air within 1 h. Electrical connections wereade using alligator clips.The OPV devices were tested using an Oriel solar simulator
tted with a 1000 W Xenon lamp filtered to give an output of00 mW/cm2 at AM 1.5. The lamp was calibrated using a standard,ltered silicon (Si) cell from Peccell Limited which was subse-uently cross-calibrated with a standard reference cell traceableo the National Renewable Energy Laboratory. The devices wereested using a Keithley 2400 sourcemeter controlled by labviewoftware. The incident photon collection efficiency (IPCE) data wasollected using an Oriel 150 W Xenon lamp coupled to a monochro-ator and an optical fibre. The output of the optical fibre was
ocussed to give a beam that was contained within the area ofhe device. The IPCE was calibrated with a standard, unfiltered Siell.
The OLED devices were tested by using a Kethley 2400ourcemeter to control and measure voltage and current. Devicerightnesses were measured using a calibrated Minolta LS-110uminance meter. Electroluminescence spectra were recordedsing an Ocean Optics USB2000 spectrometer fitted with an opticalbre.
The instrumentation for the PhotoCELIV (photo-induced chargextraction by linearly increasing voltage) measurements is thatame as that described previously [23]. In brief, the measurementsere conducted using glass-encapsulated devices and the light
ource was a pulsed nitrogen laser (Laser Science Inc. VSL-337;);aving an output at 337 nm (pulse duration ∼4 ns; pulse energy0.1 mJ). The voltage ramp was generated using a function gen-rator (Agilent 33250), and the extraction currents were recordedsing a 500 MHz oscilloscope (Tektronix DPO3054) and a variableesistor. The delay in the triggering of the voltage ramp after trigger-ng of the laser pulse was controlled using a delay/pulse generatorStanford Research Systems DG535).
. Results and discussion
A range of monomers was prepared in this work, but onlyhose monomers comprising sufficiently strong electron-donatingroups were found to polymerize successfully. For example,onomers prepared using benzaldehyde or its singly substitutedethoxy derivative failed to polymerize. This loss of reactivity sug-
ests that electronic coupling exists between the polymerizableunctionality and the electro-active functionality within a givenendant chromophore. The weight-average molecular weight ofolymer 4 (see Table 1) was somewhat greater than those of thether polymers prepared in this work which suggests that the
nclusion of the nitrogen atom as part of an imine improves theonversion rate of the polymerisation. This is possibly a result ofhe reduced susceptibility of imine double bonds to radical attacks compared with the carbon-containing analogues [24]. The poly-ers prepared in this work were found to be highly soluble in a(−4.8 eV).d ELUMO = EHOMO + �E; where �E is the band gap calculated using the absorbance
onset values from the UV–Visible absorption spectra of the films.
variety of conventional organic solvents such as dichloromethane,chloroform, chlorobenzene, etc., which is a feature that is desir-able for solution-processed organic semi-conductor devices andwhich precludes the need for inclusion of electro-actively inertalkyl chains into the pendant module.
3.1. Photo-spectroscopic properties
The photo-spectroscopic properties of the polymers were inves-tigated by measuring their UV–Visible absorption spectra inpristine spin-cast films and the spectroscopic data is summarisedin Table 1.
The marked red shift in the longest-wavelength absorptionband of 2 compared with 1 is attributed to the greater charge-transfer character in the former due to the higher electron-donatingstrength of the diethylamine moiety. Little difference is observedbetween the absorption spectrum of 2 and that of 3 indicating thatthe charge-donating strength of the triphenylamine (TPA) frag-ment is comparable with the diethylamine fragment and also thatthe introduction of the TPA fragment has minimal effect on the�-conjugation of the pendant chromophore. However, a furthersubstantial red shift of the longest-wavelength absorption bandis observed for 4, and this is attributed to the greater electronwithdrawing ability of the cyanoimine group compared with thecyanovinylidene group leading to a substantial enhancement of thecharge transfer character of 4 compared with the other polymersinvestigated in the present work. Taking account of charge donatingand hole-transporting strength of TPA functionality, only the poly-mers 3 and 4 were considered for further study and analysis. Thedramatic change in the spectroscopic properties of the polymers (3and 4) that results on replacement of the cyanovinylidene groupwith the cyanoimine group is illustrated by the absorption andphotoluminescence spectra of dilute toluene solutions of polymers3 and 4 shown in Fig. 3 and absorption spectra of spin-cast filmsfrom their chloroform solution along with P3HT is represented inFig. 4.
The Stokes shift of the photoluminescence from 4 was foundto display a substantial solvent dependence which was analysedaccording to the Lippert–Mataga model [25] (Fig. S1, Supplemen-tary data). Assuming an Onsager cavity radius of 0.5 nm (i.e. a longmolecular axis of 1 nm), the increase in dipole moment of the flu-orescent state (LUMO) compared with the ground state (HOMO)is calculated from this analysis to be 7.1 Debye. This result isconsistent with the “push–pull” nature of the components of theelectro-active pendant chromophore of this polymer being morepronounced. The solvent effect on the Stokes’ shift of photolumi-nescence from 3 is ambiguous (see Fig. S1, Supplementary data),
suggesting that intramolecular charge re-distribution in the lumi-nescent state of 3 is less favoured than for 4, consistent with thecyanovinylidene unit being a weaker electron-withdrawing agentthan the cyanoimine unit.
A. Gupta et al. / Synthetic Metals 161 (2011) 856–863 861
F tolue4
3
tgFOoeweTstrpmil
F(
ig. 3. UV–Visible absorption spectra and photoluminescence (PL) spectra of dilute06 nm and 477 nm for 3 and 4, respectively.
.2. Electrochemical properties
The electrochemical properties of all polymers were inves-igated by cyclic voltammetry of films of the polymers on alass carbon electrode. The cyclic voltammograms are included inigs. S2–5, Supplementary data. The energy levels of the Highestccupied Molecular Orbitals (HOMO) were calculated using thenset of the oxidation potentials and are given in Table 1. Thenergy levels of the Lowest Unoccupied Molecular Orbitals (LUMO)ere calculated by adding the band gap (determined from the low
nergy onset of the film UV–Visible spectra) to the HOMO values.he values for the HOMO energy levels of polymers 3 and 4 areimilar in magnitude (−5.1 eV and −5.4 eV, respectively). Introduc-ion of the cyanoimine group into the electroactive module of 4esulted in a significant lowering of the band gap to 2.2 eV (com-
ared with the band gap of 2.6 eV for 3), which is comparable inagnitude with the band gap of 2.0 eV reported [26] for P3HT. Thisn turn suggests that the LUMO energy level of 4 is significantlyower than 3, which is attributed to the effect of the replacement
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
700650600550500450400350300
Wavelength (nm)
Ab
so
rban
ce
a
b
c
ig. 4. UV–Visible absorption spectra of spin-cast films of polymers 3 (curve a), 4curve b) and P3HT (curve c).
ne solutions of polymers 3 and 4. Excitation wavelengths used for PL spectra were
of cyanovinylidine functionality with the cyanoimine functionalityin the pendant chromophore.
3.3. Device performance
Polymer 3 was evaluated as a hole-transporting mate-rial in an OLED device by blending with the polymericelectron-transport material poly(2,7-(9,9-di-n-octylfluorene)-3,6-benzothiadiazole) (F8BT). The performance of this device wascompared against a benchmark polymer-based OLED system [27]comprising poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene)) (F8TFB) (hole transport)and F8BT (electron transport). Polymer 3 has energy levels sim-ilar to those of F8TFB (see Fig. S6, Supplementary data), and thepeak luminescence of 3:F8BT devices was found to be compara-ble with that of F8TFB:F8BT devices under the same conditions(Table 2), achieving efficiencies of around 20% that of optimizedF8TFB:F8BT devices. This result suggests that polymer 3 is capableof transporting charges.
The polymers 3 and 4 were also applied in the present studyas p-type semi-conducting components in solution-processed BHJsolar cells in combination with the soluble fullerene derivative[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the n-typesemi-conductor. For each individual polymer, BHJ devices withthe general structure ITO/PEDOT:PSS (38 nm)/polymer: PCBM/Ca
(20 nm)/Al (100 nm) were fabricated. The active layer thickness wasoptimised for each material and in general was found to be around50 nm. The optimal ratio (w/w) of polymer to PCBM was found to be1:4. Performance of polymer:PCBM solar cells with optimal ratio isrepresented in Table 3. Polymers 3 and 4 afforded power conversionTable 2Comparison of OLED device efficiencies.
Active layera TOVd (V) Lmaxe (cd/m2) QEmax
f (cd/A) QE at 100 (cd/m2)
F8BT:3b 5 979 0.38 0.37F8BT:F8TFBc 4 1003 4 1.37
a w/w ratio of components = 1:1.b Active layer thickness = 56 nm.c Active layer thickness = 75 nm.d Turn-on voltage.e Maximum luminescence.f Maximum quantum efficiency.
862 A. Gupta et al. / Synthetic Meta
Table 3Performance of the investigated polymer:PCBM (1:4) BHJ solar cells.
Polymer VOCa (V) JSC
b (mA/cm2) FFc PCEd (%)
3 0.49 0.91 0.28 0.134 0.63 1.57 0.29 0.29
a VOC = open circuit voltage.b JSC = short circuit current.c FF = fill factor.d PCE = power conversion efficiency.
0
5
10
15
20
25
30
750700650600550500450400350
Wavelength (nm)
IPC
E (
%)
a
b
F(n
eittfb
apct
F(
ig. 5. Comparative IPCE curves of photovoltaic devices. Active layers: (a) 3:PCBM80%, w/w PCBM); thickness = 54 nm, (b) 4:PCBM (80%, w/w PCBM); thick-ess = 60 nm.
fficiencies (PCE) of 0.13% and 0.29%, respectively, with the max-mum incident photon-to-current conversion efficiency (IPCE) atheir absorption maxima being 10% and 22%, respectively. Notably,hermal annealing of devices had a negligible effect on device per-ormance. The comparative IPCE measurements of OPV devicesased on these pendant polymers are shown in Fig. 5.
The modest efficiencies found for these OPV systems are
ttributed partly to the limited absorption of visible light by botholymers. The open circuit voltage (Voc) in the current–voltageurves shown in Fig. 6 suggests that the band-gap energies forhe polymers investigated in this preliminary study are well posi-0.0
0.5
1.0
1.5
2.0
0.80.70.60.50.40.30.20.10
Voltage (V)
Cu
rre
nt
De
ns
ity
(m
A/c
m2)
a
b
ig. 6. Current–voltage curves of photovoltaic devices under AM1.5 illumination.a) 3:PCBM (80%, w/w PCBM); 54 nm, (b) 4:PCBM (80%, w/w PCBM); 60 nm.
ls 161 (2011) 856–863
tioned, confirming the suitability of the chemical entities chosenfor pendant polymers.
The bulk mobility, �B, of charge carriers in devices fabricatedusing polymers 3 or 4 was investigated using the technique ofPhotoCELIV (Photoinduced Charge Extraction by Linearly Increas-ing Voltage). The device structure used for these measurementswas identical to that used for the OPV cells, with an active layerthickness of 65 nm. The absence of a light-induced extraction max-imum in the transient current density profile for the devices basedon the pristine polymers suggests that the magnitudes of thebulk mobilities of charges (assumed to be holes) in these poly-mers are below the detection range of our instrument, implying�B < 10−6 cm2/V s. Such low charge mobilities are consistent withthe disordered amorphous structure expected for these polymersin the solid state. In contrast, analysis of the distinct light-inducedextraction maxima observed in the transient current density pro-files for the devices fabricated using a blend of polymer 3 or 4 withPCBM (Fig. S7, Supplementary data) yields values for �B of approx-imately 2 × 10−5 cm2/V s in each case. This value is at least an orderof magnitude larger than for the pristine films of these polymers,and given the high content of PCBM in the blends, it is believedthat the bulk mobility values measured for the blends correspondsto that of electrons in the PCBM phase. The low mobility of elec-trons in the PCBM phase is suggestive of a high degree of disorderin the morphology of these blends.
We further studied the atomic force microscopy (AFM) topo-graphic images of blend films of polymer 4:PCBM and polymer3:PCBM. Thin films were spun at 2500 rpm with chlorobenzene assolvent, as for the devices. The resulting films were very smoothand showed a homogenously mixed layer with no signatures oflarge phase separated domains indicating highly disordered nature(Fig. S8, Supplementary data).
There is clearly a need to increase the absorption profile andcharge mobility of the future pendant polymers. Strategies toimprove these parameters include the incorporation of longer�-conjugated systems in the electro-active moieties within thependant groups, as well as by further increasing the donor–acceptordipole within the pendant chromophore.
4. Conclusions
In conclusion, we have introduced new pendant-type solution-processable polymers, derived from small molecules having apolymerizable group on one end, as p-type semiconducting compo-nents for BHJ solar cells comprising PCBM as an acceptor. We haveshown that simple structural modifications in the electro-activemoiety can provide band gap tuning with subsequent improve-ments in device power conversion efficiencies. We have alsodetermined that the polymerizable group should ideally be elec-tronically isolated from the electro-active group and should onlybe contemplated if the electro-active group is particularly electronrich. The clear potential of pendant polymers results from the factthat a greater proportion of the polymer can be electro-active asthe need for solubilising and electronically inert and insulated alkylgroups can be minimised. Given the recent reports of highly effi-cient small-molecule OPV devices, free radical polymer chemistrymay be best thought of as a way of providing larger scale orderingor morphology control of pendant small molecules while minimis-ing the need for inert solubilising groups to further improve highefficiencies with ease of processability.
Further improvement in performance might be achieved onincorporation of oligothiophenes as well as more highly conju-gated light absorbing building blocks in the pendant structure andthe use of living radical polymerization methods to create block-copolymers and other architectures. It is also desirable that the
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A. Gupta et al. / Syntheti
onomers have polymerizable functionality that is electronicallysolated from the electro-active functionality of the pendant chro-
ophores.
cknowledgements
This work was supported financially by the Australian Depart-ent of Innovation, Industry, Science and Research (International
cience Linkage Grant CG100059) and the Victorian Organic Solarell Consortium (Victorian Department of Primary Industries, Sus-ainable Energy Research and Development Grant). We are alsorateful to Mr. Tino Ehlig for fabricating some of the OPV devicestudied in this work.
ppendix A. Supplementary data
Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.synthmet.2011.02.013.
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