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Direction for development of near infrared-dyes
for dye-sensitized solar cells
from the view point of electron injection and charge recombination
Kyushu Institute of Technology (National Institute) Kitakyushu, Fukuoka, Japan, 808-0196, Japan
Shuzi Hayase
Tokyo
Collaborator
Kyushu Institute of Technology Shyam S. Pandey Yuhei Ogomi
Nippon Steel & Sumikin Chemical Co., LTD Yoshihiro Yamaguchi
Universidad de Castilla-La Mancha Abderrazzak Douhal Boiko Cohen Marcin Aiolek Gustovo de Miguel Michal Zitnan, Maria Jose Marchena Barriento, Sofia Kapetanaki
3
Dye sensitized solar cells (DSCs) B. O’Regan and M. Graetzel Nature, 1991, 353, 737
Electrolyte I-/I3-
SnO2 /F
(Acetonitrile, ethylene carbonate, molten salts, etc.)
TiO2
10-20nm
TiO2
Ru N
N
CO
COOH
OTi
N
N
CO
HOOC
O
Ti
SCN NCSOHP257
Dye
TiO2 layer: 10-20 μm
Electrolyte layer: 30 μm
Photovoltaic Performance Comparison Certified efficiency AM1.5G ,1000W/m2
0
10
20
Effic
ienc
y(%
)
C-Si
25.0%
Poly-Si
20.4%
a-Si DSC CIGS
CIGS: CuInGaS(Se)
19.6%
11.9%
CdTe
16.7%
OPV
Commercially available
10.1 % 10.7%
Organic PV
Sharp Mitsubishi Chem.
15% (target)
M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Prog. Photovolt: Res. Appl. 2013; 21:1-11.
DSC:Dye-sensitized solar cell
OPV: Organic thin film PV
Spectrum matching for DSC to sun light spectrum (AM1.5)
25-26 mA・cm-2
32 mA・cm-2
36 mA・cm-2
FF:0.75
Voc: 0.75
14-15%
18%
20%
Ru dye
IR dye
Ru N
N
CO
COOH
OTi
N
N
CO
HOOC
O
Ti
SCN NCSOHP257
Increase in Voc
Increase in Jsc (New dye, or tandem, hybrid)
5
IPCE 80%, FF 0.75
Efficiency expectation
Wavelength (nm)
IPC
E
N-719 Black dye
300 400 500 600 700 800 900 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8Cocktail dyes
A
B
New NIR dyes to be developed
Conventional Ru dye
e-
hν
dye
I-/I3-
HOMO
LUMO TiO2cond. band
e-
⊿G1
0.9eV
⊿G2
Requirement for development of Near Infrared Dye
How to decrease ⊿G1 and ⊿G2 with maintaining fast electron shift Collaboration of solar cell researchers with photo-physics researchers
Dye syntheses
Molecular orbital calculation
Solar cell performance evaluation
Fundamental analyses on solar cells
Time resolved study
Analyses of electron injection and dye regeneration
Collaboration of Japan side with Spain side
Extraction of items determining sola cell efficiency Propose high efficiency dyes
Douhal group (Time resolved study)
Hayase group (Solar cell-based research)
Samples Substrates
Dynamics
A B
0 10 20 30 40 500.00
0.25
0.50
0.75
1.00
No electrolyte
ΙΙΙ
ΙΙ
Nor
mal
ized
∆A
Time / ps
No electrolyte
Ι0 1 2 3 4 5
0.00
0.25
0.50
0.75
1.00
Nor
mal
ized
∆A
Time / ps
ΙΙΙ
ΙΙΙ
450 500 550 600650 700 750-0.2
-0.1
0.0
0.1
700 725 750 775-0.02
-0.01
0.00
0 ps 1 ps 3 ps 7 ps 42 ps
∆A
Wavelength / nm
∆A
Wavelength / nm
Ground State Bleaching
Femto-second Transient Absorption: SQ-41
)()(* 221−+• +→+ eTiOSQTiOSSQ
Electrolyte Life-tim I 6.7 ps II 11.1 p III 4.9 ps No 2.5 ps
( ) exp[( ) ]f x A t βτ= −
SQ radical cation formation; Electron injection
Change in spectral shape at longer delay. Signal beyond 2 ns.
Research Collaboration
✓ Research discussions: 5 times March 2010 (Spain), May 2011 (Spain), Aug. 2011 (Japan), Sep. 2011 (Spain), June 2012 (Sweden) ✓ Dr. Gustovo de Miguel visited us in Japan and joined the research in Aug. 2011. ✓ Provided 15 dyes and 15 substrates (encapsulated) to Douhal Lab.
Development of near IR dyes for combination with TiO2 (~900 nm)
e-
hν
dye
I-/I3-
HOMO
LUMO
✔HOMO-LUMO energy level
adjustment
✔ LUMO orbital shape
✔Electron injection energy barrier
✔Excitation life time
✔Molecular orbital calculation
✔Dye syntheses tech.
✔Cell structure and cell analyses
TiO2cond. band
e-
0.2eV
0.9eV
12
✔ Time resolved spectroscopy
Simulation results of molecular orbital on organic dye
13
Side chain effect
HOOC
N+
O
O-
N
COOH
R R
SQ dyes
Model dyes: Sharp absorption spectra
HOOC
N+
O
O-
N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
SQD-2 (alkyl chain=2)
SQD-4 (alkyl chain=4)
SQD-8 (alkyl chain=8)
SQD-12 (alkyl chain=12)
SQD-18 (alkyl chain=18)
HOOC
N+
O
O-
N
COOH
SQD-0 (alkyl chain=0)
N
O
O
N
FF
F
FF
F
COOH
HOOC
SQD-4F3
Designing of molecular structures after MO calculation
Ene
rgy
vs E
Vac
(eV
)
Alkyl chain length
TiO2 CB
I3-/I-
SQ-Fluoro
SQ-Fluoro
0 5 10 15 20-6
-5
-4
-3
HOMO-LUMO level of synthesized dyes
HOMO-LUMO level can be controlled within 0.6 eV by varying substituents
ΔG1
LUMO
HOMO ΔG2
TiO2 CB
I-/I3-
HOMO
LUMO
Introduction of F alkll decreases HOMO and LUMO
0.00
0.40
0.80
1.20
1.60
2.00
2.40
2.80
0 2 4 6 8 10 12 14 16 18 20
Eff
icie
ncy
[%]
Alkyl Chain Length
Efficiency vs. Alkyl chain length
HOOC
N+
O
O-
N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
HOOC
N+
O
O-N
COOH
SQ-2 (alkyl chain=2)
SQ-4 (alkyl chain=4)
SQ-8 (alkyl chain=8)
SQ-12 (alkyl chain=12)
SQ-18 (alkyl chain=18)
Adsorption scheme for SQ dyes
N+
O
O-
NN+
O
O-
NN+
O
O-
N N+
O
O-
NN+
O
O-
NN+
O
O-
N N+
O
O-
N
TiO2 TiO2
Main structure responsible for near IR dyes
(SQD2)
N
O
-O
N
COOH
SQ-12Chemical Formula: C36H40N2O4
Exact Mass: 564.2988
Dyes with extended conjugation
0
0.4
0.8
1.2
500 600 700 800 900
Abs
orba
ce (
Nor
m.)
Wavelength (nm)
8
27
31
16
70
21
-7.3
-6.3
-5.3
-4.3
-3.3
TiO
2
8 27 31 16 70
Vacu
um le
vel [
eV]
I-/I3-
Model SQ dyes
LUMO LUMO LUMO LUMO LUMO
HOMO HOMO
HOMO HOMO
HOMO Redox potential
Conduction band
TiO2
One of results of collaboration research
Model Squaraine Sensitizers
1. Chain length ----- SQ-2 and SQ-4 2. Nature of substituents----SQ-4 and SQ-26 3. Molecular Asymmetry------------- SQ-26 and SQ-41
Electronic Absorption Spectra
550 600 650 700 7500.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
abs
orba
nce
Wavelength / nm
SQ 41 SQ 26 SQ 4 SQ 2
ε = 2-3 X 105 dm3.mole-1.cm-1
Energy Band Diagram
CB TiO2
I-/I3-
SQD8 SQ4/4F3
SQD4F6
LUMO change: Electron injection HOMO change: Dye generation Anchor group: Substitution effects:
⊿G1
⊿G2
Photovoltaic Characteristics
Photocurrent Action Spectra
Wavelength (nm)
IPC
E
SQ-2 SQ-4 SQ-26 SQ-41
400 500 600 7000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time Resolved Investigations
Time-resolved techniques Femtosecond Transient Absorption Spectroscopy Nanosecond Flash Photolysis
Experimental Conditions: SQ-dyes adsorbed on TiO2 SQ-dyes adsorbed on ZrO2 To determine the electron injection efficiency !
Time resolved investigations in
the presence of electrolyte Time-resolved investigations
in the absence of electrolyte To determine the Dye Regeneration efficiency !
-3
SQ Dye
-4
-5 I-/I3-
⊿G1
①
②
③
④
⑤
⑥
TiO2
Electron injection
-3
SQ Dye
-4
-5 I-/I3- 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
⊿G
Injection rate constant /10(-9)/s
SQ26(SQD4F6)
SQ2(SQD2)
SQ4(SQD8)
SQ41(SQ4F3)
⊿G1
No apparent relation between ⊿G1 and injection rate constant
①
②
③
④
⑤
⑥
Electron Injection (②)
Relationship between ⊿G and injection rate constant
Electron injection efficiency is governed by factors other than ⊿G1
Better TiO2
-3
SQ Dye
-4
-5 I-/I3- 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
⊿G
Injection rate constant /10(-9)/s
SQ26(SQD4F6)
SQ2(SQD2)
SQ4(SQD8)
SQ41(SQ4F3)
⊿G1
①
②
③
④
⑤
⑥
Electron Injection (②)
Relationship between ⊿G and injection rate constant
F facilitates the electron injection with the same ⊿G1
High electron injection can be realized with lower⊿G1 by using dyes with F atoms
Better
TiO2
Common trend
Dye regeneration
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.2 0.4 0.6 0.8 1 1.2
Gap
(red
ox-H
OM
O)/
eV
Regeneration efficiency
SQ41(SQ4F3)
SQ26(SQD4F6)
SQ4(SQD8)
SQ2(SQD2)
Dye generation(⑥)
Relationship between ⊿G2 and dye regeneration
-3
SQ Dye
-4
-5 I-/I3-
⊿G2
①
②
③
④
⑤
⑥
High dye generation with Low ⊿G2
Better for high efficiency Common trend
Symmetrical-SQ-26
N
O
O
N
COOH
F F
F
FF
F
HOOC
Chemical Formula: C36H34F6N2O6Exact Mass: 704.23
TiO2
I-
HOMO
Possible explanation for high efficiency dye generation with low ⊿G2
0.000.100.200.300.400.500.600.700.800.901.00
0.50 0.52 0.54 0.56 0.58 0.60 0.62
Rre
l(I 3
- /I- )
Voc (V)
SQ4
SQ4F3
SQ18
SQ4O2
SQ4O4SQ4F6
I3-
e TiO2
e TiO2
I3- 30 nm
30,000 nm
N
O
O
NHOOC COOH
R1
R2
R1 = R2 = Butyl SQ4R1 = R2 = Dodecyl SQ12R1 = R2 = Octadecyl SQ 18R1 = R2 = Trifluorobutyl SQ 4F6R1 = R2 = Methoxybutyl SQ4O2R1 = R2 = Ethylbutanoate SQ4O4R1 =Butyl & R2 = Trifluorobutyl SQ4F3
-F
-O-
A. Hayat, S. S. Pandey, Y. Ogomi, and S. Hayase, J. Electrochem. Soc., 158, B770-B771 (2011).
Porous TiO2 sheet
TiO2
F O
Dye I3
-
e
-3
SQ Dye
-4
-5 I-/I3-
⊿G
①
②
③
④
⑤
⑥ Ei eff: Electron injection efficiency: ②/②+③ Reg eff: Regeration efficiency: ⑥/④+⑥
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.2 0.4 0.6 0.8 1IP
CE
Ei eff x Reg eff
SQ4(SQD8)
SQ41(SQ4F3)
SQ26(SQD4F6)
SQ2(SQD2)
Explanation on IPCE (④)
IPCE can be explained by product of electron injection efficiency and regeneration efficiency
TiO2
I-
HOMO LUMO Polar substitute interacting with I-
High dye generation efficiency with low ⊿G2
High electron injection efficiency with low ⊿G1
High Coupling
Proposed structure realizing high efficiency dye generation with low ⊿G2 and high electron injection with low ⊿G1
Conclusions
6 model Squaraine dyes were synthesized to investigate the role, of alkyl chain length, nature of substituents and molecular asymmetry on the photovoltaic performance.
Electron injection and dye regeneration are not always governed by energy gap (⊿G) and influenced by substituents and molecular structure. This suggests that high electron injection with low ⊿G becomes possible.
Creation of molecular asymmetry and introduction of longer alkyl chain leads to facile electron injection.
Introduction of electron withdrawing Fluor-alkyl substituent leads to facile dye regeneration.
Order of IPCE value was explained by the product of charge injection efficiency and dye regeneration efficiency.
List of Publications in Academic Journals: [1] Alkyl and Fluoroalkyl Substituted Squaraine Dyes: A Prospective Approach Towards Development of Novel NIR Sensitizers; Shyam S. Pandey, Takafumi Inoue, Naotaka Fujikawa, Yoshihiro Yamaguchi and Shuzi Hayase, Thin Solid Films, Vol. 519, pp. 1066-1071 (2010). [2] Substituent Effect in Direct Ring Functionalized Squaraine Dyes on Near Infra-Red Sensitization of NanocrystallineTiO2 for Molecular Photovoltaics ; Shyam S. Pandey, Takafumi Inoue, Naotaka Fujikawa, Yoshihiro Yamaguchi and Shuzi Hayase, Journal of Photochemistry and Photobiology A: Chemistry, Vol. 214, pp. 269-275 (2010). [3] Synthesis and characterization of squaric acid based NIR dyes for their application towards dye-sensitized solar cells ; Inoue, Takafumi, Pandey, Shyam S., Fujikawa, Naotaka, Yamaguchi, Yoshihiro, Hayase, Shuzi; JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY Volume: 213 Issue: 1 Pages: 23-29 (2010). [4] Fine tuning the structure of unsymmetrical squaraine dyes towards the development of efficient dye sensitized solar cells; Shyam S. Pandey, Rie Watanabe, Naotaka Fujikawa, Yuhei Ogomi, Yoshihiro Yamaguchi and Shuzi Hayase; Proc. SPIE Vol. 8111 Page 811116 (2011).
[5] Femto to millisecond observations of indole-based squaraine molecules photodynamics in Solution.; G. de Miguel, M. Marchena, M. Ziolek, M, Zitnan, S. S. Pandey, S Hayase and A. Douhal. Physical Chemistry and Chemical Physics; Vol. 14, Pages 1796-1805 (2012). [6] Photophysics of H-and J-aggregates of Indole based squaraines in solid state.; G. de Miguel, M. Ziolek, M, Zitnan, J. A. Organero, S. S. Pandey, S Hayase and A. Douhal; Journal of Physical Chemistry-C; Vol. 116, No.-17, Pages 9379-9389 (2012). [7] Femto to Millisecond Photophysical Characterization of Indole-based Squaraines Adsorbed on TiO2 Nanoparticle Thin Films. de Miguel, Gustavo; Marchena, Maria Jose; Ziolek, Marcin; Pandey, Shyam; Hayase, Shuzi; Douhal, Abderrazzak; Journal of Physical Chemistry-C; Vol. 116, No.-22, Pages 12137-12148 (2012). [8] Novel unsymmetrical squaraine dye bearing cyanoacrylic acid anchoring group and its photosensitization behavior; Gururaj M. Shivashimpi, Shyam S. Pandey, Rie Watanabe, Naotaka Fujikawa, Yuhei Ogomi, Yoshihiro Yamaguchi and Shuzi Hayase ; Tetrahedron Letters; Vol. 53, No.40, Pages 5437-5440 (2012). [9] Relating the Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells to the Molecular Structure of the Sensitizers and to the Presence of Additives; G. de Miguel, M. Marchena, B. Cohen, S. S. Pandey, S. Hayase and A. Douhal; Journal of Physical Chemistry-C; Vol. 116, No.-42, Pages 22157-22168 (2012).
[10] Dye-Sensitized Solar Cells based on Novel Far-red Sensitizing Unsymmetrical Squaraine Dye containing Pyrroloquinoline Moiety.; Shyam S. Pandey, Naotaka Fujikawa, Rie Watanabe, Yuhei Ogomi, Yoshihiro Yamaguchi and Shuzi Hayase; Japanese Journal of Applied Physics; Vol. 51, No.-10, Issue-2, pages 10NE12-1-10NE12-5 (2012). [11] Solution processable thin film organic photovoltaic cells based on far red sensitive soluble squaraine dye; Shyam S. Pandey, Takafumi Mizuno, Sandeep K. Das, Yuhei Ogomi and Shuzi Hayase; Thin Solid Films; Vol. 522, pages 401-406 (2012). [12] Effect of extended p-conjugation on photovoltaic performance of dye-sensitized solar cells based on unsymmetrical squaraine dyes; Shyam S. Pandey, Rie Watanabe, Naotaka Fujikawa, Gururaj M. Shivashimpi, Yuhei Ogomi, Yoshihiro Yamaguchi and Shuzi Hayase; Tetrahedron, Vol. 69, No. 12, pages 2633-2639 (2013). [13] Effect of anchoring groups on photosensitization behavior in unsymmetrical squaraine dyes’ Gururaj M. Shivashimpi1, Shyam S. Pandey1, Rie Watanabe1, Naotaka Fujikawa1, , Yuhei Ogomi1, Yoshihiro Yamaguchi2, Shuzi Hayase; Synthetic Metals (Submitted). [14] Real-Time Photodynamics of Squaraine-Based DSSCs with Iodide and Cobalt Electrolytes; Maria Jose; de Miguel, Gustavo; Cohen, Boiko; Hayase, Shuzi; Pandey, Shyam; Douhal, Abderrazzak; Journal of Physical Chemistry-C (Submitted).
Thank you for your attention !