Organic dyes incorporating low-band-gap chromophores for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol050417f

(Figure Presented) Versatile dyes based on benzothiadiazole and benzoselenadiazole chromophores have been developed that perform efficiently in dye-sensitized solar cells. Power conversion efficiency of 3.77% is realized for a dye in which charge recombin

Full text of DOI:10.1021/ol050417f

ORGANIC
LETTERS
2005
Vol. 7, No. 10
1899-1902
Organic Dyes Incorporating
Low-Band-Gap Chromophores for
Dye-Sensitized Solar Cells
Marappan Velusamy,† K. R. Justin Thomas,† Jiann T. Lin,*,† Ying-Chan Hsu,‡ and
Kuo-Chuan Ho*,‡
Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, and
Department of Chemical Engineering, National Taiwan UniVersity, Taipei 106, Taiwan
jtlin@chem.sinica.edu.tw; kcho@ntu.edu.tw
Received February 25, 2005
ABSTRACT
Versatile dyes based on benzothiadiazole and benzoselenadiazole chromophores have been developed that perform efficiently in dye-sensitized
solar cells. Power conversion efficiency of 3.77% is realized for a dye in which charge recombination is probably hindered by the nonplanar
charge-separated structure.
The demand for energy conservation triggered the search
for alternate renewable energy sources.1 A practical solution
is the adoption of light-harvesting biological concepts. This
biomimetic strategy has been recently translated into tech-
nological advances such as solar cells.2,3 Although silicon-
based semiconductors4 dominated solar cell applications for
decades, recent demonstration of dye-sensitized solar cells
(DSSCs) based on nanocrystalline TiO2 by Gra¨tzel et al.
opened up the opportunity for organic and organometallic
dyes in this area.5,6 A dye with excellent light-absorbing
capability in the red and near-IR region, photostability, and
redox stability is presumed to be attractive for the functioning
of DSSCs.
recently. Though benzothiadiazole-based polymers10,11 have
been widely employed in photovoltaic applications, to the
Coumarin-,7 indoline-,8 cyanine-,9a and hemicyanine-
based9b,c dyes have been used for the construction of DSSCs
Figure 1. Structure of the dyes.
† Academia Sinica.
‡ National Taiwan University.
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S. K.; Kim, D.; Kongkanand, A.; Kuwabata, S.; Fukuzumi, S. J. Phys. Chem.
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best of our knowledge, small molecules containing benzo-
thiadiazole or benzoselenadiazole structural motifs have not
been exploited for DSSCs. In this letter, we report a series
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10.1021/ol050417f CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/20/2005

Scheme 1. Synthesis of the Dyes
of organic donor-acceptor compounds (Figure 1) that
low-energy absorption is predominantly charge transfer in
nature, and the same is true with the low-lying excited state
also.12 This will ensure and enhance the possibility of charge
separation and migration in such systems.
contain diphenylamine donors and cyano acrylic acid ac-
ceptors bridged by a benzothiadiazole or benzoselenadiazole
fragment. The carboxylic acid group is introduced so that it
will function as an anchoring group toward nanocrystalline
TiO2 in the solar cell setup. We believe that the low-band-
gap chromophores will function as a photon sink where
charge separation occurs, and their migration in opposite
directions is facilitated by the presence of donor and acceptor
units. In benzo(thia/selena)diazole-based fluorophores, the
The dyes were obtained in moderate yields in three steps
as illustrated in Scheme 1. In the first step, the donor is
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1900
Org. Lett., Vol. 7, No. 10, 2005

Table 1. Electrooptical Parameters of the Dyesa
dye
λabs/nm (ꢀ × 10-3 M-1 cm-1
)
E1/2 (ox) (∆Ep)/mV
E1/2 (red) (∆Ep)/mV
HOMO, eV
LUMO, eV
band gap, eV
S1
S2
S3
S4
491 (27.5), 388 (16.3), 309 (28.4)
502 (6.30), 351 (15.6)
541 (24.4), 398 (20.1), 312 (21.0)
544 (13.2), 363 (26.5)
581 (91)
556 (77)
322 (85)
263 (107)
-1718 (90)
-1630 (96)
-1660 (106)
-1556 (138)
5.381
5.356
5.122
5.063
3.260
3.338
3.256
3.290
2.121
2.018
1.866
1.773
a Absorption and electrochemical data were collected in tetrahydrofuran solutions. Scan rate, 100 mV/sec; electrolyte, (n-C4H9)4NPF6; ∆Ep is the separation
between the anodic and cathodic peaks. Potentials are quoted with reference to the internal ferrocene standard (E1/2 ) +265 mV vs Ag/AgNO3).
attached to the core either by Stille13 or Suzuki14 coupling
reactions. These reactions produced the disubstituted side
products as well. However monocapped compounds can be
easily separated by column chromatography. In the next
step, this bromo-exposed intermediate was reacted with
(5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributyl stannane under
Stille coupling conditions, and subsequent cleavage of the
1,3-dioxalane protecting group in aqueous acetic acid
produced the free aldehydes. These aldehydes, on reaction
with cyanoacetic acid in the presence of ammonium acetate
catalyst in acetic acid, produced the required dyes. The dyes
are dark red or black in the solid state and freely dissolve in
tetrahydrofuran to produce a red or violet solution.
linking aromatic segment was also noticed, i.e., the thiophene-
linked compounds (S3 and S4) show bathochromic shift for
the charge-transfer band when compared to the phenylene-
linked derivatives (S1 and S2). The drop in the extinction
coefficient of the CT absorption in the benzoselenadiazole
derivatives (S2 and S4) is more pronounced for the phe-
nylene-conjugated derivative S2 (4.37 times when compared
to S1) than for the thiophene-linked analogue S4 (1.85 times
when compared to S3). These observations are attributed to
the larger size and smaller electronegativity of Se when
compared to S. These factors will lead to less electron density
on Se and thus diminished aromaticity for benzoselenadiazole
when compared to benzothiadiazole.15 Similarly, the in-
creased donor property and coplanar arrangement of thiophene
with the core substantially compensate the drop in transition
probablity for S4.
The absorption spectra recorded for the tetrahydrofuran
solution of the dyes are displayed in Figure 2. All the dyes
The dyes are redox stable, exhibiting both quasireversible
oxidation and reduction couples. Dyes with good redox
stability are required for sustaining dye-sensitized solar cells.
Benzothiadiazole-based derivatives containing triarylamines
have been reported to undergo facile oxidation and reduc-
tion.16 Similarly, in the current derivatives, the oxidation and
reduction also originate from the amino and benzothiadiazole/
benzoselenadiazole functionality, respectively. Thiophene
linker slightly influence the redox potentials. Benzoselena-
diazole derivatives display negative shift in oxidation
potentials and positive shift in the reduction potentials when
compared to the benzothiadiazole derivatives. This results
in a substantial reduction in the band gap for the benzo-
selenadiazole derivatives when compared to the benzothia-
diazole analogues (see Table 1).
The dye-sensitized solar cells were constructed using these
dyes as a sensitizer for nanocrystalline anatase TiO2. Typical
solar cells, with an effective area of 0.25 cm2, were fabricated
with 0.05 M I2/0.5 M LiI/0.5 M tert-butyl pyridine in
acetonitrile solution as an electrolyte. The device perfor-
mance statistics under AM 1.5 illumination are collected in
Table 2. The incident to photon conversion efficiency data
at each wavelength are plotted for S1-S4 in Figure 3.
From the data, it is evident that dyes S1 and S2 exhibit
Figure 2. Absorption spectra of dyes S1-S4 recorded in THF.
exhibit three transitions and cover a broad range (250-
700 nm) of spectra (Figure 2 and Table 1). For the similar
structural architecture, the benzoselenadiazole derivatives
(S2 and S4) display red-shifted absorption when compared
to the benzothiadiazole analogues (S1 and S3). However,
the benzoselenathiadiazole derivatives possess less optical
density for the CT transition. Another effect due to the
(15) (a) Yang, R.; Tian, R.; Hou, Q.; Yang, W.; Cao, Y. Macromolecules
2003, 36, 7453. (b) Katritzky, A. R.; Pozharskii, A. F. Handbook of
Heterocyclic Chemistry, 2nd ed.; Pergamon: Oxford, 2000; Chapter 2, pp
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Tao, Y. T.; Chuen, C. H. Chem. Commun. 2004, 2328. (b) Justin Thomas,
K. R.; Lin, J. T.; Velusamy, M.; Tao, Y. T.; Chuen, C. H. AdV. Funct.
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(13) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
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R.; Najera, C.; Yus, M. Chem. ReV. 2004, 104, 2667.
Org. Lett., Vol. 7, No. 10, 2005
1901

Table 2. Performance Parameters of DSSCs Constructed Using
the Dyesa
dye
V
OC, mV
JSC, mA cm-2
ff
η, %
S1
S2
S3
S4
N3
546
524
517
474
708
10.44
8.35
3.21
3.57
11.28
0.66
0.67
0.69
0.66
0.66
3.77
2.91
1.15
1.11
5.30
a Experiments were conducted using TiO2 photoelectrodes with ap-
proximately 14 µm thickness and 0.25 cm2 working area on the FTO
(7 Ω/sq) substrates.
impressive photovoltaic performances. The open-circuit
photovoltage and overall yield for the four dyes lie in the
order S1 > S2 > S3 ∼ S4.
The difference in performance between the phenylene-
conjugated derivatives (S1 and S2) and thiophene-linked
analogues (S3 and S4) probably stems from the difference
in the coplanarity of the aromatic segment that bridges the
donor and acceptor units. Molecular modeling (SPARTAN,
PM3) studies reveal that the phenlylene derivatives (S1 and
S2) possess a twisted nonplanar geometry, which will
decelerate the recombination of charges in the charge-
separated state.17 Additionally, in the phenylene derivatives
(S1 and S2), the reorganization energy required for the
decoupled twisted state may be more favorable when
compared to the thiophene analogues (S3 and S4).
Figure 3. Action spectra for dyes S1-S4.
and hemicyanine dyes.9 This design opens up the possibility
of preparing new dye molecules for DSSCs utilizing low-
band-gap building blocks. The approach is unique in that it
uses an easily polarizable and electron-deficient bridge
between the push-pull chromophores, leading to push-
pull-pull architecture that is different from that used
previously, which contains a donor and an acceptor bridged
through an aromatic or aliphatic linker.
In summary, we have developed a new class of dyes
featuring donor-acceptor architecture that incorporates low-
band-gap chromophores. They are successfully adsorbed on
nanocrystalline anatase TiO2 particles, and subsequently
efficient dye senstitized solar cells have been fabricated. The
performance of these small-molecule-based, dye-sensitized
solar cells is better than that fabricated from certain cyanine
Acknowledgment. We thank CTCI Foundation, Taipei,
Research Centre for Applied Sciences, Academia Sinica
(ASCAS), and National Science Council, Taiwan, for
financial support.
Supporting Information Available: Synthetic procedures
and characterization details for the new compounds described
in this paper. This material is available free of charge via
the Internet at http://pubs.acs.org.
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1902
Org. Lett., Vol. 7, No. 10, 2005