Thiophene-fused coplanar sensitizer for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2011.03.091

We have designed and synthesized a novel ladder-type heteroacene dye consisting uniquely of thiophene segments as a photosensitizer for the dye-sensitized solar cells (DSSCs). The onset of the IPCE spectrum for the dye not only reaches up to 700 nm with a

Full text of DOI:10.1016/j.tetlet.2011.03.091

Tetrahedron Letters 52 (2011) 2764–2766
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Thiophene-fused coplanar sensitizer for dye-sensitized solar cells
Jonggi Kim a, Yimhyun Jo a, Won-Youl Choi b, Yongseok Jun a, Changduk Yang a,
⇑
a Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea
b Department of Metal and Materials Engineering, Gangneung-wonju National University, Gangneung 210-702, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have designed and synthesized a novel ladder-type heteroacene dye consisting uniquely of thiophene
segments as a photosensitizer for the dye-sensitized solar cells (DSSCs). The onset of the IPCE spectrum
for the dye not only reaches up to 700 nm with a high IPCE (>60%) but also a solar energy-to-electricity
conversion efficiency of 2.31% is achieved. Even though the efficiency is slightly lower than those of other
dyes reported previously, this work opens up a new strategy to design heteroaromatic fused photosen-
sitizers for DSSCs.
Received 21 February 2011
Revised 9 March 2011
Accepted 20 March 2011
Available online 12 April 2011
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells (DSSCs) have grown one of the most
blossoming alternatives for the photovoltaic conversion of solar
energy as compared to the typical solid p–n junction photovoltaic
devices.1,2 Up to this time, the conventional ruthenium (Ru)-based
sensitizers such as N3/N7193,4 and black dye5 have reached the
promising solar-energy-to-electricity conversion efficiencies of
11% under AM 1.5 irradiation. In addition to Ru complexes,
metal-free organic dyes have also been utilized as photosensitizers
in DSSCs because of their wide variety of the structures, facile mod-
ification, high molar absorption coefficient, low cost, and environ-
ment-friendliness.6–8 Generally, organic dyes should contain a
One can also observe in DSSCs that thiophene is an important
building unit for improved efficient devices. Thereby, in the one-
dimensionally ladder-type framework, the introduction of a thio-
phene moiety seems to provide advantages in order to achieve
high performance photovoltaic materials.18 On the basis of the re-
sults reported above, we have tried to design a fused oligoacene
dye with the combination of thiophene and phenylene units in
one molecular skeleton. Herein, we report a new ladder-type
conjugated heteroacene dye containing thiophene segments as
coplanar -conjugation system with the acceptor/anchor group
p-
p
(2-cyanoacrylic acid) and donor functionality (N,N-dimethylani-
line) for use in DSSCs. (see Fig. 1) In addition, the bulky aryl side
groups can suppress the aggregation for the dye on the TiO2 sur-
face, probably resulting in further efficient photocurrent
generation.
structure of donor (D)-to-acceptor (A) bridged by a
system and possess a broad and intense spectral absorption.
Among the various types of -conjugated compounds, the ladder
-electron systems, which have fully ring-fused cyclic skeletons,
are a particularly growing class of materials.9–12 Their rigid
-conjugated frameworks without any conformational disorder
can not only strengthen the parallel p-orbital interactions to elon-
gate effective conjugation length and facilitate -electron delocal-
p-conjugation
p
p
The novel ladder-type
p-conjugated heteroacene dye contain-
p
ing thiophenes is constructed by the stepwise synthetic protocol
illustrated in Scheme 1. The diethyl 2,5-di(thiophen-2-yl)tere-
phthalate (2) was synthesized by Stille coupling reaction of diethyl
2,5-dibromoterephthalate (1) with 2-stannylthiophene. Addition
of 4-methylphenyllithium gave the corresponding diol and the
desired thiophene-implanted coplanar chromophore (4,9-
dihydro-4,4,9,9-tetrakis(4-methylphenyl)-s-indaceno[1,2-b:5,6-b0]
dithiophene (3)), which subsequently underwent bromination
p
ization, providing an effective way to extend the visible absorption
region13,14 but also suppress the rotational disorder around inter-
annular single bonds and lower the reorganization energy, which
in turn enhances the intrinsic charge mobility.15–17
Taking these positive points into account, the first successful
examples of the ladder-type donor (D)-p-spacer-acceptor (A)
sensitizers with pentaphenylene were reported by Müllen and
co-workers.6 Although such dyes exhibited high absorption coeffi-
cients up to 75,600 Mꢀ1 cmꢀ1, the efficiencies of these dye-based
solar cells were below 2.3% in DSSCs with the low current a result
of the short wavelength absorption of the dyes (kmax = 442–
457 nm) and the low driving forces for the dye regeneration.
Thiophene coplanar spacer
R
R
S
N
COOH
S
NC
R
Donor
R
Acceptor/Anchor
R =
⇑
Corresponding author. Tel.: +82 52 217 2920; fax: +82 52 217 2909.
E-mail address: yang@unist.ac.kr (C. Yang).
Figure 1. Chemical structure of the thiophene-fused coplanar sensitizer.
0040-4039/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.091

J. Kim et al. / Tetrahedron Letters 52 (2011) 2764–2766
2765
O
O
O
O
1. n-BuLi, THF
Br
S
SnBu3
S
Br
Br
S
S
Pd(PPh3)4
DMF
77%
2. BF3 OEt2
CH2Cl2
S
O
O
O
O
70%
1
2
3
Pd(PPh3)4
POCl3,
NBS/DMF
91%
K2CO3/THF
S
Br
S
DMF/CH2Cl2
N
S
S
O
N
96%
B
O
12%
5
4
H
NC COOH
S
S
N
N
COOH
O
S
piperidine
CH3CN
S
NC
46%
6
7
Scheme 1. Synthetic route of the thiophene-fused coplanar sensitizer 7.
with NBS in DMF to generate the mono bromothiophenyl com-
pound 4. Suzuki coupling of 4 with N,N-dimethylaniline-4-boro-
nate ester yielded the key intermediate 5. The thiophene unit
was converted to the corresponding carbaldehyde 6 via Vilsme-
ier–Haack reaction.19 Upon condensation of 6 with cyanoacetic
acid by Knoevenagel reaction in the presence of piperidine, the tar-
get thiophene-fused coplanar sensitizer 7 was isolated as a dark-
red solid. Thanks to the bulky aryl substituents as the solubilizing
groups, the coplanar sensitizer 7 shows excellent solubility in com-
mon organic solvents, overcoming the planarity–solubility tradeoff
to facilitate its processability.
introduction of thiophenes as heteroaromatic electron-rich build-
ing segments instead of phenylene groups in the dye.
The absorption peak of the coplanar sensitizer 7 is blue-shifted
by adsorption on the TiO2 surface (Fig. 2). The blue shift of the
absorption spectrum by adsorption on the TiO2 surface is also ob-
served in other organic dyes,20,21 which is presumably attributed to
the deprotonation of the dye by adsorption on the TiO2 surface (i.e.,
the carboxylate anion). In addition, we cannot count out that the
interactions between TiO2 surface and the adsorbed dye molecule
may lead to the formation of H-type aggregate.22
Photovoltaic performance of DSSCs: Nano-crystalline TiO2 col-
loids prepared by a sol-gel hydrolysis method23 were used for
making the TiO2 paste. The TiO2 paste was consisted of TiO2
Optical properties: The UV–vis absorption of the coplanar sensi-
tizer 7 was investigated in chloroform and on the TiO2 film, respec-
tively, as depicted in Figure 2. The absorption of 7 shows a broad
nano-crystalline, ethylcellulose as a binder, and
a-terpineol as a
peak centered at 500 nm with extinction coefficient
(
e)
of
solvent. The TiO2 paste was printed on a F-doped SnO2 conducting
glass (TEC Glass-TEC 8, Pilkington) and subsequently sintered at
84,000 Mꢀ1 cmꢀ1, arising from p⁄ charge transfer transition in
p–
the chromophore backbone. Notably, the absorption of the copla-
nar sensitizer 7 is substantially bathochromically shifted with sig-
nificantly higher molar extinction coefficient as compared to the
mother ladder-type pentaphenylene-based dyes (the absorption
maxima of the dyes range from 442 to 457 nm and their molecular
coefficients are at less than 76,000 Mꢀ1 cmꢀ1).6 This is due to the
500 °C for 1 h. The thickness of the TiO2 thin film was 15 lm and
the active area of the TiO2 electrode was 0.25 cm2. The coplanar
sensitizer 7 was dissolved at a concentration of 0.3 mM in an abso-
lute ethanol. The TiO2 film was immersed in the dye solution and
kept at room temperature for 1 day to allow the dye to be adsorbed
to the TiO2 surface. A detailed fabrication process is described in
Supplementary data.
The photovoltaic performance of the DSSCs was obtained using
a liquid electrolyte comprising 0.6 M 1-hexyl-2,3-dimethyl-imi-
dazolium iodide, 0.1 M lithium iodide, 0.05 M iodine, and 0.5 M
4-tert-butylpyridine in acetonitrile.24 The photovoltaic perfor-
mance was recorded under the simulated AM 1.5 irradiation
(100 mW cmꢀ2).
The action spectrum of monochromatic incident photon-to-cur-
rent conversion efficiency (IPCE) for the DSSC based on the copla-
nar sensitizer 7 is presented in Figure 3. As expected, the IPCE
spectrum of the DSSC based on the coplanar sensitizer 7 is batho-
chromic-shifted, when compared to that based on the ladder-type
pentaphenylene-based dyes,6 which is in good agreement with
absorption behavior of 7 described above. The onset of the IPCE
spectrum of the DSSC based on 7 is 700 nm and a high IPCE
(>60%) is observed in the range from 440 to 520 nm with a maxi-
mum value of 65.5% at 475 nm.
100000
1.0
0.8
0.6
0.4
80000
60000
40000
20000
0.2
0
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 2. UV–vis absorption spectra of the thiophene-fused coplanar sensitizer 7 in
CHCl3. (-4-) and on the TiO2 film (-s-).

2766
J. Kim et al. / Tetrahedron Letters 52 (2011) 2764–2766
with the parent ladder-type pentaphenylene-based dyes, the bath-
ochromic shift of the absorption spectrum is achieved by introduc-
tion of thiophenes as electron-rich groups. A maximum value of
2.31% is reached under AM 1.5 irradiation (100 mW cmꢀ2) with a
70
60
50
40
30
20
10
0
g
DSSC based on the thiophene-fused coplanar sensitizer
7
(Jsc = 7.3 mA cmꢀ2, Voc = 0.53 V, and FF = 60.3%). Although the effi-
ciency of the resulting rigid coplanar dye molecule has somewhat
lower relative to other dyes such as Ru-complex and other metal-
free organic dyes, this work has launched the heteroaromatic-
fused molecular structure into the possibility of sensitizers based
on
p-spacer bridged donors and acceptors. We believe that the
300
400
500
600
700
800
development of highly efficient organic sensitizers is possible
through sophisticated structural modification based on the lad-
der-type heteroaromatic dyes, and work on these is now in
progress.
Wavelength (nm)
Figure 3. IPCE spectrum for dye-sensitized solar cell based on thiophene-fused
coplanar sensitizer 7.
Acknowledgments
8
6
4
2
0
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2010-
0002494) and the National Research Foundation of Korea Grant
funded by the Korean Government (MEST) (2010-0019408),
(2010-0026916), (2010-0026163), and (2010-0026163).
Supplementary data
0.0
0.1
0.2
0.3
Voltage (V)
0.4
0.5
0.6
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.03.091.
Figure 4. J–V curve obtained with dye-sensitized solar cell based on thiophene-
fused coplanar sensitizer 7 under AM 1.5 irradiation (100 mW cmꢀ2).
References and notes
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the following equation:
g (%) un-
3. Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska,
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g
¼ ðJsc ꢁ Voc ꢁ FFÞ ꢁ 100=I0
ð1Þ
where I0 (mW cmꢀ2) is the photon flux (100 mW cmꢀ2 for AM
1.5G), Jsc (mA cmꢀ2) is the short-circuit current density under irra-
diation, Voc (V) is the open-circuit voltage, and FF represents the fill
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Voc = 0.53 V, and FF = 60.3%) is obtained with the DSSC based on 7
g ,
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under AM 1.5 solar condition.
By considering the high molar extinction coefficient and the
broad absorption of the dye 7 in our experiments, the further en-
hanced device performance could be expected. The low perfor-
mance of DSSCs based on the dye 7 is mainly due to the low
current value, which can be the result of the relatively decreased
IPCE. Although the efficiency of the dye 7 still needs to be further
improved with respect to high-end applications, the preliminary
result can be comparable with or better than those of the opti-
mized performance of DSSCs constructed with the parent ladder-
type pentaphenylene-based dyes (g = 1.1–2.3%). We are currently
studying the molecular structures of the further extended thio-
phene-fused coplanar sensitizers to help us better understand
the DSSCs based on heteroaromatic fused molecular structures.
In summary, a novel ladder-type donor (D)-p-spacer-acceptor
(A) heteroaromatic dye is synthesized as a photosensitizer for DSSC
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applications in which the thiophene-fused coplanar building block
serves as a
p-conjugated electron spacer. The electron-donating
moiety is substituted to N,N-dimethylaniline, and the electron-
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withdrawing part/anchor is 2-cyanoacrylic acid. In comparison