Novel organic dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/b603706b

Two novel metal-free organic dyes containing thienothiophene and thiophene segments have been synthesized. Nano-crystalline TiO₂ dye-sensitized solar cells were fabricated using these dyes as light-harvesting sensitizers, and a high solar energ

Full text of DOI:10.1039/b603706b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Novel organic dyes for efficient dye-sensitized solar cells{
Shao-Lu Li,ab Ke-Jian Jiang,*a Ke-Feng Shaoab and Lian-Ming Yang*a
Received (in Cambridge, UK) 13th March 2006, Accepted 2nd May 2006
First published as an Advance Article on the web 26th May 2006
DOI: 10.1039/b603706b
Two novel metal-free organic dyes containing thienothiophene
and thiophene segments have been synthesized. Nano-crystal-
line TiO2 dye-sensitized solar cells were fabricated using these
dyes as light-harvesting sensitizers, and a high solar energy-to-
electricity conversion efficiency of 6.23% was achieved.
Dye-sensitized solar cells (DSSCs) have received wide attention
due to their high performance and low-cost of production. DSSCs
based on Ru-complex photosensitizers such as N3 and black dyes
can provide efficient solar energy-to-electricity conversion effi-
ciencies (g) of up to 10%.1–6 Compared to Ru-complexes, the
advantages of sheer organic dyes are that not only can they exhibit
high molar extinction coefficients, but that they can also be
prepared and purified easily at lower cost. Metal-free dyes such as
perylene dyes,7,8 cyanine dyes,9,10 xanthene dyes,11 merocyanine
dyes,12–14 coumarin dyes,15–17 hemicyanine dyes,18,19 and indoline
dyes20,21 have been investigated as sensitizers in DSSCs, and great
progress has been made in this field. To date the highest overall
photoelectric conversion efficiency of organic dye-sensitized solar
cells already can reach a considerable level of 8%.21 Therefore,
development of new organic dyes with photo-response in the
broad solar spectrum is promising for the improvement of the
light-to-electricity conversion in DSSCs. Generally, organic dyes
should contain a structure of donor-to-acceptor bridged by a
p-conjugation system, and possess a broad and intense spectral
absorption. A usual approach to extend the absorption region of
dyes is to increase the methine unit (–CHLCH–) to enlarge the
p-conjugation system, leading to a red-shift of dye absorption.
However, such a measure also often result in instability of dye
molecules due to the possibility of isomer formation. To obtain
organic dyes with both red-shift absorption and high stability,
introduction of new p-conjugation segments into dye molecules is
necessary, and much research has been done along this line.22–25
Here, we report two new dyes containing thienothiophene or
thiophene segments as p-conjugation system (coded as D-ST and
D-SS, respectively, Fig. 1). To the best of our knowledge there has
been no report on the application of a dye containing a
thienothiophene moiety in DSSCs, .
Fig. 1 Molecular structure of the dyes.
treated with 40 mM TiCl4 aqueous solution as reported by the
Gra¨tzel group.26 The double layer of TiO2 was prepared by the
screen-printing mode. The first TiO2 layer of 10 mm thickness was
coated with paste (PST 18 NR, CCI in Japan, TiO2 particle size:
18 nm), and the second TiO2 layer of 4 mm thickness was printed
with a different paste (PST 400 C, CCI, TiO2 particle size: 400 nm)
for the improvement of light harvesting by the photo-scattering
The dyes D-ST and D-SS were constructed by a stepwise
synthetic protocol illustrated in the ESI.{{ For the preparation of
DSSC, an FTO glass plate (Nippon Sheet Glass, 10 V sq21) was
cleaned with detergent solvents, water and ethanol, respectively.
After treatment in a UV–O3 system for 18 min, the glass was
aNew Materials Laboratory, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100080, China. E-mail: yanglm@iccas.ac.cn;
Fax: 86-010-62559373; Tel: 86-010-62565609
bGraduate School, Chinese Academy of Sciences, Beijing 100080, China
{ Electronic supplementary information (ESI) available: Full synthesis
and characterization details. See DOI: 10.1039/b603706b
Fig. 2 (a) Absorption spectra of the dyes in THF and on the TiO2 film.
(b) Absorption and emission spectra of the dyes recorded in THF.
2792 | Chem. Commun., 2006, 2792–2794
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Table 1 Optical, redox and DSSC performance parameters of the dyes
labsa/nm
Eox (DEp)b/V
(vs Ag/Ag+)
Eox*/V
(vs Ag/Ag+)
Dye
(e/M21 cm21
)
lema/nm
E0-0c/eV
Voc
Jsc
FF
g (%)
D-SS
D-ST
a
513 (38400)
488 (50100)
636
542
0.40 (0.10)
0.29 (0.05), 0.48 (0.07)
2.13
2.39
b
21.73
22.10, 21.91
0.52
0.56
10.64
15.23
0.70
0.73
3.87
6.23
Absorption and electrochemical data were recorded in THF solution. Redox potentials were measured in 0.1 M tetrabutylammonium
hexafluorophosphate in acetonitrile. Scanning rate: 20 mV s21; DEp is the separation between the anodic and cathodic peaks. Determined
from the intersection point of absorption and emission spectra.
c
which is in good agreement with the absorption behavior of the
dyes on TiO2 film. The onset of the IPCE spectrum of the DSSC
based on D-ST is 790 nm, and a high IPCE (.70%) was observed
in the range from 470 nm to 580 nm with a maximum value of
82% at 515 nm. In contrast, the DSSC based on D-SS shows a
relatively low IPCE with the maximum of 66% at 540 nm.
To judge the possibility of electron transfer from the excited dye
molecule to the conductive band of TiO2, cyclic voltammograms
were performed to determine redox potentials of the two dyes.
Both dyes are reversible at a moderately high oxidation potential.
The oxidation peak potential of cathodic current and the reduction
potential calculated from the oxidation potential and the energy at
the crossing section of absorption and emission spectra observed
under low-concentration conditions are listed in Table 1. As shown
in Table 1, the excited state oxidation potential (Eox*) of the dyes
(D-SS: 21.73 vs Ag/Ag+, 20.93 vs NHE; D-ST: 22.10, 21.91 vs
Ag/Ag+, 21.30, 21.11 vs NHE) are far more negative than the
band edge energy of the nanocrystalline TiO2 electrode (20.5 vs
NHE), indicating that the electron injection process from the
excited dye molecule to TiO2 conduction band is energetically
permitted.
property of large TiO2 particles. The detailed procedure for
sintering TiO2 was similar to that described in literature.27 After
being sintered, the TiO2 film was re-treated with 40 mM TiCl4,26
and then heated at 400 uC for 20 min. After cooling to 80 uC, the
film was immersed in a THF solution of 0.3 mM dye at room
temperature for 24 h. In order to prevent aggregation of dye
molecules on the TiO2 surface, 20 mM of chenodeoxycholic acid
was added to the dye solution.28
For the photovoltaic measurement, the dye-coated TiO2 film
was sandwiched with a Pt-sputtered FTO glass, and an organic
electrolyte was introduced between electrodes by the capillary
force. The electrolyte composed of 0.05 M I2, 0.1 M LiI, 0.6 M
1-propyl-3-methylimidazolium iodide and 0.5 M 4-tert-butyl-
pyridine in a mixed solvent of acetonitrile and valeronitrile
(1 : 1, v/v). The photovoltaic performance was recorded under
the simulated AM 1.5 irradiation (100 mW cm22).
The absorption and emission spectra of the dyes are displayed in
Fig. 2, and the data are collected in Table 1. In THF solution, both
dyes give narrow absorption bands, and the dye D-SS has a slight
red-shift absorption in comparison to D-ST. This may arise from
larger p-conjugation system of D-SS than D-ST. On the other
hand, the absorption spectra of the two dyes on TiO2 film are
broadened. D-SS produces a slight blue-shift due to the formation
of H-aggregate on the TiO2 electrode and D-ST has a red-shift due
to J-aggregation. As a result, the dye D-ST has a broader
absorption spectrum than the dye D-SS. The broadening of
absorption is favorable for harvesting solar light and leading to a
large photocurrent.
The performance statistics of the devices are also listed in
Table 1. An overall conversion efficiency (g) of 6.23% was achieved
from the DSSC based on the dye D-ST
(
short-circuit
photocurrent density, Jsc
=
15.23 mA cm22
; open-circuit
photovoltage, Voc = 0.56 V; fill factor, ff = 0.73). This efficiency
is much higher than the efficiency of 3.87% obtained from the
DSSC based on the dye D-SS, although D-SS possesses a larger
p-conjugation system.
The action spectra of monochromatic incident photo-to-current
conversion efficiencies (IPCEs) for the DSSCs based on the two
dyes are presented in Fig. 3. The IPCE spectrum of the DSSC
based on D-ST is red shifted compared to that based on D-SS,
In summary, we have designed and synthesized two novel
organic dyes featuring thienothiophene- and thiophene-bridging
structures. Both dyes show good performance for DSSCs.
Especially the conversion efficiency of the DSSC based on the
dye D-ST can reach 6.23%, a considerably high level for organic
dyes. This result strongly suggests that thienothiophene could be
an excellent p-conjugation system for organic dyes to harvest the
solar spectrum.
This work was supported by the National 863 Program of
China (715-Z36-1-98) and the National Natural Science
Foundation of China (Project no. 60276007).
Notes and references
1
{ D-SS: H NMR (DMSO-d6): d 1.10 (6H, t, J = 6.98 Hz), 3.34 (4H, q,
J = 6.96 Hz), 4.46 (2H, s), 6.64 (2H, d, J = 8.76 Hz), 6.82 (1H, d, J =
15.77 Hz), 7.03 (1H, d, J = 3.49 Hz), 7.11 (1H, d, J = 16.15 Hz), 7.13 (1H,
d, J = 15.82 Hz), 7.22 (1H, d, J = 3.77 Hz), 7.36–7.39 (3H, m), 7.43 (1H, d,
J = 15.89 Hz), 7.71 (1H, d, J = 3.86 Hz), 8.04 (1H, s). MALDI-TOF m/z:
566.1 [M+]. HRMS-SIMS (m/z): [M2] calcd for C28H26N2O3S4, 566.0826,
found, 566.0837 (100%); [M 2 COOH]2 calcd for C27H25N2O1S4,
521.0850, found, 521.0851 (100%).
Fig. 3 Absorption spectrum of the dyes absorbed on nanocrystalline
TiO2 and the action spectra for DSSCs constructed using D-SS and D-ST.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2792–2794 | 2793

View Article Online
D–ST: 1H NMR (DMSO-d6): d 1.10 (6H, t, J = 6.98 Hz), 3.38 (4H, q,
J = 7.02 Hz), 4.49 (2H, s), 6.67 (2H, d, J = 8.02 Hz), 6.99 (1H, d, J =
16.11 Hz), 7.26 (1H, d, J = 15.98 Hz), 7.44 (2H, d, J = 8.13 Hz), 7.52 (1H,
s), 8.08 (1H, s), 8.20 (1H, s). MALDI-TOF m/z: 514.4 [M+]. HRMS-SIMS
(m/z): [M2] calcd for C24H22N2O3S4, 514.0513, found, 514.0525 (100%);
[M 2 COOH]2 calcd for C23H21N2O1S4, 469.0537, found, 469.0549 (78%).
13 K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi,
Y. Abe, H. Sugihara and H. Arakawa, Chem. Commun., 2000, 1173.
14 K. Sayama, S. Tsukagoshi, T. Mori, K. Hara, Y. Ohga, A. Shinpou,
Y. Abe, S. Suga and H. Arakawa, Sol. Energy Mater. Sol. Cells, 2003,
80, 47.
15 K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and H. Arakawa,
Chem. Commun., 2001, 569.
16 K. Hara, Y. Tachibana, Y. Ohga, A. Shinpo, S. Suga, K. Sayama,
H. Sugihara and H. Arakawa, Sol. Energy Mater. Sol. Cells, 2003, 77,
89.
17 K. Hara, M. Kurashige, Y. Danoh, C. Kasada, A. Shinpo, S. Suga,
K. Sayama and H. Arakawa, New J. Chem., 2003, 27, 783.
18 E. Stathatos and P. Lianos, Chem. Mater., 2001, 13, 3888.
19 Z. S. Wang, F. Y. Li and C. H. Huang, Chem. Commun., 2000, 2063.
20 T. Horiuchi, H. Miura and S. Uchida, Chem. Commun., 2003, 3036.
21 T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, J. Am. Chem. Soc.,
2004, 126, 12218.
22 K. Hara, M. Kurashige, S. Ito, A. Shinpo, S. Suga, K. Sayama and
H. Arakawa, Chem. Commun., 2003, 252.
23 T. kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson, X. Ai,
T. Lian and S. yanagida, Chem. Mater., 2004, 16, 1806.
24 M. Velusamy, K. R. J. Thomas, J. T. Lin, Y. C. Hsu and K. C. Ho,
Org. Lett., 2005, 7, 1899.
25 K. R. J. Thomas, J. T. Lin, Y. C. Hsu and K. C. Ho, Chem. Commun.,
2005, 4098.
26 S. Ito, P. Liska, P. Comte, R. Charvet, P. Pe´chy, U. Bach, L. Schmidt-
Mende, S. M. Zakeeruddin, A. Kay, M. K. Nazeeruddin and
M. Gra¨tzel, Chem. Commun., 2005, 4351.
27 P. Wang, S. M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-
Baker and M. Gra¨tzel, J. Phys. Chem. B, 2003, 107, 14336.
28 A. Kay and M. Gra¨tzel, J. Phys. Chem., 1993, 97, 6272.
1 B. O’Regan and M. Gra¨tzel, Nature, 1991, 353, 737.
2 M. K. Nazeeruddin, A. Kay, I. Rodicio, R. H. Baker, E. Muller,
P. Liska, N. Vlachopoulos and M. Gra¨tzel, J. Am. Chem. Soc., 1993,
115, 6382.
3 M. K. Nazeeruddin, P. Pe´chy, T. Renouard, S. M. Zakeeruddin,
R. H. Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover,
L. Spiccia, G. B. Deacon, C. A. Bignozzi and M. Gra¨tzel, J. Am. Chem.
Soc., 2001, 123, 1613.
4 T. Renouard, R. A. Fallahpour, M. K. Nazeeruddin, R. Humphry,
S. I. Gorelsky, A. B. P. Lever and M. Gra¨tzel, Inorg. Chem., 2002, 41,
367.
5 M. Yanagida, T. Yamaguchi, M. Kurashije, K. Hara, R. Katoh,
H. Sugihara and H. Arakawa, Inorg. Chem., 2003, 42, 7921.
6 C. Klein, M. K. Nazeeruddin, D. D. Censo, P. Liska and M. Gra¨tzel,
Inorg. Chem., 2004, 43, 4216.
7 S. Ferrere, A. Zaben and B. A. Gregg, J. Phys. Chem. B, 1997, 101, 449.
8 S. Ferrere and B. A. Gregg, New J. Chem., 2002, 26, 1155.
9 K. Sayama, K. Hara, Y. Ohga, A. Shinpou, S. Suga and H. Arakawa,
New J. Chem., 2001, 25, 200.
10 A. Ehret, L. Stuhi and M. T. Spitler, J. Phys. Chem. B, 2001, 105, 9960.
11 K. Hara, T. Horiguchi, T. Kinoshita, K. Sayama, H. Sugihara and
H. Arakawa, Chem. Lett., 2000, 29, 316.
12 A. C. Khazraji, S. Hotchandani, S. Das and P. V. Kamat, J. Phys.
Chem. B, 1997, 103, 4693.
2794 | Chem. Commun., 2006, 2792–2794
This journal is ß The Royal Society of Chemistry 2006