Novel D-π-A type II organic sensitizers for dye sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2012.04.049

Four organic donor-π-conjugated-acceptor (D-π-A) type II dyes with different thiophene linkers are reported for dye sensitized solar cells (DSSCs). For the first time, a donor (triphenylamine) was introduced in type II sensitizers, and 2-hydroxybenzonitrile as acceptor/anchoring moiety was covalently linked TiO₂ particles. The dye LS203 in this series gives the best solar energy conversion efficiency of 3.4%, with J_sc = 7.4 mA cm^-2, V_oc = 0.67 V, FF = 0.69, the maximum IPCE value reaches 66.9%.

Full text of DOI:10.1016/j.tetlet.2012.04.049

Tetrahedron Letters 53 (2012) 3425–3428
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Tetrahedron Letters
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Novel D–p–A type II organic sensitizers for dye sensitized solar cells
Shi-Feng Li a, Xi-Chuan Yang a, , Ming Cheng a, Jiang-Hua Zhao a, Yu Wang a, Li-Cheng Sun a,b,
⇑
⇑
a State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China
b Department of Chemistry, Organic Chemistry, Royal Institute of Technology (KTH), Stockholm 10044, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 February 2012
Revised 3 April 2012
Accepted 12 April 2012
Available online 20 April 2012
Four organic donor–p–conjugated-acceptor (D–p–A) type II dyes with different thiophene linkers are
reported for dye sensitized solar cells (DSSCs). For the first time, a donor (triphenylamine) was introduced
in type II sensitizers, and 2-hydroxybenzonitrile as acceptor/anchoring moiety was covalently linked TiO2
particles. The dye LS203 in this series gives the best solar energy conversion efficiency of 3.4%, with
Jsc = 7.4 mA cmÀ2, Voc = 0.67 V, FF = 0.69, the maximum IPCE value reaches 66.9%.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Photovoltaic devices
Solar cells
Due to the increase in solar energy conversion efficiency and
decrease in cost, dye sensitized solar cells (DSSCs) have drawn
much attention during the past two decades.1,2 In most of tradi-
dyes, LS203 gives the best solar energy conversion efficiency of
3.4%, the maximum IPCE value reaches about 66.9%.
The absorption spectra of the four dyes dissolved in CH2Cl2
(Fig. 2a) and adsorbed on TiO2 (Fig. 2b) are displayed in Figure 2,
and the photophysical data are collected in Table 1. In CH2Cl2 solu-
tion, the absorption maxima are between 351 and 376 nm. When
the four dyes were absorbed on the TiO2, the absorption maxima
do not have significant changes, but the absorption peaks become
much wider and the cut-off absorptions of the four dyes changed
from 400 nm dissolved in CH2Cl2 to more than 450 nm absorbed
on TiO2. That brings about a surprising change of the colors of
the dyes from the CH2Cl2 solutions to the TiO2 absorbed films
showed in Figure 3.
The first oxidation potentials (Eox) of the four dyes were mea-
sured by cyclic voltammetry (CV) in CH2Cl2 solution; the data were
collected in Table 1. It shows that the HOMO levels of the four dyes
were sufficiently more positive than the iodine/triiodide redox po-
tential value (0.4 V vs NHE), indicating that the oxidized dyes
formed after electron injection into the CB of TiO2 could accept
electrons from I- thermodynamically.17,18 In addition, the LUMO
levels of the four dyes were more negative than the Ecb of TiO2
(À0.5 V vs NHE) indicating that the dyes all have enough driving
force for electron injection.17
tional organic D–p–A dyes, cyanoacrylic acid group was always
introduced as acceptor and anchoring group, which has limited
much more excellent dyes that were developed.2 It is well known
that the traditional DSSCs (Type I DSSCs) are the devices in which
electrons are injected from the adsorbed dyes by photoexcitation
of the dyes followed by electron injection from the excited dyes
to TiO2.3,4 Type II DSSCs in which electrons are injected not only
by electron injection from the excited states of dyes to TiO2, but
also by direct one-step electron injection from the dyes to TiO2
by photoexcitation of the dye-to-TiO2 charge-transfer bands.5–13
The DSSCs employing catechol or its derivatives in the dye struc-
tures have been considered as typical examples of Type II DSSCs.
An et al. reported a class of Type II thiophene-catechol sensitizers
used in DSSCs.14 However, the reported solar energy-to-electricity
conversion efficiencies of Type II DSSCs did not exceed 1.6%.5,14
In this Letter we report four novel Type II organic dyes LS201,
LS202, LS203, and LS204 with Donor–p–Acceptor design in struc-
ture (Fig. 1). Triphenylamine was introduced as a donor part in
Type II sensitizer, and catechol was used instead by 2-hydrox-
ybenzonitrile as the electron acceptor/anchoring group. Compared
with the traditional organic D–
p
–A dyes with cyanoacrylic acid
The J–V characteristic curves are shown in Figure 4a, and all
data are summarized in Table 2. The best solar cell we have
achieved is sensitized by LS203 dye, which has 3.4% efficiency un-
der AM 1.5 solar simulator illumination with a short circuit Jsc of
7.4 mA cmÀ2 and an open circuit voltage Voc of 0.67 V. The solar
cells sensitized by LS201, LS202, and LS204 have efficiencies of
2.8%, 3.1%, and 2.6%, respectively. All the dyes show better cell
performance compared to the compound Cat-v-Q.5 From the data,
system, the electron donor, -spacer and electron acceptors were
p
linked by C–C single bonds without the involvement of any vinyl
group, which would hopefully increase the stability of DSSCs.15,16
This is a new strategy to design Type II sensitizers. Among the four
⇑
Corresponding authors. Tel.: +86 411 88993886; fax: +86 411 83702185.
E-mail addresses: yangxc@dlut.edu.cn (X.-C. Yang), lichengs@kth.se (L.-C. Sun).
it is clear to see that introducing alkyl group to the
p-spacer is
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.049

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S.-F. Li et al. / Tetrahedron Letters 53 (2012) 3425–3428
Figure 1. Molecular structures of Type II dyes LS201, LS202, LS203, and LS204.
Table 1
Optical and electrochemical properties of LS201, LS202, LS203, and LS204
a
b
c
Dye
kmax
in
e
kmax
on
Tio2
E0–
Eox (V)
EoxÀE0–0
(V) (vs
NHE)
(104 MÀ1 cmÀ1
)
(vs
NHE)
d
0
CH2Cl2
(nm)
(V)
(nm)
LS201 376
LS202 356
LS203 351
LS204 364
3.85
3.35
2.95
3.30
376
358
356
365
2.82 0.85
2.92 0.84
2.92 0.88
2.72 0.84
À1.97
À2.08
À2.04
À1.88
a
Absorption spectra were measured in CH2Cl2 solution (2 Â 10À5 M) at room
temperature.
b
Absorption spectra on TiO2 were measured through the dye adsorbed on TiO2
film in CH2Cl2.
c
The oxidation potentials of the dyes were measured in CH2Cl2 with 0.1 M tet-
rabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working elec-
trode: glassy carbon; reference electrode: Ag/Ag+; calibrated with ferrocene/
ferrocenium (Fc/Fc+) as an internal reference and converted into NHE by addition of
440 mV, counter electrode: Pt).
d
E0–0 was estimated from the onset point of absorption spectra on TiO2.
Figure 2. The UV–vis spectra of dyes in CH2Cl2 (2 Â 10À5 M) (a) and on TiO2 films
(b).
Figure 3. The color change of the dyes dissolved in CH2Cl2 (above) and absorbed on
TiO2 (below).
beneficial for increasing the efficiency of DSSCs. The Jsc and Voc of
the devices based on LS202 and LS203 are much higher than that
of LS201. The IPCE spectra of these DSSCs are shown in Figure 4b,
the maximum IPCE obtained for LS203 was 66.9% at 440 nm, and
the maximum IPCE values for LS201, LS202, and LS204 are 66.2%,
68.9%, and 72.9%, respectively, at 420 nm. Although the maximum
IPCE of LS203 is not the highest, the IPCE curve of LS203 is broader
than the other three dyes. The integral current value of LS203 is

S.-F. Li et al. / Tetrahedron Letters 53 (2012) 3425–3428
3427
Figure 5. Electrochemical impedance spectra of DSSCs sensitized by LS201, LS202,
LS203, and LS204.
Figure 4. J–V characteristic curves (a) and IPCE spectra (b) of LS201, LS202, LS203,
and LS204.
process of the injected electrons at the interfaces between TiO2 and
the electrolyte/dye.19 It is clear to see that the device based on
LS203 has the biggest charge recombination resistance (Rrec
=
Table 2
Photovoltaic performances of DSSCs sensitized by LS201, LS202, LS203, LS204a, and
Cat-v-Qb
24.1 Ohm) on the TiO2, which can reduce the charge recombination
rate. It seems reasonable to conclude that the better performance
of the device based on LS203 can be ascribed to a reduced rate of
charge recombination, which led to highest value of Voc (0.67 V).
The similar result can be found from the Bode plots (Fig. 5b).
In summary, a new strategy to design the Type II dye sensitizers
for DSSCs was developed. For the first time, a triphenylamine donor
unit was introduced in Type II sensitizers. Catechol was replaced by
2-hydroxybenzonitrile and the vinyl group was avoided to increase
the stability of the DSSCs. By using the strategy four novel dyes were
successfully synthesized. The dye LS203 giving the best solar energy
conversion efficiency of 3.4%, with Jsc = 7.4 mA cmÀ2, Voc = 0.67 V,
FF = 0.69, the IPCE value reaches 66.9%. Improvement of the dye
structure of this type might further increase the performance of
the DSSCs. Related work is in progress.
Dyes
Jsc
Voc (V) FF
g (%) IPCEmax Integral current
(mA cmÀ2
)
(mA cmÀ2
)
LS201
LS202
LS203
LS204
6.3
6.9
7.4
6.9
0.62
0.65
0.67
0.55
0.58
0.71 2.8
0.70 3.1
0.69 3.4
0.69 2.6
0.63 1.6
66.2
68.9
66.9
72.9
6.3
7.0
7.3
7.0
Cat-v-Qb 4.3
a
Irradiating light: AM 1.5 G (100 mW cmÀ2); working area: 0.159 cm2; electro-
lyte: 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII), 0.06 M LiI,
0.04 M I2, 0.1 M 4-tert-butylpyridine (TBP) in acetonitrile.
Data taken from Ref. 5.
b
7.3 mA cmÀ2 showed in inset of Figure 4a, which match with J–V
measurement.
Acknowledgments
In order to study the electron transport and recombination
properties in the DSSCs based on LS201, LS202, LS203, and
LS204, the electrochemical impedance spectroscopy of these DSSCs
( Fig. 5) were measured at an applied bias of Voc under dark
condition. From the Nyquist plots (Fig. 5a) two semicircles were
observed clearly, the smaller one is ascribed to the charge-transfer
process at the interfaces between the redox couple and the plati-
nized counter electrode. The bigger one is related to the transport
We gratefullyacknowledgethefinancialsupport of thisworkfrom
China Natural Science Foundation (Grant21076039), the NationalBa-
sic Research Program of China (Grant 2009CB220009), the Ministryof
Science andTechnology (MOST) (Grant 2001CCA02500), the Swedish
Energy Agency, K&A Wallenberg Foundation, and the State Key Labo-
ratory of Fine Chemicals (KF0805).

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S.-F. Li et al. / Tetrahedron Letters 53 (2012) 3425–3428
6. Lu, H.; Prieskorn, J. N.; Hupp, J. T. J. Am. Chem. Soc. 1993, 115, 4927.
Supplementary data
7. Blackbourn, R. L.; Johnson, C. S.; Hupp, J. T. J. Am. Chem. Soc. 1991, 113,
1060.
Supplementary data (details of synthesis of the dyes and the
device fabrication) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.tetlet. 2012.04.049.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
8. Khoudiakov, M.; Parise, A. R.; Brunschwig, B. S. J. Am. Chem. Soc. 2003, 125,
4637.
9. Yang, M.; Thompson, D. W.; Meyer, G. J. Inorg. Chem. 2002, 41, 1254.
10. Weng, Y.-X.; Wang, Y.-Q.; Asbury, J. B.; Ghosh, H. N.; Lian, T. J. Phys. Chem. B
2000, 104, 93.
11. Ghosh, H. N.; Asbury, J. B.; Weng, Y.; Lian, T. J. Phys. Chem. B 1998, 102, 10208.
12. Walters, K. A.; Gaal, D. A.; Hupp, J. T. J. Phys. Chem. B 2002, 106, 5139.
13. Ramakrishna, G.; Ghosh, H. N. J. Phys. Chem. B 2001, 105, 7000.
14. An, B.; Hu, W.; Burn, P.; Meredith, P. J. Phys. Chem. C 2010, 114, 17964.
15. Wang, P.; Zakeeruddin, S.; Moser, J.; Nazeeruddin, M.; Sekiguchi, T.; Grätzel, M.
Nat. Mater. 2003, 2, 402.
References and notes
1. O’Regan, B.; Grätzel, M. Nature 1991, 353, 737.
2. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110,
6595.
16. Wang, P.; Zakeeruddin, S.; Humphry-Baker, R.; Moser, J.; Grätzel, M. Adv. Mater.
2003, 15, 2101.
3. Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M.; Nelson, J.; Li, X.;
Long, N. J.; Durrant, J. R. J. Am. Chem. Soc. 2004, 126, 5225.
4. Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.;
Durrant, J. R. J. Am. Chem. Soc. 2005, 127, 3456.
17. Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49.
18. Ito, S.; Zakeeruddin, M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.;
Nazeeruddin, M.; Péchy, P.; Takata, M.; Miura, H.; Uchida, S.; Grätzel, M. Adv.
Mater. 2006, 18, 1202.
5. Tae, E.; Lee, S.; Lee, J.; Yoo, S.; Kang, E.; Yoon, K. J. Phys. Chem. B 2005, 109,
22513.
19. Wang, Q.; Ito, S.; Grätzel, M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.;
Bessho, T.; Imai, H. J. Phys. Chem. B 2006, 110, 25210.