A novel pyridinium hemicyanine dye with carboxylate anchoring group and its application in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2011.12.133

A novel organic dye with N-substituted pyridinium as the acceptor and carboxylate as the anchoring group were designed and synthesized for dye-sensitized solar cells, which give solar energy-to-electricity conversion efficiency (η) of up to 3.47% in compa

Full text of DOI:10.1016/j.tetlet.2011.12.133

Tetrahedron Letters 53 (2012) 1341–1344
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Tetrahedron Letters
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A novel pyridinium hemicyanine dye with carboxylate anchoring group and
its application in dye-sensitized solar cells
Yifan Shan a, Jie Tang a, Hua Lai b, Hongwei Tan c, Fan Yang a, , Qiang Fang b, , Pierre Audebert d,
⇑
⇑
Adam Pron e
a Institutes for Advanced Interdisciplinary Research, East China Normal University, Shanghai 200062, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
c Department of Chemistry, Beijing Normal University, Beijing 100875, China
d P. P. S. M., UMR 8531, Ecole Normale Supérieure de Cachan, 94235 CACHAN, France
e CEA Grenoble INAC/SPrAM (UMR 5819), 17 Rue des Matryrs 38 38054 Grenoble, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel organic dye with N-substituted pyridinium as the acceptor and carboxylate as the anchoring
group were designed and synthesized for dye-sensitized solar cells, which give solar energy-to-electricity
Received 28 October 2011
Revised 26 December 2011
Accepted 30 December 2011
Available online 16 January 2012
conversion efficiency (
value of 5.27% under similar experimental conditions. This new dye is simple in structure, very easy to
synthesize, and gives high Voc
g) of up to 3.47% in comparison with the reference Ru-complex (N719 dye) with an
g
.
Ó 2012 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells (DSSCs) using ruthenium based dyes
have been actively investigated since Grätzel and co-workers re-
ported the first case of a highly efficient photovoltaic device with
the conversion efficiencies up to 11% at standard AM1.5 sun
light.1–7 Compared with ruthenium based dyes, metal-free organic
dyes exhibit higher molar extinction coefficients, and more impor-
tantly they can be prepared and purified in simpler procedures at
lower cost. Therefore, the development of novel metal-free organic
dyes to achieve high solar to electric power conversion efficiencies
has attracted more and more attention. Great advance in the use of
organic dyes for DSSCs has been made recently. A variety of organic
dyes such as coumarin dyes,8,9 hemicyanine dyes,10,11 indoline
dyes,12–14 merocyanine dyes,15 triphenylamine-based dyes16–19
and et al have been developed. It is very instructive that porphy-
rin-sensitized solar cells with cobalt (II/III)–based redox electrolyte
can achieve 12.3% efficiency according to the report of Grätzel and
co-workers24 in 2011. The most efficient hemicyanine dyes for
hemicyanine dyes with carboxylates as anchoring groups, which
exhibited improved IPCEs and overall photoelectric conversion effi-
ciencies in the DSSCs compared to the dyes with the same chromop-
heres, but sulfonates as anchoring groups. It was found that TiO2
absorbs carboxylates (–COOÀ) better than sulfonates (ÀSOÀ3 ) and
the interactions between TiO2 and carboxylates are also stronger.
Therefore, shorter tethering groups could be used to ensure closer
contact between dyes and TiO2.21
The major problems for low conversion efficiency of many
organic dyes in the DSSCs are the formation of dye aggregates on
the semiconductor surface and recombination of conduction-band
electrons with triiodide. However, these problems could be
solved by using appropriate molecular structural design and
modification.19
Herein, we report a new pyridinium hemicyanine dye (CP1) with
carboxylate as anchoring group (Fig. 1). To the best of our
knowledge, the application of these type of pyridinium acceptors
in DSSCs has never been reported. CP1 can be easily synthesized
with inexpensive materials and shows good efficiencies. CP1 mole-
cule employs a diphenylaniline moiety as the electron donor, a
pyridinium moiety as the electron acceptor and a carboxlate as
the anchoring group to attach to TiO2 (see Fig. 1). The ethoxy-substi-
tuted oligo-phenylenevinylene chain extends the p-conjugation and
thereby gives a red shift in the absorption spectrum without affect-
ing the stability of the dye. The hemicyanine dye PS with sulfonate
(ÀSOÀ3 ) as the anchoring group10, which was reported by Huang and
co-workers, is also synthesized for comparison.
DSSCs have been reported so far are D-p-A-type cationic dyes with
p-dialkylaminophenyl groups as donors, cationic groups as accept-
ers such as benzo- and naphthothiazolium, pridinium, and indolium
salts, and methine anchoring groups. The overall yields were im-
proved when the long alkyl chains were replaced by (–CH@CH–)
units as
p
conjugation linkers between the donors and acceptors.20
In 2000, Huang and co-workers reported pyridinium hemicyanine
dyes PS and LPS with sulfonates (ÀSOÀ3 ) as the methyl group.10
Zhang and co-workers reported five new benzothiazolium
The absorption, emission, and electrochemical properties of PS
and CP1 are listed in Table 1. Absorption spectra of these dyes in
CH2Cl2 solutions at a concentration of 1 Â 10À5 M are shown in
⇑
Corresponding authors. Tel.: +86 21 62232764; fax: +86 21 62232100.
E-mail addresses: fyang@chem.ecnu.edu.cn (F. Yang), qiangfang@mail.sioc.ac.cn
(Q. Fang).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.133

1342
Y. Shan et al. / Tetrahedron Letters 53 (2012) 1341–1344
O
N
N
N+
N+
O
COO-
-
SO3
PS
CP1
Figure 1. Chemical structures of PS and CP1.
Table 1
PS
CP1
8.0x10-5
Absorption, emission, and electrochemical properties of PS and CP1
Dye
Absorptiona
Emissiona
Oxidation potential datab
kmax/nm
e
/MÀ1 cmÀ1
kem/nm
Eo–oc/V
Eox/V
Eox –Eo–o/V
4.0x10-5
0.0
PS
CP1
514
519
46,100
36,400
601
663
2.21
2.12
0.90
1.09
À1.31
À1.03
a
Absorption and emission spectra were measured in CH2Cl2 solution
(1 Â 10À5 M) at room temperature.
b
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/AgI/IÀ; calibrated with ferrocene/
ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of
0.55 V, counter electrode: Pt).
-4.0x10-5
-8.0x10-5
c
E0–0 was estimated from the onset point of absorption spectra.
-2
-1
0
1
2
0.5
Voltage [V]
PS
CP1
Figure 3. The CV curves of dyes in CH2Cl2.
0.4
0.3
0.2
0.1
Potential vs. NHE [V]
PS
-1.31 V
CP1
-1.03 V
-1.5
-1.0
CB
-0.5 V
-0.5
2.21 V
0.0
0.5
2.12 V
I- / I3
-
0.0
350 400 450 500 550 600 650 700 750 800
0.4 V
Waveleng [nm]
Figure 2. Absorption spectra of PS and CP1 in CH2Cl2 solutions (1 Â 10À5 M).
1.0
1.5
0.90 V
1.09 V
Figure 2. Both PS and CP1 have relatively broad and strong absorp-
tions in the visible region with maxima at 514 and 519 nm, respec-
tively. CP1 has two intense absorption maxima at 363 and 519 nm,
Figure 4. Molecular orbital energy diagram of HOMO and LUMO of dyes.
indicating
p–p⁄ transition and intramolecular charge transfer (ICT)
between the donor and the pyridinium acceptor, respectively.
The molar extinction coefficients of PS and CP1 are 46,100
dm3 molÀ1 cmÀ1 (514 nm) and 36,400 dm3 molÀ1 cmÀ1 (519 nm),
respectively. These numbers are obviously larger than that of
N719, indicating good light—harvesting ability. Although PS and
CP1 have the similar absorption maxima (kmax) and molar extinc-
tion coefficients, CP1 has broader absorption range than PS does.
These results suggest that the absorption spectrum is determined
by both the length of the conjugated linker and the nature of the
donor group.
cycle, the CVs of these dyes were performed in CH2Cl2 to measure
the redox potentials (Fig. 3). The first oxidation potentials (Eox) cor-
responding to the HOMO energy levels of the dyes were summa-
rized in Table 1. The HOMO energy levels of PS and CP1 dyes are
0.90 and 1.09 V, respectively. These values are higher than that
of iodine/iodide (0.4 V)22, therefore the oxidized dyes could be re-
duced effectively by electrolyte and regenerated (Fig. 4). The LUMO
energy levels of these dyes can be calculated by using HOMO en-
ergy level and zeroth–zeroth energy (E0–0) of the dyes estimated
from the intersection between the absorption and emission spec-
tra, namely, HOMO–E0–0. To effectively inject the electron into
the CB of TiO2, the LUMO energy levels of the dyes must be much
lower than the conducting band energy (Ecb) of the semiconductor,
To further study the mechanism of electron injection from the
dye to conduction band (CB) of semiconductor, followed by the
reduction of the dye cation back to its original state for the next

Y. Shan et al. / Tetrahedron Letters 53 (2012) 1341–1344
1343
Table 2
Photovoltaic performance of DSSCs based on PS, CP1 and N719 dyesa
Dye
JSC/mA cmÀ2
VOC/V
Fill factor
g (%)
PSb
0.75
7.84
10.43
0.56
0.66
0.76
0.63
0.67
0.66
0.26
3.47
5.27
CP1b
N719c
a
Measured under irradiation of AM 1.5G simulated solar light (100 mW cmÀ2) at
room temperature, 12
l
m film thickness, 0.25 cm2 working area; electrolyte: 1.0 M
1,2-dimethyl-3-n-propylimidazolium iodide (DMPII), 0.05 M LiI, 0.03 M I2, 0.1 M
guanidinium thiocyanate (GuSCN), 0.5 M 4-tert-butylpyridine (TBP) in AcCN and
valeronitrile (85:15).
b
The concentration of pyridinium dyes are 3 Â 10À4 M in CH2Cl2.
c
The concentration of N719 dye is 3 Â 10À4 M in ethanol.
12
PS
CP1
N719
10
8
6
4
2
0
Figure 6. The frontier molecular orbitals (MOs) of the HOMO (top) and LUMO
(bottom) of the CP1 dye.
density Jsc = 7.84 mA/cm2, open circuit photovoltage Voc = 0.66 V,
and fill factor ff = 0.67) for DSSCs using PS and CP1, respectively.
Under the similar conditions, the N719 dye gives a
g value of
5.27%. The fill factor (ff) of CP1 is similar to those of PS and
N719. However, CP1 gives much higher Jsc and Voc than the hemi-
cyanine dye PS does. Noticeably, most hemicyanine dyes have low-
er Voc around 0.40–0.60 V, than that of CP1 (up to 0.66 V). The
incident photon-to-current conversion efficiencies (IPCEs) of these
dyes in DSSCs are shown in Figure 5b. CP1 gives higher IPCE value
and broader IPCE spectrum than PS does in the spectral range of
400–700 nm, with highest value of 46% at 450 nm. Compared with
N719, CP1 has lower Jsc, which might be resulted from the fact that
0.0
0.1
0.2
0.3
Voltage [V]
0.4
0.5
0.6
0.7
0.8
Figure 5a. Photocurrent density versus voltage curves for DSSCs based on PS, CP1
and N719 under irradiation of AM 1.5G simulated solar light (100 mW cmÀ2).
the methylene group in the pyridinium framework disrupts the p-
80
conjugation between the pyridinium and the carboxylate, thus de-
creases the electron injection efficiency in a dynamic manner.23
Density functional theory (DFT) calculations were performed at
a B3LYP/3-21G (d) level for the geometry optimization to observe
frontier molecular orbitals (MOs) of the HOMO and LUMO. The
frontier MOs of the CP1 reveal that HOMO–LUMO excitation moves
the electron density distribution from TPA moieties to the N-
substituted pyridinium (Fig. 6). It is noteworthy that the LUMO
electron density of the CP1 is located mainly on the N-substituted
pyridinium, therefore the excited electron can be injected into the
TiO2 electrode efficiently.
In summary, we have synthesized a new pyridinium hemicya-
nine dye CP1 for DSSCs. A prominent solar energy-to-electricity
conversion efficiency (g) of 3.47% and high Voc are achieved in a
DSSC using this dye. CP1 is simple in structure, easy to synthesize,
and has good DSSC performance. Further structural modification to
improve the DSSC performance is in progress.
70
PS
CP1
60
N719
50
40
30
20
10
0
400
450
500
550
600
650
700
750
800
Wavlength [nm]
Figure 5b. The incident photon-to-current conversion efficiencies spectra for
DSSCs based on PS, CP1 and N719.
Acknowledgments
À0.5 V (vs NHE). Both of the dyes can achieve the electron injection
and form the oxidized dyes according to their LUMO energy values
(Fig. 4). The relatively large energy gaps between the LUMO of the
dyes and Ecb of semiconductor are sufficient for efficient electron
injection.
We gratefully acknowledge the financial support of this work
from Shanghai Committee of Science and Technology of China
(Grant No. 10520710100). We also thank the Laboratory of organic
Functional molecules, Sino-French Institute of ECNU for support.
The photovoltaic properties of these dyes on DSSCs are shown
in Table 2 and J–V curves of the dyes are shown in Fig. 5a. We have
Supplementary data
achieved a
g value of 0.26% (with short-circuit photo current den-
sity Jsc = 0.75 mA/cm2, open circuit photovoltage Voc = 0.56 V, and
fill factor ff = 0.63) and 3.47% (with short-circuit photo current
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.133.

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Y. Shan et al. / Tetrahedron Letters 53 (2012) 1341–1344
12. Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126,
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