Bithiazole-bridged dyes for dye-sensitized solar cells with high open circuit voltage performance

Article abstract of DOI:10.1039/c0jm03811c

Five new metal-free organic dyes (T1-T5) containing bithiazole moieties were synthesized and used for dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic properties were fully characterized. Electrochemical measu

Full text of DOI:10.1039/c0jm03811c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 6054
www.rsc.org/materials
PAPER
Bithiazole-bridged dyes for dye-sensitized solar cells with high open circuit
voltage performance
*
Jinxiang He, Wenjun Wu, Jianli Hua, Yihua Jiang, Sanyin Qu, Jing Li, Yitao Long and He Tian
*
Received 7th November 2010, Accepted 9th February 2011
DOI: 10.1039/c0jm03811c
Five new metal-free organic dyes (T1–T5) containing bithiazole moieties were synthesized and used for
dye-sensitized solar cells (DSSCs). Their absorption spectra, electrochemical and photovoltaic
properties were fully characterized. Electrochemical measurement data indicate that the tuning of the
HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety.
All of these dyes performed as sensitizers for the DSSC test, and the photovoltaic performance data of
these bithiazole-bridged dyes showed higher open circuit voltages (745–810 mV). Among the five dyes,
T1 showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current
conversion efficiency (IPCE) of 83.8%, a short-circuit photocurrent density (Jsc) of 11.78 mA cmꢀ2, an
open-circuit photovoltage (Voc) of 810 mV, and a fill factor (ff) of 0.60, corresponding to an overall
conversion efficiency of 5.73% under standard global AM 1.5 solar light condition, which reached 93%
with respect to that of an N719-based device fabricated under similar conditions. The result shows that
the metal-free dyes based on bithiazole p-conjugation are promising candidates for improvement of the
performance of DSSCs.
oxidized dyes15 are considered to be responsible for the lowering
of the open circuit voltage, which is far below the theoretical
maximum.16 In order to restrain this charge recombination
process, double layered TiO2 electrodes, 4-tert-butylpyridine,
ruthenium cyclodextrin complexes, and saccharide blocking
layers have been used in DSSCs.17 Recently it was found that
bridge groups carrying lateral alkyl chains helped to form
1. Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable
scientific and industrial interest as a promising candidate for
a new renewable energy source during the past decades.1 DSSCs
based on Ru(II)-polypyridyl complexes photosensitizers, such as
N3, N719, and the black dye, have achieved record light-to-
power conversion efficiencies exceeding 11% under AM 1.5
simulator.2 As an economically feasible alternative to conven-
tional inorganic photovoltaic devices, they offer relatively
low-cost solar devices with a high solar-energy-to-electricity
conversion efficiency.3 However, in recent years enormous
studies have been focused on metal-free organic dyes for use in
DSSCs due to their high extinction coefficient, facile molecular
design, simple synthesis process, low cost, and no concern with
the noble metal resource. Recently, novel organic dyes based on
porphyrin,4 perylene,5 cyanine,6 xanthene,7 merocyanine,8
coumarin,9 hemicyaine,10 indoline,11 diketopyrrolopyrrole12 and
ethylenedioxythiophene,13 have been investigated as sensitizers
for DSSCs, and great progress has been made in this field.
The open circuit voltage is an important factor for the power
conversion efficiency of DSSCs. Charge recombination processes
ꢀ
a blocking layer to keep I3 ions away from the TiO2 electrode
surface, hence increasing the electron lifetime and open circuit
voltage (Voc).18
Thiazole is well-known electron-deficient unit because it
contains one electron-withdrawing nitrogen of the imine (–C]
N) in place of the carbon atom at the 3-position of thiophene.19
Alkyl chain-substituted bithiazole with two thiazole rings con-
nected together have also been used as acceptor unit to copoly-
merize with the donor unit of oligothiophene20,21 or
cyclopentadithiophene,22 and present unique electronic and
optical properties. Therefore, bithiazole-based polymers have
been widely employed in thin film transistors, photovoltaic cells,
and light-emitting diodes applications, in which solar cells based
on the bithiazole copolymers show high open circuit voltage and
a power conversion efficiency of around 3%,22b, 23 suggesting that
bithiazole-based polymers could be promising photovoltaic
donor materials. We consider that the thiazole rings with two
long alkyl chains can increase their potential applicability as dye-
sensitizers for DSSCs. However, to the best of our knowledge,
small molecular dyes containing a bithiazole moiety have not
been explored for DSSCs. On the other hand, triarylamine is
of injected electrons with I3 ions in the electrolyte14 and the
ꢀ
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, 130 Meilong Road,
Shanghai, 200237, PR China. E-mail: tianhe@ecust.edu.cn; Fax: +86-
21-64252758; Tel: +86-21-64250940; jlhua@ecust.edu.cn; +86-21-
64252288; +86-21-64252756
6054 | J. Mater. Chem., 2011, 21, 6054–6062
This journal is ª The Royal Society of Chemistry 2011

View Article Online
a well-known nonplanar compound which can improve the hole-
transporting ability of the materials and prevent the formation of
dye aggregates. Recently, triphenylamine-based dyes have been
widely used in organic photovoltaic functional materials and
have become the focus of intensive research in the field of solar
cells.24 Methoxy groups attached to the triphenylamine can
enhance the extent of electron delocalization and the ability to
donate the electrons of the sensitizer. Also, the introduction of
a thiophene moiety was expected to allow red-shift of the spec-
trum, and broaden the spectral region of absorption. Meanwhile,
the phenylethyne spacer is a semirigid conjugated linker which
should favor electron injection.25 Based on the above consider-
ation, we have designed and synthesized five new efficient organic
bithiazole-bridge sensitizers (T1–T5 shown in Fig. 1) with
triarylamine as the donor, bithiazole as a p-conjugated bridge,
and a cyanoacrylic acid moiety acting as the anchoring group.
Finally, the five new sensitizers have been applied successfully to
the sensitization of nanocrystalline TiO2-based solar cells and the
corresponding photovoltaic properties, electronic and optical
properties are also presented.
dimethyl-4-(bis(4-methoxyphenyl)amino)phenylboronate, 4-for-
mylphenyl-boronic acid and 5,50-dibromo-4,40-dihexyl-2,20-
bithiazole were prepared according to published procedures.26
All other solvents and chemicals used in this work were of
reagent grade and used without further purification.
2.2 Analytical instruments and measurements
€
NMR spectra were obtained on a Brucker AM 400 spectrometer.
Mass spectra were measured with an HP5989 mass spectrometer.
The UV–vis spectra were recorded with a Varian Cary 500
spectrophotometer and fluorescence emission spectra were con-
ducted on a Varian Cary Eclipse fluorescence spectrophotom-
eter. Cyclic voltammograms were performed with a Versastat II
electrochemical workstation (Princeton applied research) using
a normal three-electrode cell with a Pt working electrode, a Pt
wire counter electrode, and a regular calomel reference electrode
in saturated KCl solution. The electrochemical impedance
spectroscopy (EIS) measurements of all the DSSCs were per-
formed using a Zahner IM6e Impedance Analyzer (ZAHNER-
Elektrik GmbH & CoKG, Kronach, Germany). The frequency
range is 0.1 Hz–100 kHz. The applied voltage bias is ꢀ0.70 V.
The magnitude of the alternative signal is 10 mV.
2. Experimental
2.1 Materials and reagents
2.3 Synthesis of dyes
Fluorine-doped SnO2 conducting glass (FTO glass, trans-
mission > 90% in the visible, sheet resistance 15 U/square) was
obtained from the Geao Science and Educational Co. Ltd. of
China. Methoxypropionitrile (MPN), was purchased from
5-(50-Bromo-4,40-dihexyl-2,20-bithiazol-5-yl)thiophene-2-carbal-
dehyde (1). 5,50-Dibromo-4,40-dihexyl-2,20-bithiazole (850 mg,
1.73 mmol), Pd(PPh3)4 (200 mg, 0.17 mmol), and K2CO3 (1.02 g,
0.01 mol) in 10 mL of THF and 5 mL of H2O were heated to
45 ꢁC under an argon atmosphere for 30 min. A solution of
5-formylthiophen-2-yl boronic acid (270 mg, 1.73 mmol) in THF
(10 mL) was added slowly, and the mixture was refluxed for
a further 12 h. After cooling to room temperature, the mixture
was extracted with CH2Cl2 (3 ꢂ 30 mL). The combined organic
layers were washed with water and brine and dried with anhy-
drous Na2SO4. The solvent was evaporated, and the residue was
purified by column chromatography on silica gel (PE/CH2Cl2 ¼
3/1–2/1, v/v) to give a yellow solid (154 mg, yield: 15%). 1H NMR
(CDCl3, 400 MHz), d: 9.85 (s, 1H), 7.67 (d, J ¼ 4.0 Hz, 1H), 7.21
(d, J ¼ 4.0 Hz, 1H), 2.97 (t, J ¼ 7.6 Hz, 2H), 2.77 (t, J ¼ 7.6 Hz,
2H), 1.81–1.67 (m, 4H), 1.42–1.26 (m, 12H), 0.91–0.89 (m, 6H).
Aldrich.
Tetra-n-butylammonium
hexafluorophosphate
(TBAPF6), 4-tert-butylpyridine (4-TBP), and lithium iodide were
bought from Fluka and iodine, 99.999%, was purchased from
Alfa Aesar.
ꢀ
Tetrahydrofuran (THF) was pre-dried over 4 A molecular
sieves and distilled under an argon atmosphere from sodium
benzophenone ketyl immediately prior to use. Triethylamine
(TEA) and N,N-dimethyl formamide (DMF) were heated under
reflux with calcium hydride and distilled before used. The
starting materials 4-ethynyl-N,N-diphenylaniline, dimethyl-4-
(diphenylamino)
phenylboronate,
4-ethynylbenzaldehyde,
4-(50-Bromo-4,40-dihexyl-2,20-bithiazol-5-yl)-N,N-bis(4-methoxy-
phenyl)aniline (2). The synthesis method resembles that of
compound 1, and the compound was purified by column chro-
matography on silica gel (PE/CH2Cl2 ¼ 4/1–1/2, v/v) to give
a orange solid (240 mg, yield: 63.2%).1H NMR (CDCl3, 400 MHz)
d: 7.22 (d, J ¼ 8.4 Hz, 2H), 7.11 (d, J ¼ 8.8 Hz, 4H), 6.92 (d, J ¼ 8.4
Hz, 2H), 6.86 (d, J ¼ 8.8 Hz, 4H), 3.81 (s, 6H), 2.79 (d, J ¼ 6.2 Hz,
2H), 2.75 (d, J ¼ 6.0 Hz, 2H), 1.77–1.68 (m, 4H), 1.35–1.26 (m,
12H), 0.90–0.85 (m, 6H).
4-((50-Bromo-4,40-dihexyl-2,20-bithiazol-5-yl)ethynyl) benzalde-
hyde (3). To a flask containing Pd(PPh3)2Cl2 (70 mg, 0.1 mmol),
CuI (50 mg, 0.26 mmol), PPh3(15 mg, 0.057 mmol) and 5,50-
dibromo-4,40-dihexyl-2,20-bithiazole (500 mg, 1.02 mmol) was
added 4-ethynylbenzaldehyde (102 mg, 0.38 mmol) in TEA
(25 mL) under an argon atmosphere. The mixture was stirred at
Fig. 1 Molecular structures of bithiazole dyes (T1–T5).
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6054–6062 | 6055

View Article Online
reflux for 24 h. The solvent was evaporated, then the residue was
diluted with water and extracted with CH2Cl2 (3 ꢂ 30 mL). The
combined organic layers were washed with water and brine and
dried with anhydrous Na2SO4. The solvent was evaporated, and
the remaining crude product was purified by column chroma-
tography using silica gel and CH2Cl2/PE (v/v, 1/3–1/2) as the
4-(50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)benzaldehyde (7). The synthesis method resembles
that of compound 1, and the compound was purified by column
chromatography on silica gel (PE/CH2Cl2 ¼ 2/1–1/1, v/v) to give
1
a yellow solid (120 mg, yield: 96.8%). H NMR (CDCl3, 400
MHz), d: 10.06 (s, 1H), 7.96 (d, J ¼ 8.0 Hz, 2H), 7.64 (d, J ¼ 8.4
Hz, 2H), 7.25 (d, J ¼ 8.4 Hz, 2H), 7.12 (d, J ¼ 8.8 Hz, 4H), 6.94
(d, J ¼ 8.8 Hz, 2H), 6.87 (d, J ¼ 8.8 Hz, 4H), 3.81 (s, 6H), 2.85 (t,
J ¼ 5.4 Hz, 2H), 2.81 (t, J ¼ 5.4 Hz, 2H), 1.80–1.74 (m, 4H),
1.36–1.28 (m, 12H), 0.89–0.84(m, 6H).
1
eluent to give a dark yellow solid (160 mg, yield: 23.6%). H
NMR (CDCl3, 400 MHz), d: 10.04 (s, 1H), 7.89 (d, J ¼ 8.4 Hz,
2H), 7.67 (d, J ¼ 8.0 Hz, 2H), 2.92 (t, J ¼ 7.6 Hz, 2H), 2.77 (t, J ¼
7.6 Hz, 2H), 1.83–1.66 (m, 4H), 1.40–1.25 (m, 12H), 0.92–0.86
(m, 6H).
4-((50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)ethynyl)benzaldehyde (8). Compound 3 (150 mg,
0.28 mmol), Pd(PPh3)4 (32 mg, 0.028 mmol), and K2CO3 (1.02 g,
0.01 mol) in 10 mL of THF and 5 mL of H2O were heated to
45 ꢁC under an argon atmosphere for 30 min. A solution of
dimethyl-4-(bis(4-methoxyphenyl)amino)phenylboronate (234
mg, 0.62 mmol) in THF (5 mL) was added slowly, and the
mixture was refluxed for a further 12 h. After cooling to room
temperature, the mixture was extracted with CH2Cl2 (3 ꢂ 20
mL). The combined organic layers were washed with water and
brine and dried with anhydrous Na2SO4. The solvent was evap-
orated, and the residue was purified by column chromatography
on silica gel (PE/CH2Cl2 ¼ 2/1–2/1, v/v) to give a red solid (100
mg, yield: 47.2%). 1H NMR (CDCl3, 400 MHz), d: 10.03 (s, 1H),
7.89 (d, J ¼ 8.4 Hz, 2H), 7.67 (d, J ¼ 8.4 Hz, 2H), 7.24 (d, J ¼ 8.4
Hz, 2H), 7.12 (d, J ¼ 9.2 Hz, 4H), 6.93 (d, J ¼ 8.4 Hz, 2H), 6.87
(d, J ¼ 8.8 Hz, 4H), 3.81 (s, 6H), 2.94 (t, J ¼ 7.6 Hz, 2H), 2.82 (t, J
¼ 7.6 Hz, 2H), 1.83–1.72 (m, 4H), 1.43–1.23 (m, 12H), 0.88 (t, J ¼
6.8 Hz, 6H).
5-(50-(4-(Diphenylamino)phenyl)-4,40-dihexyl-2,20-bithiazol-5-yl)
thiophene-2-carbaldehyde (4). Compound 1 (100 mg, 0.19 mmol),
Pd(PPh3)4 (22 mg, 0.019 mmol), and K2CO3 (1.02 g, 0.01 mol) in
10 mL of THF and 5 mL of H2O were heated to 45 ꢁC under an
argon atmosphere for 30 min. A solution of dimethyl-4-(diphe-
nylamino)phenylboronate (120 mg, 0.38 mmol) in THF (3 mL)
was added slowly, and the mixture was refluxed for a further 12 h.
After cooling to room temperature, the mixture was extracted
with CH2Cl2 (3 ꢂ 20 mL). The combined organic layers were
washed with water and brine and dried with anhydrous Na2SO4.
The solvent was evaporated, and the residue was purified by
column chromatography on silica gel (PE/CH2Cl2 ¼ 3/1–2/1, v/v)
1
to give an orange solid (120 mg, yield: 92%). H NMR (CDCl3,
400 MHz), d: 9.92 (s, 1H), 7.74 (d, J ¼ 4.0 Hz, 1H), 7.32–7.27 (m,
7H), 7.16 (d, J ¼ 7.6 Hz, 4H), 7.11–7.06 (m, 4H), 2.99 (t, J ¼ 8.0
Hz, 2H), 2.84 (t, J ¼ 8.0 Hz, 2H), 1.84–1.72 (m, 4H), 1.38–1.24 (m,
12H), 0.91–0.87 (m, 6H).
5-(50-((4-(Diphenylamino)phenyl)ethynyl)-4,40-dihexyl-2,20-bithia-
zol-5-yl)thiophene-2-carbaldehyde (5). To a flask containing Pd
(PPh3)2Cl2 (70 mg, 0.1 mmol), CuI (50 mg, 0.26 mmol), PPh3(15 mg,
0.057 mmol) and compound 1 (200 mg, 0.38 mmol) were added 4-
ethynyl-N,N-diphenylaniline (102 mg, 0.38 mmol) in TEA (15 mL)
and THF (8 mL) under an argon atmosphere. The mixture was
stirred at reflux for 24 h. The solvent was evaporated, then the
residue was redissolved in CH2Cl2 and washed with water and brine.
The organic layer was dried with anhydrous Na2SO4. The solvent
was evaporated, and the remaining crude product was purified by
column chromatography using silica gel and CH2Cl2/PE (1/3–1/1, v/
v) as the eluent gave pure compound 5 as a dark red solid (130 mg,
yield: 48%). 1HNMR(CDCl3, 400MHz), d: 9.92 (s, 1H), 7.74 (d, J¼
4.0 Hz, 1H), 7.36 (d, J¼ 8.4 Hz, 2H), 7.29 (t, J ¼ 7.8 Hz, 4H), 7.27 (d,
J ¼ 4.0 Hz, 1H), 7.13 (d, J ¼ 7.6 Hz, 4H), 7.09 (t, J ¼ 7.4 Hz, 2H),
7.02 (d, J ¼ 8.8 Hz, 2H), 2.98 (t, J ¼ 7.8 Hz, 2H), 2.91 (t, J ¼ 7.6 Hz,
2H), 1.82–1.77 (m, 4H), 1.43–1.28 (m, 12H), 0.91–0.86 (m, 6H).
2-Cyano-3-(5-(50-(4-(diphenylamino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)thiophen-2-yl)acrylic acid (T1). Compound 4 (100
mg, 0.14 mmol), 2-cyanoacetic acid (83 mg, 0.98 mmol),
ꢁ
ammonium acetate (300 mg) and acetic acid (8 mL) at 120 C
for 12 h. After cooling the solution, water was added to quench
the reaction. The precipitate was filtered and washed with
water. The residue was purified by column chromatography on
silica gel (CH2Cl2/EtOH ¼ 20/1, v/v) to yield 133 mg of red
1
solid (yield 82.6%). H NMR (THF-d8, 400 MHz), d:8.54 (s,
1H), 8.04 (d, J ¼ 4.0 Hz, 1H), 7.62 (d, J ¼ 3.2 Hz, 1H), 7.43 (d, J
¼ 8.0 Hz, 2H), 7.36 (t, J ¼ 8.0 Hz, 4H), 7.15–7.10 (m, 6H), 7.01
(d, J ¼ 8.0 Hz, 2H), 2.98 (t, J ¼ 7.4 Hz, 2H), 2.80 (t, J ¼ 7.4 Hz,
2H), 1.78–1.65 (m, 4H), 1.30–1.23 (m, 12H), 0.85 (t, J ¼ 6.8 Hz,
6H). 13C NMR (THF-d8, 100 MHz), 165.1, 162.0, 159.0, 158.7,
156.0, 150.6, 149.7, 147.5, 144.2, 140.6, 138.8, 137.6, 132.2,
131.6, 130.2, 128.6, 127.3, 126.9, 125.9, 124.6, 118.1, 117.9,
34.2, 34.0, 33.0, 32.0, 31.9, 31.8, 31.4, 30.4, 29.4, 27.9, 27.8,
24.9, 23.8, 15.8. HRMS (m/z): [M
+
H]+ Calcd for
44H45N4O2S3, 757.2705; found, 757.2714. Anal. calcd for
44H44N4O2S3: C 69.81, H 5.86, N 7.40; found C 69.69, H 5.98,
5-(50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)thiophene-2-carbaldehyde (6). The synthesis
method resembles that of compound 1, and the compound was
purified by column chromatography on silica gel (PE/CH2Cl2 ¼
C
C
N 7.28.
1
2-Cyano-3-(5-(50-((4-(diphenylamino)phenyl)ethynyl)-4,40-dihexyl-
2/1–1/2, v/v) to give a red solid (200 mg, yield: 93%). H NMR
2,20-bithiazol-5-yl)thiophen-2-yl)acrylic acid (T2). A procedure
similar to that for the dye T1 but with compound 5 (120 mg,
0.17 mmol) instead of compound 4 giving the dye T2 as a dark red
solid (95 mg, yield: 72.5%). 1H NMR (THF-d8, 400 MHz), d: 8.27
(s, 1H), 7.77 (d, J ¼ 4.0 Hz, 1H), 7.33 (d, J ¼ 4.0 Hz, 1H), 7.26
(CDCl3, 400 MHz), d: 9.91 (s, 1H), 7.73 (d, J ¼ 4.0 Hz, 1H), 7.27
(d, J ¼ 4.0 Hz, 1H), 7.24 (d, J ¼ 8.8 Hz, 2H), 7.12 (d, J ¼ 8.8 Hz,
4H), 6.93 (d, J ¼ 8.8 Hz, 2H), 6.87 (d, J ¼ 8.8 Hz, 4H), 3.81 (s,
6H), 2.99 (t, J ¼ 8.0 Hz, 2H), 2.82 (t, J ¼ 8.0 Hz, 2H), 1.82–1.74
(m, 4H), 1.43–1.26 (m, 12H), 0.90–0.86 (m, 6H).
6056 | J. Mater. Chem., 2011, 21, 6054–6062
This journal is ª The Royal Society of Chemistry 2011

View Article Online
(d, J ¼ 8.8 Hz, 2H), 7.17 (t, J ¼ 7.8 Hz, 4H), 6.99 (t, J ¼ 7.6 Hz,
4H), 6.96 (t, J ¼ 7.4 Hz, 2H), 6.88 (d, J ¼ 8.8 Hz, 2H), 2.87 (t, J ¼
7.8 Hz, 2H), 2.81 (t, J ¼ 7.6 Hz, 2H), 1.72–1.65 (m, 4H), 1.34–1.23
(m, 12H), 0.78 (t, J ¼ 6.8 Hz, 6H). 13C NMR (THF-d8, 100 MHz),
165.1, 164.7, 161.1, 160.2, 158.9, 151.1, 149.6, 149.4, 147.4, 143.7,
140.5, 139.0, 134.6, 131.8, 131.7, 130.4, 129.4, 128.3, 127.9, 127.5,
127.2, 126.9, 126.1, 125.8, 124.4, 123.9, 121.7, 118.6, 117.9, 117.0,
102.5, 102.1, 80.6, 33.9, 33.0, 32.4, 32.0, 31.4, 31.1, 27.2, 27.0, 24.8,
15.8. HRMS (m/z): [M + H]+ Calcd for C46H45N4O2S3, 781.2705;
found, 781.2715. Anal. calcd for C46H44N4O2S3: C 70.74, H 5.68, N
7.17; found C 70.53, H 5.87, N 7.01.
2.4 Device fabrication
A layer of ca. 5 mm TiO2 (13 nm paste, T/SP) was coated on the
FTO conducting glass by screen printing and then dried for 6 min
ꢁ
at 125 C. This procedure was repeated 2 times (ca. 10 mm) and
finally coated by a layer (ca. 4 mm) of TiO2 paste (Ti-nanoxide
300) as the scattering layer. The tri-layer TiO2 electrodes were
gradually heated under an air flow at 275 ꢁC for 5 min, 325 ꢁC for
ꢁ
ꢁ
ꢁ
5 min, 375 C for 5 min, 450 C for 15 min, and 500 C for 15
min. The sintered film was further treated with 0.2 M TiCl4
aqueous solution at room temperature for 12 h, then washed with
water and ethanol, and annealed at 450 ꢁC for 30 min. After the
film was cooled to 50 C, it was immersed in a 3 ꢂ 10ꢀ4 M dye
ꢁ
3-(5-(50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)thiophen-2-yl)-2-cyanoacrylic acid (T3). A procedure
similar to that for the dye T1 but with compound 6 (150 mg, 0.20
mmol) instead of compound 4 giving the dye T3 as a dark red solid
bath in CH2Cl2 solution and maintained in the dark for 12 h at
room temperature. The electrode was then rinsed with CH2Cl2
and dried. The size of the TiO2 electrodes used was 0.28 cm2. To
prepare the counter electrode, the Pt catalyst was deposited on
cleaned FTO glass by coating with a drop of H2PtCl6 solution
1
(90 mg, yield: 55.2%). H NMR (THF-d8, 400 MHz), d: 8.38 (s,
1H), 7.91 (d, J ¼ 3.2 Hz, 1H), 7.52 (d, J ¼ 4.0 Hz, 1H), 7.28 (d, J ¼
8.4 Hz, 2H), 7.10 (d, J ¼ 8.8 Hz, 4H), 6.94 (d, J ¼ 8.8 Hz, 4H), 6.76
(d, J ¼ 8.8 Hz, 2H), 3.75 (s, 6H), 2.94 (t, J ¼ 7.4 Hz, 2H), 2.74 (t,
J ¼ 7.4 Hz, 2H), 1.73–1.64 (m, 4H), 1.30–1.26 (m, 12H), 0.85 (t, J ¼
6.6 Hz, 6H). 13C NMR (THF-d8, 100 MHz), 163.5, 161.6, 160.6,
160.5, 158.0, 154.0, 144.4, 140.5, 135.0, 133.7, 132.7, 131.5, 126.2,
123.0, 120.3, 60.5, 38.9, 36.5, 36.2, 35.4, 35.0, 34.2, 34.0, 33.9, 33.8,
33.6, 27.3, 27.2, 19.2, 19.1. HRMS (m/z): [M + H]+ Calcd for
C46H49N4O4S3, 817.2916; found, 817.2901. Anal. calcd for
C46H48N4O4S3: C 67.62, H 5.92, N 6.86; found C 67.41, H 6.06, N
5.78.
ꢁ
(0.02 M 2-propanol solution) with heat treatment at 400 C for
15 min. A hole (0.8 mm diameter) was drilled on the counter
electrode using a drill-press. The perforated sheet was cleaned
with ultrasound in an ethanol bath for 10 min. For the assembly
of DSSCs, the dye-covered TiO2 electrode and Pt-counter elec-
trode were assembled into a sandwich type cell and sealed with
a hot-melt gasket of 25 mm thickness made of the ionomer Surlyn
1702 (DuPont). The electrolyte consisting of 0.6 M 1,2-dimethyl-
3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and
0.5 M 4-tertbutylpyridine (TBP) in a mixture of acetonitrile and
methoxypropionitrile (volume ratio, 7 : 3) was introduced into
the cell via vacuum backfilling from the hole in the back of the
counter electrode. Finally, the hole was sealed using a UV-melt
gum and a cover glass.
3-(4-(50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)phenyl)-2-cyanoacrylic acid (T4). A procedure similar
to that for the dye T1 but with compound 7 (120 mg, 0.16 mmol)
instead of compound 4 giving the dye T4 as yellow solid (110 mg,
yield: 84%). 1H NMR (THF-d8, 400 MHz), d: 8.19 (s, 1H), 7.84 (d,
J ¼ 8.0 Hz, 2H), 7.34 (d, J ¼ 8.0 Hz, 2H), 7.07 (d, J ¼ 8.0, 2H), 6.97
(d, J ¼ 8.8, 4H), 6.80 (d, J ¼ 8.0, 2H), 6.75 (d, J ¼ 8.8, 4H), 3.72 (s,
6H), 2.70–2.60 (m, 4H), 1.69–1.57 (m, 4H), 1.27–1.13 (m, 12H),
0.76 (t, J ¼ 7.2 Hz, 6H). 13C NMR (THF-d8, 100 MHz), 158.8,
155.8, 155.2, 154.1, 147.6, 139.3, 133.9, 131.1, 130.3, 129.9, 128.8,
128.3, 126.0, 121.9, 118.4, 113.8, 54.4, 30.6, 29.1, 28.7, 28.2, 21.6,
13.1. HRMS (m/z): [M + H]+ Calcd for C48H51N4O4S2, 811.3352;
found, 811.3346. Anal. calcd for C48H50N4O4S2: C 71.08, H 6.21, N
6.91; found C 70.98, H 7.07, N 6.73.
2.5. Photovoltaic properties measurements
Photovoltaic measurements employed an AM 1.5 solar simulator
equipped with a 300 W xenon lamp (Model No. 91160, Oriel).
The power of the simulated light was calibrated to 100 mWcmꢀ2
using a Newport Oriel PV reference cell system (Model 91150V).
J–V curves were obtained by applying an external bias to the cell
and measuring the generated photocurrent with a Keithley model
2400 digital source meter. The voltage step and delay time of
photocurrent were 10 mV and 40 ms, respectively. Cell active
area was tested with a mask of 0.196 cm2. The photocurrent
action spectra were measured with a IPCE test system consisting
of a Model SR830 DSP Lock-In Amplifier and a Model SR540
Optical Chopper (Stanford Research Corporation, USA), a 7IL/
PX150 xenon lamp and power supply, and a 7ISW301 Spec-
trometer.
3-(4-((50-(4-(Bis(4-methoxyphenyl)amino)phenyl)-4,40-dihexyl-2,20-
bithiazol-5-yl)ethynyl)phenyl)-2-cyanoacrylic acid (T5). A procedure
similar to that for the dye T1 but with compound 8 (90 mg, 0.12
mmol) instead of compound 4 giving the dye T5 as orange solid (93
1
mg, yield: 95.9%). H NMR (THF-d8, 400 MHz), d: 8.15 (s, 1H),
7.86 (d, J ¼ 7.8 Hz, 2H), 7.45 (d, J ¼ 8.1 Hz, 2H), 7.11 (d, J ¼ 8.0
Hz, 2H), 6.95 (d, J ¼ 8.8 Hz, 4H), 6.77 (d, J ¼ 8.0 Hz, 2H), 6.75 (d, J
¼ 8.8 Hz, 4H), 3.46 (s, 6H), 2.81 (t, J ¼ 7.2 Hz, 2H), 2.67 (t, J ¼ 7.5
Hz, 2H), 1.73–1.62 (m, 4H), 1.28–1.24 (m, 12H), 0.77 (t, J ¼ 6.8 Hz,
6H). 13C NMR (THF-d8, 100 MHz), 165.6, 162.6, 159.1, 158.6,
155.4, 151.5, 142.3, 138.0, 135.2, 133.7, 132.7, 131.9, 129.4, 128.0,
124.5, 121.2, 117.0, 116.1, 57.0, 34.0, 33.9, 32.5, 32.0, 31.8, 31.4, 31.2,
31.1, 24.8, 15.8, HRMS (m/z): [M + H]+ Calcd for C50H51N4O4S2,
835.3352; found, 835.3376. Anal. calcd for C50H50N4O4S2: C 71.91,
H 6.03, N 6.71; found C 71.79, H 6.31, N 6.54.
3. Results and discussion
3.1 Synthesis
The synthetic route to the five dyes (T1–T5) containing the
bithiazole moiety is depicted in Scheme 1. The hexyl chain on the
bithiazole group can improve the solubility and form a tightly
ꢀ
packed insulating monolayer blocking the I3 anions or cations
from approaching the TiO2. The Suzuki or Sonogashira coupling
reaction of 5,50-dibromo-4,40-dihexyl-2,20-bithiazole with 5-
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6054–6062 | 6057

View Article Online
3.2 Absorption properties in solution and on the TiO2 film
Normalized UV-Vis absorption spectra of the dyes (T1–T5) in
a diluted solution of CH2Cl2 are shown in Fig. 2, and their
absorption data are listed in Table 1. All of the dyes (T1–T5)
have a relatively broad and strong absorption in the ultraviolet
and blue regions with maxima at 457, 465, 460, 410 and 426 nm,
respectively. The absorption bands at around 300 nm can be
attributed to the p–p* transition and the bands at around 410–
460 nm to the intramolecular charge transfer (ICT) between the
donor and the acceptor.17,27 T1–T3 were observed to be largely
shifted to a longer wavelength than T4–T5 because of the pres-
ence of the thiophene instead of the benzene in the p-conjugated
linker part. The absorption maximum of T2 at 465 nm was red-
shifted by 8 nm relative to that of T1, as T2, containing a triple
bond between the triphenylamine and bithiazole, has a larger
conjugation length than T1. Compared with T1, T3 has two
methoxy groups attached to the triphenylamine on the electron
donor side, which enhanced the extent of electron delocalization
over the whole molecule, so its maximum absorption peak was
red shifted a little. In addition, the absorption maximum of T5
was red-shifted by 16 nm compared with that of T4 due to the
introduction of a triple bond between the bithiazole and phenyl
moieties. In comparison with conventional ruthenium complexes
4
(for example, 1.39 ꢂ 10 Mꢀ1 cmꢀ1 for N719),28 the molar
extinction coefficients of the dyes (T1–T5) are obviously larger
than that of N719, indicating that these dyes have good light-
harvesting ability. The greater maximum absorption coefficients
of the organic dyes allows for a correspondingly thinner nano-
crystalline film so as to avoid the decrease in the film’s
mechanical strength. This also benefits the electrolyte diffusion in
the film and reduces the recombination possibility of the light-
induced charges during transportation.29
The absorption spectra of T1–T5 on TiO2 films after 3 h
adsorption are shown in Fig. 3. The maximum absorption peaks
for T1–T5 on the TiO2 film are at 471, 474, 467, 441 and 448 nm,
respectively, in which the absorption bands of the dyes on the
TiO2 films are red-shifted by 14, 9, 7, 31 and 22 nm respectively,
compared with the solution spectra. The red shifts of the
absorption spectra on TiO2 of T1–T5 are due to the J-aggrega-
tion of the dyes on the TiO2 surface, in which J-aggregation can
also occur readily because of the presence of carboxyl groups in
the molecules. Such broaden and red-shifted absorption has been
Scheme 1 The synthetic procedure of bithiazole dyes (T1–T5)
formylthiophen-2-yl-2-boronic acid, dimethyl 4-(bis(4-methoxy-
phenyl)amino)phenylboronate or 4-ethynyl benzaldehyde affor-
ded compounds 1–3, respectively. The reaction produced the di-
substituted side products as well, fortunately the monocapped
compounds can be easily separated by column chromatography.
In the next step, these bromo-exposed intermediates were reacted
with dimethyl-4-(diphenyl amino)phenylboronate, 4-ethynyl-N,
N-diphenylaniline,
dimethyl-4-(bis(4-methoxyphenyl)amino)
phenylboronate or 4-formylphenylboronic acid by Suzuki or
Sonogashira coupling reaction, respectively. Finally, the target
products (T1–T5) were synthesized via the Knoevenagel
condensation reaction of aldehydes 4–8 with cyanoacetic acid in
the presence of acetic acid and ammonium acetate. All the key
intermediates and five new organic bithiazole sensitizers (T1–T5)
1
were confirmed by H NMR, 13C NMR, HRMS and elemental
analysis.
Fig. 2 Normalized absorption spectra of T1–T5 in CH2Cl2.
This journal is ª The Royal Society of Chemistry 2011
6058 | J. Mater. Chem., 2011, 21, 6054–6062

View Article Online
Table 1 Optical and electrochemical properties of bithiazole dyes (T1–
T5)
molecular orbital (LUMO) levels, calculated from EHOMO ꢀ E0–
0, are ꢀ1.36, ꢀ1.30, ꢀ1.52, ꢀ1.64 and ꢀ1.69 V, respectively. The
examined HOMO and LUMO levels are collected in Table 1.
From these data, we found the HOMO levels of T1–T5 to be
sufficiently more positive than the iodine/iodide redox potential
value (0.4 V), indicating that the oxidized dyes formed after
electron injection into the conduction band of TiO2 could ther-
modynamically accept electrons from Iꢀ ions. The LUMO levels
of these dyes (see Table 1) were sufficiently more negative than
the conduction-band-edge energy level (Ecb) of the TiO2 elec-
trode (ꢀ0.5 V vs. NHE), which implies that electron injection
from the excited dye into the conduction band of TiO2 is ener-
getically permitted.30 Noticeably, the relatively large energy gaps
between the LUMO levels of the dyes and the conduction band
of the TiO2 semiconductor allow for the addition of the 4-tert-
butylpyridine (4-TBP) into the electrolyte, which can shift the
conduction band of TiO2 by about ꢀ0.3 V and consequently
improve the open-circuit voltage and total solar energy conver-
sion efficiency.26b
lmaxa/nm
(3 ꢂ 10ꢀ4M cm)
lmax
(nm)
HOMOc/V
(vs. NHE)
LUMOe/V
(vs.NHE)
b
/
d
E0–0 /
(eV)
Dye
T1
T2
T3
T4
T5
457(3.44)
465(3.93)
460(2.74)
410(1.92)
426(3.12)
471
474
467
441
448
1.06
1.08
0.85
0.84
0.84
2.42
2.38
2.37
2.48
2.53
ꢀ1.36
ꢀ1.30
ꢀ1.52
ꢀ1.64
ꢀ1.69
a
Absorption maximum in CH2Cl2 solution. b Absorption maximum on
c
TiO2 film.
HOMOs were measured in THF with 0.1
M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte
(working electrode: Pt; reference electrode: SCE; calibrated with
ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter
d
electrode: Pt wire). E0–0 was estimated from the intersection between
the absorption and emission spectra. LUMOs are estimated by
subtracting E0–0 from the HOMO.
e
3.4 Photovoltaic performance of DSSCs
Fig. 4 shows the action spectra of incident photon-to-current
conversion efficiency (IPCE) for DSSCs with T1–T5. The dye-
coated TiO2 film was used as the working electrode, platinized
FTO glass as the counter electrode and 0.6 M DMPII, 0.05 M I2,
0.10 M LiI and 0.5 M 4-tertbutylpyridine in acetonitrile and
methoxypropionitrile (volume ratio, 7 : 3) mixture solution as
the redox electrolyte. The losses of light reflection and absorption
by the conducting glass were not corrected. As shown in Fig. 4,
all five dyes can efficiently convert visible light to photocurrent in
the region from 400 nm to 650 nm. The IPCE exceeds 72.5% in
the spectral range 400–570 nm for T1, which reaches its
maximum of 83.8% at 460 nm. The IPCE of T2–T5 reached
a maximum 80.1% at 460 nm, 63.5% at 461 nm, 79.6% at 410 nm
and 78.9% at 459 nm, respectively. The IPCE performance of the
DSSCs with T1 is higher than those with T3–T5 due to its
broader photocurrent action spectrum. On the other hand, the
IPCE efficiencies of T1 are higher than that of T2 in the region
from 400 nm to 557 nm, though the IPCE values of T1 than T2
are lower in the region 560–700 nm (see Fig. 4), indicating that
Fig. 3 Normalized absorption spectra of T1–T5 on TiO2 film.
reported in other organic sensitizers on TiO2 electrode due to the
J-aggregation.27b
3.3. Electrochemical properties
To evaluate the possibility of electron transfer from the excited
dye to the conduction band of TiO2, cyclic voltammograms were
performed in THF solution using 0.1 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte, Pt as the
counter electrode and a saturated calomel electrode (SCE) as
reference electrode. The SCE reference electrode was calibrated
using a ferrocene/ferrocenium (Fc/Fc+) redox couple as an
external standard. The highest occupied molecular orbitals
(HOMOs) of T1–T5 corresponding to their first redox potential
are 1.06, 1.08, 0.85, 0.84 and 0.84 V vs. NHE, respectively. It is
clear that the HOMO levels of T3–T5 are higher than those of
T1–T2, which is in agreement with the order of the increase of
electron-donating ability of the donors: 4-(bis(4-methoxyphenyl)
amino)phenyl
> 4-(diphenylamino)phenyl. Generally, the
stronger electron-donating ability of the donor resulted in
a higher HOMO energy level. The band gap energies (E(0–0)) of
five the dyes were 2.42, 2.38, 2.37, 2.48 and 2.53 eV for T1–T5,
respectively, which were estimated from the intercept of the
normalized absorption and emission spectra. The estimated
excited state potential corresponding to the lowest unoccupied
Fig. 4 Photocurrent action spectra of the TiO2 electrodes sensitized by
T1–T5.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6054–6062 | 6059

View Article Online
Table 2 Optical properties and performance parameters of dye-sensi-
tized solar cells of T1–T5a
the T1 sensitized TiO2 electrode would generate the highest
conversion yield among five dyes.
Fig. 5 shows the current–voltage characteristics of DSSCs
fabricated with these thiazole dyes (T1–T5) as sensitizers under
standard global AM 1.5 solar light condition. The detailed
parameters of short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factor (ff), and photovoltaic conversion effi-
ciency (h) are summarized in Table 2. The short circuit current Jsc
is related to the molar extinction coefficient of the dye molecule,
in which a higher molar extinction coefficient have good light-
harvesting ability and yields a higher short circuit. T2 dye has the
highest light harvesting efficiency and consequently an improved
Jsc due to the largest molar extinction coefficient. Compared with
T2, T1 has a little lower molar extinction coefficient, but the
highest photovoltaic performance among the five dyes. This is
probably due to the fact that T1 has a much higher maximum
values of IPCE spectrum than T2. Conversely, compared with
the T1 dye, there is dramatic falls in both Jsc and Voc values of
T3, which is ascribed to lower IPCE value of T3. The decreased
IPCE values suggest that there are inefficient regeneration of the
oxidized dye, and accordingly lower Jsc (10.97 mA cmꢀ2) value is
observed. Therefore, it is found that the increase of electron
donating ability is not always beneficial to improve the photo-
electric conversion efficiency of DSSCs. These results agree well
with the corresponding IPCE spectra. In addition, the Jsc of T5 is
higher than that of T4, this can be attributed to its larger molar
extinction coefficient and semirigid conjugated linker of the
phenylethyne spacer in T5 which should favor the electron
injection.
Dye
Jsc/mA cmꢀ2
Voc/mV
ff
h (%)
T1
T2
T3
T4
T5
N719
11.78
12.06
10.22
8.92
10.51
15.43
810
754
745
782
790
672
0.60
0.62
0.59
0.60
0.61
0.60
5.73
5.60
4.48
4.20
5.05
6.18
a
Illumination: 100 mW cmꢀ2 simulated AM 1.5 G solar light; electrolyte
containing: 0.1 M LiI + 0.05 M I2 + 0.6 M DMPII + 0.5 M TBP in the
mixed solvent of acetonitrile and 3-methoxypropionitrile (7 : 3, v/v).
bridged dyes in improving Voc, effect of them and N719 on dark
current was studied as shown in Fig. 5. It is obvious that the
bithiazole-bridged dyes dark current onset potential shifted to
a larger value than N719. This dark current change indicates that
the introduction of two long alkyl chains into thiazole rings can
ꢀ
inhibit charge recombination between injected electrons and I3
ions in the electrolyte.
3.5. Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS) analysis was
performed to further clarify the bithiazole-bridged dyes effect on
the Voc in DSSCs sensitized by the dyes. Fig. 6 shows the elec-
trochemical impedance spectra for the DSSCs made with TiO2
electrodes dipped with the five sensitizers (T1–T5) and N719 in
the dark under a forward bias of ꢀ0.70 V with a frequency range
of 0.1 Hz to 100 kHz. Two semicircles were observed in the
Nyquist plots (Fig. 6). The smaller and larger semicircles in the
Nyquist plots are attributed to the charge-transfer at the counter
electrode and the electron transport at the TiO2/dye/electrolyte
interface, respectively. The radius of the larger semicircle
increases in the order T1 > T5 > T4 > T2 > T3 > N719, indicating
that the electron recombination resistance increases from N719,
T3, T2, T4, T5 to T111b and is reflected in the improvements seen
in the Voc. This result is in agreement with the observed shift in
the Voc value under standard global AM 1.5 illumination and
dark.
As shown in Table 2, it is noteworthy that all of five dyes have
higher open circuit voltages (Voc) (745–810 mV) than N719 (672
mV) measured under the same conditions, in which the intro-
duction of two long alkyl chains into the thiazole rings can
inhibit the charge recombination and improve Voc. DSSCs based
on T1 exhibit the best overall light to electricity conversion
efficiency of 5.73% (Jsc ¼ 11.78 mA cmꢀ2, Voc ¼ 810 mV, ff ¼
0.60) under AM 1.5 irradiation (100 mW cmꢀ2), which reached
93% with respect to that of an N719-based device fabricated
under similar conditions. To understand the role of bithiazole-
Fig. 5 Photocurrent density vs. voltage curves of DSSCs sensitized by
the dyes T1–T5 with electrolyte containing 0.1 M LiI, 0.05 M I2, 0.6 M
DMPII, 0.5 M TBP in the mixed solvent of acetonitrile and 3-
methoxypropionitrile (7 : 3, v/v) under light (100 mW cmꢀ2 simulated
AM 1.5 G solar light) and dark.
Fig. 6 Impedance spectra of DSSCs based on dye measured at ꢀ0.70 V
bias in the dark (Nyquist plots).
6060 | J. Mater. Chem., 2011, 21, 6054–6062
This journal is ª The Royal Society of Chemistry 2011

View Article Online
F. Meng, W. Zhan and H. Tian, Tetrahedron, 2008, 64, 345; (c)
W. Wu, J. Hua, Y. Jin, W. Zhan and H. Tian, Photochem.
Photobiol. Sci., 2008, 7, 63.
7 (a) J. R. Mann, M. K. Gannon, T. C. Fitzgibbons, M. R. Detty and
D. F. Watson, J. Phys. Chem. C, 2008, 112, 13057; (b) S. Hattori,
T. Hasobe, K. Ohkubo, Y. Urano, N. Umezawa, T. Nagano,
Y. Wada, S. Yanagida and S. Fukuzumi, J. Phys. Chem. B, 2004,
108, 15200.
8 (a) K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi,
Y. Abe, H. Sugihara and H. Arakawa, Chem. Commun., 2000, 1173;
(b) K. Sayama, S. Tsukagoshi, T. Mori, K. Hara, Y. Ohga, A. Shinpo,
Y. Abe, S. Suga and H. Arakawa, Sol. Energy Mater. Sol. Cells, 2003,
80, 47.
9 (a) K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and
H. Arakawa, Chem. Commun., 2001, 569; (b) K. Hara, T. Sato,
R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S. Suga, K. Sayama,
H. Sugihara and H. Arakawa, J. Phys. Chem. B, 2003, 107, 597; (c)
K. Hara, Z.-S. Wang, T. Sato, A. Furube, R. Katoh, H. Sugihara,
Y. Dan-oh, C. Kasada, A. Shinpo and S. Suga, J. Phys. Chem. B,
2005, 109, 15476; (d) Z. Wang, Y. Cui, K. Hara, Y. Dan-oh,
C. Kasada and A. Shinpo, Adv. Mater., 2007, 19, 1138; (e)
Z. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo and K. Hara,
J. Phys. Chem. C, 2007, 111, 7224.
4. Conclusions
In this paper, five metal-free organic dyes (T1–T5) comprising
a triarylamine moiety as the electron donor, a cyanoacrylic acid
as the anchoring groups, and two hexyl chain substituted
bithiazole as the bridge were designed and synthesized for use in
dye-sensitized solar cells (DSSCs). The results based on photo-
voltaic experiments showed that dyes with two hexyl chain
substituted bithiazole unit exhibited a higher open circuit voltage
(0.74–0.81V). The power conversion efficiency was shown to be
sensitive to the structural modifications of electron donor and
bridging linker. Among the five dyes, DSSCs based on T1 exhibit
the best overall light to electricity conversion efficiency of 5.73%
(Jsc ¼ 11.78 mA cmꢀ2, Voc ¼ 810 mV, ff ¼ 0.60) under AM 1.5
irradiation (100 mW cmꢀ2). All the results reveal that these
metal-free organic bithiazole dyes are promising in the develop-
ment of DSSCs, and the optimization of their chemical structure
and the device is in progress to further improve their energy
conversion efficiency.
10 (a) Z. Wang, F. Li and C. Huang, Chem. Commun., 2000, 2063; (b)
Y. Chen, C. Li, Z. Zeng, W. Wang, X. Wang and B. Zhang,
J. Mater. Chem., 2005, 15, 1654.
11 (a) W. H. Howie, F. Claeyssens, H. Miura and L. M. Peter, J. Am.
Chem. Soc., 2008, 130, 1367; (b) D. B. Kuang, S. Uchida,
Acknowledgements
Dedicated to Professor Dao-Ben Zhu on the occasion of his 70th
birthday.
€
R. Humphry-Baker, S. M. Zakeeruddin and M. Gratzel, Angew.
Chem., Int. Ed., 2008, 47, 1923.
12 (a) S. Qu, W. Wu, J. Hua, C. Kong, Y. Long and H. Tian, J. Phys.
Chem. C, 2010, 114, 1343; (b) F. Guo, S. Qu, W. Wu, J. Li,
W. Ying and J. Hua, Synth. Met., 2010, 160, 1767.
13 W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang,
C. Pan and P. Wang, Chem. Mater., 2010, 22, 1915.
14 (a) Y. Luo, D. Li and Q. Meng, Adv. Mater., 2009, 21, 1; (b) D. Li,
H. Li, Y. Luo, K. Li, Q. Meng, M. Armand and L. Chen, Adv.
Funct. Mater., 2010, 20, 3358; (c) Z. Ning, Q. Zhang, H. Pei,
J. Luan, C. Lu, Y. Cui and H. Tian, J. Phys. Chem. C, 2009, 113,
10307; (d) Z. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010,
3, 1170.
This work was supported by NSFC/China (20772031 and
61006048),
National Basic
Research
973
Program
(2011CB808400), the Fundamental Research Funds for the
Central Universities (WJ0913001) and Scientific Committee of
Shanghai (10520709700).
Notes and references
€
1 (a) B. O’Regan and M. Gratzel, Nature, 1991, 353, 737; (b)
A. Hagfeldt and M. Gratzel, Acc. Chem. Res., 2000, 33, 269; (c)
15 J. E. Kroeze, N. Hirata, S. Koops, M. K. Nazeeruddin, L. Schmidt-
€
Mende, M. Gratzel and J. R. Durrant, J. Am. Chem. Soc., 2006,
€
€
M. Gratzel, Acc. Chem. Res., 2009, 42, 1788; (d) J.-H. Yum,
P. Walter, S. Huber, D. Rentsch, T. Geiger, F. Nuesch,
128, 16376.
16 F. D. Angelis, S. Fantacci, A. Selloni, M. Gratzel and
€
€
€
F. D. Angelis, M. Gratzel and M. K. Nazeeruddin, J. Am. Chem.
Soc., 2007, 129, 10320.
€
2 (a) M. Gratzel, J. Photochem. Photobiol., A, 2004, 168, 235; (b)
M. K. Nazeeruddin, A. Kay, L. Rodicio, R. Humphry-Baker,
M. K. Nazeeruddin, Nano Lett., 2007, 7, 3189.
17 J. Song, F. Zhang, C. Li, W. Liu, B. Li, Y. Huang and Z. Bo, J. Phys.
Chem. C, 2009, 113, 13391.
18 N. Koumura, Z. Wang, M. Miyashita, Y. Uemura, H. Sekiguchi,
Y. Cui, A. Mori, S. Mori and K. Hara, J. Mater. Chem., 2009, 19,
4829.
19 (a) T. Yamamoto, S. Otsuka, K. Namekawa, H. Fukumoto,
I. Yamaguchi, T. Fukuda, N. Asakawa, T. Yamanobe, T. Shiono
and Z. G. Cai, Polymer, 2006, 47, 6038; (b) M. Mamada,
J. I. Nishida, D. Kumaki, S. Tokito and Y. Yamashita, Chem.
Mater., 2007, 19, 5404; (c) G. Sauve and R. D. McCullough, Adv.
Mater., 2007, 19, 1822.
€
€
E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am.
Chem. Soc., 1993, 115, 6382; (c) M. K. Nazeeruddin, P. Pechy,
T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte,
P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia,
€
G. B. Deacon, C. A. Bignozzi and M. Gratzel, J. Am. Chem. Soc.,
2001, 123, 1613.
3 (a) P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin,
ꢁ
€
T. Sekiguchi and M. Gratzel, Nat. Mater., 2003, 2, 402; (b) D. Shi,
N. Pootrakulchote, R. Li, J. Guo, Y. Wang, S. M. Zakeeruddin,
ꢂ ꢁ
20 W.-Y. Wong, X.-Z. Wang, Z. He, K.-K. Chan, A. B. Djurisic,
K.-Y. Cheung, C.-T. Yip, A. M.-C. Ng, Y. Y. Xi, C. S. K. Mak
€
M. Gratzel and P. Wang, J. Phys. Chem. C, 2008, 112, 17046.
4 (a) T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau and
and W.-K. Chan, J. Am. Chem. Soc., 2007, 129, 14372.
21 R. P. Kingsborough, D. Waller, R. Gaudiana, D. Muhlbacher,
€
M. Gratzel, Angew. Chem., Int. Ed., 2010, 49, 6646; (b) H. Imahori,
€
Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito,
M. Niemi, N. V. Tkachenko and H. Lemmetyinen, J. Phys. Chem.
C, 2010, 114, 10656; (c) Y. Lee and C. J. T. Hupp, Langmuir, 2010,
26, 3760.
5 (a) T. Edvinsson, C. Li, N. Pschirer, J. Schoeneboom, F. Eickemeyer,
€
R. Sens, G. Boschloo, A. Herrmann, K. Mullen and A. Hagfeldt,
J. Phys. Chem. C, 2007, 111, 15137; (b) C. Li, J.-H. Yum,
M. Morana, M. Scharber and Z. Zhu, J. Macromol. Sci., Part A:
Pure Appl. Chem., 2010, 47, 478.
22 (a) D. H. Kim, B.-L. Lee, H. Moon, H. M. Kang, E. J. Jeong, J. Park,
K.-M. Han, S. Lee, B. W. Yoo, B. W. Koo, J. Y. Kim, W. H. Lee,
K. Cho, H. A. Becerril and Z. Bao, J. Am. Chem. Soc., 2009, 131,
6124; (b) K.-C. Li, J.-H. Huang, Y.-C. Hsu, P.-J. Huang, C.-
W. Chu, J.-T. Lin, K.-C. Ho, K.-H. Wei and H.-C. Lin,
Macromolecules, 2009, 42, 3681.
23 (a) F. NarasoWudl, Macromolecules, 2008, 41, 3169; (b) M. Zhang,
H. Fan, X. Guo, Y. He, Z. Zhang, J. Min, J. Zhang, G. Zhao,
X. Zhan and Y. Li, Macromolecules, 2010, 43, 5706.
S.-J. Moon, A. Herrmann, F. Eickemeyer, N. G. Pschirer, P. Erk,
€
€
J. Schoeneboom, K. Mullen, M. Gratzel and M. K. Nazeeruddin,
ChemSusChem, 2008, 1, 615; (c) C. Li, Z. Liu, J. Schoeneboom,
F. Eickemeyer, N. G. Pschirer, P. Erk, A. Herrmann and
€
K. Mullen, J. Mater. Chem., 2009, 19, 5405; (d) Y. Jin, J. Hua,
W. Wu, X. Ma and F. Meng, Synth. Met., 2008, 158, 64.
24 Z. Ning and H. Tian, Chem. Commun., 2009, 5483.
25 A. A. Bothner-By, J. Dadok, T. E. Johnson and J. S. Lindsey, J. Phys.
Chem., 1996, 100, 17551.
6 (a) W. Zhan, W. Wu, J. Hua, Y. Jin, F. Meng and H. Tian,
Tetrahedron Lett., 2007, 48, 2461; (b) X. Ma, J. Hua, Y. Jin,
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6054–6062 | 6061

View Article Online
ꢃ
26 (a) C. Suspene, R. Barattin, C. Celle, A. Carella and J.-P. Simonato,
28 P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin and
€
29 (a) J. Tang, J. Hua, W. Wu, J. Li, Z. Jin, Y. Long and H. Tian, Energy
Environ. Sci., 2010, 3, 1736; (b) J. Tang, W. Wu, J. Hua, X. Li and
H. Tian, Energy Environ. Sci., 2009, 2, 982.
30 (a) R. Z. Li, D. Shi, D. F. Zhou, Y. M. Cheng, G. L. Zhang and
P. Wang, J. Phys. Chem. C, 2009, 113, 7469; (b) W. Wu, J. Yang,
J. Hua, J. Tang, L. Zhang, Y. Long and H. Tian, J. Mater. Chem.,
2010, 20, 1772.
J. Phys. Chem. C, 2010, 114, 3924; (b) C. Teng, X. Yang, C. Yang,
S. Li, M. Cheng, A. Hagfeldt and L. Sun, J. Phys. Chem. C, 2010,
114, 9101; (c) J. Lee, B.-J. Jung, S. K. Lee, J.-I. Lee, H.-J. Cho
and H.-K. Shim, J. Polym. Sci., Part A: Polym. Chem., 2005, 43,
1845.
M. Gratzel, J. Am. Chem. Soc., 2005, 127, 808.
27 S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M. H. Lee,
W. Lee, J. Park, K. Kim, N. G. Park and C. Kim, Chem. Commun.,
2007, 4887.
6062 | J. Mater. Chem., 2011, 21, 6054–6062
This journal is ª The Royal Society of Chemistry 2011