D-A-π-A organic sensitizers containing a benzothiazole moiety as an additional acceptor for use in solar cells

Article abstract of DOI:10.1007/s11426-012-4758-8

Three novel triphenylamine-based D-A-π-A-featured dyes (Z1-Z3) have been designed, synthesized and characterized for use in dye-sensitized solar cells. Benzothiazole was incorporated as an additional acceptor, which greatly enhanced the molar extinction c

Full text of DOI:10.1007/s11426-012-4758-8

SCIENCE CHINA
Chemistry
April 2013 Vol.56 No.4: 505–513
doi: 10.1007/s11426-012-4758-8
• ARTICLES •
D-A--A organic sensitizers containing a benzothiazole moiety as
an additional acceptor for use in solar cells
ZENG Juan1, ZHANG TengLong2, ZANG XuFeng1, KUANG DaiBin2*,
MEIER Herbert3 & CAO DeRong1*
1School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510641, China
2MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and
Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
3Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany
Received July 15, 2012; accepted August 20, 2012; published online September 15, 2012
Three novel triphenylamine-based D-A-π-A-featured dyes (Z1–Z3) have been designed, synthesized and characterized for use
in dye-sensitized solar cells. Benzothiazole was incorporated as an additional acceptor, which greatly enhanced the molar ex-
tinction coefficient of the dyes. Various conjugated linkers, such as benzene, furan and thiophene, were also introduced to con-
figure the novel D-A--A framework in order to prolong electron flow and active transportation. Among all dyes, Z2 contain-
ing a thiophene linker exhibited the maximum overall conversion efficiency () of 4.16% (Jsc = 9.27 mA cm−2, Voc = 642 mV,
FF = 0.70) under standard global AM 1.5 G solar condition.
organic dye, dye-sensitized solar cell, D-A-π-A, benzothiazole
tion coefficients, simple syntheses, low costs, and easy
1 Introduction
structural modifications [9]. A variety of organic dyes such
as coumarin [10, 11], merocyanine [12], indoline [13–15],
hemicyanine [16, 17], triphenylamine [18–25], and pheno-
thiazine [26–29] have been investigated for DSSCs and
show good performances.
The chemical structures of metal-free organic dyes are
crucial for high power-conversion efficiency. Many re-
searchers have devoted themselves to optimize chemical
structures for improvements in performance. Recently, Tian
et al. [30–32] reported a series of novel donor-acceptor-
-bridge-acceptor (D-A--A) organic dyes incorporating an
additional acceptor such as benzothiadiazole, benzotriazole
and quinoxaline into the traditional D-π-A framework. The
additional acceptor chromophore is expected to facilitate the
electron transfer from the donor to the acceptor, and has
diverse effects on the photovoltaic performances in the
D-A--A configuration [32]. Thus it is important to find
Solar energy is considered to be the most important renew-
able energy resource in the 21st century. As a new genera-
tion of photovoltaic devices, dye-sensitized solar cells
(DSSCs) have drawn immense attention since the initial
reports by Grätzel and co-workers [1–5] because of their
high efficiency, low cost, and facile fabrication. In DSSCs,
the sensitizer can be an organic ligand-metal complex or a
metal-free organic dye, and is one of the key components
for high power conversion efficiency. Nowadays, some
DSSCs based on Ru complexes have achieved remarkable
power conversion efficiencies of more than 11% [68].
However, metal-free organic dyes have recently emerged as
an alternative to Ru dyes owing to their high molar absorp-
*Corresponding authors (email: kuangdb@mail.sysu.edu.cn; drcao@scut.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

506
Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
new and appropriate additional acceptors in the D-A-π-A
framework.
MHz spectrometer (in CDCl3 or DMSO-d6, TMS as the
internal reference). Mass spectra were recorded on an Es-
quire HCT PLUS mass spectrometer. Elementary analyses
were performed using a Vario EL III Analyzer. The melting
points were measured on Tektronix X4 microscopic melting
point apparatus and were uncorrected. The absorption and
emission spectra were obtained in CH2Cl2 by using a Shi-
madzu UV-2450 spectrophotometer and a Fluorolog III
photoluminescence spectrometer, respectively. Cyclic
voltammetry measurements were carried out on an electro-
chemistry workstation (e-corder (ED 401) potentiostat) us-
ing a three-electrode cell. A TiO2 film coated with the dye
was used as a working electrode, with an Ag/AgCl (satu-
rated KCl) reference electrode and a Pt counter electrode
and a scan rate of 50 mV/s. Tetrabutylammonium perchlo-
rate (TBAPF6, 0.1 M) in acetonitrile was used as the sup-
porting electrolyte. The measurements were calibrated using
ferrocene (Fc) as a standard, and the potentials of dyes ver-
sus the normal hydrogen electrode (NHE) were calibrated
by addition of 0.63 V to the potentials versus Fc+/Fc. Current-
Benzothiazole is a well-known electron-withdrawing unit,
and has the characteristics of high chemical and photophys-
ical stability. It has been widely investigated in the con-
struction of dyes with enhanced two photon absorption [33,
34]. To the best of our knowledge, the application of ben-
zothiazole acceptors in D-A--A systems has never been
reported before. Here we introduce the benzothiazole unit
into triphenylamine-based dyes to develop a series of novel
D-A--A sensitizers (Scheme 1). The photophysical proper-
ties, electrochemical properties and photovoltaic perfor-
mance of this D-A--A system have been investigated in
detail.
2 Experimental
2.1 Instrumentation and materials
NMR (1H and 13C) spectra were recorded on a Bruker 400
Scheme 1 Synthesis of dyes Z1–Z3.

Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
507
voltage characteristics were performed on a Keithley 2400
source meter under simulated AM 1.5 G illumination (100
mW cm–2) provided by a solar simulator (Oriel, Model
91192). A 1 kW Xe (Oriel, Model 6271) lamp with optical
filter was used as light source. Incident photon-to-current
conversion efficiency (IPCEs) spectra were taken on a
Spectral Products DK240 monochromator as a function of
wavelength from 350 to 800 nm. Electrochemical imped-
ance spectroscopy (EIS) measurements were performed on
a Zahner Zennium electrochemical workstation in the fre-
quency range from 10 MHz to 1 mHz under dark conditions.
The impedance parameters were determined by fitting of the
impedance spectra using Z-view software. Intensity-
modulated photovoltage spectroscopy (IMVS) was carried
out on the electrochemical workstation (Zahner Zennium)
with a frequency response analyzer under a modulated light
emitting diode (457 nm) driven by a source supply (Zahner
PP211). The light intensity modulation was 10% of the base
light intensity over the frequency range from 100 kHz to 0.1
Hz.
leum ether (1:5, v/v) to afford the compound 4 as a yellow
solid (2.46 g, 74%). H NMR (400 MHz, CDCl3, ppm): δ
9.75 (s, 1H), 7.637.61 (m, 2H), 7.147.11 (m, 4H),
6.916.83 (m, 6H), 3.82 (s, 6H).
1
2-Amino-6-bromobenzothiazole (6)
A mixture of 4-bromoaniline (10.32 g, 60.00 mmol), KSCN
(58.20 g, 0.60 mol) and CuSO4 (48.00 g, 0.30 mol) was
stirred in methanol (80 mL) for 9 h at 70 C. After cooling
to room temperature, the suspension was filtered and the
filtrate was diluted with boiling water. Ethanol was added to
the boiling mixture until a clear yellow solution resulted.
Cooling the solution resulted in crystallization of compound
6 as a white solid (6.46 g, 47%). m.p. 217218 C (lit.
1
215217 C [36]). H NMR (400 MHz, CDCl3, ppm ): δ
7.71 (m, 1H), 7.437.38 (m, 2H), 5.25 (s, 2H).
2-Amino-5-bromobenzenethiol (7)
A mixture of 2-amino-6-bromobenzothiazole (4.58 g, 19.99
mmol) and KOH (40 g of 85% KOH in 40 mL of water)
was refluxed in ethylene glycol (5 mL) under nitrogen for
48 h. After cooling to room temperature, the mixture was
neutralized with acetic acid and extracted with ethyl acetate.
The organic phase was dried with anhydrous Na2SO4 and
the solvent was removed by evaporation to give the crude
product (3.22 g, 79%), which was directly used for the next
step.
Toluene and ethanol were dried and distilled from sodi-
um under a nitrogen atmosphere. The boiling range of pe-
troleum was 6090 C. All other solvents and reagents were
obtained from commercially available resources without
further purification.
2.2 Synthesis
Bis-(4-methoxyphenyl)phenylamine (3)
4-(6-Bromobenzothiazol-2-yl)-N,N-bis(4-methoxyphenyl)ani
line (8)
A mixture of aniline (2.28 mL, 25 mmol), 1-iodo-4-meth-
oxybenzene (13.46 g, 57.50 mmol), KOH (11.60 g, 85%,
175.73 mmol), 1,10-phenanthroline (0.91 g, 4.60 mmol) and
CuI (0.88 g, 4.60 mmol) in toluene (50 mL) was heated to
reflux under nitrogen atmosphere with stirring for 72 h. Af-
ter cooling to room temperature, CH2Cl2 was added and the
mixture was washed three times with water. The organic
phase was dried with anhydrous Na2SO4 and the solvent
was removed by evaporation. The residues were purified by
column chromatography with ethyl acetate/petroleum ether
(1:10, v/v) to afford compound 3 as a light yellow solid
(4.42 g, 58%). m.p. 108109 C (lit. 107.5108.5 C [35]).
1H NMR (400 MHz, CDCl3, ppm): δ 7.176.82 (m, 13H),
3.80 (s, 6H).
A mixture of compound 4 (2.33 g, 7.00 mmol) and com-
pound 7 (2.86 g, 14.01 mmol ) in DMSO was heated to 160
C for 1 h. After cooling to room temperature, the mixture
was poured into ice water and the precipitate was collected.
The precipitate was purified by column chromatography
with ethyl acetate/petroleum ether (1:20, v/v) to afford the
compound 8 as a pale yellow solid (1.86 g, 53%). m.p.
1
162163 C. H NMR (400 MHz, CDCl3, ppm): δ 7.98 (m,
1H), 7.867.84 (m, 3H), 7.55 (m, 1H), 7.157.13 (m, 4H),
6.966.93 (m, 2H), 6.916.89 (m, 4H), 3.84 (s, 6H). 13C
NMR (CDCl3, 100 MHz, ppm): δ 168.66, 156.76, 153.27,
151.51, 139.63, 136.37, 129.54, 128.55, 127.53, 124.11,
123.95, 123.59, 118.50, 117.81, 114.94, 55.51. ESI MS: m/z
518.1. Found: 519.0 [M
+
H]+. Anal. Calcd. for
4-Bis(4-methoxyphenyl)aminobenzaldehyde (4)
C27H21BrN2O2S: C, 62.67; H, 4.09; N, 5.41; S, 6.20%.
Found: C, 62.72; H, 4.12; N, 5.39; S, 6.23%.
To a solution of bis(4-methoxyphenyl)phenylamine (3.05 g,
10 mmol ) and DMF (3 mL) in 1,2-dichloroethane (50 mL),
POCl3 (1.5 mL, 15 mmol) was added dropwise at 0 C .
Then the mixture was stirred at 70 C for 4 h. The mixture
was neutralized with a dilute aqueous solution of sodium
hydroxide and extracted three times with CH2Cl2. The or-
ganic phase was dried with anhydrous Na2SO4 and the sol-
vent was removed by evaporation. The residues were puri-
fied by column chromatography with ethyl acetate/petro-
4-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiazol-
6-yl)benzaldehyde (9a)
Tetrakis(triphenylphosphine)palladium (84.80 mg, 0.08
mmol) was added to a mixture of compound 8 (517 mg,
1.00 mmol), 4-formylphenylboronic acid (225.00 mg, 1.50
mmol), toluene (15 mL), ethanol (10 mL) and a 2 M
aqueous potassium carbonate solution (1.50 mL) under

508
Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
To a solution of 9a (271 mg, 0.50 mmol) and 2-cyanoacetic
acid (425 mg, 5 mmol) in chloroform (20 mL) was added
piperidine (0.10 mL, 1 mmol). The mixture was refluxed
under nitrogen for 5 h. After cooling to room temperature,
the mixture was neutralized with 2 M aqueous HCl, and
extracted with CH2Cl2 three times. The organic phase was
dried with anhydrous Na2SO4 and the solvent was removed
by evaporation. The residue was purified by column chro-
matography with CH3OH/ CH2Cl2 (1:15, v/v) to afford the
compound Z1 as an orange solid (235 mg, 77%). m.p.
stirring. The suspension was heated at 85 C for 24 h under
argon atmosphere. After cooling to room temperature,
CH2Cl2 was added and the mixture was washed with water
three times. The organic phase was dried with anhydrous
Na2SO4 and the solvent was removed by evaporation. The
crude product was purified with a mixture of ethyl ace-
tate/petroleum ether (1:5, v/v) by silica column chromatog-
raphy to afford the compound 9a as a yellow solid (396 mg,
73%). m.p. 167168 C . 1H NMR (400 MHz, CDCl3, ppm):
δ 10.05 (s, 1H), 8.08 (d, J = 1.6 Hz, 1H), 8.05 (d, J = 8.4 Hz,
1H), 7.96 (m, 2H ), 7.87 (m, 2H), 7.80 (m, 2H ), 7.70 (dd, J
= 2.0, 8.4 Hz, 1H), 7.147.11 (m, 4H), 7.706.92 (m, 2H),
6.906.86 (m, 4H), 3.82 (s, 6H). 13C NMR (CDCl3, 100
MHz, ppm): δ 189.86, 169.23, 156.76, 154.50, 151.50,
146.70, 139.65, 136.14, 135.69, 135.17, 100.37, 128.61,
127.78, 127.53, 125.79, 124.39, 122.83, 120.15, 118.51,
114.95, 55.51. ESI MS: m/z 542.2. Found: 544.3 [M + 2]+.
Anal. Calcd. for C34H26N2O3S: C, 75.25; H, 4.83; N, 5.16; S,
5.91%. Found: C, 75.30; H, 4.85; N, 5.14; S, 5.93%.
1
176177 C. H NMR (400 MHz, DMSO-d6, ppm): δ 8.44
(s, 1H), 8.35 (s, 1H), 8.13 (m, 2H), 8.01 (d, J = 8.4 Hz, 1H),
7.95 (m, 2H), 7.867.85 (m, 3H ), 7.147.12 (m, 4H),
6.976.95 (m, 2H), 6.776.76 (m, 4H), 3.76 (s, 6H). 13C
NMR (DMSO-d6, 100 MHz, ppm): δ 168.33, 163.32,
156.69, 153.95, 153.51, 151.26, 143.75, 138.72, 135.12,
135.01, 131.39, 130.47, 128.48, 127.85, 127.37, 125.54,
123.16, 122.42, 120.44, 117.03, 116.29, 115.15, 103.22,
55.26. ESI MS: m/z 609.2. Found: 611.1 [M + 2]+. Anal.
Calcd. for C37H27N3O4S: C, 72.89; H, 4.46; N, 6.89; S,
5.26%. Found: C, 72.84; H, 4.51; N, 6.87; S, 5.30%.
5-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiazol-
6-yl)thiophene-2-carbaldehyde (9b)
3-(5-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiaz
ol-6-yl)thiophen-2-yl)-2-cyanoacrylic acid (Z2)
Compound 9b was synthesized according to the procedure
described for 9a, giving a yellow solid in 44% yield. m.p.
1
Compound Z2 was synthesized according to the procedure
described for Z1, giving a dark red solid in 75% yield. m.p.
172173 C. 1H NMR (400 MHz, DMSO-d6, ppm): δ
8.478.35 (m, 2H), 8.007.88 (m, 2H), 7.827.80 (m, 3H),
7.79 7.72 (m, 1H), 7.137.11 (m, 4H), 6.706.95 (m, 4H),
6.756.73 (m, 2H), 3.76 (s, 6H). 13C NMR (DMSO-d6, 100
MHz, ppm): δ 168.85, 166.70, 154.36, 151.62, 151.28,
145.44, 140.40, 138.65, 135.31, 134.88, 128.79, 128.48,
127.85, 125.03, 124.66, 122.97, 122.58, 119.48, 116.90,
115.14, 55.26. ESI MS: m/z 615.1. Found: 616.5 [M + H]+.
Anal. Calcd. for C35H25N3O4S2: C, 68.27; H, 4.09; N, 6.82;
S, 10.42%. Found: C, 68.32; H, 3.98; N, 6.84; S, 10.45%.
160161 C . H NMR (400 MHz, CDCl3, ppm): δ 9.88 (s,
1H), 8.11 (s, 1H), 7.98 (d, J = 8.4 Hz, 1H), 7.867.81 (m,
2H ), 7.747.72 (m, 2H), 7.43 (d, J = 4 Hz, 1H ), 7.137.11
(m, 4H), 6.936.91 (m, 2H), 6.896.87 (m, 4H), 3.82 (s,
6H). 13C NMR (CDCl3, 100 MHz, ppm): δ 182.72, 169.726,
156.81, 155.08, 153.94, 151.63, 142.42, 139.56, 137.57,
135.80, 129.46, 128.66, 127.56, 124.89, 124.23, 124.15,
122.92, 119.21, 118.41, 114.95, 55.52. ESI MS: m/z 548.1.
Found: 549.5 [M + H]+. Anal. Calcd. for C32H24N2O3S2: C,
70.05; H, 4.41; N, 5.11; S, 11.69%. Found: 70.16; H, 4.39;
N, 5.13; S, 11.67%.
5-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiazol-
6-yl)furan-2-carbaldehyde (9c)
3-(5-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiaz
ol-6-yl)furan-2-yl)-2-cyanoacrylic acid (Z3)
Compound 9c was synthesized according to the procedure
described for 9a, giving a yellow solid in 74% yield. m.p.
Compound Z3 was synthesized according to the procedure
described for Z1, giving a red solid in 73% yield. m.p.
1
164165 C. H NMR (400 MHz, CDCl3, ppm): δ 9.65 (s,
1
170171 C. H NMR (400 MHz, DMSO-d6, ppm): δ 8.50
1H), 8.33 (s, 1H), 8.00 (d, J = 8 Hz, 1H), 7.867.81(m, 4H ),
7.337.32 (d, J = 1.6 Hz, 1H ), 7.137.11 (m, 4H),
6.936.91 (m, 2H), 6.896.87 (m, 4H), 3.82 (s, 6H). 13C
NMR (CDCl3, 100 MHz, ppm): δ 177.06, 169.94, 159.17,
156.82, 155.20, 152.09, 151.63, 139.60, 135.61, 128.66,
127.55, 125.23, 124.21, 123.70, 122.79, 118.42, 118.36,
114.96, 107.82, 55.51. ESI MS: m/z 532.2. Found: 533.6 [M
+ H]+. Anal. Calcd. for C32H24N2O4S: C, 70.05; H, 4.41; N,
5.11; S, 11.69%. Found: C, 70.21; H, 4.37; N, 5.13; S,
11.67%.
(s, 1H), 8.007.95 (m, 3H), 7.867.84 (m, 2H), 7.417.35
(m, 2H), 7.237.15 (m, 4H), 6.786.96 (m, 4H), 6.766.75
(m, 2H), 3.76 (s, 6H). 13C NMR (DMSO-d6, 100 MHz,
ppm): δ 168.55, 163.84, 156.97, 156.75, 154.17, 151.36,
148.21, 138.64, 135.02, 128.55, 127.93, 125.27, 123.22,
122.91, 122.61, 117.97, 116.86, 115.17, 109.81, 55.26. ESI
MS: m/z 599.2. Found: 600.8 [M + H]+. Anal. Calcd. for
C35H25N3O5S: C, 70.10; H, 4.20; N, 7.01; S, 5.35%. Found:
C, 70.08; H, 4.15; N, 6.91; S, 5.37%.
2.3 Fabrication of DSSCs
3-(4-(2-{4-[Bis(4-methoxyphenyl)amino]phenyl}benzothiaz
ol-6-yl)phenyl)-2-cyanoacrylic acid (Z1)
Fluorine-doped tin oxide (FTO) coated glass plates (15

Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
509
Ω/square, Nippon Sheet Glass, Japan) were cleaned in eth-
anol, acetone and de-ionized water using an ultrasonic bath
then rinsed with water and ethanol. TiO2 powder (20 nm,
2.0 g) was added to a mixture of ethanol (16.0 mL), acetic
acid (0.4 mL), terpineol (6.0 g) and ethyl cellulose (1.0 g),
and then ground for 40 min to form a slurry. After the ul-
trasonic processing of the slurry, a white viscous TiO2 paste
was produced. Then a TiO2 film (18 μm in thickness) was
prepared using the screen-printing technique. The
TiO2-coated FTO glass was sintered by heating at 325 C
for 5 min, 375 C for 5 min, 450 C for 15 min, and then
500 C for 15 min, then soaked in 0.04 M TiCl4 solution and
calcined at 520 C for 30 min again. The prepared TiO2
electrodes were immersed into the 0.3 mM solution of the
dye in CH3OH/CH2Cl2 (1:4, v/v) and kept at room tempera-
ture in the dark for 16 h. For photovoltaic performance
measurements, the dye-adsorbed TiO2 electrode and Pt
counter electrode were assembled in a sealed sandwich-type
cell. After the injection of the electrolyte solution, which
was composed of 0.60 M 1-methyl-3-propylimidazolium
iodide (PMII), 0.10 M guanidinium thiocyanate, 0.03 M I2,
0.50 M tert-butylpyridine in acetonitrile and valeronitrile
(85:15), through the hole made on the counter electrode into
the space between the sandwiched cell, the photoelectro-
chemical properties of the DSSCs were measured.
tinction coefficients are much higher than that of the ruthe-
nium dye N719 (1.4 × 104 M1 cm1) [2]. The λmax of Z3
with the furan linker was red-shifted 3 nm compared to that
of Z2 with the thiophene linker, while the λmax of Z2 was
red-shifted 38 nm compared to that of dye Z1 with the ben-
zene linker. This is due to the better delocalization of elec-
trons over the -conjugated molecules when furan was used
as linker in this D-A--A framework. Surprisingly, the ab-
sorption max of Z1–Z3 were all blue-shifted almost 40 nm
after the introduction of benzothiazol in comparison with
the reference dye C1 without benzothiazol [37]. When the


3 Results and discussion
3.1 Photophysical properties
The UV-vis absorption and emission spectra of the dyes
Z1–Z3 in CH2Cl2 are shown in Figure 1, and the corre-
sponding data are summarized in Table 1. All the dyes show
two major absorption bands at 300–390 nm and 400600
nm. The absorption band at 300–390 nm can be ascribed to
localized aromatic -* transitions. The absorption band at
400600 nm can be attributed to the intramolecular charge
(ICT) transitions. The absorption maximum wavelength
(λmax) of Z1–Z3 was at 425, 460, and 463 nm, and the cor-
responding maximum extinction coefficient was 3.64 × 104,
4.21 × 104 and 3.17 × 104 M1 cm1, respectively (i.e., in-
creasing in the order Z3 < Z1 < Z2). These maximum ex-
Figure 1 (a) Absorption spectra of the dyes in CH2Cl2; (b) normalized
absorption and emission spectra of the dyes in CH2Cl2.
Table 1 Absorption, emission and electrochemical properties of the dyes Z1–Z3
Absorption
Emission
Oxidation potential data
c)
Egape)(V,
vs. NHE)
EHOMO/ELUMO
a)
Dye
Eox
max
E0-0 d) (V)
 (×104 M1 cm1) max b) (nm)

max (nm)
527
Eox  E0-0 (V, vs. NHE)
1.17
(vs. NHE)
(eV)
(nm)
352
425
377
460
379
463
3.00
417
Z1
Z2
Z3
1.43
2.60
0.67
0.53
0.53
5.68/3.08
5.72/3.22
5.72/3.22
3.64
2.61
419
539
546
1.47
1.47
2.50
2.50
1.03
1.03
4.21
1.91
419
3.17
a) Absorption and emission spectra of the dyes in CH2Cl2; b) absorption spectra of the dyes anchored on TiO2 films; c) Eox measured in CH3CN with 0.1
M TBAPF6 (vs. NHE); d) E0-0 was determined from the intersection of absorption and emission spectra in CH2Cl2; e) Egap is the energy gap between the Eox
E0-0 of the dye and the conductive band level of TiO2 (–0.5 V vs. NHE).
–

510
Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
three dyes were adsorbed onto TiO2 films (Table 1), the
absorption bands of Z1–Z3 were blue-shifted by 8, 41 and
44 nm, respectively, compared with the solution spectra.
This can be explained by the formation of H-aggregates of
the dye and the deprotonation of the carboxylic acid moiety
upon adsorption on the film of TiO2 [38, 39].
3.3 Theoretical calculations
The density functional theory (DFT) calculations for Z1–Z3
were carried out using the Gaussian 09 program at the
B3LYP/631 + G (d) levels. The electron distributions of
HOMO and LUMO levels of the dyes are shown in Figure 3.
It can be clearly seen that the electron density is uniformly
distributed along the D-A system (triphenylamine and ben-
zothiazole units) in the ground state; while at the LUMO
level, electrons move by an intramolecular charge transfer,
and are finally delocalized across the entire A-π-A system
(benzothiazole--cyanoacrylic acid). The results indicate
that the electrons can be successively transferred from the
donor to the benzothiazole unit, then transferred to the cy-
anoacetic acid unit, and finally to TiO2.
3.2 Electrochemical properties
To evaluate the possibility of dye regeneration and electron
transfer from the excited state to the conduction band (CB)
of TiO2, cyclic voltammetry was performed to measure the
first oxidation potentials (Eox) corresponding to the HOMO
level of the dyes. The HOMO and LUMO levels of the dyes
are listed in Table 1. The schematic energy levels of Z1–Z3
based on electrochemical data are shown in Figure 2. The
first half-wave potentials of Z1–Z3 were 0.80, 0.84 and
0.84 V. Therefore, the first oxidation potentials (Eox) corre-
sponding to the HOMO levels were 1.43, 1.47 and 1.47 V
(by addition of 0.63 V to the potential vs. Fc/Fc+ (vs. NHE)),
respectively. The estimated excited state potentials corre-
sponding to the LUMO levels were 1.17, 1.03 and 1.03
V calculated from EoxE0-0, where E0-0 values were calcu-
lated from the intersection (λint) of the normalized absorp-
tion and the emission spectra (E0-0 = 1240/λint, Figure 1(b)).
The HOMO energy levels were calculated by the equation
EHOMO = e[4.88 + E1/2 (vs. Fc/Fc+)] [40] and the ELUMO
3.4 Photovoltaic performance
The photocurrent density-photovoltage (J-V) curves of the
DSSCs based on Z1–Z3 are shown in Figure 4, and key
parameters including short-circuit current density (Jsc),
open-circuit voltage (Voc), fill factor (FF) and overall power
conversion efficiency () are summarized in Table 2. For
the three dyes, the Z2-sensitized cell exhibited a maximum
 of 4.16% (Jsc = 9.27 mA cm–2, Voc = 642 mV, FF= 0.70).
levels were calculated based on the relationship ELUMO
=
EHOMO + E0-0. The EHOMO and ELUMO data are summarized in
Table 1. All the HOMO levels of dyes Z1–Z3 (1.43, 1.47
and 1.47 V, Figure 2) were more positive than that of the
iodine/iodide redox potential value (0.4 V vs. NHE) [41],
ensuring the thermodynamic regeneration of the dyes.
However, the Eox of the dyes are slightly too positive, which
might lower the open-circuit voltage (Voc) [42]. The LUMO
levels of the three dyes were more negative than the energy
level of the TiO2 electrode (0.5 V vs. NHE) [41], allowing
excited electrons to be efficiently injected into the electrode
conduction band of TiO2.
Figure 3 The optimized structure and the frontier molecular orbitals of
the HOMOs and LUMOs of the dyes calculated by DFT at the
B3LYP/631 + G (d) level.
Figure 4 The current density-voltage curves of the DSSCs based on dyes
Z1–Z3 measured under the illumination of AM 1.5 G (100 mW cm2).
Figure 2 The schematic energy levels of Z1–Z3 based on absorption and
electrochemical data.

Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
511
Table 2 Photovoltaic parameters of DSSCs
lar extinction coefficient in solution. The higher IPCE and
Jsc could lead to an improved photovoltaic performance of
the DSSCs based on dye Z2.
Jsc
Adsorbed amount
(×107 mol cm2)
2.6
Dye
Voc (mV) FF
 (%)
(mA cm2)
6.83
Z1
Z2
Z2 a)
Z3
690
642
671
695
0.67
0.70
0.69
0.70
3.15
4.16
4.61
4.12
Among the three dyes, DSSCs based on Z2 produced the
highest Jsc but the lowest Voc, which can be explained by the
electron lifetime in the conduction band of TiO2. Electro-
chemical impedance spectroscopy (EIS) was performed to
investigate interfacial charge transfer in DSSCs under the
dark. As is shown in Figure 6, Nyquist plots typically have
three semicircles, and the large semicircle at intermediate
frequencies is attributed to charge transfer resistances at the
TiO2/dye/electrolyte interface. The radius of the larger
semicircle increases in the order Z2 < Z3 < Z1, implying
the same order of the electron recombination resistance for
the three dyes. The data of recombination resistance and
9.27
3.3
9.97

3.8
8.46
a) Dye bath: 0.3 mM Z2 and 3.0 mM CDCA in CH3OH/CH2Cl2 (1:4, v/v).
Under the same conditions, the cells sensitized with Z1 and
Z3 gave Jsc of 6.83 and 8.46 mA cm–2, Voc of 690 and 695
mV and FF of 0.67 and 0.70, corresponding to  of 3.15%
and 4.12%, respectively. The Z2-sensitized cell has the
highest  due to its relatively higher Jsc, which can be at-
tributed to its broader absorption and enhanced molar ex-
tinction coefficient in the visible region. The amounts of
dye adsorbed on the TiO2 substrate increased in the order
Z1 < Z2 < Z3. Although the absorbed amount of Z2 is
smaller than that of Z3, Z2 has the higher Jsc due to its
broader spectral response range.
The efficiencies of the DSSCs based on Z1–Z3 were not
as high as expected. We consider that the low efficiency is
due to their relatively low ability to absorb sunlight
throughout the full spectrum. In comparison with the refer-
ence dye C1 without benzothiazol [37], the absorption λmax
of Z1–Z3 are blue-shifted almost 40 nm, which reduces the
absorption range of sunlight, leading to the decrease of
short-circuit current density (Jsc) and finally resulting in low
efficiency of the solar cells. Normally the introduction of a
benzothiazol unit extends conjugation of the dye, leading to
red-shift of its light absorption. The absorption max of
Z1–Z3 are blue-shifted because the benzothiazole unit does
not conjugate well with the other units in the molecule.
Thus, structural modification of these dyes to increase the
conjugation of benzothiazole with the other units might im-
prove the DSSC performance.
We studied the performance of the DSSCs based on Z2
in the presence of chenodeoxycholic acid (CDCA) (3.0 mM
in CH3OH/CH2Cl2 (1:4, v/v). The results are collected in
Table 2. CDCA improved the device performance, with the
power conversion efficiency of the DSSC reaching 4.61%.
A possible explanation is that CDCA reduced the aggrega-
tion of dye Z2 on the TiO2 surface [43].
Figure 5 IPCE spectra of DSSCs based on Z1–Z3.
Table 3 Electrochemical parameters of DSSCs obtained from EIS
Dye
Z1
R (ohm)
134.1
49.4
C (μF)
480
Electron lifetime (ms)
64
24
82
Z2
477
Z3
103.3
797
The incident monochromatic photon-to-current conver-
sion efficiency (IPCE) spectra for the DSSCs based on dyes
Z1–Z3 are shown in Figure 5. The IPCE onsets for Z1–Z3
were extended to 600, 680 and 650 nm, respectively, in-
creasing in the order Z1 < Z3 < Z2. The solar cells based on
the Z1–Z3 dyes exhibited IPCE values above 60% in the
range 400500 nm, with a maximum value of 79.3% at 430
nm for Z1, 79.9% at 470 nm for Z2 and 75.7% at 430 nm
for Z3, respectively. The higher IPCE value of dye Z2 con-
taining the thiophene moiety can be attributed to its higher
light harvesting efficiency in the visible region, which
matched well with its broader absorption and enhanced mo-
Figure 6 Electrochemical impedance spectra of the DSSCs based on
Z1–Z3 measured in the dark.

512
Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
1
2
O'Regan B, Grätzel M. A low-cost, high-efficiency solar cell based
on dye-sensitized colloidal TiO2 films. Nature, 1991, 353: 737–740
Wang P, Klein C, Humphry-Baker R, Zakeeruddin SM, Gratzel M. A
high molar extinction coefficient sensitizer for stable dye-sensitized
solar cells. J Am Chem Soc, 2005, 127: 808–809
3
Islam A, Chowdhury FA, Chiba Y, Komiya R, Fuke N, Ikeda N,
Nozaki K, Han LN. Synthesis and characterization of new efficient
tricarboxyterpyridyl (beta-diketonato) ruthenium(II) sensitizers and
their applications in dye-sensitized solar cells. Chem Mater, 2006, 18:
5178–5185
4
5
Nattestad A, Mozer AJ, Fischer MKR, Cheng YB, Mishra A,
Baeuerle P, Bach U. Highly efficient photocathodes for dye-
sensitized tandem solar cells. Nat Mater, 2010, 9: 31–35
Seo KD, Song HM, Lee MJ, Pastore M, Anselmi C, De Angelis F,
Nazeeruddin MK, Grätzel M, Kim HK. Coumarin dyes containing
low-band-gap chromophores for dye-sensitised solar cells. Dyes
Pigments, 2011, 90: 304–310
Figure 7 IMVS spectra of the DSSCs based Z1–Z3 dyes measured under
various incident light intensities.
6
7
Nazeeruddin MK, De Angelis F, Fantacci S, Selloni A, Viscardi G,
Liska P, Ito S, Takeru B, Grätzel M. Combined experimental and
DFT-TDDFT computational study of photoelectrochemical cell
ruthenium sensitizers. J Am Chem Soc, 2005, 127: 16835–16847
Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, Robin HB, Wang
P, Zakeeruddin SM, Grätzel M. Enhance the optical absorptivity of
nanocrystalline TiO2 film with high molar extinction coefficient
ruthenium sensitizers for high performance dye-sensitized solar cells.
J Am Chem Soc, 2008, 130: 10720–10728
Grätzel M. The advent of mesoscopic injection solar cells. Prog
Photovolt: Res Appl, 2006, 14: 429–442
Mishra A, Fischer MKR, Baeuerle P. Metal-free organic dyes for
dye-sensitized solar cellsFrom structure: Property relationships to
design rules. Angew Chem Int Ed, 2009, 48: 2474–2499
chemical capacitance are listed in Table 3. The electron
lifetime values derived from curve fittings are 64, 24 and 82
ms for Z1, Z2 and Z3 based DSSCs, respectively. The same
order (Z2 < Z1 < Z3) of the electron lifetime is also shown
in the IMVS (Figure 7). Although the Z1-based DSSCs ex-
hibit a larger recombination resistance than Z3-based
DSSCs, the value of the chemical capacitance of the DSSCs
based on Z3 is much larger. As a result, the electron life-
time increases in the order Z2 < Z1 < Z3. The Z2-based
DSSCs exhibit the lowest Voc (Figure 4) value due to the
shortest electron lifetime among the three dye-based cells.
8
9
10 Hara K, Sayama K, Ohga Y, Shinpo A, Suga S, Arakawa H. A
coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell
having a high solar-energy conversion efficiency up to 5.6%. Chem
Commun, 2001: 569–570
11 Hara K, Wang ZS, Sato T, Furube A, Katoh R, Sugihara H, Dan OY,
Kasada C, Shinpo A, Suga S. Oligothiophene-containing coumarin
dyes for efficient dye-sensitized solar cells. J Phys Chem B, 2005,109:
15476–15482
4 Conclusion
12 Ono T, Yamaguchi T, Arakawa H. Study on dye-sensitized solar cell
using novel infrared dye. Sol Energy Mater Sol Cells, 2009, 93:
831–835
13 Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of
dye-sensitized solar cells based on metal-free indoline dyes. J Am
Chem Soc, 2004, 126: 12218–12219
14 Liu B, Zhu W, Zhang Q, Wu W, Xu M, Ning Z, Xie Y, Tian H.
Conveniently synthesized isophorone dyes for high efficiency
dye-sensitized solar cells: Tuning photovoltaic performance by
structural modification of donor group in donor-π-acceptor system.
Chem Commun, 2009: 1766–1768
15 Matsui M, Kotani M, Kubot Y, Funabiki K, Jin J, Yoshida T,
Higashijima S, Miura H. Comparison of performance between
benzoindoline and indoline dyes in zinc oxide dye-sensitized solar
cell. Dyes Pigments, 2011, 91: 145–152
16 Wang ZS, Li FY, Huang CH. Highly efficient sensitization of
nanocrystalline TiO2 films with styryl benzothiazolium propylsul-
fonate. Chem Commun, 2000: 2063-2064
17 Chen YS, Li C, Zeng ZH, Wang WB, Wang XS, Zhang BW.
Efficient electron injection due to a special adsorbing group's
combination of carboxyl and hydroxyl: Dye-sensitized solar cells
based on new hemicyanine dyes. J Mater Chem, 2005,15: 1654–1661
18 Kitamura T, Ikeda M, Shigaki K, Inoue T, Anderson NA, Ai X, Lian
T, Yangagida S. Phenyl-conjugated oligoene sensitizers for TiO2
solar cells. Chem Mater, 2004, 16: 1806–1812
A series of new D-A--A organic dyes Z1–Z3 containing
the benzothiazole unit as an additional electron-withdrawing
acceptor have been designed and synthesized. The photo-
physical and electrochemical properties of the D-A--A
systems have been studied. After the introduction of a ben-
zothiazole unit, all the three dyes showed enhanced molar
extinction coefficients. The effects of various conjugated
linkers (benzene, furan and thiophene) on dye-sensitized
solar cell were also investigated. It was found that dye Z2,
with a thiophene linker, gives a solar cell which exhibits the
best overall light to electricity conversion efficiency  of
4.16% under standard global AM 1.5 G solar conditions.
The efficiency is not high because the conjugation of the
benzothiazole unit is relatively poor, leading to a decrease
in the absorption range of sunlight. Further structural modi-
fications to increase the conjugation of the benzothiazole
with the other units might enhance the efficiency of such
DSSCs.
19 Tian HN, Yang XC, Chen RK, Zhang R, Hagfeldt A, Sunt LC. Effect
of different dye baths and dye-structures on the performance of
dye-sensitized solar cells based on triphenylamine dyes. J Phys Chem
C 2008, 112: 11023–11033
This work was financially supported by the National Natural Science
Foundation of China (20872038, 20873183, 21072064), and the Fund from
Guangzhou Science and Technology Project, China (2012J4100003).

Zeng J, et al. Sci China Chem April (2013) Vol.56 No.4
513
20 Hagberg DP, Yum JH, Lee H, De Angelis F, Marinado T, Karlsson
KM. Molecular engineering of organic sensitizers for dye-sensitized
solar cell applications. J Am Chem Soc, 2008, 130: 6259–6266
21 Li G, Zhou YF, Cao XB, Bao P, Jiang KJ, Lin Y, Lian MY. Novel
TPD-based organic D-π-A dyes for dye-sensitized solar cells. Chem
Commun, 2009: 2201–2203
22 Im H, Kim S, Park C, Jang SH, Kim CJ, Kim K, Park NG, Kim C.
High performance organic photosensitizers for dye-sensitized solar
cells. Chem Commun, 2010, 46: 1335–1337
23 Tian Z, Huang M, Zhao B, Huang H, Feng X, Nie Y, Ping S Tan S.
Low-cost dyes based on methylthiophene for high-performance
dye-sensitized solar cells. Dyes Pigments, 2010, 87: 181–187
24 Zeng WD, Cao YM, Bai Y, Wang YH, Shi YS, Zhang M, Wang FF,
Pan CY, Wang P. Efficient dye-sensitized solar cells with an organic
photosensitizer featuring orderly conjugated ethylenedioxythiophene
and dithienosilole blocks. Chem Mater, 2010, 22: 1915–1925
25 Liang M, Zong XP, Han HY, Chen C, Sun Z, Xue S. An efficient
dye-sensitized solar cell based on a functionalized-triarylamine dye.
Mater Lett 2011, 65: 1331–1333
32 Li W, Wu Y, Zhang Q, Tian H, Zhu W. D-A-π-A featured sensitizers
bearing phthalimide and benzotriazole as auxiliary acceptor: Effect
on absorption and charge recombination dynamics in dye-sensitized
solar cells. ACS Appl Mater Inter, 2012, 4: 1822–1830
33 Hrobarikova V, Hrobarik P, Gajdos P, Fitilis I, Fakis M, Persephonis
P, Zahradnik P. Benzothiazole-based fluorophores of donor--
acceptor--donor type displaying high two-photon absorption. J Org
Chem, 2010, 75: 3053–3068
34 Lartia R, Allain C, Bordeau G, Schmidt F, Fiorini-Debuisschert C,
Charra F, Paule M, Teulade F. Synthetic strategies to derivatizable
triphenylamines displaying high two-photon absorption. J Org Chem,
2008, 73: 1732–1744
35 Nelson RR, Adams RN. Anodic oxidation pathways of substituted
triphenylamines. II. Quantitative studies of benzidine formation. J
Am Chem Soc, 1968, 90: 3925–3930
36 Jordan AD, Luo C, Reitz AB. Efficient conversion of substituted aryl
thioureas to 2-aminobenzothiazoles using benzyltrimethylammonium
tribromide. J Org Chem, 2003, 68: 8693–8696
37 Yu QY, Liao JY, Zhou SM, Yong S, Liu JM, Kuang DB, Su CY.
Effect of hydrocarbon chain length of disubstituted triphenyl-
amine-based organic dyes on dye-sensitized solar cells. J Phys Chem
C, 2011, 115: 22002–22008
26 Tian H, Yang X, Chen R, Pan Y, Li L, Hagfeldt A, Sun L.
Phenothiazine derivatives for efficient organic dye-sensitized solar
cells. Chem Commun, 2007: 3741–3743
27 Park SS, Won YS, Choi YC, Kim JH. Molecular design of organic
dyes with double electron acceptor for dye-sensitized solar cell.
Energy & Fuels, 2009, 23: 3732–3736
38 Chen R, Yang X, Tian H, Sun L. Tetrahydroquinoline dyes with
different spacers for organic dye-sensitized solar cells. J Photochem
Photobiol A, 2007, 189: 295–300
28 Tian H, Yang X, Cong J, Chen R, Teng C, Liu J, Hao Y, Wang L,
Sun L. Effect of different electron donating groups on the
performance of dye-sensitized solar cells. Dyes Pigments, 2010, 84:
62–68
29 Tsao MH, Wu TY, Wang HP, Sun IW, Su SG, Lin YC, Chang CW.
An efficient metal-free sensitizer for dye-sensitized solar cells. Mater
Lett, 2011, 65: 583–586
30 Zhu W, Wu Y, Wang S, Li W, Li X, Chen J, Wang Z. Organic
D-A--A solar cell sensitizers with improved stability and spectral
response. Adv Funct Mater, 2011, 21: 756–763
39 Lin LY, Tsai CH, Wong KT, Huang TW, Hsieh L, Liu SH, Lin HW,
Wu CC, Chou SH, Chen SH, Tsai AI. Organic dyes containing
coplanar diphenyl-substituted dithienosilole core for efficient
dye-sensitized solar cells. J Org Chem, 2010, 75: 4778–4785
40 Jones BA, Facchetti A, Wasielewski MR, Marks TJ. Tuning orbital
energetics in arylene diimide semiconductors. materials design for
ambient stability of n-type charge transport. J Am Chem Soc, 2007,
129: 15259–15278
41 Hagfeldt A, Graetzel M. Light-induced redox reactions in
nanocrystalline systems. Chem Rev, 1995, 95: 49-68
31 Pei K, Wu Y, Wu W, Zhang Q, Chen B, Tian H, Zhu W.
42 Qu SY, Hua JL, Tian H. New D--A dyes for efficient dye-sensitized
solar cells. Sci China Chem, 2012, 55: 677–697
Constructing organic D-A--A-featured sensitizers with
a
quinoxaline unit for high-efficiency solar cells: The effect of an
auxiliary acceptor on the absorption and the energy level alignment.
Chem Eur J, 2012, 18: 8190–8200
43 Chen R, Yang X, Tian H, Wang X, Hagfeldt A, Sun L. Effect of
tetrahydroquinoline dyes structure on the performance of organic
dye-sensitized solar cells. Chem Mater, 2007, 19: 4007–4015