Triphenylamine-based organic dyes containing benzimidazole derivatives for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.05.007

The synthesis and application to dye-sensitized solar cells of two new triphenylamine-based organic dyes containing benzimidazole derivatives as secondary donors together with a simple triphenylamine derived dye for the purpose of comparison is reported. The photophysical and electrochemical properties of the dyes were investigated by UV-vis spectroscopy and cyclic voltammetry. The introduction of benzimidazole derivatives in the phenyl ring of the triphenylamine core increases the molar extinction coefficients and λ_max because of the extension of the π-conjugation structures of the dyes. Overall conversion efficiencies of ～2.5% under full sunlight (AM 1.5G, 100 mW cm^-2) irradiation were obtained for DSSCs based on these new dyes, under the same conditions, the reference dye and di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4, 4′-dicarboxylato) ruthenium(II) (N719) gave overall conversion efficiencies of 1.23% and 5.61%, respectively. Our findings demonstrate that the introduction of benzimidazole derivatives as secondary donors in triphenylamine-based dye can improve their photovoltaic performance compared to the unsubstituted reference dye in DSSCs.

Full text of DOI:10.1016/j.dyepig.2012.05.007

Dyes and Pigments 95 (2012) 743e750
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Triphenylamine-based organic dyes containing benzimidazole derivatives
for dye-sensitized solar cells
,
a
a
b
b
b
Linlei Zhou a, Chunyang Jia a , Zhongquan Wan , Zi Li , Jingying Bai , Ligong Zhang , Jiaqiang Zhang ,
*
Xiaojun Yao c
a State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China,
Chengdu 610054, PR China
b Beijing Spacecrafts, China Academy of Space Technology, Beijing 100190, PR China
c State Key Laboratory of Applied Organic Chemistry, School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and application to dye-sensitized solar cells of two new triphenylamine-based organic
dyes containing benzimidazole derivatives as secondary donors together with a simple triphenylamine
derived dye for the purpose of comparison is reported. The photophysical and electrochemical properties
of the dyes were investigated by UVevis spectroscopy and cyclic voltammetry. The introduction of
benzimidazole derivatives in the phenyl ring of the triphenylamine core increases the molar extinction
Received 13 January 2012
Received in revised form
9 May 2012
Accepted 10 May 2012
Available online 16 May 2012
coefficients and lmax because of the extension of the
p-conjugation structures of the dyes. Overall
conversion efficiencies of w2.5% under full sunlight (AM 1.5G, 100 mW cmꢀ2) irradiation were obtained
for DSSCs based on these new dyes, under the same conditions, the reference dye and di-
tetrabutylammonium cis-bis(isothiocyanato) bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium(II) (N719)
gave overall conversion efficiencies of 1.23% and 5.61%, respectively. Our findings demonstrate that the
introduction of benzimidazole derivatives as secondary donors in triphenylamine-based dye can improve
their photovoltaic performance compared to the unsubstituted reference dye in DSSCs.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cells
Organic dye
Triphenylamine
Benzimidazole derivative
Secondary donor
Cyanoacetic acid
1. Introduction
have been developed and have achieved solar electrical power
conversion efficiencies in the range 5e9 %.
Dye-sensitized solar cells (DSSCs) have received more and more
attention due to their relatively high power conversion efficiency
and low cost alternative to the conventional photovoltaic devices
[1,2]. As a key part of DSSCs, the dyes play a significant role for the
Most of the organic dyes applied in DSSCs show the same
character with the electron-donor (D) and the electron acceptor (A)
linked by a
p-conjugation bridge, which is called the DepeA
structure [14]. But it is well known that the major factors for low
conversion efficiency of DSSCs based on organic dyes are the
formation of dye aggregates on the semiconductor surface and the
recombination of conduction band electrons with triiodide in the
electrolyte [15].
Among the organic dyes, triphenylamine (TPA) and its deriva-
tives as donor units have displayed promising properties in the
development of photovoltaic devices [16e18]. Triphenylamine
derivatives are often used as the donor because of their chemical
stability, stabilized photoexcited state by charge delocalization in
the aryl substituents, and prevention of direct charge recombina-
tion between TiO2 and Iꢀ3 by the bulky aryl group covering the TiO2
[19]. So it is predicted that if introducing a secondary electron-
donor to TPA part of dye TPA-1, which is a possible alternative to
retard the interfacial charge-recombination dynamics and to retain
efficient light-induced charge separation, may enhance perfor-
mance of the dyes for DSSCs.
DSSCs gaining higher
h (photoelectric conversion efficiency) and
have been actively studied by researchers all over the world. Up
to now, DSSCs based on the ruthenium sensitizers have shown
very impressive solar-to-electric power conversion efficiencies,
reaching 11% under standard AM 1.5G sunlight [3]. However, the
manufacturing costs and environmental issues of the ruthenium
sensitizers limit their large-scale application. In order to get the
cheaper photosensitizers for DSSCs, pure organic dyes are strongly
desired because of their superior molar extinction coefficients,
simple preparation and purification procedures and lower cost.
Recently, coumarin [4], merocyanine [5], indoline [6], polyene [7],
hemicyanine [8], triphenylamine [9], fluorene [10], carbazole [11],
phenothiazine [12] and tetrathiafulvalene [13] based-organic dyes
* Corresponding author. Tel.: þ86 28 83202550; fax: þ86 28 83202569.
E-mail address: cyjia@uestc.edu.cn (C. Jia).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.007

744
L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
Based on these studies, the corresponding dye (TPA-1) for the
on Nicolet 6700. Cyclic voltammetry experiments were performed
on a CH Instruments 660C electrochemical workstation at a scan
rate of 50 mV/s in dimethylformamide (DMF) (5.0 ꢁ 10ꢀ4 M) con-
taining 0.1 M n-Bu4NPF6 as the supporting electrolyte, platinum as
counter and work electrodes and Ag/AgCl as reference electrode.
purpose of comparison and two new dyes (TPA-B1 and TPA-B2) by
introducing the benzimidazole derivatives to the framework of TPA
core, in which the cyanoacetic acid act as acceptor groups, were
designed, synthesized and applied in DSSCs. The effects of the
different secondary donors in those dyes on the optical, electro-
chemical properties and photovoltaic performances were studied.
The corresponding molecular structures of the three dyes and N719
are shown in Scheme 1.
2.4. Synthesis
Compounds 1 [21], 2 [22], 5,6-diaminobenzo[d] [1,3]dithiole-2-
thione [23] and TPA-1 [21] were prepared according to the reported
literature (as shown in Scheme 2).
2. Experimental details
2.1. Materials and characterization
2.4.1. Compound 3
A methanol (20 mL) solution of 1,2-phenylenediamine (108 mg,
0.10 mmol), compound 2 (300 mg, 0.10 mmol) were charged
sequentially in a three-necked flask and heated under reflux for 3 h.
After cooling to room temperature, solvents were removed by
rotary evaporation, and the residue was purified by silica gel
column chromatography with dichloromethane/ethyl acetate (10:1,
v:v) as eluent to afford compound 3 as a yellow solid (105 mg, yield
26%). Mp 163e166 ꢂC. FT-IR (KBr, cmꢀ1): 3059, 1690, 1588, 1491,
All solvents and other chemicals were reagent grade and used
without further purification. 1,2-phenylenediamine, cyanoacetic
acid and triphenylamine were purchased from Astatech. HRMS data
were determined with an FTICR-APEX instrument and a micro TOF-
Q II instrument. 1H and 13C NMR spectra were measured on Brücker
AM 500 NMR instrument. Mp data were obtained on X4 melting
point detector (FUKA, Beijing, China).
1163. 1H NMR (400 MHz, DMSO-d6):
d
¼ 12.87(s, 1H), 9.82 (s, 1H),
2.2. Theoretical calculations
8.16e8.18 (d, J ¼ 8.4 Hz, 2H), 7.78e7.80 (d, J ¼ 8.4 Hz, 2H), 7.44e7.52
(m, 3H), 7.19e7.30 (m, 8H), 7.04e7.06 (d, J ¼ 8.8 Hz, 2H). HRMS (m/
z): calcd for C26H19N3O: 389.1528, found 390.1590 [M þ H]þ.
Gaussian 03 package was used for density functional theory
(DFT) calculation [20]. The geometries and energies of TPA-1, TPA-
B1, and TPA-B2 were determined using the B3LYP method with the
6-31G (d) basis set. None of the frequency calculations generated
negative frequencies, being consistent with an energy minimum for
the optimized geometry.
2.4.2. Compound 4
A methanol (20 mL) solution of 5,6-diaminobenzo[d] [1,3]
dithiole-2-thione (213 mg, 0.10 mmol), compound 2 (300 mg,
0.10 mmol) were charged sequentially in a three-necked flask and
heated under reflux for 12 h. After cooling to room temperature,
solvents were removed by rotary evaporation, and the residue was
purified by silica gel column chromatography with dichloro-
methane/ethyl acetate (6:1, v:v) as eluent to afford compound 4 as
a deep yellow solid (109 mg, yield 22%). Mp 186e189 ꢂC. FT-IR (KBr,
cmꢀ1): 1688, 1589, 1487, 1063. 1H NMR (400 MHz, DMSO-d6):
2.3. Photophysical and electrochemical measurements
INSPECT
F
Scanning electron microscopy (SEM) (FEI,
Netherlands) is used to measure the thickness of the film.
Absorption spectra were measured with SHIMADZU (model
UV1700) UVevis spectrophotometer. FT-IR spectra were obtained
d
¼ 13.30 (s, 1H), 9.84 (s, 1H), 8.18e8.21 (d, J ¼ 8.4 Hz, 2H), 8.12 (s,
O
OTBA
OH
O
N
Ru
N
N
N
NCS
NCS
N
COOH
NC
HO
O
TPA-1
ABTO
N719
O
N
N
N
H
S
N
N
S
S
N
H
COOH
COOH
CN
CN
TPA-B1
TPA-B2
Scheme 1. Molecular structures of TPA-1, N719, TPA-B1 and TPA-B2.

L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
745
COOH
N
CN
TPA-1
(e)
O
1
N
COOH
CN
(
b
)
N
N
TPA-B1
N
H
(e)
O
O
3
N
O
2
(
d
)
COOH
S
S
N
S
N
CN
(e)
N
TPA-B2
H
O
4
Scheme 2. Synthetic routes of the dyes TPA-1, TPA-B1 and TPA-B2. (a) POCl3, DMF, 1,2-dichloroethane, reflux, 12 h; (b) POCl3, DMF, chloroform, reflux, 72 h (c) 1,2-
phenylenediamine, methanol, reflux, 3 h (d) 5,6-diaminobenzo[d] [1,3]dithiole-2-thione, methanol, reflux, 10 h (e) 2-cyanoacetic acid, piperidine, acetonitrile, reflux.
2H), 7.79e7.82 (d, J ¼ 8.4 Hz, 2H), 7.45e7.48 (t, J ¼ 15.6 Hz, 2H),
7.28e7.30 (d, J ¼ 8.8 Hz, 3H), 7.23e7.24 (d, J ¼ 7.2 Hz, 2H), 7.07e7.09
(d, J ¼ 8.8 Hz, 2H). HRMS (m/z): calcd for C27H17N3OS3:495.0534,
found 496.0591[M þ H]þ.
130.23, 128.42, 126.70, 126.04, 125.10, 124.98, 124.27, 120.29, 116.81.
HRMS (m/z): calcd for C30H18N4O2S3: 562.0592, found 563.0673
[M
þ
H]þ. UVevis (ethanol, nm): lmax (molar extinction
coefficient) ¼ 410 (74,900).
2.4.3. TPA-B1
2.5. Fabrication of dye-sensitized solar cells
A CH3CN (15 mL) solution of compound 3 (80 mg, 0.2 mmol),
cyanoacetic acid (57 mg, 0.67 mmol) and a few drops of piperidine
were charged sequentially in a three-necked flask and heated to
reflux for 5 h. After cooling to room temperature, solvents were
removed by rotary evaporation, and the residue was purified by
silica gel column chromatography with dichloromethane/ethanol
(1:1, v:v) as eluent to afford the dye TPA-B1 as a yellow solid
(45 mg, yield 49%). Mp 233e236 ꢂC. FT-IR (KBr, cmꢀ1): 2219, 1694,
TiO2 colloid was prepared according to the literature [24]. The
washed FTO glass substrates were immersed in 40 mM TiCl4 aq. at
70 ꢂC for 30 min to form a compact layer of TiO2, which plays an
important role in suppressing the charge recombination of DSSCs at
the interface between FTO and electrolyte, then washed with water
and ethanol. A thin film of TiO2 was prepared on the FTO substrate
with the compact TiO2 layer through blade coating with glass rod.
After drying the nanocrystalline TiO2 layer at 80 ꢂC, the TiO2 thin
film with more layers was achieved by repeating the blade coating
above process two times. Finally TiO2 electrodes were treated at
450 ꢂC for 30 min. After cooling to room temperature, the elec-
trodes were immersed in 40 mM TiCl4 aq. at 70 ꢂC for 30 min, and
washed with water and ethanol again, then recalcined at 450 ꢂC for
30 min. After the sintering, when the TiO2 electrodes cooled to
80 ꢂC, the electrodes were immersed in a dye bath containing
0.2 mM TPA-1, TPA-B1, TPA-B2 and N719 in ethanol and left
overnight. The films were then rinsed in ethanol to remove excess
dye. In our experiment, open cells were fabricated in air by
clamping the different dyed electrode with platinized counter
electrode. The electrolyte used here composed of 0.6 M 1,2-
1577, 1496, 1186. 1H NMR (400 MHz, DMSO-d6):
d
¼ 8.21e8.23 (d,
J ¼ 8.8 Hz, 2H), 8.17 (s, 1H), 7.96e7.99 (d, J ¼ 8.8 Hz, 2H), 7.58e7.60
(m, 2H), 7.45e7.49 (t, J ¼ 15.6 Hz, 2H), 7.29e7.31 (d, J ¼ 8.8 Hz, 2H),
7.25e7.27 (d, J ¼ 7.6 Hz, 2H), 7.19e7.21 (m, 2H), 7.03e7.05 (d,
J ¼ 9.2 Hz, 2H). 13C NMR
d
(400 MHz, DMSO-d6):
d
¼ 165.65, 163.99,
152.89, 151.09, 150.68, 146.78, 145.12, 132.77, 130.19, 128.04, 126.62,
126.14, 125.91, 125.24, 124.08, 122.04, 119.93, 116.99, 115.70. HRMS-
EI(m/z): calcd for C29H20N4O2 : 456.1586, found 456.1581 [M]þ.
UVevis (ethanol, nm): lmax (molar extinction coefficient) ¼ 421
(48,000).
2.4.4. TPA-B2
A procedure is similar to that for the dye TPA-B1 but with
compound 4 (70 mg, 0.14 mmol) instead of compound 3. The deep
yellow solid (38 mg, yield 48%) is TPA-B2. Mp 256e258 ꢂC. FT-IR
(KBr, cmꢀ1): 2217, 1585, 1488, 1329, 1065. 1H NMR (400 MHz,
dimethyl-3-propylimidazolium iodide (DMPII), 0.0653
M LiI,
0.03 M I2, 0.28 M 4-tertbutylpyridine (TBP) and 0.05 M guanidium
thiocyanate (GuSCN) in acetonitrile.
DMSO-d6):
d
¼ 13.38 (s, 1H), 8.22 (s, 1H), 8.19e8.20 (d, J ¼ 3.6 Hz,
2H), 8.11 (s, 2H), 7.97e8.0 (d, J ¼ 8.8 Hz, 2H), 7.45e7.49 (t,
J ¼ 15.6 Hz, 2H), 7.28e7.32 (m, 3H), 7.24e7.26 (d, J ¼ 7.2 Hz, 2H),
7.04e7.07 (d, J ¼ 8.8 Hz, 2H). 13C NMR (400 MHz, DMSO-d6):
2.6. Photovoltaic characterization
The irradiation source for the photocurrent action spectrum
measurement is a photosource (CHF-XM-500W, Trusttech Co. Ltd.,
d
¼ 213.51, 163.93, 153.21, 151.03, 147.47, 145.05, 144.61, 132.83,

746
L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
Table 1
Beijing, China) with a CH Instruments 660C electrochemical
workstation (Shanghai CH Instruments Co., China). The incident
light intensity was 100 mW cmꢀ2 calibrated with a standard Si solar
cell. The tested solar cells were masked to a working area of 0.4 cm2.
The action spectra of monochromatic incident photon-to-current
conversion efficiency (IPCE) for solar cell were performed by
using a commercial setup (QTest Station 2000 IPCE Measurement
System, CROWNTECH, USA). Electrochemical impedance spectros-
copy (EIS) data were obtained in the dark under forward bias 0.7 V,
scanning from 10ꢀ2 to 105 Hz with ac amplitude of 10 mV by using
CH Instruments 660C electrochemical workstation.
UVevis and electrochemical data.
d
Dye
lmaxa/nm
b/Mꢀ1 cmꢀ1
lmaxc/nm
Eox
Ege/eV
Eox*f/V
(vs. NHE)
(
3
)
/V (vs. NHE)
TPA-1
TPA-B1
TPA-B2
410 (40,700)
421 (48,000)
410 (74,900)
443
449
464
1.03
1.27
1.32
2.10
2.08
2.02
ꢀ1.07
ꢀ0.81
ꢀ0.70
a
Absorption is measured in ethanol solution (1.0 ꢁ 10ꢀ5 M) at room temperature.
The molar extinction coefficient at lmax of the absorption spectra.
Absorption spectra of the dyes adsorbed on TiO2 electrodes.
b
c
d
Eox was measured in DMF with 0.1 M n-Bu4NPF6 as electrolyte (scanning rate:
50 mV sꢀ1, working electrode and counter electrode: Pt wires, and reference elec-
trode: Ag/AgCl), potentials measured vs Ag/AgCl were converted to normal
hydrogen electrode (NHE) by addition of þ0.2 V.
e
Eg was estimated from the absorption spectra of TiO2 electrodes sensitized by
3. Results and discussion
the dyes.
f
Eox* was calculated from Eox ꢀ Eg.
3.1. Optical properties
As the function of light harvesting, the dye plays a significant
role for the DSSCs achieving high conversion efficiency and the
absorption spectra of the dye could affect the device performance
dramatically [25]. The UVevis spectra of the dyes TPA-1, TPA-B1
and TPA-B2 in ethanol solution were measured (Fig. 1) and the
characteristic data are collected in Table 1. The absorbance lmax at
421 nm of TPA-B1 is red-shift 11 nm compared with the lmax at
410 nm of TPA-1. And the molar extinction coefficient of TPA-B1 is
48,000 Mꢀ1 cmꢀ1, which is higher than that of TPA-1. The change of
the molar extinction coefficients of TPA-B1 and TPA-B2 is possible
owing to introducing benzimidazole derivatives to the framework
of TPA core. Although the maxima absorption peak of TPA-B2 has
not red-shift compared with that of TPA-1, the molar extinction
coefficient (74,900 Mꢀ1 cmꢀ1) of TPA-B2 is much higher than that
of TPA-1 (40,700 Mꢀ1 cmꢀ1) and the maxima absorption spectra
are generally broadened. This indicates that introducing benz-
imidazole derivatives to the framework of TPA core resulting in
TPA-B2 with an improved light harvesting ability.
Based on the Tauc relation, the energy gap (Eg) can be obtained
by plotting (ahv)2 vs. hv and extrapolating the linear portion of
(ahv)2 to zero as shown in Fig. 3 [27]. The Eg of TPA-1, TPA-B1 and
TPA-B2 are estimated to be 2.10 eV, 2.08 eV and 2.02 eV, respec-
tively. The red-shift of TPA-B1 and TPA-B2 adhering to TiO2 thin
films can be evidenced by the lower energy band-gap as compared
with that of TPA-1.
3.2. Electrochemical properties
The energetic alignment of the HOMO and LUMO energy levels
is crucial for an efficient operation of the dye in DSSCs. To ensure
efficient electron injection from the excited dye into the conduction
band of TiO2, the LUMO level must be higher in energy than the
TiO2 conduction band edge. The HOMO level of the dye must be
lower in energy than the redox potential of the Iꢀ/I3ꢀ redox couple
for efficient regeneration of the dye cation after photoinduced
electron injection into the TiO2 film. Cyclic voltammetry is
a preliminary characterization technique to determine the redox
properties of organic dyes.
Cyclic voltammetry measurements were performed in dime-
thylformamide (DMF) solution using 0.1 M tetrabutylammonium
tetrafluoroborate as supporting electrolyte (Fig. 4), and the corre-
sponding data are summarized in Table 1. The first oxidation
potentials (Eox) corresponds to the HOMO levels of the dyes (TPA-1:
1.03 V vs NHE; TPA-B1: 1.27 V vs NHE; TPA-B2: 1.32 V vs NHE) are
Fig. 2 shows the absorption spectra of TPA-1, TPA-B1 and TPA-
B2 on 6.94 mm thick TiO2 films after 12 h immersion. The maximal
absorption peaks for TPA-1, TPA-B1 and TPA-B2 on the TiO2 films
are located at 443 nm, 449 nm and 464 nm, respectively. The
absorption spectra are generally broadened and red-shifted as
compared to the dyes dissolved in ethanol, implying that most of
the dyes were adsorbed on the TiO2 surface with only partial J-type
aggregates [26]. The broadening and red-shifting of absorption
spectra are beneficial for light harvesting of the solar spectrum.
2.5
80000
TPA-1
70000
TPA-B1
TPA-1
TPA-B1
TPA-B2
2.0
60000
TPA-B2
50000
40000
30000
20000
10000
0
1.5
1.0
0.5
0.0
350
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength(nm)
Wavelength(nm)
Fig. 1. Absorption spectra of TPA-1, TPA-B1 and TPA-B2 recorded in ethanol.
Fig. 2. Absorption spectra of TiO2 films sensitized by TPA-1, TPA-B1 and TPA-B2.

L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
747
-1.5
-1.07
15
10
5
TPA-1
TPA-B1
TPA-B2
-1.0
-0.81
-0.70
-0.5
CB
TiO2
I-/I3-
-0.5
2.10
0.0
2.08
0.4
redox
2.02
0.5
1.03
1.0
1.27
1.32
1.5
TPA-1
TPA-B1
TPA-B2
2.0
0
1.6
1.8
2.0
2.2
2.4
Fig. 5. Schematic energy levels of TPA-1, TPA-B1 and TPA-B2 based on absorption and
electrochemical data.
Photoenergy hv (eV)
Fig. 3. Plot of (ahv)2 vs. hv.
performances of dye-sensitized solar cells, their geometries and
energies were optimized by density functional theory (DFT)
calculation at the B3LYP/6-31G(d) level with Gaussian 03 [20].
The ground-state geometries of the three dyes are shown in
Fig. 6, and the optimized geometries indicate that the three
benzene rings in the triphenylamine cores are non-planar, which
could reduce contact between molecules and enhance their
thermo-stability [29].
Fig. 7 shows the frontier orbital plots of the HOMOs and LUMOs
of TPA-1, TPA-B1 and TPA-B2. The HOMOs of TPA-1, TPA-B1 and
TPA-B2 dyes are delocalized in large extent over the donor parts
and the bridge, indicating that the binding energy of the electrons
more positive than the Iꢀ/I3ꢀ redox potential (0.4 V vs NHE), which
indicates that the oxidized dyes formed from respective electron
injection into the conduction band of TiO2 will favorably accept
electrons from Iꢀ ions thermodynamically. This could lead to a fast
dye-regeneration, avoiding the charge recombination between
oxidized dyes and photoinjected electrons in the nanocrystalline
TiO2 film. The excited state oxidation potentials (Eox*) of the dyes
can be obtained by the first oxidation potentials (Eox) and the
energy gaps (Eg) of the dyes, namely, Eox ꢀ Eg. The Eox* of the TPA-1,
TPA-B1 and TPA-B2 are ꢀ1.07, ꢀ0.81 and ꢀ0.70 V vs. NHE,
respectively. The Eox* of the dyes are more negative than the band
edge energy of the nanocrystalline TiO2 electrode (ꢀ0.5 V vs NHE)
[28], indicating that the electron injection process from the excited
dye molecule to TiO2 conduction band is energetically permitted.
The schematic energy levels of TPA-1, TPA-B1 and TPA-B2 based on
absorption and electrochemical data are shown in Fig. 5.
in the HOMOs is sensitive to the change in the
p-system [30].
Illumination produces an excited state (LUMOs) in these dyes, and
intramolecular charge transfer occurs, resulting in the electron
movement from the donor groups to the acceptor groups. The
electron densities of the secondary electron-donors (the benz-
imidazole derivatives) in the LUMOs of TPA-B1 and TPA-B2 are very
low. This indicates that both of the secondary electron-donors in
the framework of TPA core could enhance the photo-generated
charge separation under light illumination compared with that of
TPA-1.
3.3. Theoretical calculations
To get further insight into the effect of molecular structure and
electron distribution of TPA-1, TPA-B1 and TPA-B2 on the
3.4. Photovoltaic properties
TPA-1
TPA-B1
TPA-B2
The incident photon-to-current conversion efficiencies (IPCE) of
the DSSCs based on TPA-1, TPA-B1 and TPA-B2 are measured in the
visible region (300e800 nm), as shown in Fig. 8. The highest IPCE
values of 53% at 430 nm for TPA-B1 and 51% at 420 nm for TPA-B2
are much higher than 23% at 410 nm for TPA-1, which might be
attributable to the higher molar extinction coefficient of TPA-B1
and TPA-B2. It can be seen clearly that the photocurrent response of
TPA-B1 and TPA-B2-sensitized DSSCs is much better than that of
TPA-1 in the range of 375e525 nm, which implies the dyes would
show a relatively larger photocurrent in DSSCs. On the other hand,
the IPCE values for TPA-1, TPA-B1 and TPA-B2 obviously decrease
after 500 nm, which can be attributed to the decrease of light
harvesting properties.
15
10
5
0
Fig.
9 shows the currentevoltage characteristics of dye-
sensitized solar cells employing with TPA-1, TPA-B1, TPA-B2 and
N719 as sensitizers under a photosource with the light intensity as
100 mW cmꢀ2. The detailed photovoltaic parameters are summa-
rized in Table 2, where TPA-1 and N719 are included for compar-
-5
0.0
0.4
0.8
1.2
1.6
E/V vs. Ag/AgCl
Fig. 4. Cyclic voltammograms of TPA-1, TPA-B1 and TPA-B2 in DMF.
ison. The TPA-B1-sensitized solar cell gave a short-circuit

748
L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
Fig. 6. Optimized geometries of TPA-1, TPA-B1 and TPA-B2.
photocurrent density (Jsc) of 5.12 mA cmꢀ2, an open-circuit voltage
(Voc) of 0.659 V, and a fill factor (ff) of 0.72, corresponding to an
overall conversion efficiency of 2.43% (see Table 2). The TPA-B2-
sensitized solar cell gave a Jsc of 6.12 mA cmꢀ2, a Voc of 0.679 V, and
a ff of 0.64, corresponding to an overall conversion efficiency of
2.65%.
(HOMO) than that of the TPA-1, which offers a significantly larger
driving force for the reduction of the oxidized dye. This, in turn, will
lead to a slower back electron transfer from TiO2 to the dye and
result in a lager Voc value [31]. The higher Jsc and Voc for the DSSCs of
TPA-B1 and TPA-B2 lead to the higher efficiencies than that of TPA-
1, and increases in
h of about 97% and 115% were obtained from
According to Fig. 9 and Table 2, it is clear that the photovoltaic
performances of the DSSCs can be evidently affected by introducing
a benzimidazole unit and a substituted benzimidaole unit into the
formwork of TPA core. In comparison with TPA-1, both the Jsc and
Voc values of TPA-B1 and TPA-B2 are improved by introducing
secondary donor to TPA part. TPA-B1 and TPA-B2 have a higher Jsc
which might be due to the secondary donors increasing the ability
of TPA core’s electron-donating potential. It can be deduced from
the higher molar extinction coefficient and IPCE of TPA-B1 and
TPA-B2 than that of TPA-1. The higher Voc of TPA-B1 and TPA-B2
than that of TPA-1 might be due to the comparatively lower Eox
TPA-1 to TPA-B1 and TPA-B2, respectively.
Electrochemical impedance spectroscopy (EIS) is a useful tool
for characterizing important interfacial charge transfer processes in
DSSCs, such as the charge recombination at the TiO2/dye/electro-
lyte interface, electron transport in the TiO2 electrode, electron
transfer at the counter electrode, and Iꢀ3 transport in the electrolyte.
In this study, impedance spectra of the solar cells based on TPA-1,
TPA-B1 and TPA-B2 were performed to further elucidate the
photovoltaic properties.
Fig. 10 shows the EIS Nyquist plots for DSSCs based on TPA-1,
TPA-B1 and TPA-B2. The DSSCs were measured in the dark under
Fig. 7. The electron distribution of the HOMOs and LUMOs of TPA-1, TPA-B1 and TPA-B2.

L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
749
70
60
50
40
30
20
10
0
24
TPA-1
TPA-B1
TPA-B2
22
TPA-1
TPA-B1
TPA-B2
20
18
16
14
12
10
8
6
4
2
0
400
450
500
550
600
10
20
30
40
50
60
70
Wavelength (nm)
Z'/ohm
Fig. 8. The IPCE spectra of the DSSCs sensitized with TPA-1, TPA-B1 and TPA-B2.
Fig. 10. Impedance spectra measured in the dark under forward bias (0.70 V) with
TPA-1, TPA-B1 and TPA-B2.
TPA-1
TPA-B1
TPA-B2
N719
12
10
8
4. Conclusions
In summary, two new triphenylamine-based organic dyes
(TPA-B1 and TPA-B2) containing benzimidazole derivatives as
secondary donors were designed, synthesized and applied in
dye-sensitized solar cells. The corresponding dye TPA-1 was
prepared for the purpose of comparison. The effects of intro-
ducing benzimidazole derivatives as secondary donors into the
formwork of TPA core on the optical, electrochemical, and
photovoltaic properties were studied. Density functional theory
calculation shows that both of the secondary electron-donors in
the framework of TPA core could enhance the photo-generated
charge separation under light illumination compared with that
of TPA-1. The overall conversion efficiencies of 2.43%
(Jsc ¼ 5.12 mA cmꢀ2, Voc ¼ 0.659 V, ff ¼ 0.72) and 2.65%
(Jsc ¼ 6.12 mA cmꢀ2, Voc ¼ 0.679 V, ff ¼ 0.64) were obtained for
DSSCs based on TPA-B1 and TPA-B2, respectively. In comparing
6
4
2
0
0.0
0.1
0.2
0.3
0.4
Voltage/V
0.5
0.6
0.7
0.8
with TPA-1 (
h
¼ 1.23%, Jsc ¼ 3.16 mA cmꢀ2, Voc ¼ 0.640 V,
Fig. 9. Current densityevoltage curves of DSSCs based on TPA-1, TPA-B1, TPA-B2 and
N719.
ff ¼ 0.64), the photovoltaic performances of TPA-B1 and TPA-B2
were significantly improved. The present results strongly suggest
that benzimidazole derivatives as secondary donors in
triphenylamine-based dyes can strongly improve photovoltaic
performances of TPA-B1 and TPA-B2. Further structural optimi-
zation, such as broadening the absorption spectra and tuning the
energy levels, is very likely to generate more efficient sensitizers
and this work is currently underway in our laboratory.
forward bias (0.70 V) for the frequency ranging from 0.01 Hz to
100 kHz. With the bias voltage applied, the larger semicircle at
lower frequencies corresponded to the charge-recombination
processes at the TiO2/dye/electrolyte interface. As shown in Fig. 8,
the radius of the semicircle in the intermediate frequency regime of
the Nyquist plot is in the order of TPA-B2 > TPA-B1 > TPA-1,
indicating that the recombination rate is increased in the order of
TPA-B2 < TPA-B1 < TPA-1 in the dark [32]. This trend appears to be
consistent with the Voc values of TPA-B2 (0.679 V), TPA-B1
(0.659 V) and TPA-1 (0.640 V), which is reasonable to conclude that
the better performance of TPA-B1, TPA-B2 can be ascribed to
a reduced rate of charge recombination leading to higher values of
Voc compared with that of TPA-1.
Acknowledgments
We thank the National Natural Science Foundation of China
(Grant Nos. 20602005, 20873015), the Fundamental Research
Funds for the Central Universities (Grant No. ZYGX2010J035), the
Innovation Funds of State key Laboratory of Electronic Thin Films
and Integrated Device (Grant No. CXJJ201104) and Beijing National
Laboratory for Molecular Sciences (BNLMS) for financial support.
Table 2
Photovoltaic performances of DSSCs based on TPA-1, TPA-B1, TPA-B2 and N719.
Dye
Jsc/mA cmꢀ2
Voc/V
ff
h%
TPA-1
TPA-B1
TPA-B2
N719
3.16
5.12
6.12
0.640
0.659
0.679
0.714
0.61
0.72
0.64
0.68
1.23
2.43
2.65
5.61
Appendix A. Supplementary material
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2012.05.007.
11.56

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L. Zhou et al. / Dyes and Pigments 95 (2012) 743e750
[17] Satoh N, Nakashima T, Yamamoto K. Metal-assembling dendrimers with
References
a triarylamine core and their application to a dye-sensitized solar cell. J Am
Chem Soc 2005;127:13030e8.
[18] Wang Q, Zakeeruddin SM, Cremer J, Bauerle P, Humphry-Baker R, Grätzel M.
Cross surface ambipolar charge percolation in molecular triads on mesoscopic
oxide films. J Am Chem Soc 2005;127:5706e13.
[19] Ning Z, Tian H. Triarylamine: a promising core unit for efficient photovoltaic
materials. Chem Commun 2009;37:5483e95.
[20] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, et al. Gaussian 03,
Revision C.02: Wallingford, CT; 2004.
[21] Xu W, Peng B, Chen J, Liang M, Cai F. New triphenylamine-based dyes for dye-
sensitized solar cells. J Phys Chem C 2008;112:874e80.
[22] Behl M, Hattemer E, Brehmer M, Zentel R. Tailored semiconducting polymers:
living radical polymerization and NLO-functionalization of triphenylamines.
Macromol Chem Phys 2002;203:503e10.
[23] Jia C, Liu S-X, Tanner C, Leiggener C, Neels A, Sanguinet L, et al. An experi-
mental and computational study on intramolecular charge transfer: a tetra-
thiafulvalene-fused dipyridophenazine. Chem Eur J 2007;13:3804e12.
[24] Liu J, Yang H, Tan W, Zhou X, Lin Y. Photovoltaic performance improvement of
dye-sensitized solar cells based on tantalum-doped TiO2 thin films. Electro-
chim Acta 2010;56:396e400.
[25] Lind SJ, Gordon KC, Gambhir S, Officer DL. A spectroscopic and DFT study of
thiophene-substituted metalloporphyrins as dye-sensitized solar cell dyes.
Phys Chem Chem Phys 2009;11:5598e607.
[1] Grätzel M. Recent advances in sensitized mesoscopic solar cells. Acc Chem Res
2009;42:1788e98.
[2] Hagfeldt A, Boschloo G, Sun LC, Kloo L, Pettersson H. Dye-sensitized solar cells.
Chem Rev 2010;110:6595e663.
[3] Chen CY, Wang MK, Li JY, Pootrakulchote N, Alibabaei L, Ngoc-le CH, et al.
Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-
sensitized solar cells. ACS Nano 2009;3:3103e9.
[4] Seo KD, Song HM, Lee MJ, Pastore M, Anselmi C, De Angelis F, et al. Coumarin
dyes containing low-band-gap chromophores for dye-sensitised solar cells.
Dyes Pigm 2011;90:304e10.
[5] Ono T, Yamaguchi T, Arakawa H. Study on dye-sensitized solar cell using novel
infrared dye. Sol Energ Mater Sol Cell 2009;93:831e5.
[6] Matsui M, Kotani M, Kubot Y, Funabiki K, Jin JY, Yoshida T, et al. Comparison of
performance between benzoindoline and indoline dyes in zinc oxide dye-
sensitized solar cell. Dyes Pigm 2011;91:145e52.
[7] Hara K, Sato T, Katoh R, Furube A, Yoshihara T, Murai M, et al. Novel conju-
gated organic dyes for efficient dye-sensitized solar cells. Adv Funct Mater
2005;15:246e52.
[8] 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;15:1654e61.
[9] Im H, Kim S, Park C, Jang SH, Kim CJ, Kim K, et al. High performance organic
photosensitizers for dye-sensitized solar cells. Chem Commun 2010;46:1335e7.
[10] Kim D, Lee JK, Kang SO, Ko J. Molecular engineering of organic dyes containing
N-aryl carbazole moiety for solar cell. Tetrahedron 2007;63:1913e22.
[11] Zafer C, Gultekin B, Ozsoy C, Tozlu C, Aydin B, Icli S. Carbazole-based organic
dye sensitizers for efficient molecular photovoltaics. Sol Energ Mater Sol Cells
2010;94:655e61.
[12] Wan Z, Jia C, Zhang J, Duan Y, Lin Y, Shi Y. Triphenylamine-based starburst
dyes with carbazole and phenothiazine antennas for dye-sensitized solar cells.
J Power Sources 2012;199:426e31.
[26] Liang M, Xu W, Cai F, Chen P, Peng B, Chen J, et al. New triphenylamine-based
organic dyes for efficient dye-sensitized solar cells. J Phys Chem C 2007;111:
4465e72.
[27] Chien-Hsin Y, Han-Lung C, Yao-Yuan C, Chun-Guey W, Chiao-Pei C, Shao-
Hong L, et al. Characteristics of triphenylamine-based dyes with multiple
acceptors in application of dye-sensitized solar cells. J Power Sources 2009;2:
627e34.
[28] Hagfeldt A, Graetzel M. Light-induced redox reactions in nanocrystalline
systems. Chem Rev 1995;95:49e68.
[29] Ning Z, Zhou Y, Zhang Q, Ma D, Zhang J, Tian H. Bisindolylmaleimide deriv-
atives as non-doped red organic light-emitting materials. J Photochem Pho-
tobiol A 2007;192:8e16.
[13] Wenger S, Bouit P-A, Chen Q, Teuscher JL, Censo DD, Humphry-Baker R, et al.
Efficient electron transfer and sensitizer regeneration in stable
p-extended
[30] Qin P, Yang X, Chen R, Sun L, Marinado T, Edvinsson T, et al. Influence of
p-
tetrathiafulvalene-sensitized solar cells. J Am Chem Soc 2010;132:5164e9.
[14] Chen ZG, Li FY, Huang CH. Organic D-pi-A dyes for dye-sensitized solar cell.
Curr Org Chem 2007;11:1241e58.
[15] Liu D, Fessenden RW, Hug GL, Kamat PV. Dye capped semiconductor nanoclusters.
Role of back electron transfer in the photosensitization of SnO2 nanocrystallites
with cresyl violet aggregates. J Phys Chem B 1997;101:2583e90.
[16] Satoh N, Cho J-S, Higuchi M, Yamamoto K. Novel triarylamine dendrimers as
a hole-transport material with a controlled metal-assembling function. J Am
Chem Soc 2003;125:8104e5.
conjugation units in organic dyes for dye-sensitized solar cells. J Phys Chem C
2007;111:1853e60.
[31] Justin Thomas KR, Hsu Y-C, Lin JT, Lee K-M, Ho K-C, Lai C-H, et al. 2,3-
disubstituted thiophene-based organic dyes for solar cells. Chem Mater 2008;
20:1830e40.
[32] Kim J-J, Choi H, Lee J-W, Kang M-S, Song K, Kang SO, et al. A polymer gel
electrolyte to achieve [greater-than-or-equal]6% power conversion efficiency
with a novel organic dye incorporating a low-band-gap chromophore. J Mater
Chem 2008;18:5223e9.