New Organic Dyes Based on Biarylmethylene-Bridged Triphenylamine for Dye Sensitized Solar Cell

Article abstract of DOI:10.1002/cjoc.201500137

Three new organic dyes were synthesized and characterized for applications in dye-sensitized solar cells (DSSCs). In these dyes, diarylmethylene-bridged triphenylamine is the donor, and different acceptors and vinylthiophene are designed to get TBA-1, TBA-2 and TBA-3. Their photophysical, electrochemical and photovoltaic properties were investigated, and the effects of the acceptor structures as well as the linkage on these properties were evaluated. Results demonstrated that the vinylthiophene linkage between the donor and the acceptor is favorable for improving light harvesting ability of TBA-2. In addition, the electrochemical impedance spectroscopy experiments suggest larger R_rec and longer electron lifetime of TBA-2. Therefore, it outperforms the other dyes, exhibiting the highest power conversion efficiency of 3.87%, with J_sc of 8.25 mA·cm^-2 and V_oc of 666 mV. Unfortunately, the TBA-3 with three acceptor groups only shows efficiency of 3.52%, indicating that the design of increasing acceptor groups plays little role on enhancing the solar cell efficiency. Three new organic dyes based on bridged triphenylamine were synthesized. TBA-2 with vinylthiophene linkage shows the best performance of 3.87%, with J_sc of 8.25 mA·cm^-2 and V_oc of 666 mV.

Full text of DOI:10.1002/cjoc.201500137

FULL PAPER
DOI: 10.1002/cjoc.201500137
New Organic Dyes Based on Biarylmethylene-Bridged
Triphenylamine for Dye Sensitized Solar Cell
Fei Wu,a Haitao Liu,a Lawrence Tien Lin Lee,b Tao Chen,b Min Wang,a and Linna Zhu*,a
a Faculty of Materials and Energy, Southwest University, Chongqing 400715, China
b Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
Three new organic dyes were synthesized and characterized for applications in dye-sensitized solar cells
(DSSCs). In these dyes, diarylmethylene-bridged triphenylamine is the donor, and different acceptors and vinyl-
thiophene are designed to get TBA-1, TBA-2 and TBA-3. Their photophysical, electrochemical and photovoltaic
properties were investigated, and the effects of the acceptor structures as well as the linkage on these properties
were evaluated. Results demonstrated that the vinylthiophene linkage between the donor and the acceptor is favor-
able for improving light harvesting ability of TBA-2. In addition, the electrochemical impedance spectroscopy ex-
periments suggest larger Rrec and longer electron lifetime of TBA-2. Therefore, it outperforms the other dyes, exhib-
iting the highest power conversion efficiency of 3.87%, with Jsc of 8.25 mA•cm−2 and Voc of 666 mV. Unfortunately,
the TBA-3 with three acceptor groups only shows efficiency of 3.52%, indicating that the design of increasing ac-
ceptor groups plays little role on enhancing the solar cell efficiency.
Keywords diarylmethylene-bridged triphenylamine, dye-sensitized solar cells, acceptor, rhodanine-3-acetic acid
Introduction
excited state.[10,11] Triphenylamine (TPA) is a widely
used donor in DSSCs, and many organic dyes contain-
ing TPA have been designed and considerable efficien-
cies are achieved.[12-16] In recent years, modification of
the TPA structure has been conducted to further fine-
tune the physical and optical properties of the sensitizers.
For example, the introduction of alkoxy and aromatic
groups to TPA has been reported to increase the HOMO
energy level and increase the light harvesting ability of
the dye molecules.[17] Compared to the conventional
TPA structure, bridged TPA shows better planarity of
its core structure.[18] With two adjacent phenyl rings
connected by a substituted methylene, it has more ex-
tended conjugation and better electron delocalization.
When functionalized with bulky substituents, the
charge-separated state lifetime could be increased and
dye aggregation could be inhibited.[19,20] Initially,
bridged triphenylamine derivatives were designed as
efficient blue emitting materials in organic light emit-
ting diodes (OLEDs).[21] Only quite recently, bridged
TPAs modified with t-butyl and dimethyl groups were
utilized as the donor moieties for metal-free organic
sensitizers respectively, and high power conversion ef-
ficiencies over 8% were achieved in both kinds of mate-
rials.[22,23] Nevertheless, the investigation of bridged
TPA in photovoltaics is still in its infancy to date.
The interest in dye-sensitized solar cells (DSSCs)
utilizing nanostructured TiO2 semiconductor has in-
creased enormously since the report by O’Regan and
Grätzel in the 1990’s.[1] Compared to the inorganic sili-
con-based solar cells, DSSCs have advantages such as
low costs and facile fabrication.[2] In recent years, great
efforts concerning the semiconductors, counter elec-
trodes, electrolytes and sensitizers have been devoted to
improving the power conversion efficiency. Among the
components of DSSCs, the sensitizer plays a critical
role in their performance and remains one of the most
studied areas in the past two decades.[3,4] Despite of
their leading performance of polypyridyl Ru complexes
and metalloporphyrins, metal-free organic dyes have
become increasingly attractive owing to the following
features: low-cost, high molar extinction coefficients,
easy tuning of photophysical and electrochemical prop-
erties through molecular design.[5-8] In recent years,
enormous efforts have been dedicated to developing
efficient organic dyes for DSSCs, and great achieve-
ments have been achieved.[9]
Most of the organic sensitizers in highly efficient
DSSCs are composed of donor (D) and acceptor (A)
moieties connected by a π-conjugated spacer (D-π-A).
The push-pull structure is very popular, since it can of-
fer efficient intramolecular charge transfer (ICT) in the
Quite recently, we have reported the diarylmethyl-
ene-bridged TPA based dyes TB-1－TB-3 as sensitizers
*
lnzhu@swu.edu.cn; Tel.: 0086-023-68254957
Received February 14, 2015; accepted April 7, 2015; published online XXXX, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500137 or from the author.
Chin. J. Chem. 2015, XX, 1—9
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Wu et al.
FULL PAPER
for DSSCs. These dyes showed enhanced power con-
version efficiencies (~21% increase) compared to their
triphenylamine counterpart, suggesting rational choice
of the planar donor structure.[24] Therefore, in this article,
we further present the design and synthesis of three new
organic dyes based on diarylmethylene-bridged TPA,
namely TBA-1, TBA-2, and TBA-3. In the newly de-
signed dyes, thiophene is used as the π-spacer, and dif-
ferent acceptors and linkages are incorporated to inves-
tigate how the structure changes affect the solar cell
performance. In model compound TB-1, the bridged
TPA donor is connected to the cyanoacrylic acceptor
through a thiophene spacer. In TBA-1, rhodanine-3-
acetic acid is the electron acceptor (meanwhile the an-
choring group), which is directly linked to the thiophene
spacer. In TBA-2, the thiophene spacer is connected to
the cyanoacrylic acid acceptor by the vinyl linkage.
While in TBA-3, three cyanoacrylic acid groups are
attached on the TPA framework through the thiophene
spacer. Their chemical structures are shown in Scheme
1. Dye TB-1 with one cyanoacrylic acid group was also
presented for comparison. We attempt to investigate the
influence of the acceptor structures and the way of
linkage toward the solar cell performance. Toluene
groups on the periphery of TPA core could avoid inter-
molecular aggregation, and meanwhile increase the dye
solubility. The novel bridged triphenylamine-based sen-
sitizers are applied as sensitizers and the corresponding
photophysical and photovoltaic properties are discussed
in the present work.
Experimental
1
The H NMR and 13C NMR spectra were recorded
on a BRUKER AVANCE Ⅲ 600 MHz NMR instru-
ment in CDCl3 and DMSO-d6, using tetramethylsilane
as an internal reference. LC-Ms was recorded on a
GCMS-QP2010 Plus Spectrometer. MALDI-TOF was
performed on a Bruker Autoflex instrument, using
1,8,9-trihydroxyanthracene as a matrix. Elemental anal-
yses of carbon, hydrogen, and nitrogen were performed
on a Carlorerba-1106 microanalyzer. The HRMS was
measured on Thermo Scientific Q Exactive instrument.
UV-Vis absorption spectra were recorded on a spectro-
photometer (UV-2450, Shimadzu). Electrochemical
experiments were performed using a CH Instruments
electrochemical workstation (model 660A). The ex-
periments were carried out in CH2Cl2 solution under Ar
atmosphere with tetrabutylammonium hexafluorophos-
phate (n-Bu4NPF6) as a supporting electrolyte at a scan
rate of 50 mV•s−1. The potentials are quoted against the
ferrocene internal standard.
All starting materials were purchased from commer-
cial suppliers (Sigma-Aldrich, J&K Scientific, and En-
ergy Chemical) and used without further purification.
Tetrahydrofuran (THF) for synthesis was freshly dis-
tilled over Na-K alloy under argon atmosphere prior to
use. The general synthetic routes for the intermediates
Scheme 1 Chemical structures of new bridged TPA based dyes TBA-1, TBA-2, TBA-3 and TB-1
N
N
S
S
S
O
S
COOH
NC
N
COOH
TBA-1
HOOC
NC
TB-1
S
N
N
S
COOH
NC
S
S
HOOC
COOH
CN
NC
TBA-2
TBA-3
2
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Chin. J. Chem. 2015, XX, 1—9

New Organic Dyes Based on Biarylmethylene-Bridged Triphenylamine for Dye Sensitized Solar Cell
Scheme 2 Synthetic procedures for bridged TPA based dyes TBA-1, TBA-2 and TBA-3
(n-Bu)3Sn-CH=CH2, Pd(PPh3)4
OHC
S
1
Br
OHC
S
PhMe
Br
N
B(OH)2
OHC
S
Pd(OAc)2, XPhos
K3PO4
3 equiv. NBS
CHCl3
N
THF, H2O
Br
Br
HOOC
CN
2
OHC
S
S
, NH OAc
NC
CN
4
AcOH
N
N
S
S
CHO
COOH
S
S
NC
NC
OHC
HOOC
3
TBA-3
OHC
B(OH)2
S
Pd(OAc)2, XPhos
K3PO4
N
N
1 equiv. NBS
CHCl3
THF, H2O
Br
4
NC
CN
S
S
N
N
N
COOH
N
O
or
S
S
AcOH, NH4OAc
S
S
CHO
COOH
NC
O
S
N
TBA-1
COOH
TB-1
6
N
Pd(OAc)2, PPh3
NaOAc
, NH OAc
NC
CN
4
N
S
4 + 1
DMF
COOH
S
AcOH
CHO
NC
TBA-2
5
5-Vinylthiophene-2-carbaldehyde (1),[19] the intermedi-
and the final compounds are outlined in Scheme 2.
Chin. J. Chem. 2015, XX, 1—9
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3

Wu et al.
FULL PAPER
ates 2[21] and 4[25] were synthesized as described in the
literature. The intermediate 3 was synthesized by palla-
dium-catalyzed Suzuki coupling reaction of 5-formyl-2-
thiophene-boronic acid and the corresponding substrates.
The intermediate 5 was synthesized by palladium-cata-
lyzed Heck coupling reaction of compound 4 and
5-vinylthiophene-2-carbaldehyde (compound 1). The
syntheses of TB-1 and intermediate compound 4 and
compound 6 have been reported in our other work.[24]
The final compounds were synthesized in a similar way
to literature report.[26,27] The final products were fully
6.96－6.92 (m, 6H), 6.89 (d, J＝3.6 Hz, 2H), 6.82 (d,
J＝7.8 Hz, 12 H), 6.58 (d, J＝7.8 Hz, 12 H), 2.33 (s,
18H); 13C NMR (150 MHz, CDCl3) δ: 181.34, 152.78,
142.14, 141.89, 140.35, 136.56, 136.02, 135.36, 135.12,
134.57, 133.46, 130.68, 130.39, 130.12, 129.98, 129.74,
129.48, 129.06, 128.57, 128.42, 128.29, 128.22, 128.15,
128.10, 126.57, 125.41, 122.82, 55.24, 55.17, 29.72,
20.96, 20.87. Anal. calcd for C70H55NOS: C 87.74, H
5.79, N 1.46; found C 87.81, H 5.68, N 1.54; MALDI-
TOF-MS m/z: 958.7 (M＋).
1
Compound TBA-1
characterized by H NMR spectroscopy, MALDI-TOF
A mixture of compound 7 (140 mg, 0.15 mmol),
rhodanine-3-acetic acid (86 mg, 0.45 mmol), ammo-
nium acetate (56 mg, 0.74 mmol), and acetic acid (20
mL) was heated at reflux for 6 h under argon atmos-
phere. After cooling to room temperature, it was pre-
cipitated by pouring into water. The resulting solid was
filtered, washed thoroughly with water. Then, the crude
product was purified by silica-gel column chromatog-
raphy using chloroform/ethanol mixture (7∶1) as the
eluent to afford TBA-1 as a deep red solid (119 mg,
spectrometry, the elemental analysis and the high reso-
lution mass spectra as well. Unfortunately, the 13C NMR
spectra of the final products were not able to be ob-
tained due to their poor solubility.
Compound 3
A mixture of compound 2 (212 mg, 0.2 mmol),
5-formyl-2-thiophene-boronic acid (109 mg, 0.7 mmol),
Pd(OAc)2 (2.2 mg, 0.01 mmol), xphos (5.7 mg, 0.012
mmol) and aqueous solution of K3PO4 (2 mol/L, 3 mL)
in tetrahydrofuran (10 mL) was placed in a Schlenk tube
under an argon atmosphere and stirred at 60 ℃ for 12
h. After cooling, the reaction was quenched by adding
water, and then was extracted with chloroform. The or-
ganic layer was washed with brine and dried over anhy-
drous Na2SO4. After evaporation of solvents, the residue
was purified by column chromatography over silica gel
using hexane/chloroform mixture (2∶5) as the eluent to
1
72%). H NMR (600 MHz, CDCl3) δ: 8.02 (br, 1H),
7.74 (br, 1H), 7.60 (br, 2H), 7.52 (br, 1H), 7.16－7.06
(m, 4H), 6.96－6.94 (m, 2H), 6.90－6.83 (m, 12H),
6.64－6.57 (m, 10H), 6.46 (d, J＝7.8 Hz, 2H), 4.80 (s,
2H, CH2COOH), 2.28 (s, 18H). Anal. calcd for
C73H56N2O3S3: C 79.32, H 5.11, N 2.53; found C 79.80,
H 4.73, N 2.89; MALDITOF-MS m/z: 1104.3 (M＋).
HRMS-ESI m/z: calcd for C73H56N2O3S3 ([M] ＋
)
1
give the product (159 mg, 69%) as a yellow solid. H
1103.3380, found 1103.3387.
NMR (600 MHz, CDCl3) δ: 9.77 (s, 3H, CHO), 7.57 (d,
J＝4.2 Hz, 3H), 7.16 (dd, J＝27.1, 2.2 Hz, 6H), 6.92 (d,
J＝4.2 Hz, 3H), 6.88 (d, J＝8.4 Hz, 12H), 6.65 (d, J＝
8.4 Hz, 6H), 6.61 (d, J＝8.4 Hz, 6H), 2.32 (t, J＝6.9 Hz,
18 H); 13C NMR (150 MHz, CDCl3) δ: 182.51, 154.21,
141.97, 141.80, 141.73, 137.62, 136.05, 135.52, 133.72,
131.08, 130.69, 129.89, 129.66, 128.56, 127.60, 125.90,
123.16, 116.38, 55.28, 20.97. Anal. calcd for C78H57N-
O3S3: C 81.29, H 4.99, N 1.22; found C 81.64, H 4.79,
N 1.34; MALDITOF-MS m/z: 1152.1 (M＋).
Compound TBA-2
A mixture of compound 6 (144 mg, 0.15 mmol),
cyanoacetic acid (38 mg, 0.45 mmol), ammonium ace-
tate (56 mg, 0.74 mmol), and acetic acid (20 mL) was
heated at reflux for 6 h under argon atmosphere. After
cooling to room temperature, it was precipitated by
pouring into water. The resulting solid was filtered,
washed thoroughly with water. Then, the crude product
was purified by silica-gel column chromatography using
chloroform/ethanol mixture (5∶1) as the eluent to af-
1
Compound 5
ford TBA-3 as a deep red solid (103 mg, 70 %). H
A mixture of compound 4 (450 mg, 0.5 mmol),
5-vinylthiophene-2-carbaldehyde (69 mg, 0.5 mmol),
Pd(OAc)2 (2.2 mg, 0.01 mmol), PPh3 (5.7 mg, 0.012
mmol), TBAB (4.8 mg, 0.015 mmol) and NaOAc (205
mg, 2.5 mmol) in DMF (15 mL) was placed in a
Schlenk tube under an argon atmosphere and stirred at
110 ℃ for 2 d. After cooling, the reaction was
quenched by adding water, and then was extracted with
chloroform. The organic layer was washed with brine
and dried over anhydrous Na2SO4. After evaporation of
solvents, the residue was purified by column chroma-
tography over silica gel using hexane/chloroform mix-
ture (1∶2) as the eluent to give the product (230 mg,
48%) as a pale red solid. 1H NMR (600 MHz, CDCl3) δ:
9.67 (s, 1H, CHO), 7.49 (d, J＝3.6 Hz, 2H), 7.16 (s, 2H),
NMR (600 MHz, CDCl3) δ: 8.20 (s, 1H, CH＝C), 7.46
(br, 2H), 7.08－7.03 (m, 2H), 7.00－6.93 (m, 4H),
6.86－6.84 (m, 12H), 6.78 (d, J＝7.2 Hz, 4H), 6.66－
6.62 (m, 12H), 2.29 (s, 18H). Anal. calcd for
C73H56N2O2S: C 85.51, H 5.51, N 2.73; found C 85.69,
H 5.31, N 2.83; MALDITOF-MS m/z: 1024.4 (M＋−1).
＋
HRMS-ESI m/z: calcd for C73H56N2O2S ([M]
)
1023.3990, found 1023.3997.
Compound TBA-3
A mixture of compound 4 (173 mg, 0.15 mmol),
cyanoacetic acid (76 mg, 0.90 mmol), ammonium ace-
tate (112 mg, 1.48 mmol), and acetic acid (20 mL) was
heated at reflux for 6 h under argon atmosphere. After
cooling to room temperature, it was precipitated by
4
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New Organic Dyes Based on Biarylmethylene-Bridged Triphenylamine for Dye Sensitized Solar Cell
pouring into water. The resulting solid was filtered,
washed thoroughly with water. Then, the crude product
was purified by silica-gel column chromatography using
chloroform/ethanol mixture (7∶2) as the eluent to af-
erally, in these dyes, two absorption bands could be ob-
served. The one below 400 nm is ascribed to the π-π*
transition of the bridged TPA, and the one located at
longer wavelength (around 500 nm) is assigned to the
intramolecular charge transfer (ICT) band from bridged
TPA to the corresponding acceptors. The λmax for
TBA-1 has prominent red shift compared to the other
dyes, but with quite low absorption intensity. As sum-
marized in Table 1, the λmaxs appear at 523, 507, and
494 nm for TBA-1－TBA-3, while that of TB-1 is at
488 nm. Apparently, all these dyes show red shifted
absorption maximum compared to the model compound.
The result denotes that the structure changes made in
TBA-1－TBA-3 could enhance the ICT process. The
molar extinction coefficients (ε) at λmax of the ICT bands
for TBA-1－TBA-3 are calculated to be 3.32, 4.55, and
8.23×104 L•mol−1•cm−1, respectively. Dye TBA-3 with
three cyanoacrylic acid groups exhibits the highest light
harvesting ability, but the smallest absorption edge of
these dyes. While TBA-1 incorporating rhodanine-3-
acetic acid shows the weakest light absorption ability.
1
ford TBA-4 as a deep red solid (134 mg, 66%). H
NMR (600 MHz, CDCl3) δ: 8.18 (s, 3H, CH＝C), 7.74
(d, J＝4.2 Hz, 3H), 7.21 (d, J＝2.4 Hz, 2H), 7.10 (d,
J＝1.8 Hz, 2H), 7.07 (s, 2H), 6.98 (d, J＝4.2 Hz, 3H),
6.92－6.88 (m, 12H), 6.66 (d, J＝7.8 Hz, 4H), 6.61 (d,
J＝7.8 Hz, 8H), 2.34 (s, 6H), 2.32 (s, 12H). Anal. calcd
for C87H60N4O6S3: C 77.19, H 4.47, N 4.14; found C
77.35, H 4.08, N 4.41; MALDITOF-MS m/z: 1353.4
＋
(M ). HRMS-ESI m/z: calcd for C87H60N4O6S3
([M]＋) 1351.3602, found 1351.3607.
Fabrication and characterization of DSSCs
All of the anode films for the DSSCs were made
under the same standard condition,[28] which are com-
posed of a 12 μm thick transparent layer (TiO2 with
diameter of 20 nm) and a 6 μm thick scattering layer
(TiO2 nanoparticles with diameter of 200 nm). The
electrodes were immersed into a 0.3 mmol/L dye bath in
THF/EtOH (volume ratio, 1∶1) solution for the bridged
triphenylamine dyes (TBA-1－TBA-3), and were main-
tained in the dark for 18 h. Then, the electrodes were
rinsed with ethanol to remove the non-adsorbed dyes
and dried in the air. Pt counter electrodes were prepared
by sputtering method at 13 mA for 180 s at a power of
150 W. The liquid electrolyte consisting of 0.6 mol/L
1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1
mol/L LiI, 0.05 mol/L I2 in a mixture of acetonitrile and
4-tert-butylpyridine (volume ratio, 1∶1) was intro-
duced into the cell through the drilled holes at the back
of the counter electrode. The effective areas of all the
TiO2 electrodes were 0.196 cm2. The current-voltage
(J-V) characteristics of the assembled DSSCs were
measured by a semiconductor characterization system
(Keithley 236) at room temperature in air under the
spectral output from solar simulator (Newport) using an
AM 1.5G filter with a light power of 100 mW/cm2.
IPCEs of DSSCs were recorded in the Solar Cell
QE/IPCE Measurement System (Zolix Solar Cell Scan
100) using DC mode. CHI 760E electrochemical work-
station was used to characterize the electrochemical
properties of the DSSCs. Electrochemical impedance
spectroscopy (EIS) was recorded under dark conditions
over a frequency range of 0.1－105 Hz with an AC am-
plitude of 10 mV, and the parameters were calculated
from Z-View software (v2.1b, Scribner AssociatNe,
Inc.). For the open-circuit voltage decay measurements,
the cell was first illuminated for 20 s to a steady voltage,
then the illumination was turned off for 80 s and the
open-circuit voltage decay curve was recorded.
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
TB-1
TBA-1
TBA-2
TBA-3
In solution
400
500
600
700
Wavelength/nm
Figure
1
Absorption of bridged triphenylamine based
sensitizers in chloroform (ca. 10−6 mol/L).
Table 1 Electrochemical and photophysical properties of
TBA-1, TBA-2, TBA-3 and TB-1
λmax/nm a
λabs/nm Egopt
/
EHOMO
eV
/
ELUMO /
eV
b
c
d
Dye
[ε/(104 L•
TiO2 film
eV
mol−1•cm−1)]
TBA-1 523 (3.32)
TBA-2 507 (4.55)
TBA-3 494 (8.23)
452
465
452
453
2.10
2.08
2.18
2.18
−4.96
−5.15
−5.33
−5.22
−2.86
−3.07
−3.15
−3.04
TB-1
488 (3.23)
a Absorption spectra were measured in CH2Cl2 solution. b Calcu-
lated from Egopt ＝1240/λonset
.
E
was calculated by (4.74＋
HOMO
c,d
E(onset.ox vs, FC＋/FC)), ELUMO was calculated by EHOMO
Egopt
＋
.
When the dyes are adsorbed onto the TiO2 films,
their absorption bands become broader. The absorption
behaviors on TiO2 are consistent with the corresponding
molar extinction coefficients. Detailed photophysical
data are presented in Table 1. The IPCE spectra of the
Results and Discussion
The UV-Vis absorption spectra of the dyes in chlo-
roform (ca. 10−6 mol/L) are depicted in Figure 1. Gen-
Chin. J. Chem. 2015, XX, 1—9
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5

Wu et al.
FULL PAPER
three dyes are consistent with their photophysical prop-
erties in solution. TBA-2 exhibits higher IPCE value
and the more extended absorption, which is favorable
for the overall performance. Dye TBA-3 still shows the
strongest but a little blue-shifted absorption edge on
TiO2. The quite poor light harvesting ability of TBA-1
may account for its low short circuit current. Notably,
the absorption maxima on TiO2 are all blue-shifted more
or less as compared to that measured in chloroform
(Figure 2). This could be elucidated from the H-aggre-
gation and/or deprotonation of the carboxylic acid on
TiO2 films, according to literature report.[29]
duced electron can be effectively injected into the CB
band of the semiconductor from the acceptor unit via
the terminal anchoring groups. It is noteworthy that in
TBA-1, the LUMO orbital is mainly located on the het-
erocyclic unit, but only partially distributed on the rho-
danine-3-acetic acid group. This orbital distribution may
lead to the inefficient electron injection from LUMO
orbital of TBA-1 to the CB band of TiO2. On the con-
trary, for the dye molecules with cyanoacrylic acid ac-
ceptor (TBA-2 and TBA-3), the largely distributed
LUMO on the acceptor suggests better electron injec-
tion upon photo excitation.
10
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
TB-1
TBA-1
TBA-2
TBA-3
9
8
7
6
5
4
3
2
1
0
TB-1
TBA-1
TBA-2
TBA-3
.
On TiO2 film
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
400
500
600
700
Voltage/V
Wavelength/nm
Figure
sensitizers on TiO2 film.
2
Absorption of bridged triphenylamine based
Figure 3 Photocurrent density-photovoltage curves of TBA-1,
TBA-2, TBA-3 and TB-1 under AM 1.5 G simulated sunlight
(100 mW•cm−2) illumination.
The electrochemical behaviors of these sensitizers
are measured by cyclic voltammetry. The relevant CV
data were presented in Table 1. The ground oxidation
potentials (Eox) correspond to the highest occupied mo-
lecular orbitals (HOMO). While the lowest unoccupied
molecular orbitals (LUMO) were obtained from the
values of Eox and the band gaps ( Egopt ) estimated from
the onset of the UV-visible absorption spectra (Table 1).
The results reveal that the LUMO levels of these dyes
are sufficiently higher than the conduction band edge of
the TiO2 electrode (−4.2 eV), thus providing sufficient
driving force for electron injection from LUMO orbitals
of these dyes to the CB band of TiO2. Meanwhile, the
redox potential of I−/ I－3 (−4.9 eV) is higher than the
HOMO levels of TBA-2 and TBA-3, suggesting that the
two dyes are able to be regenerated by the liquid elec-
trolyte. However, the HOMO level of TBA-1 is only
relatively lower than the redox potential of I−/ I－3 , offer-
ing inefficient dye regeneration ability.
The dye structures have been analyzed by Gaussian
09 at B3LYP/6-31＋G(d) level for full geometrical op-
timization (Figure S3).[30] Obviously, for all of the dyes
investigated, the HOMO levels are delocalized evenly
throughout the entire core of the bridged TPA and the
adjacent π-spacers, while the LUMO levels are mainly
located on the acceptor part, and also partly on the TPA
core. The molecular distribution is favorable for elec-
tronic transition from HOMO to LUMO. When these
dyes are adsorbed onto the TiO2 surface, the photoin-
The typical current-voltage characteristics of DSSCs
sensitized with the three dyes on TiO2 films using I−/ I－3
electrolyte are measured under standard AM 1.5G con-
ditions (100 mW•cm−2). The results are shown in Figure
3. As we have reported earlier, TB-1 showed η of 3.59%,
with Jsc of 7.44 mA•cm−2 and Voc of 694 mV. Under the
same test condition, TBA-2 shows the highest power
conversion efficiency of 3.87%, with a short circuit
current density (Jsc) of 8.25 mA•cm−2, and the open cir-
cuit potential (Voc) of 666 mV. TBA-4 containing three
thiophene cyanoacrylic acid groups exhibits power
conversion efficiency at 3.52%, with Jsc of 7.38
mA•cm−2 and Voc of 650 mV. Compared to dye TB-1, it
is obvious that the vinylthiophene spacer in TBA-2 only
slightly increases the solar cell performance. Unexpect-
edly, three acceptor groups in TBA-4 hardly affect the
Jsc value, but lower the Voc, and therefore it exhibits
slightly lower efficiency relative to TB-1. TBA-1 with
rhodanine-3-acetic acid acceptor shows quite low short-
circuit current of 3.94 mA•cm−2, and an open circuit
voltage of 568 mV, therefore it only exhibits low power
conversion efficiency of 1.5%. The above results dem-
onstrate that the vinylthiophene linkage is favorable for
dye design, while the increased acceptor groups could
hardly improve the efficiency. And the rhodanine-3-
acetic acid is a very poor anchoring group compared to
the cyanoacrylic acid in this series of compounds. The
quite poor efficiency of TBA-1 may come from its weak
6
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Chin. J. Chem. 2015, XX, 1—9

New Organic Dyes Based on Biarylmethylene-Bridged Triphenylamine for Dye Sensitized Solar Cell
light harvesting ability, the inappropriate HOMO levels,
as well as the inefficient electron orbital distributions
observed from their orbital geometries.
moderate dye loading amount and broader absorption
spectrum are favorable for the corresponding solar cell
performance.
The incident photon-to-current conversion efficien-
cies (IPCEs) of these dyes are shown in Figure 4. The
trend of IPCE values of these dyes is in accordance with
their UV-Vis absorption spectra on the TiO2 films. Sim-
ilar to TB-1, TBA-2 and TBA-3 show broader IPCE
spectra from 300 to 600 nm, consistent with their higher
Jsc values.[31] It is noteworthy that although TBA-3
shows higher IPCE spectrum than TBA-2, the weaker
absorption at around 600 nm still generates lower Jsc in
TBA-3. On the other hand, the quite low IPCE values
observed from 400 to 600 nm in TBA-1 reflect lower
photocurrent and hence inferior photovoltaic perform-
ance.
Electrochemical impedance spectroscopy (EIS) is
also performed to elucidate the interfacial charge re-
combination processes in DSSCs based on these dyes
under the dark conditions. The Nyquist plots for DSSCs
based on TBA-1, TBA-2 and TBA-3 are shown in Fig-
ure 5. The larger semicircle in the lower frequency
range represents the resistance of the recombination
between electrons on the TiO2 conduction band and
I－3 species in the electrolyte at the TiO2/dye/electrolyte
interface.[32] The calculated resistance values (Rrec) are
22.35 Ω for TBA-1, 51.21 Ω for TBA-2 and 39.42 Ω for
TBA-3, respectively. The larger Rrec demonstrates
slower recombination kinetics. This trend appears to be
consistent with their open circuit voltage values.
TB-1
TBA-1
0.7
-25
TBA-1
TBA-2
TBA-3
TBA-2
TBA-3
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-20
-15
-10
-5
300
400
500
600
700
0
Wavelength/nm
10
20
30
40
50
60
70
80
90
Z'/Ω
Figure 4 IPCE spectra of TBA-1, TBA-2, TBA-3 and TB-1
Figure 5 Nyquist plots of the DSSCs fabricated using dyes
based devices.
TBA-1, TBA-2 and TBA-3.
Table 2 Photovoltaic performances of DSSCs based on TBA-1,
TBA-2, TBA-3 and TB-1
Figure 6 depicts the Bode Plots of DSSCs based on
these dyes. All EIS Bode Plots exhibit two peaks fea-
turing the frequency investigated. The one at higher
frequencies corresponds to the charge transfer at the
Pt/electrolyte interface and the other one at lower fre-
quencies corresponds to the charge transfer at TiO2/dye/
electrolyte interface, which is also related to the charge
recombination rate, and moreover, its reciprocal is asso-
Jsc/(mA•cm−2)
3.94
Dye
Voc/mV
Fill factor (ff)
η/%
TBA-1
TBA-2
TBA-3
TB-1
568
666
650
694
67.0
70.4
73.3
69.7
1.50
3.87
3.52
3.59
8.25
7.38
7.44
Dye loading amounts of the dye molecules were also
determined since the adsorbed sensitizers on TiO2 films
critically influence the Jsc. The TiO2 film was soaked in
0.3 mmol/L organic dye in THF/ethanol (1∶1 V/V) for
18 h in dark, and the UV-Vis absorbance of the remain-
ing solution was measured. The dye loading of each
device is then estimated from the Beer-Lambert’s law
with the help of the dye’s respective extinction coeffi-
cient. The dye loading of each device is estimated to be
~1.87×10−7, ~2.14×10−7 and ~3.46×10−7 mol/cm2 for
TBA-1, TBA-2 and TBA-3, respectively. The relatively
lower dye loading of TBA-1 may also account for its
low Jsc value. The larger dye loading in TBA-3 may
come from the firm binding ability due to the three
cyanoacrylic acid groups. Whereas for TBA-2, the
-30
TBA-1
TBA-2
TBA-3
-25
-20
-15
-10
-5
0
0.1
1
10
100
1000 10000
Freq./Hz
Figure 6 Bode-phase plots of the DSSCs fabricated using dyes
TBA-1, TBA-2 and TBA-3.
Chin. J. Chem. 2015, XX, 1—9
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
7

Wu et al.
FULL PAPER
ciated with the electron lifetime.[33] The electron life-
time (τ) was estimated to be in the order of TBA-2＞
TBA-3＞TBA-1, which is also in good agreement with
their open circuit voltage results.
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In summary, we have demonstrated the design and
synthesis of new D-π-A type organic sensitizers con-
taining bridged triphenylamine groups. Dye TB-1 was
also studied as the reference compound. Their photo-
physical properties were studied and the photovoltaic
applications as sensitizers were investigated. Moreover,
the effects of various acceptors, as well as vinylthio-
phene linkage were evaluated. Compared to TB-1, the
introduction of vinylthiophene linkage in TBA-2 re-
sulted in broadened absorption and IPCE spectra. In
addition, the EIS experiments suggest larger Rrec and
longer electron lifetime of TBA-2. Therefore, it outper-
forms the other dyes, exhibiting the highest power con-
version efficiency of 3.87%, with Jsc of 8.25 mA•cm−2
and Voc of 666 mV under simulated AM 1.5 irradiation
(100 mW•cm−2). Whereas, TBA-3 with three acceptor
groups only shows efficiency of 3.52%, indicating the
increased acceptor groups have little effect on perform-
ance. In contrast, the quite poor efficiency of TBA-1
should be associated with its weak light harvesting abil-
ity, the inappropriate HOMO levels, as well as the inef-
ficient electron injection ability observed from its or-
bital geometries.
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Acknowledgement
This work is financially supported by the National
Natural Science Foundation of China (No. 51203046),
and the “Fundamental Research Funds for the Central
Universities” (XDJK2014C145 and XDJK2014C052),
as well as the Starting foundation of Southwest Univer-
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9