New triphenylamine-based sensitizers bearing double anchor units for dye-sensitized solar cells

Article abstract of DOI:10.1007/s11426-015-5411-0

Two organic sensitizers (LI-33 and LI-34) with double anchoring units were synthesized and utilized for dye sensitized solar cells (DSSCs), which contained thiophene or vinyl thiophene as π-bridge. The introduction of double anchoring units can change the

Full text of DOI:10.1007/s11426-015-5411-0

SCIENCE CHINA
Chemistry
•
ARTICLES •
Ja nd uo ai :r y1 02 . 01 10 50 7/ sV1 1o 4l . 25 68 -0 1N5 o- 5. 14 :1 11 –- 08
doi: 10.1007/s11426-015-5411-0
New triphenylamine-based sensitizers bearing double anchor units
for dye-sensitized solar cells
1
,2
1
1
3
1*
1*
Jie Shi , Zhaofei Chai , Runli Tang , Jianli Hua , Qianqian Li & Zhen Li
1
2
Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials; Department of Chemistry, Wuhan University, Wuhan 430072, China
Hubei Key Laboratory of Oilcrops Lipid Chemistry and Nutrition; Oil Crops Research Institute, Chinese Academy of Agricultural Sciences,
Wuhan 430062, China
3
Key Laboratory for Advanced Materials and Institute of Fine Chemicals; East China University of Science & Technology,
Shanghai 200237, China
Received September 22, 2014; accepted October 20, 2014
Two organic sensitizers (LI-33 and LI-34) with double anchoring units were synthesized and utilized for dye sensitized solar
cells (DSSCs), which contained thiophene or vinyl thiophene as -bridge. The introduction of double anchoring units can
change their absorption spectra and energy levels in a large degree, thus, the better light-harvesting ability and the convenient
electron transfer along the whole molecule can be obtained. The solar cell based on LI-34 exhibited a broad incident photon-
to-current conversion efficiency (IPCE) spectrum and high conversion efficiency (=6.05%) with coadsorbent CDCA.
organic sensitizers, double anchoring units, dye-sensitized solar cells, triphenylamine
1
Introduction
abundant raw materials, the tailorable molecular structures
5–10]. Most common investigated structure of organic sen-
[
sitizers is the type of donor--acceptor, the acceptor group
carries an anchoring group, such as carboxylic acid to the
TiO
2
surface [11–18]. During photoexcitation, an intramo-
lecular charge transfer takes place from electron donor to
acceptor, from where the photoelectron can be injected into
The intense research on various light-harvesting devices has
emerged rapidly, since the ever-increasing demands for re-
newable energy sources. Recently, dye-sensitized solar cells
(
DSSCs), as a new kind of photovoltaic devices, have at-
tracted considerable attention due to their low cost, ease of
production and flexibility in comparison with crystalline
silicon [1,2]. In DSSCs, sensitizer is the key component,
which is like the light harvesting antennae, and plays an
important role in efficient light harvesting and electron
generation/transfer. Thus, it is necessary to design and syn-
thesize dye sensitizers with novel structures to optimize
their photovoltaic performance.
2
the conduction band of TiO [19].
Anchoring groups, which also act as the acceptor, play an
important role in electron injection, and are critical to the
conversion efficiency of the whole molecule. But only a few
literatures reported the sensitizers with non-single anchoring
units [20]. In this paper, two organic dyes with double elec-
tron acceptors (cyanoacetic acid) were synthesized and
characterized (LI-33 and LI-34, Scheme 1), and further
applied as sensitizers in DSSCs. Dyes with two anchoring
group would afford more chance for the injection of elec-
So far, ruthenium bipyridyl and zinc porphyrin complex-
es hold the record in DSSCs. In addition, much research
interest has been focused on metal-free organic dyes for the
tron into TiO
2
electrode. The thiophene or vinyl thiophene
group was selected as a conjugation bridge. In addition, to
*
Corresponding authors (email: qianqian-alinda@163.com; lizhen@whu.edu.cn,
reduce the possible intermolecular - stacking, 4-tert-
lichemlab@163.com)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Shi et al. Sci China Chem January (2015) Vol.58 No.1
Scheme 1 Synthetic route of LI-33 and LI-34.
butylbenzene moiety was chosen as an anti-aggregation
group, which was linked to the triphenylamine (TPA) unit.
Here, we report the detailed syntheses, structural character-
izations, electrochemical characters, theoretical calculations,
and their conversion efficiencies.
(phenylazanediyl)dibenzaldehyde were prepared following
the literatures [21,22].
2
.2 Instrumentation
1
A Varian Mercury 300 spectrometer (USA) conducted H
13
and C NMR spectroscopy study with using tetramethyl-
silane (TMS, =0 ppm) as internal standard. A Shimadzu
UV-2550 spectrometer (Japan) was used to obtain UV-
visible spectra. Cyclic voltammograms were carried out on
a CHI 660 voltammetric analyzer at room temperature with
nitrogen-purged anhydrous dichloromethane as the solvent.
2
Experimental
2
.1 Materials
Under an atmosphere of dry nitrogen, tetrahydrofuran (THF)
and toluene were dried over, and distilled from K-Na alloy.
N,N-dimethylformamide (DMF) was distilled from CaH
with the similar procedures. After the addition of phospho-
rus pentoxide, 1,2-dichloromethane was dried over and dis-
tilled. Before use, phosphorus oxychloride was freshly dis-
tilled. All reagents were purchased, and used as received.
6
Tetrabutylammonium hexafluorophosphate (TBAPF ) acted
2
as the supporting electrolyte, a platinum disk was used as
the working electrode, and an Ag/AgCl electrode was used
as the quasi-reference electrode. The ferrocene/ferrocenium
redox couple was applied for potential calibration. The scan-
ning rate was 100 mV/s. MALDI-TOF-MS spectra were
recorded with Voyager-DE-STR MALDI-TOF (ABI, USA).
4
-Bromo-N-(4-bromophenyl)-N-phenylaniline and 4,4'-

Shi et al. Sci China Chem January (2015) Vol.58 No.1
3
2
.3 Synthesis
mmol, 0.085 g) were mixed with vacuum-dried, then 15 mL
of MeCN, 5 mL of THF and 10 μL of piperidine were added.
The solution was stirred at 75 °C for 8 h. Then, the solution
was cooled, and the organic layer was evaporated by vac-
uum. Dye LI-34 was purified by column chromatography
over silica gel as a dark red solid (0.15 g, 67%). m.p.:
2.3.1 5,5'-(4,4'-(Phenylazanediyl)bis(4,1-phenylene))dithi-
ophene-2-carbaldehyde (1)
4
0
-Bromo-N-(4-bromophenyl)-N-phenylaniline (1.0 mmol,
.40 g), 5-formylthiophen-2-yl boronic acid (2.5 mmol, 0.39
1
g), and sodium carbonate (10.0 mmol, 1.06 g), were mixed.
After degassed and charged with nitrogen, Pd(PPh (10 mg)
was added, the solution (THF and water (2:1)) was injected
into the mixture. The reaction was refluxed at 80 °C for
193–197 °C. H NMR (CDCl , 300 MHz) δ (ppm): 8.46 (s,
3
)
3 4
2H, –CH=), 7.97 (br, 2H, ArH), 7.71–7.65 (m, 8H, ArH),
7.54–7.48 (m, 8H, ArH), 7.15–7.03 (m, 4H, ArH), 1.29 (s,
13
9H, –CH3). C NMR (DMSO-d , 75 MHz) δ (ppm): 164.9,
6
2
4 h, then, cooled to room temperature, the organic layer
164.0, 150.2, 145.7, 144.3, 143.1, 138.3, 137.5, 131.1,
was separated, and dried over anhydrous sodium sulfate.
130.4, 128.4, 127.3, 125.4, 123.3, 117.7, 34.7, 31.7.
Compound 1 was purified by column chromatography over
MALDI-TOF MS Calcd. for C H N O S [M] m/z:
731.1912; Found. C H N O S (M): 731.1924.
44 33 3 4 2
44
33
3
4 2
1
silica gel as a yellow solid (0.47 g, 35%). H NMR (CDCl
3
,
3
–
00 MHz) δ (ppm): 9.95 (s, 1H, –CHO), 9.86 (s, 1H,
CHO), 7.78–7.71 (m, 2H, ArH), 7.53 (d, J=8.4 Hz, 2H,
2
.3.5 4,4'-(4-Bromophenylazanediyl)dibenzaldehyde (4)
After 4,4'-(phenylazanediyl) dibenzaldehyde (6.6 mmol, 2.0 g)
was dissolved in CHCl (15 mL), N-bromosuccinimide (7.4
ArH), 7.40–7.31 (m, 5H, ArH), 7.26–7.21 (m, 6H, ArH),
7
.05 (d, J=8.7 Hz, 2H, ArH), 6.98 (d, J=8.4 Hz, 2H, ArH).
3
mmol, 1.3 g) was added slowly. Then, the reaction was
stirred at room temperature in dark for 10 h. After that, the
solvent was evaporated by vacuum. The product 4 was puri-
2
.3.2 5,5'-(4,4'-(4-Bromophenylazanediyl)bis(4,1-phenyl-
ene)) dithiophene-2-carbaldehyde (2)
After compound 1 (1.3 mmol, 0.81 g) was dissolved in
fied through a silica gel chromatography column as a yel-
1
CHCl
3
(15 mL), N-bromosuccinimide (1.5 mmol, 0.27 g) was
3
low solid (2.14 g, 85%). H NMR (CDCl , 300 MHz) δ
added slowly, then the mixture was stirred at room temper-
ature in dark for 10 h, the solvent was evaporated by vacu-
um. Compound 2 was purified through a silica gel chroma-
(ppm): 9.91 (s, 2H, –CHO), 7.79 (d, J=8.6 Hz, 4H, ArH),
7.50 (d, J=8.7 Hz, 2H, ArH), 7.18 (d, J=8.6 Hz, 4H, ArH),
7.06 (d, J=8.6 Hz, 2H, ArH).
1
tography column as a red solid (0.51 g, 92%). H NMR
2
.3.6 4,4'-(4'-tert-Butylbiphenyl-4-ylazanediyl)dibenzald-
(
1
CDCl
H, –CHO), 7.75–7.69 (m, 2H, ArH), 7.51 (d, J=8.4 Hz, 2H,
ArH), 7.39–7.29 (m, 4H, ArH), 7.26–7.23 (m, 6H, ArH),
3
, 300 MHz) δ (ppm): 9.93 (s, 1H, –CHO), 9.85 (s,
ehyde (5)
Compound 4 (1.3 mmol, 0.50 g), 4-tert-butylphenyl boronic
acid (1.5 mmol, 0.25 g), sodium carbonate (10.0 mmol,
7
.04 (d, J=8.7 Hz, 2H, ArH), 6.99 (d, J=8.4 Hz, 2H, ArH).
1
.06 g) were mixed, after carefully degassed and charged
2
.3.3 5,5'-(4,4'-(4'-tert-Butylbiphenyl-4-ylazanediyl)bis-
with nitrogen, Pd(PPh ) (10 mg) was added, and the sol-
3
4
(
4,1-phenylene))dithiophene-2-carbaldehyde (3)
vent of THF and water (2:1) was injected. The mixture was
stirred at 80 °C for 24 h, then, cooled to room temperature.
The organic layer was collected, dried over anhydrous so-
dium sulfate. Compound 5 was purified by column chro-
Compound 2 (0.92 mmol, 0.51 g), 4-tert-butylphenyl-
boronic acid (1.5 mmol, 0.25 g), sodium carbonate (10.0
mmol, 1.06 g) were mixed. After carefully degassed and
1
charged with nitrogen, Pd(PPh
3
)
4
(10 mg) was added, and
matography over silica gel as a red solid (0.40 g, 70%). H
the solvent of THF and water (2:1) was injected. The reac-
tion was stirred at 80 °C for 24 h. Then, cooled to room
temperature, the organic layer was collected, dried over
anhydrous sodium sulfate. Compound 3 was purified by
NMR (CDCl , 300 MHz) δ (ppm): 9.91 (s, 2H, –CHO),
3
7.77 (d, J=8.3 Hz, 4H, ArH), 7.51 (br, 4H, ArH), 7.40 (d,
J=8.5 Hz, 2H, ArH), 7.19 (d, J=8.3 Hz, 4H, ArH), 7.09 (d,
J=8.5 Hz, 2H, ArH), 1.35 (s, 9H, –CH3).
column chromatography over silica gel as a dark red solid
1
2
.3.7 4'-tert-Butyl-N,N-bis(4-((E)-2-(thiophen-2-yl)vinyl)-
(
3
0.34 g, 62%). H NMR (CDCl , 300 MHz) δ (ppm): 9.95 (s,
phenyl)biphenyl-4-amine (6)
1
H, –CHO), 9.86 (s, 1H, –CHO), 7.71 (d, J=3.3 Hz, 1H,
ArH), 7.55–7.45 (m, 8H, ArH), 7.41–7.31 (m, 4H, ArH),
.24–7.10 (m, 4H, ArH), 7.04 (d, J=8.7 Hz, 2H, ArH), 6.99
d, J=8.4 Hz, 2H, ArH), 1.36 (s, 9H, –CH ).
Under an atmosphere of dry nitrogen, diethyl thiophen-2-yl
methylphosphonate (2.0 mmol, 0.47 g) was suspended in
anhydrous tetrahydrofuran (20 mL), then t-BuOK (5.0 mmol,
7
(
3
0
.56 g) was added directly as a solid. After that, keep the
2
.3.4 3,3'-(5,5'-(4,4'-(4'-tert-Butylbiphenyl-4-ylazanediyl)-
resultant mixture stir for 10 min at room temperature. Then,
added the solution of compound 5 (0.92 mmol, 0.4 g) in 20
mL of anhydrous tetrahydrofuran dropwise, the reaction
mixture was stirred overnight at room temperature. 100 mL
bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(2-cyanoacry-
lic acid) (LI-33)
Compound 3 (0.33 mmol, 0.20 g) and cyanoacetic acid (1.0

4
Shi et al. Sci China Chem January (2015) Vol.58 No.1
of water was poured into the mixture to terminate the reac-
tion. The organic product was extracted with CHCl , and
3 Results and discussion
3
dried over anhydrous sodium sulfate. Compound 6 was pu-
3
.1 Synthesis of the sensitizers
rified through a silica gel chromatography column as a yel-
1
low solid (0.41 g, 73%). H NMR (CDCl
3
, 300 MHz) δ
The synthetic procedure is shown in Scheme 1. As to LI-33,
after the normal Suzuki coupling reaction between the bro-
mine exposed intermediate and 5-formylthiophen-2-ylboro-
nic acid, compound 2 was obtained easily. Then, 4-tert-
butylbenzene moiety was introduced to the donor side of the
molecule via Suzuki reaction. For LI-34, 5 was brominated
by N-bromosuccinimide (NBS), then a Suzuki reaction of 5
produced the corresponding compound 6, which underwent
another Wittig reaction with diethyl thio phen-2-ylmethyl
hosphonate to yield compound 7. Dialdehyde 8 was ob-
tained by the followed Vilsmeier reaction. Finally, two or-
ganic dyes were produced through the Knoevenagel con-
densation reactions in the presence of piperidinen, and from
(
7
–
ppm): 7.54–7.44 (m, 6H, ArH), 7.38–7.33 (m, 4H, ArH),
.19–7.09 (m, 8H, ArH), 6.97–6.79 (m, 8H, ArH and
CH=CH–), 1.36 (s, 9H, –CH ).
3
2
.3.8 5,5'-(1E,1'E)-2,2'-(4,4'-(4'-tert-butylbiphenyl-4-ylaz-
anediyl)bis(4,1-phenylene))bis(ethene-2,1-diyl)dithiophene-
-carbaldehyde (7)
Under an atmosphere of dry nitrogen, DMF (8.0 mmol, 0.62
mL) was injected into freshly distilled POCl (5.0 mmol,
.74 mL) at zero degree. After it complete transform into a
2
3
0
glassy solid, compound 6 (0.67 mmol, 0.40 g), which dis-
solved in 20 mL of 1,2-dichloromethane, was added drop-
wise. The reaction was stirred overnight at room tempera-
ture, then poured into an aqueous solution of sodium acetate
4
and 8 with cyanoacetic acid. All the intermediates and two
1
13
target organic sensitizers were confirmed by H and
NMR, and MALDI-TOF-MS.
C
(
1 mol/L, 200 mL), and stirred for another 2 h. The mixture
was extracted with CHCl for three times, the organic frac-
3
tions were collected, then dried over anhydrous sodium sul-
3.2 Absorption spectra
fate. After removing the solvent by vacuum, compound 7
The absorption spectra of dyes in dichloromethane solution
are displayed in Figure 1 and the corresponding data are
collected in Table 1. There are two distinct absorptions
around 340 and 500 nm. Generally, the -* transitions of
the dyes could lead to the absorption band around 340 nm,
while an intramolecular charge transfer (ICT) along the
whole molecules result in the absorption band around 500
nm. In comparison with LI-33, the absorption maximum of
LI-34 (511 nm) red shifted 41 nm, indicating that a vinyl
thiophene unit instead of thiophene, can be beneficial to the
conjugation of whole molecule. And in contrast to conven-
tional ruthenium complexes (for example, 13900 L/(mol cm)
at 541 nm for N3), the present dye molecules show about
was purified through a silica gel chromatography columnas
1
a yellow solid (0.32 g, 73%). H NMR (CDCl , 300 MHz) δ
3
(
ppm): 9.85 (s, 2H, –CHO), 7.66 (d, J=3.9 Hz, 4H, ArH),
7
–
.54–7.41 (m, 10H, ArH), 7.20–7.12 (m, 10H, ArH and
CH=CH–), 1.37 (s, 9H, –CH3).
2
.3.9 3,3'-(5,5'-(1E,1'E)-2,2'-(4,4'-(4'-tert-butylbiphenyl-4-
ylazanediyl)bis(4,1-phenylene))bis(ethene-2,1-diyl)bis(thio-
phene-5,2-diyl))bis(2-cyanoacrylic acid) (LI-34)
Compound 7 (0.31 mmol, 0.20 g) and cyanoacetic acid (1.0
mmol, 0.085 g) were mixed with vacuum-dried, then 15 mL
of MeCN, 5 mL of THF and 10 μL of piperidine were added.
The solution was stirred at 75 °C for 8 h. Then, cooled to
room temperature, the organic layer was evaporated by
vacuum. Dye LI-34 was purified by column chromatog-
raphy over silica gel as a dark red solid (0.15 g, 63%). m.p.:
1
2
2
4
4
–
1
1
1
07–210 °C. H NMR (CDCl , 300 MHz) δ (ppm): 8.39 (s,
3
H, –CH=), 7.88 (br, 2H, ArH), 7.62 (br, 4H, ArH), 7.45 (br,
H, ArH), 7.39 (br, 2H, ArH), 7.24 (br, 2H, ArH), 7.15 (br,
H, ArH), 7.01 (br, 6H, ArH and –CH=CH–), 1.31 (s, 9H,
13
CH3). C NMR (DMSO-d , 75 MHz) δ (ppm): 165.9,
6
64.1, 151.7, 150.3, 147.6, 146.8, 145.5, 140.52, 137.5,
34.6, 132.2, 131.1, 130.2, 128.7, 127.2, 127.6, 125.5,
25.9, 124.7, 123.7, 120.1, 117.7, 34.8, 31.7. MALDI-TOF
MS Calcd. for C H N O S [M] m/z: 783.2225; Found,
48
37
3
4 2
C H N O S (M): 783.2234.
4
8
37
3
4 2
2
.4 Device fabrication
The DSSCs based on these two dyes were fabricated simi-
larly with the literature [16].
2 2
Figure 1 UV-Vis spectra of dyes in CH Cl .

Shi et al. Sci China Chem January (2015) Vol.58 No.1
5
Table 1 Absorbance and electrochemical properties of dyes
a)
a)
b)
c)
d)
e)
f)

max


max
E
0-0
E
p
Eox (V) Ered (V)
Dye
(nm) (L/(mol cm)) (nm)
(eV) (V) vs. NHE vs NHE
LI-33
LI-34
470
511
42500
37300
476
520
2.18 1.02
2.07 0.87
1.22
1.07
0.96
1.20
a) Absorption spectra of dyes measured in CH
2
Cl
2
with the

5
concentration of 3×10 mol/L; b) absorption spectra of dyes adsorbed on
the surface of TiO ; c) the bandgap, E0-0 was derived from the observed
optical edge; d) E is the peak of DPV (the DPV of the dyes were measured
in CH Cl with 0.1 mol/L (n-C NPF as electrolyte (scanning rate, 100
2
p
2
2
4
H
9
)
4
6
mV/s; working electrode and counter electrode, Pt wires; reference
electrode, Ag/AgCl)); e) the oxidation potential (Eox) referenced to calibrated
Ag/AgCl was converted to the NHE reference scale: Eox=E
p
+0.197 V; f)
Ered was calculated from EoxE0-0
.
three times of absorption coefficients, 42500 L/(mol cm) for
LI-33 and 37300 L/(mol cm) for LI-34, respectively, sug-
gesting that the two dyes can harvest the sun-light efficiently.
The absorption spectra of organic sensitizers on the TiO
2
2
Figure 2 UV-Vis spectra of the dyes on TiO films.
film (6 μm) were shown in Figure 2. The absorption maxi-
ma of the two dyes were located at 476 and 520 nm, which
red-shifted by 6 and 9 nm than those in the solution, respec-
tively, hinting that the aggregates of two dyes on the TiO
2
surface was not severe. This well-organized arrangement
may be ascribed to the introduction of 4-tert-butylben- zene
moiety on the donor part, which can reduce the intermolec-
ular coupling and - stacking. Similar to the absorption in
solution, LI-34 exhibited significant broader absorption in
the visible region than that of LI-33, and extended the ab-
sorption onset to longer wavelength (>700 nm), which
would be beneficial to the light harvesting, thus, leading to
the increase of Jsc
.
Figure 3 DPV of dyes.
3.3 Electrochemical properties
Differential pulse voltammetric (DPV) was used to deter-
mine the oxidation potential of the two dyes from the peak
potentials (E ) (Figure 3). The highest occupied molecular
p
orbital (HOMO) was derived from the oxidation potential vs
NHE (Eox), while the lowest unoccupied molecular orbital
(LUMO) was derived from the reduction potential vs NHE
(
E
red), which could be calculated from Eox E0-0. The elec-
trochemical properties of the two dyes were summarized in
Table 1. The potential energy of ionization of LI-33 and
LI-34 were not very close, as a result of their different lev-
els of conjugation. As described in Figure 4, comparing to
the iodine/iodide redox potential value (0.4 V vs. NHE), the
HOMO levels of the two organic dyes were more positive,
indicating that the reduced species in the electrolyte could
regenerate the oxidized dyes. It is essential to the operation
of the corresponding solar cells. By estimated from the values
of Eox and the E0-0 band gaps, the LUMO levels of the dyes
were obtained, which were sufficiently negative than the
Figure 4 Schematic representation of the band positions in DSSCs based
on LI-33 and LI-34.
conduction-band-edge energy level (ECB) of the TiO
2
elec-
trode (0.5 V vs. NHE), suggesting that the excited dye can
3.4 Theoretical approach
inject electron into the conduction band of TiO
cally.
2
energeti-
Quantum chemistry computation was conducted, in order to

6
Shi et al. Sci China Chem January (2015) Vol.58 No.1
gain further insight in the correlation between structure and
the physical properties as well as the device performance.
Time-depending DFT (TDDFT) calculations with Gaussian
surface of semiconductor TiO
alignment of dyes on the TiO
IPCEs of solar cells, in which TiO film exposed to dye
2
2
, which was beneficial to the
surface. For comparison, the
2
0
9 program was used to optimize the sensitizers geometries
individually, and the ones exposed to 10 mmol/L CDCA
before the dye bath were shown in Figure 6. It is certain that
the IPCE spectra of the two dyes became broaden after the
addition of CDCA, which extended to 700 nm. This broad
absorption would favor the light harvesting, resulting in the
enhancement of Jsc. However, the effect of CDCA on the
IPCE values of the two dyes was different. For LI-34, the
IPCE values changed little, indicating that the degree of the
and the structures of the sensitizers were analyzed by the
B3LYP exchange-correlation functional and a 6-31G* basis
set [23]. The electron distributions of the HOMO and
LUMO of LI-33 and LI-34 were shown in Figure 5.
Similar HOMO/LUMO electron distributions for the two
dyes were seen from molecular orbital calculations. The
electron density distribution of the HOMO is mainly located
at delocalized  orbitals on spacer unit, with the greatest
density on the donor part, while the LUMO is primarily
localized on the acceptor part. Thus, when the light irradi-
ates to the dyes, the electron could move through the whole
molecules to the anchoring moieties. The optimized struc-
tures of the two dyes were also depicted in Figure 5. It re-
vealed that the conjugation system of the dyes were nearly
planar. However, the 4-tert-butylbenzene moieties, which
were linked to the electron donor group, were twisted
largely. They were just like an umbrella, which suppress the
charge recombination and - stacking of the dyes.
2
dye aggregations on the TiO surface was not very much,
which should be attributed to the introduction of the tert-
butyl group. The steric and nonplanar structure of the tert-
butyl group in LI-34 could help to suppress the dye aggre-
gates and electron recombination [24,25]. However, for
LI-33, the maximum value of the IPCE spectra increased
much more after the addition of 10 mmol/L CDCA, from
14% to 35%, meaning that the introduction of CDCA can
favor the well-organized arrangement of LI-33 on the TiO
2
surface or help them be absorbed on the TiO
2
film effi-
ciently.
Figure 7 shows the photocurrent-voltage (J-V) plots of
DSSCs fabricated with these dyes under AM 1.5G simulat-
ed sunlight at 100 mW/cm , and the photovoltaic character-
2
3
.5 Photovoltaic performance of DSSCs
istic parameters, including short circuit current (Jsc), open-
circuit photovoltage (Voc), fill factor (ff), and solar-to-
electrical photocurrent density () are summarized in Table
The IPCEs of dyes (Figure 6) were obtained with a sand-
wich cell using 1,2-dimethyl-3-propylimidazolium iodide
(
0.6 mol/L), LiI (0.1 mol/L), I
2
(0.05 mol/L), and 4-tert-
2
, where N719 is included for comparison. For the extended
butylpyridine (0.5 mol/L) in acetonitrile and methoxypro-
pionitrile (7:3). The IPCEs of the dyes illustrate that the
visible light can be converted to photocurrent efficiently in
the region from 400 to 600 nm, in good accordance with
their absorption spectra on the TiO
2
film. The IPCE spec-
trum for LI-34 exceeded 50% from 350 to 570 nm where
conjugated system, LI-34 gave the better performance with
2
J
sc of 14.95 mA/cm , Voc of 0.64 V and ff of 0.59, leading to
a  value of 5.61%, which further confirm the superiority of
vinyl thiophene uint as the conjugated bridge. The much
lower conversion efficiency of LI-33 (1.89%) may be due
to its shorter conjugated bridge, which makes the two an-
choring groups much close to each other, thus, the interac-
tion of them would adverse to the adsorption of the dyes on
the incident photon to current conversion efficiency reached
6
0% at 473 nm, however, the IPCE values were relatively
2
TiO surface, resulted in the poorer light harvesting, then to
lower for the solar cells based on LI-33, the maximum was
only 14%. In order to optimize their photovoltaic perfor-
mance, the function of co-adsorbents was investigated. As
the most popular co-adsorbent, chenodeoxycholic acid (CD-
CA) would attach to nanostructured TiO
2
strongly, thus, it
can displace dye molecules with weak adsorption from the
the lower values of Jsc and . This phenomenon was in
keeping with the IPCE spectra. With the aim to further op-
timize their photovoltaic properties, the function of CDCA
was also considered. After the addition of CDCA, the con-
version efficiencies of the two dyes increased. As to dye
2
LI-33, Jsc values increased from 4.7 to 8.8 mA/cm , and the

value reached to 3.41%. DSSC based on LI-34 with 10
mmol/L CDCA showed the highest solar to electricity con-
2
version efficiency of 6.05% (Jsc=14.61 mA/cm , Voc=0.67 V,
ff=0.62) among these devices, which reached 86% of a
N719-based DSSC (7.83%), which was fabricated and
measured under the same conditions.
4
Conclusions
Two new dye sensitizers LI-33 and LI-34, were synthe-
sized, which contain 4-tert-butylbenzene substituted TPA as
Figure 5 Frontier orbitals of the dyes LI-33 and LI-34 optimized at the
B3LYP/6-31+G (D) level.

Shi et al. Sci China Chem January (2015) Vol.58 No.1
7
Figure 6 Spectra of monochromatic incident photon-to-current conversion efficiency (IPCE) for DSSCs based on LI-33 and LI-34. (a) Without CDCA; (b)
with 10 mmol/L CDCA.
2
Figure 7 Current density-voltage characteristics obtained with a nanocrystalline TiO film supported on FTO conducting glass and derivatized with
monolayer of the sensitizers. (a) Without CDCA; (b) with 10 mmol/L CDCA.
a)
Table 2 DSSCs performance data of new dyes
This work was supported by the National Science Foundation of China
21372003, 21325416), and the National Fundamental Key Research Pro-
gram (2011CB932702).
(
J
sc
V
oc
Dye
CDCA
2
ff
 (%)
(mA/cm )
(V)
none
4.7
0.54
0.63
0.64
0.67
0.76
0.61
0.62
0.59
0.62
0.61
1.89
3.41
5.61
6.05
7.83
LI-33
LI-34
1
1
0 mmol/L
none
8.8
1
2
3
4
5
6
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