The effect of anchoring group number on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2010.03.034

Novel dyes comprising a triphenylamine donor, thiophene conjugated bridge and different numbers of anchor groups (cyanoacrylic acid acceptor) were synthesized and employed as photosensitizers in dye-sensitized solar cells. The absorbance of the dyes in solution was red shifted and the first oxidation potential decreased with increasing butoxy group:anchor group ratio. Solar cells based on the dyes exhibited energy conversion efficiencies of 6.7-7.4% at 100?mW?cm^-2. Increasing butoxy group:anchor group ratio resulted in higher open-circuit voltage and higher efficiency.

Full text of DOI:10.1016/j.dyepig.2010.03.034

Dyes and Pigments 87 (2010) 249e256
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The effect of anchoring group number on the performance
of dye-sensitized solar cells
,
,
*
Huixia Shang a b, Yanhong Luo c, Xiaozhi Guo c, Xiaoming Huang c, Xiaowei Zhan a
,
Kejian Jiang a , Qingbo Meng
,
c,
*
*
a Beijing National Laboratory for Molecular Sciences and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
c Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel dyes comprising a triphenylamine donor, thiophene conjugated bridge and different numbers of
anchor groups (cyanoacrylic acid acceptor) were synthesized and employed as photosensitizers in dye-
sensitized solar cells. The absorbance of the dyes in solution was red shifted and the first oxidation
potential decreased with increasing butoxy group:anchor group ratio. Solar cells based on the dyes
exhibited energy conversion efficiencies of 6.7e7.4% at 100 mW cmꢀ2. Increasing butoxy group:anchor
group ratio resulted in higher open-circuit voltage and higher efficiency.
Received 21 January 2010
Received in revised form
27 March 2010
Accepted 29 March 2010
Available online 18 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Dye-sensitized solar cell
Organic dye
Effect of anchoring group number
Structureeproperty relationship
Triphenylamine
Synthesis
1. Introduction
Dye anchoring in DSCs is critical in achieving high h, and typical
anchor groups are carboxylate and phosphonate. However, only
a few studies have investigated the influence, on DSC performance,
of the nature and number of anchor groups in ruthenium and dye
sensitizers. Hara et al. found that a monocarboxylate Ru-phenan-
throline complex was less efficient than that obtained using di- or
tricarboxylate and concluded that two anchoring carboxylates
were necessary for effective binding to a TiO2 surface [37,38]. Park
et al. systematically investigated the effects of the nature
(carboxylate vs phosphonate) and number (2, 4, and 6) of anchor
groups in Ru-bipyridyl complexes on their binding to a TiO2
surface and the performance of DSCs [39]. During the preparation
of this manuscript, a report on the impact of anchor group number
on the performance of DSCs appeared [40]. Yang et al. found
that in triphenylamine-based, multiple rhodanine-3-acetic acid
electron acceptor dyes, those which contained two anchor groups
(rhodanine-3-acetic acid) exhibited superior photovoltaic perfor-
mance than those with one or three anchor groups, owing to red
shifts and broadening of absorbance [40].
Dye-sensitized solar cells (DSC) have received considerable
attention as a promising low cost candidate for transferring solar
energy into electricity since the first, high-efficiency DSC was
reported by Grätzel et al. in 1991 [1e3]. Considerable research effort
has attended the design of more efficient dyes as these play a very
important role in obtaining high power conversion efficiency (h) [4].
The dyes reported so far include metal (ruthenium) complexes and
metal-free dyes. DSCs based on ruthenium dyes achieve a power
conversion efficiency of 11.1% measured using an air mass of 1.5 (AM
1.5) [5]. In contrast, dyes have received great attention in recent
years in view of the rarity and toxicity of ruthenium as well as the
high molar extinction coefficient, low cost and flexibility in terms of
molecular tailoring, of metal-free dyes, such as coumarins [6e11],
triarylamine derivatives [12e31], indolines [32,33] and others
[27,31,34e36]. In general, these dyes are of donor-conjugated
bridge-acceptor (De
focused on changing the
p
eA) architecture; the majority of research has
-conjugated linkers or donors.
p
This paper concerns the synthesis of three novel dyes
comprising a triphenylamine (TPA) donor (with 0, 1, or 2 butoxy
groups), thiophene conjugated bridge, and cyanoacrylic acid
acceptors (3, 2, or 1). The DSC based on dye S1 (Fig. 1) with two
* Corresponding authors. Tel.: þ86 10 62533098; fax: þ86 10 62559373.
E-mail address: xwzhan@iccas.ac.cn (X. Zhan).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.034

250
H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
Fig. 1. Molecular structures of the three organic dyes.
butoxy groups and one anchor group gave best performance (
h
:
CHI660C electrochemical workstation. A platinum working elec-
trode, a platinum-wire auxiliary electrode and an Ag wire anodized
with AgCl as a pseudo-reference electrode were used. S3 was
coated on the platinum working electrode due to its limited
solubility. Ferrocene was used as an internal standard. Cyclic
voltammogram measurements in TiO2 film were carried out using
dye-loaded TiO2 films on FTO glass, platinum-wire and Ag/AgCl as
working, counter and reference electrodes, respectively. The
supporting electrolyte was 0.1 mol dmꢀ1 tetra-n-butylammonium
hexafluorophosphate in acetonitrile.
S1 > S2 > S3), this differing to Yang’s experience [40]. The rela-
tionship between the number of anchor and butoxy groups and
the photophysical, electrochemical, and photovoltaic properties of
the dyes are investigated.
2. Experimental section
2.1. Materials
Unless otherwise stated, starting materials were used as
commercially purchased without further purification. Toluene and
THF were distilled from sodium-benzophenone under nitrogen
before use, DMF was distilled from CaH2. Tris[4-(5-formyl-2-
thienyl)phenyl]amine (3d) [41] was prepared according to litera-
ture procedures.
2.3. Synthesis
2.3.1. N,N-bis(4-butoxyphenyl)-aniline (1a)
A
solution of 1,10-bis(diphenylphosphino)ferrocene (dppf)
(0.32 mmol, 177 mg), Pd2(dba)3$CHCl3 (0.16 mmol, 166 mg) and
1-bromo-4-butoxy-benzene (12 mmol, 2.75 g) in anhydrous
toluene (20 cm3) was stirred under nitrogen at room temperature
for 30 min. Aniline (4 mmol, 372 mg) and t-BuONa (20 mmol,
1.93 g) were added and the ensuing reddish-brown coloured
suspension was refluxed under nitrogen for 72 h. The reaction
mixture was cooled to room temperature; the solid was filtered off
and washed with petroleum ether. The organic phase was washed
with water, extracted with petroleum ether and dried over anhy-
drous MgSO4. Chromatographic purification (silica gel, petroleum
ether) afforded colorless oil (1.01 g, 65%). 1H NMR (400 MHz,
2.2. Measurements
The 1H NMR and 13C NMR spectra were measured on a Bruker
AVANCE 400 MHz spectrometer using tetramethylsilane as an
internal standard. Mass spectra were measured on a GCT-MS
micromass spectrometer using the electron impact (EI) mode or on
a Bruker Daltonics BIFLEX III MALDI-TOF analyzer using MALDI
mode. Elemental analyses were carried out using an FLASH EA1112
elemental analyzer. The UVevis absorption spectra of the dyes in
CHCl3 and adsorbed on TiO2 films were recorded on a JASCO V-570
spectrophotometer. Cyclic voltammetry measurements in solution
were carried out under nitrogen in a deoxygenated solution of tetra-
n-butylammonium hexafluorophosphate (0.1 mol dmꢀ3) in either
acetonitrile (S1) or THF (S2) employing a computer-controlled
acetone-d6):
d
7.13 (t, J ¼ 7.9 Hz, 2H), 7.00 (d, J ¼ 8.9 Hz, 4H), 6.90 (d,
J ¼ 7.9 Hz, 2H), 6.83 (m, 5H), 3.93 (t, J ¼ 6.4 Hz, 4H), 1.74 (m, 4H),
1.49 (m, 4H), 0.98 (t, J ¼ 7.4 Hz, 6H). 13C NMR (100 MHz, acetone-
d6):
d 156.74, 150.17, 142.10, 130.14, 127.62, 121.80, 121.61, 116.46,
68.78, 32.59, 20.39, 14.74. HRMS (EI): m/z 389.2360 (calcd for

H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
251
C26H31NO2, 389.2355). Anal. Calcd for C26H31NO2: C, 80.17; H, 8.02;
N, 3.60. Found: C, 80.17; H, 8.17; N, 3.44%.
2.3.5. 3-(5-(4-(Bis(4-butoxyphenyl)amino)phenyl)thiophen-2-yl)-
2-cyanoacrylic acid (S1)
To a mixture of 1d (0.48 mmol, 240 mg), cyanoacetic acid
(0.72 mmol, 61.2 mg), and ammonium acetate (0.12 mmol, 9.24 mg)
was added acetic acid (5 cm3). The mixture was deoxygenated with
nitrogen for 30 min and was stirred at 120 ꢁC for 24 h. After cooling
to room temperature, the mixture was washed with water and
extracted with CHCl3, and the organic phase was dried over anhy-
drous Na2SO4. After removing the solvent, the residue was purified
by column chromatography on silica gel using CHCl3 as eluent
yielding a red solid (205 mg, 75%). 1H NMR (400 MHz, DMSO-d6):
2.3.2. N,N-bis(4-butoxyphenyl)-N-(4-bromophenyl)-amine (1b)
In a two-neck round-bottom flask were added 1a (0.5 mmol,
195 mg), NBS (0.5 mmol, 89 mg), and 5 cm3 carbon tetrachloride.
The mixture was deoxygenated with nitrogen for 30 min and then
refluxed for 24 h. After cooling to room temperature, the reaction
mixture was filtered, the solvent was removed using rotary evap-
oration and the residue was purified by column chromatography on
silica gel (petroleum ether/ethyl acetate ¼ 25:1) yielding a colorless
oil (227 mg, 97%). 1H NMR (400 MHz, acetone-d6):
d
7.28 (d,
d
13.61 (s, 1H), 8.43 (s, 1H), 7.96 (d, J ¼ 4.0 Hz, 1H), 7.60 (d, J ¼ 8.6 Hz,
J ¼ 8.9 Hz, 2H), 7.04 (d, J ¼ 8.9 Hz, 4H), 6.90 (d, J ¼ 8.9 Hz, 4H), 6.74
2H), 7.57 (d, J ¼ 4.1 Hz, 1H), 7.09 (d, J ¼ 8.5 Hz, 4H), 6.94 (d,
J ¼ 8.6 Hz, 4H), 6.76 (d, J ¼ 8.6 Hz, 2H), 3.95 (t, J ¼ 6.4 Hz, 4H), 1.69
(m, 4H), 1.44 (m, 4H), 0.94 (t, J ¼ 7.4 Hz, 6H). 13C NMR (100 MHz,
(d, J ¼ 8.9 Hz, 2H), 3.97 (t, J ¼ 6.4 Hz, 4H), 1.74 (m, 4H), 1.49 (m, 4H),
0.97 (t, J ¼ 7.4 Hz, 6H). 13C NMR (100 MHz, acetone-d6):
d 156.88,
149.19, 140.92, 132.46, 127.71, 122.00, 116.23, 112.14, 68.41, 32.10,
19.89, 14.14. HRMS (EI): m/z 467.1464 (calcd for C26H30BrNO2,
467.1460). Anal. Calcd for C26H30BrNO2: C, 66.67; H, 6.46; N, 2.99.
Found: C, 66.97; H, 6.61; N, 2.79%.
DMSO-d6): d 163.89, 155.90, 154.02, 149.76, 146.52, 141.80, 138.84,
132.87, 127.42, 127.26, 123.18, 123.03, 117.99, 116.70, 115.51, 96.58,
67.32, 30.79,18.75,13.68. MS (MALDI): m/z 566 (Mþ). Anal. Calcd for
C34H34N2O4S: C, 72.06; H, 6.05; N, 4.94. Found: C, 71.61; H, 6.09; N,
4.87%.
2.3.3. N,N-bis(4-butoxyphenyl)-N-(4-(2-thienyl)phenyl)-
amine (1c)
2.3.6. 4-Butoxy-triphenylamine (2a)
To a solution of 1b (1 mmol, 468 mg) and 2-tri-n-butylstannyl-
thiophene (1.5 mmol, 560 mg) in dry toluene (20 cm3) was added
Pd(PPh3)4 (0.05 mmol, 58 mg) under a nitrogen atmosphere. The
mixture was stirred at 110 ꢁC for 24 h. After cooling to room
temperature, a 10 cm3 aqueous solution of KF (5 g) was added and
the mixture then stirred for 2.5 h. The organic layer was extracted
with CH2Cl2 and washed with 10% aq HCl solution and saturated
aq NaHCO3. The organic phase was dried over anhydrous MgSO4.
The product was purified by column chromatography on silica gel
using 3:1 hexane/dichloromethane as the eluent yielding a light
A solution of dppf (0.4 mmol, 222 mg), Pd2(dba)3$CHCl3
(0.2 mmol, 207 mg) and 1-bromo-4-butoxybenzene (10 mmol,
2.29 g) in anhydrous toluene (30 cm3) was stirred under nitrogen at
room temperature for 30 min. Diphenylamine (5 mmol, 845 mg)
and t-BuONa (25 mmol, 2.4 g) were added and the resulting red-
brown suspension was refluxed under nitrogen for 72 h. The
reaction mixture was cooled to room temperature, the solid was
filtered off and washed with petroleum ether. The organic phase
was washed with water, extracted with petroleum ether and dried
over anhydrous MgSO4. Chromatographic purification (silica,
petroleum ether) afforded a white solid (1.32 g, 83%). 1H NMR
yellow solid (313 mg, 66%). 1H NMR (400 MHz, acetone-d6):
d 7.47
(d, J ¼ 8.2 Hz, 2H), 7.33 (d, J ¼ 5.1 Hz, 1H), 7.29 (d, J ¼ 3.5 Hz, 1H),
7.07 (d, J ¼ 8.6 Hz, 4H), 7.06 (s, 1H), 6.91 (d, J ¼ 8.6 Hz, 4H), 6.86
(d, J ¼ 8.2 Hz, 2H), 3.99 (t, J ¼ 6.4 Hz, 4H), 1.75 (m, 4H), 1.50 (m,
4H), 0.97 (t, J ¼ 7.4 Hz, 6H). 13C NMR (100 MHz, acetone-d6):
(400 MHz, acetone-d6):
d
7.26 (d, J ¼ 7.6 Hz, 2H), 7.24 (d, J ¼ 7.7 Hz,
2H), 7.03 (m, 2H), 6.99 (d, J ¼ 7.8 Hz, 4H), 6.93 (m, 4H), 3.99
(t, J ¼ 6.4 Hz, 2H), 1.75 (m, 2H), 1.50 (m, 2H), 0.97 (t, J ¼ 7.4 Hz, 3H).
13C NMR (100 MHz, acetone-d6):
d 157.12, 149.30, 141.43, 130.15,
d
156.79, 149.34, 145.21, 141.22, 128.92, 127.68, 127.19, 127.16,
128.41, 123.65, 122.86, 116.40, 68.58, 32.31, 20.09, 14.33. HRMS (EI):
m/z 317.1782 (calcd for C22H23NO, 317.1780). Anal. Calcd for
C22H23NO: C, 83.24; H, 7.30; N, 4.41. Found: C, 83.11; H, 7.16; N,
4.16%.
124.40, 122.61, 120.86, 116.22, 68.47, 32.17, 19.94, 14.17. HRMS (EI):
m/z 471.2235 (calcd for C30H33NO2S, 471.2232). Anal. Calcd for
C30H33NO2S: C, 76.40; H, 7.05; N, 2.97. Found: C, 76.05; H, 7.04; N,
2.99%.
2.3.7. N,N-bis(4-bromophenyl)-N-(4-butoxyphenyl)-amine (2b)
In a two-neck round-bottom flask was placed 2a (4.63 mmol,
1.47 g), NBS (9.26 mmol, 1.65 g), and 50 cm3 carbon tetrachloride.
The mixture was deoxygenated with nitrogen for 30 min and
refluxed for 24 h. After cooling to room temperature, the reaction
mixture was filtered. The solvent was removed with rotary evap-
oration and the residue was purified by column chromatography on
silica gel (petroleum ether/ethyl acetate ¼ 35:1) yielding a white
2.3.4. 5-(4-(Bis(4-butoxy-phenyl)amino)phenyl)-
thiophene-2-carbaldehyde (1d)
POCl3 (caution: reacts violently with water; incompatible
with many metals, alcohols, amines, phenol and bases;
1.66 mmol, 254 mg) was added dropwise to a solution of 1c
(0.83 mmol, 390 mg) in anhydrous DMF (5 cm3) under a nitrogen
atmosphere. The solution was stirred at 80 ꢁC overnight. After
cooling to room temperature, the resulting dark red suspension
was washed with water and extracted with dichloromethane.
The organic phase was dried over anhydrous MgSO4. The product
was purified by column chromatography on silica gel using 2:1
hexane/dichloromethane as the eluent yielding a yellow solid
solid (2.15 g, 98%). 1H NMR (400 MHz, acetone-d6):
d 7.39
(d, J ¼ 8.4 Hz, 4H), 7.07 (d, J ¼ 8.6 Hz, 2H), 6.94 (m, 6H), 4.00
(t, J ¼ 6.4 Hz, 2H), 1.75 (m, 2H), 1.50 (m, 2H), 0.97 (t, J ¼ 7.3 Hz, 3H).
13C NMR (100 MHz, acetone-d6):
d 157.66, 147.97, 140.15, 133.00,
128.67, 125.09, 116.54, 114.83, 68.49, 32.09, 19.89, 14.11. HRMS (EI):
m/z 474.9963 (calcd for C22H21Br2NO, 474.9969). Anal. Calcd for
C22H21Br2NO: C, 55.60; H, 4.45; N, 2.95. Found: C, 55.95; H, 4.57; N,
3.00%.
(311 mg, 75%). 1H NMR (400 MHz, acetone-d6):
d 9.88 (s, 1H), 7.89
(d, J ¼ 4.0 Hz, 1H), 7.59 (d, J ¼ 8.7 Hz, 2H), 7.47 (d, J ¼ 4.0 Hz, 1H),
7.11 (d, J ¼ 8.8 Hz, 4H), 6.94 (d, J ¼ 8.8 Hz, 4H), 6.84 (d, J ¼ 8.7 Hz,
2H), 4.00 (t, J ¼ 6.4 Hz, 4H), 1.75 (m, 4H), 1.50 (m, 4H), 0.97
(t, J ¼ 7.4 Hz, 6H). 13C NMR (100 MHz, acetone-d6):
d
183.37,
2.3.8. N-(4-butoxyphenyl)-N,N-bis(4-(thiophen-2-yl)phenyl)-
amine (2c)
To a solution of 2b (2.32 mmol, 1.1 g) and 2-tri-n-butylstannyl-
thiophene (7 mmol, 2.6 g) in dry toluene (30 cm3) was added Pd
(PPh3)4 (0.15 mmol, 173 mg) under a nitrogen atmosphere. The
mixture was stirred at 110 ꢁC for 24 h. After cooling to room
157.46, 155.04, 151.19, 142.09, 140.60, 139.41, 128.48, 128.10,
124.98, 123.70, 119.69, 116.49, 68.65, 32.32, 20.12, 14.37. HRMS
(EI): m/z 499.2185 (calcd for C31H33NO3S, 499.2181). Anal. Calcd
for C31H33NO3S: C, 74.52; H, 6.66; N, 2.80. Found: C, 74.29; H,
6.79; N, 2.79%.

252
H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
temperature, a 10 cm3 aqueous solution of KF (5 g) was added and
stirred for 2.5 h. The organic layer was extracted with CH2Cl2 and
washed subsequently with a 10% HCl aqueous solution and a satu-
rated NaHCO3 aqueous solution. The organic phase was dried over
anhydrous MgSO4. The product was purified by column chroma-
tography on silica gel using 15:1 hexane/dichloromethane as the
eluent yielding a yellow solid (715 mg, 64%). 1H NMR (400 MHz,
Soxhlet extracted with CHCl3 for 65 h to afford a red solid (80 mg,
69%). 1H NMR (400 MHz, DMSO-d6):
13.75 (s, 3H), 8.47 (s, 3H),
d
8.01 (d, J ¼ 4.0 Hz, 3H), 7.78 (d, J ¼ 8.5 Hz, 6H), 7.72 (d, J ¼ 5.6 Hz,
3H), 7.21 (d, J ¼ 8.5 Hz, 6H). 13C NMR (100 MHz, DMSO-d6):
d 163.69,
151.96, 147.01, 145.83, 140.94, 134.33, 127.68, 124.57, 116.86, 99.15.
MS (MALDI): m/z 776 (Mþ). Anal. Calcd for C42H24N4O6S3: C, 64.93;
H, 3.11; N, 7.21. Found: C, 63.88; H, 2.94; N, 7.48%.
acetone-d6):
d
7.57 (d, J ¼ 8.4 Hz, 4H), 7.37 (m, 4H), 7.12 (d,
J ¼ 9.0 Hz, 2H), 7.09 (d, J ¼ 4.8 Hz, 2H), 7.06 (d, J ¼ 8.3 Hz, 4H), 6.97
(d, J ¼ 8.5 Hz, 2H), 4.02 (t, J ¼ 6.4 Hz, 2H), 1.77 (m, 2H), 1.51 (dd,
J ¼ 14.8 Hz, 7.4 Hz, 2H), 0.98 (t, J ¼ 7.4 Hz, 3H). 13C NMR (100 MHz,
2.4. Dye adsorption measurements
After the preparation of the TiO2 films (as depicted in the
following device fabrication section), dye adsorption on TiO2 of
surface area 4 cm2 was carried out by immersing the films for 8 h in
20 mL of 0.1 mmol dmꢀ3 solutions of S1e3 in dry THF. The differ-
ence between the absorbance values of the solution before and
after adsorption, as well as the molar extinction coefficient and the
volume of the solutions were used to determine the amount of dye
molecules adsorbed on the TiO2 film.
CDCl3):
d 157.41, 148.14, 144.82, 140.53, 129.07, 129.02, 128.55,
127.39, 124.98, 123.70, 123.23, 116.43, 68.50, 32.14, 19.92, 14.13.
HRMS (EI): m/z 481.1538 (calcd for C30H27NOS2, 481.1534). Anal.
Calcd for C30H27NOS2: C, 74.81; H, 5.65; N, 2.91. Found: C, 74.86; H,
5.70; N, 2.91%.
2.3.9. N-(4-butoxyphenyl)-N,N-bis[4-(5-formyl-2-
thienyl)phenyl]-amine(2d)
POCl3 (4.36 mmol, 669 mg) was added dropwise to a solution of
2c (0.87 mmol, 420 mg) in anhydrous DMF (6 cm3) under a nitrogen
atmosphere. The solution was stirred at 80 ꢁC overnight. After
cooling to room temperature, the resulting dark red suspension was
washed with water and extracted with dichloromethane. The
organic phase was dried over anhydrous MgSO4. The product was
purified by column chromatography on silica gel using 2:1 hexane/
dichloromethane as the eluent yielding an orange solid (262 mg,
2.5. DSC device fabrication and measurements
Two layers of TiO2 films, main layer and scattering layer, were
prepared by doctor blading two different TiO2 pastes on FTO glass
substrate (10
layer (TiO2 particle size: 20 nm, PST-18NR, Catalysts & Chemicals
Industries Co., Ltd., Japan) was transparent with thickness of 8 m,
and the second layer (thickness: 4 m, TiO2 particle size, 400 nm,
U ,
ꢀ1, Nippon Sheet Glass). The first nano-porous
m
56%). 1H NMR (400 MHz, acetone-d6):
d 9.92 (s, 2H), 7.94 (d,
m
from CCI in Japan) was used for increasing the device light har-
vesting in the red region of the solar emission spectrum. The first
layer of TiO2 was heated at 450 ꢁC for 30 min, and the second layer
was heated at 500 ꢁC for 20 min. The resulting film was further
treated with 0.04 mol dmꢀ3 TiCl4 aqueous solution, and then sin-
tered again at 500 ꢁC for 20 min. After cooling to about 80 ꢁC, the
films were immersed into the dye solutions (0.3 mmol dmꢀ3 S1e3
J ¼ 4.0 Hz, 2H), 7.72 (d, J ¼ 8.7 Hz, 4H), 7.57 (d, J ¼ 4.0 Hz, 2H), 7.16
(d, J ¼ 8.9 Hz, 2H), 7.13 (d, J ¼ 8.7 Hz, 4H), 7.01 (d, J ¼ 8.9 Hz, 2H),
4.03 (t, J ¼ 6.4 Hz, 2H), 1.78 (m, 2H), 1.52 (m, 2H), 0.98 (t, J ¼ 7.4 Hz,
3H). 13C NMR (100 MHz, acetone-d6):
d 183.53, 158.05, 154.02,
149.38, 142.73, 139.68, 139.23, 129.21, 128.18, 127.64, 124.52, 123.53,
116.63, 68.54, 32.10, 19.91, 14.12. HRMS (EI): m/z 537.1437 (calcd for
C32H27NO3S2, 537.1432). Anal. Calcd for C32H27NO3S2: C, 71.48; H,
5.06; N, 2.60. Found: C, 71.55; H, 5.13; N, 2.80%.
and N719 in dry ethanol) for 8 h 1 mmol dmꢀ3 3
-cholanic acid (DCA) was added to the dye solutions to prevent
a, 12a-dihydroxy-
5b
the dye aggregation on the TiO2 film. In our experiment, open cells
were fabricated in air by clamping the different dyed electrode with
Pt-sputtered FTO glass as counter electrode. The electrolyte used
here composed of 0.6 mol dmꢀ3 1-propyl-3-methylimidazolium
iodide, 0.1 mol dmꢀ3 LiI, 0.05 mol dmꢀ3 I2, and 0.5 mol dmꢀ3
tert-butylpyridine in the mixture of acetonitrile and valeronitrile
(1:1, v/v). All the cells were prepared with active area of 0.2 cm2.
The action spectra of monochromatic incident photo-to-current
conversion efficiency (IPCE) for solar cells were performed using
a commercial setup for IPCE measurement (PV-25 DYE, JASCO), and
2.3.10. 3,30-(5,50-(4,40-(4-Butoxyphenylazanediyl)bis(4,1-
phenylene))bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) (S2)
To a mixture of 2d (0.37 mmol, 200 mg), cyanoacetic acid
(1.48 mmol, 125.8 mg), and ammonium acetate (0.37 mmol,
28.5 mg) was added acetic acid (5 cm3). The mixture was deoxy-
genated with nitrogen for 30 min and was stirred at 120 ꢁC for 24 h.
After cooling to room temperature, the mixture was washed with
water and extracted with CHCl3, the organic layer was dried over
anhydrous Na2SO4. After removing the solvent, the residue was
purified by column chromatography on silica gel using CHCl3 as
eluent yielding a red solid (142 mg, 57%). 1H NMR (400 MHz,
IPCE is defined by IPCE(
is the speed of light in a vacuum, e is the electronic charge,
wavelength (m), Jsc is the short-circuit photocurrent density
(A mꢀ2), and is the incident radiation flux (W mꢀ2). The photo-
l
) ¼ hcJsc/e4l, where h is Planck’s constant, c
l
is the
DMSO-d6):
d
13.46 (s, 2H), 8.47 (s, 2H), 8.00 (d, J ¼ 4.0 Hz, 2H), 7.71
(d, J ¼ 8.7 Hz, 4H), 7.67 (d, J ¼ 4.0 Hz, 2H), 7.12 (d, J ¼ 8.7 Hz, 2H),
7.07 (d, J ¼ 8.6 Hz, 4H), 6.99 (d, J ¼ 8.9 Hz, 2H), 3.98 (t, J ¼ 6.4 Hz,
2H), 1.71 (m, 2H), 1.45 (m, 2H), 0.94 (t, J ¼ 7.4 Hz, 3H). 13C NMR
4
current-voltage (IeV) characteristics were recorded using
a computer-controlled Keithley 2400 source meter under air mass
(AM) 1.5 simulated illumination (100 mW cmꢀ2, Oriel, 67 005).
(100 MHz, DMSO-d6): d 171.95,163.70,152.90,147.93,146.43,141.51,
133.71, 128.18, 127.46, 126.04, 124.16, 122.46, 116.61, 115.83, 97.63,
67.39, 30.77,18.74, 13.68. MS (MALDI): m/z 671 (Mþ). Anal. Calcd for
C38H29N3O5S2: C, 67.94; H, 4.35; N, 6.26. Found: C, 67.69; H, 4.34; N,
6.05%.
3. Results and discussion
3.1. Synthesis
2.3.11. 3,30,300-(5,50,500-(4,40,400-Nitrilotris(benzene-4,1-diyl))tris
(thiophene-5,2-diyl))tris(2-cyanoacrylic acid) (S3)
The synthetic routes of the new dyes S1e3 are illustrated in
Scheme 1. All the dyes have been synthesized starting with TPA
(with 0e2 butoxy groups) via four routine reactions, including
bromination (1b and 2b), Stille coupling (1c and 2c), Vilsmeier
formylation (1d and 2d), and Knovenagel condensation reaction
(S1e3).
To a mixture of 3d (0.15 mmol, 86.4 mg), cyanoacetic acid
(0.68 mmol, 57.4 mg), and ammonium acetate (0.11 mmol, 8.5 mg)
was added acetic acid (3 cm3). The mixture was deoxygenated with
nitrogen for 30 min and stirred at 120 ꢁC for 40 h. After cooling to
room temperature, the mixture was washed with CH2Cl2 and then

H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
253
Scheme 1. Synthetic routes of the three dyes.
3.2. Absorption spectra
absorption edge (Eg, Table 1) decreases with decreasing the cya-
noacrylic acid anchoring groups and increasing the electron-
donating butoxy groups. The increase in electron donating butoxy
groups on TPA is beneficial to ICT, resulting in red shift of the main
absorption peak. This phenomenon is in sharp contrast to that
reported for TPA-based rhodanine-3-acetic acid electron acceptor
dyes, in which the main absorption band red shifts with increasing
number of the rhodanine-3-acetic acid anchoring groups [40]. On
the other hand, the molar extinction coefficient of the dyes
increases in order of S1 < S2 < S3 with increasing the number of
The normalized absorption spectra of dyes S1e3 in dilute
chloroform solutions are shown in Fig. 2a. All absorption spectra
exhibit a weak absorption band at ca. 350 nm and a strong
absorption band at ca. 500 nm. The strong visible absorption peaks
are attributed to the intramolecular charge transfer (ICT) transition
between the TPA-based donor and the cyanoacrylic acid acceptor
[14,15,40]. It is interesting that the main absorption band red shifts
and accordingly the optical band gap estimated from the onset

254
H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
Fig. 2. The absorption spectra of S1-3 in CHCl3 (a) and adsorbed on TiO2 films (b).
Table 1
3.3. Electrochemistry
Absorption and electrochemical properties of the dyes.
Dye
Absorption
Electrochemistry
The electrochemical characteristics of the dyes were investi-
gated by cyclic voltammetry (CV) method in solution and in TiO2
films. As shown in Fig. 3a, all the dyes show the first oxidation
peaks at about 0.9e1.1 V vs NHE attributed to the oxidation of the
TPA groups. The first oxidation potential Eox (vs NHE) of the dyes
increases gradually in order of S1 (0.93 V) < S2 (1.08 V) < S3
(1.13 V), and the similar profile was found when they were
measured on dye-loaded TiO2 films (Fig. 3b); the dyes are easier to
oxidize with increasing the electron-donating butoxy groups and
decreasing the cyanoacrylic acid anchoring groups. All the Eox
values are more positive than the Iꢀ/I3ꢀ redox couple (w0.4 V vs
NHE), suggesting that there is enough driving force for the dye
regeneration reaction to compete efficiently with the recapture of
the injected electrons by the dye cation radical. The reduction
potential Ered (vs NHE) of the dyes is estimated from the Eox and Eg.
The Ered values of S1e3 (ꢀ1.15, ꢀ1.04 and ꢀ1.11 V, respectively) are
less sensitive to changing the number of the electron-donating
butoxy groups and cyanoacrylic acid anchoring groups. All the Ered
values are more negative than the conduction band edge energy
level of the TiO2 electrode, ꢀ0.5 V vs NHE [7], indicating that the
electron injection process is energetically favorable.
a
b
c
d
e
f
3max
lmax
lmax
Eg
Eox
Ered
(V)
(104 molꢀ1 dm3 cmꢀ1
)
(nm)
(nm)
(eV)
(V)
S1
S2
S3
3.3
4.6
8.4
510
506
484
448
463
460
2.08
2.12
2.24
0.93
1.08
1.13
ꢀ1.15
ꢀ1.04
ꢀ1.11
a
The molar extinction coefficient at lmax in solution.
Absorption maxima in solution.
Absorption maxima adsorbed on TiO2 photoelectrode.
b
c
d
e
Optical bandgap estimated from the absorption edge in solution.
The first oxidation potential vs. NHE. Potentials measured vs Fcþ/Fc were con-
verted to normal hydrogen electrode (NHE) by addition of þ0.63 V.
f
The reduction potential vs. NHE calculated from Eox ꢀ Eg.
anchoring groups (Table 1), due to large oscillator strength of the
charge transfer transition according to Franck-Condon principle
[42]. Fig. 2b shows the absorption spectra of the dyes adsorbed on
TiO2 films. The absorption spectra of S1e3 blue shift 62, 43, and
24 nm, respectively, compared to that in solution. The blue shift is
possibly attributed to the deprotonation of carboxylic acid in the
dye molecule upon adsorption on the TiO2 film [43,44]. However, in
TPA-based rhodanine-3-acetic acid electron acceptor dyes, when
adsorbed on the TiO2 surface, the absorption spectra generally red
shift relative to that in solution except that with 3 anchoring groups
[40]. The main absorption band of S2 and S3 red shifts compared to
that of S1, different from the profile in solution.
3.4. Photovoltaic performance
Action spectrum, incident photon-to-electron conversion effi-
ciency (IPCE) as a function of wavelength, was measured to
Fig. 3. Cyclic voltammograms of the dyes (S1 in CH3CN, S2 in THF and S3 in film) with ferrocenium/ferrocene as an internal standard (a) and of dye-loaded TiO2 film on FTO glass
(b), at 50 mV sꢀ1. The horizontal scale refers to an anodized Ag wire pseudo-reference electrode.

H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
255
Table 2
Photovoltaic performances of DSCs based on S1e3 and N719 without and with DCA.
dye
Jsc (mA cmꢀ2
)
Voc (mV) ff
h
(%) Amounta
(10ꢀ7 mol cmꢀ2
)
Without DCA S1
16.80
16.53
16.77
630
570
565
0.63 6.65 2.29
0.58 5.49 1.76
0.53 5.34 2.42
S2
S3
With DCA
S1
S2
S3
16.59
16.95
17.30
685
635
620
675
0.65 7.38
0.66 7.10
0.63 6.75
0.64 8.15
N719 18.87
a
Amount of dye adsorbed on TiO2 film.
anchoring groups upon adsorption transfers more protons to the
TiO2 surface, leading to the positive shift of the conduction band
edge of TiO2. Thus, the sensitizer with more anchoring groups
possesses larger gap between its LUMO and the conduction band
level of the TiO2, leading to more efficient electron injection into
TiO2 photoelectrode and thus to increase in the short-circuit
photocurrent density.
Fig. 4. Photocurrent action spectra of DSCs based on S1-3 with DCA.
The values of fill factor (ff) are almost the same, but open-circuit
voltages (Voc) decreased in order of S1 > S2 > S3. The decrease of
Voc is generally associated with the positive shift of conduction
band edge of TiO2 photoanode [45] and/or enhancement of charge
recombination at the TiO2/dye/electrolyte interface [42]. The
sensitizer with more anchoring groups upon adsorption transfers
more protons to the TiO2 surface, charging it more positively,
leading to lower conduction band of TiO2 and lower Voc. On the
other hand, decreasing the electron-donating butoxy groups and
increasing the cyanoacrylic acid anchoring groups could lead to
increased charge recombination and lower Voc. The good suppres-
sion function to the charge recombination of alkoxy chains has also
been reported by Wang et al. [42].
evaluate the photoresponse of photoelectrode in the whole spectral
region. Fig. 4 shows the IPCE spectra of DSCs based on S1e3 in the
presence of DCA. All three dyes can efficiently convert visible light
to photocurrent in the range of 400e700 nm. In the range of
400e500 nm, IPCE maxima of up to 84%, 82% and 83% were ach-
ieved for S1, S2 and S3, respectively, while the maximum at 535 nm
was 84% for the standard N719 based DSC under similar conditions,
indicating that efficient photon-to-electron conversion efficiencies
for S1e3 based DSCs. S2 and S3 gave broader spectra and higher
photoresponse in the long wavelength region (>500 nm) than S1,
which was consistent with the corresponding absorption spectra
on transparent TiO2 film. It should be noted that S1e3 based DSCs
gave higher IPCE values in 400e500 nm region as compared with
N719 based device.
According to the photocurrent action spectra, one may expect
higher short-circuit photocurrent densities (Jsc) for S2- and S3-
based DSCs from the overlap integral of IPCE curves with the
standard AM 1.5 solar emission spectrum. Indeed, both S2- and S3-
based DSCs give higher Jsc (16.95 and 17.30 mA cmꢀ2) than S1-based
DSC (16.59 mA cmꢀ2, see Fig. 5 and Table 2), which is consistent
with the IPCE data. The sensitizer with more cyanoacrylic acid
Although the DSC sensitized with S1 exhibits the lowest Jsc
(16.59 mA cmꢀ2) and a moderate ff (0.65), the highest Voc (685 mV)
leads to the highest
used dye N719 gave Voc of 675 mV, ff of 0.64, Jsc of 18.87 mA cmꢀ2
and of 8.15%. decreased in order of S1 > S2 > S3. Clearly, the
h (7.38%). In the same geometry DSC, the widely
,
h
h
efficiencies of the DSCs are susceptible to changing the number of
anchoring groups/butoxy groups in the dyes. The more anchoring
groups the dye has, the lower efficiency the device exhibits. Our
result is different from Yang’s example, in which the dye with two
anchoring groups exhibited the best efficiency [40].
We also investigated the performances of the S1e3 based
devices without DCA (see Table 2). The values of Jsc, Voc, ff and
h
without DCA dominantly decreased compared to those with DCA,
probably due to dye aggregation. The amount of dyes adsorbed on
TiO2 film without addition of DCA was measured (Table 2). The
adsorption amount of S2 was lower than that of S1 and S3, possibly
responsible for its lower Jsc.
4. Conclusion
We have synthesized three new organic sensitizers composed of
different numbers of anchoring groups/butoxy groups for dye-
sensitized solar cells and obtained some insight into their struc-
tureeproperty relationships. The absorption spectrum in solution
red shifts, the absorption spectrum adsorbed on TiO2 films blue
shifts, and the first oxidation potential decreases with increasing
butoxy groups/decreasing anchoring groups. Two butoxy groups on
the TPA donor of S1 (with one anchoring group) can suppress
charge recombination and increase Voc and ff of DSCs, giving the
Fig. 5. Current density-voltage curves of DSCs sensitized with S1-3 and N719 with DCA
under simulated AM 1.5, 100 mW cmꢀ2
.
highest h of 7.38%, 90% of that for N719-based device fabricated

256
H. Shang et al. / Dyes and Pigments 87 (2010) 249e256
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h
h
Disclosure statement
The authors declare no conflict of interest.
Acknowledgements
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organic sensitizers for high-performance dye-sensitized solar cells. Journal of
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This work was supported by the NSFC (Grants 20721061,
20725311 and 20673141), the MOST (Grants 2006AA03Z220,
2006CB202606 and 2006CB932100), and the Chinese Academy of
Sciences.
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acceptor system. Chemical Communications 2009;13:1766e8.
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