The structural modification of thiophene-linked porphyrin sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2010.05.003

Three donor-(π-spacer)-acceptor porphyrin dyes were synthesized for use in dye-sensitized solar cells. The dyes comprised the same donor (porphyrin derivative) and acceptor/anchoring group (2-cyanoacrylic acid) but varying π-spacer consisting of a combina

Full text of DOI:10.1016/j.dyepig.2010.05.003

Dyes and Pigments 88 (2011) 75e83
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The structural modification of thiophene-linked porphyrin sensitizers for
dye-sensitized solar cells
,
Na Xiang a, Xianwei Huang a, Xiaoming Feng a, Yijiang Liu a, Bin Zhao a b, Lijun Deng a,
,b,
Ping Shen a, Junjie Fei a, Songting Tan a
*
a College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three donore(
The dyes comprised the same donor (porphyrin derivative) and acceptor/anchoring group (2-cyanoa-
crylic acid) but varying -spacer consisting of a combination of 4-methylthiophene, 4-hexylthiophene or
3,4-ethylenedioxythiophene groups. The dyes displayed different adsorption behavior and coverage of
the TiO2 surface.
p-spacer)eacceptor porphyrin dyes were synthesized for use in dye-sensitized solar cells.
Received 17 March 2010
Received in revised form
7 May 2010
Accepted 7 May 2010
Available online 19 May 2010
p
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Porphyrin derivative
Thiophene derivatives
p
-Spacer unit
Incident photon-to-current conversion
efficiency
Dye-sensitized solar cells
Photovoltaic performances
1. Introduction
injection, and the highest occupied molecular orbital (HOMO) of the
dye must be lower than the hole-transport material (HTM) for
In recent years, increasing demands for renewable energy
sources have focused attention on the utilisation of solar energy. In
this context, dye-sensitized solar cells (DSSCs) have attracted
considerable interest owing to their ability to convert solar energy
to electricity at low cost. In dye-sensitized solar cells, light is
absorbed by a dye and charge separation takes place owing to
photoinduced electron injection from the dye into the conduction
band of TiO2, the separated charges then move toward respective
electrodes, thereby yielding a photocurrent in an external circuit [1].
To develop more efficient dyes for DSSCs, essential design require-
ments must be satisfied. Firstly, the sensitizing dyes must strongly
adhere to the photocatalyst (TiO2) surface to ensure efficient elec-
tron injection into the conduction band of TiO2. Secondly, the lowest
unoccupied molecular orbital (LUMO) of the dye must be suffi-
ciently higher than the conduction band of TiO2 for efficient charge
efficient regeneration of the oxidized dye. Finally, the dye must have
light-harvesting ability in both the visible and/or near IR regions [2].
To date, the most efficient dye sensitizers employed in DSSCs are
polypyridyl ruthenium complexes, with
a solar-to-electricity
conversion efficiency ( ) of 10e11% [3]. The advantages of such
h
ruthenium complexes are that they exhibit both broad absorption in
the near-UV and visible region and appropriate excited-state
oxidation potentials for the electron injection into TiO2 [4].
However, these dyes are not readily available because of their high
cost and the low availability of noble metals such as ruthenium.
Extensive researches have been devoted to the development of
alternative, efficient metal-free dyes, which offer advantages as
photosensitizers in that they have high molar absorption coeffi-
cients due to intramolecular pep* transitions and their structures
can be modified easily and economically. In recent years, whilst
various metal-free dyes based on coumarins [5], indolines [6],
perylenes [7], merocyanines [8], porphyrins [9], triarylamines [10],
and carbazoles [11] have been reported, such compounds display
overall conversion efficiencies in the range 5e10%; however, the
cell performances of metal-free dyes are either lower than or
comparable to that of Ru dye-based DSSCs. Recently, porphyrins
* Corresponding author. College of Chemistry, Key Laboratory of Environmentally
Friendly Chemistry and Applications of Ministry of Education, Xiangtan University,
Xiangtan 411105, PR China.
E-mail address: tanst2008@163.com (S. Tan).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.05.003

76
N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
have been recognized as the most promising dyes for such appli-
cation because of their photochemical and electrochemical
stability, strong absorbing ability in the visible region due to pep*
The introduction of 3,4-ethylenedioxythiophene (EDOT) into the
porphyrin dye was anticipated to enhance photovoltaic perfor-
mance because EDOT has a small torsional angle with the
adjoining phenyl fragment, thereby ensures efficient electronic
transitions of the conjugated macrocycle and high molar extinction
coefficient [12]. However, the narrow absorption spectra of
porphyrins, which poorly matches solar light distribution, limits
the performance of porphyrin-sensitized solar cells. In order to
solve this problem, many model compounds based on porphyrin
have been synthesized for application in DSSCs. For example,
transfer between D and A. The effects of the different
p-conju-
gated linkers on the photophysical, electrochemical properties
and photovoltaic performances were investigated.
2. Experimental
Seunghun et al. synthesized
ing carboxylic acid binding groups, which showed broadened and
red-shifted light absorption band with the aid of -extension;
b, b
0-quinoxalino porphyrins contain-
2.1. Materials and reagents
p
DSSCs based on the porphyrin dyes exhibited excellent power
conversion efficiency of 5.2% [13]. Campbell et al. also have reported
porphyrin dyes with extended p-systems by modifying a b-position
with an olefinic linkage, which resulted in broad, red-shifted
absorption bands; the dyes showed cell efficiency of 7.1% [14].
Despite major advances in porphyrin chemistry, further
improvements in photovoltaic performance are needed. This
All starting materials were purchased from Pacific ChemSource
and Alfa Aesar. DMF, CHCl3, CH3CN and POCl3 were dried and
distilled by accustomed methods before use. All other solvents and
chemicals used in this work were analytical grade and used without
further purification. All chromatographic separations were carried
out on silica gel (200e300 mesh).
paper concerns the use of
a donore(p-spacer)eacceptor
(De eA) system for metal-free dyes intended for use in DSSCs,
p
2.2. Analytical measurements
as means of securing effective photoinduced intramolecular
charge transfer [10, 15]. In this work, a series of meso-position
modified porphyrin sensitzers with porphyrin moieties as
1H and 13C NMR spectra were recorded with a Bruker Avance
400 instrument. UVevis spectra of the dyes were measured on
a PerkineElmer Lamada 25 spectrometer. The PL spectra were
obtained using PerkineElmer LS-50 luminescence spectrometer.
MALDI-TOF mass spectrometric measurements were performed on
Bruker Bifiex III MALDI-TOF. Electrochemical redox potentials were
obtained by cyclic voltammetry (CV) and differential pulse vol-
tammetry (DPV) using a three-electrode configuration and an
electrochemistry workstation (CHI660A, Chenhua Shanghai). The
working electrode was a glassy carbon electrode; the auxiliary
electrode was a Pt electrode, and saturated calomel electrode (SCE)
was used as reference electrode. Tetrabutylammonium perchlorate
(TBAP) 0.1 M was used as supporting electrolyte in dry DMF.
Ferrocene was added to each sample solution at the end of the
experiments, and was used as an internal potential reference [16].
donors, thiophene derivatives as
p-conjugated linkers, and cya-
noacrylic acids as acceptors and anchoring groups (Fig. 1) were
synthesized. The compounds were expected to have broader
absorption than the corresponding porphyrin without thiophene
cyanoacrylic acid groups, resulting in high short-circuit photo-
current (Jsc) and high solar-to-electricity conversion efficiency
(h). Herein, three meso-4-methylphenyl groups were introduced
to reduce the extent of aggregation occurring between neigh-
boring porphyrins adsorbed on TiO2 surface by means of steric
hindrance. Different thiophene derivatives employed as p-linkers
for connecting the porphyrin donors (D) and anchor (A) groups,
not only extended the light absorption region but also affected
the electron injection character of the dyes into the TiO2 surface.
Fig. 1. Molecular structures of porphyrin derivatives used in this study.

N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
77
2.3. General procedure for preparation and test of solar cells
3-methyl thiophene. A yellowebrown liquid 2 (8.25 g, 90%) was
obtained. The crude product was used in next step without further
purification. 1H NMR (CDCl3, 400 MHz, ppm):7.18 (s, 1 H, thienyl-H),
6.96 (s, 1 H, thienyl-H), 2.64 (t, 2 H, eCH2e), 1.62e1.54 (m, 8 H,
eCH2e), 1.36e1.30 (m, 12 H,eCH2e), 1.11e1.06 (m, 6 H, eCH2e),
0.93e0.87 (m, 12 H, eCH3).
Fluorine-doped SnO2 conducting glass (FTO) was immersed in
aqueous 40 mM TiCl4 solution at 70 ꢀC for 30 min, washed with
water and ethanol and then sintered at 450 ꢀC for 30 min 20e30 nm
particle size TiO2 colloid blended with 0.1% magnesium acetate
solution was applied to the FTO glass using a sliding glass rod to
obtain a TiO2 film of 10e15
particle size TiO2 colloid was applied to the electrode using the same
method, resulting in a TiO2 light-scattering layer of 4e6 m thick-
m
m thickness after drying. 200 nm
2.4.3. 4-(4-Methyl)thienyl benzaldehyde (3)
4-Bromobenzaldehyde (1.84 g,10 mmol) and 1 (3.88 g,10 mmol)
were dissolved in 100 mL of DMF, and the mixture was purged with
nitrogen for 30 min. Pd (PPh3)4 (115 mg, 0.1 mmol) was then added
after stirring at 100 ꢀC for 24 h, the ensuing mixture was poured into
100 mL of cooled water and extracted by chloroform. The organic
layer was dried over anhydrous MgSO4. Removal of the solvent was
carried outby rotary evaporation and the crude product was purified
using silica chromatography emplying petroleum ether/
dichloromethane ¼ 1/1 to give 3 as a yellow solid (1.62 g, 80%). 1H
NMR (CDCl3, 400 MHz, ppm): 10.11 (s, 1 H, CHOeH), 8.02 (d, 2 H,
phenyl-H), 7.86 (d, 2 H, phenyl-H), 7.38 (s,1 H, thienyl-H), 7.09 (s,1 H,
thienyl-H), 2.50 (s, 3 H, CH3e).13C NMR (CDCl3, ppm): 188.14,139.05,
138.17, 137.46, 137.03, 129.75, 128.44, 124.73, 120.50, 19.01.
m
ness. The TiO2 electrodes were immersed in a 1 mM DMF solution of
the porphyrin dyes at room temperature. After dye adsorption, the
dye-coated TiO2 electrodes were copiously rinsed with ethanol.
Photovoltaic measurements were performed in a sandwich cell
consisting of the porphyrin dye-sensitized TiO2 electrode as the
working electrode and a Pt foil as counter electrode. The electrolyte
comprised 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP)
in 3-methoxypropionitrile and the irradiated area of the cell was
0.196 cm2. The photocurrentevoltage (JeV) characteristics were
measured with a Keithley 2602 Source meter under 100 mW cmꢁ2
irradiation using a 500 W Xe lamp with a global AM 1.5 filter for
solar spectrum simulation. The solar-to-electricity conversion effi-
ciency (
h
) of the DSSCs was calculated from the short-circuit
2.4.4. 4-(4-Hexyl)thienyl benzaldehyde (4)
photocurrent density (Jsc), the open-circuit photovoltage (Voc), the
fill factor (FF) and the intensity of the incident light (Pin) according
to the following equation:
The synthetic procedure was similar to that described for 3,
except that 2 (4.58 g, 10 mmol) was used instead of 1. The crude
product was purified on a silica chromatograph with petroleum
ether/dichloromethane ¼ 2/1 to give 4 as a yellow solid (2.31 g,
85%). 1H NMR (CDCl3, 400 MHz, ppm): 9.98 (s, 1 H, CHOeH), 7.88 (d,
2 H, phenyl-H), 7.72 (d, 2 H, phenyl-H), 7.24 (s, 1 H, thienyl-H), 6.95
(s,1 H, thienyl-H), 2.53 (t, 2 H, eCH2e), 1.62 (t, 2 H, eCH2e), 1.23 (m,
6 H, eCH2e), 0.96 (t, 3 H, CH3e). 13C NMR (CDCl3, ppm): 191.34,
139.75, 138.43, 138.10, 137.15, 130.67, 128.01, 125.56, 122.07, 32.14,
31.87, 31.44, 29.26, 22.78, 14.15.
À
Á
Jsc mA cmꢁ2 ꢂ VocðVÞ ꢂ FF
À
Á
h
¼
Pin mW cmꢁ2
The IPCE values were plotted as a function of the excited
wavelength and defined according to the following equation:
À
Á
Jsc mA cmꢁ2
1240
À
Á
l
IPCEð Þ ¼
ꢂ
l
ðnmÞ
F
mW cmꢁ2
2.4.5. 5-(4-(4-Methyl) thienyl)phenyl-10,15,20-tris
(4-methylphenyl) porphyrin (5)
where Jsc is the short-circuit photocurrent density generated by
In a 500 mL three-necked flask, 4-methylbenzaldehyde (3.06 g,
25.5 mmol) and 3 (1.71 g, 8.5 mmol) were dissolved in 300 mL
propionic acid. The solution was heated to reflux at 140 ꢀC. Pyrrole
(2.28 g, 34 mmol) was then added dropwise and the mixture stirred
for another 30 min. After cooling to room temperature, half of the
solvent was evaporated and 150 mL CH3OH was added. The mixture
was cooled over night and filtered under vacuum. The crude
product was purified using column chromatography (petroleum
ether/dichloromethane ¼ 2/3 as an eluent). After recrystallization
from the mixture of CHCl3 and CH3OH (1:5, v/v), a desired purple
solid of compound 5 was obtained (1.60 g, 25%). 1H NMR (CDCl3,
400 MHz, ppm): 8.97 (m, 8 H, pyrrolic-H), 8.21 (d, 2 H, phenyl-H),
8.10 (d, 6 H, phenyl-H), 7.96 (d, 2 H, phenyl-H), 7.55 (d, 6 H, phenyl-
H), 7.42 (s, 1 H, thienyl-H), 6.99 (s, 1 H, thienyl-H), 2.70 (s, 9 H,
eCH3), 2.39 (s, 3 H, eCH3), ꢁ2.11 (s, 2 H, NeH). MALDI-TOF MS calcd
for C52H40N4S 752.30; found 751.94.
monochromatic light,
l
is the wavelength of incident mono-
chromatic light, and
F is the incident light intensity.
2.4. Synthesis
The synthetic routes to the three porphyrin dyes are shown in
Fig. 2. The detailed synthetic procedures were as follows.
2.4.1. 2-(Tri-n-butylstannyl)-4-methyl thiophene (1)
Under an argon atmosphere, 3-methyl thiophene (1.96 g,
20 mmol) and 50 mL freshly distilled dry THF were placed in
a 100 mL three-necked flask. n-Butyllithium (8.8 mL, 2.5 mol Lꢁ1 in
hexane, 22 mmol) was added dropwise at ꢁ78 ꢀC, and the solution
was stirred at ꢁ78 ꢀC for 1 h. Tributyltin chloride (6 mL, 22 mmol)
was then added and the mixture was allowed to reach room
temperature slowly and stirred for another 24 h. Finally, the
mixture was poured into 100 mL of cooled water and extracted by
hexane. The organic layer was dried over anhydrous MgSO4.
Removal of the solvent by rotary evaporation yielded a yellow-
ebrown liquid 1 (6.98 g, 90%). The crude product was used in next
step without further purification. 1H NMR (CDCl3, 400 MHz, ppm):
7.19 (s, 1 H thienyl-H), 6.97 (s, 1 H, thienyl-H), 2.30 (s, 3 H, eCH3),
1.53 (m, 6 H, eCH2e), 1.34 (m, 6 H, eCH2e), 1.11 (m, 6 H, eCH2e),
0.88 (m, 9 H, eCH3).
2.4.6. 5-(4-(4-Hexyl) thienyl)phenyl-10,15,20-tris(4-methylphenyl)
porphyrin (6)
The synthetic procedure for 6 was similar to that recounted for
5, except that 4 (2.31 g, 8.5 mmol) was used instead of 3. The crude
product was purified using silica chromatograph with dichloro-
methane to give 6 as a purple solid (1.68 g, 24%). 1H NMR (CDCl3,
400 MHz, ppm): 8.97(m, 8 H, pyrrolic-H), 8.21(d, 2 H, phenyl-H),
8.10(d, 6 H, phenyl-H), 7.96(d, 2 H, phenyl-H), 7.55(d, 6 H, phenyl-
H), 7.47(s, 1 H, thienyl-H), 7.01(s, 1 H, thienyl-H), 2.70(s, 9 H, eCH3),
2.53(t, 2 H, eCH2e), 1.62(t, 2 H, eCH2e), 1.23(m, 6 H, eCH2e), 0.96
(t, 3 H, CH3e), ꢁ2.11(s, 2 H, NeH). MALDI-TOF MS calcd for
C57H50N4S 822.38; found 822.59.
2.4.2. 2-(Tri-n-butylstannyl)-4-hexyl thiophene (2)
The synthetic procedure for 2 was similar to that for 1, except
that 3-hexyl thiophene (3.36 g, 20 mmol) was used instead of

78
N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
Fig. 2. Synthesis of porphyrin dyes.
2.4.7. 5-(4-(4-Methyl-5-formyl)thienyl)phenyl-10,15,20-tris
(4-methylphenyl)porphyrin (7)
(0.37 g, 40%). 1H NMR (CDCl3, 400 MHz, ppm): 10.16 (s, 1 H,
CHOeH), 8.89 (m, 8 H, pyrrolic-H), 8.28 (d, 2 H, phenyl-H), 8.13 (d, 6
H, phenyl-H), 8.07 (d, 2 H, phenyl-H), 7.59 (d, 6 H, phenyl-H), 7.52 (s,
1 H, thienyl-H), 2.73(s, 12 H, eCH3), ꢁ2.33(s, 2 H, NeH). MALDI-TOF
MS calcd for C53H40N4OS 780.29; found 779.74.
In a 100 mL three-necked flask, DMF (2.0 mL, 24 mmol) and
POCl3 (2.2 mL, 24 mmol) were added in turn at 0 ꢀC under nitrogen.
After stirring for 5 min, 5 (0.9 g, 1.2 mmol) was added and the
reaction mixture was refluxed for 12 h and then was stirred at room
temperature for 1.5 h after which it was poured into sodium acetate
solution (40 mL). The mixture was extracted with dichloromethane,
and the extract was washed with water and dried over anhydrous
MgSO4. The solvent was evaporated, and the residue was purified
by column chromatography on silica gel with petroleum ether/
dichloromethane ¼ 1/2, a purple solid of compound 7 was obtained
2.4.8. 5-(4-(4-Hexyl-5-formyl)thienyl)phenyl-10,15,20-tris
(4-methylphenyl)porphyrin (8)
The synthetic procedure for 8 was similar to that described for 7,
except that 6 (1.0 g, 1.2 mmol) was used instead of 5. The crude
product was purified on a silica chromatograph with petroleum
ether/dichloromethane ¼ 1/1 to give 8 as a purple solid (0.49 g,

N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
79
48%). 1H NMR (CDCl3, 400 MHz, ppm): 10.13 (s,1 H, CHO), 8.88 (m, 8
H, pyrrolic-H), 8.28 (d, 2 H, phenyl-H), 8.10 (d, 6 H, phenyl-H), 8.04
(d, 2 H, phenyl-H), 7.57 (d, 6 H, phenyl-H), 7.52 (s, 1 H, thienyl-H),
3.08 (t, 2 H, eCH2e), 2.70 (s, 9 H, eCH3), 1.62 (t, 2 H, eCH2e), 1.23
(m, 6 H, eCH2e), 0.96 (t, 3 H, CH3e), ꢁ2.11 (s, 2 H, NeH). MALDI-
TOF MS calcd for C58H50N4OS 850.37; found 850.58.
22.6, 21.5, 14.5.MALDI-TOF MS calcd for C61H49N5O2SZn 979.29;
found 979.28.
2.4.13. 2-(Tri-n-butylstannyl)-3,4-ethylenedioxythiophene (11)
The synthetic procedure for 11 was similar to that recounted
above for 1, except that 3,4-ethylene dioxy thiophene (2.84 g,
20 mmol) was used instead of 3-methyl thiophene. A yellow-brown
liquid 11 (6.91 g, 80%) was obtained. 1H NMR (CDCl3, 400 MHz,
ppm): 6.57 (s, 1 H thienyl-H), 4.13 (d, 4 H, eOCH2), 1.53 (m, 6 H,
eCH2), 1.34 (m, 6 H, eCH2), 1.11 (m, 6 H, eCH2), 0.88 (m, 9 H, eCH3).
2.4.9. 5-(4-(4-Methyl-5-formyl)thienyl)phenyl-10,15,20-tris
(4-methylphenyl)porphyrin Zine (9)
In a 250 mL one-necked round-bottomed flask, a mixture of 7
(0.39 g, 0.5 mmol) and Zn (OAc)2 (0.92 g, 5 mmol) in a solution of
CHCl3 (100 mL) and CH3OH (10 mL) was refluxed for 4 h. After
cooling to room temperature, the mixture was washed with water.
The organic layer was dried over anhydrous MgSO4 and concen-
trated. A purple-red solid of compound 9 was obtained (0.41 g,
97%). 1H NMR (CDCl3, 400 MHz, ppm): 10.13 (s, 1 H, CHO), 8.88 (m,
8 H, pyrrolic-H), 8.28 (d, 2 H, phenyl-H), 8.10 (d, 6 H, phenyl-H), 8.04
(d, 2 H, phenyl-H), 7.57 (d, 6 H, phenyl-H), 7.48 (s, 1 H, thienyl-H),
2.70 (s, 12 H, eCH3). MALDI-TOF MS calcd for C53H38N4OSZn
842.21; found 842.47.
2.4.14. 4-(3,4-Ethylene dioxy)thienyl benzaldehyde (12)
The synthetic procedure for 12 was similar to that described for
3, except that 11 (4.58 g, 10 mmol) was used instead of 1. The crude
product was purified on a silica chromatograph with petroleum
ether/dichloromethane ¼ 1/1 to give 12 as a yellow solid (2.09 g,
85%). 1H NMR (CDCl3, 400 MHz, ppm): 9.97 (s, 1 H, CHOeH), 7.87
(m, 4 H, phenyl-H), 6.42 (s, 1 H, thienyl-H), 4.43 (m, 2 H, eOCH2),
4.28 (m, 2 H, OCH2). 13C NMR (CDCl3, 100 MHz, ppm): 190.08,
152.45, 139.64, 136.93, 130.37, 127.75, 114.30, 94.92, 65.33.
2.4.10. -(4-(4-Hexyl-5-formyl)thienyl)phenyl-10,15,20-tris
(4-methylphenyl)porphyrin Zine (10)
2.4.15. 5-(4-(3,4-Ethylenedioxy)thienyl)phenyl-10,15,20-tris
(4-methylphenyl) porphyrin (13)
The synthetic procedure for 10 was similar to that described for
9, except that 8 (0.42 g, 0.5 mmol) was used instead of 7. A
purpleered solid of compound 10 was obtained (0.43 g, 95%). 1H
NMR (CDCl3, 400 MHz, ppm): 10.13 (s, 1 H, CHO), 8.88 (m, 8 H,
pyrrolic-H), 8.28 (d, 2 H, phenyl-H), 8.10 (d, 6 H, phenyl-H), 8.04 (d,
2 H, phenyl-H), 7.57 (d, 6 H, phenyl-H), 7.52 (s, 1 H, thienyl-H), 3.08
(t, 2 H, eCH2e), 2.70 (s, 9 H, eCH3), 1.62 (t, 2 H, eCH2e), 1.23 (m, 6
The synthetic procedure for 13 was similar to that recounted for
5, except that 12 (2.09 g, 8.5 mmol) was used instead of 3. The crude
product was purified on a silica chromatograph with dichloro-
methane to give 13 as a purple solid (1.68 g, 25%). 1H NMR (CDCl3,
400 MHz, ppm): 8.97 (m, 8 H, pyrrolic-H), 8.21 (d, 2 H, phenyl-H),
8.10 (d, 8 H, phenyl-H), 7.55 (d, 6 H, phenyl-H), 6.39 (s, 1 H, thienyl-
H), 4.43 (d, 2 H, eOCH2), 4.32 (d, 2 H, eOCH2), 2.71 (s, 9 H, eCH3),
ꢁ2.01 (s, 2 H, NeH). MALDI-TOF MS calcd for C53H40N4O2S 796.29;
found 796.50.
H, eCH2e), 0.96 (t,
3 H, CH3e). MALDI-TOF MS calcd for
C58H48N4OSZn 912.28; found 912.35.
2.4.11. 2-Cyano-3-(3-methyl-5-(4-(10,15,20-tris(4-methylphenyl))
porphyrinatozinc(II) yl)phenyl)thienyl acrylic acid (PZn-MT)
A 50 mL acetonitrile solution of 9 (0.29 g, 0.34 mmol) and cya-
noacetic acid (0.06 g, 0.68 mmol) was refluxed in the presence of
0.3 mL piperidine for 8 h under a nitrogen atmosphere. After
cooling to room temperature, the reaction mixture was poured into
a mixture of distilled water and 2.0 M HCl and then extracted with
chloroform. The organic layer was washed with water and dried
over anhydrous MgSO4. After the removal of solvent, the crude
product was purified by column chromatography over silica gel
with a dichloromethane/methanol mixture (10:1) yielding the
product as a purple solid PZn-MT (0.20 g, 65%). 1H NMR (CDCl3,
400 MHz, ppm): 8.91 (m, 8 H, pyrrolic-H), 8.29 (d, 2 H, phenyl-H),
8.22 (d, 2 H, phenyl-H), 8.15 (d, 6 H, phenyl-H), 7.85 (s, 1 H, vinyl-H),
7.55 (d, 6 H, phenyl-H), 7.43 (s, 1 H, thienyl-H), 2.70 (s, 12 H, eCH3).
13C NMR (CDCl3, ppm):162.4, 150.0, 149.9, 149.5, 140.3, 137.1, 135.7,
134.6,132.3,132.1, 131.8,129.8, 128.4, 127.7, 124.6,121.0,120.9,119.3,
115.7, 21.5, 15.0. MALDI-TOF MS calcd for C56H39N5O2SZn 909.21;
found 909.18.
2.4.16. 5-(4-(3,4-Ethylenedioxy-5-formyl)thienyl)phenyl-10,15,20-
tris(4-methylphenyl) porphyrin (14)
The synthetic procedure for 14 was similar to that described for
7, except that 13 (0.95 g, 1.2 mmol) was used instead of 5. The crude
product was purified on a silica chromatograph with petroleum
ether/dichloromethane ¼ 1/2 to give 14 as a purple solid (0.42 g,
42%). 1H NMR (CDCl3, 400 MHz, ppm): 10.05 (s, 1 H, CHOeH), 8.85
(m, 8 H, pyrrolic-H), 8.27 (d, 2 H, phenyl-H), 8.20 (d, 2 H, phenyl-H),
8.18 (d, 6 H, phenyl-H), 7.57 (d, 6 H, phenyl-H), 4.53 (s, 4 H, eOCH2),
2.70 (s, 9 H, CH3), ꢁ2.75 (s, 2 H, NeH). MALDI-TOF MS calcd for
C54H40N4O3S 824.28; found 824.73.
2.4.17. 5-(4-(3,4-Ethylenedioxy-5-formyl)thienyl)phenyl-10,15,20-
tris(4-methylphenyl) porphyrin Zinc (15)
The synthetic procedure for 15 was similar to that described for
9, except that 14 (0.41 g, 0.5 mmol) was used instead of 7. A purple-
red solid of compound 15 was obtained (0.43 g, 97%). 1H NMR
(CDCl3, 400 MHz, ppm): 10.05 (s, 1 H, CHO), 8.85 (s, 8 H, pyrrolic-H),
8.27 (d, 2 H, phenyl-H), 8.20 (d, 2 H, phenyl-H), 8.18 (d, 6 H, phenyl-
H), 7.57 (d, 6 H, phenyl-H), 4.53 (s, 4 H, eOCH2), 2.70 (s, 9 H, CH3).
MALDI-TOF MS calcd for C54H38N4O3SZn 886.20; found 885.93.
2.4.12. 2-Cyano-3-(3-hexyl-5-(4-(10,15,20-tris(4-methylphenyl))
porphyrinatozinc(II) yl)ph-enyl)thienyl acrylic acid (PZn-HT)
The synthetic procedure for PZn-HT was similar to that
described for PZn-MT, except that 10 (0.31 g, 0.34 mmol) was used
instead of 9. A purple solid of compound PZn-HT was obtained
(0.19 g, 58%). 1H NMR (CDCl3, 400 MHz, ppm): 8.95 (m, 8 H,
pyrrolic-H), 8.32 (d, 2 H, phenyl-H), 8.27 (d, 2 H, phenyl-H), 8.18 (d,
6 H, phenyl-H), 7.89 (s, 1 H, vinyl-H), 7.56 (d, 6 H, phenyl-H), 7.46 (s,
1 H, thienyl-H), 3.08 (t, 2 H, eCH2-), 2.70 (s, 9 H, eCH3), 1.62 (t, 2 H,
eCH2e), 1.23 (m, 6 H, eCH2e), 0.96 (t, 3 H, CH3e). 13C NMR (CDCl3,
ppm):161.9, 150.0, 149.9, 149.5, 144.2, 140.3, 137.1, 135.6, 134.5,
132.3, 132.1, 131.8, 127.7, 127.6, 124.6, 120.9, 119.5, 31.6, 31.1, 28.9,
2.4.18. 2-Cyano-3-(3,4-ethylenedioxy-5-(4-(10,15,20-tris
(4-methylphenyl))porphyrina- tozinc(II)yl)phenyl)thienyl acrylic
acid (PZn-EDOT)
The synthetic procedure for PZn-EDOT was similar to that
described for PZn-MT, except that 15 (0.30 g, 0.34 mmol) was used
instead of 9. A purple solid of compound PZn-EDOT was obtained
(0.19 g, 58%). 1H NMR (CDCl3, 400 MHz, ppm): 8.95 (s, 8 H, pyrrolic-
H), 8.33 (d, 2 H, phenyl-H), 8.27 (d, 2 H, phenyl-H), 8.20 (d, 6 H,
phenyl-H), 7.90 (s, 1 H, vinyl-H), 7.57 (d, 6 H, phenyl-H), 4.53 (s, 4 H,
eOCH2), 2.70 (s, 9 H, CH3). 13C NMR (CDCl3, ppm):161.9, 150.0,

80
N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
149.9, 149.5, 143.5, 140.3, 139.3, 137.1, 135.5, 134.5, 132.3, 132.0,
131.8, 130.9, 129.8, 127.7, 125.0, 121.0, 120.9, 119.6, 119.3, 115.7, 110.3,
21.5. MALDI-TOF MS calcd for C57H39N5O4SZn 953.20; found 953.17.
3. Results and discussion
3.1. Synthesis and chemical characterization
All the dyes have been synthesized according to several classical
reactions. The synthetic strategy is shown in Fig. 2. Taking 2-cyano-
3-(3-methyl-
5-(4-(10,15,20-tris
(4-methylphenyl))porphyr-
inatozinc(II)yl)phenyl)thienyl acrylic acid (PZn-MT) as an example,
the dye was synthesized as following steps. First, the starting
material, 2-(tri-n-butylstannyl)-4-methyl thiophene 1, was
obtained from 3-methyl thiophene in the presence of n-BuLi and
tributyltin chloride. Then the key intermediate 5 was obtained by
two steps: (1) 4-(4-methyl)thienyl benzaldehyde 3 was synthesized
by Stille coupling with 4-bromobenzaldehyde and compound 1; (2)
the compound 3 reacted with 4-methylbenzaldehyde and pyrrole
to obtain compound 5. Subsequently, compound 5 was converted
into 5-(4-(4-methyl-5-formyl)thienyl)-phenyl-10,15,20-tris (4-
methylphenyl)porphyrin 7 by Vilsmeier formylation with POCl3
and DMF in 1,2-dichloroethane. Finally, compound 7 was treated
with Zn(OAc)2, and then reacted with cyanoacetic acid by Knoe-
venagel reaction to gave PZn-MT. PZn-HT and PZn-EDOT were
synthesized by the same procedure as that for PZn-MT except 4-
hexyl thienyl or 3,4-ethylenedioxy thienyl group was used instead
of 4-methyl thienyl group respectively. The structures of the three
dyes were verified by 1H NMR, 13C NMR, and MOLDI-TOF mass
spectra.
3.2. Optical and electrochemical properties
Fig. 3. UVeVis absorption spectra of porphyrins in CHCl3 solution (a) and TiO2 films
(b).
The UVevis absorption spectra of PZn, PZn-MT, PZn-HT and PZn
EDOT in CHCl3 solution are shown in Fig. 3a, and the peak positions
and molar absorption coefficients ( ) of Soret and Q bands of
-
3
porphyrin dyes are listed in Table 1. The UVevis absorption spectra
of PZn exhibits a typical strong Soret band at 420 nm and moderate
Q bands at 547 nm. However, as a result of expansion of the
absorbance. This result is attributed to the introduction of a long
hexyl chain into the thiophene segment for PZn-HT, which result in
the decreasing of the absorption rate and the absorbed amounts of
the dye onto the TiO2.
p-system by introducing thiophene units, the Soret bands
(422e423 nm) and the Q bands (550 nm) of three porphyrin dyes
are both broadened and slightly red-shifted compared with those
of PZn. The molar extinction coefficients (3) of Soret bands are
The strength of intermolecular interaction for porphyrin dyes,
which may influence the solar cell performance, can be evaluated
by means of the adsorbed dye amounts. The total amounts of the
porphyrin dyes adsorbed on the TiO2 films were determined by
measuring the absorbance of the dyes, which were dissolved from
the dye-coated TiO2 films into DMF/H2O ¼ 4:1 with 0.1 mol Lꢁ1
1.21 ꢂ 105 mol Lꢁ1 cmꢁ1 for PZn-MT, 2.02 ꢂ 105 mol Lꢁ1 cmꢁ1 for
PZn-HT, and 2.55 ꢂ 105 mol Lꢁ1 cmꢁ1 for PZn-EDOT, respectively, all
of which are much higher than that of Ru(dcbpy)2(NCS)2 (N3 dye:
ca.1.6 ꢂ 104 mol Lꢁ1 cmꢁ1) [17]. As seen from Fig. 3b, the absorption
spectra of the three porphyrin dyes adsorbed onto TiO2 films are
similar to those of the corresponding solution spectra, but obvi-
ously red-shifted and broadening due to the formation of the J-type
aggregates of porphyrins on the TiO2 surface [18]. Additionally, the
absorption band of PZn-EDOT is broader and slightly red-shifted in
comparison with those of PZn-MT and PZn-HT, which should be
attributed to a more extensive conjugation of molecules containing
the EDOT moiety. It is indicated that PZn-EDOT has an enhanced
ability to harvest solar light.
NaOH. The surface density (G) values of the porphyrins absorption
in DMF are listed in Table 1. The adsorbed amounts of PZn-MT
(1.8 ꢂ 10ꢁ8 mol cmꢁ2) and PZn-EDOT (1.3 ꢂ 10ꢁ8 mol cmꢁ2) are
larger than that of PZn-HT (2.3 ꢂ 10ꢁ9 mol cmꢁ2). These results
Table 1
UV, PL spectral data of porphyrin dyes.
dye
labs
(
3
,105
Soret/
Amountc/
lem
lint
(nm)e
a
mol Lꢁ1 cmꢁ1
)
Q-band (f)b 10ꢁ8 mol cmꢁ2 (nm)d
The kinetics of adsorption of the three porphyrin dyes on TiO2
films were measured to study the procedure of porphyrin deriva-
tives absorbed onto the TiO2 surface. Fig. 4 shows the curves
obtained plotting values of absorbance at 560 nm for different
times of sensitization. The three kinds of kinetics seem to be
biphasic, which indicates that the dyes form multilayers on the TiO2
surface. PZn-MT and PZn-EDOT have about the same kinetics and
maximum absorbance with 0.6, and PZn-HT has a slower kinetic
compared to the other two dyes with a smaller value of maximum
P
P
P
Zn-MT
Zn-HT
422(1.21),550(0.08) 441/559
422(2.02),550(0.16) 438/565
1.8
0.23
1.3
608, 648 569
603, 646 563
609, 654 572
598, 645 448
Zn-EDOT 423(2.55),550(0.17) 453/567
PZn
420(2.21),547(0.07)
a
Absorption spectra was measured in CHCl3 solution.
Absorption spectra was obtained on TiO2 film.
Amount of the dyes absorbed on TiO2 film.
b
c
d
Wavelengths for emission spectra in CHCl3 solution by exciting at Soret
wavelength.
e
Measured by the intercept of the normalized absorption and emission spectra.

N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
81
Table 2
Electrochemical data for porphyrin dyes and driving forces for electron-transfer
processes on the TiO2.
Dye
E0e0a(eV) Eoxb(V) Eredc(V) Eox*d(V)
D
Ginje(eV)
D
Gregf(eV)
P
Zn-MT
2.18
2.20
0.968
1.015
0.950
1.230
ꢁ1.106 ꢁ1.212
ꢁ1.097 ꢁ1.185
ꢁ1.129 ꢁ1.220
ꢁ1.290 ꢁ1.540
ꢁ0.712
ꢁ0.685
ꢁ0.720
ꢁ0.468
ꢁ0.515
ꢁ0.450
PZn-HT
PZn-EDOT 2.17
PZn
2.77
a
Determined from the intercept of the normalized absorption and emission
spectra.
b
c
First oxidation potentials (vs NHE).
First reduction potentials (vs NHE).
Excited-state oxidation potentials approximated from Eox and E00 (vs NHE).
Driving forces for electron injection from the porphyrin excited singlet state
d
e
(Eox*) to the conduction band of TiO2(ꢁ0.5 V vs NHE).
f
Driving forces for regeneration of the porphyrin radical cation by I-/I3- redox
couple (þ0.5 V vs NHE).
Fig. 4. Kinetics of adsorption of the three porphyrin dyes on TiO2 films at 560 nm.
are summarized in Table 1. All porphyrins dyes show quasi-
reversible waves for a one-electron oxidation and reduction. The
oxidation potentials of PZn-MT, PZn-HT and PZn-EDOT ranging from
0.95 to 1.015 V vs NHE are negative relatively than that of PZn
(1.23 V vs NHE). Moreover, the oxidation potential of dye PZn-EDOT
(0.950 V) is slightly negative than those of PZn-MT (0.968 V) and
PZn-HT (1.015 V) likely because the former has a strong electron-
donating EDOT moiety. The first reduction potentials of PZn-MT
(ꢁ1.106 V vs NHE), PZn-HT (ꢁ1.097 V vs NHE) and PZn-EDOT
(ꢁ1.129 V vs NHE) are also positively shifted in comparison with
PZn (ꢁ1.19 V vs NHE). Totally, the HOMO-LUMO gaps of PZn-MT
(2.074 eV), PZn-HT (2.202 eV) and PZn-EDOT (2.079 eV) are smaller
than that of PZn (2.52 eV), which is consistent with the aforemen-
tioned trend for the zerothezeroth energies. The result indicates
that the introduction of thiophene derivatives could decrease
the electrochemical gaps of porphyrin dyes. Meanwhile, from the
UVevis and PL spectra and electrochemical measurements, the
driving forces for electron injection from the excited porphyrin
singlet state to the conduction band (CB) of TiO2 (ꢁ0.5 V vs NHE)
indicate that the porphyrin dyes with shorter alkyl or alkoxyl
chains have stronger intermolecular interaction than those with
longer alkyl chains on the TiO2 surface. In addition, the smaller
molecular sizes of PZn-MT and PZn-EDOT are also responsible for
the large adsorbed amount. As for PZn-HT, the hexyl chain linked to
the
p-conjugated segment of thiophene, which acts as a spacer to
decrease intermolecular
pep
interaction, thus reduced the adsor-
bed dye amount of PZn-HT [19].
The steady-state fluorescence spectra of PZn, PZn-MT, PZn-HTand
PZn-EDOT were measured in CHCl3 by exciting at the peak position
of the Soret band (Fig. 5), and the emission maxima are summarized
in Table 1. The emission maxima of PZn-MT (608 nm, 648 nm), PZn
-
HT (603 nm, 646 nm) and PZn-EDOT (609 nm, 654 nm) are red-
shifted with respect to those of PZn (598 nm, 645 nm), which is
consistent with the results of the UVevis absorption spectra. No
emission spectra were observed for the porphyrin dyes on TiO2 film,
which indicated that electron injection from the excited singlet state
of the dyes into the conduction band of the TiO2 completely [20].
From the intercept of the normalized UVevis absorption spectra and
(D
G
inj) and the regeneration of porphyrin radical cation by Iꢁ/I3ꢁ
couple (0.5 V vs NHE) [21] ( Greg) for the porphyrin-sensitized TiO2
D
steady-statefluorescence spectra, the zerothezerothenergies(E0e0
)
cells were determined and the corresponding data listed in Table 2.
Both of the processes are thermodynamically feasible.
are determined to be 2.18 eV for PZn-MT, 2.20 eV for PZn-HT, and
2.17 eV for PZn-EDOT, which are smaller than that of PZn (2.77 eV).
These results indicate that the HOMO-LUMO gaps of the thiophene-
linked porphyrin dyes are lower compared with that of PZn as
3.3. Photovoltaic properties of porphyrin-sensitized TiO2 solar cells
a result of the extension of
The redox potentials of the porphyrin dyes were obtained by
cyclic voltammetry and differential pulse voltammerty. The results
p system.
In the dye-sensitized solar cells, the prepared method of TiO2
electrode plays a key role. We used the double-layer TiO2 film
Fig. 6. Immersing time profiles of the
h value (close circle) and the surface density (G)
Fig. 5. PL spectra of PZn, PZn-MT, PZn-HT and PZn-EDOT in CHCl3.
of the PZn-HT dye adsorbed on a TiO2 electrode (open circle) in DMF.

82
N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
Table 3
Photovoltaic performance of DSSCs for the three dyes.
Dye
Jsc(mA/cm2)
Voc(V)
FF
h (%)
P
Zn-MT
12.83
13.71
15.59
0.64
0.63
0.64
0.68
0.67
0.65
5.55
5.80
6.47
PZn-HT
PZn-EDOT
containing 20 nm and 200 nm TiO2 particles as the photoanode to
improve the ability of light harvesting. In addition, the perfor-
mances of the porphyrin-sensitized TiO2 solar cells are strongly
affected by the solvents, immersing time and illuminating time. To
optimize the condition of the dye baths of the porphyrin sensitizer
for DSSCs, taking PZn-HT as an example, DMF, C2H5OH, CH3CN,
CHCl3 were employed in DSSCs. The best
h
of 5.80%
(Jsc ¼ 13.71 mA cmꢁ2, Voc ¼ 0.63 V, and FF ¼ 0.67) was obtained in
DMF, which can be attributed to the higher dye density on the TiO2
surface in DMF. To further improve the performance of the
porphyrin-sensitized TiO2 solar cells, the dependence of immersing
Fig. 8. IPCE plots for the DSSCs based on the three porphyrin dyes.
time on the
From the Fig. 6, it can be seen that the
G
and the
h
values for PZn-HT was measured in DMF.
values are increased rapidly
solar cells primarily result from the difference of Jsc. Because the
G
G
value of the dye-sensitized TiO2 films is an important factor to
determine the photocurrent generation. From Tables 1 and 3, we
can see that the low Jsc of the PZn-HT sensitized TiO2 cell compared
to that of the PZn-EDOT sensitized cells mainly results from the low
with increasing the immersing time until it becomes saturated
(2.3 ꢂ 10ꢁ9 mol cmꢁ2). In accordance with the initial trend of
G, the
h
values are also enhanced with increase of the immersing time, but
then the
h values decrease gradually with further increase of the
G
value of TiO2/PZn-HT. However, the highest
G value of TiO2/PZn-
immersing time. Similar behavior was reported for many other
porphyrin dyes [22]. The reasons may be associated with the
change in orientation of the porphyrin molecules on the TiO2
surface and/or in the binding strength to the TiO2 surface [22b].
Furthermore, the dependence of the illuminating time on the
values for PZn-HT was also measured in DMF. The values are
increased rapidly in the first 30 min, and then increased slowly
until the balance at 70 min. Similar results of PZn-MT and PZn-EDOT
absorbed on TiO2 electrode in DMF were obtained.
Table 3 summarizes the photovoltaic performance of the
porphyrin-sensitized TiO2 cells under the optimized conditions
which the dyes were immersed in DMF for 15 min and illuminated
for 70 min. The current-voltage characteristics of PZn-MT-, PZn-HT-
and PZn-EDOT sensitized TiO2 cells are shown in Fig. 7. PZn-MT
MT result in the lowest Jsc and , which implies that too much dyes
h
absorbed on TiO2 film produce negative effect due to serious
aggregation formation. It seems that the higher electron collection
efficiency of PZn-EDOT determined by the moderate
G value
(1.3 ꢂ 10ꢁ8 mol cmꢁ2) is suitable for preparing high efficient DSSCs.
h
h
The differences in the
h values among the porphyrin dyes can
also be explained from incident photon-to-current conversion
efficiency (IPCE) of three porphyrin-sensitized solar cells (Fig. 8).
The shapes of these spectra are slightly broader but clearly follow
the shape of the corresponding absorption spectra shown in Fig. 3b.
The maximum IPCEmax value of PZn-EDOT (65%) is higher than both
of PZn-MT (52%) and PZn-HT (58%), which is consistent well with
the trend of
h values. At longer wavelength region (gt; 500 nm),
PZn-EDOT also displays higher IPCE values than those of PZn-MT
and PZn-HT. These results are in good agreement with the photo-
voltaic behavior of porphyrin-sensitized TiO2 cells. Given the
similar driving forces of three porphyrin dyes, the influence of
sensitized cell exhibits
h
¼ 5.55% with Jsc ¼ 12.83 mA cmꢁ2
,
Voc ¼ 0.64 V, and ff ¼ 0.68, while PZn-HT sensitized one shows
a higher
h
¼ 5.80% with Jsc ¼ 13.71 mA cmꢁ2, Voc ¼ 0.63 V, and
ff ¼ 0.67, and PZn-EDOT sensitized one yields the highest
h
¼ 6.47%
electron injection efficiency in the
h values is excluded. Therefore,
with Jsc ¼ 15.59 mA cmꢁ2, Voc ¼ 0.64 V, and ff ¼ 0.65. To a large
the enhanced light-harvesting ability and higher electron collection
extent, the three porphyrin dyes display rather similar Voc and ff
efficiency of PZn- EDOT probably lead to the higher
those of PZn-MT and PZn-HT.
h value than
values. The different
h values of the three porphyrin-sensitized
4. Conclusions
In summary, we have successfully synthesized three modified
thiophene-linked porphyrin sensitizers (PZn-MT, PZn-HT and PZn
-
EDOT) in which porphyrin unit act as electron-donating moiety and
cyanoacrylic acid as electron-accepting moiety bridged by MT, HT,
and EDOT, respectivley. By introducing the electron-rich EDOT
moiety, the light absorption of PZn-EDOT has been broadened and
slightly red-shifted compared to those of PZn-MT and PZn-HT.
Therefore, PZn-EDOT exhibits the maximum power conversion
efficiency of 6.47% with Jsc ¼ 15.59 mA cmꢁ2, Voc ¼ 0.64 V and
FF ¼ 0.65. Since electron injection efficiencies of the three
porphyrin dyes are more or less similar, the light-harvesting ability
and electron collection efficiencies of porphyrin sensitizers have
strong influence on the photovoltaic performances. Our results
demonstrate the necessity and importance of modifying
p-spacers
Fig. 7. Current density-voltage characteristics for DSSCs based on the three dyes.
in De eA structure of porphyrin dyes. Inspired by the natural
p

N. Xiang et al. / Dyes and Pigments 88 (2011) 75e83
83
[10] (a) Zhang G, Bala H, Cheng Y, Shi D, Lv X, Yu Q, et al. High efficiency and stable
light-harvesting ability of porphyrin derivatives, further structural
modification of meso-substituted porphyrin for broadening the
spectral absorption is anticipated to give some new porphyrin dyes
with even better performances.
dye-sensitized solar cells with an organic chromophore featuring a binary
p-conjugated spacer. Chemical Communications 2009;16:2198e200;
(b) Xu W, Peng B, Chen J, Liang M, Cai F. New triphenylamine-based dyes for
dye-sensitized solar cells. Journal of Physical Chemistry C 2008;112:874e80;
(c) Shen P, Liu Y, Huang X, Zhao B, Xiang N, Fei J, et al. Efficient triphenylamine
dyes for solar cells: effects of alkyl-substituents and p-conjugated thiophene
unit. Dyes and Pigments 2009;83:187e97.
Acknowledgements
[11] (a) Zhang XH, Wang ZS, Cui Y, Koumura N, Furube A, Hara K. Organic sensi-
tizers based on hexylthiophene-functionalized Indolo[3,2- b]carbazole for
efficient dye-sensitized solar cells. Journal of Physical Chemistry C 2009;113:
13409e15;
This work was supported by the National Nature Science
Foundation of China (Project Nos. 50973092) and key project of
Hunan Province of China (Project No. 2008FJ2004) and Nature
Science Foundation of Hunan Province of China (09JJ3020,
09JJ4005). The authors are grateful to John N. Clifford and Emilio
Palomares at Institute of Chemical Research of Catalonia (ICIQ) in
Spain, for measuring the kinetics of adsorption of the porphyrin
dyes on TiO2 films.
(b) Liu D, Zhao B, Shen P, Huang H, Liu L, Tan ST. Molecular design of organic
dyes based on vinylene hexylthiophene bridge for dye-sensitized solar cells.
Science in China Series B: Chemistry 2009;52:1198e209.
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American Chemical Society 2003;125:9129e39;
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