Synthesis and photovoltaic properties of new branchlike organic dyes containing benzothiadiazole or triphenylamine-linked consecutive vinylenes units

Article abstract of DOI:10.1016/j.dyepig.2013.01.009

Two novel branchlike organic dyes (WD1 and FD1) based on 4,7-bis-(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazole (DTBT) and triphenylamine, containing vinylenes and consecutive vinylenes as π-conjugated unit, respectively, were designed and synthesized for

Full text of DOI:10.1016/j.dyepig.2013.01.009

Dyes and Pigments 97 (2013) 405e411
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photovoltaic properties of new branchlike organic dyes
containing benzothiadiazole or triphenylamine-linked consecutive
vinylenes units
*
Changwei Wang, Yan Fang, Zhencai Cao, Hui Huang, Bin Zhao, Haohao Li, Ying Liu, Songting Tan
College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, 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:
Two novel branchlike organic dyes (WD1 and FD1) based on 4,7-bis-(4-hexylthiophen-2-yl)benzo[1,2,5]
thiadiazole (DTBT) and triphenylamine, containing vinylenes and consecutive vinylenes as p-conjugated
Received 27 November 2012
Received in revised form
4 January 2013
Accepted 6 January 2013
Available online 29 January 2013
unit, respectively, were designed and synthesized for dye-sensitized solar cells (DSSCs). The photo-
physical, electrochemical and photovoltaic properties of the dyes were investigated. The introduction of
consecutive vinylenes into two branches of the FD1 molecule leads to a remarkably red-shifted ab-
sorption and higher molar extinction coefficient compared with the reference dye D2. The DSSCs based
on FD1 and WD1 dyes exhibited the power conversion efficiency of 7.04% and 2.18% under AM1.5G
irradiation (100 mW/cm2).
Keywords:
Branchlike dye
Benzothiadiazole
Ó 2013 Elsevier Ltd. All rights reserved.
Consecutive vinylenes
Dye-sensitized solar cells
Red-shifted absorption
Photovoltaic performance
1. Introduction
structures for light harvesting. Zhou et al. reported the DSSC based
on a triphenylamine dye C224 showed an overall efficiency up to 8%
As the sunlight is the largest and available single source of clean
energy, a lot of research is in progress to provide solar devices with
high efficiency and economical competition. Since the first report of
dye-sensitized solar cells (DSSCs) by O’Regan and Grätzel in 1991
[1], the DSSCs have been investigated as one of the most promising
methods for conversion of sunlight to electricity. Up to now, power
conversion efficiency record of 12.7% for dye-sensitized solar cells
has been reported [2,3]. The most developed DSSC dyes are based
on ruthenium polypyridyl complexes which show power conver-
sion efficiencies up to 11% [4e6]. However, the cost and environ-
mental concern of ruthenium complexes limit their wide
applications. Taking account of the limited ruthenium resource,
metal-free organic dyes [7e9] with high molar extinction co-
efficients have attracted considerable attention in recent years
owing to their cheaper and safer in molecular tailoring.
[12]. Zeng et al. reported another triphenylamine dye C219 with an
encouraging overall efficiency over 8.9% [14]. These results show
that the triphenylamine dyes are promising candidates for high
performance DSSCs. So many kinds of TPA-based molecules have
been designed and synthesized in linear, star-shaped or branchlike
structure. Tian and co-workers studied the starburst dyes and they
had made great progress [17e20]. In our previous study [8], we
have identified that branchlike tripheneylamine-based double
donor dyes featuring donoredonore
peacceptor (DeDepeA)
structure can effectively retard charge recombination of injected
electrons in the TiO2 with dye cations and also recombination of
these electrons with the electrolyte. The high performance of D2
can be attributed partly to the high molar extinction coefficient
(74,000 Mꢀ1 cmꢀ1 at 400 nm) [8]. However, the dye showed not
very high overall efficiencies (6.41%) due to the relatively narrow
absorption region. To expand the absorption range, one possible
way would be to increase the conjugation length of the molecular.
So the introduction of DTBT or consecutive vinylenes will be
expected to red-shift the spectrum and broaden the spectral region
of absorption. In this paper, two new branchlike organic sensitizers
WD1 and FD1 containing DTBT and consecutive vinylenes units
(Fig. 1), respectively, have been synthesized and applied in DSSCs.
Among dyes under investigation, triphenylamine dyes [10e16]
are considered as efficient sensitizers for DSSCs application. They
are attractive not only because of the strong absorption in the
visible region but also because of the ease of adjusting the chemical
* Corresponding author.
E-mail address: tanst2008@163.com (S. Tan).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.009

406
C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
Fig. 1. Molecular structures of WD1 and FD1. (D2 from Ref. [8]).
The effects of DTBTand consecutive vinylenes on the photophysical,
electrochemical and photovoltaic properties of the dyes were sys-
tematically studied.
the solvent was removed by rotary evaporation. The crude product
was purified using silica gel column chromatography (eluent:pe-
troleum ether/dichloromethane ¼ 5:1) to give 1.5 g 1 as a light
yellow liquid (yield: 83.6%). 1H NMR (CDCl3, 400 MHz,
d/ppm): 7.24
2. Experimental section
(d, 2H), 7.05e7.07 (d, 4H), 6.96e7.00 (m, 6H), 6.61e6.68 (dd, 1H),
5.59e5.63 (d, 1H), 5.11e5.14 (d, 1H), 2.31e2.35 (s, 6H).
2.1. Materials
2.2.2. Synthesis of (E)-3-(4-(di-p-tolylamino)phenyl)acrylaldehyde
(2)
Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (98%)
was obtained from Puyang Huicheng Chemical Co, Ltd and used
without further purification. All other starting materials were pur-
chased from Pacific ChemSource or Alfa Aesar in analytical grade.
Tetrahydrofuran (THF) and toluene were dried and distilled over
sodium and benzophenone. DMF was dried and distilled under
reduced pressure. Phosphorus oxychloride and 1,2-dichloroethane
were atmospheric distillation. All chromatographic separations
were carried out on silica gel (200e300 mesh). All other solvents
and chemicals used in this work were analytical grade and used
without further purification.
Compound 1 (1.5 g, 5 mmol) was dissolved in 25 mL of DMF and
Vilsmeier reagent that was prepared from 5 mL of DMF and 2.3 mL
of phosphorus oxychloride, and then the mixture was kept at 60 ꢁC
for VilsmeiereHaack reaction for 24 h. After neutralization with
25% NaOH solution, the product was extracted with dichloro-
methane and washed with water. The organic phase was dried over
anhydrous MgSO4, and then the solvent was removed by rotary
evaporation. The crude product was purified using silica gel column
chromatography (petroleum ether/dichloromethane ¼ 1:3) and
dried under vacuum to give 1.44 g 2 as an orange solid (yield: 88%).
1H NMR (CDCl3, 400 MHz,
d/ppm): 9.62e9.64 (d, 1H), 7.36e7.40 (m,
2.2. Synthesis
3H), 7.12e7.14 (d, 4H), 7.04e7.09 (d, 4H), 6.94e6.96 (d, 2H), 6.54e
6.60 (dd, 1H), 2.34 (s, 6H).
3-Hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[1,2,5]thiazdiazol-4-yl)
thiophene-2-carbaldehyde (5-(1,3-dioxolan-2-yl)-4-hexylthiophen-2
-yl)tributylstannane and 5-bromo-1,3-bis(diethyl-phsophonate- met
hyl)-2-(octyloxy)benzene were synthesized according to the
methods reported in the literatures [8,21]. 5-Bromo-1,3-bis(diethyl-
phsophonate-methyl)-2-(ethylhexyl)benzene was synthesized ac-
cording to literature procedure with some modification [8].
2.2.3. Synthesis of 4-(5-((1E,7E)-5-bromo-2-(2-ethylhexyloxy)-3-
((E)-2-(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)thiophen-2-yl)vinyl)styryl)-4-hexylthiophen-2-yl)-
7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (3)
5-Bromo-1,3-bis(diethyl-phsophonate- methyl)-2-(ethylhexyl)
benzene (1.414 g, 2.4 mmol) and 3-hexyl-5-(7-(4-hexylthiophen-
2-yl)benzo[1,2,5]thiazdiazol-4-yl)thiophene-2-carbaldehyde
(3.0 g, 6.0 mmol) were dissolved in THF (15 mL), and the solution
was stirred at room temperature for 30 min under nitrogen at-
mosphere. Then potassium tertbutoxide (0.673 g, 6 mmol) was
dissolved in THF (20 mL) and added dropwise to the solution. The
reaction mixture was stirred for 5 h at room temperature, and then
the mixture was heated to 45 ꢁC for 12 h. After cooling to room
temperature, the reaction mixture was extracted with dichloro-
methane and washed with dilute aqueous HCl solution. The organic
phase was dried over anhydrous MgSO4, and then the solvent was
removed by rotary evaporation. The crude product was purified
using silica gel column chromatography (eluent:petroleum ether/
dichloromethane ¼ 1:1) to give 1.58 g 3 as a dark red solid (yield:
2.2.1. Synthesis of 4-methyl-N-p-tolyl-N-(4-vinylphenyl)
benzenamine (1)
4-(Di-p-tolylamino)benzaldehyde (1.8 g, 6 mmol) and methyl-
triphenylphosphonium iodide (2.7 g, 6.78 mmol) were dissolved in
DMF (15 mL) and the solution was stirred at room temperature for
30 min under nitrogen atmosphere. Then potassium tertbutoxide
(0.76 g, 6.78 mmol) was dissolved in DMF (20 mL) and added
dropwise to the solution. The reaction mixture was stirred for 5 h at
room temperature, and then was heated to 50 ꢁC for 12 h. After
cooling to room temperature, the reaction mixture was extracted
with dichloromethane and washed with dilute aqueous HCl solu-
tion. The organic phase was dried over anhydrous MgSO4, and then

C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
407
83.2%). FT-IR (KBr, cmꢀ1): 944 (trans-vinylene). 1H NMR (CDCl3,
400 MHz, /ppm): 8.12e8.14 (d, 4H), 7.96 (s, 4H), 7.74 (s, 2H), 7.39e
ether/dichloromethane ¼ 1:1) to give 0.75 g 5 as a light yellow solid
d
(yield: 85%). FT-IR (KBr, cmꢀ1): 966 (trans-vinylene). 1H NMR
7.41 (d, 4H), 7.17 (s, 2H), 3.89e3.90 (d, 2H), 2.80e2.94 (m, 8H),
2.04e1.99 (m, 8H), 1.80e1.87 (m, 8H), 1.48e1.56 (m, 28H), 1.19e1.22
(t, 4H), 1.00e1.05 (m, 18H). MALDI-TOF MS (C70H85BrN4OS6) m/z:
calcd for 1270.744; found 1270.443.
(CDCl3, 400 MHz, d/ppm): 7.52 (s, 2H), 7.25e7.29 (d, 4H), 7.06e7.08
(d, 8H), 6.95e7.01 (m, 12H), 6.77e6.82 (m, 6H), 6.61e6.64 (d, 2H),
3.73e3.76 (t, 2H), 2.32 (s, 12H), 1.80e1.84 (m, 2H), 1.54 (m, 2H),
1.25e1.36 (m, 8H), 0.83e0.87 (t, 3H). 13C NMR (CDCl3, 100 MHz,
d/
ppm): 152.25, 146.95, 143.98, 132.59, 131.80, 130.77, 129.31, 128.90,
126.27, 126.17, 126.12, 124.02, 123.93, 123.82, 123.61, 121.12, 116.50,
73.81, 30.86, 29.16, 28.46, 25.17, 21.66, 19.80, 13.07. MALDI-TOF MS
(C62H63BrN2O) m/z: calcd for 930.412; found 930.548.
2.2.4. Synthesis of 5-(4-(2-ethylhexyloxy)-3,5-bis((E)-2-(3-hexyl-5-
(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-
2-yl)vinyl)phenyl)-3-hexylthiophene-2-carbaldehyde(4)
Compound
3 (0.75 g, 0.59 mmol), (5-(1,3-dioxolan-2-yl)-4-
hexylthiophen-2 -yl)tributylstannane (0.44 g, 1.4 mmol) and
Pd(PPh3)4 (0.1 g, 0.08 mmol) were dissolved in DMF (30 mL) and
the solution was stirred at 100 ꢁC for 30 h under nitrogen atmo-
sphere. After cooling to room temperature, the resulting solution
was treated with dilute aqueous HCl (10 mL) solution. This solution
was stirred for 30 min and then extracted with dichloromethane.
The organic phase was dried over anhydrous MgSO4, and the sol-
vent was removed by rotary evaporation. The crude product was
purified using silica gel column chromatography (eluent:petroleum
ether/dichloromethane ¼ 1:1) to give 0.47 g 4 as a red solid (yield:
57.5%). FT-IR (KBr, cmꢀ1): 1660 (nC]O), 952 (trans-vinylene). 1H
2.2.7. Synthesis of 5-(3,5-bis((1E,3E)-4-(4-(dip-tolylamino)phenyl)
buta-1,3-dienyl)-4-(octyloxy)phenyl)-3-hexylthiophene-2-
carbaldehyde (6)
Compound 6 was synthesized according to compound 4, instead
using
5
(0.88 g, 1 mmol) and (5-(1,3-dioxolan-2-yl)-4-
hexylthiophen-2 -yl)tributylstannane (0.5 g, 1.12 mmol). The
crude product was purified using silica gel column chromatography
(eluent:petroleum ether/ dichloromethane ¼ 3:1) to give 0.62 g 6
as a yellow solid (yield: 68%). FT-IR (KBr, cmꢀ1): 1637 (nC]O), 983
(trans-vinylene). 1H NMR (CDCl3, 400 MHz,
d/ppm): 10.02 (s, 1H),
7.70 (s, 2H), 7.38e7.40 (d, 4H), 7.29e7.30 (m, 2H), 7.24 (s, 1H), 7.06e
7.08 (m, 2H), 6.69e7.03 (m, 24H), 3.79e3.83 (t, 2H), 2.96e2.98 (t,
2H), 2.32 (s, 12H), 1.82e1.83 (m, 2H), 1.70e1.76 (m, 2H), 1.56 (m,
2H), 1.25e1.33 (m, 14H), 0.83e0.90 (m, 6H). MALDI-TOF MS
(C73H78N2O2S) m/z: calcd for 1046.578; found 1046.747.
NMR (CDCl3, 400 MHz, d/ppm): 10.06 (s, 1H), 8.00e8.04 (d, 4H),
7.85 (s, 4H), 7.76 (s, 3H), 7.36e7.38 (d, 4H), 7.06 (s, 2H), 3.83 (s, 2H),
3.02 (s, 2H), 2.71e2.83 (m, 8H), 1.92 (s, 1H), 1.73e1.75 (m, 10H),
1.25e1.43 (m, 36H),1.06e1.08 (m, 4H), 0.91 (s, 20H). MALDI-TOF MS
(C81H100N4O2S7) m/z: calcd for 1384.589; found 1384.636.
2.2.8. Synthesis of (E)-3-(5-(3,5-bis((1E,3E)-4-(4-(dip-tolylamino)
phenyl)buta-1,3-dienyl)-4-(octyloxy)phenyl)-3-hexylthiophen-2-
yl)-2-cyanoacrylicacid (FD1)
2.2.5. Synthesis of (E)-2-cyano-3-(5-(4-(2-ethylhexyloxy)-3,5-
bis((E)-2-(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)thiophen-2-yl)vinyl)phenyl)-3-hexylthiophen-2-yl)
acrylic acid (WD1)
The synthetic procedure for FD1 was similar to that for WD1,
except that compound 6 (0.60 g, 0.66 mmol) was used instead of
compound 4. The crude product was purified using silica gel col-
umn chromatography (eluent:CH2Cl2/CH3OH ¼ 10:1) to give 0.40 g
FD1 as a brown solid (yield: 84%). FT-IR (KBr, cmꢀ1): 2362 (nC^N),
Compound
4 (0.42 g, 0.3 mmol), cyanoacetic acid (0.26 g,
3.0 mmol), and piperidine (0.5 mL) were dissolved in CHCl3 (20 mL)
and CH3CN (10 mL), and the solution was refluxed at 80 ꢁC for 12 h
under nitrogen atmosphere. Then the solution was poured into
1601 (nC]O). 1H NMR (CDCl3, 400 MHz,
d/ppm): 8.48 (s, 1H), 7.72 (s,
a
diluted aqueous HCl (10 mL) solution and extracted with
2H), 7.29e7.30 (m, 5H), 6.70e7.07 (m, 28H), 3.78 (t, 2H), 2.87 (t, 2H),
2.32 (s, 12H), 1.84 (m, 2H), 1.56e1.69 (m, 4H), 1.26e1.36 (m, 14H),
dichloromethane. The organic phase was dried over anhydrous
MgSO4, and the solvent was removed by rotary evaporation. The
crude product was purified using silica gel column chromatography
(eluent:CH2Cl2/CH3OH ¼ 10:1) to give 0.24 g WD1 as a red solid
(yield: 54.5%). FT-IR (KBr, cmꢀ1): 2202 (nC^N), 1606 (nC]O). 1H NMR
0.86e0.99 (m, 6H). 13C NMR (CDCl3, 100 MHz,
d/ppm): 167.34,
156.28, 154.63, 153.26, 146.90, 144.16, 143.98, 132.61, 131.78, 131.40,
130.73,129.89,129.39,128.90,128.60,127.83,127.50,126.28,124.86,
124.11, 121.62, 121.12, 115.12, 93.74, 73.76, 32.95, 30.89, 29.23, 28.68,
28.10, 26.71, 25.53, 24.05, 23.69, 21.68, 19.81, 18.14, 13.09, 13.06.
Anal. Calcd for C76H79N3O3S: C, 81.09; H, 7.14; N, 3.77; S, 2.88.
Found: C, 81.07; H, 7.21; N, 3.75; S, 2.89. MALDI-TOF MS
(C76H79N3O3S) m/z: calcd for 1113.584; found 1113.725.
(CDCl3, 400 MHz, d/ppm): 8.40 (s, 1H), 7.98 (s, 2H), 7.95 (s, 2H), 7.91
(s, 4H), 7.69 (s, 2H), 7.39e7.35 (d, 2H), 7.29 (s, 2H), 7.25 (s, 2H), 7.01
(s, 1H), 3.79e3.78 (d, 2H), 2.83e2.76 (m, 6H), 2.67e2.64 (t, 4H),
1.90e1.87 (m, 1H), 1.70e1.65 (m, 10H), 1.38e1.21 (m, 32H), 1.05e
1.02 (t, 6H), 0.87e0.86 (m, 21H). 13C NMR (CDCl3, 100 MHz,
d/
ppm): 169.09, 157.53, 156.23, 154.33, 152.46, 152.37, 145.28, 144.22,
142.85,138.98,138.00,137.08,132.23,130.86,130.84,130.50,129.90,
128.85, 128.56, 125.75, 125.70, 125.27, 125.11, 125.03, 122.82, 122.21,
121.67, 121.33, 115.84, 94.77, 78.07, 68.23, 65.56, 41.13, 33.81, 31.93,
31.82, 31.77, 31.66, 30.95, 30.85, 30.67, 30.41, 30.16, 29.70, 29.53,
29.30, 29.28, 29.16, 28.97, 28.80, 23.42, 22.68, 22.67, 19.21, 14.32,
14.14, 14.10, 11.81. Anal. Calcd for C84H101N5O3S7: C, 69.43; H, 7.01;
N, 4.82; S, 15.45. Found: C, 68.79; H, 8.08; N, 4.43; S, 15.08. MALDI-
TOF MS (C84H101N5O3S7) m/z: calcd for 1451.595; found 1451.876.
2.3. Instruments and characterizations
1H NMR and 13C NMR spectra were measured with a Bruker
Avance 400 instrument. Elemental analysis was obtained on
PerkineElmer 2400 element analyzer. UVevisible spectra of the
dyes were measured on a PerkineElmer Lamada 25 spectrometer.
The PL spectra were obtained using PerkineElmer LS-50 lumines-
cence spectrometer. FT-IR spectra were obtained on a Perkine
Elmer Spectra One spectrometer. MALDI-TOF mass spectrometric
measurements were performed on Bruker Autoflex Ⅲ. Electro-
chemical redox potentials were obtained by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) using a three-
electrode configuration and an electrochemistry workstation
(ZAHNER ZENNIUM). The working electrode was a glassy carbon
electrode; the counter electrode was a Pt electrode, and saturated
calomel electrode (SCE) was used as reference electrode. Tetrabu-
tylammonium perchlorate (TBAP) 0.1 M was used as supporting
electrolyte in dry DMF. Ferrocene was added to each sample
2.2.6. Synthesis of N-(4-((1E,3E)-4-(5-bromo-3-((1E,3E)-4-(4-(di-
p-tolylamino)phenyl)buta-1,3-dienyl)-2-(octyloxy)phenyl)buta-1,3-
dienyl)phenyl)-4-methyl-N-p-tolylbenzenamine(5)
Compound 5 was synthesized according to compound 3, instead
using 5-bromo-1,3-bis(diethyl-phsophonate- methyl)-2-(octyloxy)
benzene (0.585 g, 1 mmol), compound 2 (0.645 g, 2 mmol) and
potassium tertbutoxide (0.46 g, 4 mmol). The crude product was
purified using silica gel column chromatography (eluent:petroleum

408
C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
Scheme 1. Synthesis of WD1 and FD1 dyes. Reagents and conditions: (a) CH3PPh3I, DMF, t-BuOK. (b) DMF, POCl3, VilsmeiereHaack reaction. (c) THF, t-BuOK. (d) (1) DMF, Pd(PPh3)4;
(2) HCl, H2O. (e) MeCN/HCCl3, CH3CN, Piperidine. (f) THF, t-BuOK. (g) (1) DMF, Pd(PPh3)4; (2) HCl, H2O. (h) MeCN/HCCl3, CH3CN, Piperidine.
solution at the end of the experiments, and was used as an internal
potential reference [11].
450 ꢁC for 30 min, then treated with 40 mM TiCl4 aqueous solution
at 70 ꢁC for 30 min and annealed again at 450 ꢁC for 30 min. After
the film was cooled to room temperature, it was immersed into
5.0 ꢂ 10ꢀ4 M dye solution for 15 min in the dark. The sensitized
electrode was then rinsed with ethanol, and dried. One drop of
electrolyte solution was deposited onto the surface of the electrode
and penetrated inside the TiO2 film via capillary action. The pho-
tovoltaic measurements were performed in a sandwich cell con-
sisting of the dyes-sensitized TiO2 electrode as the working
electrode and a Pt foil as the counter electrode. The electrolyte
consists of 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP)
in 3-methoxypropionitrile. The photocurrentevoltage (JeV) char-
acteristics were recorded on Keithley 2602 Source meter. Dyes-
sensitized TiO2 electrodes were measured under AM1.5G
2.4. Fabrication and characterization of DSSCs
The FTO glass (fluorine doped SnO2, sheet resistance 14 U/sq,
transmission >90% in the visible) was firstly cleaned in a detergent
solution using an ultrasonic bath for 15 min, and then rinsed with
water and ethanol. The cleaned FTO glass was immerged in 40 mM
aqueous TiCl4 solution at 70 ꢁC for 30 min, then washed with water
and ethanol. The TiO2 suspension was prepared from P25 (Degussa
AG, Germany) [22] and 1 wt% magnesium acetate solution [23] on
following a literature procedure. Then the paste was deposited onto
the FTO glass by sliding glass rod method. The film was sintered at

C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
409
Table 1
Maximum absorption and emission data of WD1 and FD1 dyes.
Dye
labs/nm
lmax/nm
lem/(nm)c
a
(ε/Mꢀ1 cmꢀ1
)
(on TiO2)b
WD1
FD1
387 (51,000), 492 (46,450)
309 (52,360), 410 (95,000)
397
414
631
457
a
Maximum absorption in CHCl3 solution (10ꢀ5 M) at 25 ꢁC.
b
c
Maximum absorption on TiO2 film.
Maximum emission of the dyes in CHCl3 solution (10ꢀ5 M) at 25 ꢁC. ε is the
extinction coefficient at labs of maximum absorption.
absorption band is attributed to the
pep* electron transitions of
the conjugated molecules, while the second band is attributed to
the intramolecular charge transfer (ICT) transition. Similarly, FD1
exhibits two distinct absorption bands, the absorption band in the
UV region (300e320 nm) is corresponding to the
pep* electron
transitions of the conjugated molecules; and the other absorption
band in the visible region (400e420 nm) can be assigned to ICT
between the di(p-tolyl)phenylamine donor and the cyanoacrylic
acid acceptor. As expected, both the absorption spectrum of WD1
and FD1 are broadened and red-shifted than D2 due to the
introduction of DTBT unit and additional vinylene unit in the
Fig. 2. Absorption spectrum of the WD1 and FD1 dyes in CHCl3 solution (10ꢀ5 M) (D2
from Ref. [8]).
irradiation (100 mW/cm2). The power conversion efficiency (
h) of
the DSSC is calculated from short-circuit photocurrent (Jsc), the
open-circuit photovoltage (Voc), the fill factor (FF) and the intensity
of the incident light (Pin) according to the following equation:
p-conjugated system. The absorption of WD1 is obviously broad-
ened (350e560 nm) and red-shifted in comparison with that of
FD1 and D2. However, the molar extinction coefficient (ε) of WD1
(51,000 Mꢀ1 cmꢀ1 at 387 nm) is lower than those of FD1
(95,000 Mꢀ1 cmꢀ1 at 410 nm) and D2 dye (74,000 Mꢀ1 cmꢀ1 at
401 nm), which indicates that the ability of light harvesting is
limitedly improved by the introduction of DTBT unit. On the con-
trary, FD1 shows broader spectrum and higher molar extinction
coefficient than D2 induced by consecutive vinylenes, so high
photovoltaic performance of FD1 would be anticipated. We also
measured the adsorption spectra of the WD1 and FD1 dye on thin
film (Fig. 3). The corresponding data are also summarized in Table 1.
The absorption spectra of the films are broadened and the peaks are
red-shifted in comparison with that in solution due to the aggre-
gate formation.
À
Á
Jsc mA cmꢀ2 ꢂ VocðVÞ ꢂ FF
À
Á
h
¼
Pin mW cmꢀ2
The action spectrum of monochromatic incident photo-to-
current conversion efficiency (IPCE) for the solar cell was also
detected with a Zolix DCS300PA data acquisition system.
3. Results and discussion
3.1. Synthesis
The detailed synthetic routes of FD1 and WD1 are shown in
Scheme 1. First, the compound 1 was synthesized from 4-(di-p-
tolylamino)benzaldehyde by Wittig reaction with methyl-
triphenylphosphonium iodide and potassium tert-butylate in DMF.
Compound 1 by VilsmeiereHaack reaction with phosphorus oxy-
chloride in DMF to give compound 2. The key compound 3 was
obtained from 5-bromo-1,3-bis(diethyl-phsophonate-methyl)-2-
(ethylhexyl)benzene and 3-hexyl-5-(7-(4-hexylthiophen-2-yl)
benzo[1,2,5]thiazdiazol-4-yl)thiophene-2-carbaldehyde according
3.3. Electrochemical properties
To evaluate the thermodynamic possibility of electron transfer
from the excited dye molecule to the conductive band of TiO2, cyclic
voltammetry (CV) was performed in CH3CN with 0.1 M tetrabuty-
lammonium perchlorate (TBAP) as a supporting electrolyte (Fig. 4).
The first oxidation potential vs. NHE (Eox), which corresponds to the
highest occupied molecular orbital (HOMO vs. NHE), was calibrated
to WittigeHorner reaction. Similarly, the compound
5 was
obtained from 5-bromo-1,3-bis(diethyl-phsophonate-methyl)-2-
(octyloxy)benzene and compound 2. Then the compounds 3 and 5
were coupled with (5-(1,3-dioxolan-2-yl)-4-hexylthiophen-2-yl)
tributylstannane through the Stille coupling reaction to obtain the
compounds 4 and 6, respectively. The WD1 and FD1 dyes were
synthesized by the Knoevenagel condensation reaction [24] of
different aldehyde derivatives 4 and 6 with cyanoacetic acid in the
presence of piperidine. The structures of the WD1 and FD1 dyes
were verified by 1H NMR, 13C NMR, MALDI-TOF mass spectra and
elemental analysis.
3.2. Optical properties
The photophysical properties of WD1 and FD1 were inves-
tigated by UVevis absorption spectroscopy in diluted (10ꢀ5 M)
chloroform solution, and the spectra and data are shown in Fig. 2
and Table 1. The WD1 dye exhibits two major prominent bands
appearing about 370e420 nm and 475e525 nm, the first
Fig. 3. UVevisible absorption spectrum of the WD1 and FD1 on thin film.

410
C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
Fig. 5. IPCE action spectrum for the DSSC based on the WD1 and FD1 dye-sensitized
transparent TiO2 films.
Fig. 4. Cyclic voltammogram of the WD1 and FD1 dyes.
by addition of 0.63 V to the potential (vs. SCE) vs. Fc/Fcþ by CV. The
zerothezeroth energies (E0e0) are determined by the intersection
of the normalized UVevis absorption spectrum and steady-state
fluorescence emission spectrum. The reduction potential vs. NHE
(Ered), which corresponds to the lowest unoccupied molecular
orbital (LUMO vs. NHE), can be calculated by Eox ꢀ E0e0. To get an
efficient charge separation, the LUMO of the dye has to be suffi-
ciently more negative than the conduction-band edge of the TiO2
(Ecb, ꢀ0.50 V vs. NHE), and the HOMO has to be more positive than
the redox potential of iodine/iodide (0.42 V) to ensure the regen-
eration of dyes [25]. As shown in Fig. 4 and Table 2, the HOMO
levels of WD1 and FD1 are 1.19 V and 1.28 V (vs. NHE), respectively.
It can be seen that the value of HOMO of FD1 is sufficiently more
positive, indicating that the electron-donating ability of triphe-
nylamine is stronger than DTBT unit [26]. As indicated in Table 2,
incident photo-to-current conversion efficiencies (IPCE) plots for
the DSSC based on WD1 and FD1 are shown in Fig. 5. DSSC based on
WD1 gives a relatively narrow IPCE action spectrum, and the
maximum IPCE value is only 47% at 335 nm because of its low molar
extinction coefficient. In contrast, DSSC based on FD1 shows a rel-
atively high IPCE values in the broad range of 300e550 nm, which
is in accordance with the spectrum of the dye on the solid film. FD1
exhibits higher IPCE values than WD1 due to its higher molar
extinction coefficient resulting in better light harvesting ability,
which implies FD1 would obtain a larger photocurrent in DSSCs.
The photocurrentevoltage curves of two dyes are shown in Fig. 6,
and the corresponding photovoltaic data are collected in Table 3.
Under the same condition, DSSCs based on WD1 without CDCA
(chenodeoxycholic acid) gave a short-circuit photocurrent density
(Jsc) of 5.83 mA/cm2, an open-circuit voltage (Voc) of 0.52 V, and a fill
factor (FF) of 0.72, corresponding to an overall conversion efficiency
the introduction of the consecutive vinylenes in the p-spacer leads
(h) of 2.18%. However, the h value of WD1 is only 1.48% when CDCA is
to the comparatively lower-lying HOMO of FD1, which increases
the gap between the HOMO level and the redox potential of iodine/
iodide. On the other hand, the LUMO level of WD1 and FD1
are ꢀ1.04 V and ꢀ1.43 V (vs. NHE), respectively. It is evident that
the value of LUMO is more negative than the conduction band of
TiO2 (about ꢀ0.5 V vs. NHE), and the Egap between the conduction
band and the Ered (vs. NHE) of WD1 and FD1 are 0.54 eV and
0.93 eV, respectively. Theoretically, an energy gap of 0.2 eV is
necessary for efficient electron injection [25,27], so the driving
force of two dyes is sufficiently large for effective electron injection.
added (Jsc ¼ 3.71 mA/cm2, Voc ¼ 0.59 V, FF ¼ 0.67). It indicates that
the addition of CDCA has less effect on reducing the formation of
aggregates because DTBT unit has enough alkyl chain [28,29]. The
DSSC based on FD1 shows higher
h
value of 6.80% (Jsc ¼ 13.26 mA/
cm2, Voc ¼ 0.70 V, FF ¼ 0.73). After the addition of CDCA, the FD1-
sensitized cell exhibits the best photovoltaic performances 7.04%
3.4. Photovoltaic properties of DSSCs
DSSCs based on WD1 and FD1 were fabricated using Pt plate as
the counter electrode with an effective area of 0.196 cm2. The
Table 2
Electrochemical data of the WD1 and FD1 dyes.a
Dye
lint/nm
E0e0/eV
Eox/V vs. NHE
Ered/V/ vs. NHE
Egap/V
WD1
FD1
557
457
2.23
2.71
1.19
1.28
ꢀ1.04
ꢀ1.43
0.54
0.93
a
E0e0 values were calculated from intersection of the normalized absorption and
the emission spectrum (lint): E0e0 ¼ 1240/lint. The first oxidation potential (vs. NHE),
Eox was measured in acetonitrile and toluene (8/2, v/v) and calibrated by addition of
0.63 V to the potential versus Fc/Fcþ. The reduction potential, Ered, was calculated
from Eox ꢀ E0e0. Egap is the energy gap between the Ered of dye and the conductive
band level of TiO2 (ꢀ0.5 V vs. NHE).
Fig. 6. JeV curves for DSSCs based on the WD1 and FD1 with and without CDCA.

C. Wang et al. / Dyes and Pigments 97 (2013) 405e411
411
Table 3
[7] Chen C, Hsu Y, Chou H, Thomas KRJ, Lin JT, Hsu C. Dipolar compounds
containing fluorene and a heteroaromatic ring as the conjugating bridge
Photovoltaic performance parameters of DSSCs based on WD1 and FD1.
for high-performance dye-sensitized solar cells. Chem Eur
J 2010;16:
Dye
Solvents
Jsc
Voc
(V)
FF
h
3184e93.
(mA/cm2)
(%)
[8] Chen H, Huang H, Huang X, Clifford JN, Forneli A, Palomares E, et al. High
molar extinction coefficient branchlike organic dyes containing di(p-tolyl)
phenylamine donor for dye-sensitized solar cells applications. J Phys Chem
C 2010;114:3280e6.
[9] Hardin BE, Yum J, Hoke ET, Jun YC, Péchy P, Torres T, et al. High excitation
transfer efficiency from energy relay dyes in dye-sensitized solar cells. Nano
Lett 2010;10:3077e83.
WD1
Chloroform
Chloroform
Toluene
5.83
3.71
13.26
12.57
0.52
0.59
0.70
0.76
0.72
0.67
0.73
0.74
2.18
1.48
6.80
7.04
WD1 þ CDCA
FD1
FD1 þ CDCA
Toluene
[10] Li G, Jiang K, Bao P, Li Y, Li S, Yang L. Molecular design of triarylamine-based
organic dyes for efficient dye-sensitized solar cells. New J Chem 2009;33:
868e76.
[11] Tian H, Yang X, Chen R, Zhang R, Hagfeldt A, Sun L. Effect of different dye baths
and dye-structures on the performance of dye-sensitized solar cells based on
triphenylamine dyes. J Phys Chem C 2008;112:11023e33.
[12] Zhou D, Cai N, Long H, Zhang M, Wang Y, Wang P. An energetic and kinetic
view on cyclopentadithiophene dye-sensitized solar cells: the influence of
fluorine vs ethyl substituent. J Phys Chem C 2011;115:3163e71.
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(Jsc ¼ 12.57 mA/cm2, Voc ¼ 0.76 V, FF ¼ 0.74). It improved by 0.63%
than the h value of 6.41% based on D2 dye [8] due to the introduction
of consecutive vinylenes units. Compared to the FD1 dye, although
WD1 has broader absorption spectrum, the value of DSSC based on
h
the WD1 is lower. This may be due to the fact that DTBT has been
developed and used as acceptor segment and WD1 shows relatively
low molar extinction coefficient (ε).
[14] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized
solar cells with an organic photosensitizer featuring orderly conjugated
ethylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22:
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materials. Chem Commun 2009;0:5483e95.
4. Conclusions
In this study, we have successfully designed and synthesized two
novel branchlike organic dyes WD1 and FD1. WD1 shows broad
absorption in the range of 350e600 nm but relatively low molar
extinction coefficient due to introduction of DTBT. The DSSC based
on the WD1 only showed a moderate
h
value of 2.18% (Jsc ¼ 5.83 mA/
cm2, Voc ¼ 0.52 V, FF ¼ 0.72). By introducing two di(p-toyl)phenyl-
amine moieties linked by consecutive vinylenes as branches, FD1
exhibits a relatively narrow absorption but very high molar
extinction coefficient of 95,000 Mꢀ1 cmꢀ1 at 410 nm. The DSSC
device based on FD1 dye exhibited the highest power conversion
efficiency of 7.04% (Jsc ¼ 12.57 mA/cm2, Voc ¼ 0.76 V, FF ¼ 0.74).
[19] Tang J, Hua J, Wu W, Li J, Jin Z, Long Y, et al. New starburst sensitizer with
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conjugated dithienylbenzothiadiazole-vinylene side chains for efficient solar
cells. Chem Commun 2011;47:9381e3.
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metal complexes in photonic and optoelectronic devices. Coordin Chem Rev
1998;177:347e414.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Nos. 51173154, 51003089).
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