Performance improvement of dye-sensitizing solar cell by semi-rigid triarylamine-based donors

Article abstract of DOI:10.1016/j.dyepig.2011.11.002

Novel organic dyes (IDB and ISB dyes), which contain 5-phenyl-iminodibenzyl (IDB) and 5-phenyl-iminostilbene (ISB) as electron donors and a cyanoacrylic acid moiety as an electron acceptor and an anchoring group, connected with a thiophene as a π-conjugated system, have been synthesized and used as the sensitizers for dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the dyes were investigated by absorption spectrometry, cyclic voltammetry and density functional theory calculations. As demonstrated, the IDB and ISB unit exhibited stronger electron-donating ability and broader absorption spectra when coated onto TiO₂. The DSSC based on ISB-2 consisting of ISB unit produced 5.83% of η (J_sc = 13.14 mA cm^-2, V_oc = 0.64 V, and ff = 0.68) under 100 mW cm ^-2 simulated AM 1.5 G solar irradiation.

Full text of DOI:10.1016/j.dyepig.2011.11.002

Dyes and Pigments 94 (2012) 40e48
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Performance improvement of dye-sensitizing solar cell by semi-rigid
triarylamine-based donors
*
Chengyou Wang, Jing Li, Shengyun Cai, Zhijun Ning, Dongmei Zhao, Qiong Zhang, Jian-Hua Su
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel organic dyes (IDB and ISB dyes), which contain 5-phenyl-iminodibenzyl (IDB) and 5-phenyl-
iminostilbene (ISB) as electron donors and a cyanoacrylic acid moiety as an electron acceptor and an
anchoring group, connected with a thiophene as a p-conjugated system, have been synthesized and used
Received 28 September 2011
Received in revised form
5 November 2011
Accepted 7 November 2011
Available online 17 November 2011
as the sensitizers for dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties
of the dyes were investigated by absorption spectrometry, cyclic voltammetry and density functional
theory calculations. As demonstrated, the IDB and ISB unit exhibited stronger electron-donating ability
and broader absorption spectra when coated onto TiO2. The DSSC based on ISB-2 consisting of ISB unit
Keywords:
produced 5.83% of
1.5 G solar irradiation.
h
(Jsc ¼ 13.14 mA cmꢀ2, Voc ¼ 0.64 V, and ff ¼ 0.68) under 100 mW cmꢀ2 simulated AM
Dye-sensitized solar cell
Semi-rigid triarylamine
Synthesis
Ó 2011 Elsevier Ltd. All rights reserved.
Characterization
Nanocrystalline TiO2
High efficiency
1. Introduction
Among various electron donors, triarylamine moieties have
been investigated widely due to their prominent electron-donating
ability, hole-transport properties, and prevention of direct charge
recombination between TiO2 and Iꢀ3 by the bulky aryl group
covering TiO2 surfaces [37]. Much work has been done to optimize
the structure of triarylamine to improve the performance of the
cells in our group [16,17,23,38]. However, one problem of triphe-
nylamine that need to be addressed is the rotation of the phenyl
rings, which causes serious energy loss. In our previous research, it
was observed that as the increase of the triphenylamine units, the
efficiency of the dyes decreases seriously. Hence, it can be specu-
lated that the efficiency of the dye can be effectively increased if the
phenyl rings were locked to prevent the rotation. Iminodibenzyl
and iminostilbene with two phenyl rings connected by alkyl and
ethylene chains have been widely used in materials of organic
light-emitting diodes (OLEDs), which contribute to the achieve-
ment of high luminescence quantum yield [39,40], however, there
are no report of utilizing iminodibenzyl and iminostilbene moiety
as electron donor for the organic sensitizers of DSSCs. Here, we
report the synthesis and characterization of four new organic dyes
that contain 5-phenyl-iminodibenzyl (IDB) and 5-phenyl-iminos-
As a potential alternative to conventional inorganic photovoltaic
devices, dye-sensitized solarcells (DSSCs), the so-called Grätzel cells
[1], have attracted ever-increasing attention in the past decades.
Several ruthenium-based sensitizers, such as N3, N719 and black
dye [2e5], have achieved remarkable power conversion efficiency of
10e11% under AM 1.5 G irradiation. Recent attentions have focused
on metal-free organic dyes as DSSC sensitizers because of their
strong molar absorption coefficient, ease of structure modification,
and low material cost [6]. Most metal-free dyes used for highly
efficient DSSCs follow a donore(p-spacer)eacceptor (DepeA)
architecture [7]. Generally, coumarin [8e10], indoline [11e14], tri-
phenylamine [15e20], tetrahydroquinoline [21,22], and carbazole
[23e27] have been widely employed as the electron donor unit with
good performance. Carboxylic acid, cyanoacrylic acid [28e32], and
rhodamine [33e35] moieties are often introduced into the (DepeA)
system as electron acceptors to fulfill these requirements described
above. The donors and the acceptors are linked by various
p-conjugated spacers such as polyene, oligophenylenevinylene,
oligothiophene and their derivatives. A series of indoline dyes have
shown an impressive high efficiency of 9% for DSSCs [32,36].
tilbene (ISB) as the electron donors, thiophene as
linkage, and cyanoacrylic acid as the electron acceptor and
anchoring group, shown in Fig. 1. Thiophene and alkene as the
conjugated systems between the donor and the acceptor were
p-conjugation
p
-
* Corresponding author. Tel./fax: þ86 21 64252288.
E-mail address: bbsjh@ecust.edu.cn (J.-H. Su).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.11.002

C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
41
Fig. 1. Molecular structures of IDB and ISB dyes.
extended to adjust the molecular HOMO and the LUMO energy
levels of the dyes, hence red-shifting and broadening the absorp-
tion spectra with the number of p-conjugations introduced [41]. On
the other hand, it was found that the bridge between the two rings
can affect the performance of the sensitizer significantly. To
understand the effect of the configuration to the performance of
the sensitizer, TDDFT calculation was performed.
2.2.3. 5-(4-Bromophenyl)-10,11-dihydro-5H-dibenzo[b,f]azepine (5)
Compound 3 (1.94 g, 7.16 mmol) was dissolved in DMF (50 mL).
While the solution was being stirred at 0 ꢁC, N-bromosuccinimide
(1.89 g, 10.74 mmol) was dissolved in DMF (50 mL) and added
dropwise to it. And the reaction mixture was then warmed to room
temperature. After being stirred for 12 h, the reaction mixture was
poured into water and extracted with diethyl ether, and the
combined extracts were washed with brine, dried over anhydrous
magnesium sulfate, and filtered. The solvent was removed by rotary
evaporation. The crude product was dissolved in hexane, and the
undissolved solid was filtered. The filtrate was evaporated to
dryness, and the resulting solid was washed with ethanol to yield
a white solid, 5 (1.56 g, 4.47 mmol, 62.4%). 1H NMR (400 MHz,
2. Experimental
2.1. General synthetic procedure and spectroscopic measurements
All chemicals were used as received from commercial sources
without purification. Solvents for chemical synthesis such as
dichloromethane (CH2Cl2), dimethylformamide (DMF) and tetra-
hydrofuran (THF) were purified by distillation. All chemical reac-
DMSO,
2H), 2.91 (s, 4H).
d
): 7.39e7.27 (m, 8H), 7.24 (d, J ¼ 8.9, 2H), 6.37 (d, J ¼ 8.9,
tions were carried out under nitrogen atmosphere. 1H NMR and 13
C
2.2.4. 5-(4-Bromophenyl)-5H-dibenzo[b,f]azepine (6)
The synthesis method resembled that of compound 5 and the
resulting solid was washed with ethanol to yield a white solid, 6
NMR spectra were recorded on Brucker AM-400 MHz instruments
with tetramethylsilane as internal standard. HRMS were performed
using a Waters LCT Premier XE spectrometer. The absorption
spectra of the dyes in solution and adsorbed on TiO2 films were
measured with a Varian Cary 500 spectrophotometer.
(1.25 g, 3.58 mmol, 68.5%). 1H NMR (400 MHz, CDCl3,
d): 7.56e7.41
(m, 6H), 7.40e7.32 (m, 2H), 7.12e7.00 (m, 2H), 6.82 (s, 2H), 6.12 (d,
J ¼ 9.1 Hz, 2H).
2.2.5. 5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)
thiophene-2-carbaldehyde (7)
2.2. Synthesis
A mixture ofcompound 3 (850 mg, 2.44 mmol), Pd(PPh3)4 (10 mg,
0.01 mmol), K2CO3 (270 mg,1.96 mmol), THF (10 mL) and H2O (5 mL)
was heated to 45 ꢁC under nitrogen atmosphere for 30 min. A stirred
solution of 5-formyl-2-thiophene-boronic acid (756 mg, 4.90 mmol)
in THF (5 mL) was added slowly, and the mixture was refluxed for
further 12 h. After cooling to room temperature, the mixture was
extracted withDCM. The organic portionwascombined and removed
by rotary evaporation. The residue was purified by column chroma-
tography on silica (DCM/PE ¼ 1/2, v/v) to give a yellow solid, 7
2.2.1. 5-Phenyl-10,11-dihydro-5H-dibenzo[b,f]azepine (3)
A mixture of iminodibenzyl (1, 4 g, 20.5 mmol), iodobenzene
(4.89 g, 23.9 mmol), copper powder (0.30 g, 4.6 mmol), potassium
carbonate (6.93 g, 50.2 mmol), and [18] crown-6 (0.33 g,1.25 mmol)
was heated in 1,2-dichlorobenzene (100 ml) at 180 ꢁC for 48 h
under an atmosphere of argon. The inorganic components were
removed by filtration after cooling. Then the solvent was distilled
under reduced pressure, and the crude product was purified by
column chromatography on silica (DCM/PE ¼ 1/3, v/v) to give
a white solid, 3 (3.57 g, 13.2 mmol, 64.4%). 1H NMR (400 MHz,
(434 mg, 1.14 mmol, 46.8%). 1H NMR (400 MHz, DMSO,
d): 9.75 (s,
1H), 7.62 (d, J ¼ 3.8 Hz,1H), 7.48 (d, J ¼ 8.9 Hz, 2H), 7.44e7.38 (m, 2H),
CDCl3,
d
): 7.45e7.38 (m, 2H), 7.27e7.18 (m, 6H), 7.09 (dd, J ¼ 8.8,
7.37e7.26 (m, 7H), 6.47 (d, J ¼ 8.9 Hz, 2H), 2.94 (s, 4H).
7.3 Hz, 2H), 6.70 (t, J ¼ 7.3 Hz, 1H), 6.58 (dd, J ¼ 8.8, 0.9 Hz, 2H), 2.98
(s, 4H).
2.2.6. 5-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)thiophene-2-
carbaldehyde (8)
2.2.2. 5-Phenyl-5H-dibenzo[b,f]azepine (4)
The synthesis method resembled that of compound 7 and the
crude compound was purified by column chromatography on silica
(DCM/PE ¼ 1/2, v/v) to yield a yellow solid, 8 (352 mg, 0.93 mmol,
The synthesis method resembled that of compound 3 and the
crude compound was purified by column chromatography on silica
(DCM/PE ¼ 1/3, v/v) to yield a white solid, 4 (2.98 g, 11.1 mmol,
49.2%). 1H NMR (400 MHz, DMSO,
d
): 9.80 (s, 1H), 7.92 (d, J ¼ 4.0 Hz,
60.8%). 1H NMR (400 MHz, CDCl3,
d): 7.54e7.46 (m, 4H), 7.44 (d,
1H), 7.64e7.54 (m, 6H), 7.51e7.43 (m, 4H), 7.42 (d, J ¼ 4.0 Hz, 1H),
J ¼ 7.4 Hz, 2H), 7.39e7.29 (m, 2H), 7.04e6.95 (m, 2H), 6.82 (s, 2H),
6.95 (s, 2H), 6.18 (d, J ¼ 8.9 Hz, 2H).
6.68 (t, J ¼ 7.3 Hz, 1H), 6.26 (dd, J ¼ 8.8, 0.9 Hz, 2H).

42
C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
2.2.7. 4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)benzaldehyde (9)
Phosphorus oxychloride (0.55 mL, 5.89 mmol) was added
dropwise to DMF (10 mL) at 0 ꢁC, and the mixture was stirred for
1 h at this temperature. Compound 3 (795 mg, 2.93 mmol) was
added and the reaction mixture was heated to 80 ꢁC for 8 h. The
mixture was subsequently cooled to room temperature, poured
into ice water, carefully neutralized with NaOH and extracted with
DCM. The combined organic extract was dried over anhydrous
MgSO4 and filtered. Solvent was removed by rotary evaporation
and the residue was purified by column chromatography on silica
(DCM/PE ¼ 1/1, v/v) to give a yellow solid, 9 (720 mg, 2.41 mmol,
47.8%). 1H NMR (400 MHz, DMSO,
d
): 9.82 (s, 1H), 7.89 (d, J ¼ 3.0 Hz,
1H), 7.56 (dd, J ¼ 15.9, 7.4 Hz, 6H), 7.46 (t, J ¼ 6.9 Hz, 2H), 7.29 (d,
J ¼ 8.0 Hz, 3H), 7.16 (d, J ¼ 16.1 Hz, 1H), 7.09 (d, J ¼ 16.1 Hz, 1H), 6.94
(s, 2H), 6.14 (d, J ¼ 8.2 Hz, 2H).
2.2.12. 3-(5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)
thiophen-2-yl)-2-cyanoacrylic acid (IDB-1)
A mixture of compound 7 (120 mg, 0.31 mmol), 2-cyanoacetic
acid (29 mg, 0.34 mmol), piperidine (0.5 mL) and THF (15 mL)
was heated to reflux under nitrogen atmosphere for 6 h. Solvent
was removed by rotary evaporation and the residue was purified by
column chromatography on silica (DCM/ethanol ¼ 20/1, v/v) to give
a red solid, IDB-1 (99 mg, 0.22 mmol, 71.3%). 1H NMR (400 MHz,
82.1%). 1H NMR (400 MHz, CDCl3,
d): 9.73 (s, 1H), 7.61 (d, J ¼ 9.0 Hz,
2H), 7.43e7.34 (m, 2H), 7.32e7.21 (m, 6H), 6.64 (d, J ¼ 8.9 Hz, 2H),
3.00 (s, 4H).
DMSO,
7.44e7.40 (m, 2H), 7.39e7.27 (m, 7H), 6.50 (d, J ¼ 8.9 Hz, 2H), 2.95
(s, 4H). 13C NMR (100 MHz, DMSO,
): 149.40, 148.99,142.46,137.66,
d
): 7.96 (s,1H), 7.58 (d, J ¼ 3.8 Hz,1H), 7.47 (d, J ¼ 8.9 Hz, 2H),
2.2.8. 4-(5H-Dibenzo[b,f]azepin-5-yl)benzaldehyde (10)
d
The synthesis method resembled that of compound 9 and the
crude compound was purified by column chromatography on silica
(DCM/PE ¼ 1/1, v/v) to yield a yellow solid, 10 (985 mg, 3.32 mmol,
136.39, 134.31, 131.13, 129.33, 127.60, 127.32, 126.96, 122.35, 122.10,
118.49, 112.43, 29.93. HRMS (ESI, m/z): [M þ H]þ calcd for
C28H21N2O2S, 449.1324; found, 449.1321.
78.4%). 1H NMR (400 MHz, CDCl3,
d): 9.69 (s, 1H), 7.55e7.46 (m, 8H),
7.43e7.37 (m, 2H), 6.86 (s, 2H), 6.35 (d, J ¼ 8.9 Hz, 2H).
2.2.13. 3-(5-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)thiophen-2-
yl)-2-cyanoacrylic acid (ISB-1)
2.2.9. 5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)
vinyl)thiophen [41]
The synthesis method resembled that of compound IDB-1 and
the crude compound was purified by column chromatography on
silica (DCM/ethanol ¼ 20/1, v/v) to yield a red solid, ISB-1 (102 mg,
A
mixture of compound 9 (650 mg, 2.17 mmol), t-BuOK
(292 mg, 2.61 mmol) and dry THF (20 mL) was stirred at ambient
temperature under nitrogen atmosphere for 1 h. 2-Thienylmethyl
triphenylphosphonium chloride (1290 mg, 3.25 mmol) was dis-
solved in THF and added dropwise to the solution, and the reaction
mixture was stirred for 1 h at ambient temperature, whereupon
the mixture was heated to reflux for 24 h. The reaction mixture
was allowed to cool to ambient temperature and a molar excess of
water was added. The mixture was concentrated by rotary evap-
orator and the water phase was extracted with DCM. The organic
phase was dried over MgSO4, filtered through a plug of silica gel
0.23 mmol, 72.3%). 1H NMR (400 MHz, DMSO,
d): 8.35 (s, 1H), 7.88
(d, J ¼ 4.1 Hz, 1H), 7.68e7.53 (m, 6H), 7.52e7.39 (m, 5H), 6.96 (s,
2H), 6.21 (d, J ¼ 8.9 Hz, 2H). 13C NMR (100 MHz, DMSO,
d): 163.95,
153.64, 149.61, 145.76, 141.55, 141.14, 135.55, 132.80, 130.51, 130.30,
130.16, 129.67, 127.69, 127.06, 122.74, 122.23, 117.06, 111.82. HRMS
(ESI, m/z): [M ꢀ H]ꢀ calcd for C28H17N2O2S, 445.1011; found,
445.1006.
2.2.14. 3-(5-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)styryl)
thiophene-2-yl)-2-cyanoacrylic acid (IDB-2)
(DCM) and
a
crude intermediate was obtained (720 mg,
The synthesis method resembled that of compound IDB-1 and
the crude compound was purified by column chromatography on
silica (DCM/ethanol ¼ 10/1, v/v) to yield a red solid, IDB-2 (90 mg,
1.90 mmol). The crude product was used in the next step without
further purification.
0.19 mmol, 68.7%). 1H NMR (400 MHz, DMSO,
d): 8.14 (s, 1H), 7.65
2.2.10. 5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)
vinyl)thiophen-2-carbaldehyde (11) [41]
(d, J ¼ 3.9 Hz, 1H), 7.44e7.33 (m, 6H), 7.30 (ddd, J ¼ 13.9, 7.0, 1.8 Hz,
4H), 7.24e7.16 (m, 2H), 7.00 (d, J ¼ 16.1 Hz, 1H), 6.44 (d, J ¼ 8.9 Hz,
5-(2-(4-(10,11-Dihydro-5H-dibenzo[b,f]azepin-5-yl)phenyl)
2H), 2.94 (s, 4H). 13C NMR (100 MHz, DMSO,
d): 164.11, 149.70,
vinyl)thiophen (720 mg, 1.90 mmol) was dissolved in dry THF
(20 mL) and was cooled to ꢀ78 ꢁC under nitrogen atmosphere. n-
Butyl lithium (0.75 mL, 2.5 M hexane solution) was added dropwise
over 10 min and the mixture was stirred at ꢀ78 ꢁC for 1 h. The
mixture was allowed to warm to 0 ꢁC and stirred for 30 min. The
mixture was once again cooled to ꢀ78 ꢁC and DMF (0.17 mL,
2.09 mmol) was added. The reaction mixture was allowed to warm to
ambient temperature and stirred for 2 h. The reaction was quenched
by the addition of aqueous HCl (10%, 100 mL) and extracted with
DCM.Thecombined organicextractwasdried overanhydrous MgSO4
and filtered. Solvent was removed by rotary evaporation and the
residue was purified by column chromatography on silica (DCM/
PE ¼ 1/1, v/v) to give an orange solid, 11 (347 mg, 0.85 mmol, 45.0%).
148.87, 142.55, 142.05, 137.68, 137.42, 134.10, 131.32, 131.12, 129.39,
128.15, 127.54, 127.29, 125.83, 125.61, 118.73, 117.17, 112.21, 29.94.
HRMS (ESI, m/z): [M þ H]þ calcd for C30H23N2O2S, 475.1480; found,
475.1485.
2.2.15. 3-(5-(4-(5H-dibenzo[b,f]azepin-5-yl)styryl)thiophene-2-yl)-
2-cyanoacrylic acid (ISB-2)
The synthesis method resembled that of compound IDB-1 and
the crude compound was purified by column chromatography on
silica (DCM/ethanol ¼ 10/1, v/v) to yield a purple solid, ISB-2
(103 mg, 0.22 mmol, 70.5%). 1H NMR (400 MHz, DMSO,
d): 13.59 (s,
1H), 8.40 (s, 1H), 7.88 (d, J ¼ 4.1 Hz, 1H), 7.58 (tt, J ¼ 7.8, 4.1 Hz, 6H),
7.50e7.41 (m, 2H), 7.31 (dd, J ¼ 6.4, 5.0 Hz, 3H), 7.21 (d, J ¼ 16.1 Hz,
1H), 7.08 (d, J ¼ 16.1 Hz, 1H), 6.95 (s, 2H), 6.14 (d, J ¼ 8.9 Hz, 2H). 13C
1H NMR (400 MHz, CDCl3,
d): 9.77 (s,1H), 7.52 (d, J ¼ 3.9 Hz,1H), 7.42
(d, J ¼ 7.6 Hz, 2H), 7.29e7.18 (m, 6H), 7.11 (d, J ¼ 8.7 Hz, 2H), 7.06 (d,
J ¼ 3.9 Hz,1H), 6.63 (d, J ¼ 12.0 Hz,1H), 6.53 (d, J ¼ 8.7 Hz, 2H), 6.45(d,
J ¼ 12.0 Hz, 1H), 3.00 (s, 4H).
NMR (100 MHz, DMSO, d): 163.86, 153.32, 149.14, 146.30, 141.76,
141.52, 135.63, 133.06, 132.91, 130.50, 130.31, 130.11, 129.78, 128.10,
127.59, 126.05, 125.82, 116.80, 116.73, 111.55. HRMS (ESI, m/z):
[M ꢀ H]ꢀ calcd for C30H19N2O2S, 471.1167; found, 471.1156.
2.2.11. 5-(2-(4-(5H-Dibenzo[b,f]azepin-5-yl)phenyl)vinyl)thiophen-
2-carbaldehyde (12)
2.3. Theoretical calculation
The synthesis method resembled that of compound 11 and the
crude compound was purified by column chromatography on silica
(DCM/PE ¼ 1/1, v/v) to yield a yellow solid, 12 (298 mg, 0.79 mmol,
Density functional theory (DFT) calculations were conducted by
using the B3LYP hybrid functional for the geometry optimizations.

C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
43
Scheme 1. Synthesis of the sensitizers (IDB-1, ISB-1, IDB-2, and ISB-2).
The molecular orbital levels of HOMO and LUMO were achieved
with the 6-31G(d) basis set implemented in the Gaussian 03
package.
0.5 M 4-tert-butylpyridine (4-TBP) in acetonitrile, the other was
composed of 0.1 M lithium iodide, 0.6 M methyl-propylimidazolum
iodide (MPII) and 0.05 M I2 in the mixed solvent of acetonitrile and
3-methoxypropionitrile (7:3, v/v).
2.4. Electrochemical measurements
2.6. Photoelectrochemical measurements
The cyclic voltammograms were determined with a Versastat II
electrochemical workstation (Princeton Applied Research) using
a three-electrode cell with a Pt working electrode, a Pt wire
auxiliary electrode, and a saturated calomel reference electrode in
saturated KCl solution. The supporting electrolyte was 0.1 M TBAPF6
(tetra-n-butylammonium hexafluorophosphate) in acetonitrile as
the solvent.
Photovoltaic measurements employed an AM 1.5 solar simulator
equipped with a 300 W xenon lamp (Model No. 91160, Oriel). The
power of the simulated light was calibrated to 100 MW/cm2 using
a Newport Oriel PV reference cell system (Model 91150 V). IeV
curves were obtained by applying an external bias to the cell and
measuring the generated photocurrent with a Keithley model 2400
digital source meter. The voltage step and delay time of photocur-
rent were 10 mV and 40 ms, respectively. Cell active area tested
with a mask of 0.158 cm2. The photocurrent action spectra were
measured with an IPCE test system consists of a Model SR830 DSP
Lock-In Amplifier and Model SR540 Optical Chopper (Stanford
Research Corporation, USA), a 7IL/PX150 xenon lamp and power
supply, and a 7ISW301 Spectrometer.
2.5. General procedure for preparation
The dye-sensitized TiO2 electrode was prepared by following the
procedure reported in the literature [3]. TiO2 colloidal dispersion
was made employing commercial TiO2 (P25, Degussa AG, Germany)
as material. Films of nanocrystalline TiO2 colloidal on FTO were
prepared by sliding a glass rod over the conductive side of the FTO,
followed by calcinations at 450 ꢁC for 30 min. Before immersion in
the dye solution, these films were immersed into a 40 mM aqueous
TiCl4 solution at 70 ꢁC for 30 min and washed with water and
ethanol. Then the films were heated again at 450 ꢁC followed by
cooling to 80 ꢁC and soaked in dye solutions (0.3 mM in chloroform)
for at least 12 h. The adsorbed TiO2 electrode and Pt counter elec-
trode were assembled into a sealed sandwich-type cell by heating
with a hot-melt ionomer film (Surlyn 1702, DuPont). The redox
electrolyte was placed in a drilled hole in the counter electrode by
capillary force, and was driven into the cell by means of vacuum
backfilling. Finally, the hole was sealed using a UV-melt gum and
a cover glass. Two electrolytes were used for device evaluation,
in which one was composed of 0.1 M lithium iodide, 0.6 M
1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M I2, and
2.7. Electrical impedance measurements
Electrical impedance experiments were performed in the dark
with CHI660C electrochemical work station, with a frequency range
from 0.01 Hz to 100 kHz and a potential modulation of 10 mV. The
applied bias potential is held at ꢀ0.60 V.
3. Results and discussion
3.1. Synthesis
The synthetic strategy, illustrated in Scheme 1, involves the
Ullmann coupling reaction, the Suzuki coupling reaction, the
Knoevenagel reaction, and the Wittig reaction to extend the
Table 1
Experimental data for spectral and electrochemical properties of the dyes.
a
Dyes
lmax (nm)a
3
(Mꢀ1 cmꢀ1
)
lmax on TiO2 (nm)b
HOMO (V) vs. NHEc
E0e0 (eV)d
LUMO (V) vs. NHEe
IBD-1
ISB-1
IDB-2
ISB-2
422
470
467
498
27,000
39,000
34,000
42,000
420
418
466
439
1.12
1.19
1.05
0.96
2.08
2.05
2.03
2.01
ꢀ0.96
ꢀ0.86
ꢀ0.98
ꢀ1.05
a
Absorption maximum in CH2Cl2 solution (3ꢂ10ꢀ5 M).
b
c
Absorption maximum on TiO2 film.
HOMO were measured in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: FTO/TiO2/dye; reference elec-
trode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an external reference. Counter electrode: Pt).
d
E0e0 was estimated from the absorption thresholds from absorption spectra of dyes adsorbed on the TiO2 film.
e
LUMO is estimated by subtracting E0e0 from the HOMO.

44
C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
6 were obtained by bromination reaction of IDB and ISB using N-
bromosuccinimide (NBS) followed by the MichaeliseArbuzor
reaction [42,43]. The Suzuki coupling reaction was made with the
respective bromides and 5-formyl-2-thiophene-boronic acid to
yield the aldehydes 7 and 8. Aldehydes 9 and 10 were prepared
from 3 and 4 by means of the VilsmeiereHaack reaction [44,45]. In
the cases of linker extension by the Wittig reaction, formylation of
the thiophene with n-BuLi and DMF was applied [41]. Finally, the
target products (IDB-1, ISB-1, IDB-2, and ISB-2) were synthesized
via the Knoevenagel condensation reaction of the respective alde-
hydes with cyanoacetic acid in the presence of piperidine, and their
chemical structures were fully characterized with 1H NMR, 13C NMR
and HRMS.
3.2. Absorption properties in solutions and on TiO2 film
Table 1 presents the experimental spectral and electrochemical
properties of the IDB-1, ISB-1, IDB-2, and ISB-2 dyes. Normalized
adsorption spectra of the dyes in dichloromethane diluted solution
and on TiO2 films were shown in Fig. 2. In dichloromethane solu-
tion, all these dyes exhibit a relatively broad and strong absorption
band in the visible region corresponding to the intramolecular
charge transfer (ICT) band (Fig. 2a). The absorption peaks (lmax) for
IDB-1, ISB-1, IDB-2, and ISB-2 located at 422, 470, 467 and 498 nm
in diluted solution, respectively. Judging from that of IDB-1 and
ISB-1, the absorption band of IDB-2 and ISB-2 showed a large red-
shift upon increased linker conjugation. Obviously, in the series of
sensitizers containing the same cyanoacetic acid unit and electron
transport channels, the ICT band is dependent upon the donor
units. As listed in Table 1, the red-shift by 48 or 31 nm was observed
when replacing IDB unit (IDB-1 and IDB-2) with ISB unit (ISB-1
and ISB-2) in the donor moiety, indicative of the more powerful
electron-donating capability of the ISB unit than that of the IDB
unit.
Generally, when sensitizers are anchored onto nanocrystalline
TiO2 surface, the deprotonation and aggregation of the dye mole-
cules affect UV-vis absorption profiles. Deprotonation and H-
aggregates always result in blue-shift of absorption peak, while J-
aggregates mainly lead to the red-shift of absorption peak. Like
Fig. 2. Normalized absorption spectra of the IDB-1, ISB-1, IDB-2, and ISB-2 dyes in
CH2Cl2 (a) and anchored on 6.5 mm TiO2 films (b).
thiophene linker. The IDB and ISB as electron donors were
synthesized via the Ullmann coupling reaction of iminodibenzyl
and iminostilbene with iodobenzene in the presence of copper
powder, potassium carbonate and [18] crown-6. Compounds 5 and
most D-
of ISB dyes (ISB-1 and ISB-2) on 6.5
blue-shift (52 and 59 nm, respectively) with respect to
p-A conjugated organic sensitizers, the ICT absorption peak
mm TiO2 films exhibit a large
Fig. 3. The optimized geometries of the IDB-1, IDB-2, ISB-1, and ISB-2 calculated with DFT on the B3LYP/6-31G(d) level.

C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
45
measurements taken in dichloromethane solution (Table 1). Since
the aggregation tendency is not obvious within such thin films of
TiO2, here the shift in absorption peak might be predominated by
deprotonation effect. Unexpectedly, the ICT absorption peaks of IDB
dyes (IDB-1 and IDB-2) are slightly blue-shifted by only 2 nm from
422 nm in solution to 420 nm on TiO2, and 1 nm from 467 nm in
solution to 466 nm on TiO2, respectively. Thus iminodibenzyl unit
in IDB dyes countervails the deprotonation effect. It is noteworthy
that the absorption spectrum of the dyes adsorbed onto 6.5 mm
TiO2 films shows a markedly broad profile, which is beneficial to
light-harvesting (Fig. 2b).
3.3. Theoretical calculation
To gain insight into the geometrical properties and scrutinize
the charge transfer on excitation, the ground-state geometries of
the protonated dyes have been optimized in the gas phase by DFT
with the Gaussian09 package [46], using the hybrid B3LYP [47]
functional and the standard 6-31G(d) basis set. The TDDFT calcu-
lations were performed with MPW1 K [48] functional and 6-31G(d)
basis set on the B3LYP optimized ground-state geometries. Solva-
tion effect was included in the TDDFT calculations in CH2Cl2 with
the nonequilibrium version of the C-PCM model [49]. As illustrated
in Fig. 3, the dihedral angles between the phenyl B1 planes and the
phenyl B2 planes of the ISB dyes are higher than those of the IDB
dyes. The phenomenon is due to the rigid iminostilbene substituent
which would increase the steric strain and cause the iminostilbene
moiety to twist slightly out of the plane defined by phenyl B1. The
angle formed between the phenyl B1 plane and the phenyl B2 plane
of the ISB-2 is as large as 82ꢁ, which can help to inhibit the close
Fig. 5. Oxidative cyclic voltammetry plots of IDB-1, ISB-1, IDB-2, and ISB-2 attached to
a 6.5 mm nanocrystalline TiO2 film deposited on conducting FTO glass.
MeCN
containing
0.1 M
tetra-n-butylammonium
hexa-
fluorophosphate (TBAPF6) with a 0.05 V sꢀ1 scan rate. CV was
employed to evaluate the redox stability and oxidation potential of
the dyes. As estimated from the absorption threshold of these dyes
onto TiO2 films, the excitation transition energy (E0e0) of IDB-1,
IDB-2, ISB-1, and ISB-2 is 2.08, 2.03, 2.05 and 2.01 eV, respectively
(Table 1). The HOMO values of IDB-1, IDB-2, ISB-1, and ISB-2 cor-
responding to their first redox potential were 1.12, 1.05, 1.19 and
0.96 V vs. NHE, respectively. Observed from these electrochemical
data of IDB-1 and ISB-1, or IDB-2 and ISB-2, it seems that the
HOMO energy level is exclusively determined by donors. The esti-
mated excited-state potential corresponding to the LUMO levels of
pe
p
aggregation. As illustrated in Fig. 4, the HOMO orbital of the
dyes is primarily located at the -framework of the donor part and
p
thiophene group, while the electron density of the LUMO is delo-
calized over the thiophene group and anchoring group. Examina-
tion of the frontier orbitals of all sensitizers suggests that the
HOMO-LUMO excitation would shift the electron density distri-
bution from the donor unit to the anchoring moiety, facilitating
efficient photoinduced/interfacial electron injection from excited
dyes to the TiO2 electrode.
IDB-1, IDB-2, ISB-1, and ISB-2, calculated from EHOMO ꢀ E0e0
,
are ꢀ0.96, ꢀ0.98, ꢀ0.86 and ꢀ1.05 V vs. NHE, respectively. Judging
from the LUMO value, the four dyes are more negative than the
bottom of the conduction band of TiO2 (ꢀ0.5 V), indicating that the
electron injection process from the excited dye molecule to TiO2
conduction band is energetically permitted [50e52].
3.4. Electrochemical properties
3.5. Performances of dye-sensitized solar cells
Fig. 5 depicts the cyclic voltammogram (CV) of IDB-1, IDB-2, ISB-
Action spectrum, incident photon-to-electron conversion effi-
ciency (IPCE) as a function of wavelength, was measured to eval-
uate the photoresponse of photoelectrode in the whole spectral
1, and ISB-2, measured with four sensitizers attached to 6.5
mm
nanocrystalline TiO2 films deposited on conducting FTO glass in
Fig. 4. Frontier molecular orbitals (HOMO and LUMO) of the IDB-1, ISB-1, IDB-2, and ISB-2 calculated with DFT on the B3LYP/6-31G(d) level.

46
C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
Table 2
DSSC performance data of the dyes.a
Dyes
Jsc/mA cmꢀ2
Voc/mV
ff
h (%)
IDB-1
ISB-1
IDB-2
ISB-2
N719
10.67
11.04
11.30
13.14
16.10
657
665
665
649
689
0.68
0.72
0.69
0.68
0.67
4.80
5.27
5.21
5.83
7.47
a
Illumination: 100 mW cmꢀ2 simulated AM 1.5 G solar light. Electrolyte con-
tained 0.1 M LiI, 0.6 M MPII, 0.1 M I2, 1.0 M 4-TBP in the mixed solvent of acetonitrile
and 3-methoxypropionitrile (7:3, v/v).
649 mV, and 0.68, respectively, yielding an overall conversion
efficiency (
on IDB-2 shows lower Jsc, leading to an inferior
(Table 2). For a fair comparison, the N719-sensitized DSSC was also
fabricated under the same conditions and yielded value of 7.47%.
h
) of 5.83%. Under the same conditions, the device based
h
value of 5.21%
h
The higher Jsc values of the IDB-2 and ISB-2-based cells than those
of the IDB-1 and ISB-1-based cells are consistent with the values
for the IPCE spectra due to the increased linker conjugation. In
comparison with the IDB-1 and IDB-2-based cells, the ISB-1 and
ISB-2-based cells show higher Jsc, respectively, reflecting the better
sunlight-harvesting ability of the ISB dyes.
Fig. 6. Photocurrent action spectra of the TiO2 electrodes sensitized by IDB-1, ISB-1,
IDB-2, and ISB-2.
region. The IDB-1, IDB-2, ISB-1, and ISB-2 sensitizers have been
used to manufacture solar cell devices by using 8 (transparent) þ 4
(scattering) mm TiO2 layers. Fig. 6 shows the IPCE obtained with
a sandwich cell by using 0.1 M lithium iodide, 0.6 M methyl-
propylimidazolum iodide (MPII) and 0.05 M I2 in the mixed
solvent of acetonitrile and 3-methoxypropionitrile (7:3, v/v) as
redox electrolyte. The photocurrent action spectra of all sensitizers
exhibit very high plateaus where IPCE values reach 85%. Consid-
ering the light absorption and scattering loss by the conducting
glass, the maximum efficiency for absorbed photon-to-collected
electron conversion efficiency is almost unity over a broad spec-
tral range, suggesting a very high electron injection efficiency of
these dyes. Solar cells based on IDB-1 and ISB-1 showed high IPCE
(85%) but not broad spectra, resulting in lower overall efficiencies
due to spectral limitations. Solar cells based on IDB-2 and ISB-2
show broad IPCEs in accordance to the broad absorption spectrum
achieved by increased linker conjugation.
Fig. 7 shows the currentevoltage (JeV) curves of the DSSCs
under standard global AM 1.5 G solar irradiation. The short-circuit
photocurrent density (Jsc), open-circuit voltage (Voc), and fill
factor (ff) of the solar cell based on ISB-2 are 13.14 mA cmꢀ2
,
Fig. 7. Photocurrentevoltage curves of dyes IDB-1, IDB-2, ISB-1, and ISB-2 sensitized
TiO2 electrodes under irradiation of AM 1.5 G simulated solar light (100 mW cmꢀ2
with liquid electrolyte.
)
Fig. 8. Impedance spectra of DSSCs based on IDB-1, ISB-1, IDB-2, and ISB-2 dyes
measured at ꢀ0.60 V bias in the dark. (a) Nyquist plots; and (b) Bode phase plots.

C. Wang et al. / Dyes and Pigments 94 (2012) 40e48
47
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Electrochemical impedance spectroscopy (EIS) analysis was
performed to elucidate the photovoltaic findings further. Fig. 8
compares the impedance spectra for IDB-1, ISB-1, IDB-2, and ISB-
2-sensitized cells measured in the dark under a forward bias
of ꢀ0.60 V with a frequency range of 0.1 Hz to 100 kHz. The
important differences for these organic dye-sensitized solar cells
are their conductivity. The Nyquist plots (Fig. 8a) show the radius of
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lifetime values derived from curve fitting are 70.18, 101.52, 73.15
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sensitized cells indicates more effective suppression of the back
reaction of the injected electron with the Iꢀ3 in the electrolyte and is
reflected in the improvements on Voc. The Bode phase plots shown
in Fig. 8b likewise support the differences in the electron lifetime
for TiO2 films derivatized with the four dyes and the result is
agreement with the observed shit in the Voc value under standard
global AM 1.5 illumination.
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4. Conclusion
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In summary, four new organic sensitizers (IDB-1, ISB-1, IDB-2,
and ISB-2) containing 5-phenyl-iminodibenzyl (IDB) and 5-phenyl-
iminostilbene (ISB) as electron donors had been designed and
synthesized for dye-sensitized solar cells. These dyes were
successfully adsorbed on nanocrystalline anatase TiO2 particles,
and subsequently efficient dye-sensitized solar cells had been
fabricated. It was found that the dyes containing 5-phenyl-imi-
nostilbene (ISB) as electron donors exhibited improved photovol-
taic performance compared with the ones containing 5-phenyl-
iminodibenzyl (IDB) as electron donors. A solar cell device based on
the sensitizer ISB-2 yielded a higher overall conversion efficiency
up to 5.83% (Jsc ¼ 13.14 mA cmꢀ2, Voc ¼ 0.64 V, and ff ¼ 0.68) under
100 mW cmꢀ2 simulated AM 1.5 G solar irradiation. The introduc-
tion of increased linker conjugation appears to contribute to higher
Jsc and IPCE in DSSCs. Our findings demonstrate that the IDB and ISB
organic sensitizers are promising for further improvement of the
conversion efficiency of DSSCs owing to the original and versatile
molecular design.
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Acknowledgments
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This work is supported by NSFC/China, National Basic Research
973 Program (2006CB806200) and Scientific Committee of
Shanghai. Wang CY thanks Prof. Hua JL, Dr. Wu. WJ, Zhang ZY,
Huang XH, Zou Q, and Zhang HY in our laboratory for their assis-
tance in the experiments.
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