Metal-free organic dyes derived from triphenylethylene for dye-sensitized solar cells: Tuning of the performance by phenothiazine and carbazole

Article abstract of DOI:10.1039/c2jm30254c

Four novel D-D-π-A configuration metal-free organic dyes (C3, P2, C2 and P3) with triphenylethylene phenothiazine moieties or triphenylethylene carbazole moieties as additional electron donors for dye-sensitized solar cells (DSSCs) have been synthesized. The cells based on C3, P2, C2 and P3 dyes with efficiencies of 2.14%, 2.69%, 5.51% and 6.55%, respectively, are obtained. The P3 based cell exhibits the highest efficiency of 6.55% accompanied by a short-circuit current density (J_sc) of 12.18 mA cm^-2, a rather high open-circuit photovoltage (V_oc) of 826 mV, and a fill factor (ff) of 0.65, performances which are remarkable in the DSSCs based on metal-free organic dyes. The twisted non-planar configuration in P3 decelerates the charge recombination in the charge-separated state and hence contributes to the improvement of the performance of DSSCs.

Full text of DOI:10.1039/c2jm30254c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 8994
www.rsc.org/materials
PAPER
Metal-free organic dyes derived from triphenylethylene for dye-sensitized
solar cells: tuning of the performance by phenothiazine and carbazole†
*
Chengjian Chen,‡ Jin-Yun Liao,‡ Zhenguo Chi, Bingjia Xu, Xiqi Zhang, Dai-Bin Kuang, Yi Zhang, Siwei Liu
*
and Jiarui Xu
Received 13th January 2012, Accepted 6th March 2012
DOI: 10.1039/c2jm30254c
Four novel D–D–p–A configuration metal-free organic dyes (C3, P2, C2 and P3) with
triphenylethylene phenothiazine moieties or triphenylethylene carbazole moieties as additional electron
donors for dye-sensitized solar cells (DSSCs) have been synthesized. The cells based on C3, P2, C2 and
P3 dyes with efficiencies of 2.14%, 2.69%, 5.51% and 6.55%, respectively, are obtained. The P3 based
cell exhibits the highest efficiency of 6.55% accompanied by a short-circuit current density (Jsc) of 12.18
mA cmꢀ2, a rather high open-circuit photovoltage (Voc) of 826 mV, and a fill factor (ff) of 0.65,
performances which are remarkable in the DSSCs based on metal-free organic dyes. The twisted
non-planar configuration in P3 decelerates the charge recombination in the charge-separated state
and hence contributes to the improvement of the performance of DSSCs.
progress. In the last decade, numerous investigations on
Introduction
p-conjugated molecules with donor–acceptor moieties, such as
Over the past two decades, dye-sensitized solar cells (DSSCs),
oligothiophene,4 indoline,5 triphenylamine6 and coumarin7 have
regarded as one of the most promising alternatives to the
been conducted.
currently used commercial silicon based solar cells, have attrac-
In general, the performance of DSSCs depends on the highest
ted increased attention because of their low-cost and acceptable
occupied molecular orbital (HOMO) and lowest unoccupied
conversion efficiency potential.1 As a critical component in
molecular orbital (LUMO) energy levels of the relative sensi-
DSSCs, the sensitizer plays a vital role in the light harvesting
tizers and the kinetics of the electron-transfer processes.8
efficiency providing electron injection into the conduction band
Therefore, the molecular structure of the organic dyes plays an
of an oxide semiconductor (e.g. TiO2) upon light excitation.
important role in DSSCs and several design principles have been
Nowadays, the maximum power conversion efficiency (h) of
DSSCs based on the Zn-complex dye has achieved 12.3%.2 Our
concluded as follows: (1) The HOMO of organic dyes should be
below the energy levels of the redox mediator (e.g. Iꢀ/I3ꢀ) and the
recent reports on hierarchical TiO2 nanostructure photo-
relative LUMO should be above the conduction band edge of the
electrodes consisting of both nanoparticles and one-dimensional
semiconductor (e.g. TiO2) electrode. (2) The electron distribution
nanorods showed significant photovoltaic performance using
of HOMO should be mainly localized on the donor part, which is
N719 and CdS/CdSe as sensitizers because of its multi-functional
further away from the semiconductor surface, and the electron
properties (excellent electron transport, large surface area and
distribution of LUMO should be mainly localized near the
superior light scattering).3 In recent years, organic dyes have
anchoring group (e.g. carboxylic acid). And (3) a wide absorp-
gained interest because of their variety of molecular structures,
tion range of organic dyes covering the whole visible and some of
high molar extinction coefficient, simple and environment
the near-infrared or infrared region is required for molecular
friendly preparation process and low cost. As a promising and
structure design, at the same time, the molar extinction coeffi-
cost-effective alternative to traditional metal-complex dyes,
cient should be high enough.9 However, the systematic study of
DSSCs based on metal-free organic dyes have made remarkable
the effects of twisted non-planar structures in organic dyes still
needs to be further improved.
PCFM Lab, DSAPM Lab, MOE Key Laboratory of Bioinorganic and
Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry,
based on triphenylamine and carbazole have been reported by
State Key Laboratory of Optoelectronic Materials and Technologies,
Very recently, the D–D–p–A type of metal-free organic dyes
Tian’s group.10 This type of dyes show a bathochromic shift,
School of Chemistry and Chemical Engineering, Sun Yat-sen University,
Guangzhou 510275, P. R. China. E-mail: chizhg@mail.sysu.edu.cn;
kuangdb@mail.sysu.edu.cn; Fax: +86 20 84112222; Tel: +86 20 84112712
† Electronic supplementary information (ESI) available: Fig. S1–S5,
Tables S1 and S2, NMR and MS spectra (Fig. S6–S38) of the
intermediates and target products. See DOI: 10.1039/c2jm30254c
‡ These authors contributed equally to the preparation of this work.
broad and intense absorption in the visible light region, as well as
better thermo-stability as compared with the D–p–A type. These
studies have suggested that the HOMO and LUMO energy level
tuning of organic dyes can be achieved by attaching additional
donor moieties to increasing the hole transporting ability of dyes.
8994 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Consequently, the D–D–p–A configuration of organic dyes has
been reported by Tian to demonstrate superior performance over
the simple D–p–A configuration in DSSCs. A further improve-
ment was expected to be accomplished by introducing triphe-
nylethylene phenothiazine or triphenylethylene carbazole
moieties at the end of each molecule to form a D–D–p–A
structure following the reports on the twisted structure of tri-
phenylethylene carbazole derivatives for aggregation-induced
emission (AIE).11 The triphenylethylene structure was expected
to extend the absorption region, reduce the tendency to aggregate
on the TiO2 surface, act as a hole transporter, as well as tune the
HOMO and LUMO energy levels at the same time. Phenothia-
zine and carbazole are aromatic heterocyclic compounds with
electron-rich nitrogen or sulfur heteroatoms, and have attracted
great interest for their excellent hole-transporting ability, rigid
structure and large p conjugated system.
Ultra-pure water was used in the experiments. The intermediates,
C2B,12 3a13 and 3b,14 were synthesized according to the literature
methods.
Measurements and characterization
1H NMR and 13C NMR spectra were measured on a Mercury-
Plus 300 spectrometer, and chemical shifts were reported in ppm
(CDCl3 as solvent and TMS as the internal standard). Mass
spectra (MS) were measured on a Thermo DSQ MS spectrom-
eter. Elemental analyses (EA) were performed with an Elementar
Vario EL elemental analyzer. UV-visible absorption spectra
(UV) were determined on a Hitachi U-3900 spectrophotometer.
Photoluminescence (PL) spectra were measured on a Shimadzu
RF-5301PC spectrometer with a slit width of 3 nm for both
excitation and emission. The DMF-water mixtures with different
water fractions were prepared by slowly adding ultra-pure water
into the DMF solution of samples under ultrasound at room
temperature. Cyclic voltammetry (CV) measurement was carried
out on a Shanghai Chenhua electrochemical workstation
CHI660C in a three-electrode cell with a Pt disk counter elec-
trode, a Ag/AgCl reference electrode and a glassy carbon
working electrode. All CV measurements were performed under
an inert argon atmosphere with supporting electrolyte of 0.1 M
tetrabutylammonium perchlorate (n-Bu4NClO4) in dichloro-
methane (MC) at a scan rate of 100 mV sꢀ1 using ferrocene (4.80
eV vs. vacuum) as standard. The highest occupied molecular
orbital (HOMO) energy levels were obtained using the onsets
oxidation potentials from the CV curves. The lowest unoccupied
molecular orbital/highest occupied molecular orbital (LUMO/
HOMO) energy gaps DEg for the compounds were estimated
from the onsets absorption of UV-vis absorption spectra in MC.
In the current study, special consideration was given to the
design of the molecular structure to determine its correlation to
the performance of DSSCs, a series of organic dyes whose
structures were slightly different from one to another were
prepared (Fig. 1). Afterwards, the effects of different donor
structure units on the photophysical, photochemical and
photovoltaic properties were investigated. To the best of our
knowledge, there are no systemic reports on the introduction of
AIE twisted structure into a dye molecule for DSSC.
Experimental section
Materials
Bis(4-fluorophenyl)methanone, 9H-carbazole, 10H-phenothia-
zine, 1-bromohexane, diethyl 4-bromobenzylphosphonate,
potassium tert-butoxide, n-butyllithium in hexane (2.2 M),
trimethylborate, bis(pinacolato)diboron, [1,10-bis(diphenyl-
phosphino)ferrocene] palladiumm(II) chloride [Pd(dppf)Cl2],
N-bromosuccinimide (NBS), cynaoacetic acid, Aliquat 336, tet-
rakis (triphenyl-phosphine) palladium(0) and tetrabutylammo-
nium perchlorate (electrochemical grade) purchased from Alfa
Aesar were used as received, All other reagents and solvents were
purchased as analytical grade from Guangzhou Dongzheng
Company (China) and used without further purification. Tetra-
hydrofuran (THF) was distilled from sodium/benzophenone.
Preparation and fabrication of DSSCs
Fluorine-doped tin oxide (FTO) glasses were cleaned in the order
of detergent solution, water and ethanol using an ultrasonic bath
overnight. Firstly, the TiO2 (20 nm particle size) paste was
screen-printed onto FTO coated glass (15 U/square, Nippon
Sheet Glass, Japan). The paste fabrication process was carried
out according to the previous paper.15 Briefly, TiO2 powder
(20 nm) (1.0 g) was ground for 40 min in the mixture of ethanol
(8.0 mL), acetic acid (0.2 mL), terpineol (3.0 g) and ethyl cellulose
(0.5 g) to form a slurry, and then the slurry was sonicated for
5 min in an ultrasonic bath, to finally form a viscous white TiO2
paste. The thickness of films can be easily controlled through
repeating screen-printing times. Afterwards, a heating process
(325 ꢁC for 5 min, at 375 ꢁC for 5 min, at 450 ꢁC for 15 min, and
then at 500 ꢁC for 15 min) to remove the organic substances was
used. The as-prepared TiO2 nanostructured films (TiO2/FTO)
were soaked in a 0.04 M aqueous solution of TiCl4 for 30 min at
70 ꢁC, which improved the photocurrent and photovoltaic
performance. The TiCl4-treated as-prepared films were rinsed
ꢁ
with deionized water and ethanol, and then sintered at 520 C
ꢁ
for 30 min. After cooling to 80 C, the films were immersed in
a 5.0 ꢂ 10ꢀ4 M solution of the C3, P2, C2, P3 and N719 dyes
(Solaronix SA, Switzerland) in MC, and sensitized for about
16 h. Afterwards, the films were rinsed with MC or acetonitrile
(N719) to remove physisorbed dye molecules. To evaluate their
Fig. 1 Molecular structures of the target organic dyes, D1 and T2-1.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 8995

View Article Online
photovoltaic performance, the dye-sensitized TiO2 nano-
structured FTO films were sandwiched together with Pt-coated
FTO glass, which was used as a counter electrode. Platinized
counter electrodes were fabricated by thermally depositing
H2PtCl6 solution (5 mM in isopropanol) onto FTO glass. The
electrolyte (0.6 M 1-methyl-3-propylimidazolium iodide (PMII),
0.10 M guanidinium thiocyanate, 0.03 M I2, 0.5 M tert-butyl-
pyridine in acetonitrile, and valeronitrile (85 : 15)) was injected
into the space (25 mm) between the sandwiched cells. The active
area of the dye-coated TiO2 film was 0.16 cm2.
(75MHz, CDCl3) d (ppm): 147.44, 142.68, 142.42, 133.73, 133.46,
132.86, 132.60, 132.35, 129.24, 128.85, 128.59, 128.27, 127.60,
127.34, 125.41, 125.00, 122.55, 121.52; MS (EI), m/z: 576 ([M]+,
calcd for C37H24N2OS2, 576).
Synthesis of 10,100-(4,40-(2-(4-bromophenyl)ethene-1,1-diyl)
bis(4,1-phenylene))bis(10H-phenothiazine) (P2Br)
A mixture of P2K (15.0 g, 26.0 mmol) and potassium tert-but-
oxide (3.5 g, 31.3 mmol) in anhydrous THF (250 mL) was stirred
under an argon atmosphere at 0 ꢁC. Diethyl-4-bromobenzyl-
phosphonate (12.0 g, 39.1 mmol) was added dropwise and the
mixture was stirred continuously for 3 h at room temperature.
Then the reaction mixture was poured into ethanol, the precip-
itate was filtered and washed with ethanol three times. A purified
P2Br (16.2 g, 86% yield) was obtained as a yellow solid by
recrystallization from MC/n-hexane. 1H NMR (300 MHz,
CDCl3) d (ppm): 7.60 (d, 2H), 7.45 (d, 2H), 7.38 (t, 4H), 7.28
(t, 4H), 7.07 (t, 4H), 6.95 (d, 4H), 6.91 (s, 1H), 6.86 (d, 4H), 6.38
(t, 4H); 13C NMR (75MHz, CDCl3) d (ppm): 144.07, 132.88,
131.38, 130.50, 129.82, 127.24, 127.09, 123.13, 117.15, 117.03;
MS (EI), m/z: 730 ([M + H]+, calcd for C44H29BrN2S2, 729).
Photovoltaic performance measurements
To measure the amount of adsorbed dye on the TiO2 film, the
dye/TiO2 film was immersed in a 0.1 M aqueous solution of
NaOH (3.0 mL) and UV/Vis absorption spectra of the desorbed
dye solution were measured by using a Hitachi U-3900 spectro-
photometer. The current–voltage characteristics of DSSCs were
recorded using a Keithley 2400 source meter under one sun AM
1.5 G (100 mW cmꢀ2) illumination with a solar light simulator
(Oriel, Model: 91192). A 1000 W xenon arc lamp (Oriel, Model:
6271) was served as a light source and its incident light intensity
was calibrated with a NREL-calibrated Si solar cell equipped
with a optical filter to approximate AM 1.5 G one sun light
intensity before each measurement. The incident photon-to-
current efficiency (IPCE) spectra were measured as a function of
wavelength from 350 to 800 nm on the basis of a Spectral
Products DK240 monochromator. The electrochemical imped-
ance spectroscopy (EIS) measurements were held on the Zahner
Zennium electrochemical workstation in dark condition, with an
applied bias of ꢀ0.75 V. A 10 mV AC sinusoidal signal was
employed over the constant bias with the frequency ranging
between 1 MHz and 10 MHz. The impedance parameters were
determined by fitting of the impedance spectra using Z-view
software. Intensity-modulated photovoltage spectroscopy
(IMVS) and intensity-modulated photocurrent spectroscopy
(IMPS) measurements were carried out on an electrochemical
workstation (Zahner, Zennium) with a frequency response
analyzer under a modulated green light emitting diode (457 nm)
driven by a source supply (Zahner, PP211), which can provide
both DC and AC components of the illumination. The modu-
lated light intensity was 10% or less than the base light intensity.
The frequency range was set from 100 KHz to 0.1 Hz.
Synthesis of 10,100-(4,40-(2-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)ethene-1,1-diyl)bis(4,1-phenylene))
bis(10H-phenothiazine) (P2B)
P2Br (7.0 g, 9.6 mmol) and bis(pinacolato)diboron (5.1 g,
20 mmol) were dissolved in 1,4-dioxane (50 mL), and potassium
acetate (5.9 g, 60 mmol) was added. The mixture was stirred for
15 min under an argon atmosphere at room temperature. Then
the Pd(dppf)Cl2 (0.05 g) catalyst was added and the reaction
mixture was stirred overnight at 80 ꢁC. After cooling to room
temperature, the reaction was quenched by adding water,
extracted with MC and dried with anhydrous sodium sulfate.
After evaporation of the solvent under reduced pressure, the
residue was chromatographed on a silica gel column with MC/n-
hexane (1/2, v/v) as eluant. Green yellow power of P2B was
obtained in 41% yield (3.1 g). 1H NMR (300 MHz, CDCl3)
d (ppm): 7.63 (d, 4H), 7.46 (d, 2H), 7.37 (dd, 4H), 7.17 (s, 1H),
7.04–7.11 (m, 6H), 6.82–7.01 (m, 8H), 6.37 (d, 4H), 1.35 (s, 12H);
13C NMR (75MHz, CDCl3) d (ppm): 144.21, 134.62, 133.12,
130.68, 129.98, 129.86, 129.13, 127.19, 127.11, 123.00, 121.43,
121.23, 117.04, 116.86, 84.09, 25.28; MS (EI), m/z: 776 ([M]+,
calcd for C50H41BN2O2S2, 776).
Synthesis of bis(4-(10H-phenothiazin-10-yl)phenyl)methanone
(P2K)
Synthesis of 6-(4-(2,2-bis(4-(10H-phenothiazin-10-yl)phenyl)
vinyl)phenyl)-9-hexyl-9H-carbazole-3-carbaldehyde (4a)
To a solution of phenothiazine (20.0 g, 101 mmol) in anhydrous
DMF (150 mL), potassium tert-butoxide (13.0 g, 121 mmol) was
added under an argon atmosphere. The mixture was heated to
60 ꢁC and bis(4-fluorophenyl)methanone (10.0 g, 45.9 mmol) was
added with viꢁgorous stirring. The reaction mixture was then
heated to 110 C for 12 h. The resulting mixture was cooled to
room temperature, poured into ice-water, filtered and washed
with acetone three times to collect the yellow solid, which was
then dissolved in CH2Cl2, dried over anhydrous Na2SO4 and
rotary evaporated. A yellow powder was obtained (21.8 g, 83%
To a solution of 3a (0.48 g, 1.3 mmol) and P2B (1.0 g, 1.3 mmol)
in THF (25 mL), 2 M aqueous K2CO3 solution (5 mL) and
Aliquat 336 (5 drops) were added. The mixture was stirred for
15 min under an argon atmosphere at room temperature. Then
the Pd (PPh3)4 (0.05 g) catalyst was added and the reaction
mixture was stirred at 80 ꢁC for 24 h. After cooling to room
temperature, the product was concentrated and purified with
silica gel column chromatography using MC/n-hexane (1/1, v/v)
as eluant. Light yellow power of 4a (0.78 g, 64% yield)
was obtained. 1H NMR (300 MHz, CDCl3) d (ppm): 10.01
(s, 1H), 8.63 (s, 1H), 8.33 (s, 1H), 8.03 (d, 1H), 7.75 (d, 1H)
1
yield). H NMR(300 MHz, CDCl3) d (ppm): 7.84 (d, 4H), 7.27
(t, 8H), 7.14 (t, 4H), 7.05 (t, 4H), 6.92 (d, 4H); 13C NMR
8996 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
7.66 (d, 2H), 7.38–7.58 (m, 10H), 7.23 (d, 2H), 7.21 (s, 1H),
7.07 (m, 4H), 6.87–7.00 (m, 8H), 6.42 (q, 4H), 4.37 (t, 2H),
1.93 (m, 2H), 1.27–1.44 (m, 6H), 0.89 (t, 3H); 13C NMR
(75MHz, CDCl3) d (ppm): 191.69, 144.64, 144.22, 142.10,141.00,
140.88, 140.80, 140.50, 139.97, 135.76, 133.26, 133.09, 130.84,
130.45, 130.03, 129.74, 129.32, 128.88, 127.47, 127.19, 127.08,
126.96, 126.09, 124.31, 123.78, 123.34, 123.00, 121.44, 119.21,
117.02, 116.85, 109.94, 109.41, 43.96, 31.88, 29.34, 27.30, 22.91,
14.39; MS (EI), m/z: 928 ([M + H]+, calcd for C63H49N3OS2,
927).
Synthesis of 3-(6-(4-(2,2-bis(4-(10H-phenothiazin-10-yl)phenyl)
vinyl)phenyl)-9-hexyl-9H-carbazol-3-yl)-2-cyanoacrylic acid
(P2)
In a three-necked flask under an argon atmosphere, 4a (0.20 g,
0.22 mmol) was dissolved in acetonitrile (35 mL). Piperidine
(0.2 mL) was added dropwise to the reaction mixture. The stir-
ring was continued for 10 min, then cyanoacetic acid (0.05 g,
0.65 mmol) was added and the mixture was heated to 90 ꢁC
for 3 h. After evaporation of the solvent under reduced pressure,
the residue was purified by column chromatography using silica
gel and dichloromthane/methanol (10/1, v/v) as the eluent to
give P2 (0.06 g, 30% yield) as a yellow power. 1H NMR
(300 MHz, CDCl3) d (ppm): 8.79 (s, 1H), 8.46 (s, 1H), 8.31
(s, 1H), 8.26 (d, 1H), 8.05 (s, 1H), 7.64–7.80 (m, 3H), 7.28–7.59
(m, 10H), 7.22 (t, 3H), 7.06 (m, 4H), 6.86–6.99 (m, 7H), 6.42
(q, 4H), 4.35 (t, 2H), 1.93 (m, 2H), 1.30–1.42 (m, 6H), 0.89
(t, 3H); 13C NMR (75MHz, CDCl3) d (ppm): 167.84, 166.28,
161.97, 157.49, 144.25, 142.14, 141.03, 139.90, 135.89, 133.07,
130.72, 130.44, 130.00, 129.75, 127.08, 123.95, 123.00, 121.44,
119.39, 117.04, 113.16, 109.97, 96.88, 87.35, 47.83, 44.02, 31.83,
29.33, 27.26, 24.88, 22.86, 14.34; MS (EI), m/z: 995 ([M + H]+,
calcd for C66H50N4O2S2, 994). Anal. Calc. for C66H50N4O2S2: C
79.65, H 5.06, N 5.63, S 6.44; Found: C 79.27, H 4.92, N 5.51,
S 6.53.
Synthesis of 7-(4-(2,2-bis(4-(10H-phenothiazin-10-yl)phenyl)
vinyl)phenyl)-10-hexyl-10H-phenothiazine-3-carbaldehyde (4b)
The synthesis method resembles that of compound 4a. Yellow
1
power of 4b (0.80 g, 63% yield) was obtained. H NMR (300
MHz, CDCl3) d (ppm): 9.79 (s, 1H), 7.59–7.66 (m, 4H), 7.51 (d,
2H), 7.23–7.43 (m, 8H) 7.16 (d, 2H), 7.13 (s, 1H), 7.04–7.09
(t, 4H), 6.82–6.98 (m, 10H), 6.40 (q, 4H), 3.91 (t, 2H), 1.85 (m,
2H), 1.47 (t, 2H), 1.32–1.37 (m, 4H) 0.90 (t, 3H); 13C NMR
(75MHz, CDCl3) d (ppm): 190.01, 150.48, 144.20, 142.88, 142.02,
141.18, 141.07, 140.87, 139.76, 138.40, 136.24, 136.04, 133.00,
131.30, 130.67, 130.37, 129.95, 129.74, 129.10, 128.57, 127.44,
127.20, 127.08, 126.18, 126.07, 125.84, 124.74, 124.44, 123.05,
121.60, 121.47, 117.06, 116.94, 116.31, 114.98, 48.46, 31.75,
27.10, 26.90, 22.96, 14.37; MS (EI), m/z: 959 ([M]+, calcd for
C63H49N3OS3, 959).
Synthesis of 3-(7-(4-(2,2-bis(4-(10H-phenothiazin-10-yl)phenyl)
vinyl)phenyl)-10-hexyl-10H-phenothiazin-3-yl)-2-cyanoacrylic
acid (P3)
Synthesis of 6-(4-(2,2-bis(4-(9H-carbazol-9-yl)phenyl)vinyl)
phenyl)-9-hexyl-9H-carbazole-3-carbaldehyde (5a)
The synthesis method resembles that of compound P2. Purple
1
red power of P3 (0.17 g, 77% yield) was obtained. H NMR
The synthesis method resembles that of compound 4a. Light
1
green power of 5a (0.64 g, 47% yield) was obtained. H NMR
(300 MHz, CDCl3) d (ppm): 8.04 (s, 1H), 6.80–7.73 (m, 31H),
6.34 (s, 4H), 3.59 (d, 3H), 0.99–1.27 (m, 10H); 13C NMR
(75MHz, CDCl3) d (ppm): 160.82, 158.11, 144.15, 141.04, 139.73,
136.05, 132.89, 130.69, 130.60, 129.82, 129.67, 127.13, 123.04,
121.60, 117.10, 114.70, 105.26, 83.27, 46.96, 30.05, 26.81, 22.95,
14.38; MS (EI), m/z: 1027 ([M + H]+, calcd for C66H50N4O2S3,
1026); Anal. Calc. for C66H50N4O2S3: C 77.16, H 4.91, N5.45, S
9.36; Found: C 77.04, H 4.83, N5.38, S 9.42.
(300 MHz, CDCl3) d (ppm): 10.09 (s, 1H), 8.65 (s, 1H), 8.38
(s, 1H), 8.17 (q, 4H), 8.03 (d, 1H), 7.42–7.78 (m, 21H), 7.27–7.35
(m, 7H), 4.37 (t, 2H), 1.93 (m, 2H), 1.31–1.40 (m, 6H), 0.89
(t, 3H); 13C NMR (75MHz, CDCl3) d (ppm): 191.71, 144.64,
142.12, 140.89, 140.46, 139.47, 137.38, 135.79, 133.25, 132.24,
130.47, 129.34, 129.13, 128.90, 127.59, 127.36, 127.08, 126.20,
124.44, 123.74, 123.36, 120.78, 120.64, 120.56, 120.32, 119.20,
110.09, 110.03, 109.41, 43.95, 31.88, 29.34, 27.31, 22.90, 14.38;
MS (EI), m/z: 864 ([M + H]+, calcd for C63H49N3O, 863).
Synthesis of 3-(7-(4-(2,2-bis(4-(9H-carbazol-9-yl)phenyl) vinyl)
phenyl)-10-hexyl-10H-phenothiazin-3-yl)-2- cyanoacryl-ic acid
(C2)
Synthesis of 7-(4-(2,2-bis(4-(9H-carbazol-9-yl)phenyl)vinyl)
phenyl)-10-hexyl-10H-phenothiazine-3-carbaldehyde (5b)
The synthesis method resembles that of compound P2. Purple
1
red power of C2 (0.06 g, 29% yield) was obtained. H NMR
The synthesis method resembles compound 4a. Light yellow
1
power of 5b (0.52 g, 46% yield) was obtained. H NMR (300
(300 MHz, CDCl3) d (ppm): 8.16 (q, 3H), 8.07 (s, 1H), 7.91
(q, 1H), 7.21–7.70 (m, 29H), 6.89 (q, 2H), 3.91 (t, 2H), 1.85
(m, 2H), 1.27–1.48 (m, 6H), 0.91 (t, 3H); 13C NMR (75MHz,
CDCl3) d (ppm): 167.92, 166.89, 154.86, 150.11, 142.10, 141.30,
140.92, 139.33, 138.18, 137.45, 136.40, 132.18, 130.41, 129.13,
127.57, 127.03, 126.21, 123.74, 120.62, 120.33, 115.99, 112.80,
110.08, 97.51, 48.58, 31.74, 26.88, 24.91, 22.96, 14.36; MS (EI),
m/z: 963 ([M + H]+, calcd for C66H50N4O2S, 962); Anal. Calc.
for C66H50N4O2S: C 82.30, H 5.23, N 5.82, S 3.33; Found: C
82.25, H 5.15, N 5.76, S 3.41.
MHz, CDCl3) d (ppm): 9.79 (s, 1H), 8.17 (q, 4H), 7.28–7.70
(m, 26H), 7.24 (s, 1H), 7.21 (s, 2H), 6.91 (q, 2H), 3.92 (t, 2H), 1.85
(m, 2H), 1.47 (m, 2H), 1.35 (m, 4H), 0.90 (t, 3H); 13C NMR
(75MHz, CDCl3) d (ppm): 190.02, 150.50, 142.86, 142.03, 141.22,
140.92, 139.34, 138.37, 137.43, 136.27, 136.05, 132.19, 131.29,
130.39, 129.14, 128.60, 127.57, 127.03, 126.30, 126.21, 124.74,
124.44, 123.74, 120.63, 120.33, 116.30, 114.96, 110.08, 110.00,
48.45, 31.76, 27.09, 26.90, 22.96, 14.38; MS (EI), m/z: 895 ([M]+,
calcd for C63H49N3OS, 895).
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 8997

View Article Online
Synthesis of 3-(6-(4-(2,2-bis(4-(9H-carbazol-9-yl)phenyl)vinyl)
phenyl)-9-hexyl-9H-carbazol-3-yl)-2-cyanoacrylic acid (C3)
The synthesis method resembles compound P2. Yellow power of
C3 (0.07g, 33% yield) was obtained. 1H NMR (300 MHz, CDCl3)
d (ppm): 8.80 (s, 1H), 8.46 (s, 1H), 8.35 (s, 1H), 8.27 (d, 1H), 8.17
(d, 4H), 8.06 (s, 1H), 7.80 (d, 1H), 7.36–7.74 (m, 20H), 7.31 (t,
6H), 4.36 (t, 2H), 1.93 (t, 2H), 1.33 (m, 6H), 0.89 (t, 3H); 13C
NMR (75MHz, CDCl3) d (ppm): 168.48, 166.75, 144.38, 140.91,
139.46, 137.39, 135.93, 133.90, 132.23, 130.46, 129.81, 129.14,
127.59, 127.16, 127.04, 126.58, 126.20, 124.00, 123.74, 122.69,
120.31, 112.75, 110.09, 96.44, 61.52, 31.83, 29.32, 27.26, 24.86,
22.86, 14.33; MS (EI), m/z: 931 ([M + H]+, calcd for C66H50N4O2,
930); Anal. Calc. for C66H50N4O2: C 85.13, H 5.41, N 6.02;
Found: C 84.76, H 5.35, N 6.12.
Results and discussion
The synthetic routes of P2B and C2B are shown in Scheme 1. The
triphenylethylene phenothiazine derivative is first introduced
into the organic dyes to reduce the tendency to aggregate, as well
as to tune the HOMO and LUMO energy levels. The interme-
diate compounds P2Br and C2B were synthesized according to
procedures in a previously published study by our laboratory.12
The boric acid ester moiety of P2B was introduced by the
addition of bis(pinacolato)diboron to a tetrahydrfuran solution
of P2Br in the presence of Pd(dppf)Cl2.
The synthetic routes of the target compounds, C3, P2, C2 and
P3, are shown in Scheme 2. The triphenylethylene derivatives,
which have a twisted structure, were introduced by the Suzuki
cross-coupling reaction. The introduction of carbazole and
phenothiazine units into the twisted structure of triphenyl-
ethylene derivatives may potentially improve the performance of
solid-state DSSCs considering its excellent hole-transporting
ability. Compounds 3a and 3b were prepared from 9H-carbazole
and 10H-phenothiazine via N-alkylation reaction with
Scheme 2 Synthetic routes for C3, P2, C2 and P3.
1-bromohexane, bromination reaction with N-bromosuccini-
mide (NBS), as well as Vilsmeier–Haack reaction with POCl3 and
N,N-dimethylformamide (DMF) according to the literature
procedures.13,14 Compounds 4a, 4b, 5a and 5b were synthesized
via Suzuki reactions. The molecular structures of the products
were confirmed using proton and carbon-13 nuclear magnetic
resonance spectra, mass spectrometry and elemental analysis.
The ultraviolet (UV)-visible absorption spectra of C3, P2, C2
and P3 dyes were measured in MC, in DMF and on TiO2 film.
Table 1 summarizes the photophysical properties of all the as-
synthesized dyes. Fig. 2(a) shows the UV-vis spectra of the dyes
in MC solutions. Dyes C3 and C2 have several absorption peaks
in the ultraviolet region of 238, 293, and 329 nm to 341 nm, as
well as in the visible light region of 412 nm for C3 and 462 nm for
C2. Evidently, the visible absorption maximum of dye C2 is
50 nm red-shifted in contrast to C3. Dyes P2 and P3 have three
prominent absorption peaks in the regions of 258, 326 nm to
335 nm, and 412 nm for P2 and 466 for P3 nm. In accordance,
the visible absorption maximum of dye P3 compared with P2 is
54 nm red-shifted.
For these four dyes, the absorption maxima (DMF solutions,
Fig. 2b) in the ultraviolet region show a bathochromic shift;
however, the absorption maxima in the visible light region show
Scheme 1 Synthetic routes for P2B and C2B.
8998 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Table 1 Photophysical properties of the as-synthesized dyes
Emission
Absorption
labs / nm (3b/10 4M ꢀ1 cm ꢀ1
a
Dye
)
lemc/nm
lemd/nm
Amounte/10 ꢀ7mol cm ꢀ2
C3
P2
C2
P3
238 (5.7), 293 (3.2), 341 (3.0), 412 (1.6)
258 (7.2), 335 (3.7), 412 (2.1)
238 (8.4), 293 (4.8), 329 (4.1), 341 (4.0), 462 (1.3)
258 (8.8), 326 (4.5), 466 (1.4)
551
538
537
629
502 (528)
422 (429)
541 (581)
545 (593)
2.8
2.1
3.4
4.1
a
b
Absorption spectra in MC at room temperature. The molar extinction coefficient at lmax of the absorption spectra. The emission spectra was
c
d
measured in MC (10 mM) at room temperature. The emission spectra was measured in DMF (10 mM) at room temperature and the data in
parentheses was measured in 80% water fraction of water–DMF mixture. e Amount of the dyes adsorbed on TiO2 film.
a blue shift compared with the spectra in MC solutions (see
Table 1). This interesting phenomenon is ascribed to the depro-
tonation of cyanoacrylic acid derivatives in DMF. Generally,
because of the basic nature of DMF the equilibrium will shift to
deprotonated species lead to blue shifted charge transfer
absorption. It was verified by solvatochromism and acid/base
studies. In various solvents, such as toluene (TOL), MC, tetra-
hydrofuran (THF), DMF, and dimethylsulfoxide (DMSO), only
the solutions of the dyes in DMF show pronounced aforemen-
tioned wavelength changes (Fig. S1, ESI†); And the aborption
spectra were strongly affected by the alkalinity or acidity of the
solutions (Fig. S2, ESI†).
could provide the thermodynamic driving for charge generation
relative to the conduction band edge of the semiconductor elec-
trode (TiO2, ꢀ4.00 eV).18 Comparing the LUMO levels of the
dyes, the values of C2 and P3 are sufficiently more negative than
those of C3 and P2, which may be the main reason for the higher
efficiencies of C2 and P3. Accordingly, there may be an exact
value of LUMO that better matches the conduction band edge of
TiO2. Compared with N719,19 the HOMO and LUMO of these
dyes have lower energy values, which indicates a higher stability
against oxidation and an advantage for solar cell applications.20
To gain insight into the molecular structure and electron
distribution, the geometries of the dyes were optimized by
density functional theory calculations at the B3LYP/6-31G
level21 (Fig. 4). As shown in Fig. 4, for C3 and C2, the electron
density of the HOMO state is distributed along the triphenyl-
ethylene carbazole moiety. However, for P2 and P3, the electron
density of the HOMO state is distributed along one of the end
phenothiazines of the triphenylethylene phenothiazine moiety.
It may imply that the nonplanar phenothiazine blocked by
triphenylethylene can make electron distribution of HOMO
separate from electron distribution of LUMO (see Fig. 4). Under
the intramolecular charge transfer along the p-conjugated skel-
eton, the LUMO of the dyes is localized near the anchoring
group (carboxylic acid) through the p-conjugated linker, which
is beneficial for efficient electron injection into the anode. Thus,
the HOMO–LUMO excitation induced by light irradiation could
move the excited electron distribution from the triphenylethylene
unit to the electron-accepting cyano acid group, and injected into
the conduction band of TiO2 through the anchoring group. The
calculated HOMO and LUMO values are summarized in
Table 2. From Table 2, a part of the experimental values is not in
agreement with the calculated values, which is a common
phenomenon in many related literatures. However, comparing
the experimental and calculated values independently reveals
that the donor close to the acceptor is a determining factor of the
electron distribution of the LUMO, whereas the donor blocked
by triphenylethylene is a determining factor of the electron
distribution of the HOMO. The dipole moments (m) of the dyes
were calculated and also summarized in Table 2. It can be seen
that the m values followed the order P3 > C3 > C2 > P2. The
polarizability of P3 is the highest among these dyes. Generally, as
more m induces more negative charge to TiO2 conduction band
and larger polarizability causes stronger interaction with
surrounding acceptor species. This factor is one of possible
reasons why P3 exhibits the highest performance of DSSC.
As shown in Fig. 2(c), the absorption spectra on TiO2 are
broadened as compared with that in MC (Fig. 2a), and the
absorption in the ultraviolet region decreases; however, the
absorption in the visible light region increases. Interestingly, this
phenomenon can also be observed in water/DMF solutions with
different volume fractions of water (Fig. 2d) and could be
explained as follows: after the addition of 50% volume fraction of
water, C3, P2, C2 and P3 in the water/DMF mixtures began to
aggregate into nanoparticles. The nanoparticle suspensions were
transparent with no precipitate and were still stable after two
days, and probably consisted of J-aggregate formation, which is
always accompanied with a bathochromic shift of the UV
absorption maximum.16 As a consequence, the emission spectra
also display a bathochromic shift in DMF as compared with the
data in 80% water fraction of water-DMF mixture (see Table 1).
Furthermore, the better absorption of P3 in contrast to C2 and
of P2 in contrast to C3 (see Fig. 2a and c) implies that the
absorption of triphenylethylene phenothiazine moiety is superior
to the triphenylethylene carbazole moiety.
To evaluate the possibility of transfer from an excited dye
ox
molecule to the conduction band of TiO2, the oxidation (Eonset
)
potentials vs. Ag/AgCl of the dyes were measured in MC
(0.5 mM) by cyclic voltammetry (CV) for the calculation of the
HOMO (Fig. 3). As shown in Table 2, the HOMO levels of C3,
P2, C2 and P3 were ꢀ5.02, ꢀ4.99, ꢀ5.03 and ꢀ5.01 eV,
respectively. All the values could lead to an efficient regeneration
of the oxidized dye in contrast to ꢀ4.60 eV17 of iodide, avoiding
the geminate charge recombination between oxidized dye mole-
cules and photo-injected electrons in the nano-crystalline TiO2
film. The optical band gaps estimated from the onset absorption
spectra were 2.45, 2.42, 1.92 and 1.89 eV, respectively, for C3, P2,
C2 and P3. The calculated LUMO levels of C3, P2, C2 and P3
were ꢀ2.57, ꢀ2.57, ꢀ3.10 and ꢀ3.12 eV, respectively, which
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 8999

View Article Online
Fig. 3 Cyclic voltammetry curves of C3, P2, C2 and P3 in MC of 0.1 mol
Lꢀ1 n-Bu4NClO4 electrolyte at a scan rate of 100 mV sꢀ1
.
Table 2 Electrochemical properties of the organic dyes
Dye
C3
P2
C2
P3
EHOMOa /eV
DEgb /eV
ꢀ5.02
2.45
ꢀ2.57
ꢀ5.31
ꢀ2.37
2.94
ꢀ4.99
2.42
ꢀ2.57
ꢀ5.06
ꢀ2.42
2.64
ꢀ5.03
1.92
ꢀ3.10
ꢀ5.33
ꢀ2.58
2.75
ꢀ5.01
1.89
ꢀ3.12
ꢀ5.01
ꢀ2.53
2.48
ELUMOc /eV
EHOMOd /eV
ELUMOd /eV
DEgd /eV
m
de/Debye
7.3554
5.8812
6.2532
8.5339
a
The HOMO is derived from a comparison with the ionization potential
of ferrocene (0.5 mmol Lꢀ1) in MC with 0.1 mol Lꢀ1 n-Bu4NClO4 as
electrolyte (scanning rate, 100 mV sꢀ1
working electrode, glassy
carbon; counter electrode, Pt disk; and reference electrode, Ag/AgCl).
;
b
The band gap is calculated from the absorption onset of the
absorption spectrum. The LUMO is calculated from the band gap
c
d
and HOMO value. Calculated at the B3LYP/6-31G level. m refers to
e
dipole moment.
Fig. 5(a). The photovoltaic properties of these dye-based DSSCs
are summarized in Table 3. The DSSC based on C3 shows an
efficiency of 2.14%, with a short-circuit photocurrent density
(Jsc) of 4.55 mA cmꢀ2, an open-circuit photovoltage (Voc) of
682 mV, and a fill factor (ff) of 0.69. However, replacing the
triphenylethylene carbazole unit with a more twisted triphenyl-
ethylene phenothiazine unit to construct P2, the performance of
DSSC based on P2 improved (h ¼ 2.69%, Jsc ¼ 5.27 mA cmꢀ2
,
Voc ¼ 711 mV, and ff ¼ 0.72). To further study the correlation
between the molecular structure and their performance in
DSSCs, we replaced 9-hexyl-9H-carbazole in C3 with a non-
planar 10-hexyl-10H-phenothiazine to construct C2, the perfor-
mance of DSSC based on C2 improved rapidly (h ¼ 5.51%, Jsc
¼
10.76 mA cmꢀ2, Voc ¼ 793 mV, and ff ¼ 0.64). Afterwards, we
replaced the triphenylethylene carbazole unit in C2 with a more
twisted triphenylethylene phenothiazine unit to construct P3,
a further increase in performance of the DSSC based on P3 was
achieved (h ¼ 6.55%, Jsc ¼ 12.18 mA cmꢀ2, Voc ¼ 826 mV, and
ff ¼ 0.64). Therefore, the increasingly twisted structures in C3,
P2, C2 and P3 match the order of their increasing performance in
DSSCs. Under the same measurement conditions, the DSSC
Fig. 2 UV-vis absorption spectra of C3, P2, C2 and P3 in MC (a), in
DMF (b), on TiO2 film (c) as well as in water and DMF mixtures (d).
The photocurrent density-photovoltage (J–V) curves of the
DSSCs based on the C3, P2, C2 and P3 performed under
simulated AM 1.5 solar irradiation (100 mW cmꢀ2) are shown in
9000 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 4 HOMO and LUMO calculated at the B3LYP/6-31G level of C3,
P2, C2 and P3.
based on dye N719 generated an efficiency of 8.84% (Jsc ¼ 16.91
mA cmꢀ2, Voc ¼ 783 mV, and ff ¼ 0.67), and clearly, the DSSCs
based on C2 and P3 possessed higher Voc.
Fig. 5 (a) Photocurrent density–photovoltage curves of C3, P2, C2 and
P3 under illumination of 100 mW cmꢀ2 AM 1.5G sunlight. (b) IPCE
spectra of the DSSCs based on the as-synthesized organic dyes.
The comparisons of P3 with P2, and C2 with C3, reveal that
the electron-richer and twisted non-planar phenothiazine may be
considered more favorable than the planar carbazole, given that
it should better match the device. Furthermore, the non-planar
structure also can decelerate the charge recombination in the
charge-separated state resulting in a higher Voc.9 The higher
open-circuit voltage values of P3 relative to C2 and P2 relative to
C3 can be predicted, given that the triphenylethylene phenothi-
azine unit possesses a better-twisted non-planar structure than
the triphenylethylene carbazole unit. Therefore, the triphenyl-
ethylene phenothiazine unit can more efficiently decelerate the
charge recombination in the charge-separated state and reduce
the tendency of aggregation, considering that aggregation leads
to unfavorable back electron transfer and decreases the Voc value
of the device.22 Most AIE derivatives have twisted non-planar
structures, hence, Voc may be improved by introducing the AIE
structure unit after comparing the open-circuit voltage of P3 and
C2 with that of D1 (595 mV)23a and T2-1 (712 mV).23b
Table 3 Photovoltaic performance parameters of the DSSCs based on
C3, P2, C2, P3, and N719 dyesa
Dye
Jsc/mA cmꢀ2
Voc/mV
Fill factor (ff)
h(%)
C3
P2
C2
P3
4.55
5.27
10.76
12.18
16.91
682
711
793
826
783
0.69
0.72
0.64
0.65
0.67
2.14
2.69
5.51
6.55
8.84
N719
a
Measured under 100 mW cmꢀ2 of simulated AM 1.5 G solar light (100
mW cmꢀ2) at room temperature.
phenomenon has already been reported and the exact reason still
needs further studies.23b In our case, a possible reason is that the
non-planar 10-hexyl-10H-phenothiazine unit is connected to the
anchoring carboxylic acid moiety only by a conducting ethylene
unit, which makes it very close to the TiO2 film. Thus, the non-
planar 10-hexyl-10H-phenothiazine unit is easily influenced by
the photo-injected electrons in TiO2 film, and this effect probably
has an effect of planarization on the non-planar phenothiazine,
which could lead to a bathochromic shift. However, carbazole is
a planar structure, as can be predicted, the onsets of the IPCE
spectra of C3 and P2 are nearly equal to that of their UV-vis
absorption spectra on the TiO2 film.
Fig. 5(b) shows the incident photon-to-current efficiency
(IPCE) data as a function of the wavelength for DSSCs based on
the C3, P2, C2 and P3 dyes. The IPCE spectra changing tendency
of the dyes are in accordance with their UV-vis absorption
spectra on the TiO2 film. However, the onsets of the IPCE
spectra for C2 and P3 are 720 and 720 nm, respectively, which are
significantly broadened, as compared with those of 663 and
665 nm for their UV-vis absorption spectra on the TiO2 film. This
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 9001

View Article Online
The IPCE values of DSSCs based on C2 and P3 exceeded 70%
from 450 nm to 480 nm (with the highest value of 71.4% at
470 nm) and from 400 to 540 nm (with the highest value of 79.3%
at 480 nm), respectively. In the DSSCs based on C3 and P2, the
IPCE values relatively exceed 70% from 410 nm to 420 nm (with
the highest value of 70.7% at 420 nm) and from 360 nm to 440 nm
(with the highest value of 78.5% at 420 nm). Comparing C2 with
P3 and C3 with P2, it is revealed that the triphenylethylene
phenothiazine donor unit in P3 and P2 can be considered as the
main reason for the higher IPCE values, which would lead to the
broadened high IPCE performance (>70%). The higher and
broader spectra response of the P3 based cell within the whole
visible region might contribute to a higher photocurrent. The
higher IPCE at a range of 350–550 nm for P2-sensitized solar cell
than that of the C3-sensitized DSSC could be attributed to the
higher molar absorption coefficient (3) in shorter wavelength of
C3. And one of the possible reasons for the higher IPCE at
a range of 350–700 nm for P3 cell as compared to C2 cell could be
ascribed to the larger adsorbed amount of P3. Comparing the
IPCE spectrum of P3 with T2-123b indicates that the introduction
of the triphenylethylene phenothiazine unit may be an effective
method of broadening high IPCE performance.
To further elucidate the photovoltaic results and obtain more
interfacial charge transfer information in DSSCs sensitized by C3,
P2, C2 and P3, electrochemical impedance spectroscopy (EIS)
was also performed in the dark under a forward bias of 0.75 V. The
Nyquist and Bode plots for C3, P2, C2 and P3 sensitized cells are
shown in Fig. 6a and 6b, respectively. In general, a typical EIS
spectrum of a DSSC exhibits three semicircles in the Nyquist
plots, which are located in order of increasing frequency and are
assigned to the Nernst diffusion within the electrolyte, the charge
transfer at the oxide/electrolyte interface, as well as the redox
reaction at the Pt counter electrode. And in a Bode phase plot,
a typical EIS spectrum of a DSSC has three characteristic
frequency peaks.24 However, only two semicircles can be observed
in the present Nyquist plots, since the Nernst diffusion within the
electrolyte is overlapped by the intermediate-frequency large
semicircle (Fig. 6a). These findings indicate the fast electron
transport and long electron lifetime in TiO2 film. After fitting the
EIS spectra to an electrochemical model,25 several important
parameters such as the series resistance (RS), charge transfer
resistance at the TiO2/dye/electrolyte interface (Rrec), and charge
transfer resistance at the Pt/electrolyte interface (RCE) can be
obtained. RCE inversely scaling with the reduction rate of I3ꢀ ions
at the Pt/electrolyte interface is estimated by the small semicircle
width. The small RCE values of C3, P2, C2 and P3 mean fast
reduction of I3ꢀ ions at Pt counter electrode. RS and RCE corre-
sponding to the first semicircle are almost the same in C3, P2, C2
and P3 dyes-based DSSCs due to using the same electrode
material and electrolyte. The simulated data of lifetime (te) can be
estimated by te ¼ Rrec ꢂ CPE2,26 herein, te value of 121.9 ms
(Rrec ¼ 89.1 U and corresponding capacitance, C ¼ 1370.5 mF),
Fig. 6 EIS Nyquist (a) and Bode (b) plots for DSSCs based on C3, P2,
C2 and P3 in the dark. The schematic circuit diagram (top) is for the
DSSCs.
phenothiazine derivative at the end of molecules, they may more
efficiently block the approach of I3ꢀ ions to the TiO2 surface, in
spite of C3 showing a larger amount of adsorbed dye than P2
(Table 1). The excellent hole transporting ability of phenothia-
zine or carbazole derivative may improve the regeneration of the
sensitizers. Therefore, the high Voc values of 826 mV for P3 and
793 mV for C2 may be explained.
To gauge the dynamics of electron transport and charge
recombination of the four DSSCs, intensity-modulated photo-
current spectroscopy (IMPS) and intensity-modulated photo-
voltage spectroscopy (IMVS) are further measured under an
illumination of a LED light source (l ¼ 457 nm) with different
light intensities from 30 to 150 W mꢀ2. Transport time (sd) and
lifetime (sr) curves as a function of light intensity derived from
IMVS and IMPS measurements are shown in Fig. 7a and b,
respectively. As shown in Fig. 7a and b, the transport time and
electron lifetime of the DSSCs based on C3, P2, C2 and P3 dyes
decrease with increasing light intensity. The transport time values
182.7 ms (Rrec ¼ 151.7 U and C ¼ 1204.3 mF), 388.6 ms (Rrec
¼
342.9 U and C ¼ 1136.4 mF) and 481.1 ms (Rrec ¼ 524.3 U and C ¼
918.2 mF) for the DSSCs based on dyes C3, P2, C2 and P3,
respectively, were obtained. The lifetime order of C3 < P2 < C2 <
P3 is consistent with the Voc results above.
In consideration of these results, given that P2 relative to C3
and P3 relative to C2 have more non-planar triphenylethylene
9002 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
of the DSSCs are in the order of C3 > P2 > C2 > P3, whereas the
electron lifetime values are in the order of C3 < P2 < C2 < P3,
which account for the remarkable increase in Voc from C3, P2,
C2 to P3. Comparison of P3 with C2 and P2 with C3 dyes
suggests that the triphenylethylene phenothiazine unit with
twisted non-planar triphenylethylene and non-planar phenothi-
azine at the end of the dye molecules could better protect the
injected electrons from the recombination process than the
triphenylethylene carbazole unit, rendering the higher Voc. The
non-planar phenothiazine close to the anchoring group or near
the TiO2 surface is indicated to be beneficial for dyes to adsorb
on TiO2 film (Table 1), which is responsible for the longer elec-
tron lifetime, as seen by comparing P3 with P2 and C2 with C3,
respectively.4a As a consequence, the higher Voc values of P3
relative to P2 and C2 to relative to C3 are ascribed to the suitable
LUMO energy levels (Table 2) and the non-planar structure near
the anchoring group.
To study the electron transport and recombination properties,
charge-collection efficiency (hcc) derived from IMPS and IMVS
measurements were apparently considered as meaningful
parameters. In sensitized solar cells, hcc can be calculated by
a expressions hcc ¼ 1 ꢀ sd/sr.27 Fig. 7(c) shows that the depen-
dence of the charge collection efficiency on the light intensity,
and the efficiency values of the DSSCs based on the as-synthe-
sized dyes are in the order of C3 < P2 < C2 < P3. However, the
measurement of the charge collection efficiency of the DSSCs
based on C2 and P3 dyes presents an interesting result, i.e., their
values approach 1 for light intensity from 30 to 150 W mꢀ2. This
may be ascribed to the cooperative effects of suitable LUMO
levels and the twisted structure of triphenylethylene phenothia-
zine moiety or triphenylethylene carbazole moiety. In contrast,
the charge collection efficiencies (96–98%) of the DSSCs based
on C3 and P2 dyes increase slightly with increasing light inten-
28
sity. According to the expression of Jsc ¼ qhlhhinjhccI0,
h is in
cc
direct proportion to Jsc of the sensitized solar cells. The above
results imply that the introduction of triphenylethylene pheno-
thiazine moiety is superior to that of triphenylethylene carbazole
moiety, both of which could considerably suppress the electron
recombination process, and the purpose of improving Voc may be
accomplished by introducing the twisted structure in the solid
state of AIE derivative.
The PL spectra of C3, P2, C2 and P3 dyes were investigated in
water-DMF mixtures (Fig. 8). A bathochromic shift combined
with decreased intensity in the C3, P2, C2 and P3 emission
spectra with increasing water fraction is observed, showing
aggregation-caused emission quenching (ACEQ) effect.
However, many triphenylethylene derivatives previously repor-
ted in our laboratory were AIE compounds.29 This observation is
ascribed to the solvent polarity effect30 and intramolecular
charge-transfer (ICT) state.31 Before the formation of aggregates,
D–A type molecules tend to adopt a more twisted conformation
in a more polar solvent, hence, the excited luminogens are in the
twisted intramolecular charge transfer state referred to as non-
radiative quenching processes.32 The large red shift with
decreased PL intensity might be related to the stabilization of the
excited state. Similar emission spectra are often observed in
highly polarized molecules exhibiting enlarged dipoles and
charge-transfer characters in their excited states, which undergo
changes from ICT states to solvent-relaxed emissive states.33
Given that C3, P2, C2 and P3 dyes are highly polarized D–A type
molecules with strong push–pull interactions that satisfy all the
given criteria, the PL spectra of C3, P2, C2 and P3 dyes could be
explained by the cooperative effects of the above. The fluores-
cence exhibited by the as-synthesized dyes in the solvent is
quenched in the aggregated and solid states because of ACEQ
effect, which is attributed to a nonradiative deactivation process.
The ACEQ of the as-synthesized dyes is aroused by the
Fig. 7 Transport time (a), electron lifetime (b), and charge collection
efficiency of the DSSCs based on C3, P2, C2 and P3 dyes.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 9003

View Article Online
ACEQ of C3, P2, C2 and P3 dyes is beneficial for the high
conversion efficiency of photovoltaic energy, after considering
that the AIE compounds use a portion of sunlight to emit longer
wavelength light back. Therefore, the current study is successful
in introducing the twisted structure of the AIE structure section
to the C3, P2, C2 and P3 dyes without importing the AIE
property, which may helpful to improve the Voc and conversion
efficiency of photovoltaic energy.
Conclusion
A series of efficient organic dyes with a twisted AIE structure as
an additional donor has been designed and synthesized. The
maximum Voc and power conversion efficiency are 826 mV and
6.55%, respectively, under standard AM 1.5 G solar conditions,
as compared with 783 mV and 8.84% for N719 dye under the
same experimental conditions. The introduction of a triphenyl-
ethylene phenothiazine unit into organic dyes is reported for the
first time, and evidence for the advantage in structure leading to
a high photovoltage in the DSSCs is given. It has been found that
the introduction of twisted non-planar AIE structure can
improve the performance of DSSCs, and its fluorescence in the
solid state can be quenched, which is beneficial for the high
power conversion efficiency. The current study presents a work-
able method to introduce the twisted structures of AIE
compounds to organic dyes, which involves increasing the donor
moiety with AIE derivatives to form a D–D–p–A configuration.
The twisted structure of the triphenylethylene phenothiazine unit
at the end of the dye molecules may result in a longer electron
lifetime, faster transport time and higher charge collection effi-
ciency as compared with the triphenylethylene carbazole unit.
Both triphenylethylene phenothiazine and triphenylethylene
carbazole units could improve the open-circuit photovoltage and
DSSCs performance, as seen by comparing the open-circuit
voltages and efficiency values of P3 and C2 with that of T2-1.
The increasing of the twisted non-planar structure in organic
dyes is a promising way to improve the performance in DSSCs.
To obtain better DSSCs performances, new molecular designs
should take into consideration to introduce other twisted AIE
structures.
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (51173210,
51073177), Construction Project for University-Industry coop-
eration platform for Flat Panel Display from The Commission of
Economy and Informatization of Guangdong Province, the
Fundamental Research Funds for the Central Universities and
Natural Science Foundation of Guangdong (S2011020001190).
Fig. 8 PL spectra of C3, P2, C2 and P3 dyes in water–DMF mixtures
(10 mM) with different water fractions. Inset: maximum PL wavelength
and intensity of corresponding dyes in DMF with varied water fractions
and emission images taken under 365 nm UV illumination at room
temperature. Excitation wavelength: C3, 365 nm; P2, 310 nm; C2, 340
nm; and P3, 415 nm.
Notes and references
cyanoacrylic acid section because compounds 4a, 4b, 5a and 5b in
the solid state still give strong fluorescent emission properties
showing high AIE effect. Accordingly, most of AIE structures
could be introduced to organic dyes to improve the photovoltaic
performance, at the same time the fluorescence in the solid state
can be quenched if the dyes use cyanoacrylic acid as an electron
acceptor. Based on the fundamental processes in the DSSCs, the
€
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737.
2 A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran,
M. Nazeeruddin, E. W. Diau, C. Y. Yeh, S. M. Zakeeruddin and
M. Gratzel, Science, 2011, 334, 629.
3 (a) J. Y. Liao, B. X. Lei, D. B. Kuang and C. Y. Su, Energy Environ.
Sci., 2011, 4, 4079; (b) X. Y. Yu, J. Y. Liao, K. Q. Qiu, D. B. Kuang
and C. Y. Su, ACS Nano, 2011, 5, 9494.
9004 | J. Mater. Chem., 2012, 22, 8994–9005
This journal is ª The Royal Society of Chemistry 2012

View Article Online
4 (a) Z. S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi,
A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008,
20, 3993; (b) Y. P. Hong, J. Y. Liao, D. R. Cao, X. F. Zang,
D. B. Kuang, L. Y. Wang, H. Meier and C. Y. Su, J. Org. Chem.,
2011, 76, 8015.
5 (a) S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska,
€
P. Comte, P. Pechy and M. Gratzel, Chem. Commun., 2008, 5194;
(b) S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador,
J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision D.01), Gaussian, Inc., Wallingford, CT, 2004.
ꢀ
R. Charvet, P. Comte, M. K. Nazeeruddin, P. Pechy, M. Takata,
H. Miura, S. Uchida and M. Gratzel, Adv. Mater., 2006, 18, 1202;
€
(c) T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, J. Am.
Chem. Soc., 2004, 126, 12218.
6 S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M. H. Lee,
W. Lee, J. Park, K. Kim, N. G. Park and C. Kim, Chem. Commun.,
2007, 4887.
7 Z. S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada and A. Shinpo,
Adv. Mater., 2007, 19, 1138.
22 G. Li, K. J. Jiang, Y. F. Li, S. L. Li and L. M. Yang, J. Phys. Chem. C,
2008, 112, 11591.
8 (a) S. A. Haque, S. Handa, K. Peter, E. Palomares, M. Thelakkat and
J. R. Durrant, Angew. Chem., 2005, 117, 5886; (b) S. A. Haque,
S. Handa, K. Peter, E. Palomares, M. Thelakkat and J. R. Durrant,
Angew. Chem., Int. Ed., 2005, 44, 5740; (c) J. N. Clifford,
€
E. Palomares, M. K. Nazeeruddin, M. Gratzel, J. Nelson, X. Li,
N. J. Long and J. R. Durrant, J. Am. Chem. Soc., 2004, 126, 5225.
23 (a) T. Y. Wu, M. H. Tsao, F. L. Chen, S. G. Su, C. W. Chang,
H. P. Wang, Y. C. Lin, W. C. Ouyang and I. W. Sun, Int. J. Mol.
Sci., 2010, 11, 329; (b) H. N. Tian, X. C. Yang, R. K. Chen,
Y. Z. Pan, L. Li, A. Hagfeldt and L. C. Sun, Chem. Commun.,
2007, 3741.
24 R. Kern, R. Sastrawan, J. Ferber, R. Stangl and J. Luther,
Electrochim. Acta, 2002, 47, 4213.
€
9 A. Mishra, M. K. R. fischer and P. Bauerle, Angew. Chem., Int. Ed.,
€
2009, 48, 2474.
10 Z. J. Ning, Q. Zhang, W. J. Wu, H. C. Pei, B. Liu and H. Tian, J. Org.
Chem., 2008, 73, 3791.
25 (a) Q. Wang, J. E. Moser and M. Gratzel, J. Phys. Chem. B, 2005, 109,
14945; (b) F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte,
G. Boschloo and A. Hagfeldt, Sol. Energy Mater. Sol. Cells, 2005,
87, 117.
11 (a) Z. Y. Yang, Z. G. Chi, T. Yu, X. Q. Zhang, M. N. Chen, B. J. Xu,
S. W. Liu, Y. Zhang and J. R. Xu, J. Mater. Chem., 2009, 19, 5541; (b)
H. Y. Li, X. Q. Zhang, Z. G. Chi, B. J. Xu, W. Zhou, S. W. Liu,
Y. Zhang and J. R. Xu, Org. Lett., 2011, 13, 556; (c) X. Q. Zhang,
Z. G. Chi, H. Y. Li, B. J. Xu, X. F. Li, S. W. Liu, Y. Zhang and
J. R. Xu, J. Mater. Chem., 2011, 21, 1788; (d) B. J. Xu, Z. G. Chi,
Z. Y. Yang, J. B. Chen, S. Z. Deng, H. Y. Li, X. F. Li, Y. Zhang,
N. S. Xu and J. R. Xu, J. Mater. Chem., 2010, 20, 4135; (e)
Z. Y. Yang, Z. G. Chi, B. J. Xu, H. Y. Li, X. Q. Zhang, X. F. Li,
S. W. Liu, Y. Zhang and J. R. Xu, J. Mater. Chem., 2010, 20, 7352.
12 X. Q. Zhang, Z. G. Chi, B. J. Xu, H. Y. Li, Z. Yang, X. F. Li,
S. W. Liu, Y. Zhang and J. R. Xu, Dyes Pigm., 2010, 89, 56.
13 M. Grigoras and N. C. Antonoaia, Polym. Int., 2005, 54, 1641.
14 S. K. Lee, T. Ahn, J. H. Park, Y. K. Jung, D. S. Chung, C. E. Park
and H. K. Shim, J. Mater. Chem., 2009, 19, 7062.
ꢁ
ꢀ
ꢁ
26 I. Mora-Sero, S. Gimenez, F. Fabregat-Santiago, R. Gomez, Q. Shen,
T. Toyoda and J. Bisquert, Acc. Chem. Res., 2009, 42, 1848.
27 A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595.
28 K. Zhu, N. R. Neale, A. Miedaner and A. J. Frank, Nano Lett., 2007,
7, 69.
29 (a) H. Y. Li, Z. G. Chi, B. J. Xu, X. Q. Zhang, Z. Y. Yang, X. F. Li,
S. W. Liu, Y. Zhang and J. R. Xu, J. Mater. Chem., 2010, 20, 6103; (b)
B. J. Xu, Z. G. Chi, H. Y. Li, X. Q. Zhang, X. F. Li, S. W. Liu,
Y. Zhang and J. R. Xu, J. Phys. Chem. C, 2011, 115, 17574; (c)
B. J. Xu, Z. G. Chi, X. F. Li, H. Y. Li, W. Zhou, X. Q. Zhang,
C. C. Wang, Y. Zhang, S. W. Liu and J. R. Xu, J. Fluoresc., 2011,
21, 433; (d) X. F. Li, Z. G. Chi, B. J. Xu, H. Y. Li, X. Q. Zhang,
W. Zhou, Y. Zhang, S. W. Liu and J. R. Xu, J. Fluoresc., 2011, 21,
1969; (e) H. Y. Li, Z. G. Chi, X. Q. Zhang, B. J. Xu, S. W. Liu,
Y. Zhang and J. R. Xu, Chem. Commun., 2011, 47, 11273; (f)
Z. Y. Yang, T. Yu, M. N. Chen, X. Q. Zhang, C. C. Wang,
B. J. Xu, X. Zhou, S. W. Liu, Y. Zhang, Z. G. Chi and J. R. Xu,
Acta Polym. Sin., 2009, 6, 560; (g) B. J. Xu, Z. G. Chi, J. Y. Zhang,
X. Q. Zhang, H. Y. Li, X. F. Li, S. W. Liu, Y. Zhang and J. R. Xu,
Chem.–Asian J., 2011, 6, 1470.
15 B. X. Lei, W. J. Fang, Y. F. Hou, J. Y. Liao, D. B. Kuang and
C. Y. Su, J. Photochem. Photobiol., A, 2010, 216, 8.
16 B. K. An, S. K. kwon, S. D. Jung and S. Y. Park, J. Am. Chem. Soc.,
2002, 124, 14410.
17 G. Zhang, Y. Bai, R. Li, D. Shi, S. Wenger, S. M. Zakeeruddin,
€
M. Gratzel and P. Wang, Energy Environ. Sci., 2009, 2, 92.
€
18 M. Gratzel, Nature, 2001, 414, 338.
19 M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni,
30 H. Y. Li, Z. G. Chi, B. J. Xu, X. Q. Zhang, Z. Y. Yang, X. F. Li,
S. W. Liu, Y. Zhang and J. R. Xu, J. Mater. Chem., 2010, 20, 6103.
31 H. Tong, Y. Q. Dong, M. Haußer, Y. N. Hong, J. W. Y. Lam,
€
G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Gratzel, J. Am.
Chem. Soc., 2005, 127, 16835.
€
20 J. H. Hou, Z. A. Tan, Y. Yan, Y. J. He, C. H. Yang and Y. F. Li, J.
Am. Chem. Soc., 2006, 128, 4911.
H. H. Y. Sung, I. D. Williams, H. S. Kwok and B. Z. Tang, Chem.
Phys. Lett., 2006, 428, 326.
21 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
32 R. Hu, E. Lager, A. A. Aguilar, J. Liu, J. W. Y. Lam, H. H. Y. Sung,
I. D. Williams, Y. Zhong, K. S. Wong, E. P. Cabrera and B. Z. Tang,
J. Phys. Chem. C, 2009, 113, 1589.
33 F. Furche and D. Rappoport, J. Am. Chem. Soc., 2004, 126, 1277.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 8994–9005 | 9005