Phenothiazine-triphenylamine based organic dyes containing various conjugated linkers for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c2jm34682f

In order to increase the electron-donating ability of the donor part of the organic dye, two phenothiazine groups, as additional electron donors, were introduced into a triphenylamine unit to form a starburst donor-donor (2D) structure in this paper. Three new organic dyes (WD-6, WD-7 and WD-8) containing this starburst 2D structure and a 2-cyanoacetic acid acceptor linked by various conjugated linkers (benzene, thiophene, and furan) have been designed, synthesized and applied in dye-sensitized solar cells (DSSCs). The introduction of a phenothiazine group with a butterfly conformation in the triphenylamine donor parts has a good influence on preventing the molecular π-π aggregation due to the starburst 2D structure of the organic dye. The conjugated linker effects on the photophysical, electrochemical and photovoltaic properties of these organic dyes were investigated in detail. The DSSCs made with these organic dyes displayed remarkable overall conversion efficiencies, ranging from 4.90-6.79% under an AM 1.5 solar condition (100 mW cm ^-2). The best performance was found for organic dye WD-8, in which a furan group was the conjugated linker. It displayed a short-circuit current (J_sc) of 14.43 mA cm^-2, an open-circuit voltage (V _oc) of 682 mV, and a fill factor (ff) of 0.69, corresponding to an overall conversion efficiency of 6.79%. The different photovoltaic behaviors of the solar cells based on these organic dyes were further elucidated by the electrochemical impedance spectroscopy.

Full text of DOI:10.1039/c2jm34682f

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PAPER
Phenothiazine–triphenylamine based organic dyes containing various
conjugated linkers for efficient dye-sensitized solar cells
a
a
b
a
b
Zhongquan Wan, Chunyang Jia, Yandong Duan, Linlei Zhou, Yuan Lin and Yu Shia
*
*
Received 17th July 2012, Accepted 27th September 2012
DOI: 10.1039/c2jm34682f
In order to increase the electron-donating ability of the donor part of the organic dye, two
phenothiazine groups, as additional electron donors, were introduced into a triphenylamine unit to
form a starburst donor–donor (2D) structure in this paper. Three new organic dyes (WD-6, WD-7 and
WD-8) containing this starburst 2D structure and a 2-cyanoacetic acid acceptor linked by various
conjugated linkers (benzene, thiophene, and furan) have been designed, synthesized and applied in dye-
sensitized solar cells (DSSCs). The introduction of a phenothiazine group with a butterfly conformation
in the triphenylamine donor parts has a good influence on preventing the molecular p–p aggregation
due to the starburst 2D structure of the organic dye. The conjugated linker effects on the photophysical,
electrochemical and photovoltaic properties of these organic dyes were investigated in detail. The
DSSCs made with these organic dyes displayed remarkable overall conversion efficiencies, ranging
from 4.90–6.79% under an AM 1.5 solar condition (100 mW cmꢀ2). The best performance was found
for organic dye WD-8, in which a furan group was the conjugated linker. It displayed a short-circuit
current (Jsc) of 14.43 mA cmꢀ2, an open-circuit voltage (Voc) of 682 mV, and a fill factor (ff) of 0.69,
corresponding to an overall conversion efficiency of 6.79%. The different photovoltaic behaviors of the
solar cells based on these organic dyes were further elucidated by the electrochemical impedance
spectroscopy.
are naturally becoming the main focus of the design of new dyes
Introduction
for DSSCs. Indeed, Diau and co-workers have developed very
€
Dye-sensitized solar cells (DSSCs), developed by Gratzel and
O’Regan, have attracted the considerable attention of many
research groups in the past two decades owing to their high effi-
ciency and low cost.1 DSSCs typically contain four components: a
mesoporous semiconductor metal oxide film, a dye, an electrolyte/
hole transporter, and a counter electrode.2 In these components,
the dye is a crucial element, exerting significant influence on the
conversion efficiency as well as the stability of the cells.
efficient push–pull porphyrin dyes, affording a record efficiency
ꢀ
of 10.17% with an I3 /Iꢀ-based electrolyte,4 and more recently a
12.3% efficiency for a cobalt electrolyte,5 which are the best
performing sensitizers reported so far. However, the synthetic
approach of porphyrin dyes is complicated and the yield is low.
For the above reasons, a lot of effort has been dedicated to the
development of pure organic dyes which exhibit not only higher
molar extinction coefficients, but also simple preparation and
purification at lower cost. Coumarin,6 squaraine,7 indoline,8
phenothiazine,9 triphenylamine,10 fluorene,11 carbazole12 and
tetrahydroquinoline13 based organic dyes have been developed
and show good performance.
DSSCs based on ruthenium dyes have shown very impressive
solar-to-electric power conversion efficiency. Recently, a new
record efficiency (11.4%) of a solar cell based on black dye with
donor–acceptor type coadsorbent was obtained.3 However, the
large-scale application of ruthenium dyes is limited because of
costs and environmental issues. Porphyrin derivatives are the
archetypal light collectors of photosynthetic organisms and they
Most organic dyes are composed of electron donor, p linker
and acceptor moieties and usually have a rod-like configuration.
However, the rod-like molecules are elongated, which may
facilitate the recombination of electrons with the triiodide and
the formation of aggregates between molecules.14 Therefore,
organic dyes with a starburst conformation were designed and
synthesized by introducing an additional electron donor group
into the D–p–A molecule to form the starburst 2D–p–A struc-
ture,15 which avoids the charge recombination process of injected
electrons with the triiodide in the electrolyte and the formation of
aggregates between dye molecules.
aState Key Laboratory of Electronic Thin Films and Integrated Devices,
School of Microelectronics and Solid-State Electronics, University of
Electronic Science and Technology of China, Chengdu 610054, P. R.
China. E-mail: cyjia@ uestc.edu.cn; Fax: +86 28 83202569; Tel: +86 28
83201991
bCAS Key Laboratory of Photochemistry, Institute of Chemistry,
BNLMS, Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: linyuan@iccas.ac.cn; Fax: +86 10 8261 7315; Tel: +86 10 8261
5031
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Recently, we reported a novel organic dye WD-1 with a star-
burst 2D–p–A configuration, in which two phenothiazine groups
are introduced into a triphenylamine donor as additional elec-
tron donors, and 2-cyanoacetic acid is used as an acceptor.16 The
solar cell based on WD-1 afforded an overall conversion effi-
ciency of 4.54%, which is higher than that of the corresponding
triphenylamine dye. The study suggested that optimizing the
triphenylamine donor part by introducing phenothiazine groups
as additional electron donors to form a starburst 2D–p–A
organic dye is a promising way for preventing unfavorable dye
aggregation and increasing the power conversion efficiency.
The linker group between the electron donor and the acceptor
in organic dyes plays an important role in terms of optical
absorption and charge transfer properties, so this work studies
the effect of different linkers on phenothiazine–triphenylamine
based starburst 2D–p–A organic dyes. We therefore designed
and synthesized three new starburst 2D–p–A organic dyes (WD-
6, WD-7 and WD-8) with different conjugated linker groups
(Scheme 1). These organic dyes comprise phenothiazine–triphe-
nylamine as a starburst electron donor, 2-cyanoacetic acid as an
electron acceptor, and the different aromatic groups as a
conjugated linker, i.e., benzene, furan, and thiophene. They were
successfully applied in DSSCs. The effects of the different
conjugated linkers on the photophysical, electrochemical and
photovoltaic properties of these organic dyes were investigated in
detail.
J & K Scientific Ltd. Lithium iodide (LiI) was purchased from
Acros. 2-Cyanoacetic acid, 4-tert-butylpyridine (TBP) and 3-
methoxypropionitrile (MPN) were purchased from Aldrich. 3-
Hexyl-1-methylimidazolium iodide (HMII) was prepared
according to the literature.17 THF was pre-dried over 4 A
ꢀ
molecular sieves and distilled under an argon atmosphere from
sodium benzophenone ketyl immediately prior to use. The
starting material, tris(4-iodophenyl)amine, was synthesized
according to the corresponding literature method.14 All other
solvents and chemicals were purchased from Aldrich and used as
received without further purification.
Preparation of DSSCs
The TiO2 colloid for liquid-state DSSCs was prepared according
to the literature.18 The FTO glass substrates were immersed in
40 mM TiCl4 aq. at 70 ꢁC for 30 min and washed with water and
ethanol. The 12 mm thick mesoporous nano-TiO2 films,
composed of 15–20 nm anatase TiO2 particles, were coated on
the FTO glass plates by doctor blade. After drying the nano-
crystalline TiO2 layer at 125 ꢁC, a 4 mm thick second layer of 300–
500 nm sized light scattering anatase particles (Shanghai Cai Yu
Nano Technology Co., Ltd) was deposited using a doctor blade
onto the first layer. The TiO2 electrodes were heated at 450 ꢁC for
30 min. After sintering, when the temperature cooled to about
90 ꢁC, the electrodes were immersed in a dye bath containing
0.5 mM N3 in ethanol or 0.2 mM WD dyes in acetonitrile and left
overnight. The films were then rinsed in ethanol to remove excess
dye. Solar cells were assembled, using a 25 mm thick thermo-
plastic Surlyn frame, with a platinized counter electrode. An
electrolyte solution was then introduced through the hole pre-
drilled in the counter electrode, and the cell was sealed with
thermoplastic Surlyn covers and a glass coverslip. The liquid
electrolyte employed was a solution of 0.3 M HMII, 0.5 M LiI,
0.05 M I2 and 0.5 M TBP in MPN.
Experimental
Equipment
€
NMR spectra were obtained with a Brucker AM 400 spectro-
meter (relative to TMS). Mass spectra were recorded with a
Waters LCT Premier XE spectrometer. The absorption spectra
of the dyes in solution and adsorbed on TiO2 films were
measured with a SHIMADZU (model UV2450) UV-vis spec-
trophotometer. The cyclic voltammograms of the dyes were
obtained with a CH Instruments 660C electrochemical work-
station using a normal three-electrode cell with a Pt working
electrode, a Pt wire counter electrode, and a Ag/AgCl reference
electrode.
Photovoltaic characterization
The irradiation source for the photocurrent-density–voltage (J–
V) measurement is an AM 1.5 solar simulator (91160A, Newport
Co., USA). The incident light intensity was 100 mW cmꢀ2 cali-
brated with a standard Si solar cell. The tested solar cells were
masked to a working area of 0.2 cm2. Volt–current characteris-
tics were performed on a Model 2611 sourcemeter (Keithley
Instruments, Inc., USA). A Keithley 2611 sourcemeter and a
Model spectrapro 300i monochromator (Acton research, USA)
Materials
Phenothiazine and triphenylamine were purchased from Asta-
tech. 4-Formylphenylboronic acid, 5-formyl-2-thiopheneboronic
acid, and 5-formyl-2-furanboronic acid were purchased from
Scheme 1 Molecular structures of the organic dyes in this study.
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equipped with a 500 W xenon lamp (Aosiyuan Technology &
Science Co., Ltd, China) were used for photocurrent action
spectrum measurements. Electrochemical impedance spectros-
copy (EIS) data were obtained by using a Solartron 1255B
frequency analyzer and a Solartron SI 1287 electrochemical
interface system.
J ¼ 8.4 Hz, 2H), 7.665 (d, J ¼ 3.6 Hz, 1H), 7.413 (s, 8H), 7.338 (d,
J ¼ 8.8 Hz, 2H), 7.214 (d, J ¼ 4 Hz, 1H), 7.098 (d, J ¼ 7.6 Hz,
4H), 7.013 (t, J ¼ 8 Hz, 4H), 6.895 (t, J ¼ 7.6 Hz, 4H), 6.378 (d,
J ¼ 7.6 Hz, 4H). HRMS (ESI, m/z): [M + H]+ calcd for
(C47H31N3O2S2): 734.1858; found: 734.1790.
(E)-3-(5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)
phenyl)benzene-2-yl)-2-cyanoacrylic acid (WD-6). A CH3CN
(10 ml) solution of 2 (186 mg, 0.25 mmol), 2-cyanoacetic acid
(43 mg, 0.5 mmol) and a few drops of piperidine was charged
sequentially in a three-necked flask and heated to reflux for 10 h.
After cooling to room temperature, solvents were removed by
rotary evaporation, and the residue was purified by silica gel
column chromatography with CH2Cl2 : CH3OH (10 : 1, v/v) as
eluent to afford the dye WD-6 as a dark red solid (132 mg, yield
65%). 1H-NMR (400 MHz, DMSO-d6) d: 8.365 (s, 1H), 8.145 (d,
J ¼ 8.8 Hz, 2H), 7.932 (d, J ¼ 8.4 Hz, 2H), 7.868 (d, J ¼ 8.8 Hz,
2H), 7.432–7.359 (m, 10H), 7.098–7.076 (dd, J ¼ 1.2 Hz, 4H),
7.028–6.985 (m, 4H), 6.906–6.866 (td, J ¼ 0.8 Hz, 4H), 6.368 (d,
J ¼ 7.6 Hz, 4H). HRMS (ESI, m/z): [M + H]+ calcd for
(C52H34N4O2S2): 811.2123; found: 811.2135.
Synthesis
N-(4-(10H-Phenothiazin-10-yl)phenyl)-N-(4-iodophenyl)-4-
(10H-phenothiazin-10-yl)benzenamine (1). Tris(4-iodophenyl)
amine (2.0 g, 3.2 mmol), phenothiazine (1.91 g, 9.6 mmol),
potassium carbonate (3.18 g, 23.0 mmol), activated copper
bronze (0.83 g, 13.0 mmol) and 18-crown-6 (0.10 g, 0.38 mmol)
were refluxed in 1,2-dichlorobenzene (30 ml) for 36 h. The
solvent was removed by decompressing distillation and the
residue was purified by column chromatography on silica gel
using dichloromethane and petroleum ether (60–90 ꢁC) in the
ratio of 1 : 2 (v/v) as the eluent to give 1 as a white solid (0.35 g,
yield 14.5%). 1H-NMR (400 MHz, DMSO-d6) d: 7.731 (d, J ¼ 8.4
Hz, 2H), 7.362 (s, 8H), 7.081 (d, J ¼ 8.4 Hz, 2H), 7.062 (d, J ¼ 8.4
Hz, 4H), 6.980–6.931 (m, 4H), 6.858–6.807 (m, 4H), 6.331 (d, J ¼
8.4 Hz, 4H). HRMS (ESI, m/z): [M
+
H]+ calcd for
(E)-3-(5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)
phenyl)thiophen-2-yl)-2-cyanoacrylic acid (WD-7). A procedure
similar to that for the dye WD-6 but with compound 3 (187 mg,
0.25 mmol) instead of compound 2. The dye WD-7 as dark red
(C42H28IN3S2): 766.0769; found: 766.0764.
5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)phenyl)
phenyl-2-carbaldehyde (2). Under N2 atmosphere, a mixture of 1
(306 mg, 0.4 mmol), 4-formylphenylboronic acid (72 mg, 0.48
mmol), Pd(PPh3)4 (14 mg, 0.0114 mmol), 2 M aqueous solution
of K2CO3 (2 ml) in dry THF (20 ml) was refluxed overnight
(Scheme 2). The reaction mixture was extracted with CH2Cl2,
water and brine. The organic layer was dried with anhydrous
MgSO4 and then the solvent was removed by rotary evaporation.
The residue was purified by column chromatography on silica gel
using dichloromethane and petroleum ether in the ratio of 2 : 1
1
solid (122 mg, yield 60%). H-NMR (400 MHz, DMSO-d6) d:
8.492 (s, 1H), 8.023 (d, J ¼ 4.4 Hz, 1H), 7.821 (d, J ¼ 8.4 Hz, 2H),
7.711 (d, J ¼ 4 Hz, 1H), 7.442–7.386 (m, 8H), 7.310 (d, J ¼ 8.4
Hz, 2H), 7.103–7.080 (dd, J ¼ 1.6 Hz, 4H), 7.029–6.986 (td, J ¼
1.6 Hz, 4H), 6.910–6.870 (td, J ¼ 1.2 Hz, 4H), 6.372 (d, J ¼ 8 Hz,
4H). HRMS (ESI, m/z): [M + H]+ calcd for (C50H32N4O2S3):
817.1687; found: 817.1785.
(E)-3-(5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)
phenyl)furan-2-yl)-2-cyanoacrylic acid (WD-8). A procedure
similar to that for the dye WD-6 but with compound 4 (184 mg,
0.25 mmol) instead of compound 2. The dye WD-8 as dark red
1
(v/v) as the eluent to give 2 as a yellow solid (74 mg, 25%). H-
NMR (400 MHz, DMSO-d6) d: 10.046 (s, 1H), 7.995 (d, J ¼ 8.0
Hz, 2H), 7.938 (d, J ¼ 8.4 Hz, 2H), 7.851 (d, J ¼ 8.8 Hz, 2H),
7.409–7.370 (m, 10H), 7.099–7.081 (m, 4H), 7.010 (t, J ¼ 7.6 Hz,
4H), 6.890 (t, J ¼ 7.6 Hz, 4H), 6.369 (d, J ¼ 7.6 Hz, 4H). HRMS
(ESI, m/z): [M + H]+ calcd for (C49H33N3OS2): 744.2065; found:
744.1808.
1
solid (128 mg, yield 64%). H-NMR (400 MHz, DMSO-d6) d:
8.048 (s, 1H), 7.929 (d, J ¼ 8.8 Hz, 2H), 7.556 (d, J ¼ 3.6 Hz, 1H),
7.433–7.379 (m, 8H), 7.356 (d, J ¼ 0.8 Hz, 2H), 7.292 (d, J ¼ 4
Hz, 1H), 7.097–7.075 (dd, J ¼ 1.6 Hz, 4H), 7.027–6.984 (m, 4H),
6.904–6.864 (td, J ¼ 0.8 Hz, 4H), 6.380 (d, J ¼ 8 Hz, 4H). HRMS
(ESI, m/z): [M ꢀ H]+ calcd for (C50H32N4O3S2): 799.1916; found:
799.1956.
5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)phenyl)
thiophene-2-carbaldehyde (3). A procedure similar to that for
compound 2 but with 5-formyl-2-thiopheneboronic acid (75 mg,
0.48 mmol) instead of 4-formylphenylboronic acid. Yield: 27%.
1H-NMR (400 MHz, DMSO-d6) d: 9.895 (s, 1H), 8.045 (d, J ¼ 4
Hz, 1H), 7.855–7.833 (m, 2H), 7.697 (d, J ¼ 4 Hz, 1H), 7.415 (d,
J ¼ 2.4 Hz, 8H), 7.316 (d, J ¼ 8.8 Hz, 2H), 7.109–7.086 (dd, J ¼
1.6 Hz, 4H), 7.034–6.991 (td, J ¼ 1.6 Hz, 4H), 6.915–6.875 (td,
J ¼ 1.2 Hz, 4H), 6.373 (d, J ¼ 8.4 Hz, 4H). HRMS (ESI, m/z):
[M + H]+ calcd for (C47H31N3OS3): 750.1629; found: 750.1716.
Results and discussion
Absorption properties in solution and on TiO2 films
Fig. 1 shows the UV-vis spectra of the three organic dyes, WD-6,
WD-7, and WD-8, measured in acetonitrile solution. The corre-
sponding spectroscopic parameters extracted are summarized in
Table 1. Each of these organic dyes exhibited two major
absorption bands at 280–360 nm and 360–550 nm. The short
wavelength bands were assigned to the localized aromatic p–p*
transitions, and the long wavelength ones to intramolecular
charge-transfer (ICT) transitions. The intensity of p–p* transi-
tions is higher than that of the ICT transitions. The absorption
5-(4-(Bis(4-(10H-phenothiazin-10-yl)phenyl)amino)phenyl)
furan-2-carbaldehyde (4). A procedure similar to that for
compound 2 but with 5-formyl-2-furanboronic acid (67 mg,
0.48 mmol) instead of 4-formylphenylboronic acid. Yield: 25%.
1H-NMR (400 MHz, DMSO-d6) d: 9.580 (s, 1H), 7.894 (d,
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Scheme 2 The synthetic procedure of the target organic dyes. Reagents: (a) phenothiazine, 1,2-dichlorobenzene, Cu, K2CO3, 18-crown-6; (b) 4-for-
mylphenylboronic acid (or 5-formyl-2-thiopheneboronic acid, or 5-formyl-2-furanboronic acid), Pd(PPh3)4, K2CO3, THF; (c) 2-cyanoacetic acid,
piperidine, acetonitrile.
maxima (lmax) of WD-6–8 in the long wavelength region were at
385, 406, and 421 nm, respectively. The lmax of WD-7 (thiophene
linker) and WD-8 (furan linker) was red-shifted 33 nm and 38 nm
compared with that of WD-6 with the benzene linker. This was
due to better delocalization of electrons over the whole molecule
when thiophene or furan was used as the conjugated linker rather
than benzene. The molar extinction coefficients at lmax for WD-
6–8 were 21 100, 23 580, and 22 890 Mꢀ1 cmꢀ1, respectively. It is
known that a wider absorption range is favorable to the
performance of DSSCs, as more photons can be harvested.
According to the lmax and molar extinction coefficients of these
organic dyes, it is clear that WD-7 and WD-8 have a better ability
to harvest light than WD-6, which would lead to a higher
photocurrent for organic dyes WD-7 and WD-8.
Fig. 2 shows the absorption spectra of the three dyes on 3 mm
thick TiO2 films after 12 h adsorption. Compared to the
Fig. 1 Absorption spectra of the three dyes in acetonitrile.
Table 1 UV-vis and electrochemical data of the three dyes
Dye
lmaxa/nm (3b/Mꢀ1 cmꢀ1
)
lmaxc/nm
Eoxd/V (vs NHE)
E0–0e/eV
Eredf/V (vs. NHE)
WD-6
WD-7
WD-8
385 (21 100)
406 (23 580)
421 (22 890)
385
418
423
1.28
1.21
1.14
2.34
2.25
2.19
ꢀ1.06
ꢀ1.04
ꢀ1.05
a
Absorption spectra were measured in acetonitrile (2.5 ꢂ 10ꢀ5 M). b The molar extinction coefficient at lmax of the absorption spectra. c Absorption
maximum on TiO2 film. d Eox was measured in CH2Cl2 with 0.1 M n-Bu4NPF6 as electrolyte (scanning rate: 100 mV sꢀ1, working electrode and counter
electrode: Pt wires, and reference electrode: Ag/AgCl), potentials measured vs Ag/AgCl were converted to normal hydrogen electrode (NHE) by addition
e
f
of +0.2 V. E0–0 values were calculated from the absorption spectral onsets of these dyes.9c Ered was calculated from Eox ꢀ E0–0
.
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spectrum in acetonitrile solution, the sharp shift of absorption
peaks for WD-6–8 on TiO2 films has not been observed (Table 1).
As illustrated with WD-8, the ICT absorption peak is slightly
red-shifted by only 2 nm from 421 nm in solution to 423 nm on
3 mm TiO2 film. This may be ascribed to the starburst 2D
structure, which prevented the strong tendency for the dyes to
aggregate on the TiO2 film, to a large degree. This method for the
prevention of aggregation might be better than the usage of the
coabsorption of the dyes with deoxycholic acid, which was also
able to prevent aggregation and hence improve the photovoltaic
performance by means of improving both photocurrent and
photovoltage, but unavoidably and significantly decreased dye
adsorption and limited the photovoltaic performance. It is
confirmed that this is an effective way to lower the tendency to
aggregate by introducing phenothiazine groups as additional
electron donors into the triphenylamine donor.
Fig. 3 Cyclic voltammograms of the three dyes.
Theoretical calculations
Electrochemical properties
To gain insight into the molecular structures and electron
distribution, the geometries of the dyes were optimized by
density functional theory calculations at the B3LYP/6-31G level
with Gaussian 03.20 As shown in Fig. 5, for the three organic
dyes, HOMO-1/HOMO are only localized on a phenothiazine
ring, and HOMO-2 is distributed along the triphenylamine and
linker system. The LUMO of the three dyes is delocalized across
the linker and acceptor system. It may imply that the nonplanar
phenothiazine blocked by triphenylamine can make electron
distribution of HOMO-1 and HOMO separate from electron
distribution of LUMO. The well-overlapped HOMO-2 and
LUMO orbitals on the linker unit suggest the well inductive or
withdrawing electron tendency from triphenylamine donor unit
to the cyanoacetic acid unit. Thus, the HOMO-2 / LUMO
excitation induced by light irradiation could move the excited
electron distribution from the triphenylamine unit to the cya-
noacetic acid unit, and injected into the conduction band of TiO2
through the anchoring group. The electronic transition in the
visible band with the largest oscillator strength was found to
correspond to the excitation of HOMO-2 to LUMO (the oscil-
lator strengths of HOMO-1 / LUMO and HOMO / LUMO
transitions are almost zero).21
To evaluate the possibility of electron transfer between the TiO2/
dye/redox electrolyte systems, cyclic voltammetry (CV) was
performed in CH2Cl2 solution, using 0.1 M tetrabutylammo-
nium hexafluorophosphate as the supporting electrolyte (Fig. 3).
The oxidation potential of the organic dyes was determined from
the peak potentials by CV, the oxidation potential vs. NHE (Eox
)
corresponds to the highest occupied molecular orbital (HOMO),
while the lowest unoccupied molecular orbital (LUMO), could
be calculated from Eox ꢀ E0–0
.
19 As shown in Table 1 and Fig. 4,
the HOMO levels are more positive than the iodine/iodide redox
potential value (0.4 V vs. NHE), indicating that the oxidized dye
molecules formed after electron injection into the conduction
band of TiO2 could be regenerated by the reducing species in the
electrolyte solution. The LUMO levels of the dyes are suffi-
ciently more negative than the CB level of the TiO2 electrode
(ꢀ0.5 V vs. NHE), which implies that electron injection from the
excited dye into the conduction band of TiO2 is energetically
permitted. Therefore, these organic dyes could be used as good
sensitizers in DSSCs. The schematic energy levels of the dyes
based on absorption and electrochemical data are shown in
Fig. 4.
Fig. 2 Absorption spectra of TiO2 electrodes sensitized by the three
Fig. 4 Schematic energy levels of the three dyes based on absorption and
dyes.
electrochemical data.
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with their UV-vis absorption spectra on the TiO2 film. However,
the onsets of the IPCE spectra for WD-6, WD-7 and WD-8 are
655, 665 and 675 nm, respectively, which are significantly
broadened compared with those of their UV-vis absorption
spectra on the TiO2 film. The IPCE values of DSSCs based on
WD-7 and WD-8 exceeded 60% from 400 nm to 535 nm (with the
highest value of 79.1% at 470 nm) and from 400 to 547 nm (with
the highest value of 79.2% at 475 nm), respectively. In the DSSC
based on WD-6, the IPCE values relatively exceed 60% from
408 nm to 493 nm (with the highest value of 69.1% at 445 nm).
The IPCE of WD-8 is higher and broader than those of WD-6
and WD-7 in the region from 470 nm to 650 nm, indicating that
the WD-8 sensitized TiO2 electrode would generate the higher
conversion efficiency among the three dyes.
The photocurrent-density–photovoltage (J–V) curves of the
DSSCs based on WD-6, WD-7 and WD-8 performed under
simulated AM 1.5 solar irradiation (100 mW cmꢀ2) are shown in
Fig. 7. The photovoltaic properties of these dye-based DSSCs are
summarized in Table 2. The solar cell based on WD-6 shows an
efficiency of 4.90%, with a short-circuit photocurrent density
(Jsc) of 10.86 mA cmꢀ2, an open-circuit photovoltage (Voc) of
635 mV, and a fill factor (ff) of 0.71. When we replaced the
benzene unit with a thiophene unit to construct WD-7, the
performance of the solar cell based on WD-7 improved (h ¼
6.02%, Jsc ¼ 12.64 mA cmꢀ2, Voc ¼ 680 mV, and ff ¼ 0.70).
Afterwards, when we replaced the thiophene unit in WD-7 with a
furan unit to construct WD-8, a further increase in performance
of the solar cell based on WD-8 was achieved (h ¼ 6.79%, Jsc
¼
14.43 mA cmꢀ2, Voc ¼ 682 mV, and ff ¼ 0.69). Under the same
measurement conditions, the solar cell based on dye N3 gener-
ated an efficiency of 8.05% (Jsc ¼ 17.55 mA cmꢀ2, Voc ¼ 685 mV,
and ff ¼ 0.67).
Fig. 5 Frontier orbitals of the three dyes optimized at the B3LYP/6-31G
level.
According to Fig. 7 and Table 2, it is clear that the photo-
voltaic performances of the DSSCs can be affected by the
different conjugated linkers in the starburst 2D–p–A organic
dyes. In comparison with WD-6, the Jsc and Voc values of WD-7
and WD-8 are improved by introducing a thiophene and a furan
linker, respectively. WD-7 and WD-8 have the higher Jsc, which
might be due to the higher light harvesting efficiency than that of
WD-6. Clearly, the DSSCs based on WD-7 and WD-8 possessed
higher Voc. The higher Jsc and Voc for the DSSCs of WD-7 and
WD-8 lead to the higher efficiencies finally.
Photovoltaic performances of DSSCs
Fig. 6 shows the incident photon-to-current efficiency (IPCE)
data as a function of the wavelength for DSSCs based on the
organic dyes WD-6, WD-7 and WD-8.
All three organic dyes could efficiently convert the light to
photocurrents in the region from 400 to 650 nm. The IPCE
spectra changing tendency of the organic dyes are in accordance
Fig. 7 J–V characteristics measured at an irradiation of 100 mW cmꢀ2
Fig. 6 IPCE spectra of DSSCs with the three dyes.
simulated AM1.5 sunlight.
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Table 2 Photovoltaic performances of DSSCs based on the three dyes
Dye
CDCA/mM Jsc/mA cmꢀ2 Voc/mV ff
h (%) se/ms
WD-6
WD-7
WD-8
0
5
0
5
0
5
10.86
10.63
12.64
12.53
14.43
14.17
635
625
680
671
682
681
0.71 4.90
39.91
56.47
63.19
0.73 4.85
0.70 6.02
0.70 5.89
0.69 6.79
0.67 6.47
In order to further probe the behavior of these organic dyes on
the surface of TiO2, the function of co-adsorbents was investi-
gated. Chenodeoxycholic acid (CDCA) is the most popular co-
adsorbent, which binds strongly to the surface of nanostructured
TiO2, being able to displace dye molecules from the semi-
conductor surface and therefore hindering the formation of
sensitizer aggregates.22 Thus, the performances of the DSSCs in
the presence of CDCA were studied. The results were collected in
Table 2, and the corresponding J–V curves are shown in Fig. 7. It
was found that the Jsc of the solar cells based on these dyes and
CDCA decreased. CDCA did not improve the corresponding
device’s performance, however, it decreased the power conver-
sion efficiency. A possible explanation is that the amount of dye
adsorbed on the TiO2 surface was reduced by the coadsorption of
CDCA, resulting in a loss of active light harvesting, indicating
that CDCA was not necessary for these dyes to improve their
performance. In addition, the Voc values of the DSSCs in the
presence of CDCA were decreased; this is because adsorption of
CDCA leaves protons on the TiO2 surface and, hence, charges
the surface positively shifted by the coadsorption of CDCA,
resulting in Voc loss.23 These results could be considered as a hint
that these dyes with a starburst 2D–p–A structure nearly did not
aggregate on the TiO2 surface.
Fig. 8 EIS spectra of DSSCs based on the three dyes measured at
ꢀ0.65 V forward bias in the dark: (a) Nyquist and (b) Bode phase plots.
injected electrons and the electrolyte, resulting in the improve-
ment of the Voc due to reduced charge recombination rate. The
combination of the highest Voc and Jsc values (vide supra)
affords the best photovoltaic performance of the solar cell based
on WD-8.
To further elucidate the photovoltaic results and obtain more
interfacial charge transfer information in DSSCs sensitized by
WD-6, WD-7 and WD-8, electrochemical impedance spectros-
copy (EIS) was also performed in the dark under a forward bias
of ꢀ0.65 V.24 The Nyquist and Bode plots for WD-6, WD-7 and
WD-8 sensitized cells are shown in Fig. 8a and b, respectively.
Nyquist plots have two semicircles and the larger semicircle
indicates charge recombination resistance at the TiO2/dye/elec-
trolyte interface. As can be seen from Fig. 8a that the radius of
the larger semicircle increases in the order of WD-8 > WD-7 >
WD-6, indicating that the charge recombination resistance is
increased in the order WD-6 > WD-7 > WD-8. This is to some
extent consistent with the order of decreasing Voc: WD-8
(682 mV) > WD-7 (680 mV) > WD-6 (635 mV).
Conclusion
In this work, two phenothiazine groups as additional electron
donors were introduced into a triphenylamine unit to form a
starburst 2D structure. A series of efficient organic dyes (WD-6,
WD-7 and WD-8) containing this starburst 2D structure and a 2-
cyanoacetic acid acceptor linked by different p-conjugated
linkers (benzene, thiophene, and furan) have been synthesized,
characterized and applied in DSSCs. The phenothiazine group
with a butterfly conformation in these organic dyes has an effect
on preventing the molecular p–p aggregation. The DSSCs made
with these organic dyes displayed remarkable overall conversion
efficiency in a range of 4.90–6.79%. The best performance of a
solar cell based on WD-8 (furan group as the conjugated linker)
was achieved (h ¼ 6.79%, Jsc ¼ 14.43 mA cmꢀ2, Voc ¼ 682 mV,
and ff ¼ 0.69). The DSSCs based on WD-7 and WD-8 have
higher Voc and Jsc values than those of WD-6 because of their
more efficient dark current suppression and light harvesting.
According to the EIS analysis, both the resistance for charge
recombination and the electron lifetime were increased from
WD-6 to WD-7 and WD-8, indicating that the introduction of
the furan unit as the linker in starburst phenothiazine–triphe-
nylamine based organic dyes could improve photovoltaic
In the Bode phase plots (Fig. 8b), the lower-frequency peak is
indicative of the charge-transfer process of injected electrons in
TiO2. The higher Voc of WD-7 and WD-8 can be further
explained by electron lifetime, calculated through the relation
se ¼ 1/(2pf) (f is the peak frequency of lower-frequency range in
EIS Bode plot). For the three devices, the peak frequency of
lower-frequency range decreased in the order of WD-6 > WD-7 >
WD-8, and the electron lifetime was enhanced in reverse with the
calculated values of 39.91, 56.47, and 63.19 ms, respectively.
Thus, WD-8 has the longest electron lifetime, indicative of a
more effective suppression of the back reaction between the
25146 | J. Mater. Chem., 2012, 22, 25140–25147
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View Article Online
and P. Baeuerle, Adv. Funct. Mater., 2012, 22, 1291; (b) M. F. Xu,
M. Zhang, M. Pastore, R. Z. Li, F. De Angelis and P. Wang,
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performance compared with the benzene and thiophene units as
the linker. Coadsorption of CDCA did not improve the
conversion efficiency of the devices based on the three dyes,
indicating that these starburst 2D–p–A dyes nearly did not
aggregate on the TiO2 surface. The results from this work
strongly indicate that the increasing of the twisted non-planar
structure in organic dyes is a promising way to improve the
performance in DSSCs.
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We thank the National Natural Science Foundation of China
(Grant nos. 20873015, 21272033), the Fundamental Research
Funds for the Central Universities (Grant no. ZYGX2010J035),
the Innovation Funds of State Key Laboratory of Electronic
Thin Films and Integrated Device (Grant no. CXJJ201104) and
Beijing National Laboratory for Molecular Sciences (BNLMS)
for financial support.
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