Influence of the terminal electron donor in D-D-π-A phenothiazine dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.05.015

We report three new molecularly engineered push-pull dyes based on phenothiazine (PTZ) as a π-conjugating linker and arylamine moiety as donor and compared with a simple phenothiazine derived reference dye. Absorption spectra, electrochemical cyclic voltammetry, theoretical calculations, current-voltage curves and electrochemical impedance spectroscopy were performed to understand the terminal electron donor influence on the performance of D-D-π-A PTZ dyes. Among these four dyes, the dye incorporating a 9-hexyl-9H-carbazole shows the best photovoltaic performance: a short-circuit photocurrent density of 14.02 mA cm^-2, an open-circuit photovoltage of 748 mV, and a fill factor of 0.68, corresponding to an overall conversion efficiency of 7.13% under standard global AM 1.5 solar light conditions.

Full text of DOI:10.1016/j.dyepig.2014.05.015

Dyes and Pigments 109 (2014) 96e104
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Influence of the terminal electron donor in DeDepeA phenothiazine
dyes for dye-sensitized solar cells
Shibin Wang*, Haoru Wang, Jianchun Guo*, Hongbiao Tang, Jinzhou Zhao
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report three new molecularly engineered pushepull dyes based on phenothiazine (PTZ) as a
p-
Received 6 April 2014
Received in revised form
8 May 2014
Accepted 10 May 2014
Available online 20 May 2014
conjugating linker and arylamine moiety as donor and compared with a simple phenothiazine derived
reference dye. Absorption spectra, electrochemical cyclic voltammetry, theoretical calculations, current
evoltage curves and electrochemical impedance spectroscopy were performed to understand the ter-
minal electron donor influence on the performance of DeDepeA PTZ dyes. Among these four dyes, the
dye incorporating a 9-hexyl-9H-carbazole shows the best photovoltaic performance: a short-circuit
photocurrent density of 14.02 mA cmꢀ2, an open-circuit photovoltage of 748 mV, and a fill factor of
0.68, corresponding to an overall conversion efficiency of 7.13% under standard global AM 1.5 solar light
conditions.
Keywords:
Dye-sensitized solar cell
Phenothiazine
© 2014 Elsevier Ltd. All rights reserved.
Organic dye
Electron donor
Photovoltaic performance
Conversion efficiency
1. Introduction
electron donor effect due to its electron-rich sulfur and nitrogen
heteroatoms. Furthermore, the phenothiazine ring is non-planar
Significant attention has been attracted by dye-sensitized solar
cells (DSSCs) as alternatives to traditional solar cells, owing to their
low-cost fabrication combined with high photovoltaic performance
[1]. Upon irradiation, light is absorbed by the sensitizer, which re-
sults in electron injection from the photoexcited dye into the con-
duction band of the semiconductor. Studies have demonstrated that
the performance of DSSCs strongly depends on the nature of sensi-
tizer(s). DSSCs based on ruthenium sensitizers have reached overall
solar-to-electric power-conversion efficiency (PCE) of 11.4% under
standard air mass 1.5 G illumination. At the same time, the research
activity has increased in the design and synthesis of metal-free
organic dyes because of their lower cost, high molar extinction co-
efficients and good flexibility of molecular tailoring [2e33].
In general, organic sensitizers are constituted with the
donorepeacceptor (DepeA) configuration. This pushepull struc-
ture can induce an efficient intramolecular charge transfer (ICT)
from the donor to the acceptor. Based on this strategy, a series of
arylamine electron donors have been employed to develop highly
efficient organic dyes. Phenothiazine (PTZ) exhibits a well-known
with a butterfly conformation in the ground state, which can
impede the molecular aggregation and the formation of intermo-
lecular excimers. The features make the PTZ-based dyes promising
candidates for high efficiency DSSCs [34e49].
Originally, Yang, Hagfeldt, Sun and co-workers have synthesized
a series of PTZ dyes for DSSCs. A prominent solar energy-to-
electricity conversion efficiency of 5.5% was achieved in a DSC
based on T2-1 dye which is simple in structure and easy to syn-
thesize [34]. To extend the range of
p-electron delocalization and
increase the molar absorptivity of the PTZ dyes,
p-linkers such as
thiophene derivates was introduced in PTZ dyes. However, it is
worth noticing that in contrast to the triphenylamine dyes, the
increasing conjugation length of the PTZ dyes decreases the per-
formance of DSSCs [3,35,36]. On the other hand, rational design of
the terminal electron donor has very recently led to consecutive
efficiency progress of DSSCs based on PTZ organic photosensitizers
with DeDepeA structure [36,37]. For example, Hua et al. devel-
oped a series of PTZ dyes their structure features of (4-hexyloxy)
phenyl donor moiety at the C(7) position of phenothiazine, which
extends the
p-conjugation of the chromophore, therefore
enhancing the performance of DSSCs [37]. This molecular design
strategy was demonstrated to effectively improve the photovoltaic
performance with the best PCE up to 8.18%, under simulated AM
* Corresponding authors. Tel.: þ86 2883032808; fax: þ86 2883032442.
E-mail addresses: wangshb07@163.com (S. Wang), guojianchun@vip.163.com
(J. Guo).
http://dx.doi.org/10.1016/j.dyepig.2014.05.015
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
97
2. Results and discussion
2.1. Design and synthesis
Carbazole and its derivatives have been widely used as a func-
tional building block in the fabrication of the organic photo-
conductors, nonlinear optical materials, and photorefractive
materials due to their specific optical and electrochemical proper-
ties [50]. The excellent photoelectric function of the carbazole unit
makes it a promising type of terminal electron donor for PTZ dyes.
On the other hand, aiming to tune both the electronic trait and
packing mode of dye molecules chemisorbed on titania nano-
crystals of DSSCs, BPPA and DPFA have been incorporated to the
terminal electron donor in WR9 and WR10, respectively.
The synthetic procedure to WR8, WR9 and WR10 is depicted in
Scheme 1. Starting from 7-bromo-10-hexyl-10H-phenothiazine-3-
carbaldehyde, the terminal electron donor was introduced by a
transition metal-mediated coupling reaction, yielding the aldehyde
precursors 3, 5 and 7. Finally, the conventional Knoevenagel
condensation yielded the dyes WR8, WR9 and WR10 with cyano-
acetic acid in the presence of piperidine.
Fig. 1. Chemical structures of WR7, WR8, WR9, WR10 and T2-1.
1.5G irradiation. These results highlight the great potential of
DeDe
p
eA PTZ arylamine dyes for high efficiency DSSCs.
To design more powerful DeDepeA PTZ dyes, it is of great
significance to understand the influences of the terminal electron
donors on the light-harvesting, electrochemical, interfacial kinetic
parameters, as well as their joint contribution to the photovoltaic
performance. Herein, we employ three PTZ dyes (WR8, WR9 and
WR10, see Fig. 1) based on different arylamine electron donors, i.e.,
9-hexyl-9H-carbazole (HCBZ), bis(4-(2-phenylpropan-2-yl)phenyl)
amine (BPPA) and N-phenyl-9,9-dipropyl-N-(9,9-dipropyl-9H-flu-
oren-2-yl)-9H-fluoren-2-amine (DPFA), to investigate the electron
donor influence on photovoltaic performance. For comparison the
2.2. Absorption spectra
UV/Vis absorption spectra of the resulting dyes in a diluted so-
lution of CH2Cl2 are shown in Fig. 2 and the corresponding pho-
tophysical data are summarized in Table 1. The maximum
absorption peaks for WR7, WR8, WR9 and WR10 are 453, 470, 478
and 464 nm, respectively. Compared with WR7, the lmax of WR8,
WR9 and WR10 is red-shifted by 17, 25 and 11 nm, respectively.
Moreover, WR8-WR10 exhibited higher molar extinction co-
efficients than that of WR7. Obviously, the introduction of a ter-
simple, amine donor free dye, WR7 with the DepeA structure was
prepared as a reference [34]. Our work provides a new insight into
the terminal electron donor related structureeproperty relation-
ship of PTZ dyes for DSSCs.
minal donor extends the
therefore red-shifts the maximum absorption peak and enhances
molar extinction coefficient. Scrutinizing the dyes with different
p-conjugation of the chromophore, and
Scheme 1. Synthetic route for WR8, WR9 and WR10.

98
S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
Fig. 2. Absorption spectra of the dyes in dichloromethane.
Fig. 3. Absorption spectra of sensitized electrodes (3 mm film).
terminal donors, it is apparent that the molar extinction coefficient
of WR10 is lower than those of the WR8 and WR9. This may be
caused by the different electronic coupling parallel to the electronic
transition dipole moment between the HOMO and LUMO (see
computational analysis).
also valuable to note that the WR9 possesses the smallest ED/Dþ of
0.82 V relative to other dyes, demonstrating the stronger electron-
donating ability of the BPPA donor.
As shown in above, the HOMO levels of these dyes are more
positive than Ie/I3e redox couples (0.4 V vs NHE [2]), indicating the
driving forces for dye regeneration are sufficient for the iodine cells.
On the other hand, the excited-state potential (ED*/Dþ), reflecting
the LUMO level of the dye, can be derived from the ground-state
oxidation potential and the zero-zero excitation energy (E0-0). The
LUMO levels of these dyes are much more negative than the con-
duction band (CB) of the TiO2 level (ꢀ0.5 V vs NHE [2]), ensuring an
efficient electron injection process from the excited state of the
dyes into the TiO2 electrode.
Fig. 3 displays the absorption spectra of the dyes anchored on
transparent mesoporous titania film (3
mm). Note that, the ab-
sorption of WR9 sensitized film was found to be lower than that of
the WR8 and WR10, despite the fact WR9 possesses the highest
maximum molar absorption coefficient in solution. This observa-
tion indicates that incorporation of BPPA unit may reduce the WR9
loading amount significantly. The validity of this deduction was
confirmed by comparing the amounts of the dyes adsorbed on the
TiO2 surface (12 mm, the thickness employed to devices). The dye
load amount was evaluated by desorbing the dyes with a 0.01 M
solution of KOH in methanol, and the dye loading amount was
2.4. Computational analysis
estimated to be 1.8
ꢁ
10ꢀ7
,
1.7
ꢁ
10ꢀ7
,
1.1 ꢁ 10ꢀ7 and
To gain further insight into the molecular structures of the PTZ
dyes, density functional theory (DFT) calculations were performed
and the resulting geometries are shown in Fig. 5. From the side
view of the PTZ dyes (Fig. 5(a)), the geometry of phenothiazine is
found to present a butterfly shape, endowing the PTZ dyes with
reduced the dye-aggregation and attenuated internal charge
recombination rate in DSSCs [37]. Note that, WR9 is more bulky
than WR8/WR10 regarding their side view, which makes the whole
molecule occupy a larger surface area than a molecule of linear
1.5 ꢁ 10ꢀ7 mol cmꢀ2 for WR7, WR8, WR9 and WR10, respectively.
Apparently, loading amount of WR9 is quite limited by its 3D
structure of BPPA unit. By contrast, WR8 adopts dense packing on
the nanoparticle surface, which can be attributed to its small ter-
minal electron donor, HCBZ.
2.3. Electrochemical properties
The redox potentials of WR7-WR10 dyes, measured by cyclic
voltammetry (CV) are shown in Fig. 4 and the values are presented
in Table 1. The first oxidation potential (ED/Dþ), corresponding to the
HOMO level of dyes, decrease in the order of WR7 (1.01 V) > WR10
(1.00 V) > WR8 (0.88 V) > WR9 (0.82 V). Attaching the terminal
donor significantly extends the
p-conjugation of the donor in
WR8eWR10, which raises the HOMO energy and facilitates their
oxidation. As a consequence, WR8eWR10 exhibit relative lower
oxidation potentials as compared to the reference dye WR7. It is
Table 1
Photophysical and electrochemical data for dyes.
Dye
lmaxa/nm (ε/Mꢀ1 cmꢀ1
)
E0-0/eV
ED/Dþb/V
ED*/Dþc/V
WR7
WR8
WR9
WR10
453 (16,500)
470 (23,200)
478 (25,800)
464 (17,600)
2.29
2.24
2.28
2.36
1.01
0.88
0.82
1.00
ꢀ1.28
ꢀ1.36
ꢀ1.46
ꢀ1.36
a
The absorption spectra were measured in CH2Cl2 solutions.
The ED/Dþ (vs NHE) were measured in acetonitrile.
b
c
ED*/Dþ were estimated from calculated from ED*/Dþ ¼ Eox ꢀ E0e0
.
Fig. 4. CV lines of dyes sensitized electrodes.

S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
99
Fig. 5. (a) The side view of the PTZ dyes; (b) B3LYP/6-31G molecular geometries of dyes as resulted from DFT calculations.
shape, therefore leading to the reduction of loading capacity.
Furthermore, the terminal electron donor alteration from HCBZ/
DPFA to BPPA results in a decreased distance between the triaryl-
amine nitrogen and the carboxylic acid of the anchoring group
(Fig. 5(b)). The distance for WR8/WR10 is about 15.46 Å, while that
for WR9 is only 12.17 Å. This may render back-electron transfer in
WR9 based DSSCs owing to a closer contacts to the TiO2 surface
[32].
To clarify the terminal donor-correlated absorption and elec-
trochemical properties in these dyes, time-dependent DFT (TDDFT)
excited state calculations at the B3LYP/6-31G level in vacuo with
the B3LYP/6-31G optimized ground-state geometries were per-
formed. The charge distribution in the frontier molecular orbitals
the terminal electron donor or the PTZ unit, and the LUMO on the
electron accepting cyanoacrylic acid anchor. Thus, the electrons of
could be successively transferred to acceptor part and then injected
into the conduction band of TiO2 after being illuminated by the
light.
Computed excitation energies, oscillator strength (f) and tran-
sition assignment of the lowest excited state for the PTZ dyes in
vacuo are summarized in Table 2. The results have shown that the
absorption characteristics and excited state features will mainly be
dominated by charge transfer transitions from the HOMO to LUMO
and HOMO-1 to LUMO. In spectroscopy, oscillator strength (f) is a
dimensionless quantity that expresses the probability of absorption
or emission of electromagnetic radiation in transitions between
energy levels of an atom or molecule. The oscillator strength of
excited state 1 for WR8, WR9 and WR10 are calculated to be 0.24,
can be depicted in Fig. 6. WR8eWR10 exhibit
a typical
donoreacceptor-type architecture with the HOMO concentrated on
Fig. 6. Computed frontier orbitals of WR8, WR9 and WR10.

100
S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
Table 2
Computed excitation energies, oscillator strength (f) and transition assignment of
the lowest excited state for the PTZ Dyes in vacuo.
Dyes
Excited
state
Calculated
energy
Oscillator
strength (f)
Transition
assignmenta
(eV, nm)
WR8
WR9
WR10
1
2
1
2
1
2
2.35, 525
2.79, 444
2.09, 591
2.84, 436
1.94, 639
2.58, 479
0.24
0.10
0.18
0.17
0.06
0.32
H / L (91.6%)
H-1 / L (88.4%)
H / L (94.1%)
H-1 / L (86.0%)
H / L (99.0%)
H-1 / L (95.2%)
a
H means HOMO, and L means LUMO.
0.18 and 0.06, respectively. Note here that the oscillator strength for
WR10 is much lower than that of WR8/WR9, indicating the prob-
ability of the corresponding absorption (HOMO to LUMO) is very
low. This may be caused by the different electronic coupling parallel
to the electronic transition dipole moment between the HOMO and
LUMO. For the WR8 and WR9 dyes HOMO and LUMO have the
overlapping extension on the PTZ fragment, while the corre-
sponding overlap is markedly reduced in WR10. As a consequence,
the molar extinction coefficient of WR10 is lower than that of WR8
and WR9. In spite of that, introduction of the DPFA unit into the dye
structure evidently influenced the absorption features of WR10,
leading to red-shifted absorption peak and an enhanced molar
extinction coefficient.
Fig. 8. JeV curves for DSSCs based on the dyes under illumination of AM 1.5G simu-
lated sunlight (100 mW cmꢀ2).
with observations from the contrasting absorption between the dye
in solution and on the TiO2 film (see Figs. 2 and 3).
The same DSSCs employed for IPCE measurements have been
tested under standard conditions (AM 1.5G, 100 mW cmꢀ2) in order
to investigate the photovoltaic performance of the dyes. To prevent
inflated photocurrent arising from stray light, a black metal mask
surrounded the active area [51]. The photocurrent density voltage
curves (JeV) of the DSSCs based on these dyes are shown in Fig. 8.
The detailed parameters of short-circuit current density (JSC), open-
circuit voltage (VOC), fill factor (ff), and power-conversion efficiency
(PCE) are collected in Table 3. When compared with WR8
(JSC ¼ 14.02 mA cmꢀ2), WR7 shows lightly lower JSC values of
12.96 mA cmꢀ2 due to the narrower IPCE responsive area.
Furthermore, DSSCs fabricated using WR8 gave a high VOC of
748 mV, achieving an overall PCE of 7.13%, an evident improvement
2.5. Photovoltaic performance of DSSCs
The incident photon-current conversion efficiencies (IPCEs) for
WR7eWR10 sensitized DSSCs are plotted in Fig. 7. The electrolyte
was composed of 0.6 M 1,2-dimethyl-3-n-propylimidazolium io-
dide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine
(TBP) in acetonitrile. These cells exhibit a broad IPCE plateau from
400 to 600 nm with comparable IPCE maxima (around 80%),
implying a saturated light absorption in this wavelength. With
respect to the WR8eWR10 sensitized DSSCs, WR7 sensitized solar
cell gives a relatively narrow IPCE action spectrum because of its
narrow absorption, resulting in a relatively low short-circuit
photocurrent density (JSC) of 12.96 mA cmꢀ2. These results revealed
that incorporation of the strong auxiliary electron-donating unit at
the end of PTZ dyes can successfully broaden the IPCE response
area. On the other hand, WR9 based DSSC shows a lower and
narrower IPCE as compared to that of WR8, which was attributed to
the lower amount of WR9 adsorbed on the TiO2 surface, consistent
compared to the WR7 (PCE ¼ 6.63%) with De
peA structure. In
contrast, the introduction of BPPA unit in WR9 resulted in signifi-
cantly attenuated photocurrent (JSC ¼ 12.63 mA cmꢀ2) and photo-
voltage (VOC ¼ 705 mV), correspondingly, relative lower efficiency
of 6.14% was attained. The influence of the DPFA unit seems to be
beneficial since the JSC and VOC of WR10 are better than those of
WR7. Nevertheless, the positive effect of the DPFA unit on the
performance of DeDepeA PTZ dyes is limited by its low molar
absorption coefficient, which intimately correlates to the proba-
bility of the corresponding absorption (HOMO to LUMO).
2.6. Electrochemical impedance spectroscopy
To elucidate the correlation between the VOC of the cell and the
dye structure, electrochemical impedance spectroscopy (EIS) was
carried out in the dark. Typical EIS Nyquist plots (Fig. 9(a)) and Bode
phase plots (Fig. 9(b)) for DSSCs based on the WR7eWR10
measured in the dark under a forward bias of 0.7 V. Some important
parameters can be obtained by fitting the EIS spectra to an elec-
trochemical model [39]. RS, Rrec, and RCE represent the series
Table 3
Photovoltaic parameters for studied DSSCs.a
Dye
JSC/mA cmꢀ2
VOC/mV
FF
PCE/%
WR7
WR8
WR9
WR10
12.96
14.02
12.63
13.01
710
748
705
734
0.71
0.68
0.69
0.68
6.63
7.13
6.14
6.49
a
Iodine electrolyte: 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in
Fig. 7. IPCE spectra of DSSCs made with the four dyes.
acetonitrile.

S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
101
Fig. 9. Electrochemical impedance spectroscopy for the studied DSSCs (a) Nyquist plots and (b) Bode plots.
resistance, the charge-transfer resistance at the dye/TiO2/electro-
lyte interface, and the charge-transfer resistance at the counter
electrode (CE), respectively. The Rrec obtained by EIS is on the order
the terminal electron donor on the light-harvesting, electro-
chemical properties and performance of PTZ dyes. The introduction
of these additional electron donors extends the conjugation of the
PTZ dyes, leading to the red-shifts of their absorption spectra and
enhanced molar extinction coefficient. Interestingly, WR10 exhibits
a lower molar extinction coefficient compared to that of WR8/WR9,
a shortcoming limits the photoperformance of WR10. Theoretical
calculations revealed that the probability of HOMO to LUMO tran-
sition for WR10 is much lower than that of WR8/WR9, which could
induce a weakly absorption. In contrast, WR8 with HCBZ unit gave a
better light-harvesting, leading to an improved photovoltage,
photocurrent and efficiency of DSSCs compared to those of the
reference dye, WR7. However, WR9 (containing BPPA unit) shows
lower JSC values and thus lower efficiency owing to a decreased
amount of dye. The electron lifetime of WR9 is even slightly lower
than that of WR7, indicating the bulky structure of the terminal
electron donor in PTZ dyes may not be advantageous. In conjunc-
tion with the iodine electrolyte, WR8 sensitized DSSCs affords a
short-circuit photocurrent of 14.02 mA cmꢀ2, an open-circuit
voltage of 748 mV, and a fill factor of 0.68, corresponding to an
overall conversion efficiency of 7.13% under standard AM 1.5 sun-
light. Our work provides a new insight into the terminal electron
donor related structureeproperty relationship of PTZ arylamine
dyes for DSSCs.
of WR9 (Rrec
¼
74
U
)
<
WR7 (Rrec
¼
75 U) < WR10
(Rrec ¼ 134 ) < WR8 (Rrec ¼ 146
U
U
), implying increasing resistance
to charge recombination. By fitting the EIS curves, another impor-
tant parameter for DSSCs, electron lifetime, could be obtained by
multiplying the Rrec by the chemical capacitance. The fitted electron
lifetime increases in the order of WR9 < WR7 < WR10 < WR8,
indicating a sequence of lifetime increasing (Fig. 10). These results
are in agreement with the observed shift in the VOC value under
standard global AM 1.5 illumination. Interestingly, the WR9 ex-
hibits much shorter electron lifetime compared to WR7/WR8
despite with the more bulky terminal electron donor, BPPA unit.
This is in striking contrast with other arylamine dyes such as tri-
arylamine dyes. For example, truxene based triarylamine dyes
prolong electron lifetime in the DSSCs, benefiting from their steric
hindrance of the alkyl functionalized truxene group. The results
from EIS reveal the dominant role of dense packing of PTZ dyes on
TiO2 surface, which in turn suppress the charge recombination and
lengthens electron lifetime in DSSCs.
3. Conclusions
In summary, three new organic dyes, WR8, WR9 and WR10,
featuring HCBZ, BPPA and DPFA terminal electron donors, respec-
tively, were designed and synthesized to investigate the effects of
4. Experimental
4.1. Materials and instruments
The synthetic routes for dyes are shown in Scheme 1 n-Butyl-
lithium, Pd(PPh3)4, Pd(OAc)2, t-BuOK, (t-Bu)3P and cyanoacetic acid
were purchased and used without purification. All other solvents
and chemicals used in this work were analytical grade and used
without further purification.
1H NMR and 13C NMR spectra were recorded on a Bruker AM-
400 spectrometer. The reported chemical shifts were against TMS.
The mass spectra were conducted on a 4700 Proteomics analyzer
spectrometer. The absorption spectra were measured with a Jasco
V-570 UVevis spectrophotometer. Fluorescence spectra were ac-
quired on Hitachi F-4500 spectrophotometers.
The cyclic voltammograms were measured on a CHI660B elec-
trochemical workstation (CH Instruments) using a normal three-
electrodes cell with a dye loaded TiO2 electrode as working elec-
trode, a Pt wire auxiliary electrode, and Ag/AgCl reference electrode
in saturated KCl solution, 0.1 M tetrabutylammonium perchlorate
was used as supporting electrolyte. The redox potential of dyes on
TiO2 was measured in CH3CN with a scan rate at 50 mV sꢀ1. After
Fig. 10. Electron lifetime of the studied DSSCs as a function of bias voltage.

102
S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
the measurement, ferrocene was added as the internal reference for
calibration. Electrochemical impedance spectroscopy (EIS) was
performed in the frequency range of 100 mHz to 100 kHz in the
dark with the alternate current amplitude set at 10 mV.
4.3.2. 2-Cyano-3-(10-hexyl-7-(9-hexyl-9H-carbazol-3-yl)-10H-
phenothiazin-3-yl)acrylic acid (WR8)
To a stirred solution of compound 3 (112 mg, 0.2 mmol) and
cyanoacetic acid (25.5 mg, 0.3 mmol) in acetonitrile (8 mL) was
added chloroform (4 mL) and piperidine (51.1 mg, 0.6 mmol). The
reaction mixture was refluxed for 10 h and then acidified with 1 M
hydrochloric acid aqueous solution (30 mL). The crude product was
extracted into CH2Cl2, washed with water and dried over anhy-
drous sodium sulfate. After removing solvent under reduced
pressure, the residue was purified by column chromatography
(CH2Cl2/methanol (15: 1) as eluent) to give desired product WR8
(Red powder, 101.7 mg, 81% yield). mp 92e94 ꢂC. 1H NMR
4.2. DSSCs fabrication and characterization
A transparent conducting substrate (F-doped SnO2, 14
U/sq,
>90% transparency in the visible region, Nippon Sheet Glass,
Hyogo, Japan) was subsequently washed with detergent, distilled
water, acetone, and EtOH ultrasonically, then treated with TiCl4
(aqueous, 40 mM) at 70 ꢂC for 30 min, followed by twice screen-
printing and once doctor-blading a paste consisted of TiO2 (18%),
ethyl cellulose (9%), and terpinol (73%). The film was successively
fired at 450 ꢂC under air for 30 min, treated with TiCl4 solution
(400 MHz, CDCl3):
d
8.25 (s, 1H), 8.14 (d, J ¼ 8 Hz, 1H), 8.07 (s, 1H),
7.90 (d, J ¼ 8 Hz,1H), 7.69 (s,1H), 7.63 (d, J ¼ 9 Hz,1H), 7.42e7.52 (m,
5H), 7.26 (t, J ¼ 8.0 Hz, 1H), 6.95 (d, J ¼ 8.0 Hz, 1H), 6.87 (d,
J ¼ 8.0 Hz, 1H), 4.31e4.34 (m, 4H), 1.82e1.95 (m, 4H), 1.25e1.39 (m,
10H), 0.86e0.94 (m, 8H); 13C NMR (100 MHz, CDCl3):
d 167.6, 141.0,
(aqueous, 40 mM), and fired again to give a ~12 mm thick meso-
scopic TiO2 film. When cooled to 80 ꢂC, the TiO2 electrode was
immersed in a dye solution (5 ꢁ 10ꢀ4 M in THF) for 24 h in the dark
and then rinsed with methanol and dried under an Air flow. The
counter electrode was prepared by spinecoating H2PtCl4 (50 mM in
isopropyl alcohol) on an FTO substrate and sintering at 390 ꢂC
under air for 40 min. The DSSCs had an active area of 0.18 cm2 and
electrolyte composed of 0.6 M 1,2-dimethyl-3-n-propylimidazo-
lium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tertbu-
tylpyridine (TBP) in acetonitrile.
139.8, 132.5, 131.0, 128.8, 126.0, 125.7, 124.4, 123.4, 122.9, 120.4,
118.9, 118.3, 108.9, 108.7, 65.6, 31.6, 30.5, 26.9, 26.6, 22.5, 19.2, 13.9.
HRMS (ESI) calcd for C40H41N3O2S (MþHþ): 628.2997, found:
628.2995.
4.3.3. 7-(Bis(4-(2-phenylpropan-2-yl)phenyl)amino)-10-hexyl-
10H-phenothiazine-3-carbaldehyde (5)
To a 100 mL two neck round-bottom flask was added compound
2 (1.63 g, 4.20 mmol), compound 4 (1.62 g, 4.00 mmol), Pd(OAc)2
(60 mg), t-BuOK (600 mg), P(t-Bu)3 (0.60 mL) and toluene (30 mL).
The reaction mixture was refluxed overnight under nitrogen. After
cooling to room temperature, saturated NH4Cl was added and
extracted with ethyl acetate (3 ꢁ 10 mL). The combined organic
layers were washed with brine and then dried over anhydrous
magnesium sulfate, filtered, and concentrated in vacuo to give the
crude product, which were purified by column chromatograph
packed with silica gel using petroleum ether/ethyl acetate (15:1) as
eluent to afford a light-yellow oil of compound 5 (1.28 g, 45%). 1H
An AM 1.5 solar simulator-Oriel 91160-1000 served as the light
source for photovoltaic measurements of the DSSCs. The photo-
currentevoltage (JeV) characteristics were recorded using
a
Keithley 2400 Source meter under 100 mW cmꢀ2 simulated air
mass (AM 1.5) solar light illumination. The action spectra of
monochromatic incident phototo-current conversion efficiencies
(IPCEs) for the solar cells were performed by using a commercial
setup (Oriel-74125 system, Newport, USA).
NMR (400 MHz, CDCl3):
d
9.73 (s,1H), 7.68 (d, J ¼ 8.4 Hz, 2H), 7.52 (s,
4.3. Synthesis
1H), 7.18e7.29 (m, 8H), 7.04e7.16 (m, 7H), 6.95 (d, J ¼ 8.5 Hz, 1H),
6.75e6.84 (m, 5H), 6.71 (s, 1H), 3.85 (t, J ¼ 6.5 Hz, 2H), 1.58 (s, 12H),
1.34 (t, J ¼ 7.0 Hz, 2H), 1.16e1.26 (m, 5H), 0.76e0.82 (m, 4H). 13C
The synthetic procedures of the dyes WR8, WR9 and WR10 are
depicted in Scheme 1. Corresponding aldehyde derivatives were
synthesized by coupling reaction. Subsequently, the target dyes
WR8, WR9 and WR10 were obtained via Knoevenagel condensa-
tion reaction of the respective aldehydes with cyanoacetic acid in
the presence of a catalytic amount of piperidine.
NMR (100 MHz, CDCl3):
d 191.0, 150.6, 150.4, 145.0, 144.9, 143.6,
138.6, 131.0, 130.7, 128.5, 128.3, 127.9, 126.8, 126.01, 123.9, 123.7,
123.2, 122.8, 117.7, 115.7, 47.5, 42.4, 31.2, 30.8, 26.4, 26.1, 22.5, 14.3.
HRMS (ESI) calcd for C49H50N2OS (MþHþ): 715.3722, found:
715.3718.
4.3.1. 10-Hexyl-7-(9-hexyl-9H-carbazol-3-yl)-10H-phenothiazine-
3-carbaldehyde (3)
4.3.4. 3-(7-(Bis(4-(2-phenylpropan-2-yl)phenyl)amino)-10-hexyl-
10H-phenothiazin-3-yl)-2-cyanoacrylic acid (WR9)
A mixture of compound 1 (1.56 g, 4.15 mmol), compound 2
(1.62 g, 4.15 mmol), Pd(PPh3)4 (50 mg, 0.042 mmol), and Na2CO3
(2.2 g, 20.75 mmol) dissolved in THF (15 mL)/H2O (8 mL) was
refluxed under nitrogen for 8 h. After cooling the solution, the
solvent was removed in vacuo. Dichloromethane was added. The
organic layer was separated and washed three times with water,
dried over anhydrous MgSO4, and filtered. The crude product was
then purified by silica gel chromatography with petroleum/ethyl
acetate (10/1) as eluent to give a yellow oil 3 (1.27 g, 55%). 1H NMR
Compound WR9 was synthesized according to the same pro-
cedure of WR8, giving a red powder (78% yield). mp 109e112 ꢂC. 1H
NMR (400 MHz, CDCl3):
d
7.92 (s, 1H), 7.75 (d, J ¼ 8.0ꢂHz, 1H), 7.63
(s, 1H), 7.18e7.26 (m, 8H), 7.10e7.15 (m, 2H), 7.03e7.07 (m, 5H),
6.89e6.94 (m, 1H), 6.68e6.81 (m, 6H), 3.82 (t, J ¼ 6.5 Hz, 2H), 1.56
(s, 12H), 1.33 (t, J ¼ 7.0 Hz, 2H), 1.15e1.25 (m, 5H), 0.75e0.82 (m,
4H). 13C NMR (100 MHz, CDCl3):
d 150.6, 147.9, 145.0, 143.4, 138.8,
131.1, 128.6, 128.0, 126.8, 126.0, 124.1, 123.6, 123.1, 122.8, 118.9, 117.4,
115.8, 47.4, 42.5, 31.2, 30.8, 26.5, 26.2, 22.7, 14.3. HRMS (ESI) calcd
for C52H51N3O2S (MþHþ):782.3780, found: 782.3786.
(400 MHz, CDCl3):
d
9.82 (s, 1H), 8.26 (s, 1H), 8.14 (d, J ¼ 8.0 Hz, 1H),
7.63e7.69 (m, 3H), 7.43e7.52 (m, 5H), 7.27 (t, J ¼ 8.0 Hz, 1H), 6.97 (d,
J ¼ 8.0 Hz,1H), 6.92 (d, J ¼ 8.0 Hz,1H), 4.33 (t, J ¼ 7.0 Hz, 2H), 3.95 (t,
J ¼ 7.0 Hz, 2H),1.85e1.94 (m, 4H),1.30e1.41 (m, 10H), 0.87e0.95 (m,
4.3.5. 7-(4-(Bis(9,9-dipropyl-9H-fluoren-2-yl)amino)phenyl)-10-
hexyl-10H-phenothiazine-3-carbaldehyde (7)
8H); 13C NMR (100 MHz, CDCl3):
d
189.9, 150.5, 141.6, 140.8, 139.8,
Compound 7 was synthesized according to the same procedure
138.0, 131.1, 130.6, 130.2, 128.6, 126.1, 125.7, 124.6, 123.4, 122.8,
120.5, 119.0, 118.3, 116.3, 114.8, 108.9, 48.2, 43.3, 31.6, 31.4, 29.0,
26.9, 26.5, 22.6, 14.0. HRMS (ESI) calcd for C37H40N2OS (MþHþ):
561.2939, found: 561.2933.
of 3, giving a yellow powder (78% yield). mp 89e92 ꢂC. 1H NMR
(400 MHz, CDCl3):
d 9.81 (s, 1H), 7.58e7.69 (m, 6H), 7.39e7.46 (m,
3H), 7.31e7.37 (m, 5H), 7.27e7.30 (m, 3H), 7.23e7.25 (m, 2H),
7.17e7.22 (m, 2H), 7.06 (d, J ¼ 8.4ꢂHz, 2H), 6.91 (t, J ¼ 8.5 Hz, 2H),

S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
103
[15] Baheti A, Tyagi P, Thomas KRJ, Hsu YC, Lin JT. Simple triarylamine-based dyes
containing fluorene and biphenyl linkers for efficient dye-sensitized solar
cells. J Phys Chem C 2009;113:8541e7.
[16] Tamilavan V, Kim AY, Kim HB, Kang M, Hyun MH. Structural optimization of
thiophene-(N-aryl)pyrrole-thiophenebased metal-free organic sensitizer for
the enhanced dye-sensitized solar cell performance. Tetrahedron 2014;70:
371e9.
[17] Cai S, Hu X, Han J, Zhang Z, Li X, Wang C, et al. Efficient organic dyes con-
taining dibenzo heterocycles as conjugated linker part for dye-sensitized solar
cells. Tetrahedron 2013;69:1970e7.
3.93 (t, J ¼ 6.5 Hz, 2H), 1.82e1.96 (m, 8H), 1.42e1.53 (m, 3H),
1.31e1.40 (m, 5H), 0.85e0.94 (m, 6H), 0.67e0.74 (m, 17H). 13C NMR
(100 MHz, CDCl3):
d 190.1, 152.2, 150.5, 147.6, 146.8, 142.1, 140.9,
136.5, 136.3, 130.2, 128.4, 127.0, 126.8, 126.3, 125.6, 125.3, 124.5,
124.1, 123.4, 122.7, 120.3, 119.1, 118.9, 116.1, 114.7, 55.3, 48.2, 42.8,
31.4, 26.6, 22.6, 17.4, 14.5, 14.1. HRMS (ESI) calcd for C63H66N2OS
(MþHþ): 899.4974, found: 899.4977.
[18] Matsui M, Shiota T, Kubota Y, Funabiki K, Jin J, Yoshida T, et al. N-(2-
Alkoxyphenyl)-substituted double rhodanine indoline dyes for zinc oxide
dye-sensitized solar cell. Tetrahedron 2012;68:4286e91.
4.3.6. 3-(7-(4-(Bis(9,9-dipropyl-9H-fluoren-2-yl)amino)phenyl)-
10-hexyl-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (WR10)
Compound WR10 was synthesized according to the same pro-
cedure of WR8, giving a red powder (65% yield). mp 113e116 ꢂC. 1H
[19] Duan T, Fan K, Zhong C, Chen X, Peng T, Qin J. A new class of organic dyes
containing b-substituted 2, 20-bithiophenene unit as a
p-linker for dye-
sensitized solar cells: structural modification for understanding relationship
of structure and photovoltaic performances. J Power Sources 2013;234:23e30.
[20] Chen H, Huang H, Huang X, Clifford JN, Forneli A, Palomares E, et al. High
molar extinction coefficient branchlike organic dyes containing di(p-tolyl)
phenylamine donor for dye-sensitized solar cells applications. J Phys Chem
C 2014;114:3280e6.
[21] Liang M, Xu W, Cai F, Chen P, Peng B, Chen J, et al. New triphenylamine-based
organic dyes for efficient dye-sensitized solar cells. J Phys Chem C 2007;111:
4465e72.
[22] Li H, Hou Y, Yang Y, Tang R, Chen J, Wang H, et al. Attempt to improve the
performance of pyrrole-containing dyes in dye sensitized solar cells by
adjusting isolation groups. ACS Appl Mater Interfaces 2013;5:12469e77.
[23] Pei K, Wu Y, Islam A, Zhang Q, Han L, Tian H, et al. Constructing high-efficiency
NMR (400 MHz, CDCl3):
d
8.04 (s, 1H), 7.83 (d, J ¼ 8.0ꢂHz, 1H),
7.53e7.69 (m, 5H), 7.41 (d, J ¼ 8.0 Hz, 2H), 7.21e7.35 (m, 9H), 7.17 (d,
J ¼ 8.0 Hz, 2H), 7.06 (d, J ¼ 8.0 Hz, 2H), 6.87 (t, J ¼ 8.5 Hz, 2H), 3.83 (t,
J ¼ 6.5 Hz, 2H), 1.78e1.95 (m, 8H), 1.39e1.49 (m, 3H), 1.26e1.37 (m,
5H), 0.84e0.93 (m, 6H), 0.64-e0.73 (m, 17H). 13C NMR (100 MHz,
CDCl3):
d 152.3, 150.7, 147.7, 147.2, 147.0, 141.0, 136.4, 127.1, 126.8,
126.3, 125.4, 125.3, 124.6, 124.1, 123.5, 123.3, 122.8, 120.3, 119.1,
118.9, 55.3, 48.0, 42.7, 31.4, 29.7, 26.6, 22.6, 17.4, 14.5, 14.0. HRMS
(ESI) calcd for C66H67N3O2S (MþHþ):966.5032, found: 966.5038.
DeAꢀ
peAefeatured solar cell sensitizers: a promising building block of 2,3-
Diphenylquinoxaline for antiaggregation and photostability. ACS Appl Mater
Interfaces 2013;5:4986e95.
Acknowledgments
[24] Li G, Liang M, Wang H, Sun Z, Wang L, Wang Z, et al. Significant enhancement
of open-circuit voltage in indoline-based dye-sensitized solar cells via
retarding charge recombination. Chem Mater 2013;25:1713e22.
[25] Lu M, Liang M, Han H, Sun Z, Xue S. Organic dyes incorporating bishex-
apropyltruxeneamino moiety for efficient dye-sensitized solar cells. J Phys
Chem C 2011;115:274e81.
[26] Zhang Y, Zhang YF, Wang Z, Liang M, Jia D, Wu Q, et al. Effects of different
alkyl chains on the performance of dye-sensitized solar cells with different
electrolytes. J Power Sources 2014;253:167e76.
This work was supported by the grant from the Applied Basic
Research Programs of the Science and Technology Planning Project
of Sichuan Province (No. 14JC0176) and the Important Project in
Developing Oil & Gas of the National Department of Science and
Technology of China (No. 2011ZX05045).
[27] Zong X, Liang M, Fan C, Tang K, Li G, Sun Z, et al. Design of truxene-based
organic dyes for high-efficiency dye-sensitized solar cells employing cobalt
redox shuttle. J Phys Chem C 2012;116:11241e50.
References
[28] Wang Z, Liang M, Hao Y, Zhang Y, Wang L, Sun Z, et al. Influence of substituent
of dithieno[3,2-b:2',3'-d]pyrrole (DTP) spacer and the adsorption time on the
photovoltaic properties of arylamine organic dyes. J Mater Chem A 2013;1:
11809e19.
[29] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized
solar cells with an organic photosensitizer featuring orderly conjugated eth-
ylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22:
1915e25.
[30] Zhang M, Wang Y, Xu M, Ma W, Li R, Wang P. Design of high-efficiency
organic dyes for titania solar cells based on the chromophoric core of cyclo-
pentadithiophene-benzothiadiazole. Energy Environ Sci 2013;6:2944e9.
[31] Cao Y, Cai N, Wang Y, Li R, Yuan Y, Wang P. Modulating the assembly of
organic dye molecules on titania nanocrystals via alkyl chain elongation for
efficient mesoscopic cobalt solar cells. Phys Chem Chem Phys 2012;14:
8282e6.
[32] Ying W, Yang J, Wielopolski M, Moehl T, Moser JE, Comte P, et al. New pyrido
[3,4-b]pyrazine-based sensitizers for efficient and stable dye-sensitized solar
cells. Chem Sci 2014;5:206e14.
€
[1] O'Regan B, Gratzel M. A low-cost high efficiency solar cell based on dye
sensitized colloidal titanium dioxide films. Nature 1991;353:737e40.
[2] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells.
Chem Rev 2010;110:6595e663.
[3] Liang M, Chen J. Arylamine organic dyes for dye-sensitized solar cells. Chem
Soc Rev 2013;42:3453e88.
[4] Hua Y, Wang H, Zhu X, Islam A, Han L, Qin C, et al. New simple panchromatic
dyes based on thiadiazolo[3,4-c]pyridine unit for dye-sensitized solar cells.
Dyes Pigments 2014;102:196e203.
[5] Wan ZQ, Jia CY, Zhang JQ, Duan YD, Lin Y, Shi Y. Triphenylamine-based star-
burst dyes with carbazole and phenothiazine antennas for dye-sensitized
solar cells. J Power Sources 2012;199:426e31.
[6] Al-Eid M, Lim SH, Park KW, Fitzpatrick B, Han CH, Kwak K, et al. Facile syn-
thesis of metal-free organic dyes featuring a thienylethynyl spacer for dye
sensitized solar cells. Dyes Pigments 2014;104:197e203.
[7] He J, Hua J, Hu G, Yin XJ, Gong H, Li C. Organic dyes incorporating a thiophene
or furan moiety for efficient dye-sensitized solar cells. Dyes Pigments
2014;104:75e82.
[33] Wang S, Guo J, He L, Wang H, Zhao J, Lu C. Influence of thiophene and benzene
unit in triphenylamine dyes on the performance of dye-sensitized solar cells.
Synth Met 2013;168:1e8.
[8] Li H, Yang L, Tang R, Hou Y, Yang Y, Wang H, et al. Organic dyes incorporating
N-functionalized pyrrole as conjugated bridge for dye-sensitized solar cells:
[34] Tian H, Yang X, Chen R, Pan Y, Li L, Hagfeldt A, et al. Phenothiazine derivatives
for efficient organic dye-sensitized solar cells. Chem Commun; 2007:3741e3.
[35] Liu J, Yang X, Zhao J, Sun L. Tuning band structures of dyes for dye-sensitized
solar cells: effect of different p-bridges on the performance of cells. RSC Adv
2013;3:15734e43.
convenient synthesis, additional withdrawing group on the
suppressed aggregation. Dyes Pigments 2013;99:863e70.
[9] Iqbal Z, Wu WQ, Zhang H, Hua PL, Fang X, Kuang DB, et al. Impact of hydroxy
and octyloxy substituents of phenothiazine based dyes on the photovoltaic
performance. Dyes Pigments 2013;99:299e307.
p-bridge and the
[36] Yang CJ, Chang YJ, Watanabe M, Hona YS, Chow TJ. Phenothiazine derivatives
as organic sensitizers for highly efficient dye-sensitized solar cells. J Mater
Chem 2012;22:4040e9.
[10] Duan T, Fan K, Fu Y, Zhong C, Chen X, Peng T, et al. Triphenylamine-based
organic dyes containing a 1,2,3-triazole bridge for dye-sensitized solar cells
via a ‘Click’ reaction. Dyes Pigments 2012;94:28e33.
[37] Hua Y, Chang S, Huang D, Zhou X, Zhu X, Zhao J, et al. Significant improvement
of dye-sensitized solar cell performance using simple phenothiazine-based
dyes. Chem Mater 2013;25:2146e53.
[38] Cheng M, Yang X, Zhang F, Zhao J, Sun L. Tuning the HOMO and LUMO energy
levels of organic dyes with N-Carboxomethylpyridinium as acceptor to opti-
mize the efficiency of dye-sensitized solar cells. J Phys Chem C 2013;117:
9076e83.
[39] Cheng M, Yang X, Chen C, Zhao J, Tana Q, Sun L. Effect of the acceptor on the
performance of dye-sensitized solar cells. Phys Chem Chem Phys 2013;15:
17452e9.
[11] Verbitskiy EV, Cheprakova EM, Subbotina JO, Schepochkin AV, Slepukhin PA,
Rusinov GL, et al. Synthesis, spectral and electrochemical properties of pyr-
imidinecontaining dyes as photosensitizers for dye-sensitized solar cells. Dyes
Pigmets 2014;100:201e14.
[12] Zhao J, Jin T, Islam A, Kwon E, Akhtaruzzaman M, Asao N, et al. Thieno[2,3-a]
carbazole-based donorepeacceptor organic dyes for efficient dyesensitized
solar cells. Tetrahedron; 2014. http://dx.doi.org/10.1016/j.tet.2014.01.001.
[13] Velusamy M, Thomas KRJ, Lin JT, Hsu YC, Ho KC. Organic dyes incorporating
low-band-gap chromophores for dye-sensitized solar cells. Org Lett 2005;7:
1899e902.
[40] Kim SH, Kim HW, Sakong C, Namgoong JW, Park SW, Ko MJ, et al. Effect of
five-membered heteroaromatic linkers to the performance of phenothiazine-
based dye-sensitized solar cells. Org Lett 2011;13:5784e7.
[14] Yu X, Ci Z, Liu T, Feng X, Wang C, Ma T, et al. Influence of different electron
acceptors in organic sensitizers on the performance of dye-sensitized solar
cells. Dyes Pigments 2014;102:126e32.

104
S. Wang et al. / Dyes and Pigments 109 (2014) 96e104
[41] Cao D, Peng J, Hong Y, Fang X, Wang L, Meier H. Enhanced performance of the
dye-sensitized solar cells with phenothiazine-based dyes containing double
DeA branches. Org Lett 2011;13:1610e3.
[47] Chen C, Liao JY, Chi Z, Xu B, Zhang X, Kuang DB, et al. Metal-free organic dyes
derived from triphenylethylene for dye-sensitized solar cells: tuning of the
performance by phenothiazine and carbazole.
J Mater Chem 2012;22:
[42] Hart AS, Bikram KCC, Subbaiyan NK, Karr PA, D’Souza F. Phenothiazine-
sensitized organic solar cells: effect of dye anchor group positioning on the
cell performance. ACS Appl Mater Interfaces 2012;4:5813e20.
[43] Wu W, Yang J, Hua J, Tang J, Zhang L, Long Y, et al. Efficient and stable dye-
sensitized solar cells based on phenothiazine sensitizers with thiophene
units. J Mater Chem 2010;20:1772e9.
[44] Chang YJ, Chou PT, Lin YZ, Watanabe M, Yang CJ, Chin TM, et al. Organic dyes
containing oligo-phenothiazine for dye-sensitized solar cells. J Mater Chem
2012;22:21704e12.
8994e9005.
[48] Marotta G, Reddy MA, Singh SP, Islam A, Han L, Angelis FD, et al. Novel
carbazole-Phenothiazine dyads for dye-Sensitized solar cells: a combined
experimental and theoretical study. ACS Appl Mater Interfaces 2013;19:
9635e47.
[49] Wan Z, Jia C, Duan Y, Zhou L, Lin Y, Shi Y. Phenothiazineetriphenylamine
based organic dyes containing various conjugated linkers for efficient dye-
sensitized solar cells. J Mater Chem 2012;22:25140e7.
[50] Zhao Z, Xu X, Wang H, Lu P, Yu G, Liu Y. Zigzag molecules from pyrene-
modified carbazole oligomers: synthesis, characterization, and application
in OLEDs. J Org Chem 2008;73:594e602.
[45] Iqbal Z, Wu WQ, Kuang DB, Wang L, Meier H, Cao D. Phenothiazine-based
dyes with bilateral extension of
p-conjugation for efficient dye-sensitized
ꢀ
solar cells. Dyes Pigments 2013;96:722e31.
[51] Ito S, Nazeeruddin MK, Liska P, Comte P, Charvet R, Pechy P, et al. Photovoltaic
[46] Iqbal Z, Wu WQ, Zhang H, Han L, Fang X, Wang L, et al. Influence of spatial
characterization of dye-sensitized solar cells: effect of device masking on
arrangements of
p
-spacer and acceptor of phenothiazine based dyes on the
conversion efficiency. Prog Photovoltaics 2006;14:589e601.
performance of dye-sensitized solar cells. Org Electron 2013;14:2662e72.