Impact of hydroxy and octyloxy substituents of phenothiazine based dyes on the photovoltaic performance

Article abstract of DOI:10.1016/j.dyepig.2013.05.032

Two novel organic dyes containing hydroxy and octyloxy substituents onto a phenothiazine skeleton were synthesized and their effects on the photovoltaic performance were studied. Hydroxy acts as an ancillary anchoring unit along with the carboxylic group, while the phenothiazine modified moiety acts as an electron donor. The photophysical and electrochemical studies revealed that maximum absorbance of the dye with the hydroxy group in the solution was blue shifted and its band gap increased, indicating that donor acceptor strength was reduced as compared to the octyloxy substituted dye. Furthermore, electron lifetime of the organic dye with the hydroxy moiety was shorter due to smaller resistance of electron recombination. Contrarily the dye with octyloxy moiety exhibited higher electron lifetime and open-circuit photovoltage leading to an overall power conversion efficiency of 6.32% under standard AM 1.5G illumination. The IPCE was over 80% in the region between 450 and 500 nm.

Full text of DOI:10.1016/j.dyepig.2013.05.032

Dyes and Pigments 99 (2013) 299e307
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Impact of hydroxy and octyloxy substituents of phenothiazine based
dyes on the photovoltaic performance
,
Zafar Iqbal a c, Wu-Qiang Wu b, Hai Zhang a, Peng-Li Hua a, Xiaoming Fang a,
**
,
a
d
a,
*
Dai-Bin Kuang b , Lingyun Wang , Herbert Meier , Derong Cao
a School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510641, China
b MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical
Engineering, Sun Yat-sen University, Guangzhou 510275, China
c PCSIR Laboratories Complex, Feroze pur Road, Lahore 54000, Pakistan
d Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel organic dyes containing hydroxy and octyloxy substituents onto a phenothiazine skeleton
were synthesized and their effects on the photovoltaic performance were studied. Hydroxy acts as an
ancillary anchoring unit along with the carboxylic group, while the phenothiazine modified moiety acts
as an electron donor. The photophysical and electrochemical studies revealed that maximum absorbance
of the dye with the hydroxy group in the solution was blue shifted and its band gap increased, indicating
that donor acceptor strength was reduced as compared to the octyloxy substituted dye. Furthermore,
electron lifetime of the organic dye with the hydroxy moiety was shorter due to smaller resistance of
electron recombination. Contrarily the dye with octyloxy moiety exhibited higher electron lifetime and
open-circuit photovoltage leading to an overall power conversion efficiency of 6.32% under standard AM
1.5G illumination. The IPCE was over 80% in the region between 450 and 500 nm.
Received 20 March 2013
Received in revised form
24 May 2013
Accepted 27 May 2013
Available online 13 June 2013
Keywords:
Organic dye
Ancillary anchoring unit
Dye-sensitized solar cells
Phenothiazine
Ó 2013 Elsevier Ltd. All rights reserved.
Open-circuit photovoltage
Photovoltaic performance
1. Introduction
promising inexpensive energy source since the first report from
Grätzel in 1991 [3]. DSSCs are being considered to be a potential
The increasing global demand for energy and depletion of fossil
fuel reserves necessitate the development of clean alternative en-
ergy sources. Among all the renewable energy resources, solar
energy is regarded as one of the most perfect energy sources that
have the largest potential to cater for the future global need
without appreciable greenhouse effect [1]. Solar radiation amounts
to 3.8 million EJ/year, which is approximately 10,000 times more
than current energy needs [2]. From the perspective of energy
conservation and environmental protection, it is desirable to
directly convert solar radiation into electrical power by the appli-
cation of photovoltaic devices.
alternative to expensive inorganic solar cells due to low material
cost, easy and inexpensive fabrication process and reasonably good
power conversion efficiency [4e6]. Furthermore, dye-sensitized
solar cells have been proved to perform better than conventional
solar cell in some aspects, i.e. in the diffused light [7] and at mod-
erate temperature e up to 50 ꢀC [8]. Above all, DSSCs have opened
up remarkable new possibilities and paradigms for producing solar
photovoltaics in a green and low-cost manner.
Metals like ruthenium and zinc based dyes have been exten-
sively studied and gave efficiencies over 10e11% [5] and 12%,
respectively [6]. Ruthenium is a rare and expensive metal and its
dyes have relatively low molar extinction coefficients. Furthermore,
these complexes need tedious purification processes, which
hamper large scale production [9]. Therefore, more efforts have
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. In this regard,
great progress has been made in this field and various electron
Dye-sensitized solar cells (DSSCs), a third generation photovol-
taic technology, have attracted a great deal of interest as a
* Corresponding author. Tel./fax: þ86 20 87110245.
** Corresponding author. Tel.: þ86 20 84113015.
E-mail addresses: kuangdb@mail.sysu.edu.cn (D.-B. Kuang), drcao@scut.edu.cn
(D. Cao).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.05.032

300
Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
donors like triphenylamine, indoline, carbazole, merocyanine,
polyene, hemicyanine, fluorene, tetrahydroquinoline, phenoxazine
and phenothiazine have been explored [10e21]. However, the
photovoltaic performance of metal-free organic dyes still lag
behind the ruthenium complexes.
Among the components of DSSCs, the sensitizer is a crucial
part, which significantly influences the power conversion effi-
ciency as well as the stability of the device. The molecular archi-
modifications have been synthesized for application in dye-
sensitized solar cells [43e48].
Keeping in view the above discussion, the current concept has
been developed. Two novel dyes POH and POCT through simul-
taneous modifications of donor and linker of PTZ were designed
and synthesized for DSSCs. Dye POH contained hydroxy and car-
boxy as anchoring units with conjugate extension at the 7-position
and an ethyl chain at the 10-position of PTZ, while dye POCT
containing an octyloxyphenyl moiety at the 7-position and an
ethyl chain at the 10-position of PTZ. (Scheme 1). The effects of
hydroxy and octyloxy substituents on the photophysical, photo-
chemical and photovoltaic characteristics were investigated in
detail.
tecture of most sensitizers includes
-bridge) and an acceptor (A), which are usually combined
following a De eA rod like configuration [22]. The efficiency of
a donor (D), a bridge
(p
p
sensitization is critically dependent on electron injection from a
photoexcited state of the dye into the conduction band of the
semiconductor and resistance of recombination. The injection of
electrons from the excited states is most likely in the vibrational
mode i.e. hot electrons injected from the dye into the metal oxide,
hence there is a mandatory requirement for proximal contact
between the dye and metal oxide [23]. This suggests that the dye-
metal oxide distance should be shorter and therefore the mode of
linkage of dye on metal oxide needs more attention. Typically, the
carboxy group has been used as an anchoring unit but it suffers
hydrolysis, which inhibits the electron injection rate [24]. The
carboxy group can either form an ester-like linkage (C¼O) or
carboxylate-like linkage (CeOeOe), resulting in the decrease of
electron density of the ligand, leading to a lower energy shift in
the band. Keeping in mind this factor, several groups have inves-
tigated more than one anchoring unit for efficient electron injec-
tion [25e31]. All authors have reported symmetric di-anchoring
groups for efficient electron injection from the excited state of the
dye to the conduction band of TiO2 but still the efficiency lags
behind. Increased carboxy groups in the dye cause decrease in
electron donor-acceptor strength and also facilitate aggregation
via hydrogen bonding on the surface of titanium oxide. The bot-
tlenecks lie in either the low electron injection efficiency or the
limited broadening of absorption spectra.
Hydroxy is another promising anchoring group because it can
form a strong chromophores-semiconductor coupling with least
spatial arrangements [32]. Hemicyanine dyes with a hydroxy group
were investigated by Yao et al. [33] and Chen et al. [34] they have
found that electron injection rate increases which gave better ef-
ficiency as compared to carboxy group alone. Similar study has also
been reported by Huang et al., in 2009 [35]. Efficient electron in-
jection, reduction of recombination and effective regeneration of
dye are the leading factors for the enhancement of power conver-
sion efficiency [36]. In order to absorb the sunlight as much as
possible, a broad absorption spectrum of the dye is desirable.
2. Experimental section
2.1. Instrumentation and materials
1H NMR spectra were obtained on a Bruker 400 MHz spec-
trometer in CDCl3 or DMSO-d6 with tetramethylsilane (TMS) as the
internal standard. Mass spectra (APCIeMS) were recorded on an
Esquire HCT PLUS mass spectrometer. Elementary analyses were
performed by using Vario ELIII Analyzer, while melting points were
taken on Tektronix X4 microscopic MP apparatus. Infrared spectra
(FTeIR) were recorded on KBr pellets with Bruker Tensor 27
spectrometer. The UVevisible and fluorescent spectra of the dyes
were determined in THF and CH2Cl2 solutions (1 ꢁ 10ꢂ5 M) and
measured at room temperature by Shimadzu UV-2450 UVeVis
spectrophotometer and Fluorolog III photoluminescence spec-
trometer, respectively.
Electrochemical redox potentials of the dyes were measured by
cyclic voltammetry (CV), using an Ingsens 1030 electrochemical
work station (Ingsens Instrument Guangzhou, Co. Ltd., China) and
three electrode cells in one compartment. The measurements were
carried out at a scan rate of 50 mV/s. The Ag/AgCl in KCl (3 M)
solution and an auxiliary platinum wire were utilized as reference
and counter electrodes, while dye loaded TiO2 films were used as a
working electrode. Tetrabutylammonium perchlorate (n-Bu4NClO4)
0.1 M was used as supporting electrolyte in acetonitrile. All elec-
trochemical measurements were calibrated by using ferrocene as
standard (0.63 V vs. NHE). The photocurrent density-photovoltage
characteristics were recorded by using a Keithley 2400 source
meter under simulated AM 1.5 G (100 mW cm2) illumination with a
solar light simulator (Oriel, Model: 91192). A 1000 W Xenon arc
lamp (Oriel, Model: 6271) served as a light source which was
calibrated with an NREL standard Si solar cell. The incident photon-
to-current conversion efficiency (IPCE) spectra were measured as a
function of wavelength from 380 to 800 nm on the basis of a
Spectral Products DK240 monochromator. The electrochemical
impedance spectra (EIS) measurements were conducted on the
electrochemical workstation (Zahner Zennium) in dark conditions
with an applied bias potential of ꢂ0.71 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 the impedance spectra using Zeview
software. Intensity modulated photovoltage spectroscopy (IMVS)
was carried out on the electrochemical workstation (Zahner Zen-
nium) with a frequency response analyzer under a modulated LED
light (457 nm) driven by a source supply (Zahner PP211). The illu-
mination light-intensity ranged from 30 to 150 mW cmꢂ2. The light
intensity modulation was 10% or less than the base light intensity.
The frequency was set in a range from 100 kHz to 0.1 Hz. The dye-
loading amount was determined by desorbing the dye from the
films with 0.1 M NaOH in ethanol/H2O and measuring UVeVis
spectrum of the obtained solution [49].
Usually, extension of
this task. However, the large
p
econjugation is a feasible strategy to resolve
econjugation system leads to poor
p
photovoltaic properties due to dye aggregation, as well as electron
recombination issues. To diminish the dye aggregation and the
electron recombination, the alkoxy or alkyl chains were linked to
the aromatic system. Various studies have been conducted by the
introduction of bulky alkoxy and aromatic rings to the donor
system and improvement in the efficiencies have been observed
[37e39]. Most likely, this is due to an increase in electron donation,
as well as a decrease in aggregation and electron recombination
reactions of the dyes on TiO2 films.
Phenothiazine (PTZ) is a well known photosensitizer with
electron-rich sulfur and nitrogen heteroatom, and its ring is non-
planar with a butterfly conformation at the ground state, which
can impede the molecular aggregation and the formation of mo-
lecular excimers [40]. The additional electron rich sulfur atom
renders PTZ a stronger donor than other amines, even better than
triphenylamine, tetrahydroquinoline, carbazole and iminodi-
benzyles [41,42]. PTZ based dyes containing various types of

Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
301
Scheme 1. Syntheses of the dyes POH and POCT. (i) DMF, POCl3, 0 ꢀC, dichloroethane, 90 ꢀC, overnight; (ii) Chloroform, NBS, 70 ꢀC, 6 h; (iii) 4-Hydroxyphenylboronic acid, DMF,
K2CO3, Pd(PPh3)4, 90 ꢀC, 36 h; (iv) Cyanoacetic acid, piperidine, acetonitrile, reflux, 7 h; (v) DMF, K2CO3, 1-bromooctane, 130 ꢀC, 24 h.
10-Ethyl-10H-phenothiazine (1) was prepared from phenothi-
azine by alkylation in the presence of N,N dimethylformamide
(DMF) followed by the synthesis of 10-ethyl-10H-phenothiazine-3-
carbaldehyde (2) via VilsmeiereHaack reactions in the presence of
DMF and phosphorus oxychloride (POCl3), according to reported
literature [50]. The reagents including phenothiazine, N-bromo-
2.2.2. 7-Bromo-10-ethyl-10H-phenothiazine-3-carbaldehyde (3)
10-Ethyl-10H-phenothiazine-3-carbaldehyde (1.28
2
g,
5.0 mmol) was dissolved in CHCl3 (25 mL) at 0 ꢀC and maintained
for half an hour. Then N-bromosuccinimide (NBS) (1.06 g,
6.0 mmol) dissolved in chloroform (10 mL) was injected slowly
under nitrogen and the temperature raised to 70 ꢀC. The reaction
was completed in 6 h. After cooling, the reaction mixture was
poured into the water and extracted with CH2Cl2 (250 mL). The
organic layer was separated and washed three times with brine
solution. Then it was dried over anhydrous Na2SO4 and solvent was
removed under reduced pressure. The desired product was purified
by column chromatography, using silica gel and PE/DCM (1:1) as
the mobile phase to obtain the required product 3 as a yellow solid
(1.36 g) in 81% yield, mp 99e101 ꢀC. 1H NMR (CDCl3, 400 MHz,
succinimide
(NBS), tetrakis(triphenylphosphine)palladium(0)
Pd(PPh3)4, 4-hydroxyphenylboronic acid, 1-bromooctane and sol-
vents like DMF, petroleum ether (60e90 ꢀC, PE), dichloromethane
(DCM), tetrahydrofuran (THF) and chloroform were purchased as
reagent grade and used without purification. All reactions were
performed under nitrogen or argon and monitored by TLC (Merck
60 F254) and purified by column chromatography on silica gel
(200e300 mesh).
ppm):
d 9.80 (s, 1H), 7.65e7.63 (m, 1H), 7.56e7.55 (m, 1H), 7.26e
2.2. Synthesis of dyes
7.23 (m, 1H), 7.21e7.20 (m, 1H) 6.91e6.89 (m, 1H), 6.74e6.71 (m,
1H), 3.96e3.91 (m, 2H), 1.43 (t, J ¼ 6.8 Hz, 3H).
2.2.1. 10-Ethyl-10H-phenothiazine-3-carbaldehyde (2)
POCl3 (6.90 g, 45 mmol) was added drop wise to the freshly
distilled N,N-dimethylformamide (3.29 g, 45 mmol) at 0 ꢀC under a
nitrogen atmosphere. Stirring was continued until a solid glassy
material appeared which was dissolved in dichloroethane (50 mL)
and stirred for an additional 1 h. Then compound 1 (2.05 g, 9 mmol)
dissolved in dichloroethane (15 mL) was injected into the flask and
maintained at 90 ꢀC overnight. When the reaction was finished (TLC
monitoring), the reaction mixture was cooled to room temperature
and poured into water. The obtained mixture was neutralized with
dilute solution of NaOH. The product was extracted with
dichloromethane (250 mL). The organic layer was separated,
washed with water and dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure and the crude product was
purified by column chromatography using silica gel and petroleum
ether/dichloromethane (PE/DCM, 1:1) as the mobile phase to
obtain the required product 2 as a yellow solid (1.69 g) in 73.5%
2.2.3. 10-Ethyl-7-(4-hydroxyphenyl)-10H-phenothiazine-3-
carbaldehyde (4)
A
mixture of the 7-bromo-10-ethyl-10H-phenothiazine-3-
carbaldehyde 3 (1.34 g, 4 mmol), DMF (20 mL), aqueous solution of
K2CO3 (2 M, 10 mL) and Pd(PPh3)4 (10 mol %) was heated to 40 ꢀC for
half an hour under argon. Then 4-hydroxyphenylboronic acid (0.64 g,
4.5 mmol) dissolved in DMF (10 mL) was injected slowly into the
reaction mixture. The temperature of the reaction mixture was
raised to 90 ꢀC and maintained for 36 h. After cooling, it was dropped
into water (500 mL) and extracted with CH2Cl2 (150 mL). The organic
layer was washed several times with water (250 mL each), then dried
over anhydrous Na2SO4. The solvent was evaporated under reduced
pressure and purification was carried out by column chromatog-
raphy using silica gel and DCM as the mobile phase to give com-
pound 4 as an orange color solid (0.57 g) in 46.9% yield, mp 183e
1
184 ꢀC. H NMR (CDCl3, 400 MHz, ppm):
d 9.81 (s, 1H), 7.67e7.65
yield, mp 82e84 ꢀC. 1H NMR (CDCl3, 400 MHz, ppm):
d
9.75 (s, 1H),
(m, 1H), 7.61e7.60 (m, 1H), 7.44e7.41 (m, 2H), 7.35e7.32 (m, 1H)
7.29e7.28 (m, 2H), 6.95e6.90 (m, 3H), 4.04e3.99 (m, 2H), 1.50 (t,
J ¼ 6.8 Hz, 3H). FTeIR (KBr pellet, cmꢂ1): 3400, 3055, 2926, 2854,
1667, 1580, 1568, 1507, 1289, 1248. APCIeMS: m/z 347.10. Found:
7.60e7.58 (m, 1H), 7.53e7.52 (m, 1H), 7.15e7.11 (m, 1H), 7.07e7.05
(m, 1H) 6.95e6.91 (m, 1H), 6.87e6.85 (m, 2H), 3.95e3.90 (m, 2H),
1.41 (t, J ¼ 6.8 Hz, 3H).

302
Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
347.90 [M þ H]þ. Anal. Calcd. for C21H17NO2S: C, 72.60; H, 4.93; N,
Table 1
Absorption and emission characteristics of the dyes POH and POCT.
4.03; S, 9.23; found: C, 72.85; H, 4.92; N, 4.23; S, 9.19.
b
Dye
Absorption ε (Mꢂ1 cmꢂ1
lmax (nm)
)
Emission
Absorption Blue shift of
a
lmax (nm)c lmax (nm)d lmax (nm)e
2.2.4. 10-Ethyl-7-(4-(octyloxy)phenyl)-10H-phenothiazine-3-
carbaldehyde (5)
POH
POCT 304, 460
299, 446
53250, 18800
37900, 16100
572
611
427
454
19
6
A mixture of compound 4 (0.26 g, 0.75 mmol), K2CO3 (0.31 g,
2.25 mmol) and DMF (30 mL) was stirred for half an hour at room
temperature and then 1-bromooctane (0.29 g,1.5 mmol) was injected
under nitrogen. Temperature was raised to 130 ꢀC and maintained for
24 h. After cooling, the reaction mixture was poured into water
(500mL)andstirredforhalfanhour. ThenCH2Cl2 (250mL)wasadded
and the organic layer was separated. The organic layer was washed
several times with water, dried over anhydrous Na2SO4 and solvent
was removed by rotary evaporator. The crude product was purified
by column chromatography using silica and PE/DCM (1:1) as the
mobile phase to yield compound 5 (0.32 g, 92%), a bright yellow
a
Absorption maximum of dyes measured in THF/DCM (1:1) with concentration
1 ꢁ 10ꢂ5 mol/L.
b
The molar extinction coefficient at lmax in solution.
c
Emission maximum of dyes in THF/DCM (1:1) with concentration 1 ꢁ 10ꢂ5 mol/
L.
d
Absorption maximum of dyes adsorbed on the surface of TiO2.
Absorption lmax
e
.
water, ethanol and acetone in an ultrasonic bath for removing un-
wanted materials. Anatase TiO2 nanoparticles (20 nm) were prepared
through a hydrothermal treatment with a precursor solution con-
taining Ti(OBu)4 (10 mL), ethanol (20 mL), acetic acid (18 mL) and
deionized water (50 mL) according to the previously reported paper
[51]. The prepared TiO2 powder (1.0 g) was ground for 40 min in a
mixture of ethanol (8.0 mL), acetic acid (0.2 mL), terpineol (3.0 g) and
ethyl cellulose (0.5 g). After that, homogenized slurry was sonicated
for 15 min in an ultrasonic bath to form a viscous white TiO2 paste.
Then the paste was screen-printed onto FTO glass and TiO2 photo-
solid, mp 92e93 ꢀC. 1H NMR (CDCl3, 400 MHz, ppm):
d 9.80 (s, 1H),
7.66e7.63 (m, 1H), 7.59e7.58 (m, 1H), 7.47e7.45 (m. 2H), 7.36e7.33
(m, 1H), 7.30e7.28 (m, 1H), 6.98e6.95 (m, 2H), 6.94e6.90 (m, 2H),
4.02e3.97 (m, 4H), 1.86e1.79 (m, 2H), 1.50e1.47 (m, 5H), 1.38e1.29
(m, 8H), 0.92 (t, J ¼ 6.4 Hz, 3H). FTeIR (KBr pellet, cmꢂ1): 3062, 2925,
2854, 1678, 1607, 1580, 1505, 1296, 1245. APCIeMS: m/z 459.20.
Found: 460.20 [M þ H]þ. Anal. Calcd. for C29H33NO2S: C, 75.78; H,
7.24; N, 3.05; S, 6.98; found: C, 75.82; H, 7.34; N, 3.27; S, 6.79.
anodes (about16 mm inthickness) were prepared. Annealationsof the
prepared films were performed through a procedure (325 ꢀC for
5 min, 375 ꢀC for 5 min, 450 ꢀC for 15 min, and 500 ꢀC for 15 min) in a
muffle furnace to remove the organic substances. TiO2 films were
soaked in TiCl4 (0.04 M) aqueous solution for 30 min at 70 ꢀC for the
improvement of photocurrent and photovoltaic performance. Then
treated films were rinsed with deionized water and ethanol and
sintered again at 520 ꢀC for 30 min. After cooling to 80 ꢀC, the films
were immersed in a 5.0 ꢁ 10ꢂ4 M solution of POH and POCT dyes for
16 h (dyes solution in a mixture of tetrahydrofuran and dichloro-
methane, 1:1 volume ratio). Films were taken out of solution and
rinsed with CH2Cl2 in order to remove physical adsorbed organic dye
molecules. The dye-sensitized TiO2/FTO glass films were assembled
into a sandwiched type together with Pt/FTO counter electrode to
evaluate their photovoltaic performance. Platinized counter elec-
trodes were fabricated by thermal depositing of H2PtCl6 (5 mM in
isopropanol) solution onto FTO glass followed by heating at 400 ꢀC for
2.2.5. 2-Cyano-3-(10-ethyl-7-(4-hydroxyphenyl)-10H-
phenothiazin-3-yl)acrylic acid (POH)
A mixture of compound 4 (0.21 g, 0.6 mmol), cyanoacetic acid
(0.51 g, 6.0 mmol), piperidine (0.3 mL, 3 mmol) in acetonitrile
(35 mL) was refluxed for 7 h under nitrogen. After cooling, the
mixture was poured into aqueous solution of HCl (2 M, 100 mL). It
was stirred for 15 min and CH2Cl2 (200 mL) was added. The organic
layer was separated and washed again two times with water
(200 mL). Then the organic layer was dried over anhydrous Na2SO4.
The solvent was removed under reduced pressure and the crude
product was purified by column chromatography using silica gel.
First, it was eluted with DCM and then by DCM/CH3OH (20:1)
to yield compound POH (0.15 g, 61%) as reddish black solid, mp
222e223 ꢀC. 1H NMR (DMSO-d6, 400 MHz, ppm):
d 7.88 (s, 1H),
7.75e7.70 (m, 2H), 7.46e7.39 (m, 3H), 7.35e7.34 (m, 1H), 7.09e7.04
(m, 2H), 6.88e6.87 (m, 2H), 3.99e3.93 (m, 2H), 1.33 (t, J ¼ 6.3, 3H).
FTeIR (KBr pellet, cmꢂ1): 3445, 3067, 2925, 2853, 2215, 1620, 1589,
1510, 1284, 1248. APCIeMS: m/z 414.10. Found: 414.90 [M þ H]þ.
Anal. Calcd. for C24H18N2O3S: C, 69.55; H, 4.38; N, 6.76; S, 7.74;
found: C, 69.65; H, 4.70; N, 6.86; S, 7.56.
2.2.6. 2-Cyano-3-(10-ethyl-7-(4-(octyloxy)phenyl)-10H-
phenothiazin-3-yl)acrylic acid (POCT)
Prepared by the same synthetic procedure as for compound
POH. Compound 5 was used instead of compound 4 to give com-
pound POCT as dark black solid in 65.6% yield, mp 158e159 ꢀC. 1H
NMR (DMSO-d6, 400 MHz, ppm):
d 8.03 (s, br, 1H), 7.87e7.85
(m, 1H), 7.79e7.78 (m, 1H), 7.59e7.56 (m, 2H), 7.47e7.45 (m, 1H),
7.41e7.40 (m, 1H), 7.16e7.10 (m, 2H), 7.00e6.98 (m, 2H), 4.03e3.98
(m, 4H),1.77e1.70 (m, 2H),1.45e1.41 (m, 2H),1.37 (t, J ¼ 6.4 Hz, 3H),
1.31e1.25 (m, 8H), 0.89 (t, J ¼ 6.8 Hz, 3H). FTeIR (KBr pellet, cmꢂ1):
3444, 3061, 2926, 2854, 2216, 1618, 1589, 1510, 1291, 1247. APCIe
MS: m/z, 526.23. Found: 527.20 [M þ H]þ. Anal. Calcd. for
C
32H34N2O3S : C, 72.97; H, 6.51; N, 5.32; S, 6.09; found: C, 72.92; H,
6.67; N, 5.37; S, 6.07.
2.3. Fabrication of the dye-sensitized solar cells
Optically transparent fluorine-doped tin oxide (FTO) conducting
glasses (Nippon SheetGlass, Japan) were cleaned with detergent,
Fig. 1. Normalized absorption spectra of the dyes POH and POCT.

Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
303
Table 3
Photovoltaic parameters of DSSCs based on dyes POH and POCT.
Dyes CDCA JSC (mA/ Voc FF (%) Electron R2
h
Dye load
cmꢂ2
)
(mV)
lifetime (ohm) amount
(
sr, ms)
(mol cmꢂ2
)
POH
0
11.28
8.54
12.47
677
779
741
787
0.69 5.23 64.0
51.6
1.56 ꢁ 10ꢂ7
a
a
1 mmol
0
1 mmol 11.18
0.75 4.96
e
e a
e
POCT
0.68 6.32 110
102.2 1.63 ꢁ 10ꢂ7
a
a
0.70 6.19
e
e a
e
a
Unmeasured.
thin TiO2 film are presented in Table 1, Figs. 1 and 2. The UV spectra
of the dyes exhibited two distinct bands, one at 299, 304 nm and
the other at 446, 460 nm. The first band corresponds to the
pep*
transition of the localized conjugated skeleton in the UV region,
while the second band was assigned to an intramolecular charge
transfer (ICT) in the visible region. UV absorption lmax increases by
14 nm from POH with hydroxy group to POCT with octyloxy group,
which imparts a strong electron donating capability. The blue shift
of dye POH with hydroxy unit was attributed to its decreased donor
and acceptor strength [52]. Molar extinction coefficients (ε) of the
Fig. 2. Normalized emission spectra of the dyes POH and POCT.
dyes POH and POCT were 18800 Mꢂ1 cmꢂ1 and 16100 Mꢂ1 cmꢂ1
,
15 min in air. The electrolyte (0.6 M 1-methyl-3-propylimidazolium
iodide (PMII), 0.10 M guanidinium thiocyanate, 0.05 M LiI, 0.03 M I2
and 0.5 M tert-butylpyridine) in acetonitrile/valeronitrile (85:15) was
injectedfromaholemadeonthecounterelectrodeintotheinterspace
betweenthe photoanode and counterelectrode. The active area of the
dye coated TiO2 film was 0.16 cm2 and aperture size of black mask
used was about 0.25 cm2.
respectively. The higher value of ε for dye POH was due to the larger
oscillator strength of the charge transition, according to Frank-
Condon principle [26,53]. After being adsorbed on the surface of
TiO2 films, lmax of both dyes POH and POCT was blue shifted to
19 nm and 6 nm as compared to the solution (Table 1). It appears to
be a common phenomenon for organic dyes, which has been
ascribed to the deprotonation of anchoring groups as well as to the
formation of aggregation on the semiconductor surface [54].
Dye load amounts of the dyes POH and POCT were
1.56 ꢁ 10ꢂ7 mol cmꢂ2 and 1.63 ꢁ 10ꢂ7 mol cmꢂ2, respectively
(Table 3). The dye POH has lower dye load amount owing to the
presence of hydroxy unit which might increase the contact area
between dye molecule and TiO2, which subsequently leads to a
decreased adsorption of the dye per unit surface area of TiO2 film
[33,34].
The dye POH has hydroxy and carboxy as anchoring groups
whereas the dye POCT has only a carboxy group. Their mode of
attachments were studied by fourier transform infrared spectrto-
scopy (FT-IR). Characteristic bands for eC^N were observed at
2219 cmꢂ1 and 2211 cmꢂ1 while the eC]O stretching bands of the
carboxyl groups were observed at 1693 cmꢂ1 and 1697 cmꢂ1 for
POH and POCT dyes, respectively. Upon dye adsorption to the TiO2
surface, eC]O stretching bands at 1693 cmꢂ1 and 1697 cmꢂ1
disappeared and new asymmetric stretching bands at 1607 cmꢂ1
and 1626 cmꢂ1 and symmetric stretching bands at 1382 cmꢂ1 and
1383 cmꢂ1 for carboxylate units appeared in the spectra of POH and
POCT dyes [28,55,56]. Additionally, compared to POCT dye an
intense band at 1116 cmꢂ1 for POH dye spectra was observed,
which was due to eCeOe stretching. This observation indicates
that the hydroxy group was also involved in anchoring with the
TiO2 surface, similar findings have also been reported [25,57].
3. Results and discussion
3.1. Synthesis and structural characterization
The synthetic route of the dyes is shown in Scheme 1. Pheno-
thiazine was purchased while 10-ethyl-10H-phenothiazine (1) was
prepared by the alkylation reaction with ethyl bromide in the
presence of DMF. Formylation of compounds 1 was carried out by
the VilsmeiereHaack reactions in the presence of DMF and POCl3 to
give compound 2. This reagent was brominated with NBS in chlo-
roform to give compound 3. Suzuki coupling reaction of compound
3 was carried out with 4-hydroxyphenylboronic acid to give com-
pound 4, which was alkylated in the presence of DMF and potas-
sium carbonate to give compound 5. Finally, the compound 4 was
converted to the dye POH, while compound 5 was converted to
dye POCT by the Knoevenagel condensation reaction. All the
intermediates and targeted dyes were confirmed by standard
spectroscopic methods.
3.2. Photophysical properties
UVeVis absorption and emission spectra of the dyes POH and
POCT in THF/DCM (1:1) solution (1 ꢁ 10ꢂ5 M) and adsorbed on a
Table 2
Electrochemical characteristics of the dyes POH and POCT.
Dyes
l
(nm)a
HOMO (V) vs. NHEb
E0e0 (eV) vs. NHEc
LUMO (V) vs. NHEd
HOMO (eV) vs. Vacuume
LUMO (eV) vs. Vacuume
E-gap (eV)
POH
POCT
494.4
508.2
1.35
1.47
2.39
2.29
ꢂ1.04
ꢂ0.82
ꢂ5.39
ꢂ5.33
ꢂ2.89
ꢂ2.91
ꢂ2.50
ꢂ2.42
a
l
intersection obtained from the cross point of absorption and emission spectra in THF/DCM (1:1) solution.
HOMO of dyes measured by cyclic voltametery in 0.1 M tetrabutylammonium perchlorate in acetonitrile solution as supporting electrolyte, Ag/AgCl as reference electrode
and Pt as counter electrode.
b
c
E0e0 ¼ 1240/ intersection.
LUMO was estimated by HOMOeE0e0
HOMO and LUMO were calculated at B3LYP-3 G(d, p) level in vacuum.
l
d
e
.

304
Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
the excited state to the conduction band of TiO2 (0.5 eV vs. NHE)
[58] and regeneration from Iꢂ/Iꢂ3 redox potential (0.4 eV vs. NHE)
[59]. The requirement for appropriate electron injection from
the excited state to the conduction band of titanium dioxide is
0.2 eV [60].
3.4. Molecular orbital calculations
To gain further insight into the above mentioned results, density
function theory (DFT) calculations were carried out by using
Gaussian 09 software at the B3LYP/6-31G (d, p) levels [61]. Opti-
mized structures and the electronic distribution in HOMO and
LUMO levels are presented in Fig. 4, Table 2. The electron distri-
butions of the POH and POCT in the HOMOs levels were mainly
localized on the phenothiazine moiety, while the LUMOs showed
localized electron distribution through the cyanoacrylic acid upon
light illumination. Therefore, HOMOeLUMO excitation by light,
induced shift of the electron from the PTZ donor moiety to the
acceptor moiety. This separation of electrons ensures the efficient
electron injection from the dye to the TiO2 film. The octyloxy chain
attatched at the 7-position and ethyl chain on the N atom of PTZ
may increase the bulkiness of the molecule, which prevents the
formation of excimers and aggregates on the TiO2 surface [62].
Fig. 3. Cyclic voltammetry of the dyes POH and POCT.
3.3. Electrochemical properties
In order to investigate the possibilities of electron transfer from
the excited states of POH and POCT dyes to the conduction band of
TiO2 and regeneration of the dyes, the redox behavior was studied
by cyclic voltammetry (Fig. 3, Table 2). The first oxidation potentials
of the dyes enabled the calculation of the highest occupied mo-
lecular orbitals (HOMO) of 1.35 V and 1.47 V (vs. NHE) for POH and
POCT, respectively. The lowest unoccupied molecular orbitals
(LUMO) were estimated by oxidation potential and the energy at
the intersection of absorption and emission spectra, which were e
1.04 V and e 0.82 V for POH and POCT, respectively. The band gap of
POH was higher than POCT, which may be due to two anchoring
units i.e hydroxy and carboxy. Introduction of more anchoring
groups results in an increased variation of molecular orbitals en-
ergies. Furthermore, increased anchoring unit numbers decrease
the donor and acceptor strength [52]. Furthermore, the LUMO level
of POH was higher than the dye POCT indicating that it has more
driving force for electron injection as compared to the dye POCT.
Contrarily, HOMO level of the dye POH was less positive than the
dye POCT, indicating that its regeneration capacity of the oxidized
dye will be affected. Overall, both LUMO levels and HOMO levels of
the dyes POH and POCT are reasonably suitable for providing suf-
ficient thermodynamics driving force for electron injection from
3.5. Photovoltaic performance of the DSSCs
In order to investigate the photovoltaic performance of the dyes,
a set of dye-sensitized solar cells was fabricated as described in
the experimental section and tested under standard conditions
(AM 1.5 G, 100 mW cmꢂ2). The incident photon-to-current effi-
ciency (IPCE) and photocurrent densityephotovoltage (JeV) curves
of the dyes POH and POCT were obtained with a sandwich cell
comprising of 16 m
m TiO2 photoanode and Iꢂ/Iꢂ3 redox electrolyte.
The IPCE spectra of POH and POCT for DSSCs are plotted as a
function of wavelength from 380 nm to 800 nm as seen in Fig.6. The
maximum IPCE values for the dyes POH and POCT were attained at
480 nm and 450 nm, respectively. The dye POH reached over 70% in
the range of 390 nme540 nm with tail off at 670 nm while dye
POCT reached over 70% in the range of 380 nme550 nm with tail off
at 700 nm. Interestingly, the IPCE values reached over 80% in the
region between 450 and 500 nm. Dye POCT has a little higher IPCE
value than POH due to its broader absorption spectra and better
light-harvesting efficiency in the visible region. This trend of IPCE
spectra (POCT > POH) is in good accordance with Jsc data obtained
in JeV measurements, indicating that dyes with higher wavelength
Fig. 4. Optimized structures and frontier molecular orbitals HOMO and LUMO calculated by DFT on a B3LYP/6-31þG (d, p) level.

Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
305
Huang et al. have reported that the dye with hydroxyl group suf-
fers more aggregation on the surface of TiO2 [35]. The dye POCT
has better short-circuit photocurrent density as compared to the
dye POH due to the extended conjugation [63]. Furthermore, the
dye loading amount of the dye POCT was higher than that of the
dye POH, which is also a reason for higher value of Jsc for the
fommer [65].
After being adsorbed on the surface of TiO2, both dyes POH and
POCT were 19 nm and 6 nm blue shifted, indicating that both dyes
form aggregations. When POH and POCT were tested with CDCA
(1 mmol), the Jsc of 8.54 mA cmꢂ2 and 11.18 mA cmꢂ2, Voc of 779 mV
and 787 mV, FF of 0.75 and 0.70, corresponding to efficiencies of
4.96% and 6.19% were obtained, respectively (Table 3). The coad-
sorption of CDCA would decrease the amount of adsorbed dye on
the TiO2 surface, leading to decrease of Jsc [66]. However, CDCA
would suppress the aggregation of the dyes, which is beneficial to
increase Voc and FF [67].
JeV results indicate that the Voc values of POH are lower than
POCT. To illustrate the difference in Voc between the DSSCs based
on these two dyes and investigate the interfacial charge transfer
process within DSSCs, the electrochemical impedance spectra (EIS)
was performed in the dark condition under a forward bias
of ꢂ0.71 V. As shown in Fig. 7, two semicircles were observed in
each Nyquist plot. The smaller (higher frequency from 103e106 Hz)
and larger semicircles (lower frequency from 1e103 Hz) in the
Nyquist plots were attributed to the charge transfer at the counter
electrode/electrolyte interface and the TiO2/dye/electrolyte inter-
face, respectively. Specifically, small circles were almost similar in
both dyes based DSSCs due to the use of the same counter electrode
and electrolyte. On the other hand, there was a substantial differ-
ence in the large semicircles, which indicates that charge transfer
behavior between TiO2 and dye or between dye and electrolyte was
significantly altered, which was likely due to the surface modifi-
cations with different dyes. The R2 value (electron recombination
resistance) obtained by fitting the large semicircles in the plots
shows that POCT has a larger electron recombination resistance
than POH [68], indicating POH is easy to suffer electron recombi-
nation. The dye POCT has octyloxy substituent which may form a
compact structure and its hydrophobic capability reduces the
recombination rates. Furthermore, octyloxy chain also plays an
important role to reduce aggregation on the TiO2 surface. The
Fig. 5. JꢂV curves of DSSCs sensitized by the dyes POH and POCT.
absorption have higher Jsc [29]. The IPCE over 80% in the range
between 420 and 580 nm with phenoxazine based dye has been
reported due to intense absorption and effective intramolecular
energy transfer process [63]. IPCE extended over whole visible
range was obtained by Sun et al., in 2009 due to better light har-
vesting capability of the dye [64].
The detailed parameters of short-circuit photocurrent density
(Jsc), open-circuit photovoltage (Voc), fill factor (FF) and overall
conversion efficiency (h) were obtained from photocurrent den-
sityephotovoltage curves and are summarized (Fig. 5 and Table 3).
The cell based on dye POCT exhibits a maximum power conversion
efficiency (
h
) of 6.32% (Jsc ¼ 12.47 mA cmꢂ2, Voc ¼ 741 mV,
FF ¼ 0.68), under standard global AM 1.5G one sun illumination
(100 mW cmꢂ2). Under similar measuring conditions, dye POH
showed Jsc of 11.28 mA cmꢂ2, Voc of 677 mV and FF of 0.69 cor-
responding to
h of 5.23%. UV absorption maxima in solution and
CV studies revealed that dye POCT has better electron donor-
acceptor strength as compared to dye POH, which influence the
overall power conversion efficiencies. Furthermore, Su et al. have
reported that alkoxy chain is able to reduce aggregation [38] while
Fig. 7. Electrochemical impedance spectra of the dyes POH and POCT measured in the
Fig. 6. IPCE spectra of DSSCs based on the dyes POH and POCT.
dark condition.

306
Z. Iqbal et al. / Dyes and Pigments 99 (2013) 299e307
corresponding to an overall power conversion efficiency of 6.32%.
Under similar measuring conditions, the dye POH sensitized cell
gave a Jsc 11.28 mA cmꢂ2, an Voc of 677 mV and an FF of 0.69, cor-
responding to an overall power conversion of 5.23%. The IPCE of
both the dyes reached over 80% in the region between 450 and
500 nm. The EIS results were in good agreement with the results of
open-circuit photovoltages of the DSSCs based on POH and POCT.
Our work demonstrate that introduction of octyloxy substituent on
PTZ could improve photovoltaic performance as compared to hy-
droxy substituent. As the spectral response of POCT was not
enough, further work to extend the spectral response of the dye is
in progress. It is expected that, by designing a balanced molecular
structure, higher efficiency could be achieved.
Acknowledgment
We are grateful to the National Natural Science Foundation of
China (21272079, 20873183 and 21072064), the Natural Science
Foundation of Guangdong Province, China (10351064101000000
and S2012010010634) and the Fund from Guangzhou Science and
Technology Project, China (2012J4100003) for the financial support.
Fig. 8. IMVS spectra of the DSCCs based on dyes POH and POCT measured under
various incident light intensities.
electron lifetime (sr) values calculated by fitting the equation
sr ¼ Rrec ꢁ CPE2 (CPE2, chemical capacitance) [69] were 64 and
110 ms for POH and POCT sensitized solar cells, respectively. Here,
the longer electron lifetime of POCT compared to POH-sensitized
solar cell is associated with more effective suppression of the back
reaction of the injected electron with the Iꢂ3 in the electrolyte, and
thus leading to increase in Voc [63]. Similar studies have been re-
ported and demonstrated that octyloxy chain plays an important
role in prolonging the electron lifetime [39,70]. These results are in
good agreement with the variation tendency of Voc (POCT > POH)
measured by the JeV curves. To study the internal dynamics of the
cells, intensity modulated phtovolatage spectroscopy (IMVS) was
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