Modified multistep electrophoretic deposition of TiO2
nanoparticles to prepare high quality thin films for dye-sensitizedsolar cell
Masood Hamadanian • Hani Sayahi •
Ali Reza Zolfaghari
Received: 2 February 2012 / Accepted: 5 April 2012 / Published online: 19 April 2012
� Springer Science+Business Media, LLC 2012
Abstract In this study, a modified procedure is intro-
duced which consists of multistep process for improving
the structure of mesoporous TiO2 films. The films were
prepared by electrophoretic deposition (EPD) on FTO (F-
SnO2 coated glass). It is shown that high quality TiO2 film
can be produced by multistep EPD method. The effect of
EPD time on the thickness and density of the films have
been investigated. The performance of dye-sensitized solar
cells (DSSCs) that were fabricated by improved layer are
tested under AM 1.5 simulated sunlight. Finally, the
structure and effective parameters of DSSCs that were
fabricated by one step and multistep EPD are investigated
precisely, using electrochemical impedance spectroscopy.
Introduction
Dye-sensitized solar cells (DSSCs) recently have been used
as promising devices to convert solar energy to electricity.
Since Gratzel dye-sensitized solar cell in 1991 [1], much
attention has been paid to the development of DSSCs
structures for better efficiency [2–8]. The operation of the
DSSC is based on electron injection from the photoexcited
dye into the conduction band of the nanocrystalline oxide.
Then, the electrons are transferred through the mesoporous
oxide layer to the external circuit. The sensitized dye is
regenerated by electron transfer from a donor, typically
iodide ions, during a redox reaction in the electrolyte [9–11].
The mesoporous oxide layer (typically semiconductor TiO2)
is one of the most important parts of the DSSCs structure.
This is due to the dye loaded on the surface of oxide and the
presence of redox electrolyte in the porous network of oxide
layer [4]. Therefore, improvements in the structure of the
oxide can enhance the DSSCs efficiency via increase of the
electron diffusion, the dye loading on the oxide and the light
harvesting in the mesoporous layer, or decrease the recom-
bination reaction [12–15]. So far, several methods have been
used to prepare mesoporous TiO2 layers: sol–gel [16],
doctor blade or screen-printing [17], electrostatic spraying
[18], chemical vapor deposition (CVD) [19, 20], and elec-
trophoretic deposition (EPD) [21, 22].
Recently, among mentioned techniques the EPD method
has gained wide attentions in DSSCs technology. The EPD
syntheses of materials have several advantages including the
cost effectiveness, preparation of homogeneous coated
layer, controllable conditions, and binder-free process
[23–31]. Nevertheless, a significant problem for EPD thick
layers is the cracks formation. The evaporation of residual
solvents can lead to cracks due to contraction of the thin
films. The appeared cracks which can be seen with naked
eyes, cause low-quality electron transfer properties and
decrease the load of dye on the TiO2 surface. As a result, the
heat treatment and the thickness of the prepared layers play
an important role in the TiO2 structure and overall efficiency
of the DSSCs. In addition, the electrical repulsions of par-
ticles in the electrophoretic electrolyte normally create films
with high porosity. In spite of that, increasing the film
porosity also dramatically increases the fraction of
M. Hamadanian (&)
Institute of Nano Science and Nano Technology,
University of Kashan, Kashan, Islamic Republic of Iran
e-mail: [email protected]
M. Hamadanian � H. Sayahi
Department of Physical Chemistry, Faculty of Chemistry,
University of Kashan, Kashan, Islamic Republic of Iran
A. R. Zolfaghari
Department of Physical Chemistry,
Chemistry and Chemical Engineering Research Center of Iran,
Tehran, Islamic Republic of Iran
123
J Mater Sci (2012) 47:5845–5851
DOI 10.1007/s10853-012-6484-1
terminating particles (dead ends) in the TiO2 film which
decreases the electron transport [4]. Many efforts have been
reported to improve the porosity and decrease the cracks in
EPD films, such as, using mixture of TiO2 fiber and nano-
particles [31], mechanical compression of titania nanopar-
ticle films [30], and applying TiO2/polymer blend as binder
[32]. Although, the mentioned optimization reports about
the mesoporous TiO2 suggest the crack reduction, their
application is limited. In mixtures different shapes of TiO2
should be used; and similar to using mechanical compress-
ing, it decreases the porosity of layer more than needed.
Hence, the reduction of porosity in excess of need causes the
decrease of the dye loading on the surface of inner TiO2
particles. In addition, the low porosity of the film decreases
the influence of the electrolyte into the network. Also, the
binder additives should be removed by high temperature
treatment which can increase the crack formation.
Although, the basic operation of DSSCs is well under-
stood, a deeper comprehension of structure, electronic, and
ionic process that governs the operation of the DSSCs is
warranted. Electrochemical impedance spectroscopy (EIS)
[7, 33–35] is an experimental method that used to scruti-
nize the DSSCs operation as well as intensity-modulated
photocurrent/photovoltage spectroscopy (IMPS/IMVS)
[36–41]. The EIS method can be used to measure the
internal impedance of electrochemical system over a range
of frequencies [42–48]. Therefore, this method is helpful to
understand the transport properties of the injected electrons
and recombination phenomena in mesoscopic titania elec-
trode in the DSSCs.
In the present study, we have explored multistep EPD of
TiO2 nanoparticles to produce high quality mesoporous
TiO2 films to apply in dye- sensitized solar cells. We have
also studied the effect of multistep EPD process and the
heat treatment on the morphological and structural features
of the mesoporous TiO2 layers without any additives or
composite. The images of multistep and one step EPD
layers are compared qualitatively. The performance of
prepared layers has been tested in the dye-sensitized solar
cell system. Finally, the reliable values of parameters
obtained from EIS measurement for one step and multistep
DSSCs have been compared to understand deeply, the role
of TiO2 structure on the DSSCs performance.
Experimental
The TiO2 nanoparticles (P-25 from Degussa, Germany),
absolute ethanol [ 99.9 %, acetylaceton [ 99.0 %, iodine
[ 99.8 % (Merck, Germany), acetone [ 99.0 % (Fluka,
Switzerland), and deionized water (produced by SG instru-
ment, Germany) were used to prepare the electrophoretic
suspension according to Zaban et al. reported [29]. In our
study, the concentration of TiO2 nanoparticles was 8 g/L
and the amount of other compounds was changed accord-
ingly, follow to Zaben et al. The I-/I3- electrolyte consisted
of mixture of 0.05 M iodine, 0.5 M lithium iodide [ 98.0 %
(Merck, Germany), 0.5 M 4-tert-butylpyridine (Sigma-
Aldrich, USA) in the acetonitrile (Fluka, Switzerland).
The multistep EPD was performed at constant potential
(10 V) for 10, 15, 20, 25, and 30 s on the FTO glasses
substrate (Dyesol, 15 X/square, Australia). In this process,
every EPD was carried out for 5 s and then the prepared
layer heated for 5 min in 150 �C. After that, the mentioned
process was repeated proportionally to the essential thick-
ness. Finally, the prepared micrometer TiO2 layers were
sintered at 500 �C for 30 min.
These photo electrodes were immersed in a 0.3 mM
N719 dye (RuC58H86N8O8S2, Dyesol, Australia) in solu-
tion tert-butyl alchol (Fluka, Switzerland)/acetonitrile
(50:50 volume ratio) for 24 h at room temperature. Then,
the electrodes were taken out of the dye solution and rinsed
in acetonitrile to make a monolayer of the dye molecules
on the surface of TiO2 nanoparticles [9].
As counter electrode, a platinum electrode was used
which had been prepared by chemical deposition of hexa-
chloro-platinic (IV) acid (Riedel–de Haen, Germany) in
ethanol solution at 350 �C.
A sandwich solar cell was assembled using dye-sensi-
tized electrode as the working electrode and a platinum
electrode as the counter electrode. The 30 lm-thick ther-
moplastic film was used to separate the electrodes and
sealed by heating. The I-/I3- electrolyte was injected in
the gap between the electrodes through the injecting hole
that was previously made in counter electrode.
The TiO2 layers were characterized by scanning electron
microscopy (SEM, Hitachi S-4160) and surface profilom-
eter (DECTAK 3). The desorption of dye on the surface of
TiO2 nanoparticles was carried out by dipping the dye-
sensitized electrode into the 0.1 M sodium hydroxide
(NaOH) solution. Then, the absorbance of the desorbed dye
in NaOH solution was measured by UV–Vis device (Agi-
lent 8453) to calculate the roughness factor [31]. Rough-
ness factor is defined as ratio of the real surface to the
active surface. Therefore, the real surface of prepared
layers can be determined by calculating the surface of TiO2
which was coated with molecules of the dye. A molecule of
dye can spread specified surface of the TiO2 layer.
Therefore, the real surface of prepared layers can be
obtained by calculation of dye quantity from UV–Vis
measurements. Also, the films porosity was obtained by
comparing the SEM thickness with the mass thickness
according to previously reported literature [31]. The active
surface of prepared layer was 0.5 9 0.5 cm2. The DSSCs
performance and EIS results were estimated under AM 1.5
simulated light (Luzchem) using potantiostat/galvanostat
5846 J Mater Sci (2012) 47:5845–5851
123
(PGSTAT 100, Autolab, Eco-Chemie). The applied bias
voltage for the impedance measurements in this study was
at the open-circuit voltage (Voc) of DSSCs. Impedance
measurement of the cells was recorded in frequency range
from 0.05 Hz to 100 kHz with an ac amplitude of 5 mV.
Due to avoid errors in our results, the syntheses and
characterization processes of the prepared cells were
repeated for several times (at least for five times).
Results
Figure 1 shows the morphological images of TiO2 micro-
layers which were synthesized during one step processes
continuously. We observed that in the one step EPD the
prepared layers clearly had cracks, even during little EPD
time and production thin layers. The cross-section images
of mentioned layers also show the cracks reached to the
FTO substrate, and the size of cracks is increased with
relation to increase of layer thickness.
Figure 2 shows the SEM images of TiO2 layers which
was prepared by multistep EPD. The synthesized layers
were uniform and without crack formation and defect even
for thick layers.
The profailometer results showed the thickness of TiO2
layers involve about 8–20 lm dependence to EPD time
from 10 to 30 s. Also, the SEM images of cross-section of
deposited layers confirmed the results from the profilom-
eter measurements.
The I–V curves of prepared cells with different thicknesses
are shown in the Fig. 3. The photovoltaic characteristics are
from the curves summarized in Table 1. In addition, the
prepared cell by similar TiO2 layers was also compared and
the obtained results shown as plus-minus quantities.
The fabricated cell made by mesoporous TiO2 film with
a thickness of around 14.47 lm achieved the best effi-
ciency of 5.67 %, fill factor (FF) 0.53, open-circuit voltage
(Voc) 0.72 V, and short-circuit current density (Jsc)
14.96 mA/cm2.
Figure 4 shows the roughness factor and porosity results
of prepared layers. The increase in thickness results in an
increase of roughness factor and a decrease of porosity.
These consequences could be raised from the higher den-
sity of thicker TiO2 layers.
Figure 5 shows the Nyquist plots of DSSCs with dif-
ferent TiO2 thicknesses fabricated by multistep EPD pro-
cesses. The typical Nyquist plots of DSSCs represent three
semicircles [35, 44–48]. The left semicircle in the high
frequency peak (103–105 Hz range) is related to the
impedance of the charge transfer process at the counter
electrode. The second semicircle (1–100 Hz range) is
attributed to the impedance of charge transfer process for
TiO2/dye/electrolyte interfaces. The last semicircle in mHz
frequency peak is related to the Nernstian diffusion within
the electrolyte [9, 35].
In this study, we used a very thin spacer in the prepared
cells (30 lm), therefore the semicircle of diffusion process
within electrolyte is not obvious clearly and overlapped by
mid semicircle. The parameters of the multistep DSSCs
that were obtained by fitting the Nyquist plots with the
Bisquert’s model are listed in Table 2.
Figure 6 and Table 3 show the impedance plots and
obtained parameters from the fitting Nyquist plots of one
step DSSCs with different thicknesses of TiO2 films,
respectively. The thicknesses of the layers prepared with one
step EPD was approximately the same as Multistep layers.
Discussions
Morphology of TiO2 films prepared by different
electrophoretic deposition
As shown in Figs. 1 and 2, the evaporation of residual
solvents causes deep crack in the one step TiO2 layers. On
the other hand, if the multistep EPD method is used, even
for long time, the prepared mesoporous layer will be very
homogeneous, without cracks or other defects. After each
Fig. 1 The SEM images of TiO2 layers which synthesized by one step EPD during different deposition times a 15 s and b 30 s
J Mater Sci (2012) 47:5845–5851 5847
123
Fig. 2 The SEM images of synthesized mesoporous TiO2 layers by multistep EPD with different electrophoretic deposition times a 10 s, b 15 s,
c 20 s and d 30 s
Fig. 3 The I–V characteristic curves of DSSCs fabricated by various
multistep layers comparing to the best one step (continues) layer that
were synthesized by EPD processes
Table 1 The summarize performance include efficiency (%), fill factor (FF), open-circuit voltage (Voc), and short-circuit current density (Jsc) of
DSSCs fabricated by multistep TiO2 layers with different thicknesses and EPD time
Time (s) Profilometer
thickness (lm)
SEM
thickness (lm)
Jsc (mA/cm2) Voc (V) FF Efficiency (%)
10 8.89 8.16 12.11 ± 0.2 0.72 0.54 4.70 ± 0.09
15 11.25 10.83 12.68 ± 0.3 0.75 0.53 5.00 ± 0.05
20 15.01 14.47 14.96 ± 0.2 0.72 0.53 5.60 ± 0.07
25 17.89 17.35 10.80 ± 0.1 0.75 0.51 4.20 ± 0.05
30 19.93 19.49 7.15 ± 0.3 0.69 0.58 2.90 ± 0.03
Fig. 4 The porosity and roughness factor versus thicknesses of
mesoporous TiO2 layers that prepared by multistep EPD processes
5848 J Mater Sci (2012) 47:5845–5851
123
step of multistep EPD, the heat treatment causes the
evaporation of the solvent through the mesouporous
structure, so in the next step the new suspension of nano-
particles interpenetrates into the layer. Therefore, the
formed cracks in the layer from the previous EPD step can
be repaired by the next EPD step.
The photoelectrical properties and performance
of DSSCs
The efficiency of the best cell fabricated by one step EPD
layer with 12.46 lm thickness was 2.29 %. Figure 3
showed the I–V curve of the best one step cell comparing to
the different multistep cells. The Jsc of the cell which was
fabricated by one step EPD was less than the multistep cell
with similar thicknesses of TiO2 layer. As shown in the
Table 1 the EPD time duration effects on the thickness of
TiO2 films and characteristics performance of DSSCs
fabricated from multistep films.
Although the increase in layer thickness leads to an
increase in the amount of dye adsorption, the correspond-
ing higher frequency of recombination of electron with I3-
ions on the TiO2 surface and the electron transfer resistance
tends to smaller Voc and efficiency.
The Jsc of the sensitized layer was increased up to
14.96 mA/cm2 for cell assembled with a layer with a
thickness of around 15 lm, but the thicker layers were
leaded to decrease Jsc and Voc due to elevation of the
recombination of electron and the electron transfer
resistance.
Porosity and roughness of the TiO2 films prepared
by multistep EPD with different thicknesses
As shown in Fig. 4, the change of EPD time and the TiO2
layer thicknesses effects on the porosity and the roughness
of the layer directly. The prepared layer with thickness
around 15 lm that was synthesized by multistep EPD for
25 s time duration showed the optimal porosity and
roughness factor among the other samples. Nevertheless,
due to the recombination reaction and resistance of electron
transfer, the raising of thicknesses and the dye loading on
the surface of TiO2 particles cannot compensate the
reduction of efficiency.
The EIS of the DSSCs with TiO2 films prepared
by different EPD methods
There are two different impedance models propounded on
the quit distinct physical descriptions proposed by Kern
et al. [33] and Bisquert [34]. Nevertheless, the investiga-
tions of results obtained from of two mentioned models by
Adachi et al. [35] showed that the same equation in
impedance was derived from Kern and Bisquert models.
In this study, we used the model proposed by Bisquert
[34] to scrutinize the transport properties of injected
Fig. 5 The Nyquist plots of DSSCs prepared by different TiO2 layers
with various thicknesses that were synthesized by multistep EPD
Table 2 Characteristic parameters determined by fitting EIS data using the proposed model by Bisquert [34] for DSSCs fabricated by multistep
TiO2 layers
Time (s) keff (s-1) Rk (X) Rw (X) Rk/Rw Deff (cms-1) s (s) b (X cms-1) ns (cm-3)
10 10.2 ± 0.1 5.8 ± 0.05 0.68 ± 0.04 8.5 5.83 9 10-3 0.097 0.487 1.32 9 1018
15 10.1 ± 0.1 5.0 ± 0.01 0.60 ± 0.02 8.2 8.67 9 10-3 0.098 0.520 1.23 9 1018
20 10.0 ± 0.1 4.1 ± 0.02 0.51 ± 0.03 8.0 1.68 9 10-2 0.100 0.593 1.08 9 1018
25 10.0 ± 0.1 4.0 ± 0.06 0.66 ± 0.05 6.0 1.81 9 10-2 0.100 0.694 9.24 9 1017
Fig. 6 The Nyquist plots of DSSCs fabricated using different one
step TiO2 layers with different thicknesses were synthesized by EPD
method
J Mater Sci (2012) 47:5845–5851 5849
123
electrons and back electron reaction in the nanostructured
film. The impedance equation is given by Bisquert as
Z ¼ RwRk
1þ ixxk
!1=2
cothxk
xd
� �1=2
1þ ixxk
� �1=2" #
ð1Þ
where
xd ¼Deff
L2;xk ¼ keff ¼
1
sð2Þ
The Z, Deff, keff, and s represent the impedance, effective
electron diffusion coefficient, effective rate constant for
recombination and electron lifetime, respectively. Also,
some parameters could be estimated from following
equations:
Rw ¼ bL
Deff
; b ¼ kBT
q2Ansð3Þ
Rk ¼xd
xkRw ¼ b
1
Lkeff
ð4Þ
Deff ¼Rk
Rw
� �L2keff ð5Þ
where Rw, Rk, kB, T, q, A, and ns represent electron trans-
port resistance in TiO2 film, charge transfer resistance
against recombination, Boltzmann constant, absolute tem-
perature, elementary charge, the electrode area, and elec-
tron density in the conduction band, respectively.
The results Tables 2 and 3 that were determined from
impedance fitting showed that the layers with different
thicknesses have similar effective rate constants, keff,
approximately. The keff is estimated from the maximum of
frequency peak of mid semicircle (1–100 Hz) of imped-
ance measurement [35, 44]. As the quantity of keff is
determined by the electron lifetime (Eq 2), it is reasonable
that the location of mid frequency peak is independence of
TiO2 thicknesses. The value of Rk is estimated from the
diameter of mid semicircle approximately, and Rk/Rw can
be obtained from the shape of central semicircle in Nyquist
plots. Thus, if the Rk/Rw is increased the mid semicircle
becomes a true circle. The Rw can be obtained from the
ratio of Rk/Rw. Since the Rk is in inverse relation to L, as
show in Eq 4, the values of Rk was decreased with an
increase in thicknesses of TiO2 films. Although the
decrease of Rk value may imply more recombination, but
the ratio of Rk/Rw should be attended in this case. The
higher value of Rk/Rw implies relatively larger recombi-
nation resistance than electron transport resistance. As
shown in Eq 5, the effective diffusion coefficient, Deff,
interestingly is increased in relation to increase of TiO2
layers thicknesses. In addition, the ns is in inverse relation
to the b; and the b value is dependent to the Rw, Deff, and
L (Eq 3). Therefore, the value of ns is decreased in relation
to increases of the TiO2 films thicknesses.
Table 3 showed that changing trend of parameters val-
ues for the one step DSSCs is similar to multistep results.
Although, the Rk, Rk/Rw and ns are decreased, the Deff and bare increased in relation to increase thicknesses of TiO2
layers.
In our impedance study, we tried to find the effects of
crack formation, in TiO2 layers, on the DSSCs parameters
and overall the cell efficiency. Comparing the parameters
of DSSCs prepared by one step and multistep EPD that
showed in Tables 2 and 3, lead us to understand the reasons
in this case, carefully. Although, the Rk values of DSSCs
prepared by one step layers is larger than the Rk values of
multistep layers with similar thicknesses, the ratio of Rk/Rw
for multistep is higher than one step layers. It implies lesser
recombination chance of electrons and less resistance to the
electron transport in the multistep TiO2 films. The higher
resistance electron transport in TiO2, Rw, for one step TiO2
layers is raised from the cracks formation during the EPD
process. As mentioned earlier, the crack formation in layers
prepared by one step EPD cause decrease of electrons
transport in TiO2 layers, layer density, dye loading on TiO2
particles and cell efficiency. The Rw values were decreased
with relation to increase of layer thicknesses in the one step
and multistep layers, except the thicker layer (25 s) that
may rise from the larger transfer length in thick layers from
inner TiO2 particles to the external circuit. The other
important parameters in DSSCs are the electron density, ns,
which is implied the concentration of electrons density in
the conduction band of the semiconductors and the effec-
tive diffusion constant, Deff. In this case, the ns and Deff
values for the multistep DSSC with the best efficiency is
respectively, about 2 and 1.5 times higher than the one step
DSSC with similar thickness.
Table 3 Characteristic parameters determined by fitted EIS data based on the Bisquert model [34] for DSSCs prepared by one step TiO2 layer
Time (s) keff (s-1) Rk (X) Rw (X) Rk/Rw Deff (cms-1) s (s) b (X cms-1) ns (cm-3)
10 10.3 ± 0.1 10.7 ± 0.05 1.78 ± 0.05 6.0 4.14 9 10-3 0.097 0.899 7.13 9 1017
15 10.0 ± 0.1 8.5 ± 0.05 1.70 ± 0.05 5.0 5.86 9 10-3 0.100 0.920 6.97 9 1017
20 10.0 ± 0.1 7.8 ± 0.05 1.59 ± 0.05 4.9 1.03 9 10-2 0.100 1.128 5.68 9 1017
25 10.7 ± 0.1 6.1 ± 0.05 1.96 ± 0.05 3.1 1.00 9 10-2 0.093 1.135 5.65 9 1017
5850 J Mater Sci (2012) 47:5845–5851
123
Conclusion
We propose a multistep possess of electrophoretic depo-
sition of TiO2 nanoparticles which creates homogeneous
layers without defects. It is an excellent method to prepare
mesoporous layers with different thicknesses without using
binder additives to provide the EPD layers against crack-
ing. The fabricated solar cells from the mentioned elec-
trodes show good efficiency up to 5.67 %. The density of
prepared layers can be changed responding to the variety of
EPD time. The investigations of impedance measurements
of the DSSCs fabricated by one step and multistep TiO2
films represented that the solar cells made by multistep
EPD have higher Rk/Rw, Deff, ns, and lower Rw than usual
(one step) EPD layers. These advantages of multistep
DSSCs cause high efficiency comparing to the one step
DSSCs.
References
1. O’Regan B, Gratzel M (1991) Nature 353:737
2. Chang H, Chen TL, Huang KD, Chien SH, Hung KC (2010) J
Alloy Compd 504:S435
3. Tirosh S, Dittrich T, Ofir A, Grinis L, Zaban A (2006) J Phys
Chem B 110:16165
4. Snaith HJ (2010) Adv Funct Mater 20:13
5. Kroon JM, Bakker NJ, Smit HJP, Liska P, Thampi KR, Wang P,
Zakeeruddin SM, Gratzel M, Hinsch A, Hore S, Wurfel U,
Sastrawan R, Durrant JR, Palomares E, Pettersson H, Gruszecki
T, Walter J, Skupien K, Tulloch GE (2007) Prog Photovolt 15:1
6. Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H (2010)
Chem Rev 110:6595
7. Wang Q, Moser JE, Gratzel M (2005) J Phys Chem B 109:
14945
8. Alivov Y, Fan ZY (2010) J Mater Sci 45:2902. doi:10.1007/
s10853-010-4281-2
9. Shin I, Seo H, Son MK, Kim JK, Prabakar K, Kim HJ (2010) Curr
Appl Phys 10:S422
10. Yu Z, Gorlov M, Nissfolk J, Boschloo G, Kloo L (2010) J Phys
Chem C 114:10612
11. Huang F, Chen D, Zhang XL, Caruso RA, Cheng YB (2010) Adv
Funct Mater 20:1301
12. Koo HJ, Park J, Yoo B, Yoo K, Kim K, Park NG (2008) Inorg
Chim Acta 361:677
13. Zhu K, Kopidakis N, Neale NR, van de Lagemaat J, Frank AJ
(2006) J Phys Chem B 110:25174
14. Meen TH, Water W, Chen WR, Chao SM, Ji LW, Huang CJ
(2009) J Phys Chem Solids 70:472
15. Zolfaghari A, Nasiri Avanaki K, Jooya HZ, Sayahi H (2007)
Semicond Sci Technol 22:653
16. Tesfamichael T, Will G, Bell J, Prince K, Dytlewski N (2003) Sol
Energy Mater Sol Cells 76:25
17. Ito S, Chen P, Comte P, Nazeeruddin MK, Liska P, Pechy P,
Gratzel M (2007) Prog Photovolt 15:603
18. Hong JT, Seo H, Lee DG, Jang JJ, An TP, Kim HJ (2010) J
Electrostat 68:205
19. Narita T, Iida T, Ogawa S, Mizuno K, So J, Kondo A, Yoshida N,
Itoh T, Nonomura S, Tanaka Y (2008) Thin Solid Films 516:810
20. Murakami TN, Kijitori Y, Kawashima N, Miyasaka T (2004) J
Photochem Photobiol A 164:187
21. Miyasaka T, Kijitori Y (2004) J Electrochem Soc 151:A1767
22. Chang H, Su HT, Chen WA, Huang KD, Chien SH, Chen SL,
Chen CC (2010) Sol Energy 84:130
23. Hanaor D, Michelazzi M, Veronesi P, Leonelli C, Romagnoli M,
Sorrell C (2011) J Eur Ceram Soc 3:1041
24. Kim GS, Seo HK, Godble VP, Kim YS, Yang OB, Shin HS
(2006) Electrochem Commun 8:961
25. Manrıquez J, Godınez LA (2007) Thin Solid Films 515:3402
26. Yuma JH, Kim SS, Kima DY, Sung YE (2005) J Photochem
Photobiol A 173:1
27. Pimanpang S, Maiaugree W, Jarernboon W, Maensiri S,
Amornkitbamrung V (2009) Synth Met 159:1996
28. Dor S, Ruhle S, Ofir A, Adler M, Grinis L, Zaban A (2009)
Colloids Surf A 342:70
29. Grinis L, Dor S, Ofir A, Zaban A (2008) J Photochem Photobiol
A 198:52
30. Tan W, Yina X, Zhoua X, Zhanga J, Xiaoa X, Lin Y (2009)
Electrochim Acta 54:4467
31. Shooshtari L, Rahman M, Tajabadi F, Taghavinia N (2011) Appl
Mater Interfaces 3:638
32. Li Y, Yoo K, Lee DK, Kim JH, Park NG, Kim K, Ko MJ (2010)
Curr Appl Phys 10:e171
33. Kern R, Sastrawan R, Ferber J, Stangl R, Luther J (2002) Elec-
trochim Acta 47:4213
34. Bisquert J (2002) J Phys Chem B 106:325
35. Adachi M, Sakamoto M, Jiu J, Ogata Y, Isoda S (2006) J Phys
Chem B 110:13872
36. Oekermann T, Yoshida T, Minoura H, Wijayantha KGU, Peter
LM (2004) J Phys Chem B 108:8364
37. Bisquert J, Vikhrenko VS (2004) J Phys Chem B 108:2313
38. van de Lagemaat J, Park NG, Frank AJ (2000) J Phys Chem B
104:2044
39. Fisher AC, Peter LM, Ponomarev EA, Walker AB, Wijayantha
KGU (2000) J Phys Chem B 104:949
40. Duffy NW, Peter LM, Wijayantha KGU (2000) Electrochem
Commun 2:262
41. Huang SY, Schlichthorl G, Nozik AJ, Gratzel M, Frank AJ
(1997) J Phys Chem B 101:2576
42. Bisquert J, Belmonte GG, Santiago FF, Ferriols NS, Bogdanoff P,
Pereira EC (2000) J Phys Chem B 104:2287
43. Bisquert J (2010) J Electrochem Chem 646:43
44. Chen HW, Huang KC, Hsu CY, Lin CY, Chen JG, Lee CP, Lin
LY, Vittal R, Ho KC (2010) Electrochim Acta 56:7991
45. Chen HW, Hsu CY, Chen JG, Lee KM, Wang CC, Huang KC,
Ho KC (2010) J Power Sources 195:622546. Weerasinghe HC, Sirimanne PM, Franks GV, Simon GP, Cheng
YB (2010) J Photochem Photobiol A 213:30
47. Jun Y, Kim J, Kang MG (2007) Sol Energy Mater Sol Cells
91:779
48. Santiago FF, Bisquert J, Palomares E, Otero L, Kuang D,
Zakeeruddin SM, Gratzel M (2007) J Phys Chem C 111:6550
J Mater Sci (2012) 47:5845–5851 5851
123