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: hamadani@kashanu.ac.ir

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