Materials Sciences and Applications, 2011, 2, 1427-1431
doi:10.4236/msa.2011.210193 Published Online October 2011 (
Copyright © 2011 SciRes. MSA
Electrophoretic Deposition of Titanium Oxide
Nanoparticle Films for Dye-Sensitized Solar
Cell Applications
Jason Bandy, Qifeng Zhang, Guozhong Cao*
Department of Materials Science and Engineering, University of Washington, Seattle, USA.
Email: *
Received July 6th, 2011; revised August 5th, 2011; accepted August 30th, 2011.
Films of titanium oxide nanocrystalline particles (P25) were deposited using an electrophoretic deposition. The films
characteristics were tuned for applications in dye-sensitized solar cells. Electrophoretic deposition allows control of
film characteristics such as porosity and thickness by changing deposition parameters, such as the electric field and
deposition time. To increase the efficiency of the dye-sensitized solar cells with films created using electrophoretic
deposition, the problem of an electrolyte contamination in the film, which occurred during deposition, was addressed.
With the proper chemical post treatment, efficiency of 2.93% with fill factor of 0.55 was obtained when the films were
annealed at 450˚C. A low annealing temperature of 150˚C resulted in efficiencys of 1.99% with fill factor of 0.68. When
the P25 was replaced by hydrothermally fabricated titanium oxide nanocrystalline particles, efficiency of 4.91% with
fill factor of 0.55 was obtained.
Keywords: Dye-Sensitized Solar Cells, Electrophoretic Deposition, TiO2 Nanoparticles
1. Introduction
Since dye-sensitized solar cells (DSCs) were first created
by O’Regan and Gratzel to convert sunlight into electri-
cal energy [1], there has been much innovation in this
emerging technology from creating dyes which absorb a
longer range of wavelengths to better quality titanium
oxide nanocrystalline particles [2-8], which are much
more advantageous in crystal structure when compared to
the commercial brand of titanium oxide nanocrystalline
particles, so-called P25 (Degussa AG, Germany). More
recently, there have been many research endeavors to-
wards creating flexible DSCs, where flexible transparent
plastics coated with a transparent conductive oxide are
used [9-13]. Flexibles DSCs offer a technological advan-
tage since they are not fragile compared to traditional
DSCs and have a versatile shape. Generally a post treat-
ment at elevated temperatures is used when sol-gel me-
thods are employed to crystallize the titanium oxide
nanoparticle film [14,15]; however, this high temperature
anneal cannot be used when flexible substrates are used
as this leads to degradation of the substrate and loss of
Electrophoretic deposition (EPD) is an electrochemical
deposition of charged particles with an applied electric
field in a suspension containing the particles, electrolyte,
additives, and solvent [16,17]. It is a two-step process,
including charging of the particles in the suspension and
drifting of the charged particles towards an electrode.
Additives such as water to the electrolyte can be added to
allow control of the packing density of the particles with
a change in current. Yum et al. pointed out the hydrogen
gas from the electrolysis of water at the cathode could
interrupt the deposition of nanoparticles resulting in a
lower packing density [18]. Therefore, the packing den-
sity was suggested to be related to the current. This gives
merits to the EPD method as this allows deposition of
particles into films with tunable characteristics that are
very reproducible. Film characteristics such as morphol-
ogy and thickness can be altered by changing the EPD
conditions such as voltage, deposition time, and electro-
lyte concentration. In addition, since the nanoparticles
deposited can be nanocrystalline, only a low temperature
post thermal treatment of for example, 100˚C to 150˚C is
required to evaporate the solvent.
In this investigation, problems of electrolyte contami-
Electrophoretic Deposition of Titanium Oxide Nanoparticle Films for Dye-Sensitized Solar Cell Applications
nation associated with EPD of nanocrystalline titanium
oxide particles were studied. By using a proper chemical
post treatment to remove the electrolyte, much higher
efficiencies can be obtained on the films composed of
commercial P25 nanoparticles. Even higher efficiencies
were achieved with the use of hydrothermally grown
nanocrystalline titanium oxide particles.
2. Experimental Details
Glass coated with Fluorine doped Tin Oxide (FTO) was
used as the substrate for creating the photoelectrode film
of dye-sensitized solar cells with an active area of 36
mm2. A dispersion of nanocrystalline titanium oxide par-
ticles (P25, 20 nm, Deguessa) with concentration of 0.25
g/L in isopropyl alcohol (IPA, 99.5%, Mallinckrodt) was
used where 5 × 105 M magnesium nitrate hexahydrate
(Aldrich, 99.995 + %) and 2 vol% deionized water (DI
water) were used as the electrolyte during the EPD proc-
ess. A standard spacing of 2 cm was used between the
FTO cathode and platinum anode. These conditions were
similarly used by Yum et al. [18]. Nanocrystalline tita-
nium oxide particle films were also created using a doc-
tor blading method for comparison.
Various applied electric fields and deposition times
were adopted to determine optimum conditions for cre-
ating nanocrystalline titanium oxide particle films for
DSCs. Scanning Electron Microscopy (SEM) was em-
ployed to characterize the structure and morphology of
the films. Cross-sections of the films were also viewed
with the SEM to obtain thicknesses of the films so as to
determine the optimum deposition time. In addition, en-
ergy dispersive X-ray (EDX) analysis was performed on
the films to determine the extent of the electrolyte con-
tamination in the films.
Soaking treatments after deposition were used to re-
move magnesium electrolyte contamination from the
nanocrystalline titanium oxide particle films. Solutions of
0.1 N H2SO4 in DI water and 0.1 N H2SO4 in IPA was
prepared. Film samples were immersed in the 0.1 N
H2SO4 in DI water solution for 1 hour before and after
annealing and immersed in the 0.1 N H2SO4 in IPA for 1
hour and 12 hours. The 0.1 N H2SO 4 in DI water treat-
ments were followed by 15 minute immersions in pure
DI water and the 0.1 N H2SO4 in IPA were followed by
15 minute immersions in pure IPA to remove excess
Post thermal treatments of 450˚C were carried out on
the sample prior to dye adsorption. Immediately after
completion of the thermal treatments, the titanium oxide
nanocrystalline films were soaked in N719 dye for 24
Once the optimum post treatment was discovered,
films created using low temperature annealing of 150˚C
were dyed and tested using the same procedure as the
other samples.
The optimum post treatment was also used in the crea-
tion of films with hydrothermally grown nanocrystalline
titanium oxide particles, which were annealed at 450˚C
as well.
The solar cell performances were characterized while
the samples were irradiated with AM 1.5 simulated sun-
light with the power density of 100 mW/cm. The elec-
trolyte was composed of 0.6 M tetrabutylammonium
iodide, 0.1 M lithium iodide, 0.1 M iodine, and 0.5 M
4-tert-butylpyridine in acetonitrile. A platinum coated
FTO glass was used as the counter electrode.
3. Results and Discussion
Using varying electric field intensities, different film
morphologies were observed. As the electric field was
increased, the film morphology became much denser.
This is in agreement with previously published data [18].
A rise in the electric field is met with an increase in the
packing density of the film and a decrease in the EPD
current, but the packing density is actually dependent on
the EPD current as there is electrolysis of water at the
cathode. A higher current results in more hydrolysis of
water resulting in more evolution of hydrogen gas. The
hydrogen gas evolution interrupts the deposition of the
nanocrystalline titanium oxide particle and creates pores
in the film. It was important to find a balance between
internal surface area of the film and the ability of the dye
to penetrate deep into the films pores. Too porous of a
film results in low surface area and poor light absorption,
while too dense of a film results in low penetration of the
dye. From the SEM images in Figure 1, it was found 20
V/cm gave the desired film morphology. Cross-sectional
views of the nanocrystalline titanium oxide particle films
revealed the desired deposition time to be 15 minutes,
leading to a film thickness of 16 μm as shown in Figure 2.
Shane et al. showed how the magnesium nitrate con-
taminated the film [3]. As the magnesium nitrate ions
approach the cathode, the electrolysis of water causes the
reactions shown in Equations (1) and (2) [19].
2H2O + 2e = H2(g) + 2OH (1)
+ + 2OH = Mg(OH)2(s) + 3
The electrolysis of the water not only produces hydro-
gen gas, but hydroxyl ions as well. These ions react with
the magnesium nitrate ions creating a magnesium hy-
droxide solid where the film is depositing [19].
EDX microanalysis of the films as shown in Figure 3
revealed a high concentration of Mg in the titanium oxide
nanocrystalline particle films. Our experiments also
showed when a film of MgO particles was created using
a doctor blading technique and soaked in the N719 dye
Copyright © 2011 SciRes. MSA
Electrophoretic Deposition of Titanium Oxide Nanoparticle Films for Dye-Sensitized Solar Cell Applications
Copyright © 2011 SciRes. MSA
(a) (b)
(c) (d)
Figure 1. SEM images of deposited films wi th EPD using electric fields of (a) 15 V/cm; (b) 20 V/cm; (c) 25 V/cm; and (d) 30
Figure 2. Cross-sectional SEM image of the nanocrystalline
titanium oxide particle film with a deposition time of 15 min.
Figure 3. EDX spectrum of titanium oxide nanocrystalline
particle films prepared using EPD.
Electrophoretic Deposition of Titanium Oxide Nanoparticle Films for Dye-Sensitized Solar Cell Applications
for 24 hours, it was found the dye did not adhere to the
MgO particles like it normally would to the titanium ox-
ide nanocrystalline particles. The magnesium hydroxide
contamination in the titanium oxide nanocrystalline par-
ticle film oxidizes during the annealing process prevent-
ing adhesion of the dye to the nanocrystalline titanium
oxide particle film during dye-sensitization. This resulted
in very low short circuit currents and energy conversion
The use of post treatments for washing the as-depos-
ited films resulted in varying levels of success as can be
seen in Table 1. No post treatment resulted in poorer
efficiencies. The use of 1 h 0.1 N H2SO4 H2O Immersion
followed by 15 min H2O immersion resulted in much
better energy conversion efficiencies; however, there was
a noticeable decrease in the thickness of the film after
immersion. Corresponding energy conversion efficien-
cies were sporadic with a standard deviation of 0.55. IPA
post treatments resulted in less reduction of the film
thickness, but required more time to remove the con-
tamination. 1 h 0.1 N H2SO4 IPA immersion followed by
15 min pure IPA immersion resulted in poor energy
conversion efficiencies since it was unable to remove the
contamination. From the color of the film, the dye did
not successfully adhere to the nanocrystalline titanium
oxide particles. 15 h 0.1 N H2SO4 IPA immersion fol-
lowed by 15 min pure IPA immersion provided the best
results, which matched the 2.94% efficiency of a DSC
whose titanium oxide nanoparticle film was created using
a doctor blading technique. The energy conversion effi-
ciencies were also very reproducible with a standard de-
viation of 0.054.
15 h 0.1 N H2SO4 IPA immersion followed by 15 min
pure IPA immersion post treatment was used in the crea-
tion of a titanium oxide nanocrystalline particle film
which had undergone a low temperature anneal of 150˚C.
Efficiencies of 1.99% with fill factors (FFs) of 0.68 were
The same optimum post treatment was also used to
purify films created with hydrothermally fabricated na-
noparticles. Efficiencies of 4.91% with fill factors of 0.55
were obtained.
4. Conclusions
Electrophoretic deposition allows good control of poros-
ity by varying the electric field intensity and thus the
current. An increase in current leads to more evolution of
hydrogen gas creating more porosity in the film. The film
thickness is also tunable by varying the deposition time.
Magnesium ions can be employed to attach on P25
(TiO2) nanoparticles for electrophoretic deposition of
TiO2 thin film for dye-sensitized solar cell application.
However, the residual of magnesium ions, which are in
the formation of magnesium oxide after the thermal treat-
ment, prevents the adhesion of dye to the TiO2 nanopar-
ticles. It was found a suitable chemical post treatment
might effectively remove the magnesium oxide from the
TiO2 thin film, leading to a significant efficiency increase
from 1.99% to 3.27%. The maximum efficiency of 4.91%
was achieved with hydrothermally grown TiO2 nanopar-
ticles forming photoelectrode film through electropho-
retic deposition.
5. Acknowledgements
This work on the characterization of microstructure and
the measurement of power conversion efficiency is sup-
ported by the U.S. Department of Energy, Office of Ba-
sic Energy Sciences, Division of Materials Sciences, un-
der Award No. DE-FG02-07ER46467 (Q.F.Z.). This work
is also supported in part by the National Science Founda-
tion (DMR 1035196), the Air Force Office of Scientific
Research (AFOSR-MURI, FA9550-06-1-0326), the Uni-
versity of Washington TGIF grant, the Royalty Research
Fund (RRF-A65796) from the Office of Research at
University of Washington, the Washington Research
Foundation, and the Intel Corporation.
Table 1. Efficiencies and fill factors obtained from samples treated with varying post treatments after EPD.
Powder Sample Conditions η (%)FF
No post treatment with 450˚C anneal 0.950.59
1 h 0.1 N H2SO4 H2O Immersion followed by 15 min H2O immersion with 450˚C anneal 3.270.56
1 h 0.1 N H2SO4 IPA Immersion followed by 15 min pure IPA immersion with 450˚C anneal 2.650.60
15 h 0.1 N H2SO4 IPA Immersion followed by 15 min pure IPA immersion with 450˚C anneal 2.930.55
15 h 0.1 N H2SO4 IPA Immersion followed by 15 min pure IPA immersion with 150˚C anneal 1.990.68
Doctor Blading 2.94 0.41
TiO2 Nanoparticles (Lab-Made) 15 h 0.1 N H2SO4 IPA Immersion followed by 15 min pure IPA immersion 4.910.55
Copyright © 2011 SciRes. MSA
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