J Mater Sci
DOI 10.1007/s10853-010-4925-2

Organic solvent based TiO2 dispersion paste for dye-sensitizedsolar cells prepared by industrial production level procedure
Ryohei Mori • Tsutomu Ueta • Kazuo Sakai •Yasuhiro Niida • Yasuko Koshiba • Li Lei •Katsuhiko Nakamae • Yasukiyo Ueda

Received: 7 May 2010 / Accepted: 13 September 2010
 Springer Science+Business Media, LLC 2010

Abstract In order to prepare the TiO2 liquid dispersions
for the electrodes of dye-sensitized solar cells with industrial mass production level at a reasonable cost, the present
study investigates the preparation of TiO2 liquid dispersions by a general industrial dispersion technique using
readily available P25. To determine the TiO2 dispersion
offering the best light–electricity energy conversion efficiency, the suitability of various types of solvents andresins for use in TiO2 dispersion are tested. In general,
organic solvent based TiO2 dispersions are found to allow
the formation of more uniform thin films in comparison
with water-based dispersions. A preparation using ethyl
cellulose as the resin and the terpineol as the solvent isfound to exhibit the best conversion efficiency. We havealso found that using two kinds of resins of different
molecular weights gave rise to better efficiency. Among 26
metal compounds tested in this study, the best metal dopant
was Ag. XRD and XPS measurements confirm that the Ag
exists as metal Ag and silver oxide.

Introduction
Regenerative dye-sensitized photoelectrochemical cells
have been under investigation for the past decades.

R. Mori (&)  T. Ueta  K. Sakai
Fuji Pigment Co., Ltd, Kawanishi, Japane-mail: moriryohei@fuji-pigment.co.jp
Y. Niida  Y. Koshiba  Y. Ueda
Graduate School of Science and Technology, Kobe University,
Kobe, Japan
L. Lei  K. Nakamae
Hyogo Science and Technology Association, Ako, JapanThe low production costs, together with a good efficiencyof energy conversion, reaching 10% in some cases, makesuch devices a promising alternative for the developmentof a new generation of solar cells [1–4]. In order to achieve
high cell energy efficiency, much previous research has
been directed toward improving the photocurrent andphotovoltage by, for example, developing new sensitizers[5–8], increasing the light-scattering properties of the film
[9, 10], suppressing the charge recombination [2, 11, 12],
improving the interfacial energetic mechanism [13, 14],
and altering the particle morphology [3, 15–17]. However,
the efficiency values obtained through these approaches
thus far are still less than half the theoretical value, and are
not large enough for practical use [18]. Against thisbackground, the inclusion of metal ion doping offers apractical way of providing additive properties such as
visible light response to a TiO2 film, since TiO2 is active
only under ultraviolet light due to its wide band gap(*3.0 eV) [19–21].
However, there is as yet insufficient fundamental
research considering the application to practical industrial
use at reasonable costs. For example, T/SP TiO2 paste(Solaronix SA, Switzerland) is currently prepared by ahydrothermal method, but this method is too expensive forindustrial use. However, the much cheaper alternative
approach of manufacturing a TiO2 liquid dispersion by ageneral dispersion technique using bead mill machines
potentially presents an ideal procedure for the mass production of TiO2 electrode film.
In this study, we have focused on preparing a TiO2
liquid dispersion with readily available P25 using a generaldispersion technique, and attempted to find the best solvent
and resin combination, including a consideration of waterbased system, as well as the best metal dopant to enhance
the light-to-electricity conversion efficiency.

123

J Mater Sci

Experimental
A range of TiO2 liquid dispersions were prepared bymixing P25 (Nippon Aerosil Co. Ltd, Japan) with various
types of organic solvent and water, while resin was also
used as a binder. Polyoxyethylene sorbitane mono laurate(Kao Corporation Co. Ltd, Japan) was used as dispersant
for the water-based mixtures, whereas alkylolammonium
salt of a block copolymer with an acidic group (BYKChemie GmbH, Germany), was used as the dispersant forthe organic solvent based mixtures. A TiO2 sol slurry was
prepared with P25 according to the following procedure.
Using a paint shaker (Red Devil Equipment Co., USA),
P25 was dispersed in water or an organic solvent. Paint
shaker can mix pigment, solvent, dispersant, various typeof additives etc., in plastic bottle with ceramic beads inside,
by simply shaking intensively for a certain period of time.
Various types of resin, such as polyethylene glycol (PEG,
Dai-ichi Kogyo Seiyaku, Co. Ltd, Japan, MW 20000) or
ethyl cellulose (Dow Chemical Inc. USA), were then added
to act as a binder and also to increase the viscosity of the
TiO2 dispersion for use in the squige printing technique,and to allow the preparation of a porous thin film after
sintering. For example, to prepare sample 3 in Table 1,
21 g of P25, 3 g of dispersant, and 46 g of terpineol(Arakawa Chemical Industries, Co.Ltd, Japan) were addedin a plastic bottle, then mixed with paint shaker. Please
note that 100 mL of ceramic (zirconia 0.6–0.8 mm) beads
should be added in order to disperse and mix the content
inside of the plastic bottle. Then, an appropriate amount of
ethyl cellulose was added to obtain the final TiO2 paste forthe electrode. The quantity of added ethyl cellulose was

9.0% for the sample (3, 4, 5 8, 11), whereas it was 6.4%
(MW 60,000) and 1.8% (MW 110,000) for the sample(1, 2, 6, 7, 9, 10) in Table 2.
Metal doping was carried out by adding metal compounds in chloride, nitrate, alkoxide, and other metal
forms, into the TiO2 dispersion. In this study, a TiO2 dispersion with doped metal content in molar ratio Ti:metal = 100:X will be referred to as a metal-X TiO2 film. For
example, TiO2 film with doped Ag content in molar ratio
Ti:Ag = 100:1 is referred to as an Ag-1 TiO2 film. For Nb
and Ag doping, niobium ethoxide and silver nitrate (forboth, Wako Pure Chemical Industries, Ltd, Japan) were,respectively, used in this study.
For morphology observation, the resultant TiO2 viscous
paste was deposited on ITO coated glass (Nippon Sheet
Glass Co., Ltd, Japan) by the squige printing techniquefollowed by heating at 450 C for 30 min, and thenallowed to cool down to room temperature. An atomic
force microscope (SPI 3800 N, Seiko Instruments Co. Ltd,
Japan) and a transmission electron microscopy (TEM,
Hitachi, H-7100) were used. Thermogravimetry (TG) anddifferential thermal analysis (DTA) were performed using
a TG-DTA system (Thermoflex TG 8110, Rigaku Co. Ltd,
Japan) to evaluate the thermal properties of the titania gel.
This dried gel was prepared just simply by leaving thetitania paste at ambient atmosphere over 2 months.
For TiO2 particle size measurement, titania paste wasdiluted into solvent 200 times, then the size of TiO2 particle
in liquid was analyzed using particle size characterization(Microtrac, UPA-EX 150, Nikkiso Co.Ltd, Japan). The
crystal phase of the TiO2 was evaluated by X-ray diffractometry (XRD, Rigaku, RINT1200 X-ray diffractometer)

Table 1 Photovoltaic characteristics of dye-sensitized solar cells prepared from TiO2 dispersion paste with various type of solvent and resin
combination prepared in this study
SampleFilmformation

TiO2 (P25)content (%)

Jsc, short circuitcurrent(mA/cm-2)

Voc, open
circuitvoltage (mV)FF, fillfactorg, energy
conversion
efficiency

Resin

Solvent

1

Poly vinyl butyral

Terpineol

s

27.4

10.79

717

0.568

4.40

2

Ethyl cellulose

s

24.8

10.93

710

0.528

4.10

13.03

708

0.496

4.57

3

Ethyl cellulose

Diethylene glycol,
mono methyl ether
Terpineol

s

24.8

4

Aclylic polymer

Terpineol

9

24.8

5

Cellulose acetate buttylate

Diethylene glycol,
mono methyl ether

s

25.7

11.57

703

0.532

4.32

6

PEG

Water

s

18.8

8.21

713

0.480

2.81

7

Hydroxy propyl cellulose

Water

9

18.8

8

Xanthan gum

Water

9

18.8

9

Methyl cellulose

Water

9

18.8

10

Poly vinyl alcohol

Water

9

18.8

11

Poly vinyl pyrrolidone

Water

9

18.8

s without cracking, 9 cracked film

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J Mater Sci
Table 2 Photovoltaic characteristics of dye-sensitized solar cells prepared from TiO2 dispersion paste (ethyl cellulose as resin, terpineol assolvent) with various type of molecular weight, dispersing time, size of the bead used in this study
SampleResin (MW)1

60,000 ? 110,000

260,000 ? 110,000

390,0004
56Dispersing
time (min)Beads
diameter

TiO2 (P25)content (%)

Jsc, shortcircuit current(mA/cm-2)

Voc, open
circuitvoltage (mV)FF, fillfactorg, energy
conversion
efficiency

24.8

12.88

709

0.489

4.47

27.4

13.03

708

0.496

4.57

24.8

11.32

729

0.507

4.18

40,000

24.8

8.62

688

0.521

3.09

200,000

24.8

9.62

681

0.533

3.49

60,000 ? 110,000

24.8

7.60

671

0.576

2.94

7

60,000 ? 110,000

890,000960,000 ? 110,000

10

60,000 ? 110,000

11

90,000

15

60

120

0.6–0.8 mm zircon

0.6–0.8 mm zircon

0.3 mm zirconia

using Cu Ka radiation operated at 40 kV and 40 mA, andthe first crystallite size of TiO2 was calculated from a halfvalue width of the XRD peak at 2h = 25.4 using Scherrer’s equation. The molar ratio of doped metal in TiO2 wasmeasured by X-ray photoelectron spectroscopy (XPS, Ulvac-phi 5500 MT X-ray photoelectron spectrometer). The
binding energy was calibrated by a thin gold film vacuumdeposited on the sample. The spectra were decomposed into
Gaussian functions with a computer program supplied by
Ulvac-Phi. The BET surface area was determined by asurface area analyzer (Shimadzu, Aquasorb 2100E). Sample was prepared by heating 10 g of TiO2 paste at 450 C
for 30 min, and then allowed to cool down to room temperature as we have treated when we prepared TiO2 film
electrode. The pore size distribution was also determined by
the BJH method of nitrogen desorption measurements(Shimadzu, Autopore IV9510).
For solar cell characterization, TiO2/ITO films were
immersed in a 3 9 10-3 M ruthenium complex dye (N-719)(ruthenium (2,20 -bipyridyl-4,40 -dicarboxylate)2 (NCS)2) in a
mixture of ethanol and tert-butanol (50/50 v/v) solution atroom temperature for 15 h in the dark. TiO2 electrode wasrinsed with acetonitrile and dried. Then, the amount ofadsorbed dye was determined by dissolving the dye fromthe TiO2 surface into a 0.1 M NaOH aqueous solution,and measuring its absorbance by a UV–visible light
spectrometer.
The electrolyte solution was composed of 0.1 M oflithium iodide, 0.05 M of iodine, 0.5 M of tert-butylpyridine, and 0.6 M of 2,3-dimethyl-1-propylimidazolium
iodide in acetonitrile.
The photovoltaic properties of the TiO2/dye/I-, I3-/Pt
cells were estimated by recording the current–voltage
characteristics of each cell (MP-160, Eko Instruments Co.,

24.8

6.31

677

0.565

2.41

24.8

7.12

680

0.572

2.77

24.8

6.21

640

0.540

2.14

24.8

7.95

647

0.528

2.72

24.8

7.34

662

0.539

2.62

Ltd) under an illumination of AMl.5 (100 mW cm-2)
using a solar simulator. The spectrum of simulated solar
light was calibrated against the JIS amorphous-Si Standard.
The area of the dye-coated TiO2 electrode was 0.28 cm2.

Results and discussions
Influence of resin and solvent
Table 1 summarizes the photovoltaic characteristics ofTiO2 thin films prepared from various types of resins andsolvents. It can be seen that the film prepared using ethyl
cellulose as the resin, and terpineol as the solvent, has
exhibited the highest light-to-electricity energy conversion
efficiency. When water was used as the solvent, only PEG
exhibited a stable film formation on transparent conductive
glass. It was found to be impossible to create uniform TiO2
films using other water-soluble resins listed in Table 1(hydroxy propyl cellulose, xanthan gum, methyl cellulose,etc…). They exhibited a cracked surface that was recognized by naked eye observation, and gave no photocurrent
under light illumination. On the other hand, when an
organic solvent was used as solvent, most of the TiO2
dispersions enabled us to form uniform thin films. One cansay that the organic solvent based TiO2 dispersions seem tooffer the most promising approach for industrial mass leveluse. It should be noted here that all of the TiO2 thin films
prepared in this study possess film thickness ranging from
7.5–9.0 lm.
Table 2 summarizes the influences of the molecularweight of resin and its concentration, the dispersion time,and bead diameter, on energy conversion efficiency.
In Table 2, all of the samples are made from TiO2

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J Mater Sci

dispersion in which the ethyl cellulose was used as resin,
and the terpineol as solvent. Energy conversion efficiency
was relatively higher when two kinds of ethyl cellulosewith different molecular weights were used. It was quite
obvious that conversion efficiency was lower when thesmallest (MW 44,000) and largest (MW 200,000) ethyl
cellulose was applied to prepare the TiO2 dispersion. Thismeans there is an optimum resin molecular size for TiO2
film electrode. The detail research is remained to investigate the influence caused by the size of the resin. Here,
MW indicates mass average MW.
Dispersion time was found to be also critical. Conversion efficiency decreased when dispersing time was morethan 1 h. When we use a ‘‘paint shaker’’ or ‘‘bead mill’’machine for liquid dispersion preparation, generally beads
of smaller diameter give rise to smaller nanoparticles aslong as the beads are made of the same material. In thepresent case either zircon or zirnonia. In our study, efficiency decreases when smaller beads and a longer dispersion time were applied (0.3 mm zirconia, 120 min). It is
deduced that when TiO2 nanoparticles size becomes
smaller, there may be insufficient space for electrolyte anddye penetration, so the TiO2–dye–electrolyte interface isunable to obtain a good electrical contact with each otherinside the TiO2 thin film, resulting in lowering the light-toelectricity conversion efficiency. ‘‘In fact, the average TiO2
particle size of the dispersion was 380 nm for the sample
which has 15 min dispersion time with 0.6–0.8 mm zircon
beads. Whereas it was 270 nm for the one of 120 min
dispersion time with 0.3 mm zirconia beads. Please notethat the particle size tends to be larger compared to actual
TiO2 particle size on film. This is probably because theparticle size characterization machine measures the TiO2
particle size in a liquid when TiO2 tends to form aggregation more in the liquid, than on the film.’’ It has beenreported that when the size of pores in the TiO2 film is too
small, the performance of dye-sensitized solar cells deteriorates due to slowed electrolyte diffusion in the film [22].
Nakade et al. [23] also reported that as the TiO2 particles
become larger, the electron diffusion coefficient increases
and electron recombination lifetime decreases, respectively, resulting in an enhancement of the solar cell performance. It should be noted here that similar phenomena
were observed when we used polyvinyl butyral, cellulose
acetate butylate, and PEG as resin.
We cannot clearly elucidate why the ethyl cellulose andterpineol combination exhibited the best performance.
However, one possible explanation is that the molecularweight of currently used ethyl cellulose enabled us to forma proper pore size among individual TiO2 particles, and asuitable size of spaces within the TiO2 aggregation.
Therefore, the electrolyte and dye penetration was optimized inside the film, leading to good electrical contact.

123

Hereafter, we will refer to this ethyl cellulose and terpineol combination of TiO2 dispersion as the ECT–TiO2
system, whereas water-based PEG TiO2 dispersion as the
PEG–TiO2 system. We proceed to compare the performance of organic solvent based TiO2 dispersions withwater-based TiO2 dispersions in order to further elucidate
the fundamental TiO2 properties of ECT–TiO2 and PEG–
TiO2 systems.
Figure 1 shows AFM observations of the PEG–TiO2 andECT–TiO2 thin films. Particle size seems much larger thanthe primary particle size of P25 (*30 nm) especially in the
PEG–TiO2 thin film. Particle surface structure was more
distinctive in the ECT–TiO2 thin film. In fact, the surface
area of the ECT–TiO2 thin film (93.4256 m2/g) was 1.77
times larger than that of the PEG–TiO2 thin film
(52.6626 m2/g) measured by BET analysis.
TG–DTA curves for the PEG–TiO2 dispersion and the
ECT–TiO2 dispersion are shown in Fig. 2. Regarding the

Fig. 1 AFM observation of the TiO2 thin films. a PEG–TiO2 thinfilm, b ECT–TiO2 thin film

J Mater Sci

than that of water, the TiO2 paste for this system burned out
slower from the substrate so that TiO2 film did not crack asmuch as water-based case, which is often the case when a
TiO2 thin film is heat-treated. These fundamental morphology and film formation differences that occur during
the heat treatment process thus result in the difference ofthe energy conversion efficiency.
Influence of metal doping

PEG–TiO2 dispersion, one can observe a large exothermic
peak at 280 C, which can be attributed to polyethylene
glycol decomposition. A small peak can also be observed
at 320 C, which can be attributed to decomposition of thepolyoxyethylene (20) sorbitan monolaurate. Regarding the
ECT–TiO2 dispersion, an endothermic peak can be seen at
175 C, which is considered to be due to evaporation of
terpineol, since it coincides with the boiling temperature of
terpineol. Precisely, the currently used industrial terpineol
contains terpineol and limonene. In fact, this small peakcoincides with the boiling temperature of limonene morethan that of terpineol. The broad peak observable at 295 C
can be explained by decomposition of the ethyl cellulose.
Because the surface area of the ECT–TiO2 thin film islarger than that of the PEG–TiO2 thin film as detected by
BET analysis, the ECT–TiO2 thin film therefore possesses
more available sites for photoelectrochemically active
points since TiO2 film surface area is a critical factor fordye-sensitized solar cells. We suggest this is the reason for
showing better performance of ECT–TiO2 system than thatof PEG–TiO2 system.
It is also inferred that because the decomposition ofresin and solvent peaks are broad in the ECT–TiO2 system,and the boiling temperature of terpineol is much higher

Doped Metal

Fig. 2 TG and DTA curves of TiO2 paste. a PEG–TiO2 system,b ECT–TiO2 system

We have chosen an ECT–TiO2 system to investigate theeffects of metal doping considering this system exhibited asuperior solar cell performance as described above.
The TiO2 powder, used in this study, P25, is composedof anatase-form crystals with a 78 wt% and rutile-form
crystals with a 22 wt%, and with particle size of 15–30 nm
in diameter. The effect of doping on solar energy conversion efficiency (g) of dye-sensitized solar cells is heavily
dependent on the particular metal used as a dopant.
Figure 3 shows the conversion efficiency of dye-sensitizedsolar cells with various metal-doped TiO2 electrodes.
It should be noted here that we have extracted the highest
conversion efficiency number irrespective of doping concentration. The doping concentration is thus different
among the doped TiO2 electrodes shown in Fig. 3. It can beseen that Nb and Ag doping led to a clear enhancement ofg. In order to clarify the effect of meal doping on energy
conversion efficiency, the structure of TiO2 powder and
Au
Pt
W
Ta
Hf
Eu
Sm
Nd
Ce
La
SbF
Sn
In
Ag
Nb
Zr
Y
Sr
Zn
Cu
Ni
Al
Na
Be
Li
undope
0

1

2

3

4

5

6

Energy Conversion Efficiency (%)
Fig. 3 Conversion efficiency of DSCs with various metal-doped
TiO2 electrodes123

J Mater Sci
Table 3 Photovoltaic and film
characteristic of dye-sensitizedsolar cells prepared from Nbdoped TiO2 thin film preparedin this study

SampleJsc, short circuitcurrent(mA/cm-2)

Voc, open
circuitvoltage (mV)FF, fillfactorg, energy
conversion
efficiency

BET surface
area (m2/g)

Undoped

11.01

703

0.496

3.84

93.4256

Nb-0.1

11.46

733

0.505

4.24

56.5474

Nb-1

12

724

0.517

4.49

53.2952

Nb-5

7.05

676

0.533

2.54

54.8359

Nb-25

4.79

583

0.563

1.57

59.7875

photovoltaic properties of dye-sensitized solar cells were
investigated in detail using Nb and Ag-doped TiO2electrodes.
Table 3 summarizes the photovoltaic characteristics ofTiO2 thin films prepared from niobium doped ECT–TiO2
systems. The best results were obtained for an Nb-1 TiO2
thin film. Energy conversion efficiency decreases as the Nbcontent increases up to 5:100 = Nb:Ti in molar ratio.
Anatase TiO2 is known to possess interstitial channels in
the c direction, and so certain transition metals diffuse
through these channels into lattices. The diffusing ionshave been found to locate preferentially on either thesubstitutional or interstitial sites. In general, for better TiO2
photoelectrochemical performance, the dopant for TiO2
should satisfy the following three conditions: (1) it shouldbe pentavalent; (2) it should have an ionic radius close to
Ti4? (0.64 Å); and (3) it should have an oxide soluble in
TiO2 [19]. Niobium seems to be an optimum choicebecause it is pentavalent (Nb5?) and it has similar radius(0.69 Å) to that of Ti4? in TiO2. In addition, Nb can dissolve in TiO2 isotopically, and niobium has been previously reported to be a good dopant for TiO2 [24–26].
In fact, in examining X-ray diffraction patterns of ECT–TiO2, Nb-1 and up to Nb-5 films, none of the Nb-doped
films showed peaks corresponding to deposited niobium
correlated substance, implying that neither niobium nor
niobium oxide is precipitated on the film surface, but isinstead doped into TiO2 (data not shown). It has beenreported that the Nb5? ions in TiO2 substitute Ti4? isotropically and act as electron donors to reduce Ti4? to Ti3?.

Table 4 Photovoltaic and film
characteristic of dye-sensitizedsolar cells prepared from Agdoped TiO2 thin film preparedin this study

123

SampleIt also has been elucidated by XPS measurement that theTi(III) shoulders grow in intensity by Nb doping, especiallyat very low doping levels [27, 28]. It seems that theincrease in the concentration of the donors, such as Ti3?, inthe TiO2 thin film by Nb doping increases the conductivity,
resulting in the higher photocurrents observed in this study.
It should be noted here that even though the surface area of
the TiO2 thin film decreases with the use of Nb doping(Table 3), Nb-1 TiO2 thin films still showed the best
performance.
Table 4 summarizes the photovoltaic characteristics ofTiO2 thin films prepared from silver-doped ECT–TiO2
systems. As the amount of Ag dopant increases, solar cell
performance improves, especially the photo voltage. The
Ag doping can be seen to be most effective for an Ag-1.5
TiO2 film, with g reaching 5.07% for this doping, andthereafter decreasing as Ag content increases up to
5:100 = Ag:Ti in molar ratio. Figure 4 shows the XRD
patterns of Ag-doped TiO2 powder at various doping
concentrations. Diffraction peaks corresponding to anataseform crystals and rutile-form crystals appear in the XRD
pattern of undoped TiO2 powder. In the case of Ag-doped
TiO2 powder, diffraction peaks corresponding to both
crystal forms again appear. However, a careful observation
also takes a notice of the appearance of new diffraction
peaks at 2h = 44.3, 64.5, and 77.5 indicated by arrows
in the figure. These peaks are identified as those of (200),(220), and (311) planes of Ag crystal. The diffraction
intensity of these peaks increases with increasing amountsof doping. XPS spectrum of Ag-doped TiO2 powder isAmount ofJsc, shortVoc, open
FF, fill g, energy
Particle BET
adsorbed dye
circuit current circuit
factor conversion
size
surfacevoltage (mV)efficiency(nm)area (m2/g) (10–8 mol/cm-2) (mA/cm-2)

Undoped 21.8

93.4

5.7

6.2

717

0.5

2.21

Ag-0.2

20.1

54.2

6.2

7.4

745

0.51

2.78

Ag-0.5

18

52

9.5

9.6

794

0.53

4.05

Ag-1.0

20.8

51.7

8.4

9.8

803

0.54

4.26

Ag-1.5

19.8

51.1

9.3

10.9

832

0.56

5.07

Ag-2.0

17.5

48.7

8.8

10

831

0.58

4.81

Ag-5.0

21.5

43.6

5.6

1.7

607

0.72

0.75

J Mater Sci

There is no difference in particle size between the undoped
TiO2 and any of the Ag-doped TiO2. This indicates that Ag
doping has no significant effect on the TiO2 particle size.
The particle size and surface area of Ag-doped TiO2
particles, along with the amount of adsorbed dye against
TiO2 particles, are summarized in Table 4. Specific surface
area decreases as the added amount of dopant increases.

Fig. 4 XRD patterns of TiO2 electrode (a) undoped, (b)Ag-1, (c)Ag-5

Fig. 5 XPS-spectra of Ag3d5/2 and 3d3/2 peak of Ag-doped TiO2
powdershown in Fig. 5. Two bands are observed at 367.8 and373.8 eV. Former band is in good agreement with
Ag?(3d3/2). However, latter band is slightly different with
Ag?(3d5/2). This suggests the coexistence of another element. This band was decomposed to two bands with anintensity ratio of 47:53. The bands with high and low
energy are assigned to Ag0(3d5/2) and Ag?(3d5/2). Therefore, one can say that Ag-doped TiO2 film possess metal
Ag and silver oxide which supports the results for X-ray
diffraction analysis.
These results indicate that Ag doping does not affect theTiO2 crystalline phase, and doped Ag exists as metal andsilver oxide on the surface of TiO2 particles. It seems thatcarbon created from the organic binder during calcination
gives rise to reduction from Ag? to Ag0.
TEM observation indicates that the primary particle sizeof TiO2 was approximately 22–25 nm in diameter (Fig. 6).

Fig. 6 TEM Image of TiO2 electrode. a undoped TiO2, b Ag-1.5,
c Ag-5123

J Mater Sci

Fig. 7 Pore size distribution of Ag-doped TiO2 electrodes withvarious concentrations. Inset indicates enlarged distribution curves in
the range from 0.01 to 0.10 lm

On the other hand, the amount of adsorbed dye molecules
per unit area increases up to 2.0% doping, and thereafter
decreases with further increases in doping.
Figure 7 shows the distribution of pore size of Ag-doped
TiO2 particles. There are two peaks in the pore size distribution: a sharp peak with a narrow size, and a broad peakwith wide size. The latter appears in the range from 5 to
80 lm independently of the doping concentration. These
are suggested to be corresponding to voids between TiO2
agglomerates. On the other hand, the former is corresponding to voids between nanocrystalline TiO2 particles.
The pore sizes are 0.0336, 0.0394, and 0.0824 lm in
Ag-0.2, Ag-1.0, and Ag-5.0 TiO2 agglomerates, respectively. Ko et al. [29] have reported that surface polarity
modified by metal ions could change the primary particle
size and dispersibility. It is thus also proposed that Ag
doping has an influence on the dispersibility of TiO2 particles. The more the dopant concentration increases, thewider the pore size in the agglomerate. However, theamount of adsorbed dye molecules per unit area increases
with dopant concentration in the range from 0.2 to 2.0% in
spite of decreases in the specific surface area across thisrange. This result seems to suggest that the adsorption
strength of dye molecules on Ag metal is stronger than thaton TiO2. However, the reason for the stronger dye
adsorption on Ag remains unknown and needs to be studied. Excess doping, on the other hand, gives rise to an
aggregation of TiO2 particles, resulting in a smaller surface
area and lower adsorption.
Figure 8 shows photovoltaic current–voltage characteristics of the present TiO2 based dye-sensitized solar cells. It
is found that Ag doping is effective at enhancing bothphotocurrent and photovoltage. The photovoltaic parameters of undoped and Ag-doped TiO2 electrodes are also

123

Fig. 8 I–V curves of DSCs using undoped and Ag-doped TiO2electrodessummarized in Table 4. The current–voltage curve can be
described using the following set of parameters: the shortcircuit current (Isc, mA cm-2), the open circuit voltage(Voc, mV), the efficiency percentage (g), and the fill factor
percentage (FF). Isc is the current that is recorded when thevoltage is zero, and Voc is the potential when the current iszero. The efficiency is given by the ratio of the electrical
power that is delivered by the cell to the power of the lightthat is illuminating the cell. The fill factor, FF, is defined as
FF ¼

ðI  V Þmax 100
Isc  Voc

the energy conversion efficiency, g, is defined as
g¼

ðI  V Þmax FF  Isc  Voc¼
PlightPlightwhere Plight is the incident light intensity.
It is clear that the Ag-1.5 TiO2 electrode shows the best
cell performance, in which Isc, Voc, FF, and energy conversion efficiency are 10.9 mA/cm2, 832 mV, 0.56, and 5.07%,
respectively. It should be noted that the concentration of Ag
in the Ag-1.5 TiO2 electrode was determined to be 1.06%
with more accurate measurement using XPS. From ourexperimental results, the enhanced solar cell performance of
the Ag-1.5 TiO2 electrode is attributed to appropriate poresize and dye adsorption in the TiO2 agglomerate.
TiO2 electrodes with dispersed silver nanoparticles andsensitized dye have been reported in previous research[30–32]. There are several effects that can be related to thechange of the photocurrent and photovoltage of the cell
between the semiconductor, dye, and the redox electrolyte.
(1) Surface metal particles and the metal particleinduced surface states can electrocatalytically promote theJ Mater Sci

transportation of UV light-generated holes from thevalence band to the electrolyte solution. (2) A strong
electromagnetic field is induced by the Ag plasmon, which
either enhances the optical absorption of the dye or thesurface states of the TiO2 electrodes. (3) Internal photoemission from the Ag particle increases the photocurrent.
On the other hand, (4) Schottky barriers at the TiO2/metalinterfaces in the interior reduce the mobility of the electrons and holes. (5) The surface metal particles and themetal particle-induced surface states act as recombination
centers of light generated electron/hole pairs. (6) The
fluctuation of the conduction band edge caused by thesurface metal particles retards the transfer of the conduction band electrons from the surface to the interior. Effects(1)–(3) increase the photocurrent, while effects (4)–(6)decrease the photocurrent. In these previous researches, thestudy by Wen et al. [32] describes how the photoresponse
in a longer wavelength in the range of 420–800 nm
increases, whereas this response is found to decrease in thestudies of Zhao et al study [30, 31]. Investigations of thephotoresponse against wavelength of the current Ag-doped
ECT–TiO2 system remains an area for future study, but inany case, it seems that the influence of factors (1)–(3)above were stronger in the present study.
Comparison with general TiO2 paste for dye-sensitizedsolar cell
It should be noted here that T/SP TiO2 paste (Solaronix SA,
Switzerland) made by the hydrothermal method resulted ina conversion efficiency of about 5.5% in our experiment
although our best result was about 5%. However, our dispersion technique can be easily transferred to mass production level ‘‘bead mill’’ machine dispersion procedure.
This means that we can manufacture a TiO2 liquid dispersion with almost identical performance to that of T/SP
at a substantially lower cost. With hydrothermal method in
an industrial level, usually only 50–150 kg of titania paste
can be manufactured in 1 day and also high temperature isnecessary for autoclave. On the other hand, our method can
produce over 5–6 tons or ever more at much shorter time.
In addition, we do not need high temperature. In a recent
research, we have achieved even higher light-to-electricity
energy conversion efficiency than present study by using
TiO2 with higher anatase crystallinity instead of P25 as araw material, and also with another dispersion technique,which will be discussed in a forth coming paper.

Conclusions
We have prepared a TiO2 liquid dispersion of P25 using ageneral industrial dispersion technique for the TiO2electrode of a dye-sensitized solar cell. Organic solvent
based TiO2 dispersions enabled the formation of more
uniform TiO2 thin films in comparison with water-based
TiO2 dispersions. The best combination was found to bethat using ethyl cellulose as the resin and the terpineol as
the solvent. It was also found that using two kinds of resinof different molecular weights, gave rise to better efficiency. To obtain high solar efficiency, hard dispersion
condition to breakdown the TiO2 nanoparticle into a small
size was not necessary. Because as particle size decreases,
dye–electrolyte diffusion is suppressed in the TiO2 thinfilm, which will lower the solar cell performance. The best
metal dopant from the perspective of enhancing the light toelectricity conversion efficiency was found to be Ag. ThisAg metal doping had an influence on pore size and dispersibility of TiO2 agglomerates, and on the amount ofadsorbed dye molecule on the TiO2 electrode, which areproposed as the reasons for the observed enhanced cell
performance. Ag-doped TiO2 film possesses metal Ag andsilver oxide which supports the results for X-ray diffraction
analysis.
Acknowledgements This study has been carried out throughCollaboration of Regional Entities for the Advancement of Technological Excellence (abbr. CREATE) organized by HYOGO Prefectureand funded by Japan Science and Technology Agency (abbr. JST) for
development of common ground of nano-particle composite.

References
1. O‘Regan B, Grätzel M (1991) Nature 353:737
2. Bach U, Lupo D, Comte P, Moser JE, Weissörtel F, Salbeck J,
Spreitzer H, Grätzel M (1998) Nature 395:583
3. Nazeeruddin M, Angelis F, Fantacci S, Selloni A, Viscardi G,
Liska IS, Takeru B, Grätzel M (2005) J Am Chem Soc 127:16835
4. Xia J, Li F, Yang H, Li X, Huang C (2005) J Mater Sci 42:6412.
doi:10.1007/s10853-006-1184-3
5. Hirata N, Lagref JJ, Palomares EJ, Durrant JR, Nazeeruddin MK,
Gratzel M, Censo DD (2004) Chem Eur J 10:595
6. Bignozzi CA, Argazzi R, Indelli MT, Scandora F (1994) Sol
Energy Mater Sol Cells 32:229
7. Argazzi R, Bignozzi CA, Heimer TA, Castellano FN, Meyer G(1994) Inorg Chem 33:5741
8. Koumura N, Wang ZS, Miyashita M, Uemura Y, Sekiguchi H,
Cui Y, Mori A, Mori S, Hara K (2009) J Mater Chem 27:4829
9. Usami A (1997) Chem Phys Lett 277:105
10. Ferber J, Luther J (1998) Sol Energy Mater Sol Cells 54:265
11. Huang SY, Schlichthörl G, Nozik AJ, Grätzel M, Frank AJ(1997) J Phys Chem B 101:2576
12. Nattestad A, Mozer AJ, Fischer MR, Cheng YB, Mishra A,
Bauerle P, Bach U (2010) Nat Mater 9:31
13. Schlichthörl G, Huang SY, Sprague J, Frank AJ (1997) J Phys
Chem B 101:8141
14. Snaith HJ (2010) Adv Funct Mater 20:13
15. Kavan L, Grätzel M, Rathousky J, Zukal A (1996) J Electrochem
Soc 143:394
16. Papageorgiou N, Barbe C, Grätzel M (1998) J Phys Chem B
102:4156
17. Lin B, Aydil ES (2009) J Am Chem Soc 131:3985

123

J Mater Sci
18. Frank JA (1997) AIP conference proceedings: future generation
photovoltaic technologies: proceedings of the first NREL conference. NICH Report No. 24469:145
19. Finklea HO (1998) Semiconductor electrodes. Elsevier,
Amsterdam
20. Zhao G, Kozuka H, Lin H, Yoko T (1996) Thin Solid Films
339:123
21. Zhao G, Kozuka H, Lin H, Yoko T (1996) Thin Solid Films
340:125
22. Barbe CJ, Arendse F, Comte P, Jirousek M, Lezmann F, Shklover
V, Grätzel M (1997) J Am Ceram Soc 80:3157
23. Nakade S, Saito Y, Kubo W, Kitamura T, Wada Y, Yanagida S(2003) J Phys Chem B 107:8607

123

24. Salvador P (1980) Sol Energy Mater 2:413
25. Kutty TN, Avudaithai M (1990) Int J Hydrogen Energy 15:621
26. Kikkawa J, O’Regan B, Anderson MA (1991) J Electroanal
Chem 309:91
27. Goodenough JB (1971) Prog Solid State Chem 5:145
28. Morris D, Dou Y, Rebane J, Mitchell CJ, Egdell RG (2000) Phys
Rev B 61:13445
29. Ko KH, Lee YC, Jung YJ (2005) J Colloid Interface Sci 283:482
30. Zhao G, Kozuka H, Yoko T (1996) Thin Solid Films 277:147
31. Zhao G, Kozuka H, Yoko T (1996) Sol Energy Mater Sol Cells
277:147
32. Wen C, Ishikawa K, Kishima M, Yamada K (2000) Sol Energy
Mater Sol Cells 61:339