Fabrication of high performance Pt counter electrodes on conductive plastic substrate for flexible dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2010.01.108

Pt counter electrodes (CEs) with different platinum loading have been prepared using chemical reduced method on flexible indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for dye-sensitized solar cells (DSSCs). H₂PtCl₆

Full text of DOI:10.1016/j.electacta.2010.01.108

Electrochimica Acta 55 (2010) 3721–3726
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of high performance Pt counter electrodes on conductive plastic
substrate for flexible dye-sensitized solar cells
a
b
b
b
a,∗
b,∗∗
Lili Chen , Weiwei Tan , Jingbo Zhang , Xiaowen Zhou , Xiaoling Zhang , Yuan Lin
a
Department of Chemistry, School of Science, Beijing Institute of Technology, Beijing 100081, China
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,
b
ZhongGuanCun No. 2, 1st North Street, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Pt counter electrodes (CEs) with different platinum loading have been prepared using chemical reduced
method on flexible indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for dye-sensitized
solar cells (DSSCs). H2PtCl6·6H2O terpineol solutions were screen printed on the transparent ITO-PEN sub-
strates. After drying, H2PtCl6 was reduced by treating it in NaBH4 solution followed by the hydrothermal
Received 20 November 2009
Received in revised form 25 January 2010
Accepted 30 January 2010
Available online 6 February 2010
◦
2
treatment at 100 C. The obtained Pt CEs with different Pt-loading (2.4–7.7 ␮g/cm ) were characterized
by SEM, XPS, electrochemical impedance and transmission spectrum measurement. The Pt CEs show
high catalytic activity, low charge transfer resistance (0.26–1.38 ꢀ cm ) and good light transmittance
Keywords:
2
Flexible Pt counter electrode
Chemical reduction
Hydrothermal treatment
Screen printing
(about 70% at 400–800 nm). The light-to-electricity conversion efficiency of the flexible DSSC fabricated
with the prepared Pt CE and the TiO2 photoanode prepared on Ti substrate by screen printing technique
attains 5.41% under the simulated AM 1.5 sunlight, which is almost same as that based on the thermal
decomposited Pt CE on FTO-glass. Compared with other methods to prepare Pt CEs, chemical reduced
method is simple and suitable for flexible polymer substrates and the large scale preparation of DSSCs.
Dye-sensitized solar cells
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
poly(3,4-ethylenedioxythiophene) (PEDOT) [18–22], poly(3,3-
diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-
Et2) [23], polypyrrole [24] and polyaniline [25], have been
introduced. However, the best candidate for the CE is Pt due to its
highly catalytic effect toward triiodide reaction, superior chemical
and electrochemical stabilities.
Since O’Regan and Grätzel firstly reported dye-sensitized solar
cell (DSSC) in 1991, it has attracted great interests in academic
research and industrial applications due to its high light-to-
electricity conversion efficiency, simple fabrication process and the
potential for low cost production [1–3]. DSSC typically consists of
redox electrolyte sandwiched by a nanocrystalline-TiO2 film cov-
ered by a monolayer of dye molecules and a counter electrode (CE)
with transparent conducting oxide (TCO) glass as substrates. The
CE is used to reduce the redox species, which act as a mediator
in regenerating the sensitizer after electron injection in a liquid-
state DSSC, or to collect holes from the hole transport material in
a solid-state DSSC [4]. In order to reduce cost and accelerate the
applications of DSSCs, it is necessary to develop DSSCs based on
flexible substrates such as metal foils and plastic substrates. How-
ever, there are few detailed reports on the preparation of CEs on
plastic substrates.
Thermal decomposition of H PtCl6 isopropanol solution is
2
widely used to prepare high performance Pt CE, because it is stable
−
−
and shows higher exchange current density for the I3 /I cou-
ple [4]. However, this process requires a heat treatment up to
◦
390 C that most plastic substrates cannot bear. Pt CEs on flexi-
ble substrates are usually prepared by sputtering, electrochemical
deposition and chemical reduction. Ikegami et al. deposited a Pt/Ti
bilayer on ITO-PEN substrates using vacuum sputtering and assem-
bled a full plastic DSSC and achieved the conversion efficiency of
4.31% [10]. Pt CEs deposited by sputtering can be well used for flex-
ible DSSC. Furthermore, Fang et al.’s investigation showed that the
catalytic effect of sputtered Pt layer is independent of Pt layer thick-
ness when it is over 7 nm [26]. Therefore, the inner Pt layer in thick
sputtered films, such as 100 nm Pt-mirror layer, has no contribu-
tion on triiodide reduction and is waste. Electrochemical deposition
method attracted much attention because it is simple and feasible.
Grätzel electrodeposited Pt catalyst on the ITO/PEN substrate to
assemble a flexible DSSC and achieved the conversion efficiency of
7.2% [27]. Chemical reduction of H2PtCl6 to prepare Pt CE on plas-
tic substrate has been studied by Kang et al. [28] and Park et al.
Up to now, several kinds of catalytic materials for CEs,
such as platinum [5–11], carbon materials (graphite, activated
carbon, carbon black, single-wall carbon nanotubes) [12–17],
∗
Corresponding author.
Corresponding author. Tel.: +86 10 82615031; fax: +86 10 82617315.
E-mail addresses: ZhangXL@bit.edu.cn (X. Zhang), linyuan@iccas.ac.cn (Y. Lin).
∗
∗
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.01.108

3
722
L. Chen et al. / Electrochimica Acta 55 (2010) 3721–3726
[
29]. They spread H PtCl ·6H O 2-propanol solution on conduc-
spectrometer with standard Al K˛ radiation. The spectra were taken
2
6
2
4+
−9
tive plastic substrates followed by reducing Pt in NaBH solution.
However, there is no detailed investigation on the chemical reduc-
tion of Pt CE as to optimize its performance.
Based on these reports, we attempted to fabricate Pt CEs on flex-
ible ITO/PEN substrates by screen printing technology followed by
chemical reduction method. The ITO/PEN substrate is introduced in
DSSCs by many scientists, but it cannot endure high temperature
(
hydrothermal method as a post-treatment to treat Pt CEs at low
temperature via a hydrothermal reaction at the solid/liquid inter-
face. The common pressure hydrothermal method is a simple low
temperature preparation method, where the hydrothermal reac-
at a working pressure of <3 × 10 mbar. The binding energy was
4
calibrated using the C 1s line (284.6 eV) from adventitious carbon.
Wide-scan spectra were recorded in the range of 0–1200 eV.
To study the interfacial charge transfer resistances (Rct), electro-
chemical impedance spectra (EIS) were measured on a symmetric
thin-layer cell composed of two identical Pt electrodes and a Surlyn
film with the thickness of 40 ␮m as the spacer. The active area of
◦
2
<150 C). Here, we reported for the first time the common pressure
the electrode was 0.25 cm . The inter-space between electrodes
−
1
−1
was filled with electrolyte, which was 0.5 mol L LiI, 0.05 mol L
−
1
I2, and 0.5 mol L 4-tert-butylpyridine in 3-methoxypropionitrile.
The measurement was performed using Solartron 1255B frequency
response analyzer and Solartron SI 1287 electrochemical interface
system at the zero bias with the frequency range of 0.05 Hz to
1 MHz.
◦
tion can take place in a non-sealed system at 100 C at common
pressure. It is convenient and cheap and can be used on the flexible
polymer substrates. The organic residues can be removed and Pt
CEs become mechanically stable after the hydrothermal process. In
addition, it is also beneficial to the preparation of large scale cells
and mass production. The prepared flexible Pt CEs were used to fab-
ricate DSSCs with nanocrystalline-TiO2 photoelectrodes deposited
on Ti foil substrates by screen printing and high temperature sin-
tering and the cells show satisfied photovoltaic performance.
Platinum loadings were determined by dissolution of the elec-
trode in nitrohydrochloric acid, and platinum content in the
subsequent diluted solution was determined by measuring its
atomic emission spectroscopy (Hitachi P-4010).
2.4. Fabrication of flexible DSSCs
A Ti foil was used as a substrate for the nanocrystalline-TiO film
2
2
. Experimental
to fabricate the flexible DSSC. The TiO colloidal was synthesized by
2
sol–gel and hydrothermal techniques from titanium isopropoxide
(97 wt%) precursor. The typical preparation process was as fol-
lowing. Titanium isopropoxide was hydrolyzed in pH 2 aqueous
2
.1. Materials
◦
All chemicals used were of analytical reagent grade without fur-
solution under strong stirring at 80 C for 2 h and then autoclaved
◦
ther purification. H PtCl ·6H O was purchased from Sinopharm
at 250 C for 13 h [31]. The colloidal was evaporated and converted
2
6
2
Chemical Reagent Beijing Co., Ltd. and other reagents from Aldrich.
ITO-PEN (15 ꢀ/ꢀ, light transmittance: 80% at 550 nm, Peccell Tech-
nologies, Inc.) and fluorine-doped tin oxide transparent conductive
glass (FTO, 20 ꢀ/ꢀ, Hake New Energy Co. Ltd., Harbin) were ultra-
sonically cleaned sequentially in detergent solution, acetone and
finally in distilled water. Ti foil (0.2 mm thickness, Baoji Mingkun
Nonferrous Metals Co., Ltd.) was washed with mild detergent and
rinsed in distilled water, then immersed in saturated oxalic acid
solution for 10 min and rinsed in distilled water again.
to a TiO2 screen printing paste [32], and then screen printed on the
Ti foil surface using an 80 mesh screen. After drying at room tem-
◦
perature, the TiO2 thin films were sintered at 450 C for 30 min in
air to give nanocrystalline-TiO2 films. As the TiO2 electrodes were
◦
cooled to 80 C, they were sensitized by immersing in an absolute
−
4
−1
ethanolic solution of 5 × 10 mol L
Ru(dcbpy) (NCS) (dcbpy:
2
2
ꢀ
ꢀ
2,2 -bipyridine-4,4 -dicarboxylic acid) (N , Solaronix) for 12 h. The
3
thickness of the nanocrystalline-TiO2 film was fixed at about
2
10 ␮m for flexible DSSC. The active cell area was 0.20 cm . The
−
1
−1
−1
electrolyte was 0.5 mol L LiI, 0.05 mol L I2, and 0.5 mol L 4-
tert-butylpyridine in 3-methoxypropionitrile.
2
.2. Preparation of Pt CEs
Current–voltage (I–V) curves of DSSCs were measured with
Potentiostat/Galvanostat Model 273 (EG&G) under light intensity
of 100 mW cm at AM 1.5 offered by a solar light simulator (Oriel,
91160-1000).
H PtCl ·6H O was dissolved in terpineol with concentration of
2
6
2
−
2
0
.4 wt%, 0.6 wt%, 0.8 wt% and 1.0 wt% to prepare the paste. As a
low temperature method, the pastes were screen printed on the
◦
ITO-PEN surface using a 200 mesh screen and then dried at 80 C
for 2 h to give H PtCl /ITO-PEN electrodes with the active area of
3. Results and discussion
2
6
1
cm × 1 cm. Then the H PtCl /ITO-PEN electrodes were immersed
2
6
◦
in 10 mM NaBH aqueous solution at 40 C to reduce Pt ions. After
3.1. Preparation of flexible Pt CEs
4
2
h, the electrodes were taken out from the NaBH4 solution and
rinsed with distilled water. Then, two different post-treatment
methods were applied. One is the common pressure hydrothermal
process to place the electrodes in a non-sealed container filled with
The H PtCl /ITO-PEN electrodes prepared by screen printing the
2
6
pastes on the ITO-PEN surface are transparent and colorless. When
the H PtCl /ITO-PEN electrodes were immersed into the NaBH
2
6
4
◦
water at 100 C for 4 h to remove organic residues then to dry them
solution, the electrode surface starts to bubble up gently and color
of the surface changes gradually from colorless to gray-white. At
◦
at 80 C for 2 h giving transparent Pt CEs (Pt/ITO-PEN). Another is
◦
4+
to sinter the electrodes at 100 C for 4 h in an oven instead of the
this point, the Pt ions were reduced to metal platinum at the
hydrothermal treatment. The thermal decomposited Pt CE was also
ITO-PEN surface according to the redox reactions,
◦
prepared at 390 C on FTO-glass for comparison [30].
PtCl 2 + 4e− → Pt + 6Cl−
−
(1)
(2)
6
2.3. Characterization of Pt/ITO-PEN films
BH4 + 3H O − 4e → BO33− + 2H + 6H
−
−
+
2
2
The surface morphology of the Pt electrodes was observed by
The overall reaction,
a scanning electron microscope (SEM, Hitachi S-4300). The trans-
mission spectrum measurement was taken on a Hitachi U-3010
spectrophotometer.
To study the formation of Pt on ITO-PEN surface, X-ray pho-
toelectron spectroscopy was obtained using an Escalab 220i-XL
PtCl6 + BH4− + 3H O → Pt + BO3 + 6Cl− + 2H + 6H
2−
3−
+
(3)
2
2
To obtain the high performance flexible Pt CEs, we carefully opti-
mized the concentration and pH of NaBH4 solution, the reaction
temperature and time. These optimized reaction conditions were

L. Chen et al. / Electrochimica Acta 55 (2010) 3721–3726
3723
stated in Section 2. In addition, the ITO/PEN substrate is so light
3.3. Analysis of XPS spectrum of the Pt/ITO-PEN electrode
that it can float on the solution surface, so the flexible electrodes
were fixed on the container bottom by adhesive tape during the
chemical reduced reaction and the hydrothermal treatment pro-
cess.
The formation of Pt on ITO-PEN surface after the hydrothermal
treatment was determined using X-ray photoelectron spectroscopy
(XPS). Fig. 2 shows a wide-scan spectrum for Pt film deposited on
ITO-PEN surface (Fig. 2a) and the Pt 4f XPS peaks (Fig. 2b). There
are no other Pt characteristic peaks except Pt 4f XPS peaks which
appeared in Fig. 2a. In Fig. 2b, the XPS peaks with binding energies of
3.2. Surface morphologies of Pt/ITO-PEN films
∼
71 eV and ∼74 eV correspond to Pt 4f7 and Pt 4f5/2 [33], respec-
/2
Fig. 1 shows the surface morphology of the bare ITO-PEN
substrate and the Pt/ITO-PEN electrode prepared using 0.6 wt%
tively. But the peak intensity ratio of Pt 4f
and Pt 4f5/2 is not the
/2
7
same as the reported value of 7:5, indicating that there exist some
other valence states of platinum on the ITO-PEN surface except Pt
H PtCl ·6H O terpineol solution. As seen in Fig. 1a, the surface of
2
6
2
the bare ITO-PEN substrate is quite flat with a few particles with
a size of ∼100 nm on the surface. Fig. 1b shows the surface mor-
phology of Pt/ITO-PEN CE after the hydrothermal treatment. It can
be found that small Pt nanoparticles formed the Pt nanoclusters
with the size of ∼10 nm on the surface of ITO-PEN substrate. The
surface morphology of the Pt/ITO-PEN electrodes based on other
concentration of H PtCl ·6H O terpineol solutions is similar to that
(0), but the main valence state of platinum was zero.
3.4. Transmission spectra and Pt-loading of Pt/ITO-PEN electrodes
Fig. 3 shows the transmission spectra of Pt/ITO-PEN CEs pre-
2
6
2
pared with different concentrations of H PtCl6 solutions. The light
transmittance for all Pt/ITO-PEN CEs is around 70%, and decreases
with increasing the concentration of H PtCl . We can observe
2
in Fig. 1b. Obviously, the ITO-PEN substrate was covered completely
by Pt nanoparticles. In comparison, Fig. 1c shows the surface mor-
2
6
◦
phology of Pt/ITO-PEN CE directly dried at 100 C for 4 h in an
that the color of Pt/ITO-PEN CEs become darker and darker with
oven instead of the hydrothermal treatment. On this Pt/ITO-PEN
electrode surface, it was found the Pt nanoclusters are inhomoge-
neous and the shape of Pt nanoclusters is indistinct. In fact, after
increasing the H PtCl6 concentration. Due to the scatter effect of
Pt nanoparticles on the ITO-PEN surface, the peaks in the transmis-
sion spectra move to long wavelength with the formation of larger
2
the H PtCl /ITO-PEN electrode was taken out from NaBH aque-
Pt nanoparticles in the higher concentration of H PtCl . Consider-
2
6
4
2 6
ous solution, there was some terpineol remaining on the surface.
During the chemical reduced process, the terpineol plays a role
as a protector to stabilize the Pt nanoclusters and prevent further
growth of the Pt nanoparticles, which benefit to increase the sur-
face area of Pt electrode. Then, in hydrothermal treatment process,
the organic residues and the loose Pt nanoparticles were removed,
as shown in Fig. 1b. In other words, the hydrothermal treatment, as
a post-treatment, is more important and efficient than the drying
ing illumination through the Pt CEs is required for DSSCs based on
the photoelectrode on Ti foil substrates, the good light transmit-
tance of the Pt/ITO-PEN CEs will be beneficial to high performance
of the flexible DSSCs.
Inset of Fig. 3 shows the change of Pt-loading with the concen-
tration of H PtCl . The Pt-loading increases with increase in the
2
6
concentration of H PtCl . When the concentration of H PtCl is
2
6
2
6
0.4 wt%, 0.6 wt%, 0.8 wt% and 1.0 wt%, Pt-loading of the prepared
◦
2
2
2
2
method at 100 C.
electrodes is 2.4 ␮g/cm , 3.5 ␮g/cm , 5.8 ␮g/cm and 7.7 ␮g/cm ,
Fig. 1. Scanning electron microscopy images of bare ITO-PEN substrate (a), Pt electrodes prepared on ITO-PEN by chemical reduced deposition with hydrothermal treatment
◦
(
b) and with drying at 100 C instead of hydrothermal treatment (c).

3
724
L. Chen et al. / Electrochimica Acta 55 (2010) 3721–3726
Fig. 2. Wide-scan XPS spectrum of Pt/ITO-PEN prepared by chemical reduced deposition with hydrothermal treatment (a) and its Pt 4f XPS spectrum (b).
−
respectively. The Pt-loading was controlled by the Pt precursor
concentration, but it is not direct proportion.
an acceleration of the I3 reduction. When the concentration of
H PtCl6 exceeded 0.6 wt%, the decreased extent of Rct becomes
2
small.
◦
3
.5. Electrocatalytic activity of Pt/ITO-PEN electrodes
It is interesting to find that Rct of the Pt CE drying at 100 C
for 4 h, was 3 times larger than that of the Pt CE with hydrother-
mal treatment for the Pt/ITO-PEN electrode using the same paste
(0.6 wt% H2PtCl6·6H2O terpineol solution). This positive effect of
hydrothermal treatment is accordant with the surface morphology
images as mentioned above. In addition, Rct of the prepared Pt CEs
was comparable to that of the conventional thermal decomposited
Pt CEs and much lower than that of the electrodeposited Pt CEs
[9].
To understand electrocatalytic activity of the Pt CEs after
the hydrothermal treatment, the electrochemical impedance
spectroscopy (EIS) was measured with the sandwich type elec-
trochemical cell consisting of two identical Pt/ITO-PEN electrodes
shown in Fig. 4a [34]. For measurement convenience, the flexi-
ble cell was clamped between two pieces of glass as shown in
Fig. 4a. The equivalent circuit for this type of cell is described
in Fig. 4b. The ohmic serial resistance (Rs) can be determined
from the impedance at the high frequency (around 100 kHz)
where the phase is zero. In the middle frequency range between
1
0 Hz and 100 kHz, the impedance was dominated by the RC net-
work of Pt electrode and electrolyte interface, which consists of
the charge transfer resistance (Rct) and capacitance of electrical
double layer (Cdl). The impedance in the low frequency can be
interpreted to the Nernst diffusion impedance (ZN). The resultant
impedance spectra are shown in Fig. 4c and the fitting data are
listed in Table 1. The values of frequency at the highest points
of the impedance spectra are indicated in the figure. The cir-
cle became small with increasing the concentration of H PtCl6 as
2
shown in Fig. 4c, indicating the decrease of Rct. Because Pt serves
as a catalyst for triiodide reduction, the decrease in Rct implies
Fig. 3. Transmission spectra of ITO-PEN (solid line) and the flexible Pt counter elec-
trodes prepared with the paste containing 0.4 wt% (dot line), 0.6 wt% (dash line),
Fig. 4. Schematic structure of cell fabricated by two identical platinized electrodes
used in EIS measurements (a), equivalent circuit for EIS (b) and the measured Nyquist
plots (c) of the symmetric cells based on Pt CEs prepared by H2PtCl6·6H2O terpilenol
solution with different concentrations as 0.4 wt% (square), 0.6 wt% (star), 0.8 wt%
(circle) and 1.0 wt% (down triangle) followed by hydrothermal treatment.
0
.8 wt% (dash dot dot line) and 1.0 wt% (short dash line) H2PtCl6·6H2O terpilenol
solutions. The inset shows the relationship of Pt-loading of Pt electrodes with the
quality fraction of the paste.

L. Chen et al. / Electrochimica Acta 55 (2010) 3721–3726
3725
Table 1
The character and performance of Pt electrodes used in DSSCs.
Pt-loading (␮g/cm2)
Rct (ꢀ cm2)
Jsc (mA cm
−2
)
Voc (mV)
Conversion efficiency (%)
FF
Preparation method
Concentration of
H2PtCl6·6H2O (wt%)
RH
RH
RH
RH
RD
TD
0.4
0.6
0.8
1.0
0.6
0.6
2.4
3.5
5.8
7.7
3.4
3.7
1.38
0.53
0.30
0.26
1.68
0.46
9.90
10.1
9.85
6.56
9.30
11.8
676
682
673
673
682
680
4.91
5.41
4.88
3.27
4.05
5.62
0.73
0.78
0.74
0.74
0.64
0.70
◦
RH, NaBH4 reduction with hydrothermal treatment; RD, NaBH4 reduction with directly drying at 100 C for 4 h; TD, thermal decomposition on ITO-glass.
As a result, the CEs prepared by the paste with higher con-
centration of H PtCl6 usually have lower Rct and lower light
2
transmittance. Low Rct is beneficial to high performance of CEs,
however, low light transmittance of the CEs will decrease the per-
formance of DSSCs in the back illumination model. Therefore, it
is necessary to balance between Rct and light transmittance. In
other words, there is an optimal concentration of H PtCl6 for high
2
performance Pt CEs.
The stability of Pt CEs was examined by characterization of elec-
trocatalytic activity in terms of monitoring the Rct with time. Rct
value was increased by 15% compared with the initial value after
1
2 weeks, which indicates a good stability on the electrocatalytic
activity of Pt CEs for triiodide reduction.
3.6. Photocurrent–voltage characteristics of flexible DSSCs
Fig. 5 shows configuration of a typical flexible DSSC and the illu-
Fig. 6. Current–voltage curves of dye-sensitized solar cells based on Pt CEs prepared
with H2PtCl6·6H2O terpilenol solutions with different concentrations as 0.4 wt%
mination direction through the CE side. The flexible Pt/ITO-PEN CE
was leveled up on a piece of ordinary glass. Fig. 6 shows the mea-
sured photocurrent–voltage curves of as-fabricated DSSCs and the
corresponding photovoltaic parameters are listed in Table 1. The
inset of Fig. 6 shows the conversion efficiency of flexible DSSCs as a
(
solid line), 0.6 wt% (dash line), 0.8 wt% (dot line) and 1.0 wt% (dash dot line) followed
by hydrothermal treatment and with 0.6 wt% H2PtCl6·6H2O terpilenol solution fol-
lowed by drying at 100 ◦C for 4 h (short dash line), and thermal deposited Pt electrode
on ITO-glass (dash dot dot line). The inset shows the relationships of conversion
efficiency of flexible DSSCs with the H2PtCl6 quality fraction concentration.
function of the H PtCl quality fraction concentration to prepare Pt
2
6
CEs. The conversion efficiency increases with the H PtCl6 concen-
2
without the hydrothermal treatment shows lower fill factor and
conversion efficiency. This is consistent with the charge trans-
fer kinetic performance as mentioned above. It is known that FF
depends strongly upon the internal resistance of the cell. A higher FF
can be obtained for the cell based on Pt CE with the lower resistance
of charge transfer on the electrolyte/Pt interface.
tration and reaches a maximum value at 0.6 wt% then decreases
with the concentration. Based on these photovoltaic parameters,
the conversion efficiency enhancement is attributable to the pro-
motion of short-circuit current density (Jsc) and fill factor (FF) by
comparing with the data of the Pt CE without hydrothermal treat-
ment, because their open-circuit voltage (Voc) values were almost
unchangeable. It was found that DSSC based on the prepared CE
However, Jsc startedtodecreasewhentheH PtCl concentration
2
6
exceeded 0.6 wt%. The light transmittance of Pt CEs is an important
factor for the back-side illumination of the flexible DSSCs. The Pt CE
prepared with higher concentration of H PtCl6 solution has higher
2
Pt-loading and darker electrode surface, which reduces light inten-
sity illumined on the photoelectrode, thus giving a lower Jsc of DSSC.
Therefore, to obtain the best performance of DSSCs, there is an opti-
mized point balancing the opposite effect of Pt-loading on Rct and
the light transmittance. The highest conversion efficiency for DSSC
based on Pt CE with Pt-loading of 3.5 ␮g/cm2 prepared by 0.6 wt%
H PtCl solution attains 5.41%, which is close to that of DSSC based
2
6
on the thermal decomposited Pt film (5.62%).
4. Conclusions
A flexible DSSC was assembled with a Pt/ITO-PEN CE and a TiO2
photoelectrode on Ti foil substrate. The Pt CE was prepared on ITO-
PEN using screen printing technology followed by NaBH reduction
4
combining with hydrothermal method. This chemical reduction
method is simple and cheap. The photovoltaic performance of the
prepared Pt CEs depends on the H PtCl concentration, which con-
2
6
trols the Pt-loading, thus affects Rct and light transmittance of the
Pt CEs. The prepared Pt CE shows high electrocatalytic activity for
triiodide reduction, and the flexible DSSC fabricated by the Pt CE
Fig. 5. Configuration of a dye-sensitized solar cell irradiated through the flexible Pt
CE (back illumination).

3
726
L. Chen et al. / Electrochimica Acta 55 (2010) 3721–3726
prepared with the paste containing 0.6 wt% H PtCl ·6H O terpineol
[12] K. Imoto, K. Takatashi, T. Yamaguchi, T. Komura, J. Nakamura, K. Murata, Sol.
Energy Mater. Sol. Cells 79 (2003) 459.
2
6
2
solution and N3 sensitized nanocrystalline-TiO2 thin film elec-
trode screen printed on Ti foil gives the highest light-to-electricity
conversion efficiency of 5.41% at AM 1.5 simulated full sunlight.
The satisfied photovoltaic performance of the prepared Pt CEs is
attributed to the hydrothermal treatment, in which process the
organic residues is removed, and thus decreasing Rct of the pre-
pared Pt CEs.
[
13] T. Kuboa, J. Tanimoto, M. Minami, T. Toya, Y. Nishikitani, H. Watanabe, Solid
State Ionics 165 (2003) 97.
[14] K. Suzuki, M. Yamamoto, M. Kumagai, S. Yanagida, Chem. Lett. 32 (2003) 28.
[
15] Z. Huang, X.Z. Liu, K.X. Li, D.M. Li, Y.H. Luo, H. Li, W.B. Song, L.Q. Chen, Q.B. Meng,
Electrochem. Commun. 9 (2007) 596.
[
16] W.J. Lee, E. Ramasamy, D.Y. Lee, J.S. Song, Sol. Energy Mater. Sol. Cells 92 (2008)
814.
[17] K.X. Li, Y.H. Luo, Z.X. Yu, M.H. Deng, D.M. Li, Q.B. Meng, Electrochem. Commun.
1 (2009) 1346.
18] Y. Saito, T. Kitamura, Y. Wada, S. Yanagida, Chem. Lett. 31 (2002) 1060.
1
[
Acknowledgements
[19] Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, J. Photochem. Photobiol.
A: Chem. 164 (2004) 153.
[
[
20] J.G. Chen, H.Y. Wei, K.C. Ho, Sol. Energy Mater. Sol. Cells 91 (2007) 1472.
21] W.J. Hong, Y.X. Xu, G.W. Lu, C. Li, G.Q. Shi, Electrochem. Commun. 10 (2008)
1555.
This work was supported by the National Nature Science Foun-
dation of China (no. 20975012), the 111 Project (B07012), the Major
State Basic Research Development Program (2006CB202605), the
High-Tech Research and Development Program of China (no.
[
[
22] P. Balraju, M. Kumar, M.S. Roy, G.D. Sharma, Synth. Met. 159 (2009) 1325.
23] K.M. Lee, P.Y. Chen, C.Y. Hsu, J.H. Huang, W.H. Ho, H.C. Chen, K.C. Ho, J. Power
Sources 188 (2009) 313.
2
007AA05Z439).
[24] J.H. Wu, Q.H. Li, L.Q. Fan, Z. Lan, P.J. Li, J.M. Lin, S.C. Hao, J. Power Sources 181
2008) 172.
(
[
25] Q.H. Li, J.H. Wu, Q.W. Tang, Z. Lan, P.J. Li, J.M. Lin, L.Q. Fan, Electrochem. Commun.
10 (2008) 1299.
26] X.M. Fang, T.L. Ma, M. Akiyama, G.Q. Guan, S. Tsunematsu, E. Abe, Thin Solid
Films 472 (2005) 242.
27] M. Grätzel, Chem. Commun. 38 (2006) 4004.
28] M.G. Kang, N.G. Park, K.S. Ryu, S.H. Chang, K.J. Kim, Sol. Energy Mater. Sol. Cells
References
[
[
[
[
[
[
[
1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.
2] M. Grätzel, J. Photochem. Photobiol. C 4 (2003) 145.
3] M. Grätzel, J. Photochem. Photobiol. A 164 (2004) 3.
4] N. Takurou, A. Murakami, M. Grätzel, Inorg. Chim. Acta 361 (2008) 572.
5] M. Grätzel, J. Photochem. Photobiol. A: Chem. 164 (2004) 3.
6] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito,
B. Takeru, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16837.
[
[
90 (2006) 574.
[
[
[
[
[
[
29] J.H. Park, Y. Jun, H.G. Yun, S.Y. Lee, M.G. Kang, J. Electrochem. Soc. 155 (2008)
F145.
30] G.Q. Wang, R.F. Lin, Y. Lin, X.P. Li, X.W. Zhou, X.R. Xiao, Electrochim. Acta 50
(
2005) 5546.
[7] Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, L. Han, Jpn. J. Appl. Phys.
31] X. Yin, W.W. Tan, J.B. Zhang, Y.X. Weng, X.R. Xiao, X.W. Zhou, X.Q. Li, Y. Lin,
Colloids Surf. A 326 (2008) 42.
32] J.M. Chen, Y.P. Zou, Y.F. Li, X.W. Zhou, J.B. Zhang, X.P. Li, X.R. Xiao, Y. Lin, Chem.
Phys. Lett. 460 (2008) 168.
4
5 (2006) L638.
[
[
8] T.C. Wei, C.C. Wan, Y.Y. Wang, Appl. Phys. Lett. 88 (2006) 103122.
9] S.S. Kim, Y.C. Nah, Y.Y. Noh, J. Jo, D.Y. Kim, Electrochim. Acta 51 (2006)
3
814.
33] G.Q. Wang, Y. Lin, X.R. Xiao, X.P. Li, W.B. Wang, Surf. Interface Anal. 36 (2004)
[
[
10] M. Ikegami, K. Miyoshi, T. Miyasak, Appl. Phys. Lett. 90 (2007) 153122.
11] F.S. Cai, J. Liang, Z.L. Tao, J. Chen, R.S. Xu, S.S. Kim, Y.C. Nah, Y.Y. Noh, J. Jo, D.Y.
Kim, J. Power Sources 177 (2008) 631.
1437.
34] N. Papageorgiou, W.F. Maier, M. Grätzel, J. Electrochem. Soc. 144 (1997) 876.