Fabrication of high performance Pt/Ti counter electrodes on Ti mesh for flexible large-area dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2011.10.008

High performance and homogeneous platinum (Pt) nanoparticles is electrodeposited onto titanium (Ti) mesh to form counter electrodes for use in large-area flexible dye-sensitized solar cells (DSSCs). The obtained Pt/Ti counter electrode shows high electrocatalytic activity for the I ₃^-/I^- redox reaction, low charge transfer resistance (49.57 Ω cm² with an area of 80 cm²) and good light transmittance (92.31%). Based on the counter electrode, a light-to-electric energy conversion efficiency of 6.13% is achieved for a flexible DSSC with an area of 80 cm², and a maximum power output of 0.443 W is reached for a flexible DSSC with an area of 160 cm², under an outdoors natural light irradiation with an intensity of 55 mW cm ^-2. The present findings should accelerate the practical application of flexible DSSCs.

Full text of DOI:10.1016/j.electacta.2011.10.008

Electrochimica Acta 58 (2011) 621–627
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of high performance Pt/Ti counter electrodes on Ti mesh for flexible
large-area dye-sensitized solar cells
∗
Yaoming Xiao, Jihuai Wu , Gentian Yue, Jianming Lin, Miaoliang Huang, Leqing Fan, Zhang Lan
Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021,
China
a r t i c l e i n f o
a b s t r a c t
Article history:
High performance and homogeneous platinum (Pt) nanoparticles is electrodeposited onto titanium (Ti)
mesh to form counter electrodes for use in large-area flexible dye-sensitized solar cells (DSSCs). The
Received 29 August 2011
Received in revised form 2 October 2011
Accepted 2 October 2011
Available online 8 October 2011
−
−
obtained Pt/Ti counter electrode shows high electrocatalytic activity for the I3 /I redox reaction, low
2
2
charge transfer resistance (49.57 ꢀ cm with an area of 80 cm ) and good light transmittance (92.31%).
Based on the counter electrode, a light-to-electric energy conversion efficiency of 6.13% is achieved for a
2
flexible DSSC with an area of 80 cm , and a maximum power output of 0.443 W is reached for a flexible
Keywords:
Pt counter electrode
Ti mesh
2
−2
.
DSSC with an area of 160 cm , under an outdoors natural light irradiation with an intensity of 55 mW cm
The present findings should accelerate the practical application of flexible DSSCs.
©
2011 Elsevier Ltd. All rights reserved.
Large area
Flexible dye-sensitized solar cell
1
. Introduction
bilayer on ITO-PEN substrates using vacuum sputtering and assem-
bled a full plastic DSSC, achieving a conversion efficiency of 4.31%
[14]. Grätzel and co-workers electrodeposited a Pt catalyst on an
ITO/PEN substrate to assemble a flexible DSSC with a conversion
efficiency of 7.2% [15]. Thomas et al. electrodeposited Pt on single-
walled carbon nanotube networks [16]. Kang et al. [17] and Park
et al. [18] used chemical reduction to prepare Pt counter electrodes
by coating conductive plastic substrates with an H PtCl ·6H O 2-
Since the first dye-sensitized solar cell (DSSC) was reported in
991 by O’ Regan and Gratzel, substantial research has been con-
1
ducted in this area, resulting in the production of a DSSC with a
photoelectric conversion efficiency that is as high as 12% [1–3].
DSSCs and polymer solar cells [4–7] are considered the third gen-
eration of photovoltaic cells to use roll-to-roll processing, which
could be a promising solution for many impending energy and envi-
ronmental problems. A typical DSSC consists of a dye-sensitized
porous nanocrystalline TiO2 film electrode, a redox electrolyte and
a platinized counter electrode. The function of the counter elec-
trode is to transfer electrons from the external circuit back to the
redox electrolyte to catalyze the reduction of the triiodide ion [8,9].
To meet the demand for renewable energy and industrialization,
flexible DSSCs have attracted wide interest because of their many
advantageous properties, including a light weight, good flexibility,
impact-proof properties and low cost. The flexible DSSC’s shape or
surface can be purposefully designed, and large-scale continuous
production and rapid coating can be used to fabricate these devices,
further decreasing their cost [10–13]. Pt counter electrodes on flex-
ible substrates are usually prepared by sputtering, electrochemical
deposition, or chemical reduction. Ikegami et al. deposited a Pt/Ti
2
6
2
propanol solution and then reducing Pt4+ in a NaBH solution.
4
Reports concerning large DSSCs have previously been published
−
−
[19–25]. A high electrocatalytic activity for the I3 /I redox reac-
tion and the low charge transfer resistance of the substrate is very
significant in improving the performance of DSSCs, especially from
the perspectives of large-scale production [22,23,25]. Metal foils,
such as stainless steel and titanium foil, have been utilized as anode
electrodes to manufacture flexible DSSCs because of their flexibil-
ity and relatively low sheet resistance compared with conducting
glass substrates [11,12,15,17,18]. In addition, the conducting grid
has been considered as a carrier collector for the large-area DSSC
to improve the electrical interconnections of the cells. Ag foil has
been actively investigated because of its low sheet resistance, low
dark current and low cost [22,25]. However, the unacceptably high
−
−
corrosion (or dissolution) of Ag in the contacting I /I electrolyte
3
has been a critical issue to overcome [23].
Ti foils and Ti mesh have relatively low sheet resistances and
−
−
∗ Corresponding author. Tel.: +86 595 22693899; fax: +86 595 22692229.
E-mail address: jhwu@hqu.edu.cn (J. Wu).
superior corrosion resistances in the contacting I /I3 electrolyte
because of the passive oxide film of TiO2 on these substrates. In this
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.10.008

6
22
Y. Xiao et al. / Electrochimica Acta 58 (2011) 621–627
Fig. 1. Photographs of the large-area flexible TiO2/Ti anode and Pt/Ti counter electrode.
paper, we report the fabrication and characterization of large-area,
flexible, dye-sensitized solar cells based on TiO2 anode electrode
on a Ti foil and Pt/Ti counter electrode on a Ti mesh (see Fig. 1).
a “Run time” of 300 s (Fig. 2). The obtained Pt/Ti counter electrode
was rinsed in distilled water and dried at 80 C in a vacuum oven
(Suzhou Jiangdong Precision Instrument Co., Ltd., China). For com-
parison, Pt/Ti counter electrodes carried out at an “Init E” of −0.35 V
for “Run times” of 200 s and 400 s were also manufactured.
◦
2
. Experimental
2.1. Materials
2.3. Preparation of flexible TiO2/Ti anode electrodes
H PtCl ·6H O, ethanol, iodine, lithium iodide, tetrabutyl ammo-
Tetrabutyl titanate (20 mL) was rapidly added to distilled water
(300 mL), leading to the immediate formation of a white precipi-
tate. This precipitate was then filtered using a glass frit and washed
three times with distilled water. The filter cake was added to a nitric
2
6
2
nium iodide, 4-tert-butyl-pyridine (TBP), acetonitrile (AN), tetra-
butyl titanate, titanium tetrachloride, hydrofluoric acid, hydrochlo-
ric acid, nitric acid, PEG-20000 and Triton X-100 were purchased
from the Shanghai Chemical Agent Ltd., China (Analytical grade
◦
acid solution (0.10 M, 200 mL) with vigorous stirring at 80 C until
ꢀ
ꢀ
purity). Sensitized-dye N719 [cis-di(thiocyanato)-N,N -bis (2,2 -
bipyridyl-4-carboxylic acid-4-tetrabutylammonium carboxylate)
ruthenium (II)] was purchased from Solaronix SA, Switzerland. The
above reagents were used without further purification.
the slurry became a translucent, homogeneous blue-white solution.
◦
The resulting colloidal suspension was autoclaved at 200 C for 12 h
2.2. Fabrication of flexible Pt/Ti counter electrodes
Pt/Ti counter electrodes on Ti mesh were electrodeposited
by using the current–time (chronoamperometry) technique from
an aqueous solution containing 2.0 mM H PtCl6 in a 0.50 M HCl
2
aqueous solution. A three-electrode cell with an Electrochemical
Workstation (CHI660D, Shanghai Chenhua Device Company, China)
consisting of the Ti mesh (0.05 mm thickness, purchased from
Anheng Wire Mesh Co., Ltd., China) working electrode, a Pt wire
counter electrode and an Ag/AgCl reference electrode was used
in these studies. Before plating, Ti mesh samples with rectangular
dimensions of 21 cm × 4.5 cm, 21 cm × 6.5 cm and 21 cm × 8.5 cm
were cleaned with mild detergent and rinsed in distilled water. The
mesh samples were then immersed in hydrofluoric acid solution
at a suitable concentration for 2 min and rinsed again in distilled
water. The current–time (chronoamperometry) plating of Pt thin
films on the Ti mesh was carried out at an “Init E” of −0.35 V for
Fig. 2. Current–time for the electrodeposition of Pt on the Ti mesh.

Y. Xiao et al. / Electrochimica Acta 58 (2011) 621–627
623
Device Company, China) using the Pt/Ti as the working electrode,
Pt-foil as the counter electrode, and an Ag/AgCl cell as a reference
electrode, dipped in an acetonitrile solution of 10 mM LiI, 1 mM
I , and 0.1 M LiClO . The CV measurements were performed using
2
4
a CHI660D electrochemical measurement system (scan condition:
−
1
1
00 mV s ). Electrochemical impedance spectroscopy (EIS) mea-
surements of the large-area flexible DSSCs were carried out using a
CHI660D electrochemical measurement system at a constant tem-
◦
perature of 20 C with an AC signal amplitude of 20 mV in the
5
frequency range from 0.1 to 10 Hz at 0 V DC bias in the dark.
2.6. Photoelectrochemical measurements
The photovoltaic performance tests of the flexible DSSC were
conducted by measuring the I–V character curves using a CHI660D
electrochemical measurement system under irradiation with a nat-
ural light intensity (measured by Radiometer FZ-A, Photoelectric
Fig. 3. Schematic diagram of large-area flexible dye-sensitized solar cells.
−2
Inst. of Beijing Normal Univ., China) of 55 mW cm out of doors.
The photovoltaic performance [i.e., fill factor (FF) and overall energy
conversion efficiency (ꢁ)] of the DSSC were calculated using the
following equations [29]:
to form a milky white slurry. The resultant slurry was concentrated
to one-quarter of its original volume, and then, PEG-20000 (10 wt%
slurry) and a few drops of the Triton X-100 emulsification regent
were added to form a TiO2 colloid. The TiO2 colloid was coated on
the titanium foil (0.03 mm thickness, purchased from Baoji Yunjie
Metal Production Co., Ltd., China) using a doctor-scraping tech-
nique. Before the scraping, Ti foils with rectangular dimensions
of 21 cm × 4.5 cm, 21 cm × 6.5 cm and 21 cm × 8.5 cm were washed
with mild detergent and rinsed in distilled water. The foils were
then immersed in a hydrofluoric acid solution of suitable concen-
tration for 2 min and rinsed again in distilled water. The thickness
of the TiO2 film was controlled by the thickness of the adhesive
tape around the edge of the cleaned Ti foil [26–28]. After drying at
Vmax × I
max
FF =
(1)
(2)
VOC × I
SC
Vmax × Imax
FF × V × ISC
OC
ꢁ
(%) =
× 100% =
× 100%
P
Pin
in
In these equations, ISC is the short-circuit current (A), VOC is the
open-circuit voltage (V), Pin is the incident light power (W), Imax
(A) and Vmax (V) are the current and voltage in the I–V curves,
respectively, at the point of maximum power output.
3. Results and discussion
◦
room temperature, the TiO2 thin films were sintered at 450 C for
3
0 min in air to produce nanocrystalline TiO2 films. The TiO2 elec-
3.1. Morphology and composition of Pt/Ti counter electrode
◦
trodes were immersed in a 0.05 M TiCl solution at 70 C for 30 min
4
and then washed three times with distilled water. The treated TiO2
Fig. 4a shows the SEM image of the Pt/Ti counter electrode fabri-
◦
films were sintered at 400 C for 20 min. When the TiO electrodes
2
cated at an “Init E” of −0.35 V for a “Run time” of 300 s. It is obvious
that a homogeneous Pt film has been electrodeposited on the Ti
mesh with Pt nanoparticles approximately 20–30 nm in diameter.
According to the EDS spectra (Fig. 4b), the Pt/Ti counter electrode
◦
were cooled to 80 C, the obtained TiO /Ti electrodes were treated
2
with ultraviolet-O2 at room temperature for 30 min. The TiO flex-
2
−4
ible film was immersed in a 2.50 × 10 M solution of dye N719
in absolute ethanol for 24 h, allowing sufficient time for the TiO2
film to absorb the dye adequately to obtain a dye-sensitized TiO2
flexible film electrode.
consists of Pt and Ti, which indicates that the H PtCl6 was elec-
2
trodeposited to form Pt using the chronoamperometry method.
3
.2. Electrodeposition times for the fabrication of Pt/Ti counter
2.4. Assemblage of flexible DSSC
electrodes
The flexible DSSC was assembled by the injection of a redox-
Fig. 5a–c shows the SEM images of Pt/Ti counter electrodes
that were electrodeposited for different times (200 s (Fig. 5a), 300 s
Fig. 5b) and 400 s (Fig. 5c)). It is obvious that the number of
active electrolyte into the aperture between the TiO film electrode
2
(
anode) and the Pt/Ti counter electrode (as shown in Fig. 3). The
(
two electrodes were clipped together with two transparent flex-
ible plastics, and a cyanoacrylate adhesive was used as a sealant
to prevent the electrolyte solution from leaking. Epoxy resin was
used to further seal the cell in order to enhance the cell stability.
The redox electrolyte consisted of 0.60 M tetrabutyl ammonium
iodide, 0.10 M lithium iodide, 0.10 M iodine and 0.50 M 4-tert-
butyl-pyridine in acetonitrile.
Pt nanoparticles (approximately 20–30 nm in diameter) increased
with increasing electrodeposition times. The surface of the elec-
trode clearly showed that the Pt nanoparticles homogeneously
and uniformly gathered to fill the space on the surface of the Ti
mesh, and the Ti mesh could completely filled after electrodeposi-
tion times of 400 s. Fig. 5d shows the cyclic voltammogram for the
three Pt/Ti counter electrodes, and these Pt/Ti counter electrodes
show two pairs of oxidation and reduction peaks. The oxidation
and reduction peaks on the left (peaks Aox and Ared) result from
2.5. Characterization
−
−
the redox reaction of 3I + 2e → 2I3 , which has little effect on
2
The surface features of the flexible Pt/Ti counter electrode were
the DSSC performance; in contrast, the redox pair on the right
−
−
−
observed using a scanning electron microscope (SEM, Hitachi S-
800, JAPAN) with an energy-dispersive X-ray spectrometer (EDS)
(peaks Box and Bred) is attributed to the reaction of I3 + 2e → 3I ,
which directly affects the DSSC performance [30,31]. The Pt/Ti
counter electrode with an electrodeposition time of 400 s showed
the largest oxidation and reduction current density, likely because
this electrode contained the largest number of Pt nanoparticles on
4
attached. The cyclic voltammetry (CV) profiles of the Pt/Ti counter
electrode were measured in a three-electrode electro-chemical cell
with an Electrochemical Workstation (CHI660D, Shanghai Chenhua

6
24
Y. Xiao et al. / Electrochimica Acta 58 (2011) 621–627
Fig. 4. SEM image (a) and EDS (b) of the Pt/Ti counter electrode.
the surface of the Ti mesh. From the SEM images in Fig. 5c and d, it
appears that the optimal electrodeposition time to fabricate Pt/Ti
counter electrodes is 400 s.
and 1.8 mm × 2.5 mm, respectively) was measured and shown in
Fig. 6. Based on the EIS, the values of R and R were obtained and
shown in the table accompanying Fig. 6, where TB is the diagonal
line breadth of the Ti mesh, TL is the diagonal line length of the
S
CT
T
i
m
e
s
h
,
R
i
s
t
h
e
s
e
r
i
e
s
r
e
s
i
s
t
a
n
c
e
o
f
t
h
e
P
t
/
T
i
e
l
e
c
t
r
o
d
e
,
a
n
d
R
3
.3. Models of Ti mesh
S
C
T
is the charge-transfer resistance on the electrolyte|electrode inter-
−
−
face for the I3 /I redox reaction [31,32]. It can be seen that the
Electrochemical impedance spectroscopy (EIS) for the Pt/Ti
electrodes with the same area of 80 cm2 and the different mod-
els (TB × TL: 1.0 mm × 2.0 mm, 1.5 mm × 2.0 mm, 1.5 mm × 2.5 mm
R increases with the increase of TB and TL. The R values increase
S
S
form 50.50 ꢀ cm for the mesh unit area 1.00 mm to 56.34 ꢀ cm2
2
2
Fig. 5. SEM images of Pt/Ti electrode electrodeposited times of 200 s (a), 300 s (b) and 400 s (c), and their CV curves (d).

Y. Xiao et al. / Electrochimica Acta 58 (2011) 621–627
625
Fig. 8. EIS of Pt/Ti electrodes with different Ti mesh areas.
Fig. 6. EIS of Pt/Ti electrodes with different Ti mesh models.
2
for the mesh unit area 2.25 mm , indicating the Pt/Ti electrode with
The photovoltaic properties of flexible DSSCs with the same
rectangular dimension, 21 cm × 4.5 cm, with different Ti mesh
models were measured under an outdoors natural light irradiation
the smaller mesh has a higher electrical conductivity that is favor-
able to electron transport [33–36]. The RCT values alos increase with
the mesh unit area increase. Moreover the increase extent is larger
−
2
with an intensity of 55 mW cm . The active area of the flexi-
2
than the R . The R values increase form 47.68 ꢀ cm for the mesh
ble DSSC was 20 cm × 4.0 cm, and the results are summarized in
S
CT
2
2
2
unit area 1.00 mm to 61.15 ꢀ cm for the mesh unit area 2.25 mm ,
demonstrating that the Ti mesh model, especially in TL (the diag-
onal line length), could significantly influence the charge-transfer
resistance on the electrolyte|electrode interface for the I3 /I redox
Table 1. These DSSCs have comparatively similar V (0.725 V) val-
OC
ues, because the VOC is mainly determined by the energy level
difference between the Fermi level of the electrons in TiO2 and the
redox potential of the electrolyte [1,2]. Because these flexible DSSCs
have the same compositions, their VOC values are similar. As shown,
the ISC values increase with increasing values of TB and TL; this
result occurs because the light transmittances of Ti mesh increase
as the size of the mesh increases. However, the fill factor (FF) values
are reduced as the size of the mesh increases, because the values
−
−
reaction.
Fig. 7 shows the structure diagram of the Ti mesh and the com-
putational formula of the Ti mesh light transmittance. Here, TB is
the diagonal line breadth, TL is the diagonal line length of the Ti
mesh, and W is the width of the Ti mesh infarction. The light trans-
mittances of the Ti mesh were calculated using a computational
formula and are summarized in Table 1. As shown in Table 1, the
light transmittance of the Ti mesh increases with increasing values
of TB and TL.
2
of R and R of the large-area (80 cm ) flexible DSSCs are enlarged
S
CT
with the increase of the values of TB and TL [19,22,23]. Therefore,
the ꢁ and Pmax values of the DSSC first increase and then decrease
as the size of the Ti mesh increases. Considering the light transmit-
tance, RS and RCT, we choose the 1.5 mm × 2.0 mm model, whose
the light transmittance is 92.31% and whose R and R values are
S
CT
2
5
2.63 ± 0.05 ꢀ cm2 and 49.57 ± 0.04 ꢀ cm , respectively.
3.4. Large-area flexible DSSCs
Electrochemical impedance spectroscopy (EIS) for the Pt/Ti
electrodes in the same model of 1.5 mm × 2.0 mm and the differ-
ent area were measured and shown in Fig. 8. These electrodes’
rectangular areas included: 21 cm × 4.5 cm, 21 cm × 6.5 cm and
2
1 cm × 8.5 cm, with active areas 20 cm × 4.0 cm, 20 cm × 6.0 cm
and 20 cm × 8.0 cm, respectively. According to the EIS, the R and
S
R
values are calculated and shown in the table accompanying
CT
Fig. 8. The values of RS and RCT increase with the increase of Pt/Ti
electrode areas, implying that the larger area Pt/Ti electrodes have a
lower electrical conductivity, which is disadvantageous to electron
transport [33–36]. However, the growth rate of the R and R val-
S
CT
ues with area is obviously less than that of other large-area flexible
electrodes, suggesting that the Ti mesh is suitable for the large-scale
preparation of Pt/Ti counter electrodes and flexible DSSCs.
Fig. 7. Structure diagram of the Ti mesh and the computational formula of the Ti
mesh light transmittance.

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26
Y. Xiao et al. / Electrochimica Acta 58 (2011) 621–627
Table 1
The photovoltaic performance of flexible DSSCs with different Ti mesh models.
TB (mm)
TL (mm)
W (mm)
Light transmittance (%)
ISC (A)
VOC (V)
FF
ꢁ (%)
Pmaxa (W)
1.0
1.5
1.5
1.8
2.0
2.0
2.5
2.5
0.05
0.05
0.05
0.05
89.94
92.31
92.79
93.59
0.486
0.518
0.520
0.525
0.725
0.729
0.726
0.728
0.717
0.714
0.705
0.684
5.74
6.13
6.05
5.94
0.253
0.270
0.266
0.261
a
Pmax = Isc × Voc × FF.
Table 2
The photovoltaic performance of the flexible DSSCs with different areas.
Size (cm2)
Solar intensity (mW cm
−2
)
ISC (A)
JSC (mA cm
−2
)
VOC (V)
FF
ꢁ (%)
Pmax (W)
8
20
60
0
55
55
55
0.518
0.770
0.930
6.48
6.42
5.81
0.729
0.722
0.711
0.714
0.703
0.670
6.13
5.92
5.23
0.270
0.391
0.443
1
1
2
resistance of 52.63 ± 0.05 ꢀ cm and a charge-transfer resistance
2
2
of 49.57 ± 0.04 ꢀ cm with an area of 80 cm . Based on the counter
electrode, a light-to-electric energy conversion efficiency of 6.13%
is achieved for a flexible DSSC with an area of 80 cm , and a maxi-
2
mum power output of 0.443 W is reached for a flexible DSSC with an
2
area of 160 cm , under an outdoors natural light irradiation with an
−
2
intensity of 55 mW cm . Compared with other materials used to
prepare Pt counter electrodes, the Ti mesh has a relatively low sheet
resistance and superior corrosion resistance for the electrolyte con-
−
−
tacting I /I3 ; therefore, it is suitable for the large-scale preparation
of flexible Pt/Ti counter electrodes. The present findings should
accelerate the practical application of flexible DSSCs.
Acknowledgments
The authors would like to thank the joint support of the National
High Technology Research and Development Program of China (No.
2
009AA03Z217) and the National Natural Science Foundation of
Fig. 9. I–V curves of the flexible DSSCs with different areas (under an outdoors
natural light irradiation with intensity of 55 mW cm and in the dark).
China (No. 90922028, 51002053).
−2
References
Fig. 9 shows the I–V curves of the flexible DSSCs with Pt/Ti
counter electrodes in the same model of 1.5 mm × 2.0 mm and dif-
ferent areas under an outdoors natural light irradiation with an
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(
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[
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