Bi-functional TiO2 cemented Ag grid under layer for enhancing the photovoltaic performance of a large-area dye-sensitized solar cell

Article abstract of DOI:10.1016/j.electacta.2011.12.056

A bi-functional TiO₂ cemented Ag grid under layer for enhanced the photovoltaic performance of a large-area dye-sensitized solar cell (DSSC) is prepared with a simple way. The conductive printing paste contains micro-sized Ag powders and nano-sized TiO₂ cementing agent. The conductive printing paste can be well cemented on the FTO glass and form high conductive grids with Ag powders sintered together by the nano-sized TiO₂ particles. The formed conductive grid is protected with a TiO₂ thin layer and TiO₂ sol treatment to avoid the iodine corrosion. The addition of the TiO₂ cemented conductive grid can decrease the internal resistance of the large-area dye-sensitized solar cell when it is prepared in the photo and counter electrodes. Furthermore, the protecting TiO₂ thin layer and the TiO₂ sol treatment can be done on the whole area of the large-area photo electrode to both play as the blocking under layer at the same time, which can also enhance the photovoltaic performance of the large-area dye-sensitized solar cell.

Full text of DOI:10.1016/j.electacta.2011.12.056

Electrochimica Acta 62 (2012) 313–318
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Electrochimica Acta
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Bi-functional TiO cemented Ag grid under layer for enhancing the photovoltaic
2
performance of a large-area dye-sensitized solar cell
∗
Zhang Lan, Jihuai Wu , Jianming Lin, Miaoliang Huang
Engineering Research Center of Environment-Friendly Functional Materials for 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:
Received 20 September 2011
Received in revised form
A bi-functional TiO2 cemented Ag grid under layer for enhanced the photovoltaic performance of a large-
area dye-sensitized solar cell (DSSC) is prepared with a simple way. The conductive printing paste contains
micro-sized Ag powders and nano-sized TiO2 cementing agent. The conductive printing paste can be well
cemented on the FTO glass and form high conductive grids with Ag powders sintered together by the
nano-sized TiO2 particles. The formed conductive grid is protected with a TiO2 thin layer and TiO2 sol
treatment to avoid the iodine corrosion. The addition of the TiO2 cemented conductive grid can decrease
the internal resistance of the large-area dye-sensitized solar cell when it is prepared in the photo and
counter electrodes. Furthermore, the protecting TiO2 thin layer and the TiO2 sol treatment can be done
on the whole area of the large-area photo electrode to both play as the blocking under layer at the same
time, which can also enhance the photovoltaic performance of the large-area dye-sensitized solar cell.
© 2011 Elsevier Ltd. All rights reserved.
2
1 November 2011
Accepted 11 December 2011
Available online 20 December 2011
Keywords:
Dye-sensitized solar cell
Large-area
Protecting layer
Conductive grid
1
. Introduction
Dye-sensitized solar cells (DSSCs) have been extensively studied
productivity because of the simple structure module, and installa-
tion area of cell modules is expected to be small because of the high
aperture ratio [9,10].
in the last decades as a promising renewable energy source because
of its potentially inexpensive manufacturing technology compared
with silicon solar cells [1]. The DSSC is made of two electrodes,
one of which is the photo electrode coated with a dye-sensitized
Commercial solar cells usually have current collecting grids
made by conductive printing paste containing Ag or Al powders
to reduce the surface resistance. However, these materials can-
not be used in fabricating large-area DSSCs directly owing to the
high corrosion of the electrolyte containing iodine. So some pro-
cedures for protecting metal grids from iodine corrosion by resin
or glass–ceramic are needed [11,12]. Here, we present a simple
mesoporous nanocrystal TiO film and the other is the counter elec-
2
trode coated with a catalyst, and an electrolyte usually containing
−
−
iodide/tri-iodide (I /I3 ) redox couple is sandwiched between the
two electrodes [2,3]. The energy conversion efficiency over 11% has
already been achieved in the laboratory scale DSSC, which attracts
more fundamental as well as commercial interests [4,5].
way to fabricate TiO cemented conductive grids used in the large-
2
area DSSC. The conductive printing paste is prepared by mixing the
micro-sized Ag powders and the nano-sized TiO particles. The con-
2
Enlarging the cell area for practical use has also been attempted
during recent years. Cell area and conductivity of the substrate
influence the internal resistance of DSSC, which consequently influ-
ences the fill factor and conversion efficiency of DSSC. Therefore,
the measures for decreasing the internal resistance of large-area
DSSC are needed to maintain the high photovoltaic performance.
There are two groups of design types. The first is to intercon-
nect many parallel single small-area DSSCs in series, so that each
constituent cell can be fabricated with optimized processes for
high performance [6–8]. The second is the large single cell made
with high conductive substrate, which is expected to have high
ductive grids are further protected with a compact TiO2 thin layer
to prevent the corrosion of the electrolyte. Moreover, the compact
TiO2 thin layer can be done on the whole area of the FTO substrate
in the photo electrode, which can both play as the blocking layer to
further enhance the photovoltaic performance [13].
2. Experimental
2.1. Materials
Titanium isopropoxide, nitric acid, glacial acetic acid,
poly(ethylene glycol) 20,000 (PEG), acetyl acetone, n-butyl
alcohol, o-phenylenediamine, AgNO , H PtCl ·6H O were all A.
3
2
6
2
∗ Corresponding author. Tel.: +86 595 22690582; fax: +86 595 22693999.
E-mail address: jhwu@hqu.edu.cn (J. Wu).
R. grade and purchased from Sinopharm Chemical Reagent Co.,
Ltd, China. The reagents were used without further treatment.
0
013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.12.056

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14
Z. Lan et al. / Electrochimica Acta 62 (2012) 313–318
Fig. 1. The morphology of Ag powders (a and b).
FTO transparent conducting glasses (20 ꢀ sq−1) were purchased
from Qinghuangdao FTO glass Co., Ltd, China. Sensitizing dye
cis-[(dcbH ) Ru(SCN) ] was purchased from Solaronix SA.
of nano-size and micro-size TiO2 particles was printed on the
whole area of the above prepared FTO conductive substrate for the
photo electrode, followed with sintering at 450 C for 30 min and
◦
2
2
2
dye-loading to form the photo electrode. The prepared Pt counter
electrode was placed above the photo electrode. The two elec-
trodes were clipped together and a cyanoacrylate adhesive was
used as sealant to seal the DSSC. Bisphenol A epoxy resin was used
in a further sealing process. The liquid electrolyte contained 0.4 M
sodium iodide, 0.1 M tetrabutyl ammonium iodide, 0.5 M 4-tert-
butylpyridine, and 0.05 M iodine in acetonitrile solution [15,16].
2
.2. Preparation of conductive printing paste and fabrication of
DSSC
The micro-sized Ag powders were prepared from an aque-
ous solution of AgNO3 (0.5 M) by a chemical reduction method
with o-phenylenediamine (0.05 M) as reducing agent at room tem-
perature. The nano-sized TiO2 particles about 10–20 nm were
synthesized according to the previously reported method [14].
The TiO2 sol was prepared by mixing 2 ml Titanium isopropox-
ide, 0.5 ml acetyl acetone in 10 ml n-butyl alcohol. The conductive
paste was prepared by mixing 1 g micro-sized Ag powders with
several amounts of nano-sized TiO2 particles dispersed in deion-
ized water. The pure nano-sized TiO2 paste dispersed in deionized
water was used for preparing the protecting layer above the con-
ductive grid. The hybrid of 10 g nano-sized TiO , 2 g sub micro-sized
TiO2 (200–300 nm) and 1 g PEG dispersed in deionized water was
used for preparing the photo electrode.
The FTO conductive substrate with TiO2 cemented conductive
grids used in counter electrode was prepared as follows. The con-
ductive paste was screen printed on the FTO conductive surface to
2.3. Measurements
The morphologies of micro-sized Ag powders and conduc-
tive grids were analyzed by a field emission scanning electron
microscopy (S-4800, HITACHI). The Energy dispersive X-ray of
the samples was measured by a Genesis XM2 instrument (EDAX,
USA). The conductivity of conductive grids was measured with
a four-probe conductivity test meter (RTS-9, Guangzhou 4probe
Co., Ltd, China). Electrochemical impedance spectroscopy (EIS)
measurements were recorded using a CHI 660C electrochemical
workstation (CH Instrument Inc., USA) according to the methods
presented previously [17]. The impedance spectra were analyzed
using an equivalent circuit model, and the characteristics of the
DSSC were interpreted with the Zview software [18]. The photo-
voltaic performance of DSSC was carried out by measuring the I–V
characteristic curves under simulated AM 1.5 solar illumination at
2
2
form the designed conductive grids about 0.25 × 10 cm . A thin TiO
2
layer was printed above the surface of the conductive grids and fol-
◦
lowed with 450 C sintering for 30 min. When the FTO conductive
substrate was cooled down to room temperature, the transparent
adhesive tapes were used to protect the naked FTO conductive sur-
face among the conductive grids, then the conductive substrate
was immersed in the TiO2 sol for 2 h. After removing the trans-
−
2
1
00 mW cm , from a xenon arc lamp (CHF-XM500, Trusttech Co.,
Ltd, China) in ambient atmosphere and recorded with CHI 660C
electrochemical workstation.
parent adhesive tapes the FTO conductive substrate was sintered
◦
at 450 C for 30 min again to form the compact TiO protecting layer
3. Results and discussions
2
above the conductive grids. The FTO conductive substrate with TiO2
cemented conductive grids used in photo electrode was prepared
with a little different method as aforementioned. Namely, after
screen printing the conductive grids on the FTO conductive sur-
face, the thin TiO2 layer was prepared on the whole area of the
3.1. Morphology and conductivity of Ag–TiO2 film
Fig. 1 shows the morphology of the prepared Ag powders used in
the conductive printing paste. It is seen that the Ag micro-spheres
with diameters about 0.5–1 ␮m are successfully synthesized, which
are formed by the aggregation of Ag nanoparticles with diameters
◦
FTO conductive substrate and sintered at 450 C for 30 min. When
cooled down to room temperature, the FTO conductive substrate
◦
was directly immersed in the TiO2 sol for 12 h and followed with
around 25–50 nm. Fig. 2 shows the morphologies of 450 C sintered
◦
4
50 C sintering for 30 min. The formed compact TiO layer can play
Ag powders and the TiO2 cemented conductive grid. It is clear that
the Ag powders can be sintered together to form well-contacted
bigger particles. The cross link conductive channel can be built and
2
both as the protecting layer for conductive grids and as the blocking
layer for the large-area DSSC. The dye-loaded nanocrystal TiO film
2
3
−1
was prepared on the above substrate and the formed active area of
shows high conductivity (7.58 × 10 S cm ). However, the adhe-
sive ability of the pure Ag layer to the FTO glass is poor, so the TiO2
nanoparticles are used as the cementing agent in the conductive
2
the photo electrode is about [1 × 10 cm ] × 6.
A Pt precursor solution containing 5 mg H PtCl ·6H O in 1 ml
2
6
2
◦
2
-propanol was coated on the above prepared FTO conductive
printing paste. As shown in Fig. 2(d), after sintered at 450 C the Ag
substrate for counter electrode, then followed with thermal decom-
position to form the Pt counter electrode. The paste with the hybrid
powders are turned to smaller particles and well dispersed in the
conductive grid.

Z. Lan et al. / Electrochimica Acta 62 (2012) 313–318
315
Fig. 2. The morphologies of sintered Ag powders (a and b) and TiO2 cemented conductive grid [c and d, TiO2/Ag = 0.4/1(wt)].
The conductive variations of the conductive grids with different
3.2. Influence of iodine corrosion on the morphology and
conductivity of the conductive grids
weight ratio of TiO nanoparticles versus Ag powders are shown in
2
Fig. 3. We observe that the conductivity decreases greatly (from the
3
2
−1
original 7.58 × 10 to 6.67 × 10 S cm with 10 wt% of TiO2 vs Ag).
And the larger conductive dropped degree is appeared when the
addition amount of TiO2 nanoparticles higher than 40 wt%, which
is owing to the formation of smaller Ag particles and the decrease
of coalescence among the particles. In order to maintain the high
conductivity of the conductive grid and the good adhesive abil-
ity between the film and the FTO glass, the conductive printing
paste containing 40 wt% TiO2 nanoparticles versus Ag powders is
selected.
The silver shows low corrosive resistance in the electrolyte with
iodine [11,19], so some procedures for protecting the conductive
grids from iodine corrosion are needed. Here, we use a compact
TiO2 thin layer as the protecting layer to prevent the corrosion
of iodine on the conductive grid. Fig. 4 shows the cross-sectional
SEM images of the conductive grid protected with the compact
TiO2 thin layer. It is seen that although the micro-sized Ag pow-
ders can be sintered together by the nano-sized TiO2 cemented
agent, the formed conductive grid still leave some big holes, so the
electrolyte can easily penetrate into the conductive grid. While the
morphology of the TiO2 sol treated TiO2 protecting layer is com-
pact, which can play as a barrier to prevent the penetration of the
electrolyte.
Fig. 5 shows the morphologies of conductive grid after immersed
in a 1 M iodine ethanol solution for 30 days at room temperature.
It can be seen that some well crystal big particles appear on the
surface of the conductive grid without a protecting layer and cover
over the whole surface, which reflects the serious corrosion of the
electrolyte on the conductive grid. These particles become smaller
when a compact TiO2 protecting layer is added on the conductive
grid. And they nearly disappear when the TiO2 protecting layer is
further treated with TiO2 sol. The EDX spectrum of the new born
particles verifies the formation of AgI by the reaction between
iodine and silver. So it means that the compact TiO2 protecting
layer with TiO2 sol treatment can efficiently prevent the iodine
corrosion.
4
1
1
1
1
1
0
0
0
0
3
2
1
0
1
0
-
10
-2
10
10
10
-3
The influence of iodine corrosion on the conductivity of the
conductive grid is measured by the method illustrated in Fig. 6,
and the resulted values of conductivity are listed in Table 1. It
is seen that the conductive grid without a compact TiO2 pro-
tecting layer losses its conductivity after immersed in iodine
ethanol solution for 30 days. The conductive grid with a com-
pact TiO2 protecting layer shows a dropped conductivity from the
-4
0
10
20
30
40
50
60
70
80
90
TiO Cont.(TiO vs Ag, wt%)
2
2
Fig. 3. The variations in conductivities of conductive grids with different weight
ratio of TiO2 nanoparticles versus Ag powders.

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Z. Lan et al. / Electrochimica Acta 62 (2012) 313–318
Fig. 4. The cross-sectional SEM images of the compact nanocrystal TiO2 protected conductive grid on the FTO substrate.
Fig. 5. Corrosive degree of iodine on the TiO2 cemented conductive grid after immersed in 1 M iodine ethanol solution for 30 days. (a) Without protecting layer, (b) with a
thin TiO2 layer above the film, (c) the protecting layer further treated with TiO2 sol, (d) EDX spectrum of the new born particles.
original 3.17 × 102 to 8.73 × 10 S cm firstly then keeps in this
value with the extended immersed time, which reflects that the
conductive channel in the conductive grid is not destroyed totally,
so a reasonable conductivity can be maintained. The formed smaller
1
−1
AgI particles can fill in the compact TiO2 protecting layer, which
avoids the further penetration of iodine into the grid to continu-
ally corrode the conductive grid. The conductive grid with a TiO2
sol treated compact TiO2 protecting layer shows high efficiency on
preventing the iodine corrosion as shown in Fig. 5(c), so the con-
ductivity of the conductive grid is nearly not affected by the iodine
solution.
3.3. Photovoltaic performance of large-area dye-sensitized solar
Fig. 6. A schematic illustration of the conductive measurement of conductive grid
after immersed in iodine ethanol solution.
cell
When the compact TiO2 protecting layer is done on the whole
area of the photo electrode, it also plays as the blocking under
layer to enhance the photovoltaic performance of the DSSC. Fig. 7
shows the I–V curves of small-area DSSCs without or with the
compact TiO2 under layer in the photo electrodes. It is seen that
the DSSC with the compact TiO2 under layer in the photo elec-
trode shows enhanced photovoltaic performance, which is mainly
originated from the increased short-circuit current density and the
open-circuit voltage. The dark I–V curves show that the onset of
applied voltage for generating the dark current in the DSSC with
Table 1
Influence of iodine corrosion on the conductivity of the conductive grid. Influence
of iodine corrosion on the conductivity of the conductive grid.
Conductivity (S cm−1
Ag–TiO2 film
)
Original
Final
2
Without a protecting layer
With a protecting layer
With TiO2 sol treatment
3.17 × 10
0
2
1
2
3.13 × 10
8.73 × 10
2
3.11 × 10
3.04 × 10

Z. Lan et al. / Electrochimica Acta 62 (2012) 313–318
317
1
1
1
1
6
4
2
0
8
6
4
2
0
2
high values of the short-circuit current density and the open-circuit
voltage [20].
Fig. 8 shows the change tendencies of photovoltaic parameters
of DSSCs without or with conductive grids in the photo electrodes
by going with the enlarged area of cells. It is seen that except for
the relatively stable values of the open-circuit voltage, the values
of the short-circuit current density, fill factor and the energy con-
version efficiency are all decreased with the enlarged area of the
DSSC without conductive grids in the photo electrode. The case is
different on the DSSC with conductive grids in the photo electrode.
Namely, the values of the open-circuit voltage and the short-circuit
current density can be maintained in the constant values. The fill
factor is decreased initially, but it can be kept in a relatively high
value with further enlarged area of the cell. So the decreased ampli-
tude of the energy conversion efficiency is smaller than that of the
former one.
-
2
No. I (mA
•
cm ) V (V)
FF
Eff(%) Vonse(t V)
sc
oc
a
b
13.57
14.73
0.738
0.767
0.680
0.694
6.81
7.84
0.558
0.620
a
b
-
Fig. 9 shows the photos of the electrodes with conductive grids
and the I–V curves of large-area DSSCs without conductive grids
in the photo electrode and composing the shown electrodes. It
is seen that the photovoltaic performance of the large-area DSSC
without conductive grids is very poor. Its fill factor is as low as
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V)
Fig. 7. I–V curves of DSSCs without (a) or with (b) the compact TiO2 under layer in
the photoelectrodes, the area of cells is 1 × 0.5 cm .
2
0
.27 and the I–V curve presents as a linear relationship. While the
same area DSSC with conductive grids shows greatly enhanced
photovoltaic performance, which is owing to the decreased inter-
nal resistance with the aid of the high conductive grids. The EIS
analysis presented in Fig. 10 shows that the resistant elements
of the large-area DSSC decrease greatly with the addition of the
the compact TiO2 under layer in the photo electrode increases
obviously compared with that of the DSSC without the compact
TiO2 under layer in the photo electrode. So the dark reaction in the
DSSC can be well suppressed, which is beneficial for obtaining the
20
18
16
14
12
10
8
6
0.80
0.75
16
14
12
10
8
6
4
2
0.8
0.7
Voc
Isc
Voc
0.70
0.65
0.60
0.55
0.50
Isc
0
.6
.5
0
FF
0.4
FF
Eff
0
.3
.2
Eff
0.45
0.40
(
a)
(b)
0
0
1
2
3
4
5
6
7
8
9
10
11
0
1
2
3
4
5
6
7
8
9
10
11
2
2
Area (cm )
Area (cm )
Fig. 8. Change tendencies of photovoltaic parameters of DSSCs without (a) or with (b) conductive grids in the photo electrodes by going with the enlarged area of cells, the
2
area of cells is 1 × N cm .
Fig. 9. Photos of the large-area photoelectrode and counter electrode with conductive grids (a), and I–V curves of large-area DSSC without conductive grids in the electrodes
and composing the above electrodes (marked as a and b in the curves, respectively) (b).

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Z. Lan et al. / Electrochimica Acta 62 (2012) 313–318
time. So the conductive grids in the photo and counter electrodes
in the large-area DSSC are stable, which are not corroded by the
electrolyte to influence the photovoltaic performance of the cell.
4. Conclusions
The well protected TiO2 cemented conductive grids are fabri-
cated and used in large-area DSSCs. It is found that the Ag powders
can be well cemented on the FTO glass and maintain high con-
ductivity by mixed with TiO2 nanoparticles. The addition of a TiO2
protecting layer on the conductive grids and further treated with
TiO2 sol can efficiently decrease the iodine corrosion on the silver.
Moreover, the TiO2 protecting layer also plays as a blocking under
layer to further enhance the photovoltaic performance of the DSSC.
The large-area DSSC with the well protected TiO2 cemented con-
ductive grids in the photo and counter electrodes shows obviously
enhanced photovoltaic performance.
Acknowledgments
Fig. 10. EIS analysis on the large-area DSSCs without and with conductive grids in
the electrodes (marked as a and b in the curves, respectively).
The authors acknowledge the support of the National High
Technology Research and Development Program of China (no.
2
009AA03Z217), the National Natural Science Foundation of China
(nos. 90922028, 51002053), the Natural Science Foundation of
Fujian province (no. E1050015), and the Special Funds for Funda-
mental Operational Expenditures of Scientific Research of Huaqiao
University (no. JB-SJ1001).
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