Preparation of n/p Tandem Type Dye Sensitized Solar Cell
Utilizing Plasma Sputtering Method
Kazuya Yamamoto, Ryo Fukuda, Takayuki Yamamoto, Tatsuhiko Sonoda, and Kenji Yamada
Department of Materials Science and Chemical Engineering, Kitakyusyu National College of Technology,
Fukuoka, Japan.
Abstract: Dye sensitized solar cell (DSC) having high efficiency with low cost
was first reported by Grätzel et al and a lot of papers about various type of DSC
were reported. The authors tried to prepare n/p tandem type composed with
titanium dioxide as n type semiconductor and p type semiconductor for high
conversion efficiency. This type DSC would be absorbed the long wavelength
region of sunlight and taken high open circuit voltage by introducing p type
semiconductor electrode. The p type semiconductor electrode was prepared by
the plasma sputtering method using magnetron type plasma system to be covered
with p type semiconductor (CuO, CuxBiyOz, etc.) on Pt sputtered FTO glass
substrate.
Keywords: n/p tandem type DSC, plasma sputtering, p type semiconductor
1. Introduction
Dye sensitized solar cell (DSC) having high
efficiency with low cost was first reported by
Grätzel et al. [1]. DSC is The semiconductor
materials such as titanium dioxide (TiO2) were used
for the electrode of DSC, and the property of DSC
would be depended on this semiconductor electrode.
Several different types of DSC have been
reported to get good performance until first paper.
One of them is tandem type DSC composed of n
type and p type semiconductor electrode was
reported to improve the photovoltaic property of
DSC [2].
In our previous work, n/p tandem type DSC
assembled with n and p type semiconductor
electrode was reported [3]. Cathode was prepared by
plasma sputtering method of CuBi2O4 for coating of
p type semiconductor on Pt sputtered FTO glass
substrate. This system was expected to be achieved
high performance of conversion efficiency by
utilizing long wavelength light that the sensitized
dye could not absorb the wavelength region.
However The cell did not show the improvement for
conversion efficiency.
In this paper, n/p tandem type DSC was also
tried to be prepared using plasma sputtering method.
As p type semiconductor, CuO and CuxBiyOz with
differencial composition ratio was used. The
optimum plasma sputtering conditions were
investigated for the better photovoltaic property of
DSC.
2. Experiments
2-1 Sample Preparation
TiO2 nanoparticles (called P25, Degussa Co.,
Dusseldorf, Germany) and CuxBiyOz were used as n
or p type semiconductor respectively. CuxBiyOz
powder was prepared following the previous report [4]
by the powder of CuO and Bi2O3 pounded in mortar
with molar ratio 95:5 and 90:10. The grained powder
was calcined in a furnace at 973 K for 24 hours.
2-2 Preparation of p type semiconductor
electrode by plasma sputtering of CuxBiyOz or Cu.
CuxBiyOz thin film on Pt sputtered FTO
glass plate was prepared by plasma sputtering
method. Figure 1 shows the schematic illustration of
Substrate
Gas inlet
Exhaust
P type
semiconductor
High frequency power source (13.56 MHz)
Figure 1. Schematic illustration of magnetron type
plasma reactor.
magnetron type plasma reactor for p type
semiconductor sputtering in this paper. The plasma
sputtering was treated by Argon plasma with 200 W
of discharge power, at 15 Pa of argon gas pressure,
for 5-20 min. To compare the property of electrode
deposited with CuxBiyOz. Another electrode
including copper oxide was also prepared by plasma
sputtering method using Cu. These substrates
annealed at 723 K for 30 min in air were used as p
type semiconductor electrode.
2-3 Preparation of n/p tandem type DSC using
CuxBiyOz sputtered electrode
The thin film of semiconductor electrode for
DSC was prepared by spin-coat method on FTO
glass plate using the TiO2 paste concocted by
blending of TiO2 particles with certain
concentrations and poly (ethylene glycol): PEG
which has 20,000 molecular weight in 1 mol/L
acetic acid aqueous solution. TiO2 casting substrate
was annealed in air at 723 K for 30 min to prepare
the n type semiconductor electrode with porous TiO2.
This electrode was immersed into ethanol solution
with 0.3 M ruthenium dye (N719) for a day and
rinsed by acetonitrile. The n/p tandem type DSC was
Cu 2p3/2
assembled using n and p type semiconductor
electrode prepared above, and the electrolyte
solution composed with I2 (0.04M), LiI (0.5M) and
4-t-butylpyrdine (0.58M) in acetnitrile was filled
between these electrodes.
2-4 Characterization of p type semiconductor
electrode.
X-ray photoelectron spectroscopy (XPS)
measurement was carried out with a Shimadzu
ESCA750 X-ray photoelectron spectrometer
(Shimadzu Co., Kyoto, Japan). XPS spectra were
collected by exciting the particles without pretreatment with a Mg Kα X-ray source. Mg Kα
radiation was generated with a voltage of 8 kV and
current of 30 mA. The spectrometer was calibrated
using the Ag3d5/2 core line. An UV-vis diffuse
reflectance spectra of p type semiconductor were
measured with a JASCO V-500 UV-vis
spectrophotometer (JASCO International Co., Tokyo,
Japan) equipped with an integral-sphere attachment.
2-5 Measurement of photovoltaic properties of
n/p tandem type DSC.
Photovoltaic properties of assembled solar
cell were investigated by
current-voltage
characteristics measurement under irradiation with
simulated solar light under AM1.5G, 100 mW/cm2.
(PEC-L11, Peccell Technologies, Inc., Yokohama,
Japan)
3. Results and discussions
3-1 Confirmation of sputtered CuxBiyOz by
plasma treatment on the electrode.
Figure 2 shows Cu2p3/2 and Bi4f XPS spectra
of FTO glass substrate after CuxBiyOz sputtering
1
Bi 4f
4.
Absorbance
0.8
5.
2.
0.6
1.
2.
3.
4.
5.
0.4
0.2
0
200
300
400
500
3.
Bi2O
CuO
CuBi2O4
CuxBiyOz(95:5)
CuxBiyOz(90:10)
1.
600
700
800
900
Wavelength / nm
Figure 2. Cu2p3/2 and Bi4f XPS spectra of FTO glass substrate
after CuxBiyOz sputtering process.
Figure 3. Photo absorption property of p type semiconductor
electrode prepared by plasma sputtering method.
-2
Photocurrent Density / mA・cm
8
1.
2.
3.
4.
5.
6
2.
Original
200W 5min
200W 10min
200W 15min
200W 20min
4
1.
2
4.
3.
5.
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage / V
Photocurrent Density / mA・cm
-2
Figure 4. I-V curves of n/p tandem type DSC prepared by
plasma sputtering using CuxBiyOz (90:10)
12
1.
2.
3.
4.
5.
10
8
4.
6
Original
200W 5min
200W 10min
200W 15min
200W 20min
3.
5.
4
1.
2
0
2.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage / V
Figure 5. I-V curves of n/p tandem type DSC prepared by
plasma sputtering using CuxBiyOz (95:5)
process at 200W discharge powers of irradiation.
The peaks related Cu molecules around 933 eV, and
Bi molecules around 160 eV appeared in the spectra
of each sample. It was confirmed that CuxBiyOz
could be sputtered on FTO substrate by plasma
irradiation.
3-2 Photo absorption spectra of p type
semiconductor.
Figure 3 shows UV-Vis spectra of p type
semiconductor electrode prepared by plasma
sputtering method. Photo absorption property of
CuxBiyOz (95:5 and 90:10) was better than that of
CuBi2O4 in visible light region. DSC using the
electrode including CuxBiyOz will be expected to
show good photo conversion efficiency compared
with using CuBi2O4 due to the absorption of visible
light.
3-3 Photovoltaic property of n/p tandem type
DSC using p type semiconductor with CuxBiyOz
Table 1 and 2 show photovoltaic property of
n/p tandem type DSC using electrode depended on
the discharge power, irradiation time, and
component ratio (CuO:Bi2O3) for plasma sputtering.
Table content shows short-circuit current density Jsc,
open circuit voltage Voc fill factor ff, and photo
conversion efficiency Figure 4 and 5 shows
current-voltage (I-V) curves of n/p tandem type DSC
samples by plasma treatment with 200W discharge
power. Original in table and graph legends means
results of normal type DSC assembled by Pt
sputtered FTO glass substrate as cathode and TiO2
films with 2m thickness on FTO substrate as anode.
From the results of DSC with CuxBiyOz
(90:10) sputtered substrate as p type semiconductor
electrode, sample treated by 300W discharge power
for 10 min only represented the improvement of
short-circuit current density and photo conversion
efficiency compared with original sample. On the
other hands, DSC sputtered with CuxBiyOz (95:5)
with 200 W discharge power showed good property
like the samples with CuxBiyOz (90:10) treated with
300W. However the cells plasma treated with 300W
did not show the improvement. The effects for shortcircuit current density after plasma sputtered of
CuxBiyOz to cathode electrode would be related with
the thickness of sputtered films. In this n/p tandem
type DSC system, we need to match the excitation
electron amount from sensitized dye adsorbed on
TiO2 particles and p type semiconductor for the
improvement of current density. Because the hole
generated after light absorption in p type
Table 1. Photovoltaic property of n/p tandem type DSC
prepared by plasma sputtering using CuxBiyOz (90:10)
Discharge
power
(W)
200
300
Treatment
time (min)
Jsc
(mA/cm2)
Voc
(V)
ff
(%)

(%)
5
10
15
20
5
10
4.70
4.94
6.93
6.86
4.75
4.56
0.67
0.67
0.71
0.72
0.68
0.65
43.9
57.6
54.3
51.0
41.6
21.2
1.38
1.91
2.70
2.53
1.35
0.63
Table 2. Photovoltaic property of n/p tandem type DSC
prepared by plasma sputtering using CuxBiyOz (95:5)
Discharge
power
(W)
Treatment
time (min)
Jsc
(mA/cm2)
Voc
(V)
ff
(%)

(%)
5
10
15
20
5
10
3.26
4.62
3.42
2.53
3.65
4.46
5.31
0.68
0.67
0.71
0.67
0.68
0.67
0.68
63.4
42.4
54.1
43.5
37.6
41.1
56.1
1.45
1.31
1.32
0.74
0.94
1.23
2.04
Original
200
300
Table 3. Photovoltaic property of n/p tandem type DSC
prepared by plasma sputtering using Cu.
Discharge
power
(W)
Treatment
times (n)
Jsc
(mA/cm2)
Voc
(V)
ff
(%)

(%)
300
1
2
3
2.85
1.94
1.9
0.81
0.79
0.78
58.7
56.3
56.3
1.68
1.15
1.05
semiconductor have to receive the excited electron
from sensitized dye passing though the external
circuit. If the hole did not generate enough in p type
semiconductor, the excited electron from dye would
be trapped and disappeared. The factor for the
amount of generated holes was the amount of
sputtered CuxBiyOz, that is the thickness of CuxBiyOz
adsorbed on the electrode. In this study, the
thickness of CuxBiyOz films in these plasma
sputtered conditions did not investigate in detail, but
the thickness of CuxBiyOz sputtered with these
conditions was expected to be adequate for the
balance of electron transaction in the circuit.
3-4 Photovoltaic property of n/p tandem type
DSC using p type semiconductor with CuO
Comparison of the effect for n/p tandem
type DSC using another kind of p type
semiconductor electrode was carried out. Table 3
shows photovoltaic property of n/p tandem type
DSC using cathode of CuO sputtered with 300W
discharge power for 5 min depended on plasma
treatment times. From the results, as the plasma
treatment times, the photo conversion efficiency was
decreasing. This treatment given the photovoltaic
property worse would be due to the film thickness of
CuO on cathode substrate. The reason why was
suggested that the hole generated in CuO films was
more difficult to move in CuO matrix and catch the
excited electrons passing though the external circuit
by large resistance from thick sputtered films.
4. Conclusions
The n/p tandem type DSC was composed
with titanium dioxide as n type semiconductor and p
type semiconductor prepared by the plasma
sputtering method using magnetron type plasma
system to be covered with CuxBiyOz or CuO on Pt
sputtered FTO glass substrate. The plasma treated
substrate by CuxBiyOz or CuO as p type conductor
showed wide absorption in visible light region. The
photovoltaic property of the n/p tandem type DSC
was improved by some plasma sputtering condition
using p type semiconductor. This results suggested
that the good effect for photovoltaic property
depended on the film thickness of plasma sputtered
components.
References
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[2] J. He, H. Lindstrom, A. Hagfeldt, S.-E. Lindquist,
Solar Energy Materials Solar Cell, 62, 272 (2000).
[3] K. Yamamoto, K. Matsuki, T. Yamamoto, Y. Maitoko,
T. Sonoda, H. Yamane, S. Matsushima, H. Nakamura, K.
Yamada, ISPC-19 Proceedings, P3-7 (2009).
[4] T. Arai, M. Yanagida, Y. Konishi, Y. Iwasaki, H.
Sugihara, K. Sayama, J. Phys. Chem. C, 111, 7574,
(2007).