World Journal of Engineering
FABRICATION AND CHARACTERIZATION OF Cu2O/TiO2 COMPOSITE FILMS FOR SOLAR
CELL APPLICATIONS
Ayib Rosdi Zainun1,2,3, Tomoya Sakamoto1, Uzer Mohd Noor2, Mohamad Rusop2, Masaya Ichimura1
1
Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Nagoya 466-8555, Japan.
Solar Cell Laboratory, Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), Shah Alam, Selangor, Malaysia.
3
Faculty of Electrical &Electronics Engineering, Universiti Malaysia Pahang (UMP), Kuantan, Pahang, Malaysia.
2
needs to flow through the TiO2 matrix. However, the TiO2
particles composing the matrix are resistive, and therefore the
current would preferentially flow from the film/substrate
interface into the deposition solution. Thus Cu2O was
preferentially deposited near the interface rather than on the
top surface.
Fig. 3 shows the X-ray diffraction spectra of the Cu2O, TiO2
and Cu2O/TiO2 composite films. All the peaks observed for
TiO2 are attributed to the TiO2 anatase structure, and those
observed for Cu2O are attributed to the Cu2O cubic structure.
For the composite film, in addition to the TiO2 peaks, the
Cu2O peaks are observed.
Introduction
Use of oxide semiconductors such as Cu2O and TiO2 as
alternatives to silicon for solar cells attract much attention
among the researchers. Cu2O is a p-type semiconductor with a
direct band gap of 2.1 eV and is regarded as a suitable
material for high-efficiency solar cells [1-4]. TiO2 is an n-type
semiconductor with a wide band gap energy of 3.2 eV and
known for its photo catalytic effects [5], and it has been
widely used for anti-fouling coating and dye-sensitized solar
cells [6]. The combination of these two materials could
contribute to efficient photoelectric conversion. Siripala et al.
fabricated a Cu2O/TiO2 heterojunction thin film and observed
its photoresponse in a photoelectrochemical cell [7]. In this
research, Cu2O/TiO2 composite thin films have been
fabricated by combination of squeegee and electrochemical
deposition (ECD) methods, and a cell has been fabricated by
forming metal electrodes on the film. Structural and optical
properties of the films have been characterized, and
photoresponse of the cell has been measured.
Experimental Procedures
(a)
TiO2 films with a thickness around 16 μm were prepared by
the squeegee method using 0.8 g/mL TiO2 paste of TiO2
powders (P25, Aerosol Japan) with addition of 0.5 mL acetyl
acetone. The paste was mixed and blended with 0.4 g
polyethylene glycol and 2.5 mL triton X for about 5 min in
each process. Then the TiO2 films were heated and annealed
at 100oC and 400oC for 30 min in air. The substrate is Fdoped SnO2 (FTO) coated glass. The deposition of Cu2O on
the TiO2/FTO substrate by ECD was conducted using an
aqueous solution containing 0.5 mol/L copper (II) sulfate and
6 mL lactic acid in 20mL of pure water. The solution pH was
adjusted to 12.5 with KOH. The galvanostatic
electrochemical deposition on the TiO2/FTO substrate was
carried out at a current density of about -1 mA/cm2, and the
deposition time is 10 min unless otherwise stated.
(b)
(c)
Fig. 1 Appearance of (a) the TiO2 film, and (b) the front
surface and (c) the back surface of the Cu2O/TiO2 composite
film.
TiO2
Cu2O
FTO Glass
Fig. 2 Hypothesis of the Cu2O/TiO2 film structure.
Cu2O/TiO2
Intensity (a.u.)
Result and discussion
Physical appearances of the films are shown in Fig. 1. After
the Cu2O deposition, the front surface remained white, while
the back side (substrate side) became orange (color of Cu2O).
Thus Cu2O seems to penetrate the TiO2 film and be
dominantly deposited near the TiO2/FTO interface rather than
on the TiO2 film surface. This would be because Cu2O
gradually filled the porous matrix of TiO2. from the bottom as
in Fig. 2. During the deposition of Cu2O, the deposition
solution would easily penetrate the TiO2 film. For Cu2O to
deposit on the top surface of the TiO2 film, the electric current
(101)
(004)
TiO2
(110)
(111)
Cu2O
10
20
30
40
50
60
2  (deg.)
Fig. 3 X-ray diffraction spectra of the Cu2O, TiO2 and
Cu2O/TiO2 composite films.
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World Journal of Engineering
10 min-deposition sample, a typical solar cell characteristics
were observed, and the short-circuit current is 0.0031 mA/cm2,
the open-circuit voltage 0.47 V, and the efficiency 5x10 −4 %.
Thus a rectifying pn junction was formed with the front
surface side acting as n-type and the film/substrate interface
side acting as p-type. This will be because the surface side is
dominantly TiO2 and the interface side dominantly Cu2O, as
shown in Fig.2.
A solar cell based on a mixture of n-type and p-type
semiconductors are commonly called a blend solar cell or a
bulk-heterojunction solar cell. In all previous works on bulkheterojunction solar cells, the photovoltaic blend film
consisted of two organic semiconductors, or one organic and
one inorganic semiconductors. We demonstrated that the
Cu2O/TiO2 composite film showed photovoltaic behavior,
and thus we can regard our composite film as an inorganicinorganic bulk-heterojunction thin film.
100
Transmission (%)
80
60
Cu2O
40
TiO2
20
Cu2O/TiO2
0
800
700
600
500
Wavelength (nm)
400
300
Fig.4 Optical transmission spectra for the TiO2, Cu2O, and
Cu2O/TiO2 composite films.
Fig. 4 shows optical transmission spectra for the TiO 2, Cu2O,
and Cu2O/TiO2 composite films. The Cu2O film has an
absorption edge around 570 nm, corresponding to its band
gap of 2.1 eV. The TiO2 film is porous and thus the
transmission is low in the visible range because of scattering.
The absorption edge is observed near 400 nm. For the
composite films, the transmission is null for wavelengths
shorter than 520 nm because of the absorption by Cu2O.
Conclusions
Cu2O films have been deposited by ECD on TiO2 films
prepared by the squeegee method, and a cell has been
fabricated by evaporating In on the film. By I-V
characterization, the cell showed electrical rectification and
photovoltaic effects. Even though the overall performance of
the cell has not been well optimized yet, we have
demonstrated that an inorganic bulk heterojunction solar cell
can be fabricated by a simple approach based on the ECD and
squeegee methods.
(−)
Indium
(+)
Acknowledgements
Cu2O/TiO2 composite film
Special thanks to all members of Prof. Ichimura lab. for their
useful discussion, cooperation, support and assistance in
completing this paper.
FTO Glass
Fig. 5 Position of the electrodes for the I-V measurement.
References
1.E-03
5 min
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15 min
0
2
Current (mA/cm )
0.E+00
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-1.E-03
-2.E-03
10 min
-3.E-03
-4.E-03
Voltage (V)
Fig. 6 I-V characterization of the Cu2O/TiO2 composite films
under AM1.5 illumination of 100mW/cm2.
For conducting the I-V characterization, indium was
evaporated as shown in Fig. 5. Fig. 6 shows the photovoltaic
effects of the Cu2O/TiO2 composite films measured during
illumination through the FTO glass substrate. Three types of
samples with different Cu2O deposition times were prepared
and measured for comparison. The AM1.5 light intensity was
maintained at 100mW/cm2 for all the measurements. For the
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