Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Photocurrent [A]
a)
b)
104
10,0p
8,0p
6,0p
4,0p
2,0p
R λ
[%]
0,0
0 500 1000 1500 2000
R λ
(%)
100
80
60
40
20
0
100
80
60
40
20
0
X [ µm]
pure, polished Si
TOS-Si
U=0 V
λ=399 nm
λ=466 nm
λ=547 nm
λ=589 nm
λ=632 nm
λ=928 nm
Fig. 4. Characteristics of photocurrent obtained in effect of single
scan “in line” of light beam across the area between the electrodes
on Ti-V (3 at. % of V) oxides/Si heterostructure with the different light
wavelength
400 600 800 1000
λ [nm]
pure, polished and texturized Si
TOS-Si
R λ
=18 %
400 600 800 1000
λ (nm)
Fig. 5. Reflection spectra of mixed Ti-V oxide thin films deposited on
different Si substrates: a) polished and b) polished, texturized
in TiO 2
–V 2
O 5
system [10] containing V 5+ vanadium ions. However
thin films with vanadium reduced to +3 valence state (V 3+ ) inV 2
O 3
structure may display p-type conduction [11]. In Ti–V mixed oxide
system with such reduced vanadium ions, formation of V 2
TiO 5
phase is possible. P-type conduction in this case is due to vanadium
vacancies, which may act as acceptor levels and introduce
holes in the valence band.
In order to complete the study it was necessary to confirm
that prepared thin TOSs films can form heterojunction with silicon
(p-type) substrate. As a hterojunction example, Ti-V oxides
(3 at. % of V) has been presented in Fig. 3. The photoelectrical
characteristics I ph
(x) were recorded using the OBIC measurements
performed at six different wavelengths of 399, 466, 547,
589, 632 and 928 nm (Fig. 4). The measurements were carried
out at room temperature, with the light beam of ca. 30 µm in
diameter and frequency modulation of f = 180 Hz. The highest
photocurrent vs. scaning beam position was recorded for infrared
light (λ = 928 nm).
In Fig. 5 the comparision of reflection spectra of Ti-V (3 at. %
of V) oxides deposited on different monocrystalline silicon substrate
have been presented. Monocrystalline silicon substrate
has a reflection coefficient (R λ
) in the range of 30-35 % and
usually it is reduced mainly through texturization process and
additionally by a special antireflective film deposited on the silicon
surface. The polished silicon substrate has R λ
=30 % in the
visible spectral range. Nevertheless, after deposition of mixed
vanadium-titanium oxides on such silicon substrate, the reflection
coefficient was reduced of about 10 % (Fig. 5a). Observed
behaviour testifies about antireflective function of Ti-V oxide thin
films in the range of 300...1000 nm. Another example concerns
polished and texturized silicon substrate. Due to more diversivied
surface of textured silicon, the minimalization of R λ
down to
10% can been achieved. Covering the polished and texturized
Si with the same film like in the previous case causes further
reduction of the reflection coefficient dwon to several percent
(Fig. 5b).
Summary
This paper briefly presents the main scientific directions and measurement
opportunities of the staff from Laboratory of Optoelectrical
Diagnostics of Nanomaterials situated at Wroclaw Univeristy
of Technology. All discussed thin oxide films based on TiO 2
were
prepared by High Energy Reactive Magnetron Sputtering method.
As it was showed, control of material composition allows to obtain
nanocrystalline, transparent semiconductors with particular
type of conductivity. Additionally, in such thin films other desirable
properties can be achieved simultaneously, e.g. photoactivity and
reduced reflection. It testifies about great application potential of
nanocrystalline TOS thin films as coatings for architectural glasses,
windscreens, solar cells application.
This work was financed from the sources granted by the NCN
in the years 2011-2013 as a supervisors research project number
N N515 4970 40.
Authors would like to thank to E.L. Prociow from our Faculty
for his help in the experimental part of this work.
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Elektronika 6/2012