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

Photoelectrical properties of photovoltaic structures
based on CdTe/ZnO
EUNIKA Zielony 1) , PAULINA Kamyczek 1) , PIOTR Biegański 1) , EWA Płaczek-Popko 1) ,
RAFAŁ Pietruszka 2) , GRZEGORZ Łuka 2) , MAREK Godlewski 2)
1)
Wrocław University of Technology, Institute of Physics
2)
Institute of Physics, Institute of Physics Polish Academy of Sciences, Warsaw
Photovoltaics continues to be one of the fastest growing industries,
with annual increase beyond 40% [1]. Photovoltaic (PV)
solar cells convert incoming solar radiation directly into electricity
and produce electricity as each conventional source of energy,
but they are very attractive for they are environment friendly.
Zinc oxide (ZnO) is nowadays worldwide extensively studied
for optoelectronics and photovoltaics application. It is predicted
that there will be a mass production of conducting ZnO layers as
transparent electrodes in solar cells. It is said that ZnO layers will
soon replace indium transparent oxide (ITO) because of high cost
and limited supply of indium. Yet there is another very attractive
field of ZnO application. It may be used as the n-type partner
for the organic materials. There is a bright future in front of the
PV cells based on such a hybrid structures for their flexibility and
a very low cost of production.
The use of ZnO in the novel electronic and PV devices demands
low or extremely low processing temperature [2–4]. There
are many different technologies used to obtain ZnO layers. These
are chemical vapor deposition (CVD), molecular beam epitaxy
(MBE), sputtering, electron-beam evaporation, pulse laser deposition
(PLD), hydrothermal method and many others, however
these techniques are unsuitable for the aforementioned applications
as they are run at too high temperature. In this context
the Low Temperature Atomic Layer Deposition method (LT ALD)
technique has proven to be the most promising [5]. Moreover,
the technique is fairly cheap and allows ZnO growth with atomic
resolution.
Experiment
The subject of investigations was a test semiconductor cell based
on ZnO grown by ALD method on CdTe substrate. The structure
of the studied sample is depicted in Fig. 1. The undoped layer of
ZnO is the n-type partner for p-type CdTe whereas the ZnO:Al
layer is transparent conductive layer. The thickness of the ZnO
layers was determined from interference spectra of reflectance
measurements [6].
oxygen precursor. Zinc oxide is created as a result of a doubleexchange
chemical reaction that takes place at the surface:
Zn(C 2
H 5
) 2
+ H 2
O → ZnO + 2C 2
H 6
Substrate temperature was varied between 60°C and 240°C. For
dimethylzinc (DMZn) as zinc precursor a growth temperature could
be reduced to a room temperature [7].
Rectifying properties of the ZnO/CdTe junction have been
investigated by current-voltage (I-V) characteristics measured
in darkness and after illumination. Spectral characteristics
of photocurrent have been carried out within spectral range of
300…1100 nm using PV Quantum Efficiency system, Bentham
U.K. From the I-V measurements series resistance (R s
) was
calculated. From the spectral characteristics of photocurrent
spectral sensitivity was determined. Additionally, based on the
spectral characteristics of photocurrent the thickness of the layers
was determined.
Results
Current-voltage characteristics obtained for the studied n-ZnO/p-
CdTe junctions are shown in Fig. 2. They were carried out at room
temperature, in darkness and under illumination. The sample was
illuminated by a Xe lamp.
The I-V curves show rectifying properties, and light-electricity
conversion (both better visible in Fig. 3. on a semilogarithmic plot)
but the I-V characteristics are shifted towards negative voltage
bias. From the linear slope of the I-V curves given in Fig. 2 for
the studied junction series resistance was determined. Its value
equals to 36 Ω. Such a high value of series resistance as well as
some leakage current (low shunt resistance) are presumably due
to the observed shift.
In future work the fill factor must be enhanced by reducing the
series resistance and increasing the shunt resistance.
Figures 4 presents spectral characteristics of photocurrent
measured at room temperature. In spite of poor efficiency of the
test diode its spectral characteristics is promising for photovoltaic
application.
Fig. 1. The layer structure of n-ZnO/p-CdTe sample
In the ALD process, precursors are sequentially introduced to
the growth chamber, where they reach a surface of the growing
film. The ALD process consists of repeating of four deposition
steps: 1) deposition of the first precursor, 2) purging the reaction
chamber with an inert gas, 3) deposition of the second precursor
and 4) purging of the reaction chamber [7]. Two precursors
are at present commonly used for ZnO growth by ALD method:
diethylzinc (DEZn) as a zinc precursor and deionized water as an
Fig. 2. I-V characteristics for n-ZnO/p-CdTe structure measured
in darkness and under illumination (white light)
100
Elektronika 6/2012