Optimization of thin transparent conductive ZnO:Al film for nanostructured
piezoelectric energy harvesters
Petr Novák
1
, Joe Briscoe
2
, Marie Netrvalová
1
, Tomáš Kozák
3
1
New Technologies – Research Centre, University of West Bohemia, Czech Republic
2
School of Engineering and Material Science, Queen Mary University of London, UK
3
Department of Physics and NTIS – European Centre of Excellence, University of West
Bohemia, Czech Republic
E-mail:
phone: +420
377 63
4 773
Transparent conductive oxides (TCOs) are a class of materials which combines high
optical transmittance in the visible spectrum with a resistivity below 10
-3
Ωcm. Nowadays,
indium tin oxide (ITO) and Al-doped zinc oxide (AZO) thin films are the most used TCOs,
which are widely applied as transparent electrodes for various applications such as liquid
crystal displays, organic light emitting diodes and thin film solar cells. The AZO films are
considered as a cheap and non-toxic alternative to ITO, but it is
more difficult
to achieve
similar values of the resistivity.
Nowadays, fabrication of inorganic TCO on polymer substrates has been of increasing
interest due to their potential applications in the field of flexible electronics. Nevertheless, the
brittleness of inorganic thin films often results in a failure of the flexible electronic devices
due to strains formed during stretching, folding or bending. The films with thickness of
several hundreds of nanometres exhibit poor structural integrity on polymer substrates when
subjected to various mechanical stresses. However the lower TCO film thickness results in
the higher sheet resistance.
The subject of presented work is replacing the preferably used ITO films by an AZO
film in nanostructured piezoelectric energy harvesters based on the ZnO nanorods. The main
limitation for the use of the AZO compared to ITO films is the presence of the interface layer
which does not contributes to the electrical transport. This layer can be tens of nanometres
thick and, therefore, is significant when the film thickness is small. Another important
limitation is related to the temperature-sensitive substrates which are often used for the
flexible devices. The AZO films with the required resistivity lower than 10
-3
Ωcm are mostly
deposited at temperatures higher than 200°C.
The investigated AZO films with thickness up to 300nm were deposited on 150µm
thick PET substrate by the (i) RF magnetron sputtering from ZnO/Al
2
O
3
target and (ii) co-
sputtering from the ZnO and Al target in argon atmosphere. Electrical properties, namely
sheet resistance, Hall mobility and carrier concentration were determined by the
by Hall
measurement system. Film thickness and optical properties were evaluated by the
spectroscopic ellipsometry. The surface morphology and the formed nanorods were
investigated by the SEM.
The transparent AZO films with the resistivity of about 10
-3
Ωcm were prepared by
co-sputtering at temperature not exceeding 100°C. Obtained values of the resistivity allows
prepared 140nm and 210nm thick films with the sheet resistance lower than 100 and 50 Ω/sq,
respectively. These films were also successfully tested as the seed layer for the growth of the
nanorods and the influence of the surface morphology on the size and density of the formed
nanorods were studied
as well.
Moreover, the co-sputtering from ZnO and Al targets allows
control the Al concentration during deposition and thereby allows further improvements of
the film properties. Our study confirms that the AZO films have great potential for replacing
more expensive ITO in flexible ZnO nanorods based harvesting devices.
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