Annual Report 2012 - Latvijas Universitātes Cietvielu fizikas institūts

Main Results
LASER CRYSTALLIZATION OF a-Si:H FOR SOLAR CELL APPLICATIONS
G. Marcins, J. Butikova, J. Pervenecka, B. Polyakov, A. Muhin, I. Tale
Manufacturing of thin film amorphous silicon solar cells has perspectives with a
condition of improving solar cell efficiency and reducing production costs. Efficiency of
amorphous silicon solar cells decreases with time due to
formation of metastable light-induced defects. Laser
crystallization helps to prevent this effect. We apply
visible laser wavelength for laser crystallization in
contrast to widely used exciton laser crystallization,
which able to process only ~100 nm thick a-silicon layer.
Primary effects of laser crystallization of a-silicon are
dopants activation, and increase of charge carrier
mobility due to higher crystallinity of processed silicon.
Additional effect is texturing of initially smooth surface, which decreases back reflection
of incident light and increases efficiency of a solar cell.
Amorphous silicon crystallization by laser beam of
visible wavelength opens additional possibilities to
process the whole p-i-n structure of thin film solar cell
I-V curves of amorphous and laser processed
a-Si:H solar cells.
simultaneously. A-Si:H p-i-n thin film solar cell was grown by PECVD on ITO-coated
glass substrates. Laser crystallization was successfully applied for processing the whole
p-i-n structure of total thickness around 300 nm. A significant improvement of solar cell
output parameters in comparison to a non-processed cell was achieved.
GRAPHENE STAMP PRINTING
J. Butikova, B. Polyakov, E. Butanovs, L. Dimitrocenko, I. Tale
Graphene as a single monolayer of covalently bonded carbon atoms is an
intriguing material for both fundamental and applied science. Due to its outstanding
electronic and thermal properties (high charge carrier mobility, high heat conductance,
etc.) graphene raised huge interest in the last decade, and became a real candidate as a
successor of silicon in future microelectronics. There is plenty of graphene synthesis
methods, however, the best electric properties were demonstrated by graphene
mechanically cleaved from HOPG. Cleaved HOPG graphene sheets, nevertheless, are
usually irregular in shape and thickness, and therefore are not suitable for integration in
microelectronics devices.
Highly oriented pyrolytic graphite (HOPG) was scribed by the pulsed laser beam
to produce square patterns. Patterning of HOPG surface facilitates the detachment of
graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted
stamp printing of a few-layers graphene was compared. Printed graphene sheets were
visualized by optical and scanning electron microscopy. The number of graphene layers
was measured by atomic force microscopy. Glue-assisted stamp printing allows printing
relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method
is easier to implement and is more flexible than the majority of existing ways of placing
graphene sheets onto a substrate.
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