Abstract
Abstract
Thin film solar cells based on chalcopyrite absorbers are now
commercially available. However, the knowledge concerning their
physical properties enabeling this success is still lacking. The
determination of basic material parameters, like doping and deep level
defect concentrations on the one hand side, and their behaviour within
heterostructures, on the other hand side can give precious hints for
further developement. Within the scope of this work the following
investigations were performed on coppergalliumdiselenide as a typical
chalcopyrite:
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To determine the morphology and chemical structure at the hetero- and
metal-semiconductor interface, laterally resolved Auger Electron
Spectroscopy was performed. Diffusion coefficients of copper and selenium in
galliumarsenide and gold in CuGaSe2 were determined.
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From admittance measurements as a function of frequency and
temperature qualitative correlations between defect concentrations at
different activation energies and open circuit voltages of solar cells
were found: If both, shallow and deep defects, are detected within the
same sample, voltage increases with the concentration of the shallow
defect. This can be explained by modifications of the band-bending at
the interface, reducing voltage-limiting effects. A universal energy
to separate this two defect level ranges cannot be given
due to the individual determination of energy scale for each particular
material system.
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The basic mechanisms of doping and charge transport were investigated by
measurement of the Hall-effect on epitaxial layers. High mobilities at
room-temperature show the high epitaxial quality and suitability as a model
system for basic material properties. From the temperature dependence of
the mobility
the carrier transport mechanisms were deduced. From temperature
dependence of the net carrier concentration the activation energy of the
major dopand was calculated, as well as the effective mass of holes in
CuGaSe2, where only rough estimates from related materials were
available so far. Two distinct acceptor
levels and significant compensation by donor type defects were found.
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