Defects in Cu(In, Ga) Se₂ semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chakopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3-5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chakopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se₂ regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current-voltage characteristics of Cu(In, Ga)Se₂-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.