Effects of Oxygen on the Microstrucutre of Cu/sub 2-X/S Thin Films

摘要

Changes in the microstructure of Cu/sub 2-x/S thin films after heat treatment in air at 180 exp 0 C were monitored by high resolution transmission electron microscopy (TEM). Specimens were prepared by the aqueous exchange of copper and cadmium in the surface layers of pre-thinned CdS single crystals, simulating the copper sulfide layer in Cu/sub 2-x/S/CdS solar cells without the complicating influence of CdS grain boundaries. Results indicate that the oxidation of copper and the oxidation of sulfur are competing processes which depend on the temperature range of the heat treatment. A kinetic model explaining this behavior is proposed, wherein the effects of the activation barrier opposing the hexagonal chalcocite to cubic digenite transformation are recognized. A sequence of superstructure images indicating a net sulfur loss in Cu/sub 2-x/S during heat treatment is presented in support of this model. Furthermore, observations of the Cu/sub 2-x/S microstructure reveal that chalcocite/djurleite interfaces are abrupt to within 20A. The structural relationships between these two phases are examined in detail, and a possible transformation mechanism involving the clustering of copper vacancies is presented. In addition, the chalcocite in the vicinity of chalcocite/djurleite interfaces was found to contain a high density of 1/4 (010) faults. Structural considerations show that these faults are likely sites for copper vacancies. Finally, the consequences of these results for solar cells based upon Cu/sub 2-x/S layers produced by the aqueous exchange method are discussed with emphasis on long-term degradation and processing heat treatments conducted in air.