Effect of CdS Processing Conditions on the Properties of CdS/Si Diodes and CdS/CdTe Thin-Film Solar Cells

Thermally evaporated n-type CdS thin films were studied to determine the influence of substrate deposition temperature and postdeposition thermal annealing on their electrical, optical, and structural properties. It is shown that although increasing substrate temperature during deposition from room temperature to 200°C results in CdS films exhibiting high optical transmittance as well as large grain size, deposition temperatures above 50°C lead to significant degradation in carrier concentration and mobility. Postdeposition thermal annealing of CdS films deposited at room temperature is shown to yield CdS films with superior electrical and optical characteristics, resulting in an electron mobility of 17.45 cm2 /Vs for films annealed at 200°C. The electrical characteristic of n-CdS/p-Si heterojunction diodes indicated that postdeposition thermal annealing reduced parasitic series resistance and decreased the diode ideality factor to a value of 1.384 for films annealed at 300°C, suggesting a reduction in recombination centers in the vicinity of the CdS/Si interface. In n-CdS/p-CdTe heterojunction solar cells, the photovoltaic cell parameters indicated that the deposition of CdS films at room temperature produces better performing cells with the substrate temperature required during the thermal deposition of CdTe acting as postdeposition annealing for the underlying CdS thin film.