Thickness dependence of structural and electrical properties of Cds films for solar cells

Abstract

Polycrystalline films of CdS were evaporated on zinc-plated foils of copper so that different thicknesses were obtained in the same evaporation. The different thicknesses were compared with respect to their surface morphologies, acid etching, grain sizes, grain structure, and orientation as well as crystallographic orientation. Concomitant measurements of the electrical properties as functions of thickness were taken. These included average resistance through the thickness, current–voltage characteristics of diodes prepared by evaporating gold dots on the surfaces, and measurements of capacitance as functions of reverse bias on the same diodes. It was established that grain size, the preferential grain orientation and conelike growth towards the source, the preferential c-axis orientation towards the source and the surface roughness, all increased as the film thickness increased. Furthermore, it was found that the concentration of free carriers increased with film thickness, the quality of the diode behavior improved considerably in the same manner, and the existence of voltage-dependent shunting paths diminished as the film thickness increased. An explanation is proposed which relates the observed electrical behavior as a function of thickness to the structural parameters.