Photo effects in cadmium sulphide

Abstract

The work described in this thesis is mainly concerned with the electrical and optical properties of three types of CdS device namely: 1. Photoconductive cells with two ohmic contacts 2. Schottky diodes with one ohmic contact, and 3. P-n heterojunctions made by the heat treatment of CdS crystals carrying deposited layers of copper. Since a high stability in use is essential for successful application of CdS photoconductors and solar cells, one of the main aims of the research has been to investigate the optically induced processes which lead to the degradation of the efficiency of CdS photosensitive devices. Using the technique of thermally stimulated currents on CdS : C1 crystals doped with monovalent and bivalent copper ions, we have shown that optically induced degradation of photoconductivity in CdS : C1 rods doped with Cu(^++) ions occurs in conjunction with a reduction in the density of shallow traps, and the simultaneous formation of deep traps. In contrast, optical illumination does not have any deleterious effect on CdS : C1 samples doped with Cu ions. With the Schottky diodes, the resistivity of the sample was found to play a significant role. The effects of ageing on such properties as the short circuit photoresponse, the C-V characteristics and the I-V characteristics of the diodes were investigated. It is shown that during ageing an increase in the thickness of the interfacial layer occurs in conjunction with a decrease in the uncompensated donor density. Devices of the third type, i.e. p-n heterojunctions were produced by heating crystals of CdS carrying layers of copper metal. Optical and electrical measurements showed that the heat treatment of a Cu-CdS contact leads to the formation of a layer Cu(_2-x)S which is a mixture of djurleite and chalcocite.