Studies on the optoelectronic characteristics of cadmium sulphide

Cadmium sulphide – an indirect band gap material is ‘n’ type with a band gap of 2.4 eV and is sensitive to visible spectrum. The property of cadmium sulphide which is of technological interest is the change in its resistivity on photo excitation which decides its commercial application. Hitherto work on single crystals and vacuum evaporated layers have been reported. From the commercial point of view large area polycrystalline CdS cells are commercially feasible only by sintering technique of which not much polished literature is available. The main objective of the present thesis is to study in detail the influence of various parameters on the preparation and fabrication of quality photoconductive CdS cell. From the results obtained on the fundamental aspects, one is facilitated to formulate conditions which are to be controlled and optimised for the commercial production of polycrystalline CdS photoconductors. A detailed account of the preparation of photograde CdS powder is given and discusses the theoretical as well as practical aspects of the influences of various parameters such as the starting material, Cd:S ratio of the reacting compounds, reaction temperature, pH nature, of alkali and the organic thio-compounds used etc. on the sensitivity of the photoconductive layer. The conditions were optimized for the preparation of CdS powder which was found to be at par with the commercial photograde CdS powder. The powder as such is not photoconductive. The various aspects involved in the sensitisation process of the CdS powder to make it photosensitive are covered. The influences of various cations like copper, silver, gold, gallium, indium and thallium, and various chlorides as flux material on the sensitisation process are discussed. Among the cation as acceptors, a 400 ppm concentration of copper was found to give maximum sensitivity and 10% CdCl2 best flux material, as it introduces donor level and non-chiometry. To a detailed investigation on its influence of the sintering conditions in relation to temperature, duration and various atmospheres like air, nitrogen, hydrogen and argon on the structural and electrical properties of polycrystalline CdS layers prepared by the silk screening technique was done. The effect of various contact materials like gold, tin, aluminium, copper, silver and aquadae on the CdS layer and suitable material for ohmic contacts are identified and discussed. From the studies, it is concluded that for all practical purposes, indium contact is to be preferred for fundamental studies and tin or aluminium for practical applications. Next the various techniques employed and measurements made to characterise the sintered layers are presented. This includes the study of the optoelectronic and photoconductive properties like lux-ampere characteristics of the system, influence of temperature and dopants on the photoconductive spectrum, response time measurements and variation of current with the applied voltage etc. in detail. Information on the use of these CdS layers in practical optoelectronic devices and systems such as Cu2S, CdS-Tl2S solar cells giving stress to the futuristic aspects of cadmium sulphide photograde powder as a semiconductor material in general and the various avenues open ahead is given