Thin films of Cds and the CdS-Cu(_2)S hetherojunction

Abstract

The thin film CdS-Cu(_2)S solar cell is potentially a cheap and efficient device for converting sunlight into electrical energy, but there are various difficulties which present its ready fabrication. These centre on the CdS base layer and concern the uncertainties in the evaporation process. Moreover, the properties of the heterojunction itself are not well understood despite the profusion of theoretical models. This thesis describes an investigation into the physical and electrical properties of vacuum evaporated CdS layers on a variety of substrates, as a function of the preparative conditions, i.e. evaporation rate, substrate temperature, film thickness, and source purity. The Hall mobility, resistivity and photosensitivity have been measured in both epitaxial and polycrystalline layers. X-ray studies revealed that a fibre-axis orientation existed in the thicker polycrystalline films, and transmission electron microscopy enabled several defects in the epitaxial layers to be identified. It was essential to use a 'hot-wall' technique to deposit reproducible films. Since source purity affected the growth and subsequent electrical properties of the films, it was necessary to use re-crystallised CdS as the evaporant, and it was preferable to employ electron beam evaporation. Dopants such as indium were readily incorporated into the films. Much of the CdS film behaviour can be explained in terms of the evaporation and condensation kinetics. An Appendix describes similar work on the properties of CdSe layers. Investigations into the electrical properties of junctions prepared by chemi-plating Cu(_2)S on boule CdS suggested that the best cells were made with copper-doped CdS. Open circuit voltage was highest if the base resistivity was a few hundred ohm cm. The existence of a photoconductive i-CdS region in heat-treated cells was demonstrated by the presence of long time constants and quenching effects. The importance of the CdS surface treatment was also shown.