Factors controlling the properties of the Cds – Cu (_2)s photovoltaic cell

Abstract

Large area sprayed or silk screen printed CdS/Cu (_2) S solar cells are potentially cheap and efficient devices for the direct conversion of sunlight into electrical energy. They do not require the expenditure of large amounts of energy during fabrication as do silicon cells. There are, however, various degradation processes associated with the operation of the cells and the major mechanism is thought to be associated with a gradual oxidation of the copper sulphide. It is clear therefore that the degradation of single crystal and evaporated thin film cells must be better understood before the more difficult problems associated with the fabrication and operation of sprayed or printed layer cells can be solved. This thesis describes an investigation into the photovoltaic properties of CdS/Cu (_2) S heterojunctions. The cells were prepared by forming layers of CU (_2) S on single crystals and thin films of CdS. Undoped crystals with both high and low resistivity have been used as have low resistivity samples containing grown-in copper, indium and chlorine impurities. A study of the spectral dependence of the open circuit voltage and of the current-voltage characteristics, after baking in air at 200 C, shows that the maximum spectral sensitivity of the cells in the range 0.6 to 0.7 nm is associated with a photo- conductive region in the cadmium sulphide caused by a diffusion of copper into the cadmium sulphide. The effects of forming the cuprous sulphide layers on basal planes of opposite polarity have also been investigated. It was shown that the conversion of cadmium sulphide to cuprous sulphide proceeds 1.5 times faster on sulphur than on cadmium planes and the photovoltages of unbaked cells with the cuprous sulphide formed on sulphur faces are some 20% Larger than the photo- voltages from cells in which the cuprous sulphide is formed on cadmium faces. It was further shown that the plating temperature must be carefully controlled at 90 C to ensure production of stoichiometric Cu (_2) S.