Trapping effects in CdS devices
In order to study the possible effect of interface states on the efficiency of the CdS/Cu(_2)S heterojunction solar cell, the simpler structure of metal-on-insulator-on-CdS has been investigated. It has proved impossible to apply the theories appropriate to MIS devices on silicon, mainly because of the difficulties of producing a uniform oxide layer. However, a hypothesis has been put forward which is consistent with the experimental observations and which may be applicable to other results reported in the literature. The use of a scanning electron microscope (S.E.M.) particularly in the induced current mode, has allowed complementary investigations of surface properties to be carried out. The chemical preparation of a copper sulphide layer on CdS under different conditions is described and the various phases of Cu(_x)S produced are identified. The optical and electronic properties of these devices have been investigated under two-beam illumination to excite trapping effects. Further use of the S.E.M. with these structures has shown how useful this instrument can be in the analysis of semiconductor junctions. Finally, a number of conclusions relevant to the production of a more efficient cell are presented and a modified band structure model of the heterojunction is proposed.