Polycrystalline CdS thin films and their role in CdS/CdTe photovoltaic devices

Abstract

This thesis represents a systematic study of polycrystalline CdS thin films and their role as an n-type window layer in CdS/CdTe photovoltaic devices. This work encompasses the growth of CdS, primarily by the solution deposition method, and the subsequent characterisation of these films in isolation and as part of thin film CdS/CdTe device structures. A novel solution deposition approach was devised in order to grow high quality CdS thin films. Structural, electrical and chemical characterisation methods applied to these have shown that in their as-grown state they are highly oriented (in either the c[l1l] or h[002] direction), possess a small grain size of approximately 10-15 nm, and contain a considerable level of compressive strain. Annealing treatments in the presence of the fluxing agent CdCl(_2) have been shown to strongly modify the properties of these films, they are converted to a polycrystalline hexagonal structure with a significantly reduced level of strain, possess a larger grain size (27-28 nm) and a considerably enhanced crystalline quality. Novel 'hybrid' films comprising two CdS layers grown by different growth methods, one grown directly upon the other, have been studied. It has been shown that there are remarkable differences in morphology between these and films grown by a single growth method alone. Complete CdS/CdTe devices have been fabricated from several types of film grown in this study. Cell efficiencies of 9.80% were attained for a limited batch of devices, suggesting that these films possess good qualities for PV device fabrication. Early results from a novel tubular photovoltaic device concept are presented. This geometry has the potential to reduce manufacturing costs, may open up new routes to enhance the efficiency of CdS/CdTe devices, and is an attractive candidate for PV/solar thermal power generation.