Preparation and characterization of close-spaced vapour transport thin films of ZnSe for heterojunction solar cells

Abstract

Thin films of ZnSe were deposited onto several types of substrates, including glass coated with indium tin oxide and semitransparent metal layers, using the close-spaced vapour transport (CSVT) method. The ZnSe film parameters such as crystal orientation, resistivity, photoluminescence and absorption spectra, and photogeneration efficiency were investigated, and the effects of the deposition conditions, doping and post-deposition heat treatment on these parameters were determined. The ZnSe films were found to be polycrystalline with (111) preferred orientation independent of the substrate used. The dark resistivity of the films was reduced by about three orders of magnitude to approximately 1000 Ω cm by post-deposition annealing. It is suggested that zinc vacancies and copper residual impurities acting as acceptors are the main recombination centres and are responsible for compensation effects in the films. The fundamental absorption edge was found to be at about 2.67 eV and the values of the absorption coefficient above the absorption edge were established. Doping had a detectable effect on the absorption curves and the photoluminescence spectra. Spectral photoresponse curves computed for the ZnSe films using the absorption coefficient values found here seem to be in a qualitative agreement with the experimental data. Finally, application of the ZnSe thin films to ZnSeZn 3P 2 n-p heterojunction solar cells is discussed.