Effects of CdCl2 treatment on deep levels in CdTe and their implications on thin film solar cells;\ud A comprehensive photoluminescence study

摘要

This work is aimed at studying defect level distributions in the bandgap of CdTe thin films,\udused for solar cell development. In particular, the effects of CdCl2 treatment on the defect\udlevels are the main objectives of this research. Four different CdTe thin films were\udelectroplated using three different Cd-precursors (CdSO4, Cd(NO3)2 and CdCl2), and bulk\udCdTe wafers purchased from industry (Eagle Pitcher and University Wafers in US) were\udstudied using low temperature photoluminescence. The finger prints of defects, 0.55 eV\udbelow the conduction band down to the valence band edge were investigated. In all of the\udCdTe layers, four electron trap levels were observed with varying intensities but at very\udsimilar energy positions, indicating that the origin of these defects are mainly from native\uddefects. CdCl2 treatment and annealing eliminates two defect levels and the mid-gap\udrecombination centres are reduced drastically by this processing step. The optical bandgap of\udall four as-deposited CdTe layers is ~1.50 eV, and reduces to ~1.47 eV after CdCl2 treatment.\udThe material grown using the CdCl2 precursor seems to produce CdTe material with the\udcleanest bandgap, most probably due to the built-in CdCl2 treatment while growing the\udmaterial.