Effects of CdCl₂ treatment on deep levels in CdTe and their implications on thin film solar cells: a comprehensive photoluminescence study

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