CdCl_2 treatment related diffusion phenomena in Cd_(1-x)Zn_xS/CdTe solar cells

摘要

Utilisation of wide bandgap Cd_(1-x)Zn_xS alloys as an alternative to the CdS window layer is an attractive route to enhance the performance of CdTe thin film solar cells. For successful implementation, however, it is vital to control the composition and properties of Cd_(1-x)Zn_xS through device fabrication processes involving the relatively high-temperature CdTe deposition and CdCl_2 activation steps. In this study, cross-sectional scanning transmission electron microscopy and depth profiling methods were employed to investigate chemical and structural changes in CdTe/Cd_(1-x)Zn_xS/CdS superstate device structures deposited on an ITO/boro-aluminosilicate substrate. Comparison of three devices in different states of completion-fully processed (CdCl_2 activated), annealed only (without CdCl_2 activation), and a control (without CdCl_2 activation or anneal)-revealed cation diffusion phenomena within the window layer, their effects closely coupled to the CdCl_2 treatment. As a result, the initial Cd_(1-x)Zn_xS/CdS bilayer structure was observed to unify into a single Cd_(1-x)Zn_xS layer with an increased Cd/Zn atomic ratio; these changes defining the properties and performance of the Cd_(1-x)Zn_xS/CdTe device.