Dual-Wavelength Time-Resolved Photoluminescence Study of CdSexTe1-xSurface Passivation via MgyZn1-yO and Al2O3

Abstract

Voltage loss is currently one of the biggest challenges facing cadmium telluride (CdTe) based photovoltaics. Determining the location(s) of major voltage loss within the device stack (e.g., front/back interface, grain boundaries) is therefore of primary interest. Here, we present a custom-built time-resolved photoluminescence system with two excitation wavelengths - 670 (standard) and 405 nm - to probe the device stack at depths of approximately 130 and 35 nm, respectively; their comparison helps differentiate interface and bulk contributions to carrier lifetime. We apply this system to examine the passivation effect of two significant recent advances in CdTe: the incorporation of Se to form graded CdSexTe1-x and the replacement of CdS with MgyZn1-yO. It is found that x = 0.2 Se is required to obtain lifetime improvements, primarily in the bulk. Additionally, evidence for trapping at the MgyZn1-yO/CdSexTe1-x interface was observed. This indicates further work is required to sufficiently passivate the front interface.