Spatially resolved photoluminescence analysis of Se passivation and defect formation in CdSe$_{x}$Te$_{1-x}$ thin films

TL;DR

This study uses spatially resolved photoluminescence to analyze how selenium passivation affects defect formation and carrier recombination in CdSe$_{x}$Te$_{1-x}$ thin films, revealing both benefits and limitations for solar cell efficiency.

Contribution

It provides direct, spatially resolved evidence of Se passivation effects and defect formation in CdSe$_{x}$Te$_{1-x}$ films, independent of interface effects.

Findings

Se passivates grain boundaries, reducing recombination.

Se treatment increases radiative efficiency when combined with Cl passivation.

Se incorporation also enhances defect-related luminescence, potentially limiting efficiency.

Abstract

CdTe is the most commercially successful thin-film photovoltaic technology to date. The recent development of Se-alloyed CdSe$_{x}$Te$_{1-x}$ layers in CdTe solar cells has led to higher device efficiencies, due to a lowered bandgap improving the photocurrent, improved voltage characteristics and longer carrier lifetimes. Evidence from cross-sectional electron microscopy is widely believed to indicate that Se passivates defects in CdSe$_{x}$Te$_{1-x}$ solar cells, and that this is the reason for better lifetimes and voltages in these devices. Here, we utilise spatially resolved photoluminescence measurements of CdSe$_{x}$Te$_{1-x}$ thin films on glass to study the effects of Se on carrier recombination in the material, isolated from the impact of conductive interfaces and without the need to prepare cross-sections through the samples. We find further evidence to support Se passivation of grain boundaries, but also identify an associated increase in below-bandgap photoluminescence that indicates the presence of Se-enhanced luminescent defects. Our results show that Se treatment, in tandem with Cl passivation, does increase radiative efficiencies. However, the simultaneous enhancement of defects within the grain interiors suggests that although it is overall beneficial, Se incorporation may still ultimately limit the maximum attainable efficiency of CdSe$_{x}$Te$_{1-x}$ solar cells.