Spatially resolved photoluminescence analysis of the role of Se in CdSeTe thin films.

Evidence from cross-sectional electron microscopy has previously shown that Se passivates defects in CdSeTe solar cells, and that this is the reason for better lifetimes and voltages in these devices. Here, we utilise spatially resolved photoluminescence measurements of CdSeTe thin films on glass to directly 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 increase in below-bandgap photoluminescence that indicates the presence of Se-enhanced defects in grain interiors. Our results show that whilst Se treatment, in tandem with Cl passivation, does increase radiative efficiencies in CdSeTe, it simultaneously increases the defect content within the grain interiors. This suggests that although it is beneficial overall, Se incorporation will still limit the maximum attainable optoelectronic properties of CdSeTe thin films.