  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.

Materials Science;Applied Physics;Optics