$Cd2+∕NH3$ treatment-induced formation of a CdSe surface layer on $CuGaSe2$ thin-film solar cell absorbers

$CuGaSe2$ (CGSe)-based high-gap thin-film solar cells have to date not reached their potential level of electrical performance. In order to elucidate possible shortcomings of the electronic interface structure, we have studied the initial stage of the $CdS∕CGSe$ interface formation by use of a simple $Cd2+∕NH3$ treatment. As in the case of low-gap chalcopyrites, we find a Cd-containing surface layer, in the present case comprised of approximately one monolayer of CdSe. The results indicate that the $CdS∕CGSe$ interface is not abrupt, but contains intermediate layers. Furthermore, they shed light on possible surface modification schemes to enhance the overall performance of high-gap CGSe chalcopyrite solar cells.