Binding and energetics of oxygen at the CuInSe2 chalcopyrite and the CuInSe2/CdS interface

Abstract

Abstract The introduction of oxygen in thin-film solar cells based on the CuInSe2 compound and related CuInSe2/CdS devices has been known to affect their electrical properties, with a tendency of neutralizing part of the donor density and favoring a p-type behavior for the CuInSe2 (CIS) absorber material. The present study employed calculations based on density-functional theory supplemented with a hybrid-functional approach to determine the energetics of oxygen incorporation in the bulk CIS compound and the CIS/CdS heterojunction interface. The latter was represented by two distinct faceted interface variants. Oxygen atoms were assumed to exist both as interstitial and substitutional impurities, in the latter case occupying vacant selenium sites. The calculations identified the structural relaxation patterns and examined the thermodynamic stability of the impurity as a function of the electron and the elemental chemical potentials. Oxygen was found to incorporate favourably at the core of the CIS/CdS interfaces, in most cases by taking up a bridging position within the nearest In–In pair. The sites of the lowest-energy oxygen configurations were found to be associated with a copper-poor local environment, owing to the presence of copper vacancies or the relaxation-induced breaking of a copper-oxygen bond. The electronic structures of the CIS/CdS interfaces were also studied by analyzing the site-projected and layer-resolved densities of states for several layers within the interfacial cores. Oxygen introduced deep-lying nonbonding levels and impurity-host bonding states in the valence-energy region.