Band offsets of CuInSe2/CdS and CuInSe2/ZnS (110) interfaces: A hybrid density functional theory study

The valence band offsets of the CuInSe2/CdS and CuInSe2/ZnS (110) interfaces are obtained based on various definitions using first-principles calculations in the framework of hybrid density functional theory. Both the strained band offset and the unstrained, or natural, band offset are investigated, where the two phases share and do not share in-plane lattice parameters perpendicular to the stacking direction, respectively. The valence band offset is determined by first obtaining the difference between the reference levels of two phases in the regions far from the interface and then adding the difference between the valence band maximum and the reference levels of bulk for the two phases. The nonfaceted (110) interface and a number of (112)/(11 (2) over bar) faceted interfaces, some containing ordered point defects in the CuInSe2 (CIS) region, are considered. The excess energies of CIS/CdS and CIS/ZnS interfaces are lower when there are no ordered point defects, in contrast to the CIS surfaces that stabilize with ordered defect formation. The valence band offset is not significantly dependent on the atomic configurations at the interface as long as there are no charged layers. Surface calculations suggest that the reference level, which is determined by the average electrostatic potential at the atomic site, is not strongly dependent on lattice strain. A definition of the natural valence band offset that assumes a strain-invariant difference in the reference levels of the two phases provides values almost independent of the in-plane lattice parameters used in the interface calculation, which are about -1.2 and -1.3 eV with respect to CIS for the CIS/CdS and CIS/ZnS interfaces that contain no charged layers, respectively. The ionization potential difference can differ from the natural valence band offset by up to 0.3 eV without any consistent tendency to overestimate or underestimate, showing that the ionization potential difference is not necessarily a reasonable measure of the natural valence band offset.