Vacancy Migration and Associated Charge-Transfer at the ZnS/CZTS Interface

Secondary phases, such as ZnS in Cu-poor/Zn-rich CZTS, affect the performance of photovoltaics by reducing the absorber volume and decreasing the short circuit current. The band-offset between CZTS and ZnS render the ZnS regions as carrier traps, and the interface between these materials is of significance to CZTS cell performance. The abruptness of the interface between CZTS and ZnS is controlled by the intrinsic diffusion mechanisms, with vacant metal sites playing a key role. To understand the mechanisms underpinning the interdiffusion between CZTS and ZnS across a [100] interface, we have performed density-functional calculations for two types of inter-diffusion process at the heterojunction interface. For each process the barrier has been estimated, and the processes aligned to a common energy scale based upon the formation energies of the defects at the interface. For some processes the valence-band offset results drives charge transfer across the interface; for example, a Zn vacancy in ZnS extracts electrons from the CZTS valence band. We find mechanism involving V_Cu in CZTS are also energetically favorable, which is of significance due to the importance of these defects in ptype doping of the absorber layer.