First-Principles Understanding of NiO and TeO₂-Based Back Passivation Layers in CdTe PV

There is progressive interest toward fundamentally understanding the roles attributed to passivated back contact layers in thin-film semiconductor PV. Within the cadmium telluride PV community, special emphasis is given to a back passivation layer capable of reducing back interface recombination losses while ensuring carrier selectivity and the possibility for bifaciality. Transparent thin-film oxide layers such as NiO_x or TeO_x may offer a viable solution for back passivation layers in CdTe PV. However, atomic scale effects for NiO_x and TeO_x in relation to possible defect interactions during copper and/or chloride-based treatments as well as energy band alignment of the absorber/passivating oxide interfaces are not well understood. The current work uses a first-principles computational approach based on density functional theory to investigate the electronic structures of NiO and TeO₂ bulk layers in the presence of relevant defects along with the formation of pristine interfacial morphologies when either oxide layer is in contact with the absorber layer in CdTe PV. For the bulk defect studies, defect pair interactions are evaluated to determine avenues of dopability and/or compensation that may affect bulk carrier transport within the oxide thin-film layers. Additionally, an atomic-scale perspective of the pristine absorber/oxide interface structures is given with respect to plane orientation, termination layer, and associated surface reconstruction to give insight on how NiO and TeO₂ differ in band alignment features influencing charge transport at the back interface. Details from the first-principles-based computational study provide further insight on the mechanisms inherent to passivating oxide layers that could potentially lead to benefits in CdTe thin-film PV device performance.