Role of Cd interstitials in limiting group-V doping efficiency in CdTe

Cadmium Telluride (CdTe) is a key material in thin-film photovoltaic technology, with a current record efficiency of 23.1%, well below the theoretical limit of 33%, indicating significant potential for improvement. Increasing the hole concentration in the p-type CdTe absorber layer is crucial to enhancing open circuit voltage and, thus, device efficiency. Group-V dopants, such as phosphorus (P), arsenic (As), and antimony (Sb), are shallow acceptors, but they exhibit low doping efficiency in thin films. The underlying causes of these low doping activations remain a topic of intense debate. Using hybrid density functional theory (DFT), we show that group-V acceptors can trap Cd interstitials, forming compensating donor complexes that limit doping activation. Thermal annealing can dissociate these complexes, allowing Cd interstitials to diffuse to the surface. However, upon cooling to room temperature, Cd interstitials may either re-enter the CdTe grain or be trapped at the surface, depending on the compound formed at the interface. A thin layer of Cd-based oxides can act as a sink for Cd interstitials, shifting the chemical potential toward the Cd-poor limit and helping extract Cd interstitials from the grain, thereby preventing the formation of compensating donor complexes. These insights could lead to an approach to improve CdTe-based photovoltaic devices.