Gradient doping in Cu2ZnSnSe4 by temperature and potential induced defect steering

Kesterite materials are among the most promising emerging photovoltaic absorbers, despite the number of challenging issues this technology presents. The use of soft thermal post-deposition treatments is key to improving the CdS/kesterite interface quality. Thermal treatments can result in a low-temperature phase transition which affects the optoelectronic properties. In this work, the effects of applied voltage during a full device thermal post-deposition treatments above the critical temperature of the phase transition are explored. The applied voltage modifies the formation energy and drives in-depth migration of ionized defects, which can generate a shallow doping density gradient. Supporting the experimental findings, the effects of a shallow doping density gradient on the current-voltage curves and the external quantum efficiency are modelled using drift-diffusion calculations. The presence of bulk recombination centers in the modelling is a key aspect to precisely reproduce the experimental results. The shallow doping density gradient in opposite directions precisely matches the experimental results for opposite voltage polarizations. The effects on the band structure of the device are presented proving this as a promising strategy for improving charge carrier selectivity. This is the first time that a defect engineering approach has been proposed and successfully proven to control shallow doping density profiling and improve the charge carrier selectivity of kesterite devices. In this sense, the results and their thorough physical interpretation presented in this work will potentially open new perspectives in other photovoltaic technologies as well as in the field of materials engineering.