High VOC Cu2ZnSnSe4/CdS:Cu based solar cell: Evidences of a metal-insulator-semiconductor (MIS) type hetero-junction

In this work we report the Cu doping of chemical bath deposited CdS for the preparation of Cu₂ZnSnSe₄/CdS:Cu hetero-junction. We demonstrate that increasing the Cu concentration in the reaction bath several fundamental properties of CdS are changed (optical, crystalline, morphological), positively impacting in the resulting devices. The improvement of the efficiency from 1.6% to 6.1% is mainly explained by the large increases on V_OC, obtaining the higher voltage value reported to now in the literature for this hetero-junction. We propose the formation of a metal-insulator-semiconductor (MIS) type device to explain our experimental results, which opens the possibility to use the MIS structure to solve, at least in part, the voltage deficit problems of kesterites.