Recovery of rectifying behavior in Cu2ZnSn(S,Se)4/Zn(O,S) thin-film solar cells by in-situ nitrogen doping of buffer layers

Abstract

The possibility of using N-doping to reduce the carrier concentration of zinc oxysulfide (Zn(O,S)) thin films grown by atomic layer deposition (ALD) was investigated. The effect of N-doping on the structural, chemical, and electrical properties of the Zn(O,S) films was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and Hall measurements. The Zn(O,S) films were applied as alternative n-type buffers replacing conventional CdS for earth-abundant Cu2ZnSn(S,Se)4 thin-film solar cells. With increased O/(O+S) ratio up to ∼0.87–0.90, the fabricated cells suffered severe drops in efficiency, mainly due to the increased shunt conductance coming from excessive carrier concentration of ZnO-like Zn(O,S). Recovery of the rectifying diode behavior and cell efficiency was demonstrated by applying N-doping to the conductive Zn(O,S) buffer layer, further supported by current–voltage and external quantum efficiency measurements.