Passivation of CdS/Cu₂ZnSnS₄ Interface from Surface Treatments of Kesterite‐Based Thin‐Film Solar Cells

Abstract

Surface treatments of Cu 2 ZnSnS 4 have shown a beneficial effect on the solar cells performance due to a reduction in the open‐circuit voltage ( V OC ) deficit. Several reasons have been suggested for the V OC deficit, including an unfavorable band alignment at the buffer/Cu 2 ZnSnS 4 interface. Herein, the influence of Cu 2 ZnSnS 4 surface treatment (air exposure and air anneal) on the electronic and chemical properties of Cu 2 ZnSnS 4 and CdS/Cu 2 ZnSnS 4 interfaces is investigated. Using hard X‐ray photoelectron spectroscopy, it is shown that the band alignment at the CdS/Cu 2 ZnSnS 4 interface is not significantly altered by the applied surface treatment. The device enhancement is instead connected to interface passivation for the surface‐treated Cu 2 ZnSnS 4 samples due to the formation of SnO x , which is shown to not be fully removed upon KCN etching prior to the buffer layer deposition. In addition, a surface treatment of the Cu 2 ZnSnS 4 absorber prior to buffer layer deposition influences the growth of CdS buffer, as a thicker CdS‐overlayer is observed to grow on a surface‐treated Cu 2 ZnSnS 4 sample as compared with a nontreated sample. This suggests that a reoptimization of the CdS thickness for a given Cu 2 ZnSnS 4 surface treatment is required.