Rear Interface Modification by the ZnTe Layer Enables High-Efficient Cu2(Zn,Cd)SnS4 Thin-Film Solar Cells

The backside reaction between the Cu2(Cd,Zn)SnS4 (CCZTS) film and Mo back contact layer is detrimental to solar cell performance and is seemingly unavoidable for a long time since this reaction is thermodynamically driven by the high-temperature annealing process used in light absorber formation. In this work, a zinc telluride (ZnTe) passivation layer is introduced between the precursor and metallic back contact to suppress the back reaction. The effects of the ZnTe layer on the crystallized absorber after annealing and resulted devices are carefully studied. First, the quality of the back contact is improved due to the efficient elimination of secondary phases (including MoS2, SnS, and ZnS) and voids as well as shunting pathways. Second, the open-circuit voltage (VOC) and the fill factor (FF) are improved from 598 to 607 mV and 60.5 to 65.50%, respectively. Finally, the highest efficiency of 9.89% for CCZTS solar cells without an antireflection layer is achieved with an intentional insertion of ∼5 nm ZnTe layer between the CCZTS precursor and Mo, with a nearly 9% enhancement.