Optimization of Zn1-xSnxO Buffer Layer for Application in CZTSe Solar Cells with H2-Assisted Reactive Sputtering

CdS thin films are commonly used as buffer layer in Cu2ZnSnSe4 (CZTSe) solar cells inherited from CuInGaSe2 (CIGS) solar cells. However, in addition to the toxicity problems with Cd in conventional CdS buffer layers, the unfavorable band alignment at the CZTSe/CdS heterojunction confines the further improvement of CZTSe solar cells. ZnSnO (ZTO) is studied in this work as promising buffer layers for CZTSe solar cells. The stoichiometric composition and thickness of the ZTO layer are first optimized by sputtering. Subsequently, the improvement of the performance is demonstrated by sputtering ZTO in Ar gas mixed with H-2. The effect of H-2 on sputtering ZTO is investigated. The presence of O vacancies in ZTO buffer layer and its effect on carrier transport and device performance are presented. Through optimization, comparable V-OC and higher fill factor (FF) are obtained for the CZTSe/ZTO solar cells compared with CZTSe/CdS reference.