Efficiency Enhancement of CZTSSe Solar Cells via Thermal Treatment of (Zn, Mg)O Buffer Layers for Improving Crystallinity and Reducing Point Defects

The application of (Zn, Mg)O buffer layers significantly improves the energy band alignment and the interface quality of the heterojunction of CZTSSe solar cells, leading to a breakthrough in power conversion efficiency (PCE). However, (Zn, Mg)O thin films prepared by sputtering typically exhibit poor crystallinity, limiting their application. Rapid thermal processing (RTP) and substrate heating during the sputtering are investigated to address this issue. Our study demonstrates the effectiveness of RTP in reducing oxygen vacancies (V_O) and adsorbed oxygen (O_ad). Furthermore, it is identified that both thermal treatments increase the Mg_Zn/(Mg_Zn + Zn) ratio of (Zn, Mg)O thin films, thereby increasing their band gap. A notable improvement in the device performance of CZTSSe solar cells, particularly in fill factor (FF) and open-circuit voltage (V_OC), is achieved by adopting optimal thermal treatment processes. Power conversion efficiencies (PCEs) of 12.4% and 12.3% are obtained through RTP and substrate heating, which are remarkably improved compared with the untreated CZTSSe solar cells with the maximum PCE of 9.5%. Notably, 12.4% is the highest PCE for CZTSSe solar cells with (Zn, Mg)O buffers to date.