Sodium-assisted passivation of grain boundaries and defects in Cu2ZnSnSe4 thin films

Quite a long stagnation of photo-conversion efficiency of kesterites below 13% casts frustration onto the community. In this study, we investigated effects of sodium for passivation of grain boundaries and defects in Cu2ZnSnSe4 (CZTSe) grown on soda-lime glass (SLG) and borosilicate (BS) glass. Since BS glass does have no sodium inherently, we put a NaF thin-layer between CZTSe and Mo. The composition of the samples is Cu-poor and Zn-rich. Distribution of sodium and its contribution to phase formation and defects are examined by cross-sectional energy-dispersive X-ray profiling, Raman scattering spectroscopy and imaging, surface potential and photoluminescence. From the experimental results, it is strongly claimed that sodium ions segregate predominantly near grain boundaries and reduce CuZn related defects. Further studies with distinct alkali ions and optimal processing conditions will pave a way to improve the performance of kesterites solar cells.