Defect passivation in Cu2ZnSnSe4 thin-film solar cells by novel sodium doping

Abstract Sodium (Na) alkali doping is one of the effective ways to improve Cu2ZnSnSe4 (CZTSe) device performance by passivating detrimental native defects caused by off-stoichiometric composition and Cu–Zn cationic disordering. Various methods to incorporate Na in CZTSe films have been considered to realize the ideal distribution of Na in CZTSe films. Na content, regardless of Na incorporation method, is significantly increased in bulk CZTSe. Interestingly, when a NaF precursor is supplied after CZTSe film growth, the surface Na content is slightly increased, resulting in improved device performance. For the first time, a sodium bifacial treatment was employed to obtain an optimal profile of Na in CZTSe films. Using this novel route for Na doping, the photovoltaic (PV) device exhibited a maximum efficiency of 6.02%, which was higher than that compared to CZTSe devices with single Na doping and CZTSe device with Na diffused from soda-lime glass. According to defect analyses, the improved device performance is associated with a reduction in the defect density from 1.1 × 1016 (without Na doing) to 5.7 × 1014 cm−3 (with optimal Na profile). These results indicate that the proposed bifacial treatment method is a desirable means to fabricate high-efficiency, flexible CZTSe solar cells.