Synergistic effect of Na and Se on CZTSe solar cells through a soft chemical process

Alkali metal can improve the chalcopyrite structured device performance. However, such alkali metal is normally doped by diffusing Na into absorber layer from soda-lime glass or by vacuum method. In this study, a soft chemical method is applied to dope Na into kesterite structured compound film. Combined by XRD, SEM, SIMS, and EDS mapping, it indicates that the NaF aqueous solution treatment provides a unique Na source, which can promote the film growth and the elements distribution, especially Se element distribution. The surface decomposition of CZTSe is suppressed because of the Na participating in crystallization at the beginning stage of the selenization process, which can improve the interface between CdS and CZTSe. The average conversion efficiency of solar cell is improved from 6.3% without such treatment to above 7.8% with such chemical treatment and the best 8.4% efficiency has been achieved for the 0.005 mol/L NaF treated CZTSe device. This facile Na aqueous solution treatment afford a unique insight and also enlarge the understanding on the Na treatment on compound semiconductor solar cells, which will promote the fabrication of chalcopyrite solar cell with high performance. •A soft chemical method is applied to dope Na into kesterite structured compound film.•NaF aqueous solution treatment provides a unique Na source, which can promote the film growth and elements distribution.•The liquid phase Na2Sex suppresses the surface decomposition of the CZTSe during the high-temperature selenization.•This facile NaF treatment afford a unique insight on the Na treatment on compound semiconductor solar cells.