Photo-excited carrier transport and secondary phases of Na-engineered kesterite flexible thin films

Kesterite [Cu2ZnSn(S,Se)4 (CZTSSe)], which is one of the most promising solar cell materials, can be used as a light absorber owing to its suitable optical and electrical properties. In this study, the optimization process of NaF layers was performed in the deposition of the precursor stacks of Sn/Cu/Zn/Mo to form CZTSSe thin films. The distribution of the secondary phases on the CZTSSe surfaces strongly depends on the insertion of the NaF layer. In particular, the Cu2-xS binary phase increases with the closeness of the insertion location of the NaF layer to the front side. Moreover, the grain boundary properties can be altered by varying the deposition conditions since Na segregation and defect formation affect the surface electrical properties and carrier transport mechanism. Photo -assisted conductive-atomic force microscopy was used to measure the local variation in the surface photocurrent flow into the CZTSSe grains, and the incorporation of sodium increased the surface current formation. This study exhibits the effects of Na on the optimal growth of kesterite formation and carrier transport of the relevant photovoltaic devices.