Effect of ammonium sulfide treatments on the surface properties of Cu2ZnSnSe4 thin films

In this contribution, we report on the impact of ammonium sulfide ((NH4)2Sx) chemical treatments done at room temperature on the surface properties of Cu2ZnSnSe4 (CZTSe) thin film. The first approach is based on the immersion of the absorber layer in a (NH4)2Sx solution (20 wt.%). This method results in the preferential etching of Se, Sn and Zn atoms from the absorber surface. After a treatment time of 1 min, the performances of the solar cells are found to be improved. The second approach consists in the exposure of the CZTSe layer to ammonium sulfide vapors. In this case, it was found that the progressive sulfurization of the surface of the absorber leads to the decomposition of the CZTSe phase into Sn(S,Se)x and Se0 secondary phases. As a consequence, the short circuit current of the corresponding solar cells is reduced.