Homogeneous and Graded Ag Alloying in (Cu 1‐ x Ag x ) 2 ZnSnSe 4 Solar Cells

Cu$_2$ZnSn(S,Se)$_4$ (CZTSSE)-based solar cell performances are limited by band tailing due to a large amount of CuZ$_n$ antisite defects. Partially replacing the Cu atoms by larger Ag ones can significantly reduce the prevalence of these defects,which are particularly detrimental close to the front interface. Herein, the possibility of synthesizing (Cu$_{1‐x}$Ag$_x$)$_2$ZnSnSe$_4$ absorbers with various Ag contents by vacuum-based processes is demonstrated. Although the synthesis of high-quality materials is demonstrated, their use in thin film photovoltaic devices does not exhibit performance improvement compared with efficient pure CZTSSE-based solar cells. Moreover, the comparison with literature data reopens the debate of the beneficial effect of homogeneous Ag alloying in kesterite. On the contrary, a new method is proposed to fabricate graded (Cu$_{1‐x}$Ag$_x$)$_2$ZnSnSe$_4$) absorbers with increased Ag content at the interfaces. The solar cells with graded absorbers exhibit better performances than the reference Ag-free ones. Particularly, improved current collection at the back contact and slight reduction of the front interface recombination are demonstrated