Suppressing Element Inhomogeneity Enables 14.9% Efficiency CZTSSe Solar Cells

Abstract Kesterites, Cu₂ZnSn(S_xSe_1−x)₄ (CZTSSe), solar cells suffer from severe open‐circuit voltage (V_OC) loss due to the numerous secondary phases and defects. The prevailing notion attributes this issue to Sn‐loss during the selenization. However, this work unveils that, instead of Sn‐loss, elemental inhomogeneity caused by Cu‐directional diffusion toward Mo(S,Se)₂ layer is the critical factor in the formation of secondary phases and defects. This diffusion decreases the Cu/(Zn+Sn) ratio to 53% at the bottom fine‐grain layer, increasing the Sn‐/Zn‐related bulk defects. By suppressing the Cu‐directional diffusion with a blocking layer, the crystal quality is effectively improved and the defect density is reduced, leading to a remarkable photovoltaic coversion efficiency (PCE) of 14.9% with a V_OC of 576 mV and a certified efficiency of 14.6%. The findings provide insights into element inhomogeneity, holding significant potential to advance the development of CZTSSe solar cells.