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.

Keywords: CZTSSe solar cell; Cu‐directional diffusion; VOC loss.