S/Ge dual gradient spontaneous construction for improved CZTSSe solar cells

A key reason for Cu₂ZnSn(S,Se)₄ (CZTSSe, 15.8%) solar cells lagging far behind Cu(In,Ga)Se₂ (CIGS, 23.6%) in efficiency is its inability to autonomously form a dual-gradient bandgap via Ga gradient, critical for the simultaneous efficient light absorption and directed carrier transport. Herein, this study proposes a novel strategy for the spontaneous construction of dual gradients CZTSSe with a S-rich front interface and a Ge-rich back interface based on SnS-GeSe co-sulfoselenization. SnS releases S vapor and Sn₂Se₃ intermediate phase during selenization, synchronously compensating for Sn volatilization loss and forming a S-rich surface layer, leading to a synergistic composition stability and interface defect passivation. Meanwhile, GeSe, by virtue of its eutectic property, promotes the migration and enrichment of Ge toward the back interface for an efficient back surface field and suppressing defects. The S-rich front widens the surface bandgap to improve open-circuit voltage (<em>V</em>_OC), and the Ge-rich back elevates the back conduction band minimum (CBM) and suppresses the Sn-related defect to facilitate carrier transport. As a positive result, the optimized devices achieve a 26% enhancement in photovoltaic efficiency, offering a new insight for the development of high-efficiency kesterite-based solar cells.