Dual-Functional GeSe–Se Coselenization Enabling Synergistic Defect-Interface Engineering for High-Efficiency Electrodeposited Flexible CZTSe Solar Cells

Flexible Cu₂ZnSnSe₄ (CZTSe) solar cells hold great potential for low-cost green fabrication and portable applications, yet electrodeposited devices suffer from low efficiency (∼6% vs 12.84% for solution-processed ones), primarily due to defect-induced nonradiative recombination and carrier loss at back interfaces. Herein, a dual-functional GeSe-Se coselenization strategy is proposed to simultaneously achieve defect regulation and back-interface engineering. Ge substitution for Sn during selenization induces lattice contraction, effectively suppressing Sn-related deep defects and band-tail states while minimizing the secondary phase. Simultaneously, Ge diffuses into the MoSe₂ interface layer to optimize the energy-level alignment and reduce nonradiative recombination. Consequently, the optimized flexible CZTSe solar cells achieve a record efficiency of 9.01%, the highest among electrodeposited flexible CZTSe devices. This study elucidates the synergistic role of Ge in simultaneously mitigating bulk defects and refining interfacial energetics, highlighting a remarkable achievement for electrodeposition-based flexible CZTSe solar cells.