Surface Microstructural Reconstruction of CdS Buffers Enables Low‐Voltage‐Loss Kesterite Solar Cells With >15% Efficiency

The performance of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells is critically governed by the quality of the CZTSSe/CdS heterojunction; however, the morphology, crystallinity, and defect landscape of CdS buffer layers are intrinsically constrained by the inevitable competition between homogeneous and heterogeneous nucleation in widely used chemical bath deposition (CBD). Here, we report a simple yet effective surface microstructural reconstruction strategy based on chemical polishing that overcomes these challenges beyond the reach of conventional CBD process regulation. Specifically, polishing CZTSSe/CdS films with a Na 2 S/thiourea aqueous solution selectively removes low‐quality CdS particulates while inducing surface recrystallization and sulfur‐vacancy compensation. As a result, the CdS films exhibit markedly improved microstructure, enhanced crystallinity, and more homogeneous surface electrical properties. Benefiting from suppressed interfacial charge recombination and accelerated charge transport, kesterite solar cells achieve a champion efficiency of 15.3% with a high open‐circuit voltage ( V OC ) of 560 mV and a record‐low V OC deficit ( E g / e ‐ V OC ) of < 0.5 V, significantly advancing kesterite photovoltaics toward low voltage loss. Moreover, this work establishes a broadly applicable post‐deposition paradigm for improving CBD‐CdS–based optoelectronic devices across a wide range of material systems and applications.