Dual‐Group Surface Functionalization of TiO 2 Enabling Efficient Cd‐free Sb 2 (S,Se) 3 Solar Cells

Sb 2 (S,Se) 3 is a promising absorber for next‐generation thin‐film solar cells owing to its excellent stability, tunable bandgap, and high absorption coefficient. In the device application, CdS is currently the most efficient electron transport layer, while its narrow bandgap (Eg = 2.4 eV) causes parasitic light absorption, limiting device efficiency. Wide‐bandgap TiO 2 (Eg = 3.2 eV) has been considered as a preferable alternative interfacial material, but its chemically inert surface usually leads to poor Sb 2 (S,Se) 3 nucleation, resulting in incomplete film coverage and severe interfacial defects. Herein, we develop a sulfur‐ethylenediamine post‐treatment that introduces both sulfate and amino groups onto TiO 2 surface, which creates high‐density nucleation sites that allow the formation of compact, pinhole‐free Sb 2 (S,Se) 3 films. Simultaneously, the sulfur anchoring suppresses the formation of sulfur‐vacancy deep‐level defects. As a direct consequence, the post‐treated device achieves a champion power conversion efficiency of 9.33%, setting a new record for Cd‐free Sb 2 (S,Se) 3 solar cells. This study establishes a surface‐functionalization strategy that links surface chemistry with nucleation control and defect suppression, offering a viable pathway toward high‐performance and environmentally benign chalcogenide photovoltaics.