In Situ Construction of n‐Type SnO x Interlayer by Converting Surface Sn ⁴⁺ for Efficient Tin Perovskite Solar Cells

Tin perovskites are promising lead‐free candidates for perovskite solar cells (PSCs), yet their practical application is hindered by the facile oxidation of Sn 2+ to Sn 4+ , which induces p‐type self‐doping, defect accumulation, and progressive surface‐to‐bulk degradation. Here, we propose a novel surface treatment strategy to convert detrimental surface Sn 4+ into a functional inorganic semiconducting interlayer, thereby significantly enhancing the efficiency and stability of tin PSCs. Tin(II) chloride dihydrate (SnCl 2 ·2H 2 O) is introduced onto the as‐deposited perovskite surface, where it selectively complexes surface‐enriched Sn 4+ via Cl − coordination, enabling effective dedoping, while the supplied Sn 2+ simultaneously fills tin vacancies. During subsequent thermal annealing, the extracted Sn 4+ species undergo hydrolysis triggered by the released water from SnCl 2 ·2H 2 O, leading to the in situ formation of an n‐type SnO x layer. This process effectively suppresses non‐radiative recombination, mitigates film degradation, and promotes charge extraction. Consequently, the power conversion efficiency of tin PSCs increases from 13.21% to 16.02%, while the device retains 95% of its maximum efficiency after 75 days of storage and exhibits negligible degradation under continuous one‐sun illumination for 307 h. This work provides a new surface engineering paradigm to realize efficient and stable tin PSCs.