In Situ Construction of n-Type SnOx Interlayer by Converting Surface Sn4+ 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 Sn2+ to Sn4+, 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 Sn4+ into a functional inorganic semiconducting interlayer, thereby significantly enhancing the efficiency and stability of tin PSCs. Tin(II) chloride dihydrate (SnCl2·2H2O) is introduced onto the as-deposited perovskite surface, where it selectively complexes surface-enriched Sn4+ via Cl- coordination, enabling effective dedoping, while the supplied Sn2+ simultaneously fills tin vacancies. During subsequent thermal annealing, the extracted Sn4+ species undergo hydrolysis triggered by the released water from SnCl2·2H2O, leading to the in situ formation of an n-type SnOx 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.