In Situ Construction of n-Type SnO_x 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 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.