LAUSR

Tin‐based halide perovskites have attracted great attention in the perovskite solar cells (PSCs) community with their suitable band gaps, excellent optoelectronic properties, and non‐toxicity. However, because of their poor chemical stability, it is challenging to fabricate highly stable and efficient tin PSCs (TPSCs). In this study, the origin of the Sn2+ oxidation ahead of film formation is concentrated on, and it is found that the ionization of SnI2 in precursor plays a decisive role. Accordingly, SnI2 dissociation and the subsequent Sn2+ oxidation can be restricted in precursor by employing reductive complexes as additives. This dual‐functional source‐regulating strategy effectively helps prepare high‐quality perovskite films with low Sn4+ defect densities. As a result, the unencapsulated TPSCs show a considerable power‐conversion efficiency of 10.03% (certified 9.38%) and maintain 90% of its initial efficiency after 1000 h of light aging testing.