A Buried Functional Layer for Inorganic CsPb_0.75Sn_0.25I₂Br Perovskite Solar Cells

The enlightening inorganic Sn‐based metal halide perovskites hold promise for environment‐friendly and efficient energy conversion. However, the undesired Sn²⁺ oxidation and uncontrollable crystallization of the perovskite absorber slow the development of highly efficient Sn‐based inorganic perovskite solar cells. Herein, an ionic liquid layer of 1‐butylpyridinium bromide (BPB) is employed as a buried functional template for the growth of the inorganic CsPb_0.75Sn_0.25I₂Br perovskite absorber. The buried functional layer provides lone electron pairs from N atom to coordinate with the unsaturated metal ions (Pb and Sn) via the coupling effect. In addition, the electronegative atom from the hydrogen bond acceptor offers an electron‐rich environment for the perovskite growth to suppress Sn²⁺ oxidation. More importantly, this positive effect transduces from the interface to the bulk perovskite growth, leading to enhanced crystallinity and thus reduced nonradiative trap defects. Consequently, the efficiency of the inorganic CsPb_0.75Sn_0.25I₂Br PSCs is improved from 6.80% to 11.28%, and the unencapsulated device exhibits superior ambient stability, maintaining 62% of its initial power conversion efficiency in dried air for 200 h. The buried ionic liquid functional layer approach provides an avenue for the development of high‐efficiency Sn‐based optoelectronics.