Spatial Ionic Engineering Enables Depth‐Independent Homogeneity and Strain Relaxation in Efficient Inverted Perovskite Solar Cells

ABSTRACT Mixed‐halide perovskites are central to high‐efficiency inverted perovskite solar cells (PSCs), yet the spatial inhomogeneity of multiple halides remains an unresolved challenge that induces strain, defect formation, and interfacial recombination. Here, we uncovered the depth‐dependent distribution of halide ions within a triple‐halide perovskite, wherein the bromine content progressively increases with increasing depth, while chloride contents are preferentially enriched near both the top and bottom interfaces. To rectify such compositional gradients, we introduced methylammonium halides (MAX; X = I − , Br − , Cl − ) post‐treatments, with MABr identified as the most efficient agent for reestablishing halide compositional uniformity. Depth‐resolved analyses demonstrated that MABr treatment effectively homogenizes the halide distribution across the perovskite film, thereby alleviating lattice strain and promoting enhanced crystallographic order. The relaxation of lattice strain drives epitaxial‐like crystallization, producing films with reinforced (100) orientation, enlarged and fully merged grains. Moreover, the treatment‐induced n‐type surface enables an energetically favorable interface with C 60 , which promotes effective hole extraction. Such structural and interfacial refinement translate into a champion efficiency of 25.72% and superior operational stability, retaining over 96% of the initial efficiency after 700 h of continuous illumination. Overall, MABr‐mediated ionic homogenization provides a universal framework for composition‐strain coupling, enabling scalable and intrinsically stable multi‐halide perovskite photovoltaics.