Topochemical assembly minimizes lattice heterogeneity in polycrystalline halide perovskites

Solution-processable polycrystalline hybrid halide perovskite solar cells have achieved extraordinary efficiencies. However, severe film heterogeneity is prevalent at multiple scales, including composition, lattice structures, and defects, which significantly affects device lifetime. To date, the molecular assembly over lattice-sublattice transformations during film growth is not fully understood. Herein, we reveal the mechanisms of topochemical assembly, wherein a solid-solid transition occurs habitually along the PbI₂/perovskite interface. By introducing intermediates, crystal growth follows an alternative pathway along a different coherent interface. As a result, we obtained an optimal (001)-oriented film with minimized lattice heterogeneity, microstructure defects, and electronic disorder. The corresponding inverted device passed the light-induced degradation test certified by the independent third party following the IEC61215 protocols, which retained over 95% of original power conversion efficiency (PCE) after 500 h (AM 1.5G, one sun). Our work unveils the underlying mechanism that governs perovskite crystal synthesis, which is universally obeyed in two-dimensional and inorganic perovskites.