A Universal Grain “Cage” to Suppress Halide Segregation of Mixed-Halide Inorganic Perovskite Solar Cells

Bandgap-tunable mixed-halide perovskites offer exciting opportunities to construct efficient multijunction tandem solar cells. However, the ion migration always causes halide segregation, which inevitably creates detrimental defects and deteriorates the photovoltaic performances. Here, we report a universal caging strategy to suppress halide segregation by in situ formation of conjugated covalent organic frameworks (COFs) catalyzed by PbX2 (X = Br and I) during the formation of mixed-halide perovskite. Through theoretical calculation and systematic investigation, the strong electron-donating feature of COFs is shown to effectively solidify the soft lattice and impede the iodide ion transport from bulk to grain boundary, decelerating the light-induced halide-demixing process. Finally, the nonradiative recombination is significantly reduced, boosting efficiency up to 11.50% for an inorganic CsPbIBr2 perovskite solar cell and 14.35% for a CsPbI2Br cell with a prolonged shelf life and an improved photostability.