4‐Methylpyridine‐Mediated Homogenization of Wide‐Bandgap Perovskite Films for Efficient All‐Perovskite Tandem Solar Cells

ABSTRACT Mixed‐halide wide‐bandgap (WBG) perovskites are promising top‑cell materials for multi‐junction photovoltaics owing to their tunable bandgap and excellent photoelectronic properties. However, their solution processing often suffers from mismatched crystallization kinetics between iodine and bromine species, leading to compositional inhomogeneity and limited device performance. Herein, we report a solvent engineering strategy by introducing 4‐methylpyridine (4‐MePy) as a coordinating modulator. 4‐MePy possesses strong coordinating ability and a moderate boiling point. It selectively retards the rapid crystallization of bromine‐rich components by interacting more strongly with lead bromide, thereby homogenizing the halide distribution. The resulting perovskite films exhibit low defect density, reduced lattice strain, and uniform composition and morphology. These improvements suppress carrier recombination and increase the halide migration barrier. Consequently, single‐junction WBG cells with a bandgap of 1.77 eV achieve a champion power conversion efficiency (PCE) of 20.68% and a high open‑circuit voltage ( V OC ) of 1.35 V. When integrated into all‐perovskite tandem solar cells, this strategy delivers PCEs of 29.70% (certified 29.17%) on 0.05 cm 2 and 29.00% on 1 cm 2 devices.