Brominated Bismaleimides With Aryl Functionalization Toward Efficient Tin Perovskite Solar Cells

Tin perovskite solar cells (Sn‐PSCs) are promising lead‐free photovoltaics, but their performance is limited by facile Sn 2+ oxidation and severe non‐radiative recombination at the buried interface. Herein, a series of tetrabrominated bismaleimides ( MI, 1–4 ) bridged by phenyl ( MIP, 1 ), dimethylphenyl ( MIP’, 2 ), dimethylbiphenyl ( MIBp, 3 ), and tetramethylbiphenyl ( MIBp’, 4 ) units were synthesized and employed as multifunctional interface modifiers at the NiO x /FASnI 3 buried interface. The Lewis‐basic imide carbonyls and bromine substituents jointly improve precursor wetting, enlarge perovskite grains, and passivate the buried interface, suppressing the Sn 4+ fraction from 23.4% to 15.9%, while the aryl bridge tunes the energy‐level alignment for hole extraction. The champion MIBp device delivers a power conversion efficiency of 7.09% with a V OC of 0.551 V, a ~54% improvement over the NiO x control (4.61%), and the unencapsulated device retains 90% of its initial efficiency after 2000 h of storage. This work demonstrates tetrabrominated bismaleimides as a simple, anchoring‐group‐free molecular platform for regulating buried interfaces in lead‐free Sn‐PSCs.