Downward Homogenized Crystallization for Inverted Wide‐Bandgap Mixed‐Halide Perovskite Solar Cells with 21% Efficiency and Suppressed Photo‐Induced Halide Segregation

Abstract Mixed‐halide perovskite has an irreplaceable role as wide‐bandgap absorber in multi‐junction tandem solar cells. However, large open‐circuit voltage ( V oc ) loss due to non‐uniform halide distribution and compromised device stability due to photo‐induced halide segregation has significantly limited the applications. Here, it is introduced 4‐(2‐aminoethyl)‐benzenesulfonyl fluoride hydrochloride (ABF) with multifunctional groups (sulfonyl, ammonium, and fluoride) to the mixed‐halide precursor to demonstrate a downward homogenized crystallization strategy for suppressing the initial vertical halide phase separation during perovskite crystallization and reducing V oc loss. Furthermore, fluoride with strong electronegativity effectively fixes anions and cations, while sulfonyl and ammonium are used to passivate positive charged (halide vacancies) and negative charged (FA/MA vacancies) defects, respectively, thereby reducing the generation of ion vacancies that lead to subsequent photo‐induced halide segregation. As a result, the 1.63 and 1.68 eV wide‐bandgap perovskite solar cells with inverted structures exhibit the champion power conversion efficiency (PCE) of 21.76% and 20.11% with V oc of 1.18 and 1.21 V, respectively. Most importantly, the optimized devices without encapsulation preserve 86% of initial efficiency after 240 h of continuous illumination under AM 1.5G, showing excellent light stability. Thus, the homogenized crystallization strategy provides highly efficient performance and stability for future tandem solar cell applications.