Diminishing Defect and Ion Migration in Perovskite‐Organic Tandem Solar Cells by Ordered Anion‐Cation Modification Approach

ABSTRACT The efficency and stability of perovskite/organic tandem solar cells are fundamentally limited by two interconnected challenges in wide bandgap (WBG) perovskite top cells, namely high‐density surface defects and photo‐induced halide phase segregation. Both issues originate from anionic vacancies—specifically halogen vacancies—which act as shallow traps and serve as sites for ion migration, undermining device performance and stability. Herein, we demonstrate a novel ordered anion‐cation modification (OACM) approach—anionic vacancies are repaired before cation passivation. This order is determined by kinetic accessibility, where small and mobile Br − ions can easily diffuse into the lattice to fill vacancies. If a molecular passivation layer is deposited first (reversed sequence), a uniform barrier forms that blocks subsequent anion repair, significantly degrading device efficiency. By applying the OACM approach, we fabricated a 1.83 eV WBG perovskite solar cell that achieves an open‐circuit voltage of 1.33 V and a stabilized efficiency of 19.21%, while suppressing photo‐induced phase segregation. When integrated into perovskite/organic tandem devices, the stabilized top cell yields a champion efficiency of 26.20% with an improvement in storage stability. This work contributes to developing a sequence‐defined design principle for defect management in perovskite photovoltaics, transforming interface engineering from empirical trial‐and‐error into a rational, kinetically guided strategy.