<p>Perovskite solar cells have achieved remarkable progress in photovoltaic efficiency. However, interfacial defects at the buried and upper interfaces of perovskite layer remain a critical challenge, leading to charge recombination, ion migration, and iodine oxidation. To address this, we propose a novel all-in-one modification strategy employing ammonia borane (BNH₆) as a multifunctional complex. By incorporating BNH₆ at both buried and upper interfaces simultaneously, we achieve dual-interfacial defect passivation and iodide oxidation suppression through three key mechanisms: (1) hydrolysis-induced interaction with SnO₂, (2) coordination with Pb²⁺, and (3) inhibition of I⁻ oxidation. This approach significantly enhances device performance, yielding a champion power conversion efficiency (PCE) of 26.43% (certified 25.98%). Furthermore, the unencapsulated device demonstrates prominent enhanced operation stability, maintaining 90% of its initial PCE after 500&#xa0;h under continuous illumination. Notably, our strategy eliminates the need for separate interface treatments, streamlining fabrication and offering a scalable route toward high-performance perovskite photovoltaics.</p>