• An all-in-one modification strategy using ammonia borane (BNH6) simultaneously at both buried and upper interfaces of perovskite solar cells achieves dual-interfacial defect passivation and iodide oxidation suppression.
• BNH6 interacts with SnO2 via hydrolysis, coordinates with Pb2+, and inhibits I− oxidation, addressing key degradation mechanisms.
• The optimized devices achieve a champion efficiency of 26.43% (certified 25.98%) with negligible hysteresis and enhanced thermal and light stability.
• This strategy simplifies fabrication by eliminating separate interface treatments, offering a scalable route for high-performance perovskite photovoltaics.