Suppressing Interfacial Deprotonation and Ion Migration via a Proton‐Ion Capture Strategy for Stable and Efficient Perovskite Solar Cells

Abstract Perovskite solar cells (PSCs) have achieved rapid efficiency improvements, yet their practical deployment is hindered by intrinsic instability, as environmental stress‐particularly thermal and light exposure‐not only induces halide ion migration but also triggers deprotonation of organic cations, ultimately accelerating material degradation. Here, a proton‐ion capture (PIC) strategy is proposed to suppress interfacial proton loss and halide migration by introducing inorganic potassium salts at the buried interface. Anionic groups capture protons and coordinate with organic cations, while released potassium cation (K + ) coordinates with iodine anion (I − ), forming a synergistic ionic environment that suppresses proton‐loss‐related degradation and stabilizes the perovskite structure. Experimental characterizations demonstrate that this strategy suppresses interfacial voids, promotes oriented crystal growth, and alleviates residual strain in perovskite films, while experimental and theoretical analyses confirm the reduction in interfacial defects. The optimized interfacial energy level alignment and improved carrier transport efficiency across the interface are also confirmed. As a result, the champion PSC achieves a power conversion efficiency (PCE) of 25.1% with an extremely low open‐circuit voltage ( V OC ) loss of 0.35 V. The as‐prepared device shows enhanced long‐term stability, retaining 92.8% of its initial efficiency after 2000 h in an N 2 glove box (ISOS‐D‐1I) and 89.6% following 800 h of continuous illumination (ISOS‐L‐1).