Mitigating Stress‐Induced Nonphotoactive Phase Transition Through Sodium Sulfonate Engineering for Stable and Efficient Perovskite Solar Cells

ABSTRACT Formamidinium lead triiodide (FAPbI 3 ) perovskite solar cells (PSCs) have attracted significant attention due to their outstanding optoelectronic properties. However, their long‐term stability remains limited by lattice strain‐induced transition from the photoactive α‐phase to the nonphotoactive δ‐phase. In this work, first‐principles calculations reveal that the incorporation of Na + into interstitial sites between adjacent FA + cations significantly reduces the formation energy of the α‐phase, thereby promoting its thermodynamic stabilization. Then, experimental results confirm that the introduction of 2 mol% Na + effectively alleviates lattice strain while simultaneously suppressing δ phase. Moreover, the accompanying sulfonate groups interacting with PbI 2 can regulate the crystallization and improve film quality. As a result, the optimized PSC achieved power conversion efficiency (PCE) as high as 26.67% (certificated 26.44%), ranking among the highest reported for the n‐i‐p structured devices. Notably, the bare device without encapsulation retained over 90% of its initial efficiency after continuous heating at 85°C for 1200 h and maintained 80% after 800 h continuous illumination. This study demonstrates that metal cation doping is an effective strategy for stabilizing the perovskite lattice and enhancing long‐term operational stability of perovskite‐based optoelectronic devices.