Mitigating Stress-Induced Nonphotoactive Phase Transition Through Sodium Sulfonate Engineering for Stable and...

Formamidinium lead triiodide (FAPbI₃) 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 PbI₂ 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.