Researchers at Purdue University has reported that achieving long-term operational stability in halide perovskite solar cells had remained a major barrier to commercialization. The study examined ionic liquids as bulk modifiers and focused on clarifying their role during perovskite crystallization. The team engineered methoxyethoxymethyl-1-methylimidazole chloride (MEM-MIM-Cl) with an ethylene glycol ether side chain to regulate crystal growth and stabilize buried interfaces through interactions with NiOx. MEM-MIM-Cl was found to form a distinct intermediate phase by chelating undercoordinated Pb(II), which reduced defect formation and defect-related degradation. Devices incorporating MEM-MIM-Cl reached a power conversion efficiency of 25.9% and retained about 90% of initial performance after 1,500 h under continuous 1-sun illumination at 90 °C. Diurnal cyclic ageing tests also indicated strong resistance to performance fatigue.