Additive‐Driven Phase Control for Stable and Efficient CsPbI₃ Solar Cells Via Ambient Low‐Temperature Processing

Abstract All‐inorganic cesium lead triiodide perovskite (CsPbI₃) has emerged as a potential pathway to address stability challenges associated with organic components in hybrid‐halide perovskite solar cells. However, the high annealing temperatures required for their photoactive phase formation and the susceptibility to spontaneous phase conversion into a photoinactive phase, accelerated by moisture, remain critical challenges toward commercialisation. A common phase‐stabilising approach involves dimethylammonium iodide (DMAI), but this approach still requires high processing temperatures (>200 °C) and an inert or low‐humidity processing environment. This study introduces butylammonium acetate (BAAc) as an additive, enabling the stabilised phase formation of CsPbI₃ at 160 °C under an ambient processing environment (30–60% relative humidity, RH). BAAc promotes controlled crystallisation, suppresses defects, and facilitates the phase evolution from β‐DMA x Cs 1‐ x PbI 3 to γ‐CsPbI 3 , enhancing processability, efficiency, and stability. PSCs fabricated in 30–60% RH achieve 18.6% best PCE, maintaining >81% efficiency after 1,000 h (ISOS‐L‐1), a seven‐fold improvement over control devices, and >95% efficiency after 10,000 h of storage.