Complete DMA ⁺ /Cs ⁺ Exchange and Rapid Phase Conversion via FAAC-Mediated Intermediate Phase Engineering for Efficient and Stable CsPbI ₃ Perovskite Solar Cells

The crystallization kinetics of all-inorganic perovskites critically influence the photovoltaic performance. Here, we introduce formamidinium acetate (FAAC) as a multifunctional additive to promote the conversion of DMAPbI 3 and Cs 4 PbI 6 intermediates into high-quality black-phase γ-CsPbI 3 thin films. FAAC reduces the formation energy of the Cs 4 PbI 6 intermediate, enabling its early formation during initial heating. Concurrently, FA + cations rapidly intercalate into the DMAPbI 3 lattice, forming a mixed (FA, DMA)Pb(I, AC) 3 phase that facilitates complete cation exchange between DMA + and Cs + and accelerates DMAI removal. This synergistic effect effectively reduces residual DMA + and structural defects. The optimized FAAC-based CsPbI 3 perovskite solar cells (PSCs) achieve a power conversion efficiency (PCE) of 21.84%. Furthermore, the unencapsulated devices exhibit excellent operational stability, retaining 90% of their initial efficiency after 500 h under ambient conditions and 88.7% after 120 h of thermal aging at 85 °C.