Complete DMA+/Cs+ Exchange and Rapid Phase Conversion via FAAC-Mediated Intermediate Phase Engineering for Efficient and Stable CsPbI3 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 DMAPbI3 and Cs4PbI6 intermediates into high-quality black-phase γ-CsPbI3 thin films. FAAC reduces the formation energy of the Cs4PbI6 intermediate, enabling its early formation during initial heating. Concurrently, FA+ cations rapidly intercalate into the DMAPbI3 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 CsPbI3 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.