The supporting information for the article "Room Temperature Crystallized Phase-Pure α-FAPbI3 Perovskite with In-Situ Grain-Boundary Passivation" provides detailed experimental methods, characterization techniques, and supplementary notes. The study focuses on the synthesis and characterization of phase-pure α-FAPbI3 perovskite films at room temperature, with an emphasis on in-situ grain-boundary passivation. Key aspects include: 1. **Materials and Synthesis**: The materials used, such as ITO glasses, various chemicals, and solvents, are described. The synthesis of 3,4,5-trifluoroaniline iodide (TFAI) is detailed, involving the reaction of 3,4,5-Trifluoroaniline with hydroiodic acid. 2. **Device Fabrication**: The fabrication process for the perovskite solar cells (PSCs) is outlined, including the cleaning and preparation of ITO substrates, the deposition of SnO₂ layer, and the spin-coating of perovskite solution. The addition of TFAI at different concentrations is also discussed. 3. **Characterization**: Various characterization techniques, such as SEM, EDX, GIWAXS, XRD, FTIR, ToF-SIMS, UPS, XPS, C-AFM, absorption spectroscopy, PL mapping, and electrochemical measurements, are used to analyze the perovskite films and devices. Detailed results and figures are provided to illustrate the effects of TFAI incorporation on the film morphology, crystal structure, and performance. 4. **Supplementary Notes**: Additional notes cover topics such as the energy release during phase conversion, micro strain calculation from Williamson-Hall plots, Urbach energy calculation, TRPL carrier lifetime fitting, Mott-Schottky derived built-in potential, and SCLC (Space-Charge Limited Current) calculation. The supporting information aims to provide a comprehensive understanding of the experimental setup, material properties, and device performance, supporting the main findings of the study.The supporting information for the article "Room Temperature Crystallized Phase-Pure α-FAPbI3 Perovskite with In-Situ Grain-Boundary Passivation" provides detailed experimental methods, characterization techniques, and supplementary notes. The study focuses on the synthesis and characterization of phase-pure α-FAPbI3 perovskite films at room temperature, with an emphasis on in-situ grain-boundary passivation. Key aspects include: 1. **Materials and Synthesis**: The materials used, such as ITO glasses, various chemicals, and solvents, are described. The synthesis of 3,4,5-trifluoroaniline iodide (TFAI) is detailed, involving the reaction of 3,4,5-Trifluoroaniline with hydroiodic acid. 2. **Device Fabrication**: The fabrication process for the perovskite solar cells (PSCs) is outlined, including the cleaning and preparation of ITO substrates, the deposition of SnO₂ layer, and the spin-coating of perovskite solution. The addition of TFAI at different concentrations is also discussed. 3. **Characterization**: Various characterization techniques, such as SEM, EDX, GIWAXS, XRD, FTIR, ToF-SIMS, UPS, XPS, C-AFM, absorption spectroscopy, PL mapping, and electrochemical measurements, are used to analyze the perovskite films and devices. Detailed results and figures are provided to illustrate the effects of TFAI incorporation on the film morphology, crystal structure, and performance. 4. **Supplementary Notes**: Additional notes cover topics such as the energy release during phase conversion, micro strain calculation from Williamson-Hall plots, Urbach energy calculation, TRPL carrier lifetime fitting, Mott-Schottky derived built-in potential, and SCLC (Space-Charge Limited Current) calculation. The supporting information aims to provide a comprehensive understanding of the experimental setup, material properties, and device performance, supporting the main findings of the study.