Researchers have developed a method to fabricate high-quality formamidinium lead iodide (FAPbI₃) perovskite layers through intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI₂ with formamidinium iodide (FAI). This process produces FAPbI₃ films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI₂. Using these films, FAPbI₃-based perovskite solar cells (PSCs) with maximum power conversion efficiency (PCE) exceeding 20% were fabricated. FAPbI₃ has a broader solar spectrum absorption than methylammonium lead iodide (MAPbI₃), making it potentially more efficient. However, forming stable FAPbI₃ phases and high-quality films is more challenging than with MAPbI₃. Various methods, including sequential deposition, solvent engineering, and vapor-assisted deposition, have been used to produce high-quality MAPbI₃ films. However, these methods have not been as effective for FAPbI₃. The intramolecular exchange process involves replacing DMSO molecules in PbI₂ with FAI, leading to the formation of FAPbI₃. This process produces high-quality, dense, and uniform FAPbI₃ films with preferred (111) orientation. The resulting FAPbI₃-based PSCs showed high PCE, with a certified PCE of 20.2% under standard AM 1.5G illumination. The films exhibited excellent performance with minimal hysteresis, indicating their stability and efficiency. The study demonstrates that the intramolecular exchange process is an effective method for fabricating high-quality FAPbI₃ films, leading to efficient and stable PSCs. This method offers a promising approach for developing high-performance perovskite solar cells with improved efficiency and stability.Researchers have developed a method to fabricate high-quality formamidinium lead iodide (FAPbI₃) perovskite layers through intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI₂ with formamidinium iodide (FAI). This process produces FAPbI₃ films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI₂. Using these films, FAPbI₃-based perovskite solar cells (PSCs) with maximum power conversion efficiency (PCE) exceeding 20% were fabricated. FAPbI₃ has a broader solar spectrum absorption than methylammonium lead iodide (MAPbI₃), making it potentially more efficient. However, forming stable FAPbI₃ phases and high-quality films is more challenging than with MAPbI₃. Various methods, including sequential deposition, solvent engineering, and vapor-assisted deposition, have been used to produce high-quality MAPbI₃ films. However, these methods have not been as effective for FAPbI₃. The intramolecular exchange process involves replacing DMSO molecules in PbI₂ with FAI, leading to the formation of FAPbI₃. This process produces high-quality, dense, and uniform FAPbI₃ films with preferred (111) orientation. The resulting FAPbI₃-based PSCs showed high PCE, with a certified PCE of 20.2% under standard AM 1.5G illumination. The films exhibited excellent performance with minimal hysteresis, indicating their stability and efficiency. The study demonstrates that the intramolecular exchange process is an effective method for fabricating high-quality FAPbI₃ films, leading to efficient and stable PSCs. This method offers a promising approach for developing high-performance perovskite solar cells with improved efficiency and stability.