This study presents an innovative method for fabricating high-performance formamidinium lead iodide (FAPbI3) perovskite layers through intramolecular exchange (IEP) of dimethylsulfoxide (DMSO) molecules intercalated in lead iodide (PbI2) with formamidinium iodide (FAI). The IEP process results in FAPbI3 films with a (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using this technique, FAPbI3-based perovskite solar cells (PSCs) were fabricated, achieving a maximum power conversion efficiency (PCE) of over 20%. The study highlights the advantages of the IEP method over conventional sequential deposition processes, which often lead to volume expansion and less uniform films. The IEP process not only improves film quality but also enhances device performance, making it a promising approach for the development of high-efficiency PSCs.This study presents an innovative method for fabricating high-performance formamidinium lead iodide (FAPbI3) perovskite layers through intramolecular exchange (IEP) of dimethylsulfoxide (DMSO) molecules intercalated in lead iodide (PbI2) with formamidinium iodide (FAI). The IEP process results in FAPbI3 films with a (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using this technique, FAPbI3-based perovskite solar cells (PSCs) were fabricated, achieving a maximum power conversion efficiency (PCE) of over 20%. The study highlights the advantages of the IEP method over conventional sequential deposition processes, which often lead to volume expansion and less uniform films. The IEP process not only improves film quality but also enhances device performance, making it a promising approach for the development of high-efficiency PSCs.