Solution-processed α-FAPbI3 perovskite films frequently exhibit structural defects and impurities that impede the durable operation of solar cells. In this study, we introduced a conformationally stable, sterically bulky molecular passivator, bicyclo[1.1.1]pentane-1,3-diammonium iodide (BCPDAI), by spin-coating from an isopropanol solution onto the perovskite film surface, followed by thermal annealing. This treatment effectively converted PbI2 and δ-FAPbI3 into a two-dimensional perovskite structure and significantly enhanced the crystal quality of α-FAPbI3. The BCPDAI-treated perovskite films exhibited smoother surface morphology and reduced trap densities for charge carriers, leading to improved power conversion efficiency in the solar cells. Notably, the BCPDAI-modified perovskite films provided the solar cells with enhanced operational stability. Theoretical calculations demonstrated that the high positive charge density of BCPDA2+ confers greater binding energy at the perovskite surface and elevates the diffusion activation energy of the iodide anion.