Conformationally Stable and Sterically Hindered Bicyclo[1.1.1]pentane‐1,3‐diammonium Modification of FAPbI₃ Enhances the Performance of Perovskite Solar Cells

Abstract Solution‐processed α ‐FAPbI 3 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 PbI 2 and δ ‐FAPbI 3 into a two‐dimensional perovskite structure and significantly enhanced the crystal quality of α ‐FAPbI 3 . 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 BCPDA 2+ confers greater binding energy at the perovskite surface and elevates the diffusion activation energy of the iodide anion.