Highly Efficient and Stable FA‐Based Quasi‐2D Ruddlesden–Popper Perovskite Solar Cells by the Incorporation of β‐Fluorophenylethanamine Cations

2D Ruddlesden-Popper (2D RP) perovskite, with attractive environmental and structural stability, has shown great application in perovskite solar cells (PSCs). However, the relatively inferior photovoltaic efficiencies of 2D PSCs limit their further application. To address this issue, β-​fluorophenylethanamine (β-​FPEA) as a novel spacer cation is designed and employed to develop stable and efficient quasi-2D RP PSCs. The strong dipole moment of the β-​FPEA enhances the interactions between the cations and PbI6 4- octahedra, thus improving the charge dissociation of quasi-2D RP perovskite. Additionally, the introduction of the β-​FPEA cation optimizes the energy level alignment, improves the crystallinity, stabilizes both the mixed phase and a-FAPbI3 phase of the quasi-2D RP perovskite film, prolongs the carrier diffusion length, increases the carrier lifetime and decreases the trap density. By incorporating the β-​FPEA, the quasi-2D RP PSCs exhibit a power conversion efficiency (PCE) of 16.77% (vs phenylethylammonium (PEA)-based quasi-2D RP PSCs of 12.81%) on PEDOT:PSS substrate and achieve a champion PCE of 19.11% on the PTAA substrate. It is worth noting that the unencapsulated β-​FPEA-based quasi-2D RP PSCs exhibit considerably improved thermal and moisture stability. These findings provide an effective strategy for developing novel spacer cations for high-performance 2D RP PSCs.