Alkyl Fluorination Aromatic Acetamidine Cation Enhances the Lattice Stability and Performance of 2D Ruddlesden‐Popper Perovskite Solar Cells

Fluorinated organic cations are used to enhance the power conversion efficiency (PCE) of quasi-2D Ruddlesden-Popper perovskite solar cells (2DRP PSCs), however, most of them focus fluorine substitution on the benzene ring. Herein, a novel α-fluorophenylethanimidamide (α-FPEIA) spacer cation with a fluorine atom at α-carbon of the amidine group is designed and employed to enhance the performance and stability of 2DRP PSCs. The single-crystal structure of (α-FPEIA)₂PbI₄ exhibits stronger interaction between the organic cations and inorganic skeleton, with a larger angle of Pb-I-Pb, thus benefiting the charge transfer and improving lattice stability of perovskite film. Furthermore, the theoretical results indicate that (α-FPEIA)₂PbI₄ exhibits a reduction in the quantum confinement effect, which is conducive to carrier transfer. Additionally, the α-FPEIA-based perovskite film exhibits excellent crystal quality, suitable energy level arrangement, decreased trap states, and extended carrier lifetime. These enhancements boost the PCE of (α-FPEIA)₂(Cs_0.05FA_0.8MA_0.15)₄Pb₅I₁₆ (n = 5) device reaching 18.15% (vs phenethylammonium (PEA)-based PSCs of 15.38%) along with improving the stability of unencapsulated PSCs. The findings demonstrate that introducing a fluorine atom into the alkyl group of the PEA and replacing the ammonium with the amidine group represents a novel approach for fabricating high-performance and stable inverted 2DRP PSCs.