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(Cs0.05FA0.8MA0.15)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.