Fluorinated Aromatic Formamidinium Spacers Boost Efficiency of Layered Ruddlesden–Popper Perovskite Solar Cells

Layered two-dimensional (2D) perovskites are emerging photovoltaic materials due to their good environmental and structural stability thanks to the bulky organic spacers incorporated in the crystal lattice. Formamidine (FA) is an indispensable organic cation in high-performance 3D perovskite materials, whereas FA derivative-based spacers have remained largely unexplored in 2D perovskite. Here, we demonstrated a class of aromatic formamidinium (ArFA) spacers, namely, benzamidine (PhFA) and para-fluorobenzamidine (p-FPhFA), for efficient 2D Ruddlesden–Popper (RP) perovskite solar cells. It is found that the 2D perovskite with the fluorinated spacer p-FPhFA shows significantly improved charge carrier lifetime, enhanced mobility, and reduced trap density in comparison with an unfluorinated PhFA spacer. As a result, the p-FPhFA-based 2D perovskite ( n = 5) device yields a champion efficiency of 17.37%, which is much higher than that of the PhFA-Pb device (12.92%), representing a record value for 2D PSCs with FA-based spacers. These results highlight the great potential of ArFA spacers, especially the fluorinated ArFA spacer, for high-performance 2D perovskite solar cells.