Elucidating Surface Passivation and Giant Rashba Splitting in FAPbI₃ Using Ammonium Derivatives

Organic-inorganic hybrid perovskites offer strong potential for photovoltaic and single-photon source applications owing to their exceptional optoelectronic properties and remarkably bright ground state. However, surface FA-vacancy induced trap states remain a major obstacle to their broader commercial deployment. Here, we employ first-principles calculation to investigate the passivation behavior of six ammonium ligands on FA-site cation defects in FAPbI₃ and simultaneously tune its electronic and spin properties. The ammonium ligands efficiently eliminate trap states introduced by the surface FA vacancies. Ligands with electron-withdrawing -CF₃ groups (An-CF₃, PEA-CF₃, and PMA-CF₃) show higher work functions due to negative polarization. These ligands also induce pronounced Rashba splitting at the conduction band minimum, with PEA-CF₃, An-CF₃, and PMA-CF₃ showing the helical spin texture along (S_z) due to inversion symmetry breaking. The incorporation of -CF₃ ligands enhances the overall optical absorption as compared to that of unmodified structures. These results clarify how functional ligands passivate defects and show that targeted surface chemistry can simultaneously improve the electronic, optical, and excitonic properties of perovskite solar cells.