Surface　defects　induce　non-radiative　charge　recombination　and　reduce　the　photovoltaic　performance　of　perovskite　solar　cells　(PSCs).　To　realize　highly　efficient　and　stable　PSCs,　surface　passivation　is　an　effective　strategy　to　reduce　defects　in　mixed　halide　perovskite　films.　Organic　iodides　have　emerged　as　valuable　candidates　for　perovskite　surface　passivation　owing　to　their　facile　preparation,　optimal　lattice　matching　with　perovskites,　and　robust　defect　passivation　capability.　Here,　a　novel　phthalimide　N-alkylammonium　iodide　(PAE-I)　is　introduced　as　a　passivating　agent　for　the　defects　on　the　surface　of　FAPbI3　perovskite　films.　The　incorporated　PAE-I　layer　effectively　passivates　interface　traps　and　reduces　non-radiative　recombination,　improving　carrier　extraction　and　transfer.　As　a　result,　the　power　conversion　efficiency　(PCE)　of　PSCs　increases　from　19.76%　to　21.30%.　Furthermore,　the　PAE-I-passivated　PSCs　without　encapsulation　exhibit　excellent　long-term　stability,　retaining　80%　of　the　initial　efficiency　after　2400　hours　under　an　ambient　atmosphere.　This　work　provides　deep　insights　into　the　reduction　of　non-radiative　recombination　and　the　enhancement　of　device　performance　through　the　application　of　phthalimide　as　a　superior　and　promising　passivator.　©　2024　The　Royal　Society　of　Chemistry.