Quasi-2D　perovskite　light-emitting　diodes　(PeLEDs)　are　promising　candidates　to　realize　superior　luminescent.　However,　the　poorly　controlled　phase　distribution　and　surface　defects　hinder　the　improvement　of　the　device＇s　performance.　Here,　by　introducing　rubidium　bromide　(RbBr)　to　tune　the　crystallization　kinetics　of　quasi-2D　perovskites,　more　uniform　phase　distribution　is　achieved　through　the　suppression　of　medium-n　phases,　resulting　in　narrower　emission　spectrum　and　more　efficient　energy　transfer.　Meanwhile,　the　defects　are　effectively　passivated　by　the　addition　of　RbBr.　As　a　result,　the　photoluminescence　quantum　yield　(PLQY)　of　quasi-2D　perovskite　films　increases　significantly　from　45.6%　to　81.3%,　and　the　maximum　external　quantum　efficiency　(EQE)　of　PeLEDs　reaches　18.92%.　This　finding　provides　a　new　insight　into　the　phase　distribution　control　of　quasi-2D　perovskites　and　the　further　improvement　of　PeLEDs.　©　2023　The　Authors.　Advanced　Electronic　Materials　published　by　Wiley-VCH　GmbH.