Surface Chemical Reconfiguration by PBr ₃ Enables Electrostatic Ligand Locking and High‐Efficiency Green and Deep‐Blue CsPbBr ₃ Emitters

Halide vacancies in CsPbBr 3 nanostructures create deep trap states that limit emission efficiency, particularly in strongly confined blue emitters. Existing passivation strategies often rely on poorly soluble halide sources or bulky ligands that complicate post‐synthetic processing. Here, we introduce phosphorus tribromide (PBr 3 ), a molecular halide source readily soluble in nonpolar media, as a chemically precise surface‐engineering agent that simultaneously replenishes bromide vacancies and reconfigures ligand binding. In green‐emitting CsPbBr 3 nanocrystals, PBr 3 converts surface‐bound oleic acid into oleoyl bromide, while in situ‐generated HBr protonates oleylamine to form electrostatically stabilized OAm + Br − ion pairs. This surface chemical reconfiguration suppresses deep‐trap‐mediated recombination, increasing the photoluminescence quantum yield (PLQY) from 30% to 74% and preserving high emission efficiency even after ligand‐free washing. The optimized interfacial chemistry further renders the nanocrystals sufficiently stable for single‐molecule spectroscopy, enabling direct probing of individual emitter dynamics while preserving single‐photon characteristics. Extending this soluble halide strategy to strongly quantum‐confined 3 monolayer nanoplatelets via controlled direct addition yields efficient defect passivation without spectral shifts, boosting PLQY from 9.4% to 94%. This work establishes a simple and generalizable route for achieving highly efficient green and deep‐blue perovskite emitters through controlled surface ionic locking.