Halide Defect Passivation in CsPbBr 3 Nanocrystals Mediated by the Friedel–Crafts Reaction: Enhanced PLQY and Structural Stability Without Bandgap Modulation

Halide perovskite nanocrystals (PNCs) have emerged as highly promising emissive materials for next‐generation displays due to their outstanding optical properties. Here, we post‐treated CsPbBr 3 NCs with a toluene solution comprising AlBr 3 and CH 2 Br 2 (denoted as AlBr 3 /CH 2 Br 2 –CsPbBr 3 NCs) and elucidated the roles of both components in modulating the optical properties of the AlBr 3 /CH 2 Br 2 –CsPbBr 3 NCs. Contrary to previous studies, no evidence for Al 3+ doping at Pb 2+ sites was observed in the AlBr 3 /CH 2 Br 2 –CsPbBr 3 NCs after post‐treatment, with the photoluminescence emission remaining unchanged. Instead, the photoluminescence quantum yield (PLQY) of AlBr 3 /CH 2 Br 2 –CsPbBr 3 NCs was significantly enhanced to ∼89.8% ± 2.2% via halide defect passivation by Br − ions generated via the Friedel–Crafts reaction. HBr generated from the Friedel–Crafts reaction is readily oxidized to Br 2 , which yields Br − ions. The Friedel–Crafts reaction occurred exclusively in aromatic solvents, and its reactivity was determined by the property of the Lewis acid used. The AlBr 3 /CH 2 Br 2 –CsPbBr 3 NCs demonstrated superior structural stability under ambient conditions, retaining ∼68.8% of the initial PLQY (∼89.8%) after 14 days of ambient exposure. Our study demonstrates that post‐treatment of CsPbBr 3 NCs with AlBr 3 does not involve the substitution of Pb 2+ with Al 3+ , whereas it significantly enhances the PLQY and structural stability through halide defect passivation.