Balancing Ultrafast Decay and High Light Yield in PEA 2 PbBr 4 Crystals Through Strain‐Regulated Exciton Localization

ABSTRACT Ultrafast scintillators are indispensable for advanced radiation detection. Nevertheless, reconciling sub‐nanosecond temporal response with high light yield remains a longstanding bottleneck, since rapid de‐excitation is usually accompanied by excessive exciton localization and dominant nonradiative recombination losses. Herein, we report a localized strain regulation strategy to tailor the environments of localized excitons in 2D PEA 2 PbBr 4 , thereby addressing the competition between ultrafast scintillation decay and high light yield. Specifically, robust Cu‐I coordination acts as local structural compensation in PEA 2 PbBr 4 , offsetting the structural disorder and strain amplification triggered solely by iodide incorporation. Such local structural reconstruction preserves the fast de‐excitation channels while enhancing radiative recombination. Benefiting from this precise control, CuI‐PEA 2 PbBr 4 exhibits a fast photoluminescence (PL) decay component of 0.44 ns, while achieving a light yield of 14,200 ± 600 photons MeV −1 under γ‐ray excitation. This work demonstrates that strain regulation of localized exciton environments serves as a powerful pathway toward high‐efficacy perovskite scintillators and provides a brand‐new design paradigm for diverse perovskite scintillators.