High‐Quality Cs₃Cu₂I₅ Single‐Crystal is a Fast‐Decaying Scintillator

Abstract Copper(I) halide perovskite Cs₃Cu₂I₅ has recently attracted tremendous interest and has been considered an exceptionally promising scintillator due to its high quantum yield. However, its slow scintillation decay time, previously considered an intrinsic property, is a significant drawback preventing its practical application and commercialization. But in this work, it is found that the high‐quality Cs₃Cu₂I₅ single‐crystal grown by the designed aqueous solution method surprisingly exhibits a fast scintillation decay time of 39 ns (82%), quite different from previous reports. This subversive change may be due to the high crystal quality with denser 0D structure and low trap density (6.41 × 10⁸ cm^–3), changing the excited‐state structural deformation and accelerating the recombination of electrons and holes while directly reducing the prolongation of decay time caused by inherent defects. Moreover, the single‐crystal with a rocking curve FWHM value of only 50.4˝ has a high optical transmittance (85%), a very high absolute photoluminescence quantum yield (PLQY, 97.76%), superior environmental stability, and excellent light‐yield and energy resolution under different γ‐ray sources (²⁴¹Am, ⁶⁰Co, ²²Na, ¹³⁷Cs, ¹⁵²Eu). These extraordinary properties pave the way for the subsequent development of practical applications.