Additive‐Mediated Redox Reconstruction and Excitonic Regulation in Cs 3 Cu 2 I 5 for X/γ‐Ray Detection

ABSTRACT Cs 3 Cu 2 I 5 has emerged as a promising candidate for X/γ‐ray detection owing to exceptional scintillation performance. Growing high‐quality, centimeter‐sized Cs 3 Cu 2 I 5 single crystals via the cost‐effective solvent evaporation technique remains challenging, primarily due to the susceptibility of Cu + and to oxidation, which impedes crystal growth and introduces defect. Here, acidic additives, including formic acid (FA), oleic acid (OA), and hypophosphorous acid (H 3 PO 2 ), were introduced during the crystal growth process to reconstruct the redox environment and regulate growth kinetics. High‐transparency Cs 3 Cu 2 I 5 crystals with low defect density and maximum dimensions of 15 × 15 × 5 mm 3 were fabricated, revealing the effects of different chemical functional groups on the growth habit. Crucially, additive‐mediated defect suppression reduces nonradiative recombination and facilitates lattice relaxation, leading to stronger exciton‐phonon coupling and thereby promoting enhanced self‐trapped exciton emission. Specifically, the FA‐Cs 3 Cu 2 I 5 exhibits a large Huang‐Rhys factor of 95.4 and PLQY of 76.68%. In room‐temperature γ‐ray detection, the FA‐Cs 3 Cu 2 I 5 achieves a competitive energy resolution of 6.02%@662 keV. For X‐ray applications, the FA‐Cs 3 Cu 2 I 5 crystal demonstrates a low detection limit of 218.39 nGy air ·s −1 . An X‐ray imaging system based on a 1‐mm‐thick FA‐Cs 3 Cu 2 I 5 wafer achieves a spatial resolution of 21.6 lp/mm@MTF = 0.2. These findings highlight the substantial potential of additive‐regulated Cs 3 Cu 2 I 5 crystals as multifunctional scintillators.