Achieving Ultrahigh Photoluminescence Quantum Yield\nin Highly Stable Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> Perovskite\nSingle Crystals Through Melt Growth

Low-dimensional\nhalide perovskites Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> have\ngarnered increasing attention for their exceptional\nluminescent properties, attributed to their stability, high photoluminescence\nquantum yield (PLQY), and scintillation efficiency. Here, we report\nthe growth of transparent Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> crystals via the Bridgman method, emphasizing precise control of\nmelt stoichiometry for optimal performance. Through melt composition\noptimization, significant improvements in crystal clarity and structural\nintegrity were achieved, enhancing transmittance from 30% to over\n80% and achieving nearly 100% PLQY. Notably, it exhibited self-absorption-free\nbehavior due to a large Stokes shift of 1.15 eV, indicative of a substantial\nexciton binding energy (383 meV) associated with exciton self-trapping.\nLastly, under <sup>241</sup>Am γ-ray irradiation at 59.5 keV,\nthe energy resolution improved to 14.9% with the transparent Cs<sub>3</sub>Cu<sub>2</sub>I<sub>5</sub> crystals, demonstrating its potential\nfor high-performance scintillation applications.