Near‐Unity Green Luminescent and Stable Lead‐Free Cs₃Cu₂Cl₅ Nanocrystals Assisted by MnC₂O₄ Treatment

The toxicity and chemical instability shortcomings of lead halide perovskites are major concerns that continue to motivate the search for next generation alternatives. Copper halides are known to be lead‐free, earth‐abundant, low‐cost, and environmentally friendly, and affordable in optoelectronic performance due to their bright luminescence, tunable emission wavelength, and excellent stability. However, the controlled synthesis of low‐dimensional copper halide nanocrystals (NCs) with near‐unity photoluminescence quantum yield (PLQY) and high stability remains a great challenge. In this work, we report a reasonable optimization approach for the preparation of Cs 3 Cu 2 Cl 5 NCs by hot‐injection colloidal synthesis technology. Specifically, the Cs 3 Cu 2 Cl 5 NCs synthesized with MnC 2 O 4 as the defect repair agent exhibited negligible self‐absorption, bright green luminosity with PL emission at about 535 nm and PLQY can be increased from 68.7 % to 100 %. The Cs 3 Cu 2 Cl 5 NCs shows excellent stability and outstanding optical properties, and is expected to be used as highly efficient green light emitter in high‐efficiency X‐ray scintillator.