Role of Pseudohalide for Enhancements of Emission Efficiency and Stability of Lead-Free Blue-Emitting Cs₃Cu₂I₅ Nanocrystals

Cs₃Cu₂I₅ has been considered a promising lead-free perovskite material for blue light emission due to its environmental friendliness and unique optical properties. Although Cs₃Cu₂I₅ nanocrystals (NCs) exhibit excellent structural stability, their high specific surface area at the nanoscale inevitably leads to the formation of point defects caused by structural vacancies during the self-assembling process. In this study, the Cs₃Cu₂I₅ structure using pseudohalide thiocyanate (SCN⁻) is modified, which has an effective ionic radius similar to I⁻. The strong binding energy between Cu⁺ ions and the lone electron pairs of the S and N atoms in linear SCN⁻ ions enhance the crystallinity of the Cs₃Cu₂I₅ NCs. In addition, the SCN⁻ ions improve size uniformity by adjusting the chemical potential in the solvent system. The SCN⁻ incorporation also modified the electronic structure of the Cs₃Cu₂I₅ by changing the optical bandgap energy and improving the photoluminescence quantum yield from 62.5% to 76.7%. Finally, a fluorescent blue light-emitting diode is demonstrated utilizing the excellent blue light emission, and white light emitting diode (white-LED) are produced by combining this with a yellow-emitting layer of CsCu₂I₃, exhibiting good operational stability.