Role of Pseudohalide for Enhancements of Emission Efficiency and Stability of Lead‐Free Blue‐Emitting Cs ₃ Cu ₂ I ₅ Nanocrystals

Abstract Cs ₃ Cu ₂ I ₅ has been considered a promising lead‐free perovskite material for blue light emission due to its environmental friendliness and unique optical properties. Although Cs ₃ Cu ₂ I ₅ 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 Cs ₃ Cu ₂ I ₅ 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 Cs ₃ Cu ₂ I ₅ 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 Cs ₃ Cu ₂ I ₅ 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 CsCu ₂ I ₃ , exhibiting good operational stability.