Bright Blue Emission Lead-Free Halides with Narrow Bandwidth Enabled by Oversaturated Europium Doping

Eu²⁺-based lead-free metal halide nanocrystals (LFMH NCs), including CsEuCl₃ NCs and CsX:Eu²⁺ NCs (X = Cl or Br), exhibit highly efficient narrow-band blue photoluminescence, making them competitive candidates for next-generation lighting and displays. However, the growing mechanism of the aforementioned NCs lacks in-depth study, which hinders the development of Eu²⁺-based nanomaterials. Herein, we demonstrate the colloidal synthesis of CsBr:Eu²⁺ NCs based on an air-stable europium source. The NCs show deep blue photoluminescence centered at 444 nm, with a maximum photoluminescence quantum yield (PLQY) reaching 53.4% and a fwhm of 30 nm. We further reveal the mechanism that determines CsBr host growth and Eu²⁺ doping in CsBr:Eu²⁺ nanocrystals, especially dopant trapping and self-purification, that determine the PLQY level. Pure white, warm white, and cold white LEDs are fabricated based on CsBr:Eu²⁺ NCs, red and green phosphors, and their performance suits the needs of high-quality lighting.