Converting Self-Trapped Exciton from Dark to Bright State via Excessive Eu Incorporation in CsEuCl₃ Nanocrystals

CsEuCl₃ nanocrystals (NCs) are novel lead-free rare-earth perovskite NCs with low toxicity and intrinsic optical activity, exhibiting potential as promising alternatives to lead halide perovskites. However, the limited understanding of their dual-photoluminescence (PL) emission mechanism impedes further material optimization and device integration of CsEuCl₃ NCs. In this work, the origin of the dual PL emission peaks in CsEuCl₃ NCs is elucidated through comprehensive spectroscopic analyses, including femtosecond transient absorption and temperature-dependent PL spectroscopy. The narrow emission at 435 nm is identified as the band-edge free-exciton recombination, while the broad emission centered at 559 nm arises from extrinsic self-trapped exciton (STE) emission. The excessive Eu suppresses lattice vibrations and facilitates the conversion of the dark STE state into a radiative bright state, thereby enhancing the PL quantum yield (PLQY) to 34.1%, which is record-high for CsEuCl₃ NCs. By systematic tuning of the Eu content, the intensity ratio between dual emission bands is regulated. On the basis of the tunable emission, white-light-emitting diodes with correlated color temperatures of 8788 to 4371 K and high operation stability are successfully fabricated. This work unveils the fundamental mechanism governing dual emission behavior in CsEuCl₃ NCs, establishes an effective strategy for PL modulation, and highlights their potential as robust, lead-free materials for next-generation solid-state lighting applications.