Revealing the Role of Organic Ligands in Deep-Blue-Emitting Colloidal Europium Bromide Perovskite Nanocrystals

Europium halide perovskites are promising candidates for environmentally benign blue-light emitters with their narrow emission line width. However, the development of high-photoluminescence quantum yield (PLQY) colloidal europium halide perovskite nanocrystals (PNCs) is hindered by limited synthetic methods and elusive reaction mechanisms. Here, we provide an effective synthetic route for achieving high-PLQY deep-blue-emitting colloidal CsEuBr₃ PNCs. Using two Br-organic ligand precursors, oleylammonium bromide (OLAMHBr) and trioctylphosphine dibromide (TOPBr₂), we identified distinct phase evolution routes involving Eu²⁺:CsBr, Cs₄EuBr₆, and CsEuBr₃. The OLAMHBr precursor initially promotes the formation of the Eu²⁺:CsBr phase, which reorganizes into the CsEuBr₃ perovskite phase via proton transfer. In contrast, the TOPBr₂ precursor induces the formation of core/shell Cs₄EuBr₆/CsBr PNCs, which subsequently transform into CsEuBr₃ through nucleophilic addition. The TOPBr₂ route exhibited enhanced CsEuBr₃ phase homogeneity, resulting in a significantly higher PLQY (40.5%; full width at half-maximum (fwhm) = 24 at 430 nm), compared to the OLAMHBr route (16.5% at 418 nm). Notably, the phase-pure CsEuBr₃ PNCs demonstrated a world-record PLQY among the reported blue-emitting lead-free PNCs that exhibit a narrow emission line width (fwhm <25 nm). This work highlights the significant role of organic ligands in the colloidal synthesis of CsEuBr₃ PNCs and their potential as nontoxic, solution-processable blue-light emitters.