Electro-generated excitons for tunable lanthanide electroluminescence

Lanthanide nanocrystals offer unique advantages for electroluminescence (EL) applications, including narrow-band emission, high colour purity and compositionally tunable output^1-4. However, their insulating nature poses a challenge for carrier transport and injection, impeding their application in electrically driven optoelectronic devices⁵. Here we demonstrate efficient EL from insulating lanthanide fluoride nanocrystals (4 nm; NaGdF₄:X; X = Tb³⁺, Eu³⁺ or Nd³⁺) coated with a series of functionalized 2-(diphenylphosphoryl)benzoic acids (ArPPOA). These ligands, featuring donor-phosphine oxide acceptor hybrids with carboxyl and P=O coordination sites, effectively sensitize the luminescence of lanthanide nanocrystals by modulating the intraligand charge transfer characteristics. Ultrafast spectroscopic investigations reveal that strong coupling between ArPPOA and lanthanide nanocrystals facilitates intersystem crossing (ISC; <1 ns) and highly efficient triplet energy transfer to nanocrystals (up to 96.7%). Through careful control of dopant composition and concentration in nanocrystals, we also achieve wide-ranging multicolour EL without altering the device architecture, reaching an external quantum efficiency exceeding 5.9% for Tb³⁺. This ligand-functionalized nanocrystal platform provides a modular strategy for exciton control in insulating nanocrystal systems, offering a pathway for spectrally precise electroluminescent materials.