Harnessing electro-generated excitons for tunable lanthanide nanocrystal electroluminescence

Lanthanide nanocrystals are promising candidates for electrically powered light-emitting diodes (LEDs) due to their narrow-band emission, high color purity, and broad color turnability. However, their insulating nature poses a challenge for carrier transport and injection, impeding their application in optoelectronic devices. Here we demonstrate efficient electroluminescence from insulating lanthanide fluoride nanocrystals (5 nm; NaGdF4:Tb3+ or NaGdF4:Eu3+) 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 intensity of intra-ligand charge transfer, the first singlet (S1) and triplet (T1) energy levels of ArPPOA are precisely adjusted to optimize intersystem crossing (ISC), reverse ISC, and the interplay between singlet Förster resonance energy transfer and triplet Dexter energy transfer to the nanocrystals. Through careful control of dopant composition and concentration in nanocrystals, we also achieve wide-ranging multicolor electroluminescence without altering the device architecture, reaching an external quantum efficiency exceeding 5% for Tb3+. This ligand-functionalized nanocrystal platform provides a versatile and efficient approach for realizing wide-band tunable electroluminescent emissions.