Orange-Red-emitting Ca9Gd(PO4)7:Eu3+ Phosphors: Judd-Ofelt Analysis and Investigation on the Thermal Stability, Quantum Efficiency for WLED

Abstract

Orange-red-emitting Eu3+-doped Ca9Gd(PO4)7 phosphors have been obtained by a sol-gel technique. These phosphors, excited by UV (270 nm) and near UV light (394 nm), emit orange-red light in the range of 580–710 nm. The Ca9Gd(PO4)7:7 mol% Eu3+ phosphor annealed at 1100 °C shows the highest PL intensity. Judd-Ofelt analysis reveals that the neighbor surroundings of Eu3+ ions are asymmetric, and the lowest asymmetry value is found at the dopant ratio of 7 mol%. The optimal phosphor excited by the wavelength of 394 nm displays an excellent thermal stability (∼76.4 % at 160 °C) with an activation energy of 0.163 eV. The corresponding CIE chromaticity coordinates (x; y) and color purity are calculated to be (0.5849; 0.3635) and 84.6 %, respectively. The quantum efficiency values of the Ca9Gd(PO4)7:Eu3+ phosphors coated on UV and near-UV chips are 87.4 % and 5.1 %, respectively. It is the first time that the QE values of Ca9Gd(PO4)7:Eu3+-based red and blue-emitting LEDs are reported.