Abstract:

Red phosphors play a vital role in improving the photoluminescence properties of white LEDs (w-LEDs). In order to prepare high efficiency and stability red phosphor for commercial LED chips, a series of tetragonal scheelite structure Na_1-xM_xCaEu(WO₄)₃ (M=Li, K) red phosphors were synthesized by a solid state reaction method. The effect of Li⁺ and K⁺ doping on the crystal structure, photoluminescence properties and thermal quenching characteristics of NaCaEu(WO₄)₃ phosphor were investigated systematically. Rietveld refinement results show that the doping of Li⁺ and K⁺ exerts change on the tetragonal scheelite structure of the NaCaEu(WO₄)₃ host, but forms the solid solution, and results in the regular change in the lattice constant. The photoluminescence spectra show that under the excitation of near ultraviolet light at 395 nm, the phosphors exhibit a typical red emission, with the strongest emission peak at 617 nm, corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ ion, which indicates that Eu³⁺ is situated in non-central symmetric lattices in NaCaEu(WO₄)₃ host. It is noteworthy that the doping of Li⁺ and K⁺ effectively improves the emission intensity of the NaCaEu(WO₄)₃ phosphor. When the doping concentration (molar percent) of Li⁺ and K⁺ is 100% and 30%, respectively, both the emission intensity and the color purity realize optimum. In addition, thermal stability and thermal quenching characteristics of Na_1-xM_xCaEu(WO₄)₃ (M=Li, K) phosphors were studied. The results show that both Li⁺ and K⁺ doped phosphors display excellent thermal quenching, and when the Li⁺ doping concentration (molar percent) is 100%, LiCaEu(WO₄)₃ phosphor processes the best thermal quenching. All results demonstrate that Na_1-xM_xCaEu(WO₄)₃ (M= Li, K) red phosphors can be applied to high-power NUV (Near ultraviolet) excited white light emitting diodes potentially.

Key words: phosphor, scheelite structure, thermal quenching characteristics, photoluminescence property