Color Conversion Materials for White-Light-Emitting Diodes: Rare-Earth-Free, Single-Crystal-Structured, and Nano-sized Phosphors

Abstract

Four kinds of color conversion materials for white-light-emitting diodes were presented in this dissertation: (1) rare-earth-free heavy Mn doped phosphors, (2) single crystal garnet phosphors, (3) nanosized phosphors, and (4) heavy Ce-doped garnet phosphors. First, heavily-doped yellow ZnS:Mn2+ and green Zn2SiO4Mn2+ phosphors showed a strong 464 and 422 nm photoluminescence excitation from 6A1 → 4A1 forbidden transitions of Mn2+ ions, and an intensive 582 and 530 nm peak emission from 4T1 → 6A1 forbidden transitions with a quantum efficiency of 75 and 50%, and an excellent thermal stability of 97 and 90% @ 200 °C, respectively. The drastic enhancement of quantum efficiency under the blue light excitation was explained in terms of the relaxation of selection rule on the forbidden intra-transitions of Mn2+ ions. It was caused by the spin-spin interactions, electron-phonon interactions, and increased opposite-states intermixing confirmed by electron spin resonance signal and Raman spectroscopy. The optimized heavily-doped ZnS:Mn2+ and Zn2SiO4:Mn2+ phosphors were applied to white-light-emitting diode as a color conversion phosphor. Secondly, single crystal yellow Y3Al5O12:Ce3+ and green Lu3Al5O12:Ce3+ phosphors were grown through a floating zone method. The single crystal phosphors showed a broader blue excitation spectra with lower quantum efficiency due to large internal reflections, but they had the higher lumen maintenance at 200 °C of 97 and 103% due to the generation of less phonons than the reference powder, respectively. With pulverizing the as-grown single crystal down to micron powder, their quantum efficiencies were enhanced, while their lumen maintenance at high temperature (200 °C) were reduced due to a generation of more phonons confirmed by temperature-dependent Raman spectra. The as-grown and pulverized single crystal samples were applied to a high-power blue laser diode, and thus they gave an excellent luminescence feasibility to a high-power lighting source. Thirdly, nanosized yellow Y3Al5O12:Ce3+ and EuSi2O2N2 phosphors were prepared by high-energy planetary mill. Their photo-luminescence excitation spectra were sharpened. It can be explained in terms of a decrease of electron-vibrational interaction causing the decrease in the spectral overlap between PL and PLE spectra so as to reduce the reabsorption loss from the emission light to the absorption band. The fabricated white-light-emitting diode with a remote-phosphor structure using the nanophosphor films exhibited the better color rendering index than that of the bulk phosphor, which is similar with that with YAG:Ce phosphor. Finally, Y/Gd-free pure yellow Lu3-xCexAl5O12 (LuAG:Ce3+) powder phosphors were sintered at significantly high temperature (> 1450 °C) for heavy doping and structure stabilization. As an increase of Ce concentrations, the samples showed the gradually redshifted PL spectra from 537 nm for x = 0.1 to 550 nm for x = 1.0 together with decreasing their quantum efficiencies (Q.E.) from 97 to 87% and spectral narrowing behavior in PLE width at the 465 nm peak together with decreasing PLE intensity at the 345 nm peak. The significant red shift was explained in terms of distortion of the eight-coordinated Ce3+ sites in the dodecahedral garnet structure. The Ce-optimized sample (x = 0.40) has the Q.E. of 80%, absorbance of 91%, and lumen maintenance at 200 °C, which were comparable with those of the standard YAG:Ce reference powder. The heavily-Ce-doped LuAG:Ce3+ yellow phosphors were applied to white light-emitting diodes, and thus they gave an excellent luminous efficacy (138 lm/W) with slightly lower color rendering index (Ra = 76.4 for x = 0.4).