Preparation And Luminescence Properties Of Phosphors Excited By Ultraviolet For White Light Emitting Diodes (WLED)

White light emitting diodes (WLED) have been considered to be new generation light source due to their advantages such as energy saving, environmental protection and long life time. For white LED sources, the most promising devices are phosphor-coated LEDs. There are two kinds of approaches to get white LED:(a) blue LEDs plus yellow phosphors and (b) UV LEDs plus tricolor phosphors. The purpose of this project is to search for new efficient trichroamtic phosphor which can be excited effectively by ultraviolet LED chips.In this paper, four different series of phosphors have been synthesized. Their phase composition, structure and luminescence properties were carefully investigated.(1) Ba1.95Ca1-x(BO3)2:0.05Eu2+,xMn2+ and BaCa0.85(BO3)2:0.15Mn2+ phosphors with single phase Ba2Ca(BO3)2 structure were synthesized by high temperature solid state reaction method. Fluorescence spectra show that Ba1.95Ca(BO3)2:0.05Eu2+ emit strong green fluorescence under the excitation of 360nm, while BaCa0.85(BO3)2:0.15Mn2+yield very weak red emission excited by 360nm. Effective energy transfer occurs from Eu2+ to Mn2+ in Eu2+/Mn2+ co-doped Ba1.95Ca1-x(BO3)2:0.05Eu2+,xMn2+ phosphors and the energy transfer mechanism is demonstrated to be via the dipole-quadrupole interaction. The bicolor phosphors have the potential application in white light emitted LED excited by UV/NUV chips.(2)Li2Sr1-x-ySiO4:xCe3+,yTb3+ phosphors with single Li2SrSi04 phase structure were synthesized by high temperature solid state reaction method. Fluorescence spectra show that Ce3+ doped Li2SrSiO4 phosphor emit strong blue fluorescence with the exciting peak located at 360nm, while Tb3+ doped Li2SrSiO4 phosphors emit strong green light with the exciting peak at 243nm and no exciting peak exits in the range of 350～410nm.A resonant energy transfer from Ce3+ to Tb3+ was observed and a wide exciting spectrum with peak at 360nm appeared for Tb3+ ions. The bicolor of blue and green can be tuned by the relative content of Ce3+ and Tb3+. The bicolor phosphors have the potential application in white light emitted LED excited by UV/NUV chips.(3) K2CaP2O7:Mn2+,Eu3+ phosphors with single K2CaP2O7 phase structure were synthesized by high temperature solid state reaction. Fluorescence spectra show that Mn2+ doped K2CaP2O7 phosphors emit strong green fluorescence with the exciting peak located at 399nm, while Eu3+ doped K2CaP2O7 phosphors emit strong orange light with the exciting peak at 236nm and the exciting peaks exit in the range of 350～410nm are weak. A resonant energy transfer from Mn2+ to Eu3+ was observed for the fist time and an exciting spectrum with peak at 399nm appeared for Eu3+ ions. The bicolor of green and orange can be tuned by the relative content of Mn2+ and Eu3+.(4) A series of red phosphors Ca10-x Li(PO4)7:xEu3+ were synthesized by high temperature solid-state reaction method. Their luminescence properties were characterized by means of photoluminescence excitation and emission spectra, CIE chromaticity and quantum efficiency. Results indicated that the phosphors can be effectively excited by the near ultraviolet (NUV) light (393nm). The CIE chromaticity of Ca9.9Li (PO4)7:0.10Eu3+ was (0.638,0.361) and the quantum efficiency of this phosphor is 75% excited by 393nm. Therefore, this phosphor could be a promising red component for the applications in white LEDs.