Luminescence Properties Of Ce³⁺, Eu²⁺ Doped Three Kinds Of Phosphors For White LEDs

Light emitting diodes (LED) has attached much attention due to low consumption of energy and environmentally friendly, especially for phosphor convert LED (pc-LED). Phosphors as an important part of pc-LED affected LED devices properties. Generally, two ways can be employed in obtaining white light:one is using yellow phosphor to convert blue light to obtain white light; the other is using three phosphors named as blue, green and red, to convert ultraviolet light into white light. Both of them have serious drawbacks. In this thesis, we mainly discussed photoluminescent properties of Ce3+doped garnet and melilite structure silicate-aluminum compounds. And also we investigated photoluminescent properties of Eu2+doped Ca7(PO4)2(SiO4)2. The main results are as follows:The first part:In view of the darwbacks of commercial YAG:Ce yellow phosphor, our work was done from two aspects like improvement of commercial YAG:Ce phosphor and looking for new materials which was suit for blue light excitation.1. The energy transfer phenomenon of Ce3+-Mn2+in YAG and how the charge compensators affect energy transfer efficiency was investigated. The emission spectra of Mn2+singly doped YAG samples show three bands:green, orange and deep red bands. The color can be tuned from yellow-green to red, even to deep red by increase Mn2+content. Besides, Quadrivalence ions are introduced to balance the charge difference between Mn2+and Al3+. Among them, Si4+as charge compensator exhibits the best tunable effect on controlling the Mn2+emissions in YAG:Ce,xMn. Y3Al5O12:0.06Ce,0.04Mn,0.04Si sample, which has higher brightness, little thermal quenching and higher quantum efficiency, can be used as candidate to obtain warm white light.2. Y2.94Ceo.o6Mg2AlSi2012was obtained by solid state reaction with little amount of impure phase, and its photoluminescent properties were also studied. The photoluminescent spectra show that Y2.94Ceo.o6Mg2AlSi2O12can be effectively excited by460nm and give red light peaked at600nm. White light cann’t be obtained by single Y2.94Ceo.o6Mg2AlSi2O12sample. Besides, Y2.94Ceo.o6Mg2AlSi2O12sample has worse thermal stability. Thus, we introduce Mg2+-Si4+and Lu3+into Y2.94Ce0.06Mg2AlSi2O12sample. The best photoluminescent properties can be obtained by Lu3+completely substituted Y3+.3. We have presented the preparation and characterization of Lu3MgAl3SiO12:xCe phosphor for blue LEDs. XRD refinement results show LusMgAl3SiO12:xCe phosphor in garnet structure. The excitation and emission spectra show this new garnet phosphor can be excited efficiently by450nm and emit yellow color. Thermal quenching test indicates this new garnet phosphor has better thermal stability. All these characterization indicate Lu3MgAl3SiO12:xCe has high possibility use in LED.The second part:Luminescence spectra of LaCaAl3O7:Ce3+have been tuned by series of cationic and anionic group substitution. Three different types of isovalent substitutions in LaCaAl3O1:Ce3+was investigated:for trivalent La, by the smaller trivalent rare-earth, Gd3+, Y3+and Tb3+; for divalent Ca2+, by the bigger divalent Sr2+and Ba2+; for trivalent Al2+, by the smaller trivalent B3+, as well as by the larger trivalent Ga3+. The results show the substitution on trivalent La3+has the most obvious effect on Ce3+’s spectra. In addition, Y3+substitute La3+gives higher emission intensity, indicates YCaAl3O7:Ce3+is a better blue phosphor in ultraviolet LED use than LaCaAl3O7:Ce3+. And the emission intensity increase with increasing of Si4+amount, which demonstates that silicate melilite compounds is better as Ce3+doped host than aluminum melilite compounds.The third part:A novel green emitting phosphor of Eu2+doped Ca7(PO4)2(SiO4)2was synthesized. Ca7(PO4)2(SiO4)2:Eu2+showed an strong absorption band ranging from240to440nm. Under365nm excitation, all samples gave bright green emission peaked at522nm. The concentration quenching mechanism and the activation energy for thermal quenching are investigated. However, it is very hard to improve its photoluminescent properties due to easily produce impurity phases when we introduce other ions.