Preparation And Luminescence Mechanism Of Ultraviolet Excitation Phosphors For White Leds

As the mainstream of solid-state lighting(SSL),phosphor conversion white light-emitting diode(pc-WLED)has become a promising lighting source to replace conventional incandescent and fluorescent lamps,because of its advantages including energy saving,long lifetime,and no mercury pollution.Nowadays,phosphor conversion WLEDs can be fabricated mainly by combination the blue emission from InGaN chips and the yellow emission from phosphors(e.g.YAG:Ce³⁺).However,the white light obtained by this approach exhibits a low color rendering index(CRI)and high correlated color temperature(CCT),due to the lack of red emission.Multi-phase phosphors method of employing green and red emitting phosphors with blue LEDs and of pumping blue,green,and red emitting multi-phase phosphors with ultraviolet(UV)LEDs have been suggested as alternatives for displays and general illumination light sources due to their excellent CRIs.However,the shortcoming of re-absorption of blue light by green and/or red phosphors will bring the difficulty of phosphor content ratio regulation,which limits the application of the multi-phosphor approach.Because ions replacement of phosphor matrix can regulate the sites environment of luminescent ions,then make the spectra red shift or broad.In recent years,the strategy of ions replacement was widely studyed by researchers to control and optimize the luminescence properties of phosphors.The spectra positions of phosphors can be regulated by this strategy,the other luminous properties could be also optimized at the same time.On the other hand,the emergence of UV-LED provides another way for white LED,i.e.single matrix white light emitting under ultraviolet excitation.As the ultraviolet light is not visible to the naked eye,the color of the ultraviolet excitation model white LED is only determined by phosphor itself.This program can effectively avoid the weaknesses of reabsorption and different degradation rates existence in multi-phosphor approach.This thesis explores and synthesize of four new types of phosphors.These phosphors are suitable for down-converting white LED,excited by ultraviolet InGaN base chip.The basic conceptions and the recent development of the down-conversion white LED phosphors were summarized in chapter 1.The followed four chapters present the methods of cation replacement,double doping and tri-doping of activitors to regulate the light emitting of phosphors.The main contents are as follows:(1)Substitution is widely employed to design single-host phosphor with multiple sites for one activator,which could emit more than one band.BaSrMg(PO₄)₂:Eu²⁺was reported as a potential single-host white light-emitting phosphor for white LEDs,possessing multiple sites for Eu²⁺doping.The samples with nominal formula BaSrMg(PO₄)₂:Eu²⁺prepared at low temperature contain Sr₂P₂O₇ coexistent with the main phase BaSrMg(PO₄)₂.BaSrMg(PO₄)₂ phase prepared at 1250 ^oC was shown to be isostructural with Ba₂Mg(PO₄)₂,which was confirmed by XRD and TEM.Hence,the blue band was assigned to the 4f⁶5d¹?4f⁷ transition of Eu²⁺occupying the Sr²⁺sites in Sr₂P₂O₇ and the yellow band from the Eu²⁺occupying Sr²⁺and Ba²⁺sites in BaSrMg(PO₄)₂.The relative intensity ratio of yellow/blue bands is consistent with the content ratio of two phases in the samples with different synthesis temperatures,which is further proof for the original of the blue band.The thermal quenching data of the Sr²⁺substituted phosphor were better than that of Ba₂Mg(PO₄)₂,and interpreted by the crystal structure parameters.In addition to this,an unexpected spectrum blue shift of the yellow emission was observed for solid solution phosphors Ba_2-xSr_xMg(PO₄)₂:Eu²⁺(x=0-1.5),when bigger ions Ba²⁺were substituted by smaller ions Sr²⁺in Ba₂Mg(PO₄)₂lattices.DFT calculation and photoluminescence show that the activator ions Eu²⁺in BaSrMg(PO₄)₂ occupy Sr²⁺and Ba²⁺sites at an equal probability and both generate an anomalous yellow emission.The yellow emission from Ba_2-xSr_xMg(PO₄)₂:Eu²⁺was gradually blue shift with increasing Sr²⁺substitution concentration x from 0.1 to 1.Such unusual blue shift is interpreted based on the evolution of crystal structure parameters due to Sr²⁺substitution,and subsequently,a site environmental expansion mechanism is proposed.The proposed mechanism could serve as a general model to reveal the underlying factors for spectrum shift caused by cation substitution and contribute to design new solid solution phosphors.(2)Even though multi-phosphor strategy is employed widely to achieve the high color-rendering index WLEDs,this approach accompanies with the shortcoming of the re-absorption of emission bands,resulting in the cumbersome fabrication of phosphor content ratio regulation.Based on the principle of coexistence-phase,a color tunable white light-emitting interdependence phase phosphor was prepared by solid state reaction to simplify the lamp production processes.Photoluminescence of the phosphor show full-color light-emitting ranged from 390 to 750 nm by coupling three bands with blue,yellow and red emission under an ultraviolet radiation of 350 nm.Particularly,effective energy transfer from Eu²⁺to Mn²⁺between coexistence phases was revealed to be resonance type by luminescence lifetime analysis,instead of through radiation process,which occurs dominantly in multi-phosphor system.Consequently,the relative intensity of these three bands could be adjusted by tuning the concentrations of Eu²⁺and Mn²⁺ions,in addition to by controlling the content ratio of the two interdependence phases.These results suggest that the coexistence-phase phosphor exhibits tunable color emission and may open up a new alternative for multi-phosphor approach.(3)A series of single-phase white-light-emitting phosphors Ba₂Mg(BO₃)₂:Ce³⁺,Na⁺,Eu²⁺,Tb³⁺were synthesized by conventional solid-state reaction.Adding charge compensation agent into the phosphor enhanced the emission intensity.The crystal structure of the host was characterized by X-ray diffraction and investigated by Rietveld refinement.Photoluminescence properties were studied in detail.The energy transfer from Ce³⁺to Tb³⁺in Ba₂Mg(BO₃)₂ host was investigated and demonstrated to be a resonant type via a quadrupole–quadrupole mechanism.White light with wavelength tunable was realized by coupling the emission bands peaking at 417,543,and 626 nm at tribute to Ce³⁺,Eu²⁺and Tb³⁺,respectively.The emission color of Ba_1.92-zMg_0.94(BO₃)₂:0.04Ce³⁺,0.04Na⁺,z Eu²⁺,0.06Tb³⁺(z=0.01,0.02,0.03,0.04 and0.05)could be tuned from white to orange–yellow based on the effective energy transfer from Ce³⁺to Tb³⁺and Eu²⁺.At properly relative composition of Ce³⁺(Na⁺)/Tb³⁺/Eu²⁺,optimized Commission Internationale del'Eclairage(CIE)chromaticity coordinates(0.363,0.295),high color rendering index(CRI)90 and low correlated color temperature(CCT)3793 K were obtained from the phosphor of Ba_1.90Mg_0.94(BO₃)₂:0.04Ce³⁺,0.04Na⁺,0.02Eu²⁺,0.06Tb³⁺upon the excitation of 296nm UV radiation.(4)A series of single-phase full-color emitting Ba₂Ca(BO₃)₂:Ce³⁺,Eu²⁺,Mn²⁺phosphors were synthesized by conventional solid-state reaction.The crystal structure of the host was characterized and confirmed by X-ray diffraction and Rietveld refinement.Photoluminescence properties and refinement results illustrated that vacancies and synthesis temperature would influence on site preference of Ce³⁺in the host,and the latter plays the dominant role.Energy transfer of Ce³⁺?Eu²⁺,Ce³⁺?Mn²⁺,and Eu²⁺?Mn²⁺are investigated based on the analysis of photoluminescence properties and fluorescence lifetimes.Owing to the energy transfer from Ce³⁺to Eu²⁺,Ce³⁺to Mn²⁺,and Eu²⁺to Mn²⁺in the Ba₂Ca(BO₃)₂ host,white light was realized by properly tuning the relative composition of Ce³⁺/Eu²⁺/Mn²⁺.Upon exciting at 345 nm UV radiation,phosphor Ba_1.92Ca_0.994(BO₃)₂:0.06Ce³⁺,0.02Eu²⁺,0.006Mn²⁺exibited well Commission Internationale de l'Eclairage(CIE)chromaticity coordinates(0.335,0.326),low correlated color temperature(CCT=4802 K),and high color rendering index(CRI=89).