Synthesis And Luminous Characteristics Of Phosphors For High Color Rendering Index White LED

White light-emitting diodes are considered as the fourth generation of solid-state lighting sources for their excellent properties such as high luminous efficiency,long lifetime,energy saving,subsize,stability and lack of toxic mercury.Compared with incandescence and fluorescence,they meet our demands for safety,energy saving and environment protection in a higher level for illumination products.Using a blue LED chip coated with a yellowish phosphor of YAG:Ce³⁺ is the widely used technique to produce white light LED.However,for lacking of red component in the emission spectrum,this kind of white LED suffers from the drawbacks of high color temperature,blue/yellow color separation and poor color rendering index(CRI).The drawbacks of high color temperature and low color rendering index become two barriers in using white LED as general illuminations sources.It is important to broaden the YAG's emission spectrum or to find out other yellow/yellow-orange phosphor with wider emission spectrum than what YAG has for the new generation of solid-state illumination.High temperature solid reaction method is widely used in phosphor synthesizing for advantages of easy operation and high success rate;while microwave radiation method is a newly-developed method which has been adopted since 1980s,showing advantages of high efficiency and rapid,uniform heating,and so on.In this work, series of phosphors for white LED were synthesized by combining the two methods, i.e.,the microwave radiation and high temperature solid reaction methods.Firstly,series of Ba²⁺ codoped Sr₃SiO₅:Ce³⁺,Li⁺ samples were synthesized and studied.It was found that the excitation wavelengths peaking at 350nm and 415nm attribute to the transition of 4f→²D_3/2 and 4f→²D_5/2 of the Ce³⁺ ions,respectively. Through transitions of 5d→4f in Ce³⁺,the phosphors showed a very broad and strong yellow emission under near ultraviolet(UV) or blue light excitation.Red shift of emission band from 530nm to 590nm has been achieved by adding the element Ba²⁺ in the Sr₃SiO₅host.The optimum doping concentration of Ba²⁺ is 0.35mol.Under a forward bias current of 20mA,white LED with a color rendering index of 86 was obtained by combining an InGaN LED chip(460nm) with 0.35Ba²⁺codoped Sr₃SiO₅:0.018Ce³⁺,0.018Li⁺.Secondly,series of Ba²⁺,Ca²⁺,Mg²⁺ codoped Sr₂SiO₄:Eu²⁺ yellow phosphors were synthesized and studied.These samples can be excited by light ranging from 340nm to 450nm efficiently.Among them,the phosphor Sr_1.73Ba_0.2SiO₄:0.07Eu²⁺ has the best excitation and emission efficiency.Sr_1.73Ba_0.2SiO₄:0.07Eu²⁺ has the emission spectrum ranging from 520nm to 620nm,attributing to the transition of 4f⁶5d→⁸S_7/2 of Eu²⁺, which occupies the site of sexadentate Sr²⁺(â…¡) in the host crystal.The optimum doping concentrations of Ba²⁺ and Eu²⁺ are 0.2mol and 0.07mol.Under a forward bias current of 20mA,white LED with color rendering index of 85 was obtained by combining an InGaN LED chip(460nm) with Sr_1.73Ba_0.2SiO₄:0.07Eu²⁺.Thirdly,series of high efficiency BaMgAl₁₀O₁₇:Eu²⁺(BAM) blue phosphors were studied.The luminescence and the principle of the degradation of BAM were studied. Through analysis of the crystal structure of the BAM,we found that the BAM's thermally stability can be improved by adding additional Mg²⁺ ion,and the best sample was achieved with 1.05mol Mg³⁺.Fourthly,two series of m_3-xSiO₅:xEu³⁺(M=mg,Ca,Ba,Sr) and Ca(La_1-xEu_x)₄Si₃O₁₃ silicate red phosphors were studied.The two phosphors can be used as the compensation of the red spectrum for the white LED using a blue LED chip plus YAG yellow phosphor.In addition,they can be used as the red phosphor for the three primary colors white LED excited by a near ultraviolet LED chip.Finally,series of Ba_2-xMg_1-ySi_2-zAl₂O₇:xEu²⁺,yMn²⁺ were studied.As the concentration of the Al³⁺ is 0.3mol,the sample has the optimum emission spectrum ranging from 400nm to 650nm.The emission band consists of three peak wavelengths located at 623nm,501nm and 438nm,respectively.The emission peaks at 438 and 501nm originate from the transition of 5d→4f of Eu²⁺ ions that occupy sites of Ba(â…¡) and Ba(â… ) in the host crystal,while the emission at 623nm is attributed to the transition of ⁴T₁→⁶A₁ of Mn²⁺.The relative emission intensity of the peaks at 438nm and 501nm can be modulated by changing the concentration of Al³⁺.Three white LEDs were obtained by combining the sample phosphor with LED chips at 380nm, 405nm and 460nm,respectively.Among them,the 380nm case has the best color rendering index of 86,and the CIE chromaticity coordinates is(0.3183,0.3136).