Studies On Full-color-emitting Scandium Silicate Phosphors For Excellent Color Rendering White LEDs

Phosphor-converted white light emitting diodes (pcWLEDs) are regarded as anew lighting source for the next generation due to their high efficiency, long lifetime,and environment friendly. The most current pcWLEDs employ the yellow emittingY₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺) garnet phosphor and blue InGaN LEDs. Due to deficientemission in the red spectral region, YAG:Ce³⁺phosphor limits the color renderingindex (CRI) of pcWLEDs below80, in comparison to CRIs of100for incandescentlamps and82⁸5for fluorescent lamps. To achieve high CRIs (>80), a phosphorblend of yellow or green emitting phosphors with a red emitting phosphor isgenerally applied. The phosphor mixture gives fluorescence reabsorption andnon-uniformity of luminescent properties, resulting in a loss of luminous efficiencyand time-dependent shift of the color point. To achieve a single phase phosphor withfull color emissions is therefore desirable.The main purpose of this work is to achieve a full-color-emitting phosphor forwhite LEDs with high color rendering. Therefore, studies on the novel scandiumsilicate host matrix are carried out.First, we investigate the novel scandium silicate Ca₃Sc₂Si₃O₁₂(CSS) withgarnet structure. Ce³⁺doped green-emitting CSS:Ce³⁺phosphor has attracted muchattention for its high emission intensity and high thermal stability superior to YAG:Ce³⁺. However, under blue excitation, CSS:Ce³⁺exhibits green emissions atabout505nm, which lacks yellow and red emissive components and cannot generatewhite light by this single phosphor. In this work, to enrich the longer wavelengthvisible emission and achieve white light in the single CSS host, the approaches ofmodification by cations, nitriding, and codoping are employed. The main resultsobtained are listed as follow:For the Lu-Mg modified Ce³⁺:Ce³⁺(CSS:Ce³⁺) garnet phosphor.(1) In (Ca_2.94-xLu_xCe_0.06)(Sc_2-yMg_y)Si₃O₁₂(x=0⁰.94, y=0¹), controllableluminescent intensity and emitting color of the Ce³⁺are studied as a function of Luand Mg contents. With increasing value of x and y, the Ce³⁺emission shift from505to565nm, correspondingly resulting in the luminescence color of the phosphorchanges from green to yellow. Fixing the Mg content to be1, the effects of Lu ongarnet crystal structure formation, luminescence properties, and temperaturecharacteristics are discussed. It is revealed that the Lu-induced luminescentenhancement is the result of an increase in absorbance of Ce³⁺rather than theinternal quantum efficiency. Intense and broadband emission is realized bycontrolling the Lu content to obtain a pure garnet phase. The maximumluminescence intensity is obtained at x=0.54, which is as high as156%of theLu-free phosphor. The Lu-containing phosphor also exhibits better temperaturecharacteristics for its bigger activation energy (0.20eV) than the Lu-free one (0.18eV). Combining the present phosphor with a blue LED chip, a white LED with anexcellent color rendering index of86and a high luminous efficiency of86lm/W isobtained.(2) The effect of charge balance on the garnet crystal structure formation isstudied. The changes of bond length and covalence caused by the replacement ofLu3+and Mg²⁺ for Ca²⁺ and Sc3+are analyzed. The shift of the Ce³⁺emission andexcitation can be attributed to the combined results from crystal field splitting effectand centroid shift of Ce³⁺5d levels.(3) In (Ca₂Lu_1-xCe_x)(ScMg)Si₃O₁₂(CLSM:xCe³⁺x=0.01⁰.15), luminescence properties for various Ce³⁺concentrations are investigated. Different Ce³⁺emissionsite and energy transfers between them are observed, resulting in a red shift of theemission spectra from530to575nm with increasing x from0.01to0.15. Theoptimum concentration is x=0.09. Combining with blue (460nm) LEDs,CLSM:Ce³⁺shows excellent performances for pcWLEDs with higher colorrendering index of87.4⁸7.9and lower color temperature of5034⁵814K,especially for warm pcWLEDs with a high color rendering (CRI>80) and a lowcolor temperature (CCT <4000K). Energy transfers and thermal quenchingbehaviors depending on Ce³⁺concentrations are discussed.For the N^3- incorporated CSS:Ce³⁺phosphor.(4) Adding Si3N4into green emitting CSS:Ce³⁺garnet phosphor generates anadditionally red emission band peaking around610nm that are assigned to Ce³⁺ionshaving N3-in their local coordination. The excitation spectrum of the red bandconsists of not only a distinct band at510nm of itself but also an intense blue bandat450nm that belongs to the typical Ce³⁺ions with green emission, indicating anotable energy transfer from the green emitting Ce³⁺ions to the red ones. The energytransfer significantly enables the achievement of a broad emission spectrum coveringa red and green spectral region suitable for generating white light upon a blue LEDexcitation. The decay patterns of the red and green fluorescence are discussed inrelation to the effect of energy transfer. A white LED with a high color rendering of86and a low color temperature of4700K is fabricated using the present singlephosphor.For the energy transfer modified CSS:Ce³⁺, Mn²⁺phosphor.(5) Mn²⁺ may not only occupy Ca2+sites to generate a yellow emission (Mn²⁺(I))at574nm but also Sc3+sites to generate a red emission (Mn²⁺(II)) at680nm.Remarkable energy transfers from the green emitting Ce³⁺to both Mn²⁺(I) andMn²⁺(II) occur upon blue excitation into Ce³⁺. Concentration dependence of Mn²⁺emission is analyzed based on Ce³⁺→Mn²⁺energy transfer, steady state rateequations, and fluorescence lifetimes. Energy transfer efficiency (ηT) and rate (W) are calculated with values as high as45%and14.01×106s-1, respectively.Considerable Mn²⁺substitution for Sc3+can be performed through balancing theircharge difference by introducing trivalent rare earth ions Ln³⁺(Ln=La, Gd, Lu, Y)to replace Ca²⁺, making tunable full color emission available in CSS:Ce³⁺, Mn²⁺.White LEDs with excellent color rendering index of91⁹2are achieved by usingthe present single phosphor.Secondly, the investigations of the novel scandium silicate Ba9Sc2(SiO4)6:Ce³⁺,Mn²⁺(BSS:Ce³⁺, Mn²⁺) with rhombohedra structure are carried out. The main resultsare listed as follow:(6) Analysis of XRD patterns indicates that Ce³⁺and Mn²⁺in BSS are located atSc3+and Ba2+sites, respectively. BSS:Ce³⁺exhibits an intense UV emission bandpeaking at383nm, indicating this phosphor could be used for phototherapy andphotocopying. Codoping Mn²⁺into this material can generates a red emission bandat615nm of Mn²⁺through Ce³⁺→Mn²⁺energy transfer. BSS:Ce³⁺, Mn²⁺could beused for UV-based white LEDs as the red light source.