| White light emitting diodes (LEDs) are considered to be a promising candidatefor the future lighting system because of their higher efficiency, longer lifetime, andlack of requirement for pollutants compared with the conventional light sources suchas incandescent lamps or fluorescent lamps. Until now, the most widely usedcommercial white LEDs are fabricated by combining high performance blue-emittingInGaN chip with Y3Al5O12:Ce3+(YAG:Ce3+) yellow emitting aluminate garnetphosphor. YAG:Ce3+has a high converting efficiency, but the deficient red emittingcomponent leads to the color rendering index (CRI) of the white LEDs below80. Toresolve this problem, the method of mixing green and red phosphors instead ofYAG:Ce3+phosphor has been proposed. Unfortunately, the phosphor mixture givesfluorescence reabsorption that result in loss of luminous efficiency. In addition,non-uniformity of luminescent properties for different phosphors will result intime-dependent shift of the color point. Therefore, to achieve single phase phosphorwith full color emission is expected.The green-emitting Ca3Sc2Si3O12(CSS):Ce3+phosphor has attracted muchattention due to its higher luminous efficiency and excellent thermal stability.CSS:Ce3+phosphor exhibits a green-emitting band with a peak at505nm. In order toachieve single phase phosphor with full color emission, we have modified thisphosphor by employing approaches of nitriding and codoping. The main studies andresults are listed as follow:(1) We performed N3-incorporation into CSS:Ce3+and then achievedred-emitting Ce3+centers (peaked at610nm) that have N3-in their local coordination.Diffuse reflectance and photoluminescence spectra for O2-fully coordinated green emitting Ce3+and N3-partially coordinated red Ce3+in CSS:Ce3+, N3-are studied as afunction of CeO2and Si3N4contents in the raw material. Our results indicate that thepresence of N3-can enhance the Ce3+solubility in CSS by Ce3+-N3-substitution forCa2+-O2-. At low Ce3+concentration, the green Ce3+forms preferentially while the redCe3+hardly forms even if N3-content in the raw material is sufficient. There exists athreshold green Ce3+concentration (0.008) for generating red Ce3+. Only beyond thethreshold, the red Ce3+can form. We obtained color tunable luminescence withenhanced red/green intensity ratios through energy transfer from the green Ce3+to redCe3+as only the green Ce3+is excited by blue light. A white LED with CRI of92,CCT of6345K and chromaticity coordinates of (0.31,0.30) is obtained by combiningour single CSS:0.15Ce3+,0.6N3-phosphor with a blue-emitting InGaN LED chip (450nm).(2) In the Ce3+and Pr3+co-activated CSS, Ce3+and Pr3+both occupy the Ca2+sites. We have investigated the effect of Ce3+-Pr3+energy transfer on the luminescenceproperties. The luminescence spectra exhibit a red emission around610nm originatedfrom1D2â†'3H4transition of Pr3+through the energy transfer from Ce3+to Pr3+. Thered emission of Pr3+gradually enhances with the increase of energy transfer efficiencythat is caused by the increasing concentration of Pr3+in CSS. The amount of Pr3+incorporated into the phosphor is very limited due to the charge mismatch betweenPr3+and Ca2+. In order to explore the approach for enhancing the Pr3+concentration inCSS, we have tentatively added Mg2+in Sc3+site to compensate the residual positivecharge caused by the substitution of Pr3+for Ca2+in CSS. Our results indicate that theaddition of Mg2+can not only increase Ce3+concentration in CSS to make emissionspectra move toward longer wavelength, but also promote Pr3+incorporation into CSSlattices to enhance the Pr3+emission. Finally, A white LED with CRI of80, CCT of8715K and chromaticity coordinates of (0.28,0.32) was fabricated by combining thesingle CSS:0.05Ce3+,0.01Pr3+,0.3Mg2+phosphor with a blue-emitting InGaN LED(450nm) chip. However, this white LED needs more red emission component to beexcellent lighting source. In order to improve the performance of this white LED,further exploration to enhance the red emission of this phosphor is necessary.(3) The introduced Mn2+in CSS lattices can occupy not only Ca2+site to generatea yellow emission band around574nm (named Mn2+(I)) but also Sc3+site to generatea red emission band around680nm (named Mn2+(II)). Through the efficientCe3+â†'Mn2+(Mn2+(I) and Mn2+(II)), Ce3+â†'Pr3+and Mn2+(I)â†'Pr3+energy transfer, we have realized color tunable luminescence in the Ce3+, Pr3+and Mn2+co-activatedCSS. With the enhancement of Mn2+(Mn2+(I) and Mn2+(II)) emissions caused byincreasing Mn2+nominal content, the red emission (around610nm) of Pr3+alsoenhances. The reason may be that the formation of Mn2+(II) that substitutes for Sc3+can enhance the concentration of Pr3+in Ca2+site due to charge compensation effect.In addition, the emission of our present phosphor is also adjusted by addition of Mg2+,due to the Mg2+incorporated into Sc3+site can influence the concentrations of Ce3+,Pr3+and Mn2+(Mn2+(I) and Mn2+(II)) in our phosphor. Finally, A white LED withCRI of90, CCT of4980K and chromaticity coordinates of (0.34,0.31) is obtained bycombining the single CSS:0.08Ce3+,0.01Pr3+,0.3Mn2+,0.2Mg2+phosphor with ablue-emitting InGaN LED chip. |
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