Studies On Luminescence Properties Of Ce³⁺,Tb³⁺ Activated High Efficiency Blue And Green Phosphors For White LED

The first commercialized w LED was manufactured by combining a blue LED chip and a yellow-emitting phosphor(YAG:Ce³⁺).The 1-pc wLEDs possess many advantages such as low energy consumption,high efficiency,long life and environmentally friendly and have being popular in the market,leading to the era of solid state lighting.The approach to achieve white light is combining the blue LED chip with the yellow phosphor(YAG:Ce³⁺),however,lacking of red and green component in the emission spectra lead to high correlated color temperatures?CCT>5000K?and low color rendering indexes?Ra<75?of 1-pc wLEDs.To occupy the high-end lighting market,the wLED with high color rendering indexes?Ra>90?is indispensable.So far there are two approaches possessing different advantages and being develop to achieve wLED with high color rendering indexes:one is blue LED+red and green phosphors,the other is UV LED+blue,green and red phosphors.Therefore,discovery and synthesis of new luminescent materials with excellent properties are fundamental.Moreover,promoting the development of LED phosphor need the progress of preparation methods as well as theoretical guidance.In this dissertation,an efficient blue-emitting from Ba₂Lu₅B₅O₁₇:Ce³⁺phosphor stabilized by La₂O₃,a series of highly efficient green-emitting phosphor with low thermal quenching,a model for elucidating the high thermal stability of the Tb³⁺emission,the comment on“Zero-thermal-quenching and photoluminescence tuning with assistance of carriers from defect cluster traps”and a highly efficient and thermally stable green phosphor designed through chemical unit co-substitution for full-spectrum white LEDs are reported.The main research contents are as follows:?1?The pure phase of Ba₂Lu₅B₅O₁₇ is difficult to obtain.To stabilize the crystal structure of Ba₂Lu₅B₅O₁₇,the larger ion radius of La³⁺was added to substitute Lu³⁺in raw materials.It is found that pure-phase Ba₂Lu₅B₅O₁₇ is successfully obtained by adding 2%La³⁺.Upon 348nm excitation,a luminescence internal quantum efficiency of BLLB:Ce³⁺can reach as high as 92%.The BLLB:Ce³⁺phosphor shows a blue emission band peaked at 443nm with a main excitation band peaked at 348nm.The dependence of luminescence intensity on Ce³⁺concentration was studied.The maximal luminescence intensity was observed at 1%Ce³⁺,which gives a critical distance of2.41nm between two Ce³⁺ions for luminescence quenching.The emission of the present phosphor is stable with increasing temperature up to its quenching temperature of 403K.?2?A highly thermally stable and efficient green-emitting Ba₂Y₅B₅O₁₇:Ce³⁺,Tb³⁺phosphor prepared by high temperature solid-state reaction.The phosphor exhibits a blue emission band of Ce³⁺and green emission lines of Tb³⁺upon Ce³⁺excitation in the near-UV spectral region.The effect of Ce³⁺to Tb³⁺energy transfer on blue to green emission color tuning and on luminescence thermal stability is studied in the samples co-doped with 1%Ce³⁺and various concentrations?0-40%?of Tb³⁺.The fast energy transfer to thermally stable Tb³⁺emitters was discovered in Ba₂Y₅B₅O₁₇:Ce³⁺,Tb³⁺phosphors.The internal and external quantum yield of the sample with Tb³⁺concentration of 20%measured to be as high as 76%and 55%,respectively.?3?Green phosphors co-doped with Ce³⁺and Tb³⁺have been studied extensively for application in UV-based white LEDs,but only few of them show both high luminescence efficiency and thermal stability.The Tb³⁺emission endowed the green phosphor with a high quantum yield of 86%,and a high thermal stability over the 303–483K range was achieved.A model has been proposed to explain a general phenomenon observed in Ce³⁺and Tb³⁺co-doped phosphors that Tb³⁺emission is thermally more stable than the Ce³⁺emission.The thermally stable Tb³⁺emission was well predicated using the model.The new phosphor was employed for fabricating a UV-based white LED.A high color-rendering index at a low correlated color temperature is obtained,indicating promising application of Ba₂Lu₅B₅O₁₇:Ce³⁺,Tb³⁺in solid-state lighting.?4?A hot paper focusing on Tb³⁺doped Sr₈ZnSc?PO₄?₇ phosphor with zero-thermal-quenching for high-power pc-wLEDs aroused our attention.The zero-thermal-quenching proposed by Kim et al.for Eu²⁺activated phosphors was explained by energy transfer from thermally activated traps to luminescent centers as temperature is raised.Some questionable views such as the application potential of Sr₈ZnSc?PO₄?₇:Tb³⁺phosphor and evidences of energy transfer for zero-thermal-quenching will be discussed in this paper.?5?Traditional phosphor-converted white LEDs suffer from the so-called blue-green cavity in their emission spectra.Hence,satisfactory phosphors with sufficient emission around 490nm are still lacking.Herein,we report a newly designed garnet-type phosphor,Lu₂SrAl₄SiO₁₂:Ce³⁺(LSAS:Ce³⁺),through chemical unit co-substitution,whose emission can well fill the blue-green cavity.Importantly,LSAS:Ce³⁺exhibits nearly zero thermal quenching even at 473K?200??as well as a high internal quantum efficiency of 93.3%.Finally,the as-fabricated warm white LED device shows a high color rendering index of 97.6 with R9=94,indicating its great potential for white LEDs.