Novel High-efficiency Ce³⁺/Tb³⁺ Doped Green Phosphors For Application In Warm-white LEDs With High Color Rendering Index

At present,white light-emitting diodes(LEDs)are regarded as the next-generation solid-state lighting source to replace those conventional lamps because of their more excellent features,for instances,compactness,low energy loss,high luminescence efficiency,and eco-friendliness.And the white LEDs with high color rendering index(CRI)and low correlated color temperature(CCT)can be effectively fabricated by using a near-ultraviolet(UV)chip combined with tri-color phosphors.In this case,tri-color-emitting phosphors that can be excited by near-UV light and fulfil the requirements for fabricating white LEDs are urgently needed to be explored.In this paper,a series of Ce³⁺activated or Ce³⁺/Tb³⁺co-doped aluminate and borate phosphors that can be excited by near-UV light were synthesized,and we systematically investigated their preparation processes,crystal structures,morphologies,luminescence properties,energy transfer,and thermal stabilities as welle as some other related contents.The specific results were summarized as follows:(1)A series of green-emitting Ce³⁺-activated Ca Sr₂Al₂O₆(CSA)phosphors were synthesized.Under 375 nm near-UV light excitation,the CSA:Ce³⁺phosphors exhibited an intense broad green emission centering at 545 nm.The CIE color coordinates and internal quantum efficiency(IQE)of the optimal sample CSA:0.01Ce³⁺were found to be(0.3553,0.5541)and 31%,respectively.Besides,the CSA:0.01Ce³⁺phosphors possessed a high thermal stability,the emission intensity at 423 K still maintained 71.3%of that at initial temperature.A white LED lamp with a superb CRI of 92 was successfully achieved by using a 365 nm near-UV-emitting LED chip combined with CSA:0.01Ce³⁺green phosphors,commerical blue phosphors,and red phosphors.(2)A series of Ca₃Lu(Al O)₃(BO₃)₄:Ce³⁺,Tb³⁺(CLAB:Ce³⁺,Tb³⁺)phosphors with blue-green tunable colors were successfully prepared.Under near-UV light excitation,the emission spectrum of the sample contained a wide emission band in range of 360-450 nm and four sharp emission peaks,which were attributed to the 4f-5d transition of Ce³⁺ions and the ⁵D₄?⁷F_J(J=6-3)transitions of Tb³⁺ions,respectively.Owing to the energy transfer effect of Ce³⁺?Tb³⁺,the characteristic green emission of Tb³⁺ions at 543 nm was gradually dominant in the emission spectra of CLAB:0.02Ce³⁺,x Tb³⁺with increase of Tb³⁺doping concentration,and the energy transfer efficiency was up to as high as 96%.In addition,the IQE value of these phosphors can reach the maximum up to 69.3%.(3)We synthesized novel Ce³⁺/Tb³⁺co-doped Ca₂Gd Zr₂Al₃O₁₂(CGZA)phosphors.The CGZA:Ce³⁺,Tb³⁺samples showed an intense and broad excitation band in 370-470 nm near-UV light range.Under 408 nm excitation,the characteristic green light emissions of Tb³⁺owing to ⁵D₄?⁷F_J(J=6-0)transitions can be gradually observed in the emission spectrum of CGZA:0.02Ce³⁺,x Tb³⁺phosphors,but the blue emissions of Ce³⁺ions were gradually reduced,which was attributed to the efficient energy transfer of Ce³⁺to Tb³⁺ions.The Ce³⁺?Tb³⁺energy transfer mechanism was investigated to be the dipole-dipole interaction.And the highest IQE value of these green phosphors reached up to 54%.Finally,we successfully fabricated a warm-white LED lamp with high CRI of 91.2 using a 400 nm near-UV LED chip combined with tri-color phosphors(CGZA:0.02Ce³⁺,0.8Tb³⁺as a green component).(4)A series of novel Ce³⁺/Tb³⁺co-activated Ca₂YHf₂Al₃O₁₂(CYHA)phosphors with narrow-band green emission were successfully achieved.Under408 nm excitation,the characteristic green emission at 543 nm of Tb³⁺dominated in the emission spectra of CYHA:Ce³⁺,Tb³⁺phosphors.And the Ce³⁺?Tb³⁺energy transfer efficiency was up to 93.2%.The energy transfer mechanism was found to be quadrupole-quadrupole interaction.The optimal sample CYHA:0.03Ce³⁺,0.6Tb³⁺possessed much higher IQE value than that of Tb³⁺single-doped CYHA:0.6Tb³⁺phosphors.Under the same conditions,the emission intensity of CYHA:0.03Ce³⁺,0.6Tb³⁺phosphors was significantly stronger than that of commercial green(Ba,Sr)₂Si O₄:Eu²⁺phosphors.Finally,compared with commercial(Ba,Sr)₂Si O4:Eu²⁺green phosphors,CYHA:0.03Ce³⁺,0.6Tb³⁺green phosphors can significantly improve the CRI of white LEDs(CRI=92.3).