Structural Design And Luminescence Property Of Eu²⁺/Ce³⁺ Doped Oxide-based Red Phosphors For Lighting Application

As fourth-generation lighting sources,phosphor-converted light-emitting diodes（pc-LEDs）,are widely used due to their advantages,including high efficiency,small volume,and tunable wavelength.Developing novel,efficient and stable rare earth luminescent materials is the key to improving the quality of pc-LEDs and further promoting the rapid development of the LED lighting and display industry.However,the study of red phosphors used for white LED（w-LED）light sources has always been a challenge in this field,especially the red-emitting ceramics with laser lighting technology driven by a blue laser diode（LD）are seriously lacking.Therefore,it is an urgent task to develop novel and efficient red materials for lighting applications.Based on these,this dissertation selects several oxide substrates innovatively and uses Eu²⁺/Ce³⁺with 4f-5d transition as the activation ion to design several new red fluorescent materials.The main innovative achievements can be divided into the following aspects:（1）A novel SrLaScO₄:Eu²⁺phosphor was designed and synthesized,showing red emission peaking at 620 nm excited by blue light.It was found that the reduction of Eu³⁺to Eu²⁺was further promoted by（NH₄）₂SO₄ assisted sintering,and the external quantum efficiency（EQE）was successfully increased from 8.4%to 43%,meanwhile,the thermal stability of the phosphor was maintained.The w-LED based on the red phosphors and Y₃Al₅O₁₂:Ce³⁺（YAG:Ce）yellow phosphors show a color-rendering index（R_a）of 86.7 and correlated color temperature（CCT）of 4005 K.It is also found that the emission peak of SrLaScO₄:Ce³⁺is located at 650 nm,which has longer wavelength emission than SrLaScO₄:Eu²⁺.This anomalous luminescence phenomenon is caused by the Ce³⁺competitively occupying the[La O₈]polyhedron,which results in a more significant Stokes shift and strong crystal field splitting.The study also found that the introduction of Ga³⁺with strong electronegativity into SrLaScO₄:Ce³⁺phosphor,Ga³⁺can attract more charges from the coordination groups,thus reducing the electron occupancy at the bottom of the conduction band（CB）to increase the band gap,helping to improve the thermal stability of SrLaScO₄:Ce³⁺.This study provides feasible strategies for exploring Ce³⁺doped oxide-based red phosphors to improve thermal stability.（2）Novel oxide-based Sr₃TaO_5.5:Eu²⁺（STOE）,Sr₃Nb O₆:Eu²⁺（SNOE）and Sr₂La TaO₆:Eu²⁺（SLTOE）red-emitting phosphors and ceramics with double perovskite structure were designed and synthesized.It is found that Eu²⁺is beneficial for obtaining good thermal stability（>95%,150 ^oC）and high quantum efficiency（EQE>40%）in ordered A₂BB’O₆ framework,while in the disordered and pseudo-symmetric A₂BB’O₆ framework,Eu²⁺has poor luminescence performance.In addition,X-ray emission and absorption spectroscopy measurements of non-resonant synchrotron accelerator show that the separation of Eu²⁺5d¹from the CB can effectively reduce the thermoelectric ionization in the ordered A₂BB’O₆structure,which further indicates the origin of high thermal stability.A white LD（w-LD）device was fabricated by combining the STOE red and Lu₃Al₅O₁₂:Ce³⁺（Lu AG:Ce）green-emitting ceramics on a wheel.The w-LD shows a high luminous flux（1115 lm）and a high R_a（≈90）.（3）The Sr₂Sc_0.5M_1.5O₅:Eu²⁺（M=Al,Ga）phosphor was prepared by the high-frequency induction heating sintering method.Under 440 nm excitation,Sr₂Sc_0.5Ga_1.5O₅:Eu²⁺shows red emission peaking at 614 nm with EQE of 52.5%.The researches demonstrate that Eu²⁺occupies the Sc/Ga O₆ polyhedrons with small coordination numbers,resulting in red emission.Further,Sr_2-xBa_xSc_0.5Ga_1.5O₅:Eu²⁺phosphors were synthesized by vicinal cations Sr/Ba substitution,and the Sr/Ba substitution generates the emission spectrum to redshift from 614 nm to 728 nm.The relationship between cationic substitution and tuned emission was investigated by analyzing the structural and local environmental changes around the Eu²⁺ion.Finally,the w-LED device manufactured by blending commercial YAG:Ce yellow powder and Sr_1.7Ba_0.3Sc_0.5Ga_1.5O₅:Eu²⁺red powder shows high color rendering index（R_a=91.1）.（4）Red-emitting SrY₂O₄:Eu²⁺phosphor(λ_em=620 nm)with good thermal stability was designed and synthesized.The structural and spectral analysis results show that Eu²⁺ions occupy Y1,Y2 and Srsites in different proportions in SrY₂O₄:Eu²⁺.Eu²⁺ions mainly occupy the distorted[Y1O₆]and[Y2O₆]octahedrons causing a strong crystal field splitting.Furthermore,a series of（Sr,Ba）Y₂O₄:Eu²⁺phosphors were synthesized by vicinal cations Sr/Ba substitution.The emission spectrum redshift from 620 nm to 773 nm,attributed to the local structure changes of[Y1/Eu1O₆]and[Y2/Eu2O₆],resulting in increased covalent bond effects,crystal field strength and Stokes shift.Sr_0.5Ba_0.5Y₂O₄:0.06Eu²⁺is a novel near-infrared phosphor and the emission wavelength peak at 773 nm with an FWHM of 195 nm.A NIR pc-LED（NIR-pc-LED）device that combines near-infrared phosphors can be used in night vision,proving its practical application potential.