Controlled Synthesis,Tuned Performance,and Lunminescence Mechanisms Of The Key Materials For LEDs And QLEDs

Luminescent materials are the key functional materials for converting high-energy photons into low-energy photons or converting electrical energy directly into light energy in LEDs and quantum dot display applications,and directly affect the performance of LEDs and QLEDs.For the optimization of phosphors for LEDs lighting and the development of QLEDs quantum dot materials,three parts of research work are carried out in this paper:research on controlled synthesis method,phase composition,and luminescence properties of SrSi₂O₂N₂:Eu²⁺green phosphor,macrostructure,local structure,electronic structure,and luminescence quenching of CaAlSiN₃:Eu²⁺red phosphors,g-C₃N₄ quantum dot luminescence control and its application in QLED display devices.The content and results of the paper are listed as follows:(1)The control synthesis and phase structure conversion of green phosphor SrSi₂O₂N₂:Eu²⁺for LED are studied.The results show that the different stronium sources,such as Sr O,Sr CO₃,Sr C₂O₄ or Sr(NO₃)₂,affect the particle morphology,crystal morphology and N/O ratio of the SrSi₂O₂N₂:Eu²⁺phosphor,which will subsequently affect the absorption,scattering and luminescence performance of the phosphor.When the ratio of raw materials of Si₃N₄ and Si O₂ is 1:1,the first phase that appears during the reaction is the intermediate phase,then Si₃N₄ increases in content,and thus obtain e obtains an orthorhombic stable phase.Reducing the proportion of Si O₂ in raw materials and increasing the content of Si₃N₄ contribute to the synthesis of high-quality SrSi₂O₂N₂:Eu²⁺phosphors.(2)The macrostructure,local crystal structure,electronic structure and photoluminescence properties of nitride red phosphor CaAlSiN₃:Eu²⁺with high CRI for semiconductor lighting are studied.The result shows that the crystal structure of CaAlSiN₃:Eu²⁺is isostructural with ASi₂N₃.The sites of Si⁴⁺are replaced by Ca²⁺and Al³⁺to form a solid solution.When the Ca content remains constant,the component is regulated according to the chemical formula CaAl_xSi_1-xN₃:Eu²⁺,and the pure phase is obtained in the range of Al/Si ratio of 0.8-1.0.The distribution of electrons in the Eu²⁺5d orbital gradually shifts from the high energy level 5d₄,5d₅ and 5d₃to the low energy levels5d₁,5d₂ as the Al/Si ratio increases.The thermal ionization of the electrons from the 5d₅to conduction band is suppressed,thereby causing the luminescent efficiency of the phosphor to be improved and the emission wavelength to be red-shifted.However,when the Al/Si content is more than 1.0,Al N unpure phase appears in the phosphor,and the luminescence property is more poor than whose Al/Si ratio below 1.0.In order to maintain the charge balance,the phosphor component is regulated by the chemical formula(Ca_0.996Eu_0.004)_0.05+0.5yAl_ySi_1-yN₃:Eu²⁺.It is found that the simutanenously increase of Ca and Al/Si is not conducive to improving the comprehensive performance of phosphors.(3)The g-C₃N₄ powders are synthesized by two methods:one is the doping of a variety of valence metal ions,and the other is high pressure synthesis.g-C₃N₄ quantum dots are obtained by ultrasonic exfoliation and used to prepare QLED display devices The metal ions are doped into the g-C₃N₄ triazine ring cavity and bridged with the surrounding N atoms.The doping of metal ions results in the luminescence quenching and red shift of the emission peaks of the g-C₃N₄ powder.The luminescence intensity of g-C₃N₄ powder increases as synthetic pressure increases.The reason for this phenomenon is The reason for this phenomenon is that high pressure affects the sp³ hybridization of C and N atoms.The quantum dots are used in QLED devices,and the electroluminescence proportion of green light emission peaks in the K⁺and Zn²⁺doped g-C₃N₄ quantum dots increase as the doping concentration increased.Defects induced by Li doping could enhance the green and red emission in g-C₃N₄ quantum dots.Although the doping of metal ions causes different degrees of quenching of photoluminescence,a new defect level is introduced,so that the electroluminescence of g-C₃N₄ quantum dots exhibits multiple color emission.The emission peak of the quantum dot electroluminescence is red-shifted from the blue-violet region to the blue region as the pressure rises.