Preparation And Performance Of Red/green Light-emitting Materials For White LED

WLED have been developing rapidly for decades because of their high efficiency,long life,energy saving as well as environmental protection,and the phosphors with high quality and efficience play a pivotal role for phosphor converted white LED(pc-WLED).Although suitable phosphors of various colours are available on the market,there are still some problems such as low colour rendering index,poor thermal stability and harsh preparation conditions.Near-ultraviolet chip-excited trichromatic phosphors and red light-emitting materials can reduce the correlated color temperature and improve the color rendering index,which is conducive to obtaining high-quality white light.So far,people have developed a variety of luminescent materials,but not many of them can be industrialized.The Eu²⁺doped phosphor systems are diverse and has multitude of structures,and its 5d-4f energy level transition is more sensitive to the crystal field changes.As a result,the phosphors are more flexible in their spectral regulation and have a wide range of emission wavelength to meet the needs of different applications such as lighting and display.In order to improve the performance defects of red&green phosphors for white LEDs,two kinds of representative systems doped Eu²⁺were selected in this thesis,one is narrow-band nitride phosphor SrLiAl₃N₄:Eu²⁺and the other is green commercial phosphor(Sr Ba)₂SiO₄:Eu²⁺.The main results of improving their luminescence properties around the spectral modulation and thermal stability through ionic substitution and optimization of the preparation process are as follows:1.SrLiAl₃N₄:Eu²⁺red phosphor was successfully obtained through high-temperature solid-phase reaction and pressure sintering and its photoluminescence as well as cathodoluminescence(CL)properties were tested.The results show that SrLiAl₃N₄:Eu²⁺not only emits bright narrow-band red light under blue light excitation,but also has a narrow-band red emission with a peak of 647 nm and a smaller FWHM under electron beam excitation,and its CL intensity increases with accelerating voltage and increasing probe current without saturation effect.Due to the highly dense crystal structure,the SrLiAl₃N₄:Eu²⁺phosphor has excellent thermal stability and anti-aging properties.After 90 minutes of continuous electron beam bombardment,91%of the initial CL intensity can still be maintained,and the CIE colour coordinate almost unchanged,which effectively increases the colour gamut area of the trichromatic phosphors for field emission displays(FED).In addition,it was found that different anion substitutions led to the broadening of the spectrum of SrLiAl₃N₄:Eu²⁺,which may be the original highly dense crystal structure of the matrix and the symmetry of the local environment surrounding the luminous centre were destroyed.And a small amount of substitution would lead to the appearance of impurity phases in the synthesized sample,making it difficult to achieve the modulation of the luminescence spectrum.2.The Eu²⁺-doped(Sr La Ba)₂Si(ON)₄ green phosphor was prepared by the conventional high-temperature solid-phase method,and the refinement results showed that the prepared sample was a good single phase.The luminescence properties of the phosphors were tested and found that the presence of La³⁺is contribute to their luminescence intensity,quantum efficiency and thermal stability.The decay curves and time-resolved spectra of the samples confirmed the existence of energy transfer between the two Eu²⁺luminous center in the system.As the concentration of La³⁺and the temperature increases,the proportion of the luminous intensity in the short wavelength emission band increases which lead to the blue shift of the spectrum,while a smaller Stokes shift leads to a larger energy barrier,resulting in the increase of thermal stability.