The Preparation And Luminescence Properties Of Phosphor Glass Composites And Their Application For High Power LEDs

The white light emitting diode (LED) is the major development trend of the fourth generation solid-state lighting source due to its energy saving, long life characteristic in current global energy crisis. With increasing luminous efficiency and cost performance in recent years, LEDs have more and more applications in general lighting and special lighting. The current most widely used package is the silicone/epoxy resins phosphor converted LED. The LED power increasing causes heat not to be able to be diffused leading to silicone/epoxy resins degeneration, luminous declining, color deviation, shorter life etc. These problems propose new demands on phosphor materials:(1) phosphor materials uniform dispensing;(2) high thermal conductivity, stable physical and chemical properties;(3) increasing the red light radiation of phosphor materials. Recently, the reports about various types of phosphor are frequently reported, but the reports focus on the novel phosphor development, and the above mentioned problems of LED package are rarely considered. Materials achieving the above three requirements are rarely reported. The reason is that the majority of the phosphors have inherent physical and chemical instability. Therefore, the development of a shape controllable, stable properties phosphor material is important, to replace silicone/phosphor mixture.The phosphor glass composites and correlated color temperature (CCT) uniformity enhancement of LED are focused on in this study. This paper proposed three methods of phosphor glass composites preparation, the microstructure and optical properties of fabricated composites are investigated. The phosphor glass composites are used to package LED samples, the LED samples are also measured and the results are analyzed. The effects of diffusion loaded encapsulation on CCT uniformity of LED are also investigated. The detailed contents of this thesis are as follows:Firstly, the YAG:Ce3+phosphors are prepared through the solid state reaction method. The effects of reaction atmosphere, synthesis temperatures and concentration of doped Ce3+on the luminescent properties are systematically studied. The crystal field splitting changes with the increasing content of doped Ce3+. Thus, the emission wavelength firstly shifts to red region and then to blue. The YAG:Ce3+phosphor with highest emission intensity is prepared by sintering the precursor with precursor/flux ratio at10:1in the range of1450-1550℃in a reducing atmosphere.Secondly, a low temperature method for preparing phosphor ceramic is proposed. Slices of YAG:Ce phosphor ceramic are fabricated through solid state reaction at1000℃. The phosphor ceramic is detected to confirm crystal phase by XRD and microstructure by SEM. The phosphor particles dispersed in glass matrix is vividly observed and their distributions are relatively uniform. The phosphor ceramic packaged LED sample shows relatively lower CCT than the conventional LED sample with phosphor dispensing. White color of various CCT can be achieved in the phosphor ceramic slice under465nm LED excitation by controlling the phosphor concentrations or ceramic thickness. The highest luminous efficacy of the phosphor ceramic packaged white LED is75.61Lm/W at350mA.Thirdly, Gel glass containing yellow, green and orange phosphors is fabricated via sol-gel process. Effects of different temperatures on condensation time of the sol and density of the gel glass are investigated. The gel glass is detected to confirm microstructure by SEM. The phosphors are well dispersed in the glass without phosphor degradation. The white light is successfully observed by packaging the glass to the blue LED chip. This study reveals that glasses prepared by the sol-gel process have the ability to be dispersed with phosphor or phosphor mixture at low temperature. The gel glass containing different phosphors in proper ratio applied in LED package realizes high color rendering index more than95.Fourthly, a simple and practical screen printing method for preparing phosphor glass is proposed. Phosphor distribution and element analysis are investigated by optical microscope and FE-SEM. The phosphor particles dispersed in matrix is vividly observed and their distributions are uniform. Spectrum distribution and color coordinates are determined by thickness of screen printed phosphor layer coupled with blue LED chip are studied. The luminous efficacy of the75μm printed phosphor layer phosphor glass packaged white LED is81.24Lm/W at350mA. This study opens up many possibilities for applications using the phosphor glass on a selected chip which emission is well absorbed by all phosphors. The screen printing technique also offers possibilities for the design and engineering of complex phosphor layers on glass substrates. Phosphor screen printing technology allows the realization of high stability and thermal conductivity for phosphor glass. And this simple phosphor glass method provides many possibilities for LED packing, including TFFC (thin film flip chip) and remote phosphor technology.Fifthly, this chapter explored the effects of air gap between LED chip and phosphor glass on CCT and Ra. According to the result, the ratio between blue light and yellow light of the LEDs becomes larger as the air gap increased. The CCT of the WLED increased from5162K to10516K when the air gap increases from h=0μm to h=25μm. Therefore, the color of the LEDs deviated from white to cool white. The air gap between phosphor glass and LED chips enables blue light emitted from the LED chip in large view angles, escaping more easily from the edge of the phosphor glass. This result can be a reference for the design of glass phosphors on high power LED applications.Sixthly, the ordinary widely used phosphor free dispersion method is adopted to package the diffuser loaded encapsulation white LEDs. Using widely used packaging method is most effective to explore effects of diffuser loaded encapsulation on CCT variance and luminous efficiency drop of white LEDs. The results show that proper diffuser loaded encapsulation has much evident effect on CCT variance reduction. Spatial CCT variance and luminous efficiency decrease with the increase of the diffuser content. Among the conditions for achievement of the equivalent in color uniformity, the luminous efficiency drop in the case of1%MF resin diffuser is similar with10%CaCO3diffuser. These phenomena result from difference of the light scattering loss caused by different diffuser mixed in the encapsulation layer. Scattering enhancement leads to lower light transmission. Light transmission and actual scattering needs to achieve a balance. Stronger scattering leads to more uniform CCT, but too much light scattering results in luminous efficiency declining. The improvement in angular color uniformity for the electrically injected InGaN LEDs emitting with MF resin and CaCO3loaded encapsulation can be explained by the increasing in photon scattering. The utility of this low cost and controllable mineral diffuser packaging method provides a practical approach for enhancing the angular color uniformity of LEDs. The diffuser content of1%MF resin or10%CaCO3is the optimum condition to obtain low angular CCT variance and high luminous efficiency.