Study Of White LED Packaging Phosphor Properties

Abstract：With the global energy crisis and environmental pollution has beenincreasing, LED (Light Emitting Diode, LED) for its energy-saving,environmentally friendly features gets more and more attention. Home andabroad are vigorously promoting the development of LED industry. To makethe LED is widely used, quality is essential. There are two major concepts forthe generation of white LEDs: the mixing of the light from monochromaticLED chips, e.g., red-green-blue (RGB) LED chips, and the color conversionapproach, in which some portion of light from UV or blue LED die excites acolor conversion element,e.g., phosphor, which is placed in the direction ofthe light emitted from the die, and which is today’s most common approachfor white light generation. For phosphor-converted white LED package,phosphor is an indispensable material. Its characteristics are closely related tothe quality of the LED devices. Study the effects of phosphor differentcharacteristics on LED devices in order to resolve some key package issuesare study core of the paper. The study includes the following components:1. Based on Mie’s theory, we set up the physical model of phosphor.Theoretical calculation is use to probe the absorption and scatteringproperties of phosphor particle. At the same time taking advantage of opticalsimulation software LightTools constructing simple double-integrating spheretest system, which is used to make a preliminary study for the influence of thephosphor particles, phosphor layer thickness, phosphor concentration on lightscattering and absorption properties.2. Light-emitting-diode (LED) packaging is crucial for the development ofsolid-state lighting. The common method of making white light-emitting-diode(WLED) is to mix the blue light from the LED die and the wavelengthconverted yellow light from the phosphor layer. In this paper, we discuss thatinfluence of phosphor concentration and drive current intensity on whitelight-emitting-diode characteristics. Software simulations and the realpackaging are adopted for the study. Influence of phosphor concentration onluminous flux and correlated color temperature (CCT) for whitelight-emitting-diode are investigated with three dimension ray tracing simulations and the results are verified in use of the real packaging. Inaddition, junction temperature and color rendering of white LED are alsoinvestigated. The results show that CCT decreases with an increase ofphosphor concentration, however luminous flux achieves an optimal valueand then decreases. The increased junction temperature caused by phosphorconcentration and drive current intensity change degrades phosphorconversation efficiency. For color rendering of white LED, phosphorconcentration higher has a lower color rendering index, color rendering indexincreases with an increase of drive current intensity and achievesstabilization.3. In this section, we study the effect of phosphor particle sizes on opticalproperties of phosphor-converted (pc) white light emitting diode(LED).Based on optical ray tracing theory, for the pc-white LEDs that are packagedin use of phosphor of different particle sizes at nanometer and micrometerscale are investigated with three dimension ray tracing simulations. Theresults show that when the phosphor particle sizes are nanometer, the effectof phosphor particle sizes on the lumen output of pc-white LED package islittle; when the phosphor particle sizes are micrometer, the lumen outputincreasing as phosphor particle size increasing. At the same time the studyalso show that in order to get the same correlated color temperature(CCT),using a larger phosphor particle size’s white LED requires a higher phosphorconcentration.