Study Of Surface Modification Of YAG Phosphor For WLED

Every year, about20%of the electricity is used for illuminating in the world. In order to save energy, the main districts in the world are planning to eliminate the high consumption light sources, such as incandescent lamp, and meantime, expand the high efficiency light sources. Because of the advantages, such as high efficiency, long life and green light source, LED light sources have already become the fourth generation of illuminating. Currently, the LED light sources are mainly used for outdoor illuminating, but still far away from the indoor illuminating due to its high cost. Therefore, it is very important and significant for enhancement of efficiency of LED at a low cost. Nowadays, the dominated white light LED sources in the market are made of blue LED chip and yellow phosphor Y3Al5O12:Ce3+, that is, to Improve the efficiency of YAG:Ce3+phosphor is as important as LED chip for white light LED sources and its spreading.In this thesis, the photoluminescence of commercial YAG phosphor is enhanced by surface modification. Today, the commercial YAG phosphor is prepared by the solid state reaction thanks to its mass production. However, it is necessarily to repeat milling and grinding to achieve micro-particles, which will create many edge angles on the surface, destroy the surface crystallize and bring many surface defects. These features are greatly harmful for the efficiency of YAG phosphor and LED. By comparing the plasma dry etching ability and chemical wet etching ability for YAG:Ce3+phosphor, the chemical wet etching method is chose to modify the surface of YAG phosphor for improving its performance.Firstly, YAG phosphor was modified by40%HF acid solution in a ultrasonic bath with water temperature of80℃for three hours. Thanks to the good defects-selective ability of HF acid solution, the edge angles which contain the highest density of defects are totally removed and the surface becomes smooth. As a result, the modified phosphor has a better crystal lattice (from the XRD results), higher photoluminescence power (increased by5.5%) and less backscatter loss power (reduced by3.05%).Secondly, the etching abilities of different volume ratio of99%NH4F and99% CH3COOH for YAG phosphor have been studied. It is found that99%NH4F+CH3COOH (1:1) has a highly defects-selective ability for YAG phosphor. After being etched for about4hours at ultrasonic water bath with temperature of80℃by the solution, the sharp edge angle of the surface of YAG phosphor become passivation and the smooth surface becomes roughness. The micro-roughness of surface is first time to introduce into the YAG phosphor to increase the multi-reflection on the surface in order to enhance the absorption of the YAG phosphor particles. As a result, the surface defect density is reduced by more than50%thanks to the highly defects-selective ability of99%NH4F+CH3COOH (1:1) solution. And the photo luminescence enhances by13.2%with backscatter loss reducing by5.3%.Another work of this thesis is to prepare remote phosphor for white light LED by spin coating method. The advantages of white light LED with remote phosphor are less glare and higher thermal stability because phosphors are away from the LED chip. Therefore, this method is also an important way for making white light LED plane source.Firstly, the dependents of the thickness of remote phosphor with the spin speed and spin time are studied. And then different thickness and phosphor concentration of remote phosphor have been prepared to study the influences of these two parameters on the photochromic of the white light LED plane source. It was found that the influence of the concentration is more evident than the thickness, and so one can reach different photochromic demands of white light LED plane source by suitably adjusting these two parameters.Secondly, three different kind of remote phosphor were prepared by using the phosphor without modified (Ya)、the phosphor modified by40%HF (YHF) and the phosphor modified by99%NH4F+CH3COOH(1:1)(YFC) at the same thickness and concentration to study the influence of phosphor morphology on efficiency of white light LED plane source. The results show that the modified phosphors have a better transparent for blue light than the unmodified phosphor. The transparent powers of YFc and YHF are increased by12.6mW and14.6mW respectively, and the efficiencies of LED are increased by5.5%for YFC and5.0%for YHF. These results indicate that LED light source has a higher efficiency by using phosphor without edge angle or the phosphor with a rough surface and passivation edge angle. In a word, the surface morphology、photoluminescence and thermal performance are greatly improved by chemical wet etching method. So it is very significant for enhance the efficiency of LED light source. The spin coating method is an efficient method for preparing remote phosphor. Moreover, the work of this thesis including both surface modification and preparing remote phosphor methods which are easy operation and controlling, mass treatment and production and low cost for industry manufacture is very significant for the development and spread of LED light sources.