Research On The Reliability Of Low Color Temperature Phosphor-free LED Light Source

With the continuous breakthrough of LED technology,the application of LED in the field of lighting continues to expand and deepen.At the same time,the market also puts forward higher requirements for LED light source,so as to promote the development of LED light source in the direction of energy-saving,environmental protection,health and intelligent high-quality lighting.The white LED on the market is mostly composed of blue LED and phosphor,among which the blue LED takes up a large proportion and the color temperature is on the high side.In order to meet the healthy and comfortable lighting demand,the color temperature of LED light source should be further decreased.At present,most of the low color temperature LED light sources in scientific research institutions and industries are phosphor-converted LED.Because of the problems of energy loss,phosphor failure and hard to reach low color temperature.Pure LED lighting technology is a better technical route to achieve low color temperature lighting,which has not been developed for a long time because of the restriction of yellow LED efficiency.In recent years,the light efficiency of InGaN yellow LED with Si substrate developed by our company has been significantly improved,which lays a foundation for practical low color temperature pure LED lighting.On this basis,low color temperature phosphor-free LED light source is made by mixing high light efficiency yellow LED and red LED,which shows many advantages such as energy saving,health and high light efficiency.However,as a new artificial light source,its reliability is not clear.In this paper,the reliability and aging mechanism of low color temperature phosphor free LED light source under different current and temperature stress are analyzed,and compared with the commercial low color temperature LED light source of blue light chip with phosphor.The main research results are as follows:1.In order to find out whether the aging characteristics of AlGaInP red LED can match the aging characteristics of InGaN based LED in practice,the aging research of AlGaInP red LED was carried out.The results show that in the early stage of aging,the light power of AlGaInP red LED with different package has been improved.This is due to the thermal annealing effect in the early stage of aging,bring the decrease of defect concentration and the increase of radiation compound.It is also possible to improve the bonding performance of Ni/Ag mirror and SiO2 film,and the increase of light extraction efficiency,which leads to the increase of light power.The optical power attenuation caused by the increase of defects in the active region is mainly concentrated in the first 500 hours of aging.The optical power variation of AlGaInP red LED is similar to that of InGaN based LED with the aging time,which lays a good foundation for mixing the two LED light sources to obtain low color temperature phosphor-free LED light source.2.The high temperature accelerated aging experiments of low color temperature phosphor-free LED light source and phosphor-converted LED light source were carried out to test the changes of their photoelectric parameters.The results show that the performance of each LED remained stable when it was aged at 85?working current.After 2000 hours of aging,the magnitude of luminous flux decline is only 1.6%-2.1%.The increase of current stress or temperature stress will make the influence of phosphor layer on LED reliability gradually obvious.When the aging power is 5.5 W,after aging,the luminous flux attenuation of low color temperature phosphor-free LED is 6.5%,and that of 2000 K phosphor-converted LED is 19%.The difference of attenuation is mainly caused by the poor performance of phosphor under the aging condition of high power.At the same time,the color temperature of all samples shows a decreasing trend after aging at this power.In the low color temperature phosphor-free LED,red light decays more slowly and the proportion increases.In the phosphor-converted LED,the transmittance of phosphor layer decreases and the proportion of blue light decreases.3.In order to explore the working performance of the samples under high temperature stress,several groups of stress conditions were set up,and three kinds of LED light sources were conducted temperature step-stress test.The results show that when the aging current density is 20 A/cm2,with the increase of temperature stress,the luminous flux of 2000 K and 3000 K phosphor-converted LED light sources decreases rapidly near 175?,and 50%of the experimental samples of 2000 K phosphor-converted light sources have failed at 195?,when the temperature is higher than 175?,the phenomenon of silica gel carbonization can be observed in some phosphor LED light sources.The luminous flux of phosphor-free LED light source with low color temperature has no significant attenuation up to 200? in the whole aging process,and the upper temperature limit that can work reliably is higher.The increase of defects in the active region results in the attenuation of luminous flux of low color temperature phosphor-free LED,the degradation of performance of blue light chip and phosphor layer,which together results in the attenuation of luminous flux of phosphor-converted LED,in which the degradation of phosphor layer is more serious,the proportion of blue light increases,and the color temperature of phosphor-converted LED increases.From the above,the performance of all light sources studied in this paper is stable when aging under the working current stress and a certain temperature stress.When the aging current stress or temperature stress is at a higher level,the photoelectric performance of the low color temperature phosphor-free LED light source remains stable without obvious degradation,which has higher reliability than that of the low color temperature phosphor-converted light source.Pure LED light source without phosphor is the development trend of lighting.Low color temperature phosphor-free LED light source is more suitable for road lighting or indoor lighting at night.In order to meet the lighting needs of different occasions,it is necessary to further develop multi-color LED light source and conduct a comprehensive reliability study in the future.