Effect of phosphor layer’s location on LED’s luminous depreciation under elevated temperature

Abstract

High power white light-emitting diode (LED) has been widely used in our daily life due to its extraordinary characteristics of high luminous efficiency, low power consumption, long lifetime and environment protection. Among all the technologies to produce white light, phosphor converting scheme is most popular. In this mode, part of the blue light emitted by LED chip is converted into yellow light by phosphor layer. Then a white output light of blue and yellow light mixture is obtained. The conversion ability of phosphor layer can determine LED’s performance parameters including luminous efficiency, correlative color temperature, angular color uniformity, etc. When one of the parameters does not meet the requirement (like relative light output drops below 70%), the LED fails. Therefore, LED’s reliability is greatly affected by phosphor layer’s property. Previous study shows that the conversion efficacy of phosphor exhibits exponential degradation as temperature increases. The decrease of phosphor’s conversion efficacy can induce light output decrease and deteriorate the reliability. Thus, for phosphor converted LED (pc-LED), thermal management is of great importance. In conventional phosphor coating process, phosphor gel is directly dispensed onto the LED chip. When the LED is working, heat generated by the chip is accumulated due to the poor heat conduction of phosphor layer, resulting in the phosphor temperature rise. Besides, the phosphor’s self-heating can also cause phosphor temperature rise. To solve this problem, some researchers used remote phosphor coating to reduce the influence of heat produced by chip. The phosphor layer was moved away from the chip to avoid contact. It means that the phosphor layer’s location can have an effect on pc-LED’s reliability. In this paper, the phosphor layer’s location was further controlled by phosphor slice. The phosphor slices were prepared by mold to keep specific size. An online testing method was adopted to investigate the effect of phosphor layer’s location on LED’s lumen maintenance under accelerated temperature life test. Three cases were aged under temperature of 120oC, namely blue LED module without phosphor layer, white LED module by conventional phosphor coating and white LED module by combining blue LED module and phosphor slice. The lumen maintenance was monitored and recorded every 30 s. After 30 h of aging, the light output of three cases of modules got stabilized. The results showed that the relative light output of conventional packaged white LED module had a 30% drop, while that of the white LED module by using phosphor slice only dropped 2.7%. The reason behind the phenomenon was well explained. Compared with the conventional phosphor layer, phosphor slice was away from the chip so that the heat generated by chip would not affect the phosphor temperature. Apart from this, the phosphor slice was in the air and the good heat dissipation can reduce the influence of phosphor self-heating. Thus, the temperature of phosphor slice was much lower than that of the conventional one and the reliability was somewhat enhanced. To sum up, the effect of phosphor layer’s location on LED’s luminous depreciation was studied by online testing system in this paper. Phosphor slices were prepared to achieve various phosphor location and comparison experiments were conducted. The results showed that white LED module with phosphor slice had better reliability. The reason was attributed to the good heat dissipation property of phosphor slice. This work presented an attempt for improving LED’s reliability. For conventional packaged LED, better heat dissipation design was needed to improve the performance.