Decay mechanisms of lumen and chromaticity for high-power phosphor-based white-light-emitting diodes in thermal aging

The high-power phosphor-based white-light-emitting diodes (LEDs) with different thicknesses and concentrations of silicone YAG phosphor after thermal aging at 150 degree C and 500 hours are studied experimentally. The results showed that the lumen loss and chromaticity (CIE) blue shift of the high-power white-light LEDs increased as the silicone YAG phosphor thickness and concentration increased. These were due to that the transmittance loss of the silicone YAG phosphor increased and the spectrum intensity of the white- light LEDs decreased as the aging time increased and resulted in lower emission efficiency of the high-power white-light LEDs. The likely cause for transmittance reduction is the refractive index decreased in the silicone YAG phosphor after thermal aging. As a result, the key module package related degradation mode under thermal aging was identified as the decay of the silicone YAG phosphor. Therefore, in order to maintain lumen power and minimize color shift of the high- power white-light LEDs modules for use in low-cost lighting and commercial applications, the optimized thickness and concentration of the YAG phosphor are 2 mm and 2%, respectively.

I. Introduction

GaN-based compound semiconductor materials have been used widely to develop high-performance optical devices, such as high-power blue, green, and ultraviolet light-emitting diodes (LEDs). These LEDs are used extensively as back lighting in liquid-crystal displays, traffic lights, and indoor or outdoor displays^1–5, Intensity degradation of high-power white-light LEDs is a well-known issue, which has been until recently mainly attributed to the degradation of the chip material. With the introduction of new improved chip materials the problem has shifted to the LED packaging and association with reliability. However, the high-power LED packaging and association with reliability have been less investigated in the literatures.