Pump chip and phosphor reliability of broadband light-emitting diodes

Abstract

We present a study of the degradation of phosphor-based broadband (~90nm spectral peak width) colour and white LEDs. Specifically, our study looked at the reliability of the blue-emitting GaN/InGaN pump chip and the overlying phosphor in these LEDs. We have investigated thermal degradation arising from heat generation in both the pump chip and the colour-converting phosphor. The robustness of the pump chip in 1W broadband power LEDs was examined by driving them with various dc and pulsed waveforms at different temperatures. Both catastrophic and long term degradation of pump chips was investigated. Long term degradation behaviour of phosphors was studied by both ex situ and in situ heating of phosphors for hundreds of hours while their total light output was monitored. Optical energy storage in phosphors and its bearing on phosphor degradation is also discussed.