Long-term degradation mechanisms of mid-power LEDs for lighting applications

Abstract

This paper reports the results of a reliability investigation performed on four different groups of commercially available mid-power white LEDs. In order to determine the robustness of this continuously growing class of lighting devices we arranged an experiment of accelerated aging: the four groups of LEDs (from different manufacturers) were submitted to a series of stress tests in environmental chambers with set-point temperatures ranging from 45°C to 105°C, in accordance to the IES LM-80-08 lumen maintenance measurement standard. The experimental data gathered all along the 4000h of stress accumulated up to now suggest the presence of multiple degradation mechanisms that may limit the useful lifespan of the light-emitting diodes under test. In particular we observed the following phenomena: i) a decay of the luminous flux; ii) an increase in the reverse and forward leakage current; iii) the worsening of the chromatic properties of the emitted light; and iv) the presence of a thermally activated degradation mechanism. The results provide a first insight into the reliability of those widely used LEDs; the results on the temperature-dependence of the degradation kinetics can be used as a guideline for the thermal design of modern distributed-light lamps.