Stability and performance evaluation of high-brightness light-emitting diodes under DC and pulsed bias conditions

This paper presents an experimental analysis of high brightness light emitting diodes (HBLEDs) performance and stability under dc and pulsed current bias. Three different families of HBLEDs from three leading manufacturers have been considered. The analysis was carried out by means of current-voltage, integrated optical power and electroluminescence measurements, and failure analysis. After an initial characterization of the electrical, optical and thermal behavior of the devices, a set of ageing tests was carried out, both under dc and pulsed bias conditions. Identified degradation modes were efficiency decrease, series resistance increase, leakage current increase, and modifications of the emitted spectrum. Characterization of devices behavior during stress indicated (i) generation of non-radiative components, (ii) degradation of the anode contacts and bonding wires, (iii) degradation of the phosphorous layer conversion efficiency and (iv) of the plastic package as possible responsible of the electrical and optical degradation of the LEDs. Comparison between dc and pulsed stress carried out using the same average current level and different duty cycle values showed that the use of pulsed bias can reduce the degradation rate with respect to dc bias. However, for duty cycles lower than 20%, fast degradation and abrupt ruptures can take place, due to the high peak current levels.