Chip and package-related degradation of high power white LEDs

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three different kinds of commercially available samples, from leading manufacturers, were submitted to stress under various current and temperature levels. Based on an accurate estimation of the thermal resistance of the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separately evaluating the role of current and temperature in determining the degradation of the LEDs. Results indicate that state-of-the-art LEDs can show a significant degradation of their electrical and optical characteristics, when they are operated close to their current/temperature limits. In particular, data reveal the presence of two different degradation mechanisms: (i) the degradation of the blue semiconductor chip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant concentration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequent worsening of its optical properties. Results suggest that even high-performance LEDs can suffer from limited lifetime: thermal management and driving conditions must be carefully optimized with the aim of achieving high reliability for LEDs to be adopted in high efficiency lighting systems. (C) 2011 Elsevier Ltd. All rights reserved.