Optical degradation of phosphor-converted white gan-based light-emitting diodes under electro-thermal stress

We have investigated the optical degradation of phosphor converted white GaN-based light-emitting diodes submitted to electro-thermal stress (dc of 250 mA and the ambient temperature of 85°C). The optical output degraded rapidly with stress time, accompanied by the change of chromatic properties. After stress time of 300 h, the luminous flux (radiant flux) was reduced by a factor of 0.43 (0.39) and the color temperature was increased by 340 K. Based on analyses of electroluminescence spectra, optical microscopy, and electrical, optical, and thermal properties, optical degradation was found to originate from the temperature-driven darkening of packaged materials and the injected current-driven generation of nonradiative recombination defects.