Abstract
The degradation and failure mechanisms of nitride phosphor in GaN-based white LEDs are studied using an in situ multi-functional accelerated life test and different analytical technologies. The electroluminescence results show that both the intensities of the blue emission (BL) from the LED chips and the yellow emission (YL) from the phosphors decrease with the stress time increasing. However, the YL/BL intensity ratio increases with a yellow shift for the chromaticity coordinates. The reason is not only due to the degradation of the LED chip but the nitride phosphors. The generated microcracks and delamination of phosphor layer lead to the increase of blue light absorption. Except for that, we find that the reason for the decrease of the phosphor conversion efficiency is related to the oxidization of the phosphor's luminescence center Eu2+ ions instead of phosphor thermal quenching. These findings help further understand failure mechanism of phosphors during the operation, and improve the long-term color quality for the white LEDs.