18 August 2009 Extensive analysis of the degradation of phosphor-converted LEDs
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Abstract
This paper reports an extensive analysis of the degradation of Phosphor-Converted Light-Emitting Diodes (pc-LEDs) submitted to accelerated stress tests. We describe the degradation modes of state-of-the-art white LEDs, and the dependence of the Time-To-Failure on the adopted stress conditions. The analysis was carried out on two different sets of commercially available low-power LEDs, stressed under different current (in the range 30-50 mA) and temperature (in the range 80-140 °C) levels for 1600 hours. We have carried out an extensive analysis of the thermal properties of the LEDs, and extrapolated the junction temperature of the devices under the different applied stress conditions. The results of this work indicate that stress can induce both a gradual and a catastrophic degradation of the LEDs. The failure modes detected during gradual degradation are a decrease in the luminous flux and the worsening of the chromatic properties of the LEDs. Experimental data indicate that the degradation kinetics can be strongly influenced by the stress temperature level, and do not significantly depend on the stress current level. On the basis of the results presented within this paper, gradual degradation can be ascribed to the worsening of the optical properties of the package and phosphors-encapsulant of the LEDs. Furthermore, we show that high stress conditions can also determine an increase in the thermal resistance of the samples: by means of X-Ray analysis, we demonstrate that this effect is due to the partial detachment of the LED-phosphors system from device package. Finally, we show that catastrophic degradation is mostly correlated to the shortening of the junction. This effect is due to the generation of parasitic conductive paths that can be identified by means of emission microscopy.
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Matteo Meneghini, Matteo Meneghini, Lorenzo-Roberto Trevisanello, Lorenzo-Roberto Trevisanello, Francesco de Zuani, Francesco de Zuani, Nicola Trivellin, Nicola Trivellin, Gaudenzio Meneghesso, Gaudenzio Meneghesso, Enrico Zanoni, Enrico Zanoni, } "Extensive analysis of the degradation of phosphor-converted LEDs", Proc. SPIE 7422, Ninth International Conference on Solid State Lighting, 74220H (18 August 2009); doi: 10.1117/12.826062; https://doi.org/10.1117/12.826062
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