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.
In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal
and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In
the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this
thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels
without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete
electrical and optical characterisation of aged devices was performed, in order to find a correlation between different
aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical
power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage
at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were
well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism
of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a
package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup
and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also
the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial
detachment and degradation of the ohmic contacts.
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.
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