Degradation mechanisms of high-power white LEDs activated by current and temperature
Highlights
► Analysis of the degradation of white LEDs, for different operating conditions. ► Two different types of state-of-the-art white LEDs were submitted to reliability tests. ► Full characterization of the degradation of the package at high temperatures. ► Analysis of the degradation of the package reflectivity, as a function of wavelength. ► First report of wavelength red-shift after high current stress.
Introduction
The role of high-power white LEDs in general lighting applications is becoming, day-by-day, increasingly important. The reliability of these devices, compared to that of conventional light sources, represents one of the keys for their development and their market penetration. It is then of fundamental importance to deeply understand the various degradation mechanisms that affect the operation of LEDs, in terms of lifetime, chromaticity characteristics and efficiency. The elements that contribute to limit the reliability of white LEDs are of different types and nature, and include chip-level modifications, degradation of the optical properties of the encapsulant materials, delaminations, phosphor degradation. The two main driving forces that contribute to LED degradation are temperature and current. In recent years, many papers reported studies on the degradation mechanisms of white LEDs submitted to high-current and/or high-temperature stress [1], [2], [3]. In most of the cases described in the literature, both the flow of current and high temperatures contribute to the optical power (OP) decay: from these studies it is therefore difficult to separately evaluate the role of these two driving forces in determining device degradation.
The aim of this work is to study the degradation mechanisms of high-power white LEDs by using two different sets of experiment:
- (i)
a set of pure thermal stresses, where devices are exposed to high temperatures, with no applied bias;
- (ii)
a set of iso-thermal biased stresses, where devices are stressed at fixed junction temperature, with different applied current levels.
This allow us to separately evaluate the thermally-activated degradation mechanisms, from the current-driven ones.
The results of these degradation tests suggest that high temperatures have no effects on the electro-optical characteristics of the blue semiconductor chip. On the other hand, thermal storage can significantly degrade the optical properties of the package materials. Finally, injected current is proven to be the main cause for the generation of defects and shunt paths, and for the subsequent loss of optical power.
Section snippets
Experimental details
The devices tested in this work are 1 W power white LEDs, produced by a leading manufacturer. The electro-optical characteristics of these devices are reported in Table 1.
Tested devices were acquired through commercial suppliers, with no additional selection, in order to study the characteristics of the LEDs which are currently available on the market for the end user.
All the LEDs were mounted on an aluminum heat sink by means of a bi-adhesive thermal interface in order to ensure a proper heat
Results
Fig. 1 shows the degradation of the optical power of LEDs submitted to iso-thermal stress-tests, and the comparison with the correspondent thermal stress at 160 °C (black line); as can be noticed, the degradation kinetics of devices stressed at 160 °C with no bias stabilizes after 500/750 h. On the other hand, degradation curves of devices stressed with both bias and temperature (junction temperature equal to 160 °C) showed a stronger degradation for long stress times.
Results therefore suggest that
Discussion and conclusion
With this paper we have presented an extensive analysis of the degradation of 1W-power white LEDs submitted to several stress conditions.
Devices aged at high temperature, with no applied bias, showed a remarkable optical power decrease, with no variation in their electrical characteristics. Optical degradation was found to be significantly correlated to the decrease in the reflectivity of the package (Fig. 3, Fig. 4, Fig. 5), with subsequent worsening in the extraction efficiency of the LEDs.
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