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. This thesis reports the results of a research activity focused on several issues related to the reliability of LED-based lighting systems.
After an initial overview on the most important theoretical concepts necessary for the understanding of the physical results, three main sections can be identified in this thesis, concerning the presentation of research activity:
• The first section reports an extensive study on one of the most critical Electrical Over Stress (EOS) phenomena, called “Hot-plugging”, which occurs when an LED module is directly connected to an energized power supply and can generate current spikes up to several tens of amperes that can potentially destroy or damage the LEDs. The aim of this section is to analyze, for the first time, the nature of the current spikes generated during hot-plugging and to present a simplified model to explain the hot plugging phenomenon. The study is based on transient electrical measurements, carried out on several LED modules (fabricated by different manufacturers), connected to three different power supplies. Results reveal that the amplitude and the time constants of the current spikes are directly determined by the number of LEDs connected in series and by the output capacitance of the current driver.
• Afterwards, the second section presents an extensive study on the effects of Electrostatic Discharges (ESD) on state-of-the-art GaN based LEDs, based on optical and electrical measurements carried out during the ESD events. ESD events were simulated through a Transmission Line Pulser (TLP), which generates voltage pulses with a duration of 100ns and increasing amplitude: during each pulse, spatially resolved electroluminescence measurements were carried out through a high speed EMCCD camera. These measurements allowed to identify the chip region where the discharge is localized and the change in the damaged area induced by consecutive ESD events. In addition, the current and voltage waveforms at the LED terminal were monitored during the tests; this analysis provided important information about modifications the impedance of the devices. The analysis was carried out on different types of commercially available low-power GaN-based LEDs with several differences in the manufacturing technology. Thanks to these tests, we have identified two different failure behaviors during a destructive ESD event, clearly related to the different defects in the semiconductor lattice and to structure of the chip.
• The last section investigates the thermal stability of remote phosphor plates to be used in solid-state lighting systems, for the conversion of the blue light emitted by GaN-based LEDs into white light. A preliminary thermal characterization revealed that in normal conditions of blue light irradiance the phosphor plates could reach temperature levels higher than 60°C, which can affect both performance and reliability. The results of accelerated thermal stress tests indicate that high temperature levels can trigger a relevant degradation mechanism (estimated activation energy is 1.2 eV), that drastically reduces the phosphor conversion efficiency and modifies the photometric and colorimetric characteristics of the emitted white light
