With a rapid development of solid state technology, Light Emitting Diodes (LEDs) - one type of solid state devices, has got a stake in solid state lighting application. High brightness Light emitting diodes are the modern light source that has tendency of replacement of conventional light sources including incandescence lamps, fluorescence lamps due to its high efficiency, long lifespan, safe operation because the devices deal with a very low electrical parameters and its environment friendly. However, while the large extension of its application in the field, LEDs reliability become a main factor that can restrict its development.
This thesis entitled "Identifying and Evaluating Ageing Signatures in Light Emitting Diode Lighting Systems: incidence on the system reliability and lifespan" deals with several activities and tasks in order to reach the objective of evaluation the characteristic of the devices and observation of the degradation mechanism of LEDs. The whole experimental bench is designed and realized at Laplace Laboratory by using four different LED types coming from different manufacturers as pre-defined samples with high and poor quality to evaluate their rate speed of their degradation during the same period of stress. There are 32 LED samples for each group so in total there are 128 samples to be tested. And each group is also divided into two different group of electrical stress with the stress current at its nominal current 350mA and the accelerated high stress current 1000mA. There are various methods to evaluate the degradation of the devices including the observation of the modification of electrical and optical parameters of the device that is considered as a primary method and the secondary method by observing its physical symptom by using some microscopic scanning equipment such as Scanning Electron Microscopes (SEM), transmission electron microscope (TEM). However, in this this thesis we mainly focus on the primary method in the evaluation process for the devices by measure the electrical characteristic current-voltage (I-V) for both forward and reverse bias operation by using source-meter unit Kethley 2602A and capacitance-voltage (C-V) characteristic by using impedance analyzer Solartron Modulab. And we also perform photometric measurement by using spectrometer Specbos 1201. The optical parameters include luminance, spectrum, correlative color temperature (CCT) etc. that are the main parameters can indicate the depreciation of the devices. The measure data are also cross compare among the difference groups of samples from different manufacturers to justify the quality of the pre-selected products. |