Since their first use, solid-state lighting devices have received a growing interest and popularity from engineers, physicists, industry and consumers. Their promising internal efficiencies, i.e. the high conversion rates of injected current to produced light, make them very appealing for the replacement of incandescent or fluorescent light sources. The major challenge for the development of these light sources is to enhance the external light extraction: the active material of light emitting diodes is in general a semiconductor with a high refractive index, which considerably limits the extraction of light into free space due to total reflection. This lack of high external efficiency has generated an important amount of research and publications. Very different approaches have been considered to enhance the external light extraction efficiency: photonic crystals, photonic structures inside and above the active material, graded refractive index layer to reduce the high index contrast between the active material and air, plasmonic resonances and more. In the present research, the bioluminescent organ’s morphology of a Panamanian firefly is used as a source of inspiration to improve the light extraction efficiency of light-emitting devices. The internal efficiencies of light-emitting devices are nowadays very high, yet, the overall efficiency is limited by the total internal reflection due to the high refractive index of the incident medium. Fireflies emit light in their dense bioluminescent organ and light extraction is equally limited by total internal reflection. During their million years of evolution, the bioluminescent organ had the time to improve both, its internal functioning and its light extraction efficiency. The thorough analysis of the lantern’s morphology shows several structures added on the light path, that could influence the light extraction efficiency. The jagged scales form a micrometric, two-dimensional and asymmetric structure which is shown, through simulations, to influence the light extraction positively. The new arrangement of the interface leads to a considerable increase in the light extraction efficiency. A similar analysis is then carried out with a two-dimensional and a three-dimensional symmetric structure. From the simulations it has been concluded that the firefly inspired structure is the most suitable for fabrication on light-emitting diodes. In the framework of a collaboration with microelectronics experts, the jagged-scale structure inspired from fireflies has been adapted to enhance the light-extraction efficiency of an GaN-based LED. Measurements confirm tendency that light extraction increases, as predicted by the simulations.
|Date of Award||25 Apr 2013|
|Supervisor||Jean-Pol VIGNERON (Supervisor), Dimitri D. DEHEYN (Jury), Laurent Francis (Jury), Serge Berthier (Jury), Olivier DEPARIS (Jury) & Laurent HOUSSIAU (President)|