Research On Enhancing Light-emitting Efficiency Of Light-emitting Diodes By Ellipsoid Structure

Light-emitting diode (LED) as a new green light source plays an important role in traffic signals, lighting, backlighting in liquid crystal displays because of small volume, reliably long lifetime, low energy consumption and short response times. With increasingly prominent energy crisis, LED can promotes the implementation of energy saving and emission reduction, due to efficient use of energy conversion and huge market. But lower external quantum efficiency restricts its rapid development. Improving the internal quantum efficiency and light extraction efficiency are two facts to enhance external quantum efficiency. Improvement of internal quantum efficiency may not be readily achieved given its value has reached nearly 80%. Therefore, the best way to improve the luminous efficiency is to increase their light extraction efficiency. The Fresnel reflection and total internal reflection are main factors to influence the LED light extraction efficiency, and overcoming these facts become the focus of research at home and abroad.In order to increase light extraction efficiency in LED, some approaches are proposed such as photonic crystals, surface roughening, patterned sapphire substrates, composite structures, surface micro structure. Because surface micro structure can inhibit total internal reflection and improve the light extraction efficiency, it is paid widely attention. Experimental research plays an important role to enhance light extraction efficiency, but sometimes it has high consumption. In order to reduce unnecessary consumption in experiments, numerical calculation is used to optimize the structure parameters, provide theoretical guidance for the experiment.In this paper, ellipsoid surface periodic structure of gallium nitride (GaN) thin-film flip chip LED becomes research object. Numerical simulation is devoted to optimize ellipsoid structure parameters of thin-film flip chip LED. Internal energy distribution pattern and diffraction efficiency are calculated for analyzing the reason of light extraction efficiency enhancement in thin-film flip chip LED. The main contents are as follows:(1) Finite difference time domain method (FDTD) are applied to analyze the structural parameters (the period and ellipsoid heig ht)effects on thin-film-flip-chip LED light extraction efficiency. At the same time, the internal energy distribution pattern proves ellipsoids structure can gather the incident light in the critical angle and weaken the guide mode. The light extraction efficiency of thin-film flip chip LED is increased to 2.8 times.(2) In order to further study the mechanism of light extraction efficiency enhanced by ellipsoid structure, rigorous coupled wave analysis (RCWA) methods is proposed to calculate diffraction efficiency of periodic structures. Through the analysis of diffraction efficiency found that ellipsoid structure can effectively inhibit thin-film flip chip LED total internal reflection, extend the critical angle, enhanced the LED light extraction efficiency...