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Investigarion On Increasing Light-Emitting Effeiency Of Light-Emitting Diodes By Using Nano Dielectric Structures

Posted on:2015-02-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q Y YueFull Text:PDF
GTID:1268330431955128Subject:Radio Physics
Abstract/Summary:PDF Full Text Request
Since the21st century, the consumption of energy has increased dramatically, the energy shortage is getting more and more serious. In order to provide sufficient energy, irrational energy development and utilization has become more and more serious, which leads to environmental pollution. Dealing with the relationship between energy and environment is the key to ensure human survival and development. Except for developing new clean energy addition, energy conservation has become an important means to solve environmental problems. Among the energy, nearly20%is used for light. However, the efficiency of the conventional lighting is very low, resulting in a large waste of energy. As a novel electro-optical conversion device, the efficiency of light-emitting diodes (LED) has exceeded2001m/W, the efficiency of organic light-emitting diodes has also exceeded1001m/W. Compared with conventional light sources, LED has much advantages, such as energy saving,,long life, high color quality, good direction, etc.,and the LED will become the next generation light source.However, the applications of commercial LED production will need a long time. And the most important reason is that the efficiency of the LED be very low. As we know, the efficiencies of the LED include internal quantum efficiency and the light extraction efficiency. With the breakthrough of semiconductor technology, internal quantum efficiency of LED get closer to the theoretical limit. Owning to the high refractive index contrast between semiconductor material and air, the total internal reflection and Fresnel reflection phenomenon occurs at the semiconductor/air interface, resulting in low light extraction efficiency of the LED. Therefore, improving the external quantum efficiency of LED has become a research highlights, which is the focus of this dissertation.There are a number of approaches aimed to the increasing of light extraction efficiency from the LED chip:chip-shaped LED design, placing a back-surface mirror and surface modification. Surface modification technique, including surface roughening and photonic crystal technology, is an very effective method to improve light efficiency of LED. Now, the main means of enhancing the light extraction efficiency of LED experiment, but it has a certain blindness. Therefore, theoretical research on the issue is an urgent task. Nowadays, the fabricaton of the micro/nano structure on LED can enhance the light efficiency of LED. However, how different geometric parameters change the energy distribution within the LED structure and how to affect the extraction efficiency of LED are still not clear. Moreover, the optimization of LED structure parameters is lack of an efficient method. As a popular electromagnetic analysis method, FDTD method can provide help for LED design to some extent. The traditional FDTD method meets two problems, a large amount of calculation and lengthy computation time. In addition, all the parameters of LED structure are interrelated, the task of calculation becomes more difficult. The graphics processor units (GPU) can solve this problem to some extent.In this dissertation, the nano-scale dielectric structures were chosen to improving the light-emitting efficiency of LED. We did some study on these problems mentined above using electromagnetic fields numerical calculation methods and electromagnetic field theory analysis methods. The main contents of this dissertation include:1. This dissertation analyzed the luminescence mechanism and structure characteristics LED. This paper also illustrated the QWs of LED could be equivalent to a dipole source and established the revelent model in FDTD simulation. GPU is employed to accelerate the calculation. This provide a theoretical basis for the next simulation.2. The LED can be modeled by a slab waveguid and the energy distribution of LED is illustrated. The physical mechanism that the photonic crystals can improve the light extraction efficiency is given. On the basis of optimal photonic crystal structure, we study the Influence of position perturbation on the light extraction efficiency.3. The influence of the periodic defect in the photonic crystal LED is analyzed. We take Fourier transform for the dielectric constant distribution map, and found that it has a better direction than perfect photonic crystals.4. The typical OLED is multi-layer planer structure, the embedded PhCs and surface PhCs are used to extract low order modes and high order modes. The optimized OLED structure with double PhCs is proposed.5. The liquid-like amorphous PhCs is simulated by the use of the Metropolis Monte Carlo method with the purely repulsive short-range linear spring potential. The2D FDTD method is employed to investigate the frequency dependence of light propagation characteristics in the amorphous PhCs. The mechanisms by which amorphous PhCs can effectively improve LEE were explored, and optimized amorphous PhCs LED structure is proposed.6. Analyzing the emitted power distribution of the dipole in the semiconductor nanowire, we found that the guided mode has an important role in enhancing the LEE of LED. The parameter such as radius of nanowires and the distance between nanowires were discussed.The work of this dissertation is to design high light-emitting efficiency LED model with nano-scale dielectric structures, analyze and optimize the parameters using electromagnetic numerical methods, and provide important theoretical guidance for the designing and fabricating of high performance LED.The main innovations of the dissertation are as follows:(Ⅰ) A high efficiency LED model with periodic defect photonic crystal is designed, a method to design photonic crystal structure by using a Fourier transform of the dielectric constant pattern is presented. First, the LED planar waveguide model is given to discuss the energy distribution of the LED in various modes, the principle that PhCs can improve the light extraction efficiency of LED is also given. The simulation shows that a small amount of defects in photonic crystal LED does not reduce the LED light extraction efficiency. Instead, the light extraction efficiency of LED will be increased by adding some defects in the photonic crystals. We designed a kind of photonic crystals with defects, and its’light extraction efficiency achieves1.6times of the perfect photonic crystals.(Ⅱ) An OLED structure model with double PhCs is proposed, which can extract low order modes and high order modes from OLED, the mechanism is also discussed. A structure model of the real OLED for analyzing the mechanisms that affect the light out-coupling characteristics is provided, and the embedded PhCs and surface PhCs are used to extract these guide modes. The influence of the PhCs parameter on enhancing the efficiency of the OLED is analyzed by FDTD method. The enhancement of the extraction efficiency in excess of290%is observed for the optimized double photonic crystal OLED.(Ⅲ) FDTD method and Fourier transform technology are empolyed to investigate Anderson localization phenomena in amorphous photonic crystals, and the amorphous strong local mode is demonstrated to improve light extraction efficiency of LED. The liquid-like amorphous PhCs is simulated by the use of the Metropolis Monte Carlo method, and2D FDTD method is employed to investigate the frequency dependence of light propagation characteristics. These strong localization modes confirm that the corresponding modes are sufficiently well located inside a small area for a long time, and the light that propagates in the horizontal direction is redirected and escapes from the LEDs to increase the light extraction.(IV) FDTD method and Electromagnetic theory are used to analyze the influence of parameters of nanowire LED, such as the position of the active layer, the radius of the nanowire, distance between nanowires, on the light extraction efficiency. The guided mode is proved to improve LED light extraction efficiency. First, we analyze the emitted power distribution of the dipole in the semiconductor nanowire. we found that the LEE of LEDs agrees with guided power portion of the total power, that is respect with the radius of the nanowire and the distance between the nanowires. With the geometrical parameters optimized, a radial QWs structure and axial QWs structure are proposed.This work is supported by the project of National basic science research:The coupling effect in the structure of Metal/dielectric nanoscale heterogeneity and its application in photoelectric conversion device. Through our research, The mechanisms by which amorphous PhCs can effectively improve LEE were clearified; Our GPU computing platform is further improved; high performance LED model was proposed; all the conclusions drawn in this dissertation provided very important theriotical reference value in the designment and fabrication of high efficient white LED.
Keywords/Search Tags:Light emitting diodes, Light extraction efficiency (LEE), Finite-difference time-domain, Waveguide theory
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