Multichannel Online Lifetime Accelerating and Testing System for Light-Emitting Diodes

LED寿命长达数万小时，具有高效、节能、环保、高可靠性的优点，是当前国家正在提倡和推广的照明光源。然而，LED长寿命的优点却成为寿命评估的短板，需要耗费大量时间。加速寿命可大大缩短实验时间，同时也是探索LED失效机理，进一步提升LED性能的有效手段。传统的LED加速寿命方法往往采用离线测试方法，将样品连同夹具置于高温箱中，通过高温箱控制样品的整体环境温度，但是在老化过程期间需要中断老化，将样品冷却取出对其各个参数进行测试。为了更加便捷地连续性测试，一些科研机构也提出了在线测试方法，将光电探测器置于高温箱内，进行简单的光电在线测试，但系统易受高温影响。目前，加速寿命测试耗费大量的时间和人力，测量...As the advocated and promoted lighting source in China, LED shows excellence in high efficiency, energy saving, environmental protection, high reliability, and extra-long lifetime which can reach up to hundreds of thousands of hours. However, the advantage of long lifetime is the bottleneck for lifetime evaluation, which would consume a lot of time. LED lifetime acceleration experiments are hopefu...学位：工学硕士院系专业：物理科学与技术学院_工程硕士(电子与通信工程)学号：3332014115283

High Temperature Wide Bandgap Light Emitting Diodes for Harsh Environment Applications

The need for high temperature, high power density power modules for applications such as electric vehicles and space exploration has driven the research into wide bandgap LEDs due to their potential operation at elevated temperatures. Wide bandgap LEDs offer an attractive solution due to properties such as high temperature tolerance, strong radiation hardness and good thermal conductivity. In this thesis, the electrical properties of GaN-on-SiC heterojunctions are studied as a precursor to an LED study, and the optical characterization of an InGaN/GaN MQW is reported. The GaN-on-SiC study revealed that these wide bandgap LEDs have linear sensitivity at high temperatures. The InGaN/GaN MQW PL results revealed that as the temperature increased, the bandgap decreased as well, thus affecting the overall intensity of the material. The results of this study indicate the feasibility of the integration of wide bandgap LEDs into high temperature power modules

Performance Enhancement of Organic Light-Emitting Diodes with an Inorganically Doped Hole Transport Layer

Organic light-emitting diodes (OLEDs) are generally considered as the next generation display and lighting sources owing to their many attractive properties, including low power consumption, wide viewing angle, vibrant color, high contrast ratios and compatibility with flexible substrates. The research and development of OLEDs has attracted considerable interest and has led to significant progress during the last two decades. The use of OLEDs in small-area displays such as cell phone screens, digital cameras, and wearable devices has become a reality. However, the OLED technology is still far from mature, posing a challenge for their widespread acceptance for applications in large-area displays and solid-state lighting. In particular, the lifetime of OLEDs is too short for many commercial applications, and the degradation mechanisms are still under debate. This work aims to improve the OLED device lifetime by doping of organic hole transport materials with inorganic transition metal oxides (TMOs), and to reduce the cost by simplifying the device layer structure and manufacturing procedure.;First, stress tests under continuous wave and pulsed currents were conducted to gain a better understanding of the key factors governing the degradation process of phosphorescent OLEDs. Through comparative studies of the aging behaviors of OLEDs with different hole transport layers (HTLs) under different stressing conditions, we have found that joule heating plays an important role in device degradation when a large energy level misalignment exists at the indium-tin-oxide (ITO) anode/HTL interface. The heating was effectively suppressed by reducing the interfacial energy barrier, leading to a prolonged lifetime of the OLEDs.;P-type doping of hole transport materials with TMOs was then developed as an effective way to reduce the interfacial energy barrier and the operational voltage of OLED devices. A systematical study was carried out on the effects of doping 4,4\u27-Bis(N-carbazolyl)-1,1\u27-biphenyl (CBP), a wide bandgap organic hole transport material, with WO3 and MoO3. The optimal doping conditions including the doping level and doping thickness have been determined by fabricating and characterizing a series of hole-only devices. Integrating the doped HTL into green phosphorescent OLEDs has resulted in a simplified structure, better optoelectronic characteristics, and improved device reliability.;Finally, selective doping of organic materials with the TMOs was developed and the concept of delta doping was applied to OLEDs for the first time. Selective doping was achieved by simple sequential deposition of the organic host and TMO dopant. Hole-only devices with a HTL comprising alternative 0.5 nm TMO-doped/3-10 nm undoped CBP layers exhibited greatly enhanced hole transport and had a turn-on voltage as low as 1.1 V. Simple fluorescent tris-(8-hydroxyquinoline) aluminum (Alq3)-based green OLEDs with a selectively doped CBP HTL showed a lower voltage and longer lifetime under constant-current stressing compared to similar OLEDs with an undoped HTL. Furthermore. delta doping was realized in more thermally stable organic materials, resulting in a marked conductivity increase along the plane of the doped layers by several orders of magnitude. The delta doping effects were explained by hole accumulation in potential wells formed in nanometer-thick doped regions, as revealed by high-resolution secondary ion mass spectrometry (SIMS) measurements

Microbial Quality and Pathogen Decontamination Strategies for Locally-Grown, Fresh Produce from West Virginia and Kentucky

This study aimed to evaluate the microbiological quality/safety of fresh produce from farmers\u27 markets (FM) and assess the post-harvest washing practice with antimicrobials to inactivate Salmonella and Listeria monocytogenes on fresh produce. In study I, 212 produce samples were tested for the presence of Salmonella and Listeria spp. using modified FDA-BAM methods. Aerobic plate counts (APCs), total coliforms (TCCs), and yeast/molds were analyzed on petri-films. Among the 212 samples, the APCs, TCCs, and yeast/molds were 3.72-5.63, 3.67-5.47, and 3.07-4.13 log CFU/g, respectively, with spinach containing the highest (P\u3c0.05) populations. Among all tested samples, Salmonella enterica spp. enterica was detected on 18.6% of spinach, 10.9% of tomatoes, 18.5% of peppers, and 56.3% of cantaloupes, which is much higher than previous reported. Only 3.78% of the samples were confirmed for Listeria spp., and 50% of them were identified as L. monocytogenes, based on multiplex PCR results. Due to the high percentage of pathogens detected on the farmers\u27 marker produce, an evaluation of post-harvest produce washing with various antimicrobials was conducted in study II. Specifically, spinach, tomatoes, green peppers and cucumbers were inoculated with S. Typhimurium and Tennessee or L. monocytogenes and washed in tap water, vinegar water (10%), lactic acid (5%), a lactic and citric acid blend (2.5%), and sodium hypochlorite (200 ppm) for 30 sec or unwashed. Vinegar water (10%) showed better (P\u3c0.05) reduction of S. Typhimurium and Tennessee on tomatoes and cucumbers, and L. monocytogenes on tomatoes and peppers than tap water. The three antimicrobials reached an additional reduction level of 0.9 to 2.7 (S. Typhimurium and Tennessee) and 0.2 to 1.4 log CFU/g ( L. monocytogenes) compared to tap water. Lactic acid caused the greatest (P\u3c0.05) reduction of S. Typhimurium and Tennessee on spinach and green peppers, and sodium hypochlorite showed the great (P\u3c0.05) reduction of L. monocytogenes on cucumbers. The results supplied important information for FM vendors to develop post-harvest protocols to control foodborne pathogens

Deep defects in InGaN LEDs: modeling the impact on the electrical characteristics

Deep defects have a fundamental role in determining the electro-optical characteristics and in the efficiency of InGaN light-emitting diodes (LEDs). However, modeling their effect on the electrical characteristics of the LED is not straightforward. In this paper we analyze the impact of the defects on the electrical characteristics of LEDs: we analyze three single-quantum-well (SQW) InGaN/GaN LED wafers, which differ in the density of defects. Through steady-state photocapacitance (SSPC) and light-capacitance-voltage measurements, the energy levels of these deep defects and their concentrations have been estimated. By means of a simulation campaign, we show that these defects have a fundamental impact on the current voltage characteristic of LEDs, especially in the sub turn-on region. The model adopted takes into consideration trap assisted tunneling as the main mechanism responsible for current leakage in forward bias. For the first time, we use in simulations the defect parameters (concentration, energy) extracted from SSPC. In this way, we can reproduce with great accuracy the current-voltage characteristics of InGaN LEDs in a wide current range (from pA to mA). In addition, based on SSPC measurements, we demonstrate that the defect density in the active region scales with the QW thickness. This supports the hypothesis that defects are incorporated in In-containing layers, consistently with recent publications

Modeling the electrical characteristics of InGaN/GaN LED structures based on experimentally-measured defect characteristics

Defects can significantly modify the electro-optical characteristics of InGaN light-emitting diodes (LEDs); however, modeling the impact of defects on the electrical characteristics of LEDs is not straightforward. In this paper, we present an extensive investigation and modeling of the impact of defects on the electrical characteristics of InGaN-based LEDs, as a function of the thickness of the quantum well (QW). First, we demonstrate that the density of defects in the active region of III-N LEDs scales with increasing thickness of the InGaN QW. Since device layers with high indium content tend to incorporate more defects, we ascribed this experimental evidence to the increased volume of defects-rich InGaN associated to thicker InGaN layers. Second, we demonstrate that the current-voltage characteristics of the devices are significantly influenced by the presence of defects, especially in the sub turn-on region. Specifically, we show that the electrical characteristics can be effectively modeled in a wide current range (from pA to mA), by considering the existence of trap-assisted tunneling processes. A good correspondence is obtained between the experimental and simulated electrical characteristics (I-V), by using-in the simulation-the actual defect concentrations/activation energies extracted from steady-state photocapacitance, instead of generic fitting parameters

Development of hybrid inorganic-organic light-emitting devices with metal oxide charge transport layers

Organic light emitting diodes (OLEDs) are currently being considered as the next generation technology in flat panel displays and solid state lighting applications. Among which, phosphorescent organic light emitting diodes (PhOLEDs) with nearly 100% internal quantum efficiency including other properties such as self emitting, high luminescence efficiency, broad wavelength range, wide viewing angle, high contrast, low power consumption, low weight, and large emitting area are gaining popularity in both academic and industrial research. Although development and commercialization of OLED technology is growing, there are still several key issues that need to be addressed---the external quantum efficiency (EQE) needs to be improved and the biggest technical challenge is to increase the device operational lifetime. Balanced charge injection and transport is vital for improving the device efficiency which demands for selection of better charge injection and transport materials. In addition imbalanced charge injection also degrades the device via joule\u27s heating and charge accumulation thereby limiting the device lifetime. Sensitivity of organic materials to the ambient atmosphere, particularly oxygen and moisture impedes the device performance.;This thesis work attempts to address these issues in the PhOLEDs through selection of proper charge injection and transport material as well as device structure optimization. At first we prepared thin films of thermally evaporated zinc-tin oxide (ZTO) with various ZnO and SnO2 compositions and studied its optical, electrical and morphological properties. After optimization of transparency and conductivity, these ZTO films showed promising materials for alternate transparent conducting oxides and electron transport layer (ETL) functions. Similarly, thin films of thermally evaporated tungsten oxide (WO 3) were prepared and their optical and electrical properties were studied and evaluated as a hole transport layer (HTL) material. We then fabricated and characterized various hybrid light emitting diode (HyLED) structures comprising of---ZTO as an ETL, WO3 as a HTL, and MoO3 as a hole injecting layer (HIL). The device structures were optimized for better performance in terms of efficiency and operational lifetime. Significant enhancement in EQE and operational lifetime were obtained in HyLEDs having WO3 as a HTL than of PhOLEDs with organic HTL. This is because WO3 improved hole injection as well as enabled facile hole transport thereby maintaining the balance of charge injection into the device. Finally, we also prepared inverted HyLEDs using WO3 as HTL and several metals including Ca, Ca/LiF, and Al/LiF as a cathode and their electron injecting capability were studied. Balanced charge injection was observed when a nanometer thick Ca was used as a cathode and WO3 as a HTL. As a result, inverted HyLED with better EQE and operational lifetime were fabricated

Wide Bandgap Based Devices

Emerging wide bandgap (WBG) semiconductors hold the potential to advance the global industry in the same way that, more than 50 years ago, the invention of the silicon (Si) chip enabled the modern computer era. SiC- and GaN-based devices are starting to become more commercially available. Smaller, faster, and more efficient than their counterpart Si-based components, these WBG devices also offer greater expected reliability in tougher operating conditions. Furthermore, in this frame, a new class of microelectronic-grade semiconducting materials that have an even larger bandgap than the previously established wide bandgap semiconductors, such as GaN and SiC, have been created, and are thus referred to as “ultra-wide bandgap” materials. These materials, which include AlGaN, AlN, diamond, Ga2O3, and BN, offer theoretically superior properties, including a higher critical breakdown field, higher temperature operation, and potentially higher radiation tolerance. These attributes, in turn, make it possible to use revolutionary new devices for extreme environments, such as high-efficiency power transistors, because of the improved Baliga figure of merit, ultra-high voltage pulsed power switches, high-efficiency UV-LEDs, and electronics. This Special Issue aims to collect high quality research papers, short communications, and review articles that focus on wide bandgap device design, fabrication, and advanced characterization. The Special Issue will also publish selected papers from the 43rd Workshop on Compound Semiconductor Devices and Integrated Circuits, held in France (WOCSDICE 2019), which brings together scientists and engineers working in the area of III–V, and other compound semiconductor devices and integrated circuits

Comparison Of Thermal And Optical Behaviors Of Pre-Molded And Ceramic Package Light Emitting Diodes

The project signifies thermal and optical characterizations of pre-molded and ceramic package light emitting diodes (LEDs) that are conducted by using thermal transient and light emission recording techniques. It was observed that the luminous intensity of the LEDs is dependent on the input current. However, the LEDs efficiency is reduced for about 15% with the current increases, which due to the high current density and heating of LED junction. In terms of package-level and system-level, pre-molded package LED shows approximately to 13% and 15% lower in junction-to-case and junction-to-ambient thermal resistances respectively than ceramic package LED. Despite, the heat dissipation of ceramic package LED is improved after mounting on a metal-core board (board-level). Here, the effects of packaging material properties and ratio of contacting surface area are applied. Next, the results shows that the application of thermal compound as thermal interface material yields about 6% lower junction-to-ambient thermal resistance than thermal tape. Further investigation shows that the optimum heat convection for pre-molded package LED in an open-air environment is obtained as the cooling fan facing upward; while in terms of still-air environment, it is achieved as the cooling fan facing downward. Lastly, the accuracy of the captured thermal transient is improved by considering the optical power into the determination of real thermal resistance. In sequence, it was seen that the use of direct current reveals more accurate and reliable measurement results where the occurrence of electrical disturbance is minimized