The thesis is devoted to the frequency-resolved investigation of the relaxation processes in optoelectronic materials and devices. Conventional fluorescence decay time measurement technique in the frequency domain was adapted for the use with light-emitting diode (LED) excitation and signal registration by a lock-in amplifier. Inorganic phosphors synthesized by aqueous sol-gel combustion method for using as wavelength converters in white LEDs as well as advanced organic semiconducting materials were investigated by photoluminescence decay time and quantum yield measurement techniques. The photoluminescence decay time measurement technique with extremely low quasi-continuous UV LED excitation was applied for the carrier dynamics research in GaN epitaxial layers. The investigation under such a low excitation conditions revealed the contribution of donor-acceptor recombination in the yellow luminescence of GaN. The techniques for in-situ thermal characterization of encapsulated LEDs, including the measurements of phosphors converter temperature and heat relaxation time constants inside a LED, were developed and demonstrated for the investigation of commercial low- and high- power LEDs. / Disertacija yra skirta relaksacijos procesų, vykstančių optoelektronikos medžiagose ir prietaisuose, tyrimui dažninės skyros metodu. Įprastas fluorescencijos gesimo trukmės tyrimo metodas, veikiantis harmoniškai moduliuoto žadinimo režimu, buvo adaptuotas žadinimui puslaidininkiniais šviestukais ir signalų registravimui radijo dažnių faziniu detektoriumi. Neorganiniai vandeniniu zolių-gelių metodu susintetinti fosforai, skirti baltų puslaidininkinių šviestukų gamybai, ir modernios organinės optoelektronikos medžiagos buvo ištirtos fotoliuminescencijos gesimo dėsnio ir kvantinės išeigos matavimo metodikomis, siekiant nustatyti krūvininkų rekombinacijos savybes ir optimizuoti sintezės parametrus. Realizuotas GaN epitaksinių sluoksnių liuminescencijos kinetikos tyrimas labai žemo kvazitolydinio sužadinimo atveju patvirtino rekombinacijos per priemaišas svarbą šio tipo medžiagose. Buvo sukurtos naujos prekinių šviestukų šiluminių savybių in-situ tyrimo metodikos, veikiančios dažninės skyros režimu. Bangos ilgio keitiklių temperatūra baltuose šviestukuose ir šilumos relaksacijos konstantos įvairaus tipo šviestukų konstrukciniuose elementuose buvo išmatuotos esant vardiniam šviestukų veikimo režimui.

Disertacija yra skirta relaksacijos procesų, vykstančių optoelektronikos medžiagose ir prietaisuose, tyrimui dažninės skyros metodu. Įprastas fluorescencijos gesimo trukmės tyrimo metodas, veikiantis harmoniškai moduliuoto žadinimo režimu, buvo adaptuotas žadinimui puslaidininkiniais šviestukais ir signalų registravimui radijo dažnių faziniu detektoriumi. Neorganiniai vandeniniu zolių-gelių metodu susintetinti fosforai, skirti baltų puslaidininkinių šviestukų gamybai, ir modernios organinės optoelektronikos medžiagos buvo ištirtos fotoliuminescencijos gesimo dėsnio ir kvantinės išeigos matavimo metodikomis, siekiant nustatyti krūvininkų rekombinacijos savybes ir optimizuoti sintezės parametrus. Realizuotas GaN epitaksinių sluoksnių liuminescencijos kinetikos tyrimas labai žemo kvazitolydinio sužadinimo atveju patvirtino rekombinacijos per priemaišas svarbą šio tipo medžiagose. Buvo sukurtos naujos prekinių šviestukų šiluminių savybių in-situ tyrimo metodikos, veikiančios dažninės skyros režimu. Bangos ilgio keitiklių temperatūra baltuose šviestukuose ir šilumos relaksacijos konstantos įvairaus tipo šviestukų konstrukciniuose elementuose buvo išmatuotos esant vardiniam šviestukų veikimo režimui. / The thesis is devoted to the frequency-resolved investigation of the relaxation processes in optoelectronic materials and devices. Conventional fluorescence decay time measurement technique in the frequency domain was adapted for the use with light-emitting diode (LED) excitation and signal registration by a lock-in amplifier. Inorganic phosphors synthesized by aqueous sol-gel combustion method for using as wavelength converters in white LEDs as well as advanced organic semiconducting materials were investigated by photoluminescence decay time and quantum yield measurement techniques. The photoluminescence decay time measurement technique with extremely low quasi-continuous UV LED excitation was applied for the carrier dynamics research in GaN epitaxial layers. The investigation under such a low excitation conditions revealed the contribution of donor-acceptor recombination in the yellow luminescence of GaN. The techniques for in-situ thermal characterization of encapsulated LEDs, including the measurements of phosphors converter temperature and heat relaxation time constants inside a LED, were developed and demonstrated for the investigation of commercial low- and high- power LEDs.

Les matériaux conjugués ont fait l’objet de beaucoup de recherches durant les dernières années. Les nouveaux matériaux présentent des propriétés intéressantes que ce soit au niveau optique, électrique, mécanique ou même les trois en même temps. La synthèse reste la difficulté principale dans la fabrication de dispositifs électroniques. Les méthodes utilisées pour y parvenir sont l’électropolymérisation, le couplage de Suzuki ou de Wittig. Ces techniques comportent encore de nombreuses contraintes et s’avèrent difficilement réalisables à grande échelle. Les thiophènes, les pyrroles et les furanes ont démontré une bonne conductibilité et une bande de conduction basse due à une conjugaison accrue. L’objectif ici est de synthétiser des oligomères principalement composés de thiophènes dans le but d’en caractériser les propriétés spectroscopiques, électrochimiques et de conduction. La synthèse est souvent l’étape délicate de la fabrication de matériaux conjugués. Nous présentons ici une méthode de synthèse simple par modules avec des unités hétérocycliques. Les modules complémentaires sont attachés par condensation entre un aldéhyde et une amine menant à la formation d’un lien robuste, l’azomethine. Les résultats des propriétés photophysiques et électrochimiques de ces matériaux conjugués seront présentés. En ayant recours à différents groupes électrodonneurs et électroaccepteurs, en variant le degré de conjugaison ou en utilisant différents hétérocycles, les propriétés spectroscopiques, électrochimiques et de bande de conduction peuvent être adaptées à volonté, ce qui en fait des matériaux aux propriétés modelables. Ces nouvelles molécules seront analysées pour en déceler les propriétés recherchées dans la fabrication d’OLED. Nous explorerons les domaines de l’oxidation electrochimique réversible et de la polymérisation menant à la fabrication de quelques prototypes simples. / Conjugated materials have received much attention recently as they show promise for industrial applications. These materials are interesting because of the many new possibilities for devices combining unique optical, electrical and mechanical properties. The synthesis is the major difficulty in the fabrication of electronic devices. Usual methods to do so are electropolymerisation, Suzuki or Wittig coupling. Those techniques are full of constraints and are difficult to scale-up. Thiophenes, pyrroles and furans demonstrated good conductibility and low band-gap due to increased conjugation. Our main goal is to synthesize oligomers made principally of thiophene to characterize their spectroscopic, electrochemical and conduction properties. Synthesis is the most important step in the making of conjugated material. A synthetically simple and modular route to novel conjugated material consisting of heterocyclic units is presented. These complementary modules are linked by condensing aldehydes and amines leading to robust azomethine bonds. The resulting photophysical and electrochemical properties of these conjugated materials will be presented. Through the use of different electron donor and acceptor groups, degree of conjugation or by using different heterocycles, the spectroscopic, electrochemical and band-gap properties can be tailored leading to materials with tunable properties. Those new molecules will be analysed to detect properties suitable for OLED fabrication. This presentation will also address the electrochemical reversible oxidation and polymerization of these compounds leading to the making of simple devices.

Thesis (M.S.)--Florida State University, 2006. / Advisor: James R. Simpson, Florida State University, College of Engineering, Dept. of Industrial Engineering. Title and description from dissertation home page (viewed Sept. 22, 2006). Document formatted into pages; contains ix, 95 pages. Includes bibliographical references.

Dans ce travail, les dégradations des diodes électroluminescentes (DEL) ont été étudiées en identifiant et en évaluant leurs signatures électriques et photométriques en vieillissement accéléré sous stress thermique et électrique. Un prototype de banc de test expérimental a été développé et construit spécifiquement pour cette étude ce qui nous a permis de tester 128 échantillons en appliquant différentes conditions de stress thermiques et électriques. Quatre types différents de DEL ont été étudié avec des caractéristiques techniques similaires (température de couleur, courant nominal, mono-puce,...) mais avec des technologies différentes couvrant les principaux acteurs du marché (Cree, Osram, Philips et Seoul Semiconductor). Les échantillons ont d'abord été caractérisés à leur état initial, puis soumis à des conditions de stress électrique (à 350mA ou 1050mA) et thermique (fixé à 50°C). Les mécanismes de défaillance ont été analysés en étudiant l'évolution des signatures électriques et photométriques. Ces caractérisations ont permis d'évaluer et de déterminer l'origine des dégradations à différents niveaux : puce semi-conductrice, interconnexions, phosphore ou encapsulation du dispositif. Les caractérisations électriques nous ont permis d'identifier les mécanismes de dégradation de la puce semi-conductrice et de déterminer la nature des dégradations au niveau du contact ohmique du dispositif (sous fort courant injecté). Les caractérisations photométriques complètent cette étude en évaluant les dégradations associées à l'optique (encapsulation et packaging). / In this work, the degradation of light emitting diodes (LEDs) is studied by identifying and evaluating their aging signature during the stress time. The custom-made experimental test bench is built for realization of the test measurement. Through this experimental test bench, it allows to test a large amount of LED samples and enable to select different temperature condition and different current stress level. There are four different types of LED with similar characteristic in term of their color temperature, IF, VF, power (1W) and as monochip, but different technology coming from Cree, Osram, Philips and Seoul Semiconductor. The devices are firstly characterized their electrical and photometrical characteristic at their initial state, then they are submitted to different current stress condition at low current stress (350mA) and high current stress (1000mA) while the thermal stress is fixed at one temperature (50°C). The study of these devices failure mechanism is archived by using the primary method based on the electrical and photometrical characterization of the devices that allows to evaluate their degradation at different locations of the device components such as semiconductor chip, interconnection and device's package. The electrical characteristic of the device's I-V curve: at low injected current level and reverse bias allow us to identify the degradation characteristic of device's semiconductor chip, at high injected current level allows us to determine the degradation of device's ohmic contact and photometric characteristic allows us to evaluate the degradation of device's package system.

Ce travail porte sur l'amélioration des performances en vieillissement de micro-écrans vidéo à matrice active en technologie OLED sur Silicium. La perte d'efficacité lumineuse et la dérive en tension induites par le vieillissement des OLED restent depuis plusieurs années un point faible inhérent à cette technologie et représente toujours un verrou dans la commercialisation des micro-écrans sur un marché encore jeune. Nous proposons ici une étude d'optimisation d'un empilement OLED blanc bi-émetteurs à émission vers le haut basée sur l'amélioration systématique des modes de défaillances reconnus des OLED et adaptée aux spécificités de réalisation de micro-écrans à très forte résolution. Des outils originaux tels que des structures simplifiées de types monocouche ou monoporteur seront utilisés pour comprendre et réduire ces différents mécanismes de dégradation. Du fait de la complexité des structures OLED actuelles, des structures simplifiées seront notamment développées et analysées dans le but d'accéder à la compréhension des phénomènes intrinsèques de dégradation opérant au sein des couches organiques, à leurs interfaces ou encore aux interfaces avec les électrodes. Une méthode de caractérisation électrique encore peu utilisée dans le cas des LED organiques, la spectroscopie d'impédance, sera également développée. Cette technique de caractérisation électrique très prometteuse et surtout non destructive permet à travers l'étude des comportements capacitifs des dispositifs l'accès à de nombreuses informations relatives à la dynamique des charges liées ou mobiles dans les zones de bulk ou interfaciales des matériaux. / The present study deals with active-matrix OLED microdisplays, based on a white top-emitting bi-emitters structure. The optimization of these devices lifetime is the main point of this manuscript. The luminous efficiency loss and the voltage drift induced while ageing of the device under constant current driving conditions are indeed key parameters. A first part consists in understanding the main degradation mechanisms known to operate in OLED devices. A focus on intrinsic mechanisms is here chosen to improve devices lifetime. Extrinsic mechanisms like encapsulation issues or other process optimization are not developed in this work. We propose here a systematic study on the influence of OLED structure parameters on initial but especially on aging performances. The optimization of anode electrical contact through plasma treatments and a thin oxide interlayer show very interesting results for reducing operating bias and voltage drift induced while aging under constant current. The enhancement of doping percentage in doped injection layers also show significant improvement on devices performances, with the great advantage of being a useful tool for controlling devices efficiency. We also find that an optimization of the emission layers thicknesses could lead to great lifetime improvement. Those results are also combined and confirmed by a Design Of Experiments meant to determine the influence of the main process parameters on devices performances. Finally, we initiate the characterization of our OLED devices using impedance spectroscopy measurements. From the modeling of single-layer structures to the understanding of simple bipolar devices through analysis of capacitive evolution of full-stack devices with time, we here show that this technic appears very useful for the understanding of charge carrier dynamic and could help reducing charge accumulation.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The understanding of the interrelationships among the photometrical, electrical and thermal (photo-electro-thermal) parameters is extremely important to design lighting systems based on light-emitting diodes (LEDs), because the LEDs are highly influenced by these conditions. The efficacy and lifespan of an LED system that uses even the best LEDs, commercially available, will be only a fraction of what it would be if the electrical, thermal and optical design is poorly designed. This work proposes an improvement in the estimative of the luminous flux using photo-electro-thermal parameters, without needing any laboratory test. When designing an LED lighting system, it is possible to get almost identical results in terms of luminous flux, luminous efficacy and lifetime by scaling the parameters: intensity of the LED current, the amount of LEDs and the system design cooling. Therefore, a design methodology for LED lighting systems is also proposed. This methodology provides the creation of a design space where the designer can directly identify: the value of the luminous efficacy, thermal resistance of the heatsink and junction temperature when varying the number of LEDs and the direct current, without performing any laboratory testing or using some simulation software with high computational cost. Thus, the designer can set the initial design faster and relate it to the costs involved. Experimental tests were performed in order to validate the proposed design methodology. / O entendimento das inter-relações entre os parâmetros fotométricos, elétricos e térmicos (fotoeletrotérmicos) é extremamente importante para sistemas de iluminação que utilizam diodos emissores de luz (LEDs), pois os LEDs são extremamente sensíveis a estas condições. A eficácia e vida útil de um sistema que utiliza até mesmo os melhores LEDs do mercado será apenas uma fração do que seria se o projeto elétrico, térmico ou óptico for mal concebido. Este trabalho propõe um aprimoramento da estimativa do fluxo luminoso utilizando parâmetros fotoeletrotérmicos, sem a necessidade de realizar nenhum ensaio laboratorial. Ao projetar um sistema de iluminação a LEDs pode-se chegar a resultados quase idênticos em termos de fluxo luminoso, eficácia luminosa e vida útil dimensionando-se os seguintes parâmetros: intensidade da corrente direta aplicada, o número de LEDs utilizados e o dimensionamento do sistema de arrefecimento. Portanto, uma metodologia de projeto para sistemas de iluminação a LED também é proposta. Essa metodologia proporciona a criação de um espaço de projeto no qual o projetista pode identificar diretamente qual é o valor da eficácia luminosa, da resistência térmica do dissipador e da temperatura de junção quando são variados o número de LEDs e a corrente direta, sem realizar qualquer ensaio laboratorial ou usar algum software de simulação com alto custo computacional. Dessa maneira, o projetista pode definir o projeto inicial mais rapidamente e já relacionar os custos envolvidos. Testes experimentais foram realizados para validar o aprimoramento da estimativa de fluxo e da metodologia de projeto proposta.

No description available.

A utilização de luminárias baseadas em diodos emissores de luz (LED) é uma tendência para iluminação de interiores e para iluminação pública devido a sua alta durabilidade e eficiência. Apesar destas características desejáveis impulsionarem a utilização de LEDs na iluminação, a vida útil dos drivers utilizados para o acionamento de LEDs de potência, baseados em topologias tradicionais, pode ser inferior a vida útil do próprio LED. Estes drivers são, geralmente, constituídos de topologias que utilizam capacitores eletrolíticos, que tem baixa vida útil, principalmente, em altas temperaturas de operação. Assim, este trabalho propõe uma luminária LED para iluminação pública que seja baseada em drivers modulares. Esta metodologia, visa o aumento da vida útil dos drivers, bem como a manutenção do funcionamento da lâmpada mesmo quando um dos módulos falhar. O projeto dos drivers eletrônicos e a modelagem matemática dos conversores é apresentada. Além disso, são apresentados resultados experimentais obtidos através de um protótipo com 50W (dois módulos com potência média de 25W, cada um). / The use of LED based luminaires is a tendency for both indoor and street lighting, due to its high durability and efficiency. Despite these desirable features propel the use of LEDs on lighting systems, the lifespan of its drivers can be lower than the LED itself. Such drivers are usually based on topologies that make use of electrolytic capacitors, which present low durability, mainly in high temperatures. Therefore, this work proposes a LED luminaire for street lighting based on modular drivers. This approach intends to increase the lifetime of the drivers, as well as maintain the luminaire in operation even with one or more modules failure. The design of the electronic drivers and the technique used for the mathematical modeling of the converters are presented. Furthermore, the obtained experimental results from a prototype with rated power of 50W (two modules with rated power of 25W each one) are presented.

A utilização de luminárias baseadas em diodos emissores de luz (LED) é uma tendência para iluminação de interiores e para iluminação pública devido a sua alta durabilidade e eficiência. Apesar destas características desejáveis impulsionarem a utilização de LEDs na iluminação, a vida útil dos drivers utilizados para o acionamento de LEDs de potência, baseados em topologias tradicionais, pode ser inferior a vida útil do próprio LED. Estes drivers são, geralmente, constituídos de topologias que utilizam capacitores eletrolíticos, que tem baixa vida útil, principalmente, em altas temperaturas de operação. Assim, este trabalho propõe uma luminária LED para iluminação pública que seja baseada em drivers modulares. Esta metodologia, visa o aumento da vida útil dos drivers, bem como a manutenção do funcionamento da lâmpada mesmo quando um dos módulos falhar. O projeto dos drivers eletrônicos e a modelagem matemática dos conversores é apresentada. Além disso, são apresentados resultados experimentais obtidos através de um protótipo com 50W (dois módulos com potência média de 25W, cada um). / The use of LED based luminaires is a tendency for both indoor and street lighting, due to its high durability and efficiency. Despite these desirable features propel the use of LEDs on lighting systems, the lifespan of its drivers can be lower than the LED itself. Such drivers are usually based on topologies that make use of electrolytic capacitors, which present low durability, mainly in high temperatures. Therefore, this work proposes a LED luminaire for street lighting based on modular drivers. This approach intends to increase the lifetime of the drivers, as well as maintain the luminaire in operation even with one or more modules failure. The design of the electronic drivers and the technique used for the mathematical modeling of the converters are presented. Furthermore, the obtained experimental results from a prototype with rated power of 50W (two modules with rated power of 25W each one) are presented.