Dissertations / Theses on the topic 'Diodes'

Le developpement de l'electroluminescence de couches minces composees de matieres organiques debute dans les annees 80 avec les travaux de tang et van slyke. Plus tard la technique d'auto-assemblage couche par couche permet l'incorporation de polymeres conducteurs dans les films minces. Le poly(p-phenylene vinylene) (ppv) est le polymere electroluminescent le plus utilise dans les etudes portant sur les diodes organiques polymeriques. Son incorporation dans l'architecture du film se fait via son precurseur cationique non conjugue et soluble qui est converti in-situ en la forme finale du polymere conjugue. Nous nous sommes attaches a montrer que la variation de l'architecture interne de ces films pouvait avoir une influence sur le comportement optoelectronique de ces diodes. Ainsi, en partant d'un systeme connu (ito/pei/(ppv/pma) 2 0/al), nous avons incorpore divers materiaux (polyaniline sulfonee, nanoparticules de tio 2, montmorillonite). La presence de poly(aniline sulfonee) (pani) connue pour ses proprietes injectrices de trous, augmente l'efficacite des diodes par rapport au systeme reference. L'interpenetration entre les couches de ppv et de pani est un facteur determinant au niveau du transport de charges a l'interieur du film. L'incorporation de tio 2 nous a montre clairement que la nature cet oxyde influe de maniere significative les performances du systeme puisqu'en presence de quasi-tio 2 cristallin aucune electroluminescence n'a ete detectee. Des nanoparticules amorphes augmentent l'efficacite des diodes en modifiant les proprietes de transport des charges et la probabilite de recombinaison electron-trou. Enfin, l'introduction de montmorillonite provoque un effet de compartimentation par rapport au transport d'ions demontre par reflectivite de neutrons et par spectroscopie uv-visible. D'autre part les caracteristiques electroluminescentes des diodes ont evolue significativement en presence du mineral argileux. Le positionnement et le nombre de couches d'argile jouent un role determinant, et l'efficacite du systeme augmente avec ce nombre de couches. Nous pensons que les charges circulant dans le film peuvent subir un ralentissement ou une deviation engendres par la couche de mineral isolante. Nous avons donc clairement l'influence de l'architecture interne sur les caracteristiques electroluminescentes des diodes organiques a travers l'incorporation de divers materiaux.

This thesis describes the development of III-Nitride materials for light emitting applications. The goals of this research were to create and optimize a green light emitting diode (LED) and laser diode (LD). Metalorganic chemical vapor deposition (MOCVD) was the technique used to grow the epitaxial structures for these devices. The active regions of III-Nitride based LEDs are composed of InₓGa₁₋ₓN, the bandgap of which can be tuned to attain the desired wavelength depending on the percent composition of Indium. An issue with this design is that the optimal growth temperature of InGaN is lower than that of GaN, making the growth temperature of the top p-layers critical to the device performance. Thus, an InGaN:Mg layer was used as the hole injection and p-contact layers for a green led, which can be grown at a lower temperature than GaN:Mg in order to maintain the integrity of the active region. However, the use of InGaN comes with its own set of drawbacks, specifically the formation of V-defects. Several methods were investigated to suppress these defects such as graded p-layers, short period supper lattices, and native GaN substrates. As a result, LEDs emitting at ~532 nm were realized. The epitaxial structure for a III-Nitride LD is more complicated than that of an LED, and so it faces many of the same technical challenges and then some. Strain engineering and defect reduction were the primary focuses of optimization in this study. Superlattice based cladding layers, native GaN substrates, InGaN waveguides, and doping optimization were all utilized to lower the probability of defect formation. This thesis reports on the realization of a 454 nm LD, with higher wavelength devices to follow the same developmental path.

Ce travail concerne la croissance par épitaxie sous jets moléculaires avec NH3 comme source d'azote de diodes électroluminescentes (DELs) blanches monolithiques. La méthode proposée au laboratoire consiste à insérer dans la zone active de la DEL des puits quantiques (Ga,In)N émettant dans le bleu et le jaune. Bien que la concentration en In de l'alliage InxGa1-xN soit limitée à 20-25%, la présence d'un champ électrique interne dans les hétérostructures nitrures permet d'obtenir des longueurs d'onde balayant tout le spectre visible. Malheureusement, ce champ électrique diminue aussi la force d'oscillateur de puits quantiques de largeur de plus de 2nm. Ainsi, l'efficacité radiative de puits quantiques émettant dans le jaune est faible comparé à un puits quantique émettant dans le bleu. L'émission dans le jaune pose donc problème et il nous a fallu tenté de contrebalancer les effets du champ électrique afin d'accroître le rendement quantique des puits larges. D'autre part, la puissance des DELs EJM est faible par rapport à celles épitaxiées en EPVOM. Une des grandes différences entre ces deux techniques est la croissance du GaN de type p. Différents paramètres ont été testés afin de déterminer les conditions de croissance adéquates à l'épitaxie du GaN de type p de bonne qualité optoélectronique. Finalement, ces différents affinages ont été vérifiés par l'élaboration de DELs et en particulier d'une DEL blanche monolithique à large spectre.

Research on wide band gap semiconductors suitable for power electronicdevices has spread rapidly in the last decade. The remarkable results exhibited bysilicon carbide (SiC) Schottky batTier diodes (SBDs), commercially available since2001, showed the potential of wide band gap semiconductors for replacing silicon (Si)in the range of medium to high voltage applications, where high frequency operationis required. With superior physical and electrical properties, diamond became apotential competitor to SiC soon after Element Six reported in 2002 the successfulsynthesis of single crystal plasma deposited diamond with high catTier mobility. This thesis discusses the remarkable properties of diamond and introducesseveral device structures suitable for power electronics. The calculation of severalfigures of merit emphasize the advantages of diamond with respect to silicon andother wide band gap semiconductors and clearly identifies the areas where its impactwould be most significant. Information regarding the first synthesis of diamond,which took place back in 1954, together with data regarding the modern technologicalprocess which leads nowadays to high-quality diamond crystals suitable for electronicdevices, are reviewed. Models regarding the incomplete ionization of atomic dopantsand the variation of catTier mobility with doping level and temperature have beenelaborated and included in numerical simulators. The study introduces the novel diamond M-i-P Schottky diode, a version ofpower Schottky diode which takes advantage of the extremely high intrinsic holemobility. The structure overcomes the drawback induced by the high activationenergies of acceptor dopants in diamond which yield poor hole concentration at roomtemperature. The complex shape of the on-state characteristic exhibited by diamondM-i-P Schottky structures is thoroughly investigated by means of experimentalresults, numerical simulations and theoretical considerations. The fabrication of a ramp oxide termination on a diamond device is for thefirst time reported in this thesis. Both experimental and simulated results show thepotential of this termination structure, previously built on Si and SiC power devices. A comprehensive comparison between the ramp oxide and two other versions of thefield plate termination concept, the single step and the three-step structures, has beenperformed, considering aspects such as electrical performance, occupied area,complexity of technological process and cost. Based on experimental results presented in this study, together withpredictions made via simulations and theoretical models, it is concluded that diamondM-i-P Schottky diodes have the ability to deliver significantly higher performancecompared to that of SiC SBDs if issues such as material defects, Schottky contactformation and measurement of reliable ionization coefficients are carefully addressedin the near future.

A relatively simple, yet complete analytical model for predicting the performance of illuminated or unilluminated (dark) pn diodes with arbitrary doping profiles is developed and presented in this dissertation. It can be used to calculate the saturation current, minority carrier density, short circuit current, spectral response, and effective low-high (p-p⁺) junction recombination velocities of such diodes. The model is applied to dark or illuminated n⁺-p-p⁺ diodes as a function of the front and back surface recombination velocities and the bulk doping profiles. The analysis includes heavy doping effects. The results predicted by this model are compared with those predicted by numerical simulation programs. Both results agree well with each other and with the experimental data available. The complete analytical expressions produced by the model can be reduced to simpler forms for the transparent and quasi-transparent cases. These forms agree with the special case expressions developed by others. The new model is a substantial contribution leading to improved understanding of such devices.

Le diamant est souvent défini comme le matériau ultime pour la réalisation de composants à semi-conducteurs pour des applications d'électronique de puissance. Bien que plusieurs interrupteurs de puissance en diamant soient parus à l'échelle mondiale, ils sont à l'heure actuelle à l'état de prototype et de preuve de concept. Il est donc nécessaire de comprendre leurs mécanismes de fonctionnement afin de pouvoir utiliser tout leur potentiel dans des convertisseurs de puissance. Dans cette thèse, l'analyse se focalise sur des diodes Schottky en diamant de structure pseudo-verticale. Des caractérisations statiques et en commutation des diodes Schottky ont tout d'abord été réalisées. Elles ont permis d'extraire les caractéristiques des composants et de les intégrer dans des convertisseurs de puissance afin d'analyser leur comportement en commutation. L'utilisation et la gestion des diodes dans des convertisseurs ont ensuite été étudiées. Ces études ont permis de proposer des modifications de la structure des diodes afin d'améliorer la performance de leur intégration dans des convertisseurs de puissance. Finalement l'analyse théorique des performances d'une diode Schottky en diamant dans un convertisseur est réalisée. La comparaison entre ces performances et celles d'une diode Schottky en SiC a permis de mettre en évidence les particularités des composants en diamant ainsi que les bénéfices qu'ils peuvent apporter à l'électronique de puissance
Diamond is considered as the ultimate semiconductor for power electronics applications. Even if diamond semiconductor devices have been realized worldwide, it is still prototype or proof of concept devices. It is then necessary to understand how do they operate to use their entire benefits in power converters. In this thesis, we focused the analysis on pseudo-vertical diamond Schottky diodes. Firstly, static and switching characterizations have been realized. They allow us to extract devices characteristics in the way to integrate them in power converters to analyze their switching abilities. Management of diodes in power converters is then studied. These studies allow us to propose device structure modifications in the way to improve diodes performances and their integration in power converters. Finally, a theoretical analysis on a diamond Schottky diode performances in a power converter is realized. It has been compared to the performances of a SiC Schottky diode. It highlights the particularities of diamond devices and the benefits they might bring to power electronics applications

This thesis addresses the design, development and operational analysis of a D.C. powered semiconductor laser diode drive unit. A laser diode requires an extremely stable power supply since a picosecond spike of current or power supply switching transient could result in permanent damage. The design offers stability and various features for operational protection of the laser diode. The ability to intensity modulate (analog) and pulse modulate (digital) the laser diode output for data transmission was a major design consideration. Laser optical power is controlled via a closed loop system using a monitor photodiode. Laser diode temperature stabilization is accomplished with the use of a thermoelectric cooler. Laboratory and remote applications were considered in the design of this unit. (rh)

No presente trabalho foi estudada a resposta de diodos epitaxiais (EPI) e fusão zonal (FZ) como dosímetros on-line em tratamentos radioterápicos com feixe de fótons na faixa de megavoltagem (diodo tipo EPI)e ortovoltagem (diodos tipo EPI e FZ). Para serem usados como dosímetros os diodos foram encapsulados em sondas de polimetilmetacrilato preto (PMMA). Para as medidas da fotocorrente os dispositivos foram conectados a um eletrômetro Keithley® 6517B no modo fotovoltaico. As irradiações foram feitas com feixes de fótons de 6 e 18 MV (acelerador Siemens Primus®),6 e 15 MV (acelerador Novalis TX®) e 10, 25, 30 e 50 kV de um equipamento de Radiação X Pantak/Seifert. Nas medidas com o acelerador Siemens Primus® os diodos foram posicionados entre placas de PMMA a uma profundidade de 10,0 cm e com o acelerador Novalis TX® mantidos entre placas de água sólida a uma profundidade de 5cm. Em ambos os casos, os diodos foram centralizados em campos de radiação de 10 × 10 cm2, com distância fonte superfície (DFS) mantida fixa em 100 cm. Para as medidas com os feixes de fótons deortovoltagem os diodos foram posicionados a 50 cm do tubo em um campo de radiação circular de 8 cm de diâmetro. A linearidade com a taxa de dose foi avaliada para as energias de 6 e 15MV do acelerador Novalis TX® variando-se a taxa de dose de 100 a 600 unidades monitoras por minuto e para o feixe de 50 kV variando-se a corrente do tubo de 2 a 20 mA. Todos os dispositivos apresentaram respostas lineares com a taxa de dose e dentro das incertezas, independência da carga com a mesma. Os sinais de corrente mostraram boa repetibilidade, caracterizada por coeficientes de variação em corrente (CV) inferiores a 1,14%(megavoltagem) e 0,15%(ortovoltagem) e em carga menores que 1,84% (megavoltagem) e 1,67% (ortovoltagem). Foram obtidas curvas dose-resposta lineares com coeficientes de correlação linear melhores que 0,9999 para todos os diodos.
In the current work it was studied the performance of epitaxial (EPI) and float zone (FZ) silicon diodes as on-line dosimeters for megavoltage (EPI diode) and orthovoltage (EPI and FZ diode) photon beam radiotherapy. In order to be used as dosimeters the diodes were enclosed in black polymethylmethacrylate (PMMA) probes. The devices were then connected, on photovoltaic mode, to an electrometer Keithley® 6517B to allow measurements of the photocurrent. The irradiations were performed with 6 and 18 MV photon beams (Siemens Primus® linear accelerator), 6 and 15 MV (Novalis TX®) and 10, 25, 30 and 50 kV of a Pantak / Seifert X ray radiation device. During the measurements with the Siemens Primus® the diodes were held between PMMA plates placed at 10.0 cm depth. When using Novalis TX® the devices were held between solid water plates placed at 50 cm depth. In both casesthe diodes were centered in a radiation field of 10 x 10 cm2, with the source-to-surface distance (SSD) kept at 100 cm. In measurements with orthovoltage photon beams the diodes were placed 50.0 cm from the tube in a radiation field of 8 cm diameter. The dose-rate dependency was studied for 6 and 15 MV (varying the dose-rate from 100 to 600 monitor units per minute) and for the 50 kV beam by varying the current tube from 2 to 20 mA. All devices showed linear response with dose rate and, within uncertainties the charge collected is independent of dose rate. The current signals induced showed good instantaneous repeatability of the diodes, characterized by coefficients of variation of current (CV) smaller than 1.14% (megavoltage beams) and 0.15% for orthovoltage beams and coefficients of variation of charge (CV) smaller than 1.84% (megavoltage beams) and 1.67% (orthovoltage beams). The doseresponse curves were quite linear with linear correlation coefficients better than 0.9999 for all diodes.

O presente trabalho teve como objetivo o estudo da aplicação de um novo polímero, o poli [2,7(9,9dioctilfluoreno)alt1,4fluorfenileno], dopado com o 5,6,11,12tetrafenilnaftaceno (rubreno), como camada ativa de diodos emissores de luz poliméricos, os PLEDs. O polímero semicondutor polifluoreno fluorado (PFF), da classe dos polifluorenos, foi sintetizado por meio da reação de acoplamento via rota de Suzuki, onde ao final da síntese apresentou um rendimento de processo de 95%. O polímero foi caracterizado por diversas técnicas analíticas a fim de comprovar a formação da estrutura molecular prevista. A análise de cromatografia de permeação em gel (GPC) indicou valores de massa molar numérica média (Mn) e massa molar ponderal média (Mw) de 4230 g/mol e 21490 g/mol, respectivamente. As análises térmicas de calorimetria diferencial exploratória (DSC) e análise termogravimétrica (TGA) indicaram a presença de um pico de fusão a 145°C, e início de degradação em torno de 200°C, respectivamente. As análises espectroscópicas no infravermelho (IR), espectrometria de energia dispersiva de raios X (EDX) e ressonância magnética nuclear de hidrogênio (1HRMN) comprovaram a presença de anéis aromáticos, grupos CH2, CH3 e hidrocarbonetos fluorados na estrutura do polímero, enquanto a difração de raios X (DRX) indicou 9% de grau de cristalinidade, tratando-se, portanto, de um polímero semicristalino, conforme previamente indicado pela análise DSC. O poli[2,7(9,9dioctilfluoreno)alt1,4fluorfenileno] foi dopado com o 5,6,11,12tetrafenilnaftaceno (rubreno) em diferentes proporções mássicas, a fim de avaliar a interação entre ambos os materiais por meio das respostas obtidas pelos sistemas dopados e não-dopados através das caracterizações por espectroscopia de absorção e fotoluminescência no UV-Visível, realizadas nas amostras em solução e em filme. Os materiais puros, em solução, apresentam picos de absorção em 360 nm e 300 nm e de emissão em 410 nm e 560 nm, para o polímero e para o rubreno, respectivamente. O rubreno apresentou pico de emissão de menor intensidade em torno de 400 nm quando excitado em 360 nm. Os sistemas foram dopados nas seguintes proporções em porcentagem de massa de polímero:rubreno, 100:0, 98:2, 95:5, 90:10, 80:20, 50:50, e quando analisados em solução e excitados em 300 nm, apresentam fluorescência em duas regiões diferentes do espectro eletromagnético, em 410 nm e em 560 nm, ou seja, tanto o polímero quanto o rubreno apresentam resposta emissiva, cada um em sua região característica do espectro eletromagnético. Já quando excitados em 360 nm apresentaram picos de emissão somente em 410 nm, região característica do polímero, porém os espectros das soluções dopadas apresentaram aumento da intensidade do pico de emissão em 410 nm, fato este atribuído à contribuição do rubreno, que quando excitado em 360 nm apresenta uma pequena emissão em torno de 410 nm. Os materiais dopados nas mesmas proporções, quando analisados em filme e excitados em 360 nm, apresentaram bandas de emissão, na faixa entre 410 nm e 450 nm e outra com máxima em 560 nm, diferente do observado nas amostras dopadas em solução e excitadas no mesmo comprimento de onda. Esta alteração no comportamento das amostras pode ser atribuída à maior proximidade entre as moléculas quando se encontram na forma de filme, melhorando a interação e a transferência de energia entre os cromóforos. Os sistemas também foram estudados quanto ao seu comportamento eletroluminescente, para isso foram fabricados dispositivos eletroluminescentes nas seguintes proporções em porcentagem de massa de polímero:rubreno, 100:0, 95:5, 90:10, 80:20, 50:50. Na confecção dos dispositivos utilizou-se o poli(3,4etilenodioxitiofeno) dopado com poli(4sulfonato de estireno), conhecido como PEDOT:PSS, como camada transportadora de lacunas (HTL Hole Transport Layers) e o alumínio como cátodo responsável pela injeção de elétrons. Foram realizadas caracterizações elétricas e ópticas dos dispositivos, por meio do levantamento da curva de densidade de corrente em função da tensão, dos espectros de eletroluminescência, das coordenadas de cromaticidade e pela caracterização visual através de fotografias. Os dispositivos preparados com o polímero puro, quando caracterizados eletricamente, apresentaram valores de tensão de limiar e luminância de 14 V e 18,7 cd/m2, respectivamente, enquanto os dispositivos dopados apresentaram tensão de limiar variando entre 14 V e 7 V, e luminância entre 125 e 278 cd/m2, aproximadamente. Observou-se ainda que com o aumento da concentração de rubreno na camada ativa houve um aumento da luminância, atingindo o máximo em menores tensões. Todos os dispositivos apresentam resposta elétrica típica de diodos. A intensidade luminosa dos dispositivos se intensificou à medida que a densidade de corrente aumentou, atingindo um máximo em tensões entre 10 V e 20 V, aproximadamente, seguidas de uma redução. Os espectros colhidos dos dispositivos poliméricos emissores de luz (PLEDs-Polymer Light Emitting Diode) mostram que os dispositivos preparados a partir do polímero puro apresentaram baixa intensidade de emissão quando comparados aos dispositivos dopados. A dopagem intensificou os picos de emissão em 560 nm em aproximadamente 20 vezes.
The present work aimed to study the application of a new polymer, poly[2,7-(9,9-dioctylfluorene)-alt-1,4-fluorophenylene], doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) as the active layer of polymer light emitting diodes, the PLEDs. The semiconductor fluorinated polyfluorene (PFF) was synthesized by Suzuki coupling reaction, with a yield of 95%. The polymer was characterized by various analytical techniques in order to confirm the formation of the expected molecular structure. The analysis by gel permeation chromatography (GPC) indicated numerical average molar mass (Mn) and ponderal average molar mass (Mw) of 4230 g/mol and 21490 g/mol, respectively. Thermal analysis of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) indicated the presence of a melting peak at 145°C and the beginning of degradation around 200°C, respectively. The infrared spectroscopy (IR), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction spectrometry (XRD), hydrogen nuclear magnetic resonance (1HNMR) confirmed the presence of aromatic rings, CH2 and CH3 groups and fluorinated hydrocarbons in the polymer structure, while the X-ray diffractometry (XRD) showed a degree of crystallinity of 9%, therefore, a semicrystalline polymer, as previously indicated by DSC analysis. The poly[2,7-(9,9-dioctylfluorene)-alt-1,4-fluorphenylene] was doped with 5,6,11,12-tetraphenylnaphthacene (rubrene) at different mass ratios in order to evaluate the interaction between both materials by means of responses for systems doped and non- doped. The systems thus formed were characterized by absorption and photoluminescence spectroscopy in the UV-Visible region and both their solutions in chloroform as well as the solid films were studied. The neat materials, polymer and rubrene, in solution showed absorption peaks at 360 nm and 300 nm and emission at 410 nm and 560 nm, respectively. An emission peak of lower intensity at around 400 nm after excitation at 360 nm was also observed for rubrene. The systems were doped in the following proportions in percent of the polymer:rubrene , 100:0 , 98:2 , 95:5 , 90:10 , 80:20 , 50:50 , and fluorescence spectroscopy were carried out in solution after excitation at 300 nm, resulting in two different regions of emission, at 410 nm and 560 nm, demonstrating independent and characteristic spectra. However, when excited at 360 nm, only emission peaks at 410 nm region, characteristic of the polymer, was observed, furthermore, the spectra of the doped solutions showed increased emission intensity, due to the contribution of rubrene, as a small emission around 410 nm, when excited at 360 nm, has been observed. Films of the doped systems at the same proportions, excited at 360 nm, showed emission bands in the range 410 - 450 nm and, at the same time, another band with maximum at 560 nm. This behavior is different from that observed for the same samples in solution under an equal condition of analysis. This behavior can be explained by the greater proximity between the molecules when they are in the solid state, increasing the segmental interactions and energy transfer between chromophores. The electroluminescence was also analysed by building devices having an active layer of the polymer:rubrene at different mass ratio, 100:0 , 95:5 , 90:10 , 80:20 , 50:50. The devices were built using poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate), PEDOT:PSS, as a hole transporting layer (HTL) and aluminum as the cathode. The devices having neat polymer as the active layer presented luminance of 14 cd/m2 at 18.7 V, while the devices for the doped active layers presented threshold voltage ranging from 14 V to 7 V, and luminance between 125 and 278 cd/m2, approximately. It was observed that by increasing the concentration of rubrene in the active layer, increased luminance was reached at lower voltages. All devices feature typical electrical response of diodes. The luminous intensity of the device is intensified as the current density increased, reaching a maximum at voltages between 10 V and 20 V. The electroluminescence spectra showed lower emission intensity for the neat polymer compared to those observed for the doped. The emission intensity at 560 nm increased approximately 20 times after doping.

Die vorliegende Arbeit beschäftigt sich mit drei Ansätzen der hocheffizienten Erzeugung von weißem Licht mit organischen Leuchtdioden (OLEDs) auf der Basis kleiner Moleküle. Ein Ansatz kombiniert die Emission eines fluoreszenten und zweier phosphoreszenter Emitter in einer einzelnen Emissionsschicht. Da das Triplettniveau des verwendeten Blauemitters niedriger ist als die Triplettniveaus der phosphoreszenten Emitter, werden die Konzentrationen der Emitter so gewählt, dass ein Exzitonenübertrag zwischen ihnen unterbunden wird. Die strahlungslose Rekombination von Tripletts auf dem fluoreszenten Blauemitter begrenzt die Effizienz dieses Ansatzes, jedoch besticht die resultierende weiße OLED durch eine bemerkenswerte Farbstabilität. Der zweite Ansatz basiert auf dem “Triplet Harvesting” Konzept. Ansonsten ungenutzte Triplett Exzitonen werden von einem fluoreszenten Blauemitter auf phosphoreszente Emitter übertragen, wodurch interne Quanteneffizienzen bis zu 100 % möglich sind. Der zur Verfügung stehende Blauemitter 4P-NPD erlaubt aufgrund seines niedrigen Triplettniveaus nicht den Triplett übertrag auf einen grünen Emitter. Daher wird das “Triplet Harvesting” auf zwei unterschiedliche phosphoreszente Emitter, anhand des gelben Emitters Ir(dhfpy)2acac und des roten Emitters Ir(MDQ)2acac untersucht. Es wird gezeigt, dass beide phosphoreszente Emitter indirekt durch Exzitonendiffusion angeregt werden und nicht durch direkte Rekombination von Ladungsträgern auf den Emittermolekülen. Eine genaue Justage der Anregungsverteilung zwischen den phosphoreszenten Emittern ist durch Schichtdickenvariation in der Größenordnung üblicher Schichtdicken möglich. Spätere Produktionsanlagen brauchen daher keinen speziellen Genauigkeitsanforderungen gerecht zu werden. Der dritte und zugleich erfolgreichste Ansatz beruht auf einer Weiterentwicklung des zweiten Ansatzes. Er besteht zunächst darin den Tripletttransfer auf den Übertrag von einem fluoreszenten blauen auf einen phosphoreszenten roten Emitter zu beschränken. Die sich ergebende spektrale Lücke wird durch direktes Prozessieren einer unabhängigen voll phosphoreszenten OLED auf diese erste OLED gefüllt. Verbunden sind beide OLEDs durch eine ladungsträgererzeugende Schicht, in welcher durch das angelegte Feld Elektron/Loch-Paare getrennt werden. Dieser Aufbau entspricht elektrisch der Reihenschaltung zweier OLEDs, welche im Rahmen dieser Arbeit individuell untersucht und optimiert werden. Dabei ergibt sich, dass die Kombination von zwei verschiedenen phosphoreszenten Emittern in einer gemeinsamen Matrix die Ladungsträgerbalance in der Emissionszone sowie die Quanteneffizienz der vollphosphoreszenten OLED stark verbessert. Als Ergebnis steht eine hocheffiziente weiße OLED, welche durch die ausgewogene Emission von vier verschiedenen Emittern farbstabiles Licht mit warm weißen Farbkoordinaten (x, y) = (0.462, 0.429) und ausgezeichneten Farbwiedergabeeigenschaften (CRI = 80.1) erzeugt. Dabei sind die mit diesem Ansatz erreichten Lichtausbeuten (hv = 90.5 lm/W) mit denen von voll phosphoreszenten OLEDs vergleichbar.

This study focuses on the investigation of the key parameters that determine the optical and electrical characteristics of inverted top-emitting organic light emitting diodes (OLED). A co-deposition of small molecules in vacuum is used to establish electrically doped films that are applied in n-i-p layered devices. The knowledge about the functionality of each layer and parameter is important to develop efficient strategies to reach outstanding device performances. In the first part, the thin film optics of top-emitting OLEDs are investigated, focusing on light extraction via cavity tuning, external outcoupling layers (capping layer), and the application of microlens films. Optical simulations are performed to determine the layer configuration with the maximum light extraction efficiency for monochrome phosphorescent devices. The peak efficiency is found at 35%, while varying the thickness of the charge transport layers, the semitransparent anode, and the capping layer simultaneously. Measurements of the spatial light distribution validate, that the capping layer influences the spectral width and the resonance wavelength of the extracted cavity mode, especially for TM polarization. Further, laminated microlens films are applied to benefit from strong microcavity effects in stacked OLEDs by spatial mixing of external and to some extend internal light modes. These findings are used to demonstrate white top-emitting OLEDs on opaque substrates showing power conversion efficiencies up to 30 lm/W and a color rendering index of 93, respectively. In the second part, the charge carrier management of n-i-p layered diodes is investigated as it strongly deviates from that of the p-i-n layered counterparts. The influence of the bottom cathode material and the electron transport layer is found to be negligible in terms of driving voltage, which means that the assumption of an ohmic bottom contact is valid. The hole transport and the charge carrier injection at the anode is much more sensitive to the evaporation sequence, especially when using hole transport materials with a glass transition temperature below 100°C. As a consequence, thermal annealing of fabricated inverted OLEDs is found to drastically improve the device electronics, resulting in lower driving voltages and an increased internal efficiency. The annealing effect on charge transport comes from a reduced charge accumulation due to an altered film morphology of the transport layers, which is proven for electrons and for holes independently. The thermal treatment can further lead to a device degradation. Finally, the thickness and the material of the blocking layers which usually control the charge confinement inside the OLED are found to influence the recombination much more effectively in inverted OLEDs compared to non-inverted ones.

This thesis discusses the wavelength stabilisation of semiconductor lasers for WDM applications. The motivation is to explore the potential of a novel athermal WDM laser with much less wavelength drift than conventional lasers, and without the traditional external temperature compensation units. The study is mainly theoretical, but some experimental work is included. Based on the Cambridge time domain model, intensive simulation has been done to investigate the design space of DBR lasers. Although the structure of the initial prototype DBR model is simple, the simulation output results are rather complex. All the main parameters such as grating length, grating coupling strength, total laser cavity length and waveguide scattering loss are varied. Results obtained agree with current measurements on DBR lasers, and support dimensions and structures derived empirically. Based upon the above results, simulation work then goes further towards the athermal WDM laser idea which employs negative temperature coefficient material to physically compensate the frequency variation with temperature. A polymer widely used in optical waveguides is one candidate. Having reviewed temperature dependent parameters in semiconductor lasers, we incorporated these into the Cambridge Time Domain model. It predicts a wavelength shift of about 0.75 nm over a 100^o C range at wavelength of 1500 nm where about 9nm would be expected from a normal DFB laser. It also reveals other complex and interesting behaviour, which is likely to give several useful insights into the behaviour of tuneable and other forms of DBR and BFB lasers. An experiment to verify the athermal effect of a polymer has been done. Focussed ion beam etch was employed to form a deep grating in near-conventional FP laser, and the grating voids were filled with the polymer. Lasing action with good wavelength stability was observed, though the particular structure employed is probably not capable of single mode operation over a wide temperature range.

Optical communication was developed to allow high-speed and long-distance data transmission and is currently a £6bn market. This has also led to the adoption of optical technologies in other areas including the CD, DVD and medical imaging systems. Standardisation of components means that these systems require light sources that operate near the 1310 and 1550 nm telecommunications windows but existing lasers here are expensive due to their high temperature sensitivity. The exploitation of quantum con¯nement has led to the development of \quan- tum dot" (QD) laser material because of predictions of huge gains in performance. Emission wavelengths of InAs/GaAs QD lasers have been extended to the telecom- munications window near 1300 nm by various growth technologies and the first commercial devices have recently been brought to the market. However, progress to longer wavelengths has been stalled for several years as well as the speed and tem- perature sensitivity of these devices falling short of the predictions; partly because QDs are grown by self-assembly resulting in a random distribution of sizes, compo- sitions and strain-states, leading to inhomogeneous broadening which is a departure from the ideal \atom-like" system. This work details the growth, design and development of QD bilayer laser devices, which o®er a unique approach to fixing these shortcomings. When two QD layers are grown close together; the first layer provides a template that allows larger, more uniform QDs to be grown in the second layer, giving greater uniformity and deeper confinement. This has the potential to increase the efficiency and to achieve emission wavelengths out towards the more-commonly used telecommunications window at 1550 nm directly on GaAs substrates. Multiple bilayer laser diodes with inhomge- neous broadening of less than 30meV, lasing at up to 1430 nm and room-temperature photoluminescence at 1515 nm are shown. Despite the vastly reduced inhomogeneous broadening of QD bilayers, it is still found to be a relevant factor due to the change from de-localised geometries of quantum wells to an ensemble of separate QDs. It will be shown that understanding this is essential for describing the observed optical and electrical behaviour of the laser diodes.

Advances in fabrication and analysis tools have allowed the synthesis and manipulation of functional materials with features comparable to fundamental physical length scales. Many interesting properties inherently due to quantum size effects have been observed in nanometre scale structures. It is hoped that these nanoscale structures will play a key role in future materials and devices that exploit their unique properties. Zinc oxide (ZnO) is a wide band-gap transparent and piezoelectric semiconductor material. It also has a large exciton binding energy which allows for stable ultraviolet light emission at room temperature. There are therefore foreseeable applications in optoelectronic devices which include ultraviolet photosensitive devices and light emitting diodes. Nanoscale structures formed from ZnO are interesting as they possess many of the properties inherent form the bulk but are also subject to various quantum size effects that may occur at the nanoscale. To date, the study of ZnO nanostructures is a relatively recent endeavour with the vast majority of reports being made within the last five years. ZnO is unique in that it forms a family of nanoscale structures. These structures include nanoscale wires, rods, hexagons, tetrapods, ribbons, rings, flowers and helixes. This work is focussed on the study of zinc oxide tetrapod crystalline nanoscale structures and their devices. We have synthesised ZnO tetrapods using chemical vapour transport techniques. Photoluminescence characterisation revealed the presence of optically active surface defects that could be quenched with a simple surface treatment. We have also for the first time observed resonant cavity modes in a single ZnO tetrapod nanocrystal. An ultraviolet sensitive Schottky diode was fabricated from a single ZnO tetrapod using focussed ion-beam assisted deposition techniques. The device characteristics observed were modelled and successfully shown to result from an illumination induced reduction in the Schottky barrier height at the metal-semiconductor interface.

A rectifier is an electrical device, comprising one or more semiconductor devices arranged for converting alternating current to direct current by blocking the negative or positive portion of the waveform. The purpose of this study would be to evaluate dynamic and static electrical characteristics of rectifier chips fabricated with (a) DY8 process and (b) YI8 process and compare them with the existing DI8 process rectifiers. These new rectifiers were tested to compare their performance to meet or exceed requirements of lower forward voltages, leakage currents, reverse recovery time, and greater sustainability at higher temperatures compared to diodes manufactured using boron as base (DI8 process diodes) for similar input variables.

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xxv, 201 p.; also includes graphics Includes bibliographical references (p. 188-201). Available online via OhioLINK's ETD Center

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Bachelors
Engineering and Computer Science
Electrical Engineering

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
FAPESP:03/10164-9

Cette étude a pour objectif l'analyse théorique et expérimentale des performances potentielles d'hétérostructures IMPATT qui sont obtenues à partir de matériaux qui présentent des caractéristiques physiques favorables par la réalisation des différentes zones du dispositif. Pour cela, deux méthodes sont envisageables : association d'un matériau fortement ionisant pour la zone d'avalanche à un autre faiblement ionisant pour la zone de transit (hétérostructure Ge/AsGa ou GaInAs/InP) ; conception d'une zone de transit où l'évolution spatio-temporelle de la vitesse des porteurs est optimale (hétérostructure AlGaAs/AsGa). Ces composants nouveaux nécessitent une bonne connaissance des propriétés des hétérojonctions et une compréhension des phénomènes physiques qui interviennent en champ élevé. Dans ce but, une étude complète des structures spécifiques du dispositif IMPATT a été effectuée afin d'établir les limitations que pouvaient apporter les hétérojonctions et la nature des matériaux utilisés.

Photochromism has been investigated extensively during recent years. The large interest for information storage in memory applications is associated with the bi-stable character of the photochromism phenomena. In molecular photochromics, two isomers with different absorption spectrum can be obtained according to the specific wavelength of the light exposure. This reversible transformation process can be considered as optical writing/erasing step of a memory.

Here we first report the absorption spectra of solid-state films based on the blends consisting of PC molecules, the spirooxazine 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3’-[3H]phenanthr[9,10-b](1,4)oxazine] (PIII, Sigma-Aldrich, 32,256-3) and a polymer matrix host, poly(2-methoxy-5(2’-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). The bi-stability in conjugated polymer matrix is studied by following the time evolution of the optical properties of the blends.

Thereafter, the electrical performance of PC-polymer diodes is characterized and reported. While the PIII molecules in the blend bulks are switched to their low energy gap state, forming external energy levels above the valence band of MEH-PPV, the injected charges (hole-dominated) will be trapped by the low energy gap isomer of PIII and that leads to current modulation. PIII molecules can be switched between two energy gap states upon the photo-stimulation, and the I-V characteristics of the device can also be controlled reversibly via the photoisomerization. The retention time of the diode’s electrical switching fits quite well with the absorption characteristics of the blend films; this correspondence builds a good link between the film property and the device behavior.

Furthermore, we observed a two-trap system in the blend diodes from the I-V curves, and a model is proposed which can explain the schematic concept of the trap-limited current modulation. To combine the knowledge and information from the investigations above, we tested a novel device design based on a bi-layer of the PC and polymer materials, and the promising result for future work is presented in the end.

In this thesis light emission from sulphur related impurity in silicon has been reported. Although, sulphur related luminescence from silicon has been stated since the 1980's, no room temperature luminescence has been achieved and no compatible devices that can be integrated to the silicon technology have been invented. Photoluminescence and electroluminescence experiments were made on a set of samples implanted with only with sulphur at doses ranging from 1011-1014 S cm-2 at 30 keV, annealed at 1000 °C or 1100 °C for 10 s and on another set of samples implanted with sulphur as above and further implanted with boron at 1015 B cm-2 at 30 keV, further annealed at 950 °C for 1 min. The experiments revealed two major emissions at 1129.5 nm (1.0997 eV) which is due to the Si TO phonon assisted transition and at 1363 nm (0.9097 eV) which is due to sulphur related impurities. Variable temperature experiments were done at both PL and EL experiments. From the EL variable temperature measurements, it was observed that the two main lines were shifting towards longer wavelengths with the increase of temperature. Sulphur emission was present at room temperature with low intensity compared to the silicon emission which was more dominant at room temperature. Of great interest was the effect of power on silicon and sulphur emission. It has revealed a sublinear and a superlinear behaviour for the sulphur and silicon integrated intensity respectively with the increase of the injection condition, which can be attributed to the saturation of sulphur related levels responsible for the 1.33 nm emission at the high excitation levels. A model of the diffusion of sulphur concentration after the annealing treatments was presented, introducing the two cases of perfect reflection and perfect loss from the samples surfaces. Finally a model explaining our PL and EL power dependence experiments was provided which showed that there are two major radiative routes via the silicon and the sulphur that take place, which are competing at each other along with a non-radiative route coming from the sulphur related level. Our model describes the trends in our experimental data well. Finally, the energy related to the sulphur peak quenching was calculated to be 32.2 +/-1.4 meV.

Erbium, a rare earth element, has been shown to exhibit characteristic luminescence at 1.54mum due to its internal 4f transition from the first excited state (4pi3/2) to the ground state (4pi5/2). As this emission wavelength falls inside the maximum transmission window of silicon based optical fibers, erbium doped silicon might lead to the opportunity of silicon based optoelectronics. The introduction of erbium in silicon allows excitation through electron-hole recombination and subsequent radiative emission from the rare earth centers. The works reported here describe the structural, electrical and optical properties of crystalline silicon codoped with erbium and boron by ion implantation technique. Four sets of samples, co-implanted with erbium and boron at different Er dose, implantation energy and at different conditions, were prepared. Post-implantation annealing has been performed to recover the implantation damage to an acceptable value and to activate the dopant atoms optically and electrically. PL and EL measurements have been performed in the temperature range between 80K to room temperature. The sample with the lowest erbium concentration and energy gives the best PL and EL results. The observed emission peaks in both PL and EL measurements were at around 1.129mum, ~1.303mum, 1.50mum and 1.597mum at 80K. At higher temperatures, a broader peak at around 1.50mum with long tail towards the both end of wavelength has been observed. The peak at 1.129mum corresponding to the Si band edge emission, the reason for the peaks at around l.303mum has not been identified while the remaining two peaks correspond the Er3+ emission. Virtually no temperature quenching of Er luminescence is observed in some samples rather room temperature intensity is higher than that at 80K. The improvement of the temperature quenching effect on Er luminescence at room temperature has been attained in our results, which is significant improvement in comparison to the result found in the literature. The structural properties were studied by TEM in both cross-sectional and plan view configurations. TEM analyses showed dislocation loops and other defects of random size and distribution from the surface to 600nm below the surface. Er precipitates defects were also seen in the sample doped with Er comparatively at higher dose (1x1015Er/cm2) and energy (1.0 MeV). No detectable room temperature PL and EL signals were observed from the sample implanted at higher doses and energies.

L'objectif de ce travail est d'une part, de connaître et d'identifier les paramètres de construction qui engendrent, le plus, le phénomène d'empiétement des diodes tournantes des excitatrices brushless. D'autre part, il vise à proposer une structure de contrôle PID afin d'améliorer le temps de réponse en tension des excitatrices à diodes tournantes. Les résultats attendus vont contribuer à un meilleur dimensionnement des excitatrices pour des générateurs hydrauliques. Pour traiter ce problème, nous avons mis à contribution une revue de littérature détaillée et développé un modèle mathématique de l'excitatrice brushless qui tient compte des hypothèses de son analyse fonctionnelle. Par ailleurs, l'influence des paramètres de construction de la brushless sur l'angle d'empiétement a été évaluée (réactance synchrone, nombre de phases, nombre de pôles, courant d'excitation de l'excitatrice, courant d'excitation de l'alternateur principal, entrefer, diamètre et longueur de l'armature, nombre et géométrie des encoches, etc...) Une des tâches importantes de ce travail est l'amélioration du temps de réponse en charge de l'excitatrice. Le chapitre 4 de cette étude propose une structure simple de contrôle PID. Le contrôleur proposé doit permettre, entre autre, d'éviter la surexcitation de l'excitatrice brushless. Les résultats de cette étude permettent de conclure que les paramètres les plus influents de l'empiétement sont: - Le courant redressé absorbé par l'inducteur de l'alternateur principal, - La réactance synchrone, - La force électromotrice de la brushless. Nous déduisons des principes établis de cette étude, qu'un dimensionnement adéquat de l'excitatrice réduirait de façon suffisante les effets de l'empiétement et maintiendrait une valeur sûre et constante de la tension de sortie de l'excitatrice.

In this work a series of new semiconducting liquid crystals (LCs), which are applicable for organic light emitting diodes (OLEDs), were investigated. Semiempirical calculations were carried out on monomers and anti-cofacial dimers built from our molecules, representing molecules in solution and thin film respectively. Compared to the monomer a doubling of the oscillator strength in the dimer was found for longitudinal offsets larger than 20 A. Smaller shifts showed a forbidden absorption transition from ground to the lowest excited state. Assuming that the absorption transition is equivalent to the emissive transition, this might explain the reduced optical quantum efficiency observed for all of our materials in the solid state. OLEDs made from blends of three different blue/green emitters with a red component showed white light emission with voltage independent CIE coordinates close to the ideal white. With polarised microscopy nematic phases frozen in a glassy state at room temperature were observed for all blends. Thus the blends were homogeneous and no phase separation occurred. This is important for homogeneous white emission and the alignment of the LCs due to a rubbed alignment layer below. Polarised white electroluminescence with an average polarisation ratio of 8:1 was shown from an OLED made with a blend deposited onto an alignment layer. Polarised background light for LC displays is desirable as this minimises the losses at the polarisers in the display and thus increases its brightness or lowers the power consumption. The low efficiency of the red emitter however limited the OLED performance. Surface relief gratings (SRGs) with periods of a few hundred nm and a maximum depth of 66 nm and periods in the nm-range with a depth of 140 nm were spontaneously induced on our films. They were formed through molecular mass transport from the dark to bright regions during crosslinking by irradiation with a sinusoidal light pattern created by a phase mask. The anisotropic properties of LCs are shown to enhance transport. SRGs were formed at room temperature and an elevated sample temperature of 65deg. They are suitable feedback structures for optically pumped organic lasers and can also be employed to enhance the outcoupling of OLEDs.

Significant advances have been made in the past five years in realizing photon sources based on various kinds of emitters in the 500-1000 nm spectral region. However, reliable sources emitting at wavelengths compatible with standard telecommunication optical fibre are still lacking. Self-assembled quantum dots offer a way to generate wavelength-tunable photons. They offer also relatively high radiative efficiencies, short spontaneous emission lifetimes (~1 ns) and, importantly, can easily be incorporated into compact semiconductor devices fabricated with established technologies. In this thesis, single photon sources based on InAs/GaAs quantum dots were studied and developed for applications at the technologically important wavelength around 1.3 μm compatible with standard telecommunication optical fibre. A single photon source suitable for use in a quantum key distribution system was demonstrated by optically pumping an InAs/GaAs quantum dot incorporated inside a micropillar optical cavity (chapter 4). Single photon emission driven by short electrical pulses was then realised for the first time at λ ~ 1.3 μm using an InAs/GaAs quantum dot embedded in a planar optical microcavity (chapter 5). Electrically driven single photon emission was also achieved for quantum dot devices based on the micropillar geometry, demonstrating the viability of such compact cavities with small optical mode volumes at both λ ~ 900 nm and 1.3 μm (chapters 6 and 7). The realization of these devices surmounted the difficulty of contacting the small cross-sectional diameters (~ 2 – 2.5 μm) of the micropillars. Positioning quantum dots accurately inside optical microcavities can improve the efficiency of single photon sources. Chapter 3 presents results of a study of semiconductor wafers with intentionally positioned quantum dots, demonstrating the viability of a technique for placing quantum dots reproducibly.

Ce mémoire concerne l'étude de systèmes laser utilisant le cristal de cr3+:lisaf comme milieu amplificateur. Ce dernier posséde une large bande d'émission entre 800 et 1000 nm, ainsi qu'une large bande d'absorption entre 600 et 700 nm. Depuis 1993, des diodes laser de puissance sont disponibles a ces longueurs d'onde (670 nm). Malgre leurs mauvaises qualites spatiales, il est possible, en choisissant correctement les systèmes optiques, de realiser un pompage efficace de ce cristal. Nous avons développé des oscillateurs cr3+:lisaf pompes par diodes et délivrant quelques centaines de mw en regime continu, entre 800 et 900 nm. Nous avons teste differentes configurations de pompage et de cavite, et recueilli des informations sur ce cristal, en particulier sur son efficacite comme materiau laser. Les résultats étant satisfaisants, nous avons teste le fonctionnement de ce laser pour deux régimes impulsionnels différents. Le premier oscillateur impulsionnel réalise nous a permis d'obtenir des impulsions de quelques dizaines de picosecondes, limitées par la transformée de fourier et accordables sur 100 nm. Ces impulsions sont produites par blocage de modes actif a l'aide d'un modulateur acousto-optique. Le deuxieme oscillateur realise, nous a permis d'obtenir des impulsions femtosecondes en utilisant l'effet kerr optique. Ainsi nous avons obtenu des impulsions de 50 fs pour une puissance moyenne de 40 mw. Les caracteristiques obtenues placent cet oscillateur parmi les premiers sur la scene internationale. Les energies produites par ces oscillateurs etant faibles, il est necessaire d'utiliser un amplificateur. Pour optimiser les performances de ce dernier, nous avons développé une simulation theorique permettant de calculer le gain disponible dans le laser, en tenant compte de facteurs limitant tels que l'upconversion ou les effets de l'échauffement du cristal (quenching). Cette modelisation, permet d'evaluer l'influence de divers paramètres, et en particulier du pompage, sur le gain. Sa validite a ete confirme par la realisation d'un oscillateur déclenche délivrant quelques j.

Optical rectennas, which are micro-antennas to convert optical-frequency radiation to alternating current combined with ultrahigh-speed diodes to rectify the current, can in principle provide high conversion efficiency solar cells and sensitive detectors. Currently investigated optical rectennas using metal/insulator/metal (MIM) diodes are limited in their RC response time and poor impedance matching between diodes and antennas. A new rectifier, the geometric diode, can overcome these limitations. The thesis work has been to develop geometric diode rectennas, along with improving fabrication processes for MIM diode rectennas. The geometric diode consists of a conducting thin-film, currently graphene, patterned into a geometry that leads to diode behavior. In contrast with MIM diodes that have parallel plate electrodes, the planar structure of the geometric diode provides a low RC time constant, on the order of 10^-15 s, which permits operation at optical frequencies. Fabricated geometric diodes exhibit asymmetric DC current-voltage characteristics that match well with Monte Carlo simulations based on the Drude model. The measured diode responsivity at DC and zero drain-source bias is 0.012 A/W. Since changing the gate voltage changes the graphene charge carrier concentration and can switch the majority charge type, the rectification polarity of the diode can be reversed. Furthermore, the optical rectification at 28 THz has been measured from rectennas formed by coupling geometric diodes with graphene and metal bowtie antennas. The performance of the rectenna IR detector is among the best reported uncooled IR detectors. The noise equivalent power (NEP) of the rectenna detector using geometric diode was measured to be 2.3 nW Hz^-1/2. Further improvement in the diode and antenna design is expected to increase the detector performance by at least a factor of two. Applications for geometric diodes and graphene bowtie antennas include detection of terahertz and optical waves, ultra-high speed electronics, and optical power conversion.

Organic Light Emitting Diodes (OLEDs) are extremely attractive candidates for flexible display and lighting panels due to their high contrast ratio, light weight and flexible nature. However, the materials in an OLED get oxidized by extremely small quantities of atmospheric moisture and oxygen. To obtain a flexible OLED device, a flexible thin-film barrier encapsulation with low permeability for water is necessary.

Water permeates through a thin-film barrier by 4 modes: microcracks, contaminant particles, along interfaces, and through the bulk of the material. We have developed a flexible barrier film made by Plasma Enhanced Chemical Vapor Deposition (PECVD) that is devoid of any microcracks. In this work we have systematically reduced the permeation from the other three modes to come up with a barrier film design for an operating lifetime of over 10 years.

To provide quantitative feedback during barrier material development, techniques for measuring low diffusion coefficient and solubility of water in a barrier material have been developed. The mechanism of water diffusion in the barrier has been identified. From the measurements, we have created a model for predicting the operating lifetime from accelerated tests when the lifetime is limited by bulk diffusion.

To prevent the particle induced water permeation, we have encapsulated artificial particles and have studied their cross section. A three layer thin-film that can coat a particle at thicknesses smaller than the particle diameter is identified. It is demonstrated to protect a bottom emission OLED device that was contaminated with standard sized glass beads.

The photoresist and the organic layers below the barrier film causes sideways permeation that can reduce the lifetime set by permeation through the bulk of the barrier. To prevent the sideways permeation, an impermeable inorganic grid made of the same barrier material is designed. The reduction in sideways permeation due to the impermeable inorganic grid is demonstrated in an encapsulated OLED.

In this work, we have dealt with three permeation mechanisms and shown solution to each of them. These steps give us reliable flexible encapsulation that has a lifetime of greater than 10 years.

Photochromism has been investigated extensively during recent years. The large interest for information storage in memory applications is associated with the bi-stable character of the photochromism phenomena. In molecular photochromics, two isomers with different absorption spectrum can be obtained according to the specific wavelength of the light exposure. This reversible transformation process can be considered as optical writing/erasing step of a memory. Here we first report the absorption spectra of solid-state films based on the blends consisting of PC molecules, the spirooxazine 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3’-[3H]phenanthr[9,10-b](1,4)oxazine] (PIII, Sigma-Aldrich, 32,256-3) and a polymer matrix host, poly(2-methoxy-5(2’-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV). The bi-stability in conjugated polymer matrix is studied by following the time evolution of the optical properties of the blends. Thereafter, the electrical performance of PC-polymer diodes is characterized and reported. While the PIII molecules in the blend bulks are switched to their low energy gap state, forming external energy levels above the valence band of MEH-PPV, the injected charges (hole-dominated) will be trapped by the low energy gap isomer of PIII and that leads to current modulation. PIII molecules can be switched between two energy gap states upon the photo-stimulation, and the I-V characteristics of the device can also be controlled reversibly via the photoisomerization. The retention time of the diode’s electrical switching fits quite well with the absorption characteristics of the blend films; this correspondence builds a good link between the film property and the device behavior. Furthermore, we observed a two-trap system in the blend diodes from the I-V curves, and a model is proposed which can explain the schematic concept of the trap-limited current modulation. To combine the knowledge and information from the investigations above, we tested a novel device design based on a bi-layer of the PC and polymer materials, and the promising result for future work is presented in the end.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 129-135).
The work presented here is a study of thermally enhanced injection in light-emitting diodes (LEDs). This effect, which we refer to as "thermal pumping", results from Peltier energy exchange from the lattice to charge carriers when current is injected into an LED. For an applied voltage V such that qV < (hw), where q is the electron charge and (hw) is the average emitted photon energy, thermal pumping can greatly enhance the wall plug efficiency of an LED. Thermal pumping can even give rise to LED wall plug efficiency greater than one, which corresponds to electroluminescent cooling of the diode lattice. Thermal pumping and electroluminescent cooling will be studied through numerical modeling and experiment. Our results include the first ever experimental demonstration of electroluminescent cooling in an LED. Finally we use the intuition gained from the study of thermal pumping to design an LED for maximized optical power output with 100% wall plug efficiency.
by Dodd Gray.
M.Eng.