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Technological developments in the area of high power LED light sources have enabled their utilization in general illumination applications. Along with this advancement comes the need for progressive thermal management strategies in order to ensure device performance and reliability. Minimizing an LED's junction temperature is done by minimizing the total system's thermal resistance. For actively cooled systems, this may essentially be achieved by simultaneously engineering the conduction through the heat sink and creating a well-designed flow pattern over suitable convective surface area. While such systems are routinely used in cooling microelectronics, their use in LED lighting systems encounter additional constraints which must be accounted for in the design of the cooling system. These are typically driven by the size, shape, and building codes involved with the lighting industry, and thus influence the design of drop-in replacement LED fixtures. Employing LED systems for customary down-lighting applications may require shrouded radial fin heat sinks to increase the heat transfer while reducing the space requirement for active cooling. Most lighting is already in some form of housing, and the ability to concurrently optimize these housings for thermal and optical performance could accelerate the widespread implementation of cost-efficient, environmentally-friendly solid-state lighting. In response, this research investigated the use of conical, cylindrical, square, and pyramidal shrouds with pin/radial fin heat sink designs for the thermal management of high power LED sources. Numerical simulations using FLUENT were executed in order to account for details of the air flow, pressure drop, and pumping power, as well as the heat transfer and temperature distributions throughout the system. The LEDs were modeled as a distributed heat source of 25 - 75 W on a central portion of the various heat sinks. Combinations of device junction temperature and pumping power were used to assess the performance of shrouded heat sink designs for their use in air-cooled, down-lighting LED fixtures.

Photovoltaics (PV) is fast emerging as an attractive renewable energy technology due to concerns of global warming, pollution and scarcity of fossil fuel supplies. However to compete in the global energy market, solar cells need to be cheaper and more energy efficient. Silicon is the favorite semiconductor used in solar photovoltaic cells because of its ubiquity and established technology, but due to its indirect bandgap silicon is a poor absorber and light emitter. Thin film cells play an important role in low cost photovoltaics, but at the cost of reduced efficiencies when compared to wafer based cells. There remains much untapped potential in thin-film solar cells which this work has attempted to exploit through exploring novel approaches of enhancing the efficiency of thin film cells using the optical properties of sub-wavelength metal nanoparticles. Metals are considered as strong absorbers of light because of their large free-electron density. How can metals improve light trapping in solar cells? This question has raised several eyebrows and this thesis is an attempt to show that metal nanoparticles can be useful in producing efficient solar cells. Subwavelength metal particles support surface modes called surface plasmons when light is incident on them, which cause the particles to strongly scatter light into the underlying waveguide or substrate, enhancing absorption. The process of coupling thin film silicon waveguide modes to plasmonic metals using unpolarised light at normal incidence is applied to silicon-based solar cells and light emitting diodes, and enhanced photocurrent and electroluminescence is realized with potential for further optimisation and improvement. The results from this study correspond to a current increase of up to 19% from planar wafer based cells and up to 33% increase from 1.25 micron thin-film silicon-on-insulator structures for the AM1.5 global spectrum. We also report for the first time an up to twelve fold increase in electroluminescence signal from 95nm thick light-emitting diodes. From the results we conclude that this method which involves simple techniques of nanoparticle deposition and characterization could hold important implications in the improvement of thin-film silicon cell absorption / emission efficiencies where conventional methods of light trapping are not feasible, resulting in promising near-term applications of surface plasmons in photovoltaics and optoelectronics.

Thesis (Ph.D.) -- McMaster University, 1999. / Includes bibliographical references (leaves 98-103). Also available via World Wide Web.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2006. / Buck, John, Committee Co-Chair ; Ferguson, Ian, Committee Chair ; Krishnamurthy,Vikram, Committee Member ; Chang, Gee-Kung, Committee Member ; Callen, W. Russell Jr., Committee Member ; Summers, Christopher, Committee Member.

This dissertation investigates various design metrologies on designing freeform surfaces for LED illumination applications. The major goal of this dissertation is to study designing freeform optical surfaces to redistribute the radiance (which can be simplified as intensity distribution for point source) of LED sources for various applications. Nowadays many applications, such as road lighting systems, automotive headlights, projection displays and medical illuminators, require an accurate control of the intensity distribution. Freeform optical lens is commonly used in illumination system because there are more freedoms in controlling the ray direction. Design methods for systems with rotational and translational symmetry were well discussed in the 1930's. However, designing freeform optical lenses or reflectors required to illuminate targets without such symmetries have been proved to be much more challenging. For the simplest case when the source is an ideal point source, the determination of the freeform surface in a rigorous manner usually leads to the tedious Monge-Ampère second order nonlinear partial different equation, which cannot be solved with standard numerical integration techniques. Instead of solving the differential equation, ray mapping is an easier and more efficient method in controlling one or more freeform surfaces for prescribed irradiance patterns. In this dissertation, we investigate the ray mapping metrologies in different coordinate systems to meet the integrability condition for generating smooth and continuous freeform surfaces. To improve the illumination efficiency and uniformity, we propose a composite ray mapping method for designing the total internal reflective (TIR) freeform lens for non-rotational illumination. Another method called "double pole" ray mapping method is also proposed to improve system performance. The ray mapping designs developed for the point source do not work well for extended sources, we have investigated different design methodologies including optimization method, deconvolution method and feedback modification method to design freeform optical surfaces for extended sources.

Ce travail de doctorat est dédié à l’étude des mécanismes régissant l’interaction lumière-matière dans des dispositifs optoélectroniques à base d’alliages (Al,Ga,In,)N. Diverses compositions de ces alliages sont assemblées en structures multicouches de matériaux d’épaisseurs nanométriques afin d’obtenir une émission à plus grande longueur d’onde que le bleu, couleur pour laquelle la technologie est mature depuis plus de vingt ans. Il s’agit de réaliser des émetteurs efficaces de lumière jaune, verte ou blanche (avec une approche alternative au pompage optique de luminophore par une diode bleue). Les solutions solides assemblées pour obtenir des émissions à grandes longueur d’onde sont à base de GaN et de Ga0.8In0.2N, matériaux pour lesquels les mailles cristallographiques sont désaccordées. Lorsqu’elles sont déposées sur un substrat ou un pseudo-substrat de GaN, matériau pour lequel les dopages n et p sont maitrisés et permettent l’injection électrique des porteurs, les tranches de Ga0.8In0.2N subissent de très fortes compressions dans leur plan de croissance et l’énergie élastique est relaxée par la formation de défauts délétères pour le rendement lumineux. Nous avons construit des hétéro-structures plus complexes en intercalant une couche complémentaire d’Al0.2Ga0.8N afin de réduire la densité d’énergie élastique globale. L’insertion de telles couches améliore la qualité cristalline et augmente leur rendement optique. Nous avons mesuré le rendement quantique interne en utilisant la spectroscopie de photoluminescence résolue en temps et une analyse des temps de déclins par une approche de type Lotka-Volterra des équations de recombinaison pour obtenir les taux de recombinaisons radiatifs et non-radiatifs. Nous avons montré quantitativement comment les champs électriques internes résultant du contraste de polarisation électrique aux interfaces et les recombinaisons non-radiatives de type Schockley-Read-Hall contribuent à définir le rendement quantique à faible densité d’excitations (optique ou électrique). L’objectif est l’obtention d’une émission spontanée intense pour une densité d’excitation modérée. Nous avons donc conduit une campagne d’expériences en variant l’intensité d’injection. Nous avons montré que l’effet Auger est le facteur dominant régissant la chute du rendement quantique interne sous fortes densités d’injection. Nous avons étudié diverses architectures à simple puits quantique ou à puits quantiques multiples émettant à des longueurs d’onde identiques pour quantifier l’influence spécifique de l’effet Stark confiné quantique. Nous avons corrélé l’apparition d’un seuil d’excitation au-delà duquel domine la recombinaison non-radiative de type Auger avec l’augmentation du temps de recombinaison radiative et de l’énergie de localisation des porteurs dans l’alliage inhomogène. Nous arrivons à la conclusion que la localisation des porteurs produite par le champ électrique aux interfaces et les fluctuations de la composition chimique de Ga0.8In0.2N contribuent de concert, facilitant l’interaction répulsive électron-électron et la recombinaison non-radiative Auger nnp. Nous avons montré que le modèle ABC permet de bien décrire la physique du phénomène si ses trois paramètres tiennent compte des effets d’écrantages sous injections modérées et des effets de remplissage de l’espace des phases sous fortes injections. Enfin, nous nous sommes écartés de l’étude des structures traditionnellement épitaxiées selon le plan polaire (0001) pour choisir des plans d’épitaxie semi-polaire (11-22). Dans ces conditions, il a été nécessaire de fabriquer des puits quantiques en Ga0.65In0.35N. Nous avons montré que la quasi-absence d’effet Stark confiné quantique augmente de manière très significative le seuil d’excitation au-delà duquel domine la recombinaison non-radiative de type Auger. Cette amélioration par rapport aux échantillons épitaxiés selon le plan (0001) est d’autant plus marquée que la longueur d’onde émise est grande. / Non renseigné

This thesis is about new ways to experimentally realise materials with desired nano-structures for solution-processable optoelectronic devices such as solar cells and light-emitting diodes (LEDs), and examine structure-performance relationships in these devices. Short exciton diffusion length limits the efficiency of most exciton-based solar cells. By introducing nano-structured architectures to solar cells, excitons can be separated more effectively, leading to an enhancement of the cell’s power conversion efficiency. We use diblock copolymer lithography combined with solvent-vapour-assisted imprinting to fabricate nano-structures with 20-80 nm feature sizes. We demonstrate nanostructured solar cell incorporating the high-performance polymer PBDTTT-CT. Furthermore, we demonstrated the patterning of singlet fission materials, including a TIPS-pentacene solar cell based on ZnO nanopillars. Recently perovskites have emerged as a promising semiconductor for optoelectronic applications. We demonstrate a perovskite light-emitting diode that employs perovskite nanoparticles embedded in a dielectric polymer matrix as the emissive layer. The emissive layer is spin-coated from perovskite precursor/polymer blend solution. The resultant polymer-perovskite composites effectively block shunt pathways within the LED, thus leading to an external quantum efficiency of 1.2%, one order of magnitude higher than previous reports. We demonstrate formations of stably emissive perovskite nanoparticles in an alumina nanoparticle matrix. These nanoparticles have much higher photoluminescence quantum efficiency (25%) than bulk perovskite and the emission is found to be stable over several months. Finally, we demonstrate a new vapour-phase crosslinking method to construct full-colour perovskite nanocrystal LEDs. With detailed structural and compositional analysis we are able to pinpoint the aluminium-based crosslinker that resides between the nanocrystals, which enables remarkably high EQE of 5.7% in CsPbI3 LEDs.

In recent years there is growing research in optical wireless communication. This growing popularity isdue to several characteristics like such as large bandwidth that is also not having spectrum regulationsimposed, low cost and license-free operation. Since visible light communications (VLC) is a branch ofoptical wave communications (OWC), it is used for replacing RF communications. The other primaryreason for the use of visible light communications [1], because it uses 400 THz of unlicensed secure andradio free media for wireless communications which are 1000 times more than that of radiocommunications. For transmission of VLC, we use LED as light sources.Due to the high efficiency and less power consumption LED have replaced the oldfluorescence lamps, LED provide the dual functionality they can provide lighting and can providecommunications (transfer of data) just like Wi-Fi. In LED the on and off state is so fast that the humaneye can’t even perceive it. The on and off state can be taken as 1 and 0’s and through this we can transferthe data, this type of modulation is called OOK keying modulation it is used for single carrier modulationscheme. We can interpret the data that is received from the transceiver side with the help of thephotodiode at the receiver’s side. This communication technique can provide better security as there is nointerference, as light can't penetrate through walls leaving the data transfer to the room itself. ThroughVLC we can offer better security to data over RF communications.In this thesis, the implementation process has been performed in MATLABsimulations where we analyse different modulation techniques and parameters. We design a room withdimensions as 5m*5m*3m as length, width and height. We take multiple LED’s at the top and determinethe illumination parameters in the room due to the light emitted from the LED. The receiver is located ona desk and we calculate the number of data rates received at the receiver. The modulation techniques usedin this thesis are OOK keying modulation. We estimate the data rates in two methodologies directdetection (Line of sight) and also, we take reflections from the wall into consideration (Non-line of sight).The effect of data rates due to illumination and distance are also determined. In this thesis we transfer dataover the transmitter and receive the information at the receiver for obtained information the calculation of bit error rate (BER) is performed for both single LED and multiple LED array. The analysis is performed between the performance metrics of a single LED’s and multiple LED’s arrays to determine better-LEDarray.Key Words: OOK modulation scheme, MATLAB-Simulation, Light Emitting Diodes .