Benzobisazoles are a class of molecules that initially found their use in high-performance materials as high tensile strength fibers. Recent modifications to the syntheses of benzobisazoles have allowed for the materials to be studied as an n-type material to be used in organic semiconductors, more specifically organic light-emitting diodes (OLEDs). The high molecular stability required to produce blue light gives an opportunity for benzobisazoles to fulfill the requirement. Prior work on benzobisazoles, more specifically, the oxygen analog benzobisoxazole, has been used to try to achieve blue (<450 nm) but fell short in terms of efficiency due to molecular design choices. The following describes new design strategies such as utilizing single-bond linkage between the electron rich and deficient molecules, as well as transitioning from polymer to small molecules to fine-tune the properties of the materials for semiconductor applications. Utilizing a new design strategy, we demonstrate the ability to blue-shift the emission on two benzobisoxazole-based polymers by adopting single bond linkage between the benzobisoxazole and electron rich moieties fluorene and carbazole and achieve a usable brightness (> 1000 Cd/m2) when incorporated into OLEDs. With further modification of the benzobisoxazole core piece by adding dual conjugation along both axes to produce small molecules, we were able to achieve a deeper blue emission at higher efficiencies due to the reduced conjugation and aggregation than our previous systems experienced. Development of the small molecules led us to adopt a modular synthetic strategy for the high-efficiency material design of benzobisoxazole-based materials. In combination with Density Functional Theory calculations, we show the viability of performing computer-backed molecular design to develop materials to be used in all types of semiconductor applications. From calculations, we synthesize benzobisoxazole cruciforms that have both electron rich and electron deficient moieties. These products we then compared to experimental data to confirm the validity of computer-based rational design of molecules for not only blue OLEDs but for all semiconductor applications. The extremely high number of possible combinations of electron rich and electron deficient moieties allows for extensive future studies for the most optimal substituents for proper energy leveling tuning.

Micro light-emitting diode (microLED) technologies have been rapidly developing in the past decade and stand to be the prominent display technology for high-brightness applications. MicroLED microdisplays are particularly well-suited for systems that compete with high ambient light, such as augmented reality headsets and smartwatches that reflect light from the sun. However, there are several technological issues to overcome before microLED cost can be driven to a point that enables widespread commercial use. This dissertation covers the current microLED technological landscape, key issues to overcome, and an in-depth discussion on microLED performance and applications using modeled and experimentally fabricated microLEDs. The first experiment focuses on microLED fabrication fidelity and methods to overcome the challenge of defect-free displays. Current ultra-high definition display resolution standards require approximately 25 million individual microLED emitters with an expected zero dead pixels. To better identify defect states at early stages of fabrication, this dissertation presents methods using photoluminescence and cathodoluminescence that can identify dry-etching related damage to GaN/InGaN microLEDs that result in dead pixels. Expanding on fabrication fidelity, the second study in this dissertation examines surface recombination losses in etched GaN/InGaN microLEDs from nitrogen vacancy trap states. As microLED emitter size decreases, the ratio of etched surface area to emitter area size increases and injected current recombining at surface trap states increases causing large efficiency losses. To combat this, this study examines pGaN contact geometry selections and the influence on surface recombination losses. In particular, the results show that there is a strong dependence on efficiency for a desired output power in relation to current density. Utilizing the fabrication knowledge from the first two studies, applications and implementations of microLED microdisplays as a structured illumination microscopy light source within miniaturized microscopes are presented. There is discussion on future miniaturization strategies and next steps to improve device performance. Finally, this dissertation includes a short discussion on a display-adjacent technology, organic field-effect transistors (OFETs). An investigation on the electrostatic discharge resilience of parylene in OFETs is presented for applications in flexible high-voltage thin-film transistors.

Thesis: Elec. E., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1975 / Includes bibliographical references. / Elec. E. / Elec. E. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

In conventional optics, curved lenses focus light rays to a focal point after light passes through them. These lenses have been designed to shape the wavefront of the incident beam as it emerges from the curved surface of the lens. Conventional lenses suffer from many limitations, such as limited optical quality for imaging and integration difficulties with other optical components due to their large size, huge thickness, as well as being difficult to manufacture. Using subwavelength structure, it is possible to fabricate flat, thin lenses (metalenses) with new optical properties not found in nature, in which many fundamental properties of light (like polarization, focal point, and phase) can be controlled with high accuracy. This results in high resolution and high quality of optical imaging. This thesis demonstrates a new design of reflective metalens, in which the metalens structure is integrated with another optical component: Distributed Bragg Reflector (DBR). The metalens planer is a two-dimensional ultrathin planer arranged as an array with subwavelength separation distance. In recent works, a metalens was integrated with (metal/dielectric)-mirrors to form reflective metalenses. Simulation results show that, high-focusing efficiency is obtained for the lens (> 60%) with the ability to reflect96% of total incident optical power. In comparison, the new metalens-DBR design - processes maintain the same high-focusing efficiency, but with a reflectance of 99.99%, which makes it promising for optoelectronic integration and perfectly suitable for integration with Vertical Cavity Surface Emitting Lasers (VCSEL) technology. This study of the optical properties: focal length; optical aberration; insensitivity to light polarization; and focusing efficiency of demonstrated metalens was done mainly by Finite Difference Time Domaine (FDTD) by using Lumerical FDTD solution.

Metal halide perovskite semiconductors offers bright future for optoelectronic applications due to their excellent optical and electrical properties and their low-cost solution-based facile fabrication. The most of the perovskite application are based on the defective polycrystalline films and they offer inadequate moisture/thermal chemical stability. Therefore, this dissertation is dedicated to find new strategies to deploy the single-crystal perovskites to photovoltaics and new methods to reduce the moisture/thermal instability of inorganic perovskite light-emitters. In first part of this dissertation, we aimed to reveal the potential of the single crystal in photovoltaics. Single-crystal semiconductors can outperforms their polycrystalline forms in terms of photovoltaic performance due to their better structural quality and less electronic traps. However, the most efficient perovskite solar cells are based on polycrystalline films. While single crystals can perform beyond the limits of polycrystalline films, their synthesis and device integration are complex. Therefore, we aimed to create new synthetic methods to unveil the potential of the single-crystal perovskites in photovoltaics. We developed new strategies leading the perovskite single crystals to go beyond 20% power conversion efficiency in Chapter 2. Also fundamental limits of the perovskite single crystals are investigated in Chapter 3 by fabricating single crystal cells with varying thicknesses, and the electron diffusion length is calculated to be 520 μm. In Chapter 4, we propose surface modification and compositional engineering techniques to bring the perovskite single crystal photovoltaic one step beyond of the previous point by reaching 21.9% and 22.8% efficiencies, respectively. In the second part of this dissertation (Chapter 5), a novel synthetic method is offered to achieve highly stable light-emitting perovskite-related materials since the fast degradation of perovskites in the presence of water and moisture is a challenge for perovskite-based technologies and hinders the material’s potential. We demonstrated that these a direct transformation of 3D CsPbBr3 films to CsPb2Br5 exhibiting excellent stability against humidity and heat while keeping the high photoluminescence quantum yield. We believe the strategies offered in this dissertation will open an avenue in photovoltaic and light emitting applications, and can be utilized in new optoelectronic applications in future.

In recent years, Light-emitting Diodes (LEDs) have become a promising candidate for backlighting Liquid Crystal Displays [1] (LCDs). Compared with traditional Cold Cathode Fluorescent Lamps (CCFLs) technology, LEDs offer not only better visual quality, but also improved power efficiency. However, to fully utilized LEDs' capability requires dynamic independent control of individual LEDs, which remains as a challenging topic. A FPGA-based hardware system for LED backlight control is proposed in this work. We successfully achieve dynamic adjustment of any individual LED's intensity in each of the three color channels (Red, Green and Blue), in response to a real time incoming video stream. In computing LED intensity, four video content processing algorithms have been implemented and tested, including averaging, histogram equalization, LED zone pattern change detection and non-linear mapping. We also construct two versions of the system. The first employs an embedded processor which performs the above-mentioned algorithms on pre-processed video data; the second embodies the same functionality as the first on fixed hardware logic for better performance and power efficiency. The system servers as the backbone of a consolidated display, which yields better visual quality than common commercial displays, we build in collaboration with a group of researchers from CREOL at UCF.

This project successfully designed, fabricated and characterized two highly reflective distributed Bragg reflectors for use with long wavelength vertical cavity surface emitting lasers. The first reflector consisted of 20 pairs of alternating lnP/Ino.64Gao.36Aso.777Po.223 layers grown on an InP substrate with a theoretically predicted normal incident reflectivity of 96.6% at a center wavelength of 1550nm. The second DBR had 20 pairs of alternating GaAs/Ino.484Gao.5i6P layers grown on a GaAs substrate with a theoretically predicted reflectivity of 94.9% at a center wavelength of 1550nm for normal incident light. Experimental results obtained using a specially designed reflectivity measurement setup confirmed reflectivity models and predictions at both normal and variable incident light angles. However, these measurements revealed a discrepancy between theoretical and experimental layer thickness values for both DBR structures. Applying perturbations to the theoretical models, the actual layer thicknesses ofthe DBRs were determined. X-ray analysis was employed to examine the periodicity of the super-lattices along with the accuracy of lattice matching to the substrate. Transmission electron microscopy revealed that no detectable drift in layer thickness was apparent during growth of the DBR structures. Photoluminescence was used to investigate any compositional variations ofthe quaternary layers in the first DBR stack. / Thesis / Master of Applied Science (MASc)

Light-emitting diode (LED) is an improved light delivery technology that provides better energy efficiency, longer lifespan, and the ability to select a specific wavelength of light. We evaluated the effect of blue and red-LED on performance, behavior, egg quality, hormonal concentration, and prevalence of Avian pathogenic E. coli (APEC) like E. coli in pullets and laying hens raised in cageree housing system. Results showed that birds raised in blue-LED during the pullet phase had higher body weight and earlier onset of early lay. Red-LED increased the yolk percentage, illustrating that red-LED can stimulate reproduction. Red-LED also decreased the relative spleen percentage, which is an indication of decreased immunity. Overall egg production was not affected by the light treatment. Irrespective of the light treatment, a higher prevalence of APEC like E. coli was found in colonies isolated from the trachea suggesting a possible route of transmission in cageree housing system.

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