Defect passivation and surface modification of perovskite semiconductors play a key role in achieving highly efficient and stable perovskite solar cells (PSCs) and light-emitting diodes (LEDs). This dissertation describes three novel strategies for such defect passivation and surface modification. In the first strategy, we demonstrate a facile approach using inorganic perovskite quantum dots (QDs) to supply bulk- and surface-passivation agents to combine high power conversion efficiency (PCE) with high stability in CH3NH3PbI3 (MAPbI3) inverted PSCs. This strategy utilizes inorganic perovskite QDs to distribute elemental dopants uniformly across the MAPbI3 film and attach ligands to the film’s surface. Compared with pristine MAPbI3 films, MAPbI3 films processed with QDs show a reduction in tail states, smaller trap-state density, and an increase in carrier recombination lifetime. The strategy results in reduced voltage losses and an improvement in PCE from 18.3% to 21.5%, which is among the highest efficiencies for MAPbI3 devices. The devices maintain 80% of their initial PCE under 1-sun continuous illumination for 500 h and show improved thermal stability. In the second strategy, we reduce the efficiency gap between the inverted PSCs and regular PSCs using a trace amount of surface-anchoring, long-chain alkylamine ligands (AALs) as grain and interface modifiers. We show that long-chain AALs suppress nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films. These translate into a certified stabilized PCE of 22.3% (23.0% PCE for lab-measured champion devices). The devices operate for over 1000 hours at the maximum power point (MPP), under simulated AM1.5 illumination, without loss of efficiency. Finally, we report a strategy to passivate Cl vacancies in mixed halide perovskite (MHP) QDs using non-polar-solvent-soluble organic pseudohalide (n-dodecylammonium thiocyanate (DAT)), enabling blue MHP LEDs with enhanced efficiency. Density-function-theory calculations reveal that the thiocyanate (SCN-) groups fill in the Cl vacancies and remove deep electron traps within the bandgap. DAT-treated CsPb(BrxCl1-x)3 QDs exhibit near unity (~100%) photoluminescence quantum yields; and their blue (~470 nm) LEDs are spectrally stable with an external quantum efficiency (EQE) of 6.3% – a record for perovskite LEDs emitting at the 460-480 nm range relevant to Rec. 2020 display standards.

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 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.

No description available.

This dissertation focuses on the design, synthesis, characterization of platinum (II) pyridylazolate complexes and develop high performance organic light emitting diodes (OLEDs) and design and execute high-sensitivity humidity sensors based on the luminescent metal-organic complexes of platinum. A majority of existing platinum compounds do not dissolve in organic solvents, making it difficult to analyze the photophysical characteristics of complexes in solution, a key part of understanding chemical photophysical properties. Furthermore, due to the poor quantum yield, it is inefficient for use in devices such as OLEDs. Chapter 2 reports the synthesis and characterization of a novel heteroleptic platinum(II) pyridylazolate complex with high solubility and quantum yield. The photochemistry of the complex is studied, including efficiency, emission profiles, and lifetimes at different temperatures. Chapter 3 reports the power efficiency (lm/W), current efficiency (cd/A), external quantum efficiency (EQE), luminance and operating voltage (V) of OLED devices made with the heteroleptic platinum(II) pyridylazolate complex. The relation between thickness of hole transport layer and electron transport layer on performance of devices has been studied through building a variety of devices. Chapter 4 includes application of a homoleptic platinum(II) pyridylazolate complex in humidity sensor. In many environments, the relationship between moisture content and emissive wavelength has been investigated. This research reveals that regardless of the humidity level, there is a link between increasing the temperature and decreasing the moisture absorption capacity of the complex.

An experiment was conducted to evaluate the potential of producing dwarf tomatoes (Solanum lycopersicum L.) in an indoor vertical system. Cultivars 'Micro Tom', 'Jochalos', and 'Venus' were grown under the three daily light integral (DLI) levels of 16.9, 22.7, and 27.9 mol·m-2·day-1. Cultivar had an effect on height, with Jochalos being the tallest. Plants grown under 27.9 mol·m-2·day-1 were the shortest. The time of first flower was affected by cultivar, appearing earliest in Micro Tom. Regarding DLI, the first flower was earliest in plants under 22.7 and 27.9 mol·m-2·day-1. The time of first harvest was only affected by cultivar, in which Micro Tom was earliest. There was an interaction effect for the total and marketable fruit count harvested. Micro Tom had the highest overall number of fruits harvested amongst cultivars, and Micro Tom grown under 22.7 and 27.9 mol·m-2·day-1 had more total and marketable fruit counts than those grown under 16.9 mol·m-2·day-1. There was no interaction effect for fruit weight harvested. The effect of cultivar differed between the total and marketable fruit weights harvested, but Jochalos produced the highest weights in both. The effect of DLI was the same for total and marketable fruit weights harvested, with plants under 22.7 and 27.9 mol·m-2·day-1 having the highest weights. Cultivar had an effect on fruit Brix, with Micro Tom fruit having the lowest Brix and Jochalos fruit having the highest Brix. The effect of DLI resulted in fruits produced by plants under 27.9 mol·m-2·day-1 having the highest Brix and fruits produced by plants under 16.9 mol·m-2·day-1 having the lowest. Based on these results, there is potential for dwarf tomatoes to be grown in vertical farm systems. / Master of Science in Life Sciences / An experiment was conducted to evaluate the potential of producing dwarf tomatoes (Solanum lycopersicum L.) in an indoor vertical system. Cultivars 'Micro Tom', 'Jochalos', and 'Venus' were grown under the three daily light integral (DLI) levels of 16.9, 22.7, and 27.9 mol·m-2·day-1. Cultivar had an effect on height, with Jochalos being the tallest. For DLI, all plants grown under 27.9 mol·m-2·day-1 were the shortest. The time of first flower was affected by cultivar, appearing earliest in Micro Tom. Regarding DLI, the first flower was earliest in plants under the higher DLIs. The time of first harvest was only affected by cultivar, in which Micro Tom was earliest. There was an interaction effect for the total and marketable fruit count harvested. Micro Tom had the highest overall number of fruits harvested under the highest DLIs. There was no interaction effect for fruit weight harvested. In cultivars, Jochalos produced the highest weights for both total and marketable fruits. Plants under 22.7 and 27.9 mol·m-2·day-1 had the highest weights for total and marketable fruits harvested. Cultivar had an effect on fruit Brix, with Micro Tom fruit having the lowest Brix and Jochalos fruit having the highest. Uninfluenced by cultivar, fruits produced by plants under 27.9 mol·m-2·day-1 had the highest Brix, while those under 16.9 mol·m-2·day-1 had the lowest. Based on these results, there is potential for dwarf tomatoes to be grown in vertical farm systems.

abstract: White organic light emitting diodes (WOLEDs) are currently being developed as the next generation of solid state lighting sources. Although, there has been considerable improvements in device efficiency from the early days up until now, there are still major drawbacks for the implementation of WOLEDs to commercial markets. These drawbacks include short lifetimes associated with highly efficient and easier to fabricate device structures. Platinum (II) complexes are been explored as emitters for single emissive layer WOLEDs, due to their higher efficiencies and stability in device configurations. These properties have been attributed to their square planar nature. Tetradentate platinum (II) complexes in particular have been shown to be more rigid and thus more stable than their other multidentate counterparts. This thesis aims to explore the different pathways via molecular design of tetradentate platinum II complexes and in particular the percipient engineering of a highly efficient and stable device structure. Previous works have been able to obtain either highly efficient devices or stable devices in different device configurations. In this work, we demonstrate a device structure employing Pt2O2 as the emitter using mCBP as a host with EQE of above 20% and lifetime values (LT80) exceeding 6000hours at practical luminance of 100cd/m2. These results open up the pathway towards the commercialization of white organic light emitting diodes as a solid state lighting source. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016

A novel solid state illumination source has been developed. A two terminal dual LED has been created with the ability to control the relative intensities of the two emission peaks by varying drive current. Doping profiles have been used to extend the dynamic range of the dual LED over other reported devices. Operation of the dual LEDs is explained as a function of drive current. In addition, novel use of phosphor mixtures allows the creation of a broadband spectral power distribution that can be varied using a dual LED as an excitation source. Combinations of phosphors that have varied excitation spectra provide the ability to selectively excite different phosphors with the different LED emission peaks. First and second generations of the two terminal dual LED and the phosphor combination are discussed. The final source has the ability to mimic the light of a blackbody radiator over a range of 3200 K - 5300 K. The development of a three terminal dual LED as a pump source was prohibited by the need for a III-nitride tunnel junction, that proved unattainable in the scope of this work. However, several novel doping schemes were investigated toward this end. Finally, a circadian light source has also been developed that can affect physiological changes in humans, and a light box for entrainment of circadian rhythms in rats has been built.