Browsing by Subject "Optoelectronics"

Now showing 1 - 5 of 5
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    3D Printing Multifunctional Optoelectronic and Microfluidic Devices
    (2020-10) Su, Ruitao
    Functional materials encompass different classes of materials possessing intrinsic or synthetic properties that are responsive to external stimuli. A few examples include semiconducting polymers/crystals, electroluminescent polymers, polymers with controlled cross-linking mechanisms and printable metallic inks with tunable sintering mechanisms and conductivity. The technology of additive manufacturing, or 3D printing, has been extensively investigated with structural plastics and metals to realize rapid prototyping of irregular/customized geometries, demonstrating a few successful examples of commercialization. Yet, a further systematic study is demanded to investigate the methodologies to incorporate multiple functional materials in the 3D printed multifunctional devices. This will lay important foundations for the fabrication of a range of devices under ambient conditions that were conventionally accessible exclusively to the cleanroom-based microfabrication. More importantly, the capability of 3D printing to integrate materials in a freeform manner will facilitate novel device form-factors and functionalities that are challenging to realize with microfabrication. In this work, the methodologies of 3D printing optoelectronic and microfluidic devices were investigated with an emphasis on material selection, device configuration, alignment, performance optimization and scalable fabrication. To this end, a custom-built 3D printing system was utilized to accurately pattern functional materials that possess varying rheological properties. Over the past several decades, 3D printing has demonstrated an array of electronic devices such as batteries, capacitors, sensors, wireless transmitters etc. This progress renders an expectation for fully 3D printed integrated circuits that can be rapidly prototyped and adopt more complicated spatial architectures. However, fully 3D printed optoelectronic devices are still a relatively unexplored paradigm. One major challenge of 3D printed optoelectronics is to optimize the device performance by controlling the thickness and uniformity of the solution-processed layers. An optimized layer thickness maintains the balance between charge injection and light extraction for light emitting diodes (LEDs) or light absorption and charge separation for photodetectors. Layer uniformity affects the contact between adjacent layers and therefore the charge carrier transport. In this work, electroluminescent semiconductors, including silicon nanocrystals (SiNCs) and conjugated polymers, were 3D printed as the active layers of LEDs and photodetectors. The effect of printed layer thickness on the device performance was investigated for the extrusion-based printing. A spray printing method was integrated in the 3D printing system and an improved device performance was observed. Significantly, for the 3D printed polymer photodetectors, an external quantum efficiency (EQE) of 25.3%, comparable to that of spin-coated devices, was achieved by controlling the concentration of the active ink. For the device integration, photodetector arrays were printed on flexible and spherical substrates for a freeform and wide field-of-view image sensing. Novel multifunctional optoelectronic devices consisting of integrated LEDs and photodetectors in a side-by-side layout was printed on the same platform, demonstrating potential applications of wearable physiological sensors. Next, for the 3D printed microfluidic devices, this work demonstrates that yield-stress fluids, such as viscoelastic gels, can be extruded to construct self-supporting hollow microstructures that are highly flexible and stretchable. Several additive manufacturing methods, such as stereolithography and multi-jet printing, have demonstrated 3D printed microfluidic devices with improved automation compared to the conventional soft lithography. However, it remains a challenge to directly incorporate electrical and biological sensing elements in the microfluidic devices. In this study, because of the yield strength of the viscoelastic ink, mechanical equilibrium states were found to exist for the inclined standing walls. Self-supporting microfluidic channels and chambers were 3D printed by stacking silicone filaments according to prescribed toolpaths. Since no sacrificial material was demanded to realize the hollow structures, the microfluidic structures can be directly aligned and printed onto microfabricated circuits without contaminating the electrodes. The high modeling precision of this method was demonstrated via fully 3D printed chemical species mixers that were embedded with herringbone ridges. In addition, automation components, including microfluidic valves and peristaltic pumps, were also 3D printed with overlapping silicone channels that were encapsulated by UV-curable resins. Most compellingly, microfluidic networks integrated with valves transcended the conventional planar form-factors and were directly printed on 3D surfaces. The 3D microfluidics suggests a potential application of microfluidics-based physiological sensors that can be directly printed onto freeform surfaces such as human bodies. Lastly, this work demonstrates that the above two distinct systems can be seamlessly integrated together via 3D printing, yielding fully encapsulated and flexible LED matrices. Liquid metals such as eutectic GaIn are promising candidates for soft and stretchable electronics. As the cathode material of 3D printed optoelectronic devices, it has the desired work function and a high mechanical compliance. However, current challenge of patterning liquid metals lies in the design of a robust encapsulation for the cathodes and simultaneously creating an effective interface with interconnects. To this end, self-supporting microfluidic networks that are highly adaptable and aligned to the layout of LED matrices were printed to encapsulate the liquid metal. The 3D printed liquid metal microfluidics enabled the scalable fabrication of flexible and individually addressable LED matrices. In summary, this research expanded the scope of ink composition for 3D printed multifunctional devices. Transferring these materials from microfabrication to 3D printing significantly improves the manufacturability of optoelectronic and microfluidic devices. The intrinsic capabilities of 3D printing to pattern 3D structures in a freeform manner facilitated novel functionalities for both types of devices, including spherical image sensors, 3D microfluidic networks, flexible organic LED matrices etc.
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    Electrical and optical characterization of colloidal silicon nanocrystals
    (2013-12) Li, Meng
    Colloidal silicon nanocrystals (SiNCs), due to their high photoluminescence efficiency and tunable bandgap, can be used to fabricate highly efficient hybrid nanocrystal-organic light-emitting-devices (NC-OLEDs) that emit in red or near infrared spectrum. Despite reports of outstanding device performance, the underlying mechanism of this high efficiency remains unknown. Consequently, this thesis focuses on studying the electrical and optical properties of SiNCs. The electrical conductivity and mobility of electrons and holes are successfully extracted in order to explain the observed dependence of device efficiency on SiNC surface ligand coverage. Steady-state and transient photoluminescence is also examined to better understand the connection between surface ligand coverage and molecular photophysics. In addition, these measurements are used to better understand the mechanisms for non-radiative exciton decay in SiNCs. This work elucidates the relationship between SiNC properties and device performance, potentially guiding the design of future NC materials for high performance.
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    Luminescent Probes of Emergent Physics from Organic Semiconductor Interfaces
    (2022-12) Concannon, Nolan
    To prevent the most harmful effects of the present climate crisis, development ofhigher performance energy conversion devices is needed to accelerate the adoption of renewable energy and energy efficiency technologies. Organic semiconductor materials have demonstrated exciting efficiency gains in a variety of emerging and in-production devices. These materials exhibit a variety of emergent material and device physics, requiring additional research to understand and design next-generation energy conversion technologies. Thin films of organic semiconductors, common in large-area optoelectronics such as consumer displays, present rich photophysics due to forming room-temperature stable excitons, unlike silicon or III-V semiconductors. A plethora of emergent phenomena of excitons at organic semiconductor interfaces requires a detailed understanding of such processes to optimally design devices such as energy-efficient lighting, flexible or transparent solar cells, photodetectors and displays. This dissertation focuses on investigating the novel optical physics of excited states at organic donor-acceptor interfaces through emission spectroscopy of organic mixtures and bilayer devices. In one study, exciplex diffusion is investigated in several donor-acceptor pairings toward an improved understanding of the mechanism of nanoscale energy transport in organic semiconductor mixtures. Additionally, the effect of electric field on exciplex emission spectra is studied to detail the effect of field on exciplex energy and electron-hole separation. Finally, preliminary data displaying the effect of binary dilution on exciplex energy in a two-component mixture is presented. All together, these findings present new insights into the behavior of key device properties such as exciton diffusion length and excited state energies to aid further study of device performance.
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    Thin-Film Synthesis of Metal Halide Perovskites for Optoelectronics
    (2020-08) Clark, Catherine
    Metal halide perovskites (MHPs), like the archetypal methylammonium lead iodide (MAPbI3), have emerged in the last decade as promising materials for efficient, low-cost optoelectronics. MHP solar cells have already reached efficiencies >25%, rivaling established technologies like single-crystal Si. Yet several challenges prevent the widespread commercialization of MHPs, including their instability in ambient conditions, their toxicity, and the need for scaleable fabrication techniques. Fundamentally, the origins of important material properties relating to carrier transport and recombination are still not well understood. Thin film deposition techniques that enable detailed study of process-structure-property relationships and are commercially relevant are consequently becoming increasingly essential. This thesis seeks to address these challenges through the design, implementation, and utilization of a carrier-gas assisted vapor deposition (CGAVD) method that can grow MHP films with highly tunable stoichiometries and morphologies. Alongside the design of a CGAVD system with six independently controllable experimental parameters, an analytical model is developed and experimentally validated that allows the determination of robust and repeatable growth regimes and the prediction of material deposition rates. Harnessing this technique, we demonstrate the ability to deposit MASnI3 and MASnBr3 films and to systematically vary their compositions across a wide range, and realize corresponding changes in film microstructures (grain size, coverage) and electronic properties (resistivity, carrier concentration, mobility). Control of grain size and film texturing is also achieved independent of stoichiometry via modulation of chamber pressure and substrate temperature. The benefits of CGAVD are further highlighted by the successful growth of novel all-MHP heterojunctions. Two stable pairings are identified: MAPbBr3/MASnBr3 and CsPbBr3/MASnBr3. Design rules to control the mixing of heterojunctions are developed by exploring the dependence of mixing rate on MHP layer composition and grain size. Finally, through a collaboration with Physical Electronics, we optimize the use of XPS depth-profiling for MHPs and investigate which ions are diffusing in a layered structure that exhibits mixing. Moving forward, the incorporation of CGAVD-grown heterojunctions and Pb MHPs into optoelectronic devices will harness the tunability of this system towards a deeper understanding of process-structure-property relationships in MHP thin films and novel layered structures.
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    Waveguide Integrated Optoelectronics Using Two-Dimensional Materials
    (2016-11) Youngblood, Nathan
    The focus of this dissertation is the integration of 2D materials on a silicon photonics platform for optoelectronic applications. The current state of waveguide integrated photodetectors and modulators is reviewed and provides a context for the work detailed in the following chapters. The first dual-function graphene photodetector and modulator is demonstrated in a simple geometry that allows simultaneous use of both functionalities. Next, the first waveguide integrated black phosphorus photodetector is demonstrated with superior dark current to its graphene counterparts. Operation speeds six orders of magnitude higher than any previous black phosphorus detector is demonstrated together with a clear understanding of the photocurrent mechanisms that dominate the device. Finally, the nonlinear response of black phosphorus was used to investigate the intrinsic speed of a photodetector. Subsequent observation of third-harmonic generation led to characterization of chi(3) in black phosphorus for the first time.