Examining thermal and charge transport in organic materials with pi-electron interactions
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Atassi, Amalie
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Abstract
Organic materials continue to be explored as the active component in wearable, flexible electronics, and thermal devices. Their low-temperature thermal transitions and extensive chemical tunability makes them straightforward to process for a variety of applications, including thermal switches, devices that dynamically control the flow of heat, and thermoelectric generators, devices that convert heat into electrical energy (or vice versa). Despite these applications, the physical and electronic characteristics that lead to different thermal and charge transport in organic materials require further understanding. This dissertation examines the physical and electronic features of conjugated organic materials through a characterization of three unique chemistries. Factors that affect the electronic structure include morphology, such as the extent and quality of crystallinity, and doping, or reduction- oxidation processes.
I demonstrate how these factors further alter thermal and charge transport by showcasing three distinct chemical structures. In each case study, the morphological changes in response to chemical tuning differs, allowing for the deduction of chemical design guidelines for potential thermal and energy applications. In the first case study, I show how a crystalline molecule that decreases its intermolecular π-electron overlap yet maintains a high degree of structural order undergoes a fourfold decrease in thermal conductivity. Because a high extent of crystallinity is maintained in the material, the diminished thermal transport is in part due to the decreased conjugation measured in the material and the decreased heat capacity. For the second case study, I address changes in charge transport in highly disordered amorphous conjugated polymers. In these highly disordered materials, I increase the π-electron interactions by increasing the main-chain planarity in a series of poly(dioxythiophenes). This increased planarity, in turn, increases the polymer’s susceptibility to oxidation, increases the electrical conductivity, and rapidly decreases the Seebeck coefficient. The decrease in the Seebeck coefficient is related to the electronic structure. The chemical structure can be further modified to remove the side chains; this side chain modification increases the carrier density of the material, which further increases the electrical conductivity and decreases the Seebeck coefficient. In the final case study, I consider a system with both ordered phases and disordered phases—semicrystalline conjugated polymers. For this study, I show how the phase behavior of these materials at the macroscale affects the electronic structure at the local scale. Furthermore, the phase behavior (and resulting electronic structure) can be altered by combining the two materials. This phase interaction of each polymer can result in an enhancement of the thermoelectric power factor, which I demonstrate at intermediate doping levels.
Overall, this dissertation examines the complex relationships between phase morphology and electronic structure in conjugated organic materials, establishes structure-property guidelines for materials with ranging degrees of order, and contextualizes these relationships for potential thermal and energy management applications.
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2025-01-13
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Dissertation