(Georgia Institute of Technology, 2022-08-08)
Advincula, Abigail A.
This thesis highlights charge transport in solution-processable conjugated polymers (CPs) as a central theme, with an emphasis on the development of structure-property relationships to tune materials properties. Side chain and backbone structures of CPs are functional handles by which electron density and carrier localization are modified, with concomitant changes to the optical, thermal, and microstructural properties. Subtle changes in molecular structure additionally give rise to differences in device performance, allowing for the development of design rules for materials optimization. In this work, structure-property relationships are explored for several applications (i.e., organic solar cells for energy generation, reversible redox materials for mixed ionic and electronic conduction, and solution-processable materials for electrically conductive films in the solid-state) with each presenting a unique landscape for charge transport.
Chapter 1 introduces the fundamentals of conjugated materials, with the first half overviewing the history of CPs and molecular design motifs to target materials properties. The didactic second half covers basic operating principles of organic solar cells, oxidative and reductive processes in CPs, and charge development and transport for chemically doped films in the solid-state. Chapter 2 discusses details of the experimental techniques, beginning from a fundamental physical understanding and progressing to practical application and implementation. Chapters 3, 4, 5, and 6 detail the main projects that I have worked on during my graduate career, with each discussing these characterization techniques in more detail and context. Chapter 3 recounts our efforts to evaluate the fullerene docking hypothesis for organic solar cells with a series of thienopyrroledione-based donor-acceptor polymers with side chains of varying bulk. In this system, charge transport was not found to be drastically influenced by modulation of the side chain substituents, though changes to thermal properties, microstructural ordering, and UV-vis solution spectroscopy were observed. Chapter 4 is an in-depth analysis of three new polar side chain substituted copolymers with varied dioxythiophene comonomers. Spectroscopic, electrochemical, and computational data allows us to gain insight into the effects of comonomer planarity on redox properties. Additionally, the polymers are studied for their suitability in aqueous electrochromic and model Type I supercapacitor devices. Chapter 5 utilizes the same copolymer series, demonstrating how comonomer choice allows the electronic and thermoelectric properties of polymer films to be tuned. At optimal doping conditions, electrical conductivities of the polymer series span over two orders of magnitude, with the highest electrical conductivity approaching 200 S/cm. In Chapter 6, various electrolyte anions were studied to probe how their size, hydrophilicity, and relative “hardness” affect the swelling and redox cycling properties of a polar side chain functionalized dioxythiophene homopolymer. In this work, we observe differences in redox kinetics (e.g., charge/discharge times and redox capacity) and onsets of oxidation of the CP due to anion selection, as well as highlight the unusual swelling properties of the CP in the gluconate electrolyte. Finally, Chapter 7 will provide a brief outlook on how the completed projects could be further developed, proposing future projects based upon work presented in this dissertation.