Understanding the Effects of Structure on the Charge Transport Properties and Doping of Dioxythiophene Polymers
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Pittelli, Sandra
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Abstract
Redox-active conjugated polymers have been investigated for a multitude of applications including electrochromic displays, transistors, and transparent conducting electrodes. As with all fields of polymer science, the material properties observed for these conjugated polymers are determined by their structure. While the groundwork for probing their fundamental structure-property relationships has been established, there is still much to be explored in order to have a holistic understanding of these systems. This dissertation aims to investigate how the structure of dioxythiophene polymers can affect the processes of chemical and electrochemical oxidation, their charge transport properties, and their use in electrochemical and solid-state applications.
Chapter 1 provides an introduction to the fundamental properties and concepts of conjugated polymers, and their use in potential applications. Characterization methods used in this work, including electrochemical experimentation and dry-film characterizations are highlighted in Chapter 2. Chapter 3 describes the work exploring how the use of chemical oxidants and phosphonic acids can be used as pre-treatment steps in the construction of electrochemical devices. A family of polymers including two dixoythiophenes (DOTs) and one dioxypyrrole (DOP) were exposed to a series of chemical oxidant salts with the oxidizing agent either being Ag+ or Fe3+. The structure and redox properties of the conjugated polymers were shown to affect the overall extent of oxidation. Specifically, the DOT polymer that had the highest onset of oxidation in the family was oxidized to a lesser extent when using Ag+ as the chemical oxidant rather than Fe3+. In contrast, the DOT polymer with the lowest onset of oxidation in the family was oxidized to the same extent using both dopants. Using Fe3+ was shown to disrupt the interface between films of each polymer in the family and the conducting substrate, however this was rectified through modification of the surface energy of the substrate using phosphonic acids. Ultimately, it was shown that devices constructed after chemical pre-oxidation had comparable electrochromic contrasts to those using an electrochemical pre-oxidation step. Chapter 4 explores the effects of side chain alteration on the charge transport properties of a family of either 3,4-propylenedioxythiophene (ProDOT) or acyclic dioxythiophene (AcDOT) homopolymers. It was shown that the incorporation of linear side chains rather than branched side chains for both the ProDOT and AcDOT polymers allowed for a decrease in the onset of electrochemical oxidation by 300 mV and a 2 order of magnitude increase in the solid-state in-plane conductivity (10-3 vs 10-1 S/cm) after chemical oxidation. This study ultimately showed that there is a delicate balance in the degree of ordering of the polymer film and effective charge transport. Chapter 5 further supported this concept of balance by probing a family of soluble dioxythienothiophene (DOTT) polymers for their structure-property relationships. These polymers also showed that too much intermolecular ordering inhibits the processes of oxidation and charge transport. Specifically, the DOTT homo polymer showed higher degrees of intermolecular ordering according to GIWAXS measurements, and had an electrochemical onset of oxidation that was almost 700 mV higher (0.05 vs -0.62 V) than the less ordered DOTT co-polymerized with two EDOT units (DOTT-BiEDOT). The introduction of two EDOT units also raised the solid-state in-plane conductivity by 4 orders of magnitude (10-3 vs 101 S/cm).
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2019-11-06
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Dissertation