Multiscale (Spectro)electrochemical Methods for Understanding Charge Transfer at Conducting Polymer Interfaces

Abstract

Conductive polymer (CP) based semiconductors offer advantages over inorganic counterparts such as tunable redox properties, intermolecular packing structures, facile processing methods, and hybrid ionic-electronic transport. These attribute make CPs especially attractive for implementation in photoelectrochemical cells, and require an understanding of interactions such as (1) ion and solvent ingression into the polymer lattice, (2) formation of charge carriers on the polymer backbone, and (3) interactions with a redox species to drive a redox process. Each of these interactions is integral to the implementation of polymers as electrodes and pose many questions about their individual contributions to the overall redox process that must occur. Unlike classical semiconductors, which have a discrete interface, the soft nature of CPs leads to a interphase-like region at the boundary of the polymer and electrolyte, allowing ion and solvent ingression into the system. Additionally, structural and ordering differences between polymers have direct implications on electrochemical properties within a given system and its subsequent use for driving redox processes. The primary questions guiding the work in this dissertation is (1) how does the mobility of ions and presence of mobile solvent molecules affect the formation of polarons in the polymer lattice?, (2) how does rigidity in in polymer electrodes affect the ability to drive charge transfer reactions?, and (3) can ordering within a system improve the redox properties of a given polymer system? To begin addressing these questions, Chapter 4 aims to provide an understanding of ion interactions in a polymer electrode. In this study, P3HT was subjected to a variety of electrolyte conditions comprising of a solvent-based electrolyte, solid gel-electrolyte, and an ionic-liquid only electrode, each of which have differing ion mobilities and mobile solvent molecules. Spectroelectrochemical and X-ray photoelectron spectroscopy (XPS) analysis of these conditions provided an understanding of how ion mobility and degree of solvent-induced swelling affects the interphase of the polymer, and the ability to form polarons in the electrode. Changes in the binding energy of marker atoms provided an opportunity to begin quantifying ion-polaron interactions within each of the electrolyte systems. To provide context to these findings, the use of a transistor as a testing-platform revealed the device-level impacts that modulation of the interphase can have on functionality of the polymer. To begin understanding the impact that rigidity has on the redox properties of CPs, Chapter 5 uses two classes of polymers: Flexible, semicrystalline rr-P3HT and rigid, liquid-crystalline PBTTT-C12 for comparison. The results of these studies suggest that singular, macroscale rate coefficients may not provide a full picture of the impacts that rigidity and flexibility have on the electrochemical properties of the polymer electrodes. Submicron electrochemical measurements were then performed using SECCM, which helped elucidate the electrochemical heterogeneity in each of the systems. This work was expanded in Chapter 6 by focusing only on PBTTT-C12 and the impacts that ordering regimes have on the redox properties in this system. Ordering within PBTTT-C12 can be tuned using a thermal annealing treatment, and has been found to impact charge mobility in the system. Electrochemical analysis of the resulting three different ordering schemes in PBTTT-C12, with an emphasis on density of state comparisons. Focusing on a thiophene-based polymer family provides a comprehensive understanding the multilengthscale properties governing the interphase interactions and the electrochemical processes while building upon a breadth of work already performed on these model systems.