Establishing Design Rules for Polythiophenes Used in Electrochemical Applica
Savagian, Lisa Renee
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The electrochemical functionality of π-conjugated polymers has been leveraged for a number of applications, including optical displays, bio-electronic interfaces, and energy storage devices. Among these, electrochromic devices (ECDs) and organic electrochemical transistors (OECTs) have attracted considerable contemporary interest. The operation of such devices hinges on the rapid, reversible electrochemical doping of the polymeric active layer. This redox process is essential to effectuate the desired optical change in ECDs or conductivity modulation in OECTs. This dissertation aims to establish materials design strategies for tuning the optical and electrochemical properties of redox-active, π-conjugated polymers for use in OECTs and ECDs. First, solution co-processing of dioxythiophene-based copolymers is explored as a straightforward and scalable technique for accessing high-contrast black-to-transmissive electrochromic films for ECDs. These blends require low driving voltages while exhibiting extended functional lifetimes and minimal transient chromaticity. This work demonstrates how judicious blend formulation, particularly in the choice of the mid- and high-gap chromophore components, can be used to control the long-term, intermediate, and transient coloration of achromatic, black-to-clear, polymer-based electrochromes for ECDs. Next, this thesis develops a comprehensive understanding regarding the structural factors governing the properties of two distinct classes of aqueous-compatible OECT active materials—traditional polythiophenes (PTs) and poly(3,4-propylenedioxythiophenes) (PProDOTs). A combination of optical, electrochemical, and x-ray techniques are used to probe the redox response, capacitance, neutral and charged state absorbance properties, and solid-state microstructure of polymers with varying backbones and side chain structures. The properties of PT-based active material are shown to be highly dependent on the length of the polar side chains. Moreover, the redox capacitance and stability of these PTs can be enhanced simply by changing the side chain distribution along the backbone, even when the overall electroactive mass of the material is maintained or even reduced. Meanwhile, the properties of polar-functionalized PProDOTs are remarkably less sensitive to the side chain length. These amorphous materials show considerable redox activity in aqueous electrolytes, yet they do not show enhanced steady-state capacitance or redox stability when compared to the top PT analogue. The studies also demonstrate how a post-polymerization side chain modification strategy can be used to access an alcohol-functionalized PProDOT with enhanced capacitance in a saline electrolyte. The cumulative results of the studies outlined in these studies indicates that underlying structure-property trends in OECT active materials cannot be generalized across material classes. The final work presented in this thesis utilizes in situ specular neutron reflectivity to track actuation and aqueous electrolyte uptake by PProDOTs while electrochemically doping. Contrast-matching methods reveal that electrolyte irreversibly penetrates the polymer film, even prior to application of an electrochemical bias, as indicated by changes in film thickness and neutron scattering length density. After repeated cycling, the structure and composition of the films are permanently altered. The extent of potential-dependent reorganization and swelling is found to depend on the side chain functionalization of the polymer. Physical insights provided by the neutron reflectivity shed light on the diffusional limitations associated with the electrochemical doping reaction. Such work sets a precedent for using neutron reflectivity to study the volumetric and interfacial characteristics of electrochemically doped conjugated polymers in a more general sense.