Georgia Institute of Technology Materials Science and Engineering

Committee Members: 

Prof. John Reynolds, Advisor, CHEM/MSE

Prof. Natalie Stingelin, MSE/CHBE

Prof. Carlos Silva, CHEM/PHYS

Prof. Blair Brettmann, CHBE/MSE

Prof. Vladimir Tsukruk, MSE

"Establishing Design Principles for Polythiophenes Used in Electrochemical Applications"

Abstract:

Among the many applications that capitalize on the electrochemical functionality of π-conjugated polymers, electrochromic devices (ECDs) and organic electrochemical transistors (OECTs) have attracted considerable interest. The operation of these devices hinges on electrochemical doping reactions that effectuate the desired color change or conductivity modulation. Understanding relationships between polymer structure and redox properties is essential for the rational design of active materials and the development of viable application spaces. This dissertation aims to explore materials design strategies for tuning the functional properties of polythiophenes for use in ECDs and OECTs. 

First, solution co-processing of poly(dioxythiophenes) is presented as a straightforward and scalable technique for accessing high-contrast black-to-transmissive electrochromic films with low driving voltages, extended functional lifetimes, and minimal transient chromaticity. These efforts demonstrate how judicious blend formulation, particularly in the choice of the mid- and high-gap chromophore components, can be leveraged to control the coloration of ECDs. 

Next, this work establishes a comprehensive understanding regarding the structural factors that dictate the properties of polymers for application in aqueous-based OECTs. A combination of electrochemical, spectroscopic, and X-ray techniques are used to probe the redox response, capacitance, thin film microstructure, and associated performance metrics in two distinct classes of materials: conventional polythiophenes (PTs) and poly(3,4-propylenedioxythiophenes) (PProDOTs).  In a family of PTs, the length and substitution pattern of the ethylene glycol-based side chain is found to drastically impact the properties of the active materials. Meanwhile, the properties of polar-functionalized PProDOTs are found to be remarkably less sensitive to side chain length. This work demonstrates the balance of material characteristics that must be achieved for maximal performance. The cumulative results indicate that underlying structure-property trends in OECT channel materials cannot be generalized across material classes. 

Finally, this research utilizes a novel application of in situ specular neutron reflectivity to track actuation and electrolyte uptake in PProDOTs while electrochemically doping in aqueous media. Contrast-matching methods reveal that electrolyte irreversibly penetrates the polymer film, even prior to application of an electrochemical bias. 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-based techniques to study the redox dynamics of electroactive polymers in a more general sense.