Title: "The role of ionic interactions on electronic and ionic transport in conjugated polymeric mixed conductors" 

Abstract: Simultaneous ion and electron conduction is a critical property of many material systems and is essential for applications ranging from batteries and supercapacitors to bioelectronics. Designing mixed ion and electron conducting materials is challenging because ion and electron transport follow different design rules. Electron transport is often higher in ordered, crystalline materials while ion transport is often higher in structurally dynamic, polar materials. Conjugated polymers are uniquely suited to support both ionic and electronic transport because organic synthesis and molecular design strategies can be readily implemented to tailor their ionic, electronic, and structural properties. One important question in the design of mixed conducting conjugated polymers, however, is how electronic carriers and mobile ions affect the transport of each other. For instance, electrostatic interactions between mobile ions and electronic carriers have the potential to affect their stability and mobility. Furthermore, the structure, size, and spatial distribution of ions in a conjugated polymer likely affects electronic properties.

Utilizing the design of well controlled model systems, this work focuses on the structural, electronic, electromagnetic, and dynamical factors that affect the properties of materials with mobile electronic and ionic carriers. First, an ion-gated transistor model system is developed and employed to change the charge density of counterions to electronic charges in conjugated polymers. This reveals the inter-relation between ion-electron Coulomb interactions, structure, and electronic mobility. This model system is then employed to probe the role of ion distribution on the mechanism of electron transport in conjugated polymers on the semicrystalline scale. With this understanding of the relation between electronic transport and ion-electron interactions, we design a model conjugated polymeric mixed conductor to study the role of ion-electron interactions on macroscopic ionic and electronic transport by correlating macroscale transport properties to molecular scale structural and chemical characterization. 

Committee members: Rachel Segalman, Michael Chabinyc, Glenn Fredrickson, Todd M. Squires