THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, November 22, 2024 

at 10:00 AM
in Kendeda 210
and via Teams

https://teams.microsoft.com/l/meetup-join/19%3ameeting_OTE1N2JjYTEtMDM4MS00N2Q0LWFkNzgtOTdjMzk1YjVkMGE2%40thread.v2/0?context=%7b%22Tid%22%3a%22482198bb-ae7b-4b25-8b7a-6d7f32faa083%22%2c%22Oid%22%3a%2290f62797-bfce-4ec7-b0fd-9764001ca91a%22%7d (Meeting ID: 246 906 891 138

Passcode: 3VPa4F)

will be held the

DISSERTATION THESIS DEFENSE

for

Amalie Atassi
  
“Examining thermal and charge transport in organic materials with pi-electron interactions”

  Committee Members:

Prof. Natalie Stingelin, MSE/ChBE

Prof. Shannon Yee, ME

Prof. John Reynolds, CHEM/MSE

Prof. Faisal Alamgir, MSE

Prof. Mark Losego, MSE

Prof. Erin Ratcliff, The University of Arizona

Abstract:

Organic materials continue to be explored as the active component in wearable, flexible electronics and, more recently, in thermal devices. Their low-temperature thermal transitions and extensive chemical tunability makes them straightforward to process for a variety of applications, including thermal switches, devices that dynamically control the flow of heat, and thermoelectric generators, devices that convert heat into electrical energy (or vice versa). Despite these applications, the physical and electronic characteristics that lead to different thermal and charge transport in organic materials require further understanding. This thesis examines the physical and electronic features of conjugated organic materials through a characterization of three unique chemistries. I begin by constructing the electronic structure of conjugated materials and identifying the pi-bond electrons that are crucial to transport. Factors that further affect the electronic structure include morphology, such as the extent and quality of crystallinity, and doping, or reduction-oxidation processes. 

I demonstrate how these factors further alter thermal and charge transport by showcasing three distinct chemical structures. In each case study, the morphological changes in response to chemically tuning these organic materials differs, allowing us to deduce chemical design guidelines for potential thermal and energy applications. In the first case study, I show how a crystalline molecule that decreases its intermolecular pi-electron overlap yet maintains a high degree of structural order undergoes a fourfold decrease in thermal conductivity. Because a high extent of crystallinity is maintained in the material, the diminished thermal transport is in part due to the decreased conjugation measured in the material. For the second case study, I address changes in charge transport in highly disordered materials—amorphous conjugated polymers. In these highly disordered materials, I increase the pi-electron interactions by increasing the main-chain planarity in a series of poly(dioxythiophenes). This increased planarity, in turn, increases the polymer’s susceptibility to oxidation, increases the electrical conductivity, and decreases the Seebeck coefficient. We can further modify the chemical structure to remove the side chains; this side chain modification increases the carrier density of the material, which further increases the electrical conductivity and decreases the Seebeck coefficient. In the final case study, I consider a system with both ordered phases and disordered phases—semicrystalline conjugated polymers. For this study, I show how the phase behavior of these materials at the macroscale affects the electronic structure at the local scale. Furthermore, the phase behavior (and resulting electronic structure) can be altered by combining the two materials. This phase interaction of each polymer can result in an enhancement of the thermoelectric power factor, which I demonstrate at intermediate doping levels. 

Overall, this dissertation examines the complex relationships between phase morphology and electronic structure in conjugated organic materials, establishes structure-property guidelines for materials with ranging degrees of order, and contextualizes these relationships for potential thermal and energy management applications.