Controlling Charge-Transfer Interactions in Doped Semiconducting Polymers and Directly Measuring Charge Carrier Localization with the Vibrational Stark Effect
Author: Dane Andrew Stanfield
Publisher:
Published: 2021
Total Pages: 230
ISBN-13:
DOWNLOAD EBOOKSemiconducting polymers show promise for use in a variety of applications such as photovoltaic cells, light emitting diodes, and thermoelectric generators. For many of these devices, the electronic properties are tuned through the introduction of chemical dopants. This dissertation is focused on understanding several key aspects of the chemical doping process. The first chapter gives an overview of semiconducting polymers, introduces doping by sequential processing methods and looks at how the chemical doping process works on a basic level. We also explore dopant transport methods, discuss the electrical and thermoelectrical characterization of these materials, and finally consider the structural morphology of conjugated polymer thin films. Chapter 2 takes an analytical approach to understanding how the underlying morphology and electrical/thermoelectrical properties of doped polymer films are affected when introducing the dopant either via the solutionphase or using vapor transport. Chapter 3 explores the fundamental charge transfer interactions that occur between polymer and dopant. We introduce a novel processing technique that enables the tunable production of dopant-polymer charge transfer complexes (CTCs), which represent a poorly understood but widely seen doping mechanism in these materials. We provide the first comprehensive picture of the forces that drive CTC formation and offer guidelines for limiting CTC occurrence in doped conjugated polymers as their electrical properties are usually undesirable. Finally, in Chapter 4 we solve a long-standing mystery in the literature of the highly variable vibrational spectra of certain dopant molecules, which should nominally show consistent and predictable frequencies. We show that the wide range of vibrational energies observed for these dopant molecules can be fully understood through the framework of the vibrational Stark effect. Our experimental evidence shows a clear and predictable shift for these modes as a function of their locally experienced electric field, which arises due to Coulomb interactions with the charge carriers on the polymer. Thus, the vibrational shifts of these dopant molecules are actually exquisite reporters on the local environment of the charge carriers in doped conjugated polymers. We use our experimentally-measured shifts to quantitatively estimate the change in polaron coherence length, the extent to which the charge carriers on the polymer spread over multiple polymer repeat units. These chapters cover a variety of themes which highlight the sometimes unexpected path from experiment to manuscript. I sincerely hope they can be of use to others who study similar systems and motivate additional works in the future.
