UCLA

Conjugated polymers (CPs) are organic semiconducting materials that offer a promising alternative to inorganic semiconductors as they are flexible, inexpensive, and solution-processable. The biggest challenge in integrating CPs into thermoelectric and other devices is their limited electrical conductivity, especially in comparison to their inorganic counterparts. Recent efforts have focused on enhancing the performance of CPs by using another molecule to dope the polymer and create charge carriers. The sequential processing (SqP) method [Aguirre et. al. Adv. Energy Mater. 2015, 1402020], developed by our group, works by casting the polymer and dopants from semi-orthogonal solvents. By separating the polymer and doping casting steps, the polymer has enough time to aggregate, allowing control over the polymer crystallinity by changing the regioregularity or polymer processing conditions. Using SqP, we can study the effects of tuning the polymer crystallinity with anion-exchange (AX) doping, in which a high concentration of inert salt anion accompanies the introduction of the dopant [Yamashita et. al. Nature 572,634-638 (2019)]. In this dissertation, we will explore how introducing a molecular oxidant with and without an inert salt affects the conductivity of polymers with different crystallinities; we also will examine new spectroscopic features associated with AX doping that to date have been unassigned in the literature. The first chapter of this dissertation gives an overview of conjugated polymers, different doping mechanisms, and the tools we use to analyze doped-polymers including absorption, x-ray diffraction, and electrical Hall effect measurements. In chapter two we focus on AX doping and how the new doping mechanism affects the electrical properties of a well-studied polymer with varying crystallinities. Chapter three explores two new spectroscopic features that appear when using high concentration dopant and salt during the AX doping process. Using Resonance Raman spectroscopy and quantum chemistry calculations, we are able to assign these peaks to a salt-dopant complex that is formed during the exchange process. Chapter four explores the doping mechanism of a very strong oxidant that has been recently introduced to the polymer literature. Although there is debate on if the dopant counterion infiltrates the entirety of the polymer or just the amorphous regions, we show that the dopant does affect the crystal structure, indicating its intercalation into crystalline sites. Finally, chapter five discusses theoretical calculations on tungsten tetraboride, a synthetic inorganic material that has been discovered to have superhard characteristics. This chapter explores the various bonding properties of tungsten tetraboride to understand if the hardening affects are due to extrinsic or intrinsic affects. Although many unsolved questions remain in the conjugated polymer field, this dissertation breaks ground as foundational work in understanding new doping methods and molecular oxidants that have been introduced into the field.