UCLA

Conjugated polymers are organic semiconductors with extended π-electron systems that enable charge transport in these lightweight, flexible, and solution-processable materials. Their low thermal conductivity and relatively high Seebeck coefficients make them promising candidates for soft thermoelectric devices. However, pristine conjugated polymers possess low electrical conductivity due to their wide bandgaps and limited free carrier density at room temperature. Improving their performance therefore requires strategies that increase carrier density while supporting efficient charge delocalization.This dissertation investigates chemical doping as a route to enhance the electrical and thermoelectric properties of semiconducting polymer films, with a focus on understanding how the dielectric environment, environmental stability, dopant identity, doping method, and pristine microstructure shape charge transport. A combination of four-point probe conductivity, Seebeck coefficient, Hall effect, temperature-dependent conductivity, and X-ray scattering measurements, as well as steady-state and time-resolved spectroscopies, is used to develop a multiscale picture of transport in doped semiconducting polymer systems.The first half of this dissertation focuses on understanding anion-exchange doping, a doping method that has been shown to boost the doping levels achieved with relatively weak molecular dopants while allowing precise control over the counterion that intercalates into the polymer film. Chapter 2 examines the origin of the enhanced carrier generation observed during anion-exchange doping. By separating the initial molecular doping step from subsequent electrolyte exposure, we show that the high-dielectric electrolyte environment can increase the effective oxidizing power of an already-reduced dopant anion, enabling a second electron-transfer event with the polymer backbone. This double-doping process nearly doubles the carrier density, demonstrating that the dielectric environment plays a central role in the enhanced doping efficiency associated with anion-exchange doping.Chapter 3 builds on this mechanistic understanding by examining how the choice of counterion affects electrical conductivity and environmental stability under ambient conditions. Although anion-exchange doping enables control over the choice of counterion, many electrolyte salts used for this process are hygroscopic and can introduce counterion-dependent ambient stability limitations. We demonstrate that hygroscopic counterions incorporated during anion exchange can draw ambient water into the polymer, leading to strong reductions in electrical conductivity. Hall effect and optical measurements reveal that humidity primarily suppresses carrier mobility rather than carrier density, indicating that absorbed water introduces additional trapping sites that localize polarons.Chapter 4 investigates how dopant identity and doping method influence the Seebeck–conductivity relationship and charge transport in doped poly(3-hexylthiophene-2,5-diyl) (P3HT). By comparing the classic dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), two large (~2-nm diameter) dodecaborane-based dopants, and anion-exchange doping, we show that different dopants modulate carrier transport through distinct mechanisms. Large counterions reduce Coulomb trapping and lower hopping barriers, while anion-exchange doping enhances mesoscale connectivity by enabling doping of amorphous regions. Using temperature-dependent conductivity and the Semi-Localized Transport (SLoT) model, we show that both routes can improve the thermoelectric performance depending on how they reshape the energetic and structural landscape of the doped film.Chapter 5 explores how the microstructure of pristine P3HT governs carrier transport after doping. By systematically varying the polymer regioregularity, we can tune crystallinity and domain connectivity before introducing carriers through anion-exchange doping. UV–vis–NIR and transient absorption spectroscopy show that higher-crystallinity polymers support greater intradomain delocalization. Surprisingly, Hall effect and temperature-dependent conductivity measurements reveal that at low doping levels, less crystalline P3HT samples exhibit higher macroscopic carrier mobility due to superior mesoscale connectivity. At higher doping levels, these mesoscale-connectivity-driven mobility differences become less dominant, and the higher-regioregularity, more crystalline P3HT films exhibit more favorable charge transport. These results highlight the interplay between local order and long-range domain connectivity in determining charge transport.Collectively, this dissertation establishes how the carrier environment—whether altered by the dielectric constant of a doping solution, relative humidity, dopant–counterion interactions, or the intrinsic polymer structure—governs carrier delocalization and mobility in doped conjugated polymers. Understanding these multiscale relationships provides a foundation for designing higher-performance thermoelectric materials and, more broadly, for controlling charge transport in soft electronic systems.