UCLA

Chemically-doped conjugated polymers are a class of emerging semiconducting materials with unique and advantageous properties distinct from inorganic semiconductors, including solution processability, mechanical flexibility, highly tunable electronic behavior, and intrinsically low thermal conductivity. These properties make them promising candidates for thermoelectric applications, where temperature gradients are converted into electrical energy or vice versa. Such devices are relevant for applications ranging from industrial waste heat recovery to wearable electronics powered by body heat. Research on chemically-doped conjugated polymers has historically focused on maximizing the thermoelectric power factor, which depends on the electrical conductivity and the Seebeck coefficient. However, full quantification of thermoelectric performance also requires knowledge of the thermal conductivity to calculate the thermoelectric figure of merit, a measure of the degree of energetic conversion efficiency. Despite the importance of the figure of merit, the thermal conductivity of chemically-doped conjugated polymers remains poorly characterized. Most calculations of the figure of merit incorporate either thermal conductivity values that are representative of the undoped polymer or values from measurements embedded with a multitude of poor assumptions. This lack of thermal conductivity measurements largely arises from experimental challenges. Conjugated polymers exhibit very low thermal conductivities (below 1 W m⁻1 K⁻2), resulting in weak measurement signals. Additionally, these materials are typically studied as thin films (tens to hundreds of nanometers thick), requiring careful modeling of adjacent layers that often have much higher thermal conductivities and thus dominate the thermal response. Many measurement techniques operate in high-frequency regimes, probing thermal diffusivity rather than conductivity directly, which necessitates the accurate knowledge of volumetric heat capacity to extract the thermal conductivity. Because volumetric heat capacity is rarely measured in doped polymer systems due to separate measurement challenges, researchers often unphysically assume it remains unchanged from the undoped polymer. Furthermore, doped conjugated polymers may be both heat-sensitive and chemically reactive when exposed to certain environmental conditions and materials, respectively, during various measurement techniques. This thesis addresses these challenges by focusing on fully understanding the nature of changes in conjugated polymer thermal conductivity upon doping. The focus is on doped poly(3-hexylthiophene-2,5-diyl) (P3HT), a widely studied p-type conjugated polymer. The approach combines experimentally-measured volumetric heat capacities with thermoreflectance techniques that probe thermal diffusivity or effusivity. Structural analysis using X-ray diffraction is used to interpret changes in thermal transport, while electrical conductivity and Seebeck measurements enable a full evaluation of thermoelectric performance. Chapter 2 of this thesis specifically examines how volumetric heat capacity, thermal conductivity, and the thermoelectric figure of merit vary as a function of different amounts of doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Conventional time-domain thermoreflectance (TDTR) is employed to measure cross-plane thermal conductivity. The results show that both the volumetric heat capacity and thermal conductivity vary non-monotonically with doping, contradicting the common assumption that these properties remain unchanged from the undoped state. These variations are linked to doping-induced changes in crystallinity, morphology, and the incorporation of dopant molecules at higher concentrations. Chapter 3 of this thesis introduces a new technique, femtosecond transient thermo reflectance (fs-TTR), which uses a low-repetition-rate, high-pulse-energy amplified ultrafast laser, as well a newly-derived thermal model to interpret fs-TTR data. The new method has improved measurement accuracy compared to conventional TDTR of the cross-plane thermal conductivity, particularly for low-thermal-conductivity polymeric thin-films. For example, the lack of pulse accumulation-induced heat built-up from the low repetition rate allows control over the thermal penetration depth, preventing substrate effects from influencing the thermoreflectance signal. The easy achievability of large pump-probe spot-size ratios allows the measurement to achieve rigorous one-dimensional axial heat transport in materials with thermal anisotropy. The ability to spectrally probe the thermoreflectance signal simultaneously at multiple wavelengths yields an improved signal-to-noise ratio compared to single-wavelength measurements that rely on lock-in amplification. These advantages allow fs-TTR to measure both the thermal conductivity and thermal boundary conductance with an accuracy comparable to or slightly exceeding that of conventional TDTR, making it well suited for chemically-doped polymer thin-film systems. Chapter 4 of this thesis presents preliminary fs-TTR measurements of P3HT films doped with dodecaborane clusters functionalized with 3,5-bis(trifluoromethyl)benzyloxy functional groups (referred to as DDB-F72), a bulkier and much stronger oxidizing dopant than F4TCNQ. The use of DDB-F72 as a dopant not only improves carrier mobility, but also may achieve very low cross-plane thermal conductivity and higher thermoelectric figure of merit values compared to F4TCNQ-doped P3HT. Preliminary results suggest that the cross-plane thermal conductivity of DDB-F72-doped P3HT films monotonically decreases by an order of magnitude by high doping levels under the assumption that the volumetric heat capacity remains the same as the undoped polymer. Preliminary density measurements show that the film density monotonically increases upon doping with DDB-F72, suggesting that the volumetric heat capacity of these composite systems differs from that of the undoped polymer, although specific heat measurements are still needed to confirm this. The thermoreflective decays of DDB-F72-doped P3HT films show a large-amplitude damped oscillation of unknown origin that dominates the signal during the timeframe needed to extract information about heat transfer within the doped polymer film. This means that thermal conductivity values for these doped P3HT systems cannot be defined until the damped oscillation is fully understood, a subject for future work. However, combining our preliminary thermal conductivity results with measured power factors reveal that DDB-F72-doped P3HT may reach zT values of up to 0.18.