Thermoelectric Transport in Organic Materials.

Abstract

Organic materials are well-suited to certain applications due to their low cost, low weight, and mechanical flexibility, but are less desirable for other applications due to poor conduction of electricity and heat. The contributions of my work focus on strategies to mitigate these limitations in current and emerging organic materials, developing techniques to improve charge carrier mobility in doped organic semiconductors (OSCs) and improve thermal conductivity in common commercial plastic materials.

Understanding of charge carrier transport is a prerequisite to developing a good electrical conductor. In OSCs, charge carriers are historically assumed to be strongly localized, which bears heavily on their assumed mechanism of transport; however, the degree of localization in emerging high-conductivity OSCs has been the subject of intense study. My work develops a model that can be used with thermoelectric measurements to quantitatively determine the degree of carrier localization in an OSC, and applies this technique to high-conductivity polymers and iodine-doped pentacene films. The model also suggests a strategy to improve energy conversion efficiency in OSC-based thermoelectric materials by reducing dopant volume. This strategy was confirmed experimentally to vary all three thermoelectric parameters (Seebeck coefficient, electrical conductivity, and thermal conductivity) in a manner that increases thermoelectric efficiency, in sharp contrast to their trade-offs in common inorganic semiconductor based thermoelectric materials. This method led to 70% increase in the thermoelectric efficiency from the previous record for an OSC.

Finally, I propose and study methods to increase inter-chain bonding in polymer mixtures as an efficient engineering route to improve their thermal conductivity. By controlling the mole fractions of components to favor (strong) hydrogen bonds over weaker van der Waals bonds, the thermal conductivities of mixtures of common commercial polymers are increased by an order of magnitude, reaching 1.72 W/mK, the highest value yet reported among non-crystalline polymer materials without the incorporation of fillers.