The idea of converting low-grade waste heat into useful electrical energy is appealing for a range of applications, from powering wearable electronics to increasing the energy conversion efficiency of thermal power plants. Thermoelectric devices based on electrically conductive polymers are attractive for wearable electronic applications due to their mechanical flexibility and low toxicity; whereas, in large scale applications of waste heat recovery, such as in thermal power plants, a primary advantage of conductive polymers is their potential low cost. We have investigated several strategies for developing organic thermoelectric materials: including energy filtering using organic-inorganic composites, polymer blends for maximizing the power factor relative to pure polymers, and creating n-type thermoelectric materials through p-type doping. Energy filtering is theoretically straightforward, that is, inclusion of an energy barrier between nanoscale components in a material that allows for selective transmission of higher energy charge-carriers. However, in polymer-nanoparticle composites the story is more complicated and our investigations into a model system suggest that increased power factors are not due to energy filtering. Polymer blends provide another route to higher thermoelectric performance. Through a combination of charge transport models and experimental measurements a polymer blend is demonstrated to show a 50% improvement in the power factor relative to the pure polymers. The majority of high-performing electrically conductive polymers are p-type (e.g., PEDOT:PSS); however, both n-type and p-type materials are required to maximize the performance of a thermoelectric module. Interestingly, at high p-type doping concentrations we demonstrate that many polymers will transition from displaying p-type characteristics (i.e., a positive Seebeck coefficient) to n-type characteristics. This finding opens a new route to developing n-type organic thermoelectric materials through p-type doping.

Pathways to high-performing organic thermoelectric materials

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