(645d) Interface Engineering for High Performance Thermo-Electric Nanocomposites

Thermoelectric devices convert thermal energy directly into electrical energy and vice-versa and hold promising contributions in waste heat recovery and refrigeration. Due to their low cost, potential for high throughput manufacturing and unique transport mechanisms; solution-processed conducting polymer/nanoparticle composite films provide an environmentally clean and efficient route to harness electricity from low-grade heat sources. However, we still lack a fundamental understanding of these hybrid systems that would provide a general and robust framework to guide materials design. In this work, we use a model organic/inorganic hybrid system to demonstrate how engineering of nanoscale interfaces can drive novel macroscale transport behavior.

Here, we study a hybrid organic/inorganic thermoelectric model system comprising of Poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Tellurium nanowires as a function of nanowire loading and chemical doping. Interesting non-monotonic electrical conductivity is observed at intermediate loadings, suggesting non-effective medium behavior, in contrast to the values of the Seebeck coefficient that increase linearly with mass fraction of inorganic. These trends in Seebeck coefficient and electrical conductivity lead to an optimized power factor in the composites that exceed that of the individual components. The surprising electrical conductivity behavior can be explained with a model where carrier transport is primarily through a highly conductive volume of polymer that exists at the nanoparticle-polymer interface. Additionally, by doping the individual components separately, we can boost the electrical conductivities even further and obtain higher power factors. Thus, a mix of traditional doping mechanisms and innovative interface engineering at the nanoscale allows us to generate high performance thermoelectric materials.

In a complementary study, modifying the interface of hybrid polyvinylpyrollidone (PVP) and Tellurium nanowires by controlled addition of small molecules, allows us to tune the nature of carrier transport from hole-dominated to electron-dominated in thin film devices of these materials. To create a thermoelectric module, complementary hole (p-type) and electron (n-type) transporting materials are necessary, connected electrically in series and thermally in parallel. Typically, one needs two different materials with matching conductance which presents an engineering challenge. However, our approach provides a n-type thermoelectric material from essentially a p-type material simply by modifying the interface while preserving the conductivity and still rendering it solution processable. This enables use of a single material for both legs of the thermoelectric module. The approach is general and we can use it to tune the transport behavior in other promising thermoelectric materials too.

In summary, we provide novel design routes which highlight the role of the interface for optimizing performance in nanoscale organic/inorganic composites and provide guidelines for improving efficiencies of existing devices even further and observing novel transport behavior as well.