(Invited) Less Is More: Thermoelectric Performance Enhancements in Polymer-Free Semiconducting Single-Walled Carbon Nanotube Networks

Abstract

There is a growing interest in the use of carbon nanostructures for a variety of electronic and optoelectronic technologies, including energy harvesting applications such as photovoltaics (PV) and thermoelectrics (TE). We will present a series of studies aimed at improving the TE performance of semiconducting single-walled carbon nanotube (s-SWCNT) thin film networks. The optical and electrical performance of SWCNT ensembles is often limited by the presence of metallic single-walled carbon nanotube (m-SWCNT). We will demonstrate a polymer-based purification strategy that effectively eliminates these, and other impurities, from the raw material, leaving s-SWCNTs in extremely high purity. Modification of this extraction process produces s-SWCNT thin film networks where the polymer can be completely removed, resulting in close tube-tube contacts in a dense s-SWCNT network. Removal of the polymer in the solid state, rather than in solution, also minimizes nanotube bundling during network formation. By controlling the bundle size and extent of polymer remaining in the s-SWCNT network we demonstrate TE power factors that almost double the performance of s-SWCNT networks previously demonstrated. We trace the improved performance to an enhanced electrical conductivity, resulting from improved doping and strongly enhanced charge carrier mobility, and analyze our data within the framework of a recently developed thermoelectric transport model. Finally, we demonstrate that the removal of the polymer from the s-SWCNT network has negligible impact on the thermal conductivity, which appears to be limited by dopant-induced phonon scattering processes. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.