Ion-Exchange Doping of Semiconducting Single-Walled Carbon Nanotubes

Due to their unique electronic and thermoelectric properties, networks of purely semiconducting single-walled carbon nanotubes (SWNTs) are a promising material for thermoelectric generators, which convert a temperature difference produced by waste heat into an electrical current. Dense films of SWNTs can be deposited from dispersion (e.g., by printing) and are mechanically flexible. They reach high electrical conductivities upon p- or n-doping and retain a high Seebeck coefficient. To introduce the charge carrier densities relevant for thermoelectric applications, chemical doping is typically used. However, the remaining dopant counterions are often unstable and the impact of the counterion on the thermoelectric properties of the SWNTs is still not fully explored. The recently developed technique of ion-exchange doping provides a means to exchange the dopant counterion with anions or cations from a huge library of commercially available salts and ionic liquids that do not possess the redox potential to reduce or oxidize the SWNTs themselves. In this work, we fabricated thin film networks of polymer-sorted small and large diameter SWNTs (>99% semiconducting) and chemically p-doped them using ion-exchange doping with a range of dopant counterions at low and high doping levels. Optical and electrical measurements reveal the ion-exchange efficiency and the influence of the counterion size on the electrical and thermoelectric properties.