Alternative Doping Strategies for Semiconducting Single-Wall Carbon Nanotube Thin Films for Thermoelectrics

Dense films of semiconducting single-walled carbon nanotubes (SWCNTs) are a promising thermoelectric material with high electrical conductivities and power factors after p- or n-doping. To introduce the charge carrier densities required for thermoelectric applications, chemical doping is typically used. However, the remaining dopant counterions are often unstable and residual impurities can affect the doping stability. Here, we present two novel strategies to p-dope thin films of semiconducting SWCNTs that enable the replacement of the dopant counterion with redox-inactive anions from an electrolyte.<br/>First, we employ ion-exchange doping with an excess of several different electrolytes (e.g. [BMP][TFSI]) to systematically vary the exchanged counterion size and investigate its impact on conductivity, Seebeck coefficients and power factors. Larger anions lead to higher electrical conductivities and improved doping stability, while no significant effect on the power factors is observed. These experimental trends can be reproduced by a random resistor model for the nanotube network that accounts for overlapping Coulomb potentials caused by the anions at high doping concentrations leading to the formation of an impurity band whose depth depends on the carrier density and counterion size.<br/>A second new doping method is proton-coupled electron transfer (PCET), which enables tunable p-doping of SWCNTs with the mild oxidant quinone in aqueous electrolyte solutions. At low pH and in the presence of TFSI anions, large-diameter SWCNTs are oxidized and the quinone is reduced to hydroquinone via two-electron/two-proton PCET. The reversible quinone/hydroquinone redox couple further enables precise doping control by tuning the pH of the doping solution. Importantly, the charge transport and thermoelectric properties of PCET-doped and ion-exchange doped SWCNT films are similar to those of chemically doped nanotube films.