Composites of Isotropic and Aligned Semiconducting Single-Walled Carbon Nanotubes with Conjugated Polymers for Air-Processed Thermoelectrics

Abstract

Thermoelectric (TE) materials exploit the Seebeck effect to transform a temperature gradient into a voltage difference and a net current flow. Carbon-based semiconductors such as polymer-sorted single-walled carbon nanotubes (SWCNTs) and conjugated polymers are promising TE candidates that combine unique processing and cost advantages against traditional inorganic counterparts. After the introduction of extrinsic charge carriers via doping to improve the TE properties, various strategies exist to further boost the TE performance of organic semiconductors including their alignment and blending in composites. Here, the TE power factor (PF) of isotropic and aligned networks of semiconducting SWCNTs, PBTTT, and their composites are benchmarked for common p-doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Composites of SWCNTs and CPs aligned by hot rubbing exhibit superior performance compared to isotropic films. Moreover, the PF increases with the SWCNTs mass ratio in the composites. A PF of 150 ± 20 μW m^-1 K^-2 is achieved in these composites upon F4TCNQ doping, which exceeds the figures attained in neat and densely packed SWCNT networks doped with that same molecular acceptor, thus confirming the positive effect of SWCNT/PBTTT blending on TE performance. While F4TCNQ doping is transient, when doping these composites via proton-coupled electron transfer in air, the PF remains high (140 ± 30 μW m^-1 K^-2) and stable over at least 60 hours of storage in inert atmosphere. These promising results bring organic TE composites closer to open-air processing as required for applications in printed TE generators.