Sorting-free utilization of semiconducting carbon nanotubes for large thermoelectric responses

Abstract

Semiconducting carbon nanotubes (s-CNTs) are promising organic thermoelectric materials mainly due to their large thermopower (or Seebeck coefficient), but it is impractical to pick only s-CNTs out of a mixture of different chirality tubes in mass-produced CNTs. Here we report a sorting-free method for getting the large thermopower by suppressing electronic transport from metallic CNT (m-CNT). This study employed an organic electrochemical transistor (OECT) configuration where poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) channel was disposed between two separated CNT films. Based on the experimentally constructed band diagrams and theoretical estimation, the PEDOT:PSS channel could create energy barriers for abating the contribution of m-CNT to thermopower as well as injecting holes to CNT films. As the gate bias voltage was raised up to 20 V, thermopower was noticeably increased, resulting in the maximum power factor. For practical applications without an externally supplied bias voltage, nanoscale PEDOT:PSS were deposited on top of one end of CNT films for the out-of-plane hole transport, and then PEDOT:PSS was chemically de-doped to adjust the Fermi level like the OECT experiment. With six CNT-PEDOT connections, the thermopower was raised up to ~150 μV/K and a remarkably high PF was obtained up to ~1.3 × 103 μW/m-K2 at room temperature, which is ~460% improvement compared with that of pristine CNT and is comparable to those of inorganic counterparts. This study provides not only better understanding of thermoelectric behaviors for organic thermoelectric materials, but also a practical method for suppressing the electronic transport from m-CNT, which would be widely applicable to other organic materials for thermoelectrics and beyond.