Effects of Connecting Polymer Structure and Morphology at Inter-Tube Junctions on the Thermoelectric Properties of Conjugated Polymer/Carbon Nanotube Composites

Conjugated polymer (CP)/carbon nanotube (CNT) composites have been actively used for thermoelectrics for more than a decade. Thermoelectric performance of CP/CNT composites is greatly improved compared with that of the individual components; however, the underlying origin of the performance improvement remains vague, without clear explanations at the molecular scale. Moreover, the nature of the heterogeneous system limits quantitative analysis and restricts physical understanding of the thermoelectric effect in the composites. By combining experimental approaches with molecular dynamics simulations, the contribution of the CPs to the thermoelectric properties at inter-tube junctions between adjacent CNTs is revealed. Indacenodithiophene-co-benzothiadiazole (IDTBT), which has a highly planar backbone and does not aggregate at CP/CNT interfaces, can better mediate effective intramolecular charge transport along backbone chains at inter-tube junctions than poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT). The isotropic and continuous distribution of IDTBT backbone chains enables both holes and phonons to be transported effectively at inter-tube junctions; this effect greatly increases electrical conductivity, but also increases thermal conductivity. Thus, to obtain a high thermoelectric figure of merit, the balance between the two effects must be optimized. These results may enable CP/CNT composites, whose development is currently stagnating, to be developed into commercially available thermoelectrics, complementing their conventional inorganic counterparts.