Origin of unusual thermoelectric transport behaviors in carbon nanotube filled polymer composites after solvent/acid treatments
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2017 Elsevier B.V. Composites made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and carbon nanotube (CNT) have shown unchanged or even increased thermopower when electrical conductivity was raised by altering the concentration of CNT and/or treating the composites with a polar solvent, dimethyl sulfoxide (DMSO) or an acid, formic acid (FA). Unlike typical adversely correlated thermopower and electrical conductivity, remarkable increases in electrical conductivity by the treatment did not decrease thermopower. Furthermore, it is necessary to use an optimum CNT concentration to maximize thermopower and the power factor. In this study, an intermediate CNT concentration of 6.7 wt% showed the highest thermopower and power factor unlike other reports with typical organic composites. The origin of the unusual transport properties was suggested by studying changes in the relative ratio of conducting PEDOT core and insulating PSS, morphology, and carrier concentration and mobility. Our results indicate that the PSS removal by DMSO and FA could alter the carrier transport barrier, and CNT-PEDOT:PSS-CNT junctions could increase thermopower for composites with a low CNT concentration by avoiding direct contacts between CNTs.
