The thermoelectric properties of carbon nanotube (CNT)-filled polymer composites can be enhanced by modifying junctions between CNTs using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), yielding high electrical conductivities (up to approximately 40000 S/m) without significantly altering thermopower (or Seebeck coefficient). This is because PEDOT:PSS particles are decorated on the surface of CNTs, electrically connecting junctions between CNTs. On the other hand, thermal transport remains comparable to typical polymeric materials due to the dissimilar bonding and vibrational spectra between CNT and PEDOT:PSS. This behavior is very different from that of typical semiconductors whose thermoelectric properties are strongly correlated. The decoupled thermoelectric properties, which is ideal for developing better thermoelectric materials, are believed to be due to thermally disconnected and electrically connected contact junctions between CNTs. Carrier transport at the junction is found to be strongly dependent on the type and concentration of stabilizers. The crucial role of stabilizers was revealed by characterizing transport characteristics of composites synthesized by electrically conducting PEDOT:PSS and insulating gum Arabic (GA) with 1:1-1:4 weight ratios of CNT to stabilizers. The influence of composite synthesis temperature and CNT-type and concentration on thermoelectric properties has also been studied. Single-walled (SW) CNT-filled composites dried at room temperature followed by 80 degrees C exhibited the best thermoelectric performance in this study. The highest thermoelectric figure of merit (ZT) in this study is estimated to be approximately 0.02 at room temperature, which is at least one order of magnitude higher than most polymers and higher than that of bulk Si. Further studies with various polymers and nanoparticles with high thermoelectric performance may result in economical, lightweight, and efficient polymer thermoelectric materials.

Typical organic materials have low thermal conductivities that are best suited to thermoelectrics, but their poor electrical properties with strong adverse correlations have prevented them from being feasible candidates. Our composites, containing single-wall carbon nanotubes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and/or polyvinyl acetate, show thermopowers weakly correlated with electrical conductivities, resulting in large thermoelectric power factors in the in-plane direction of the composites, ∼160 μW/m 3 K 2 at room temperature, which are orders of magnitude larger than those of typical polymer composites. Furthermore, their high electrical conductivities, ∼10 5 S/m at room temperature, make our composites very promising for various electronic applications. The optimum nanotube concentrations for better power factors were identified to be 60 wt % with 40 wt % polymers. It was noticed that high nanotube concentrations above 60 wt % decreased the electrical conductivity of the composites due to less effective nanotube dispersions. The thermal conductivities of our 60 wt % nanotube composites in the out-of-plane direction were measured to be 0.2À0.4 W/m 3 K at room temperature. The in-plane thermal conductivity and thermal contact conductance between nanotubes were also theoretically estimated.

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Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

Hybrids of carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene) (PEDOT) treated by tetrakis(dimethylamino)ethylene (TDAE) have large n-type voltages in response to temperature differences. The reduced carrier concentration by TDAE reduction and partially percolated CNT networks embedded in the PEDOT matrix result in high thermopower and low thermal conductivity. The high electron mobility in the CNTs helps to minimally reduce the electrical conductivity of the hybrid, resulting in a large figure-of-merit.