In-Plane Thermoelectric Properties of Flexible and Room Temperature Doped Carbon Nanotube Films

Thermoelectric (TE) energy conversion is a highly reliable method of solid-state energy harvesting and thermoregulation, without the use of moving parts or bulky fluids. Flexible TE generators (FTEGs) are especially attractive for powering the next generation of wearable devices by harvesting the thermal gradient between the human body and the environment, via the Seebeck effect. State of the art bulk TE materials include alloys based on Bi-Te-Sb-Pb which have high figure of merit ZT but are heavy, brittle, expensive, and contain toxic elements such as tellurium and lead. Moreover, to create efficient TE devices for energy harvesting or thermoregulation, ZT is not the only important criterion – room temperature and solution processable doping, air stability, scalable synthesis, low cost, and efficient near-ambient temperature performance are all crucial parameters to create truly wearable and flexible TE devices. Soft materials with high power factors and low thermal conductivity (κ) are critically important for integration of thermoelectric (TE) modules into flexible form factors for energy harvesting or cooling applications. Carbon nanotubes (CNTs) are a promising class of TE materials due to their flexibility and strength, high electrical conductivities, low density, and their oxygen sensitivity which makes them behave as p-type semiconductors in ambient conditions. Moreover, their high surface area provides for facile physisorption of electron rich polymers to create n-type CNT materials. In the current literature, electrical measurements are carried out in in-plane direction while thermal properties are measured in through plane directions, which can lead to erroneous ZT. Thermal conductivity measurement in the in-plane direction is not commonly reported due to the complex methods required for measurement with a reasonably low uncertainty. Here we report an accurate in-plane thermal conductivity measurement i.e. the direction of heat transport is similar to the current flow.  In this work, air stable p- and n-type multiwalled carbon nanotube (MWCNT) films with high power factors (up to 521 µW/mK2) are reported, with the n type doping carried out in a facile two-step process. The maximum figure of merit (ZT) of the p type and n-type CNTs are obtained as 0.019 and 0.015 at 300 K, respectively with all three transport properties – Seebeck coefficient, electrical conductivity, and κ – measured in-plane, providing a more accurate ZT. Using time-domain thermoreflectance we report a fast and non-contact measurement of κ without complex microfabrication or material processing. Moreover, there is no material mismatch between the p- and n-type legs of the TE module. Such materials have the potential for widespread applications in inexpensive and scalable wearable harvesting and localized heating/cooling.