Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes

Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (S). However, a maximum value of S in a semiconductor is normally accompanied by a minuscule electrical conductivity (sigma), and hence, the thermoelectric power factor (P = S-2 sigma) remains small. An attempt to increase a by increasing the Fermi energy (E-F), on the other hand, decreases S. This trade-off between S and sigma is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show that the use of metallic carbon nanotubes (CNTs) with tunable E-F solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the E-F dependence of S, sigma, and P in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both S and sigma monotonically increased with E-F, continuously boosting P while increasing E-F. Particularly, in an aligned metallic CNT film, the maximum of P was similar to 5 times larger than that in the highest-purity (>99%) single-chirality semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced S and a of one-dimensional conduction electrons near the first van Hove singularity.