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 (σ), and hence, the thermoelectric power factor (P = S2σ) remains small. An attempt to increase σ by increasing the Fermi energy (EF), on the other hand, decreases S. This trade-off between S and σ 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 EF solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the EF dependence of S, σ, and P in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both S and σ monotonically increased with EF, continuously boosting P while increasing EF. Particularly, in an aligned metallic CNT film, the maximum of P was ∼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 σ of one-dimensional conduction electrons near the first van Hove singularity.
Keywords: electrical double-layer technique; ionic liquid; nanomaterials; one dimension; single-wall carbon nanotubes; thermoelectric.