Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes

Abstract

Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (<i>S</i>). However, a maximum value of <i>S</i> in a semiconductor is normally accompanied by a minuscule electrical conductivity (σ), and hence, the thermoelectric power factor (<i>P</i> = <i>S</i>²σ) remains small. An attempt to increase σ by increasing the Fermi energy (<i>E</i>_F), on the other hand, decreases <i>S</i>. This trade-off between <i>S</i> 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 <i>E</i>_F solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the <i>E</i>_F dependence of <i>S</i>, σ, and <i>P</i> in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both <i>S</i> and σ monotonically increased with <i>E</i>_F, continuously boosting <i>P</i> while increasing <i>E</i>_F. Particularly, in an aligned metallic CNT film, the maximum of <i>P</i> 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 <i>S</i> and σ of one-dimensional conduction electrons near the first van Hove singularity.