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 σ, and hence, the thermoelectric power factor P = S²σ remains small. An attempt to increase σ by increasing the Fermi energy (E_F), 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 using metallic carbon nanotubes (CNTs) with tunable E_F solves this long-standing problem, demonstrating higher thermoelectric performance than semiconducting CNTs. We studied the E_F dependence of S, σ, and P in a series of CNT films with systematically varied metallic CNT contents. In metallic CNTs, both S and σ monotonically increased with E_F, continuously boosting P with increasing E_F. Particularly, in an aligned metallic CNT film, the maximum P was ~5 times larger than that in the high-purity (>99%) 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.