Materials design criteria for ultrahigh thermoelectric power factors in metals

Metals have high electronic conductivities, but very low Seebeck coefficients, which traditionally make them unsuitable for thermoelectric materials. Recent studies, however, showed that metals could deliver ultrahigh thermoelectric power factors (PFs) under certain conditions. In this work, we theoretically examine the electronic structure and electronic transport specifications that allow for such high PFs. Using Boltzmann transport equation (BTE) simulations and a multiband electronic structure model, we show that metals with (i) a high degree of transport asymmetry between their bands, (ii) strong interband scattering, and (iii) a large degree of band overlap, can provide ultrahigh power factors. We show that each of these characteristics adds to the steepness of the transport distribution function of the BTE, which allows for an increase of the Seebeck coefficient to sizable values, simultaneously with an increase in the electrical conductivity. This work generalizes the concept that transport asymmetry (i.e., a mixture of energy regions of high and low contributions to the electrical conductivity), through a combination of different band masses, scattering strengths, energy filtering scenarios, etc., can indeed result in very high thermoelectric power factors, even in the absence of a material band gap. Under certain conditions, transport asymmetry can overcompensate for any performance degradation to the PF due to bipolar conduction and the naturally low Seebeck coefficients that otherwise exist in this class of materials.