Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Low bandgap thermoelectric materials suffer from bipolar effects at high temperatures, with increased electronic thermal conductivity and reduced Seebeck coefficient, leading to a reduced power factor and a low ZT figure of merit. In this work we show that the presence of strong transport asymmetries between the conduction and valence bands can allow high phonon-limited electronic conductivity at finite Seebeck coefficient values, leading to largely enhanced power factors. The power factors that can be achieved can be significantly larger compared to their maximum unipolar counterparts, allowing for doubling of the ZT figure of merit. We identify this behavior in low-bandgap cases from the half-Heusler material family. Using both advanced electronic Boltzmann transport calculations for realistic material band structures and model parabolic electronic bands, we elaborate on the parameters that determine this effect. We then develop a series of descriptors that can guide machine learning studies in identifying such classes of materials with extraordinary power factors at nearly undoped conditions. For this we test more than 3000 analytical band structures and their features, and more than 120 possible descriptors, to identify the most promising ones that contain: (i) only band structure features for easy identification from material databases and (ii) band structure and transport parameters that provide much higher correlations, but for which parameter availability can be somewhat more scarce.