  Low band gap thermoelectric materials suffer from bipolar effects at high
temperatures, with increased electronic thermal conductivity and reduced
Seebeck coefficient, leading to reduced power factor and 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 band
gap cases from the half-Heusler materials family. Using both, advanced
electronic Boltzmann transport calculations for realistic material
bandstructures, as well as model parabolic electronic bands, we elaborate on
the parameters that determine this effect. We then develop a series of
descriptors which can guide machine learning studies in identifying such
classes of materials with extraordinary power factors at nearly pristine
conditions. For this we test more than 3000 analytical bandstructures 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 scarce.
