Why do half-Heusler materials often have large thermoelectric power factor?

Advancements for thermoelectric materials benefit from understandings of the underlying charge transport mechanism. One example is given by the “band engineering” approach, which seeks to enhance the power factor by tuning the electron density of states. Despite tremendous work that shows enhanced electrical properties via this strategy, the large power factors often possessed by the half-Heusler system – with a highest value ever reported for semiconductors at room temperature (>100 μW/cm-K² in Ti-doped NbFeSb) – remains unclear. Using first principles electron transport calculation, we reveal that such high power factor results from a distinct transport regime where the electron scattering by acoustic phonons can be strongly suppressed, making half-Heusler a unique system that contrasts traditional viewpoints that acoustic phonons often limit the charge transport. The electron-phonon interaction is rationalized via chemical bonding concepts, through which we find that the weak coupling strength is protected as a result of the crystal symmetry of half-Heusler phases. Large room temperature power factors well above 100 μW/cm-K² are predicted for several compositions. We believe the results will stimulate future work into discovering new thermoelectric materials with exceptional power factors.