Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides

PbTe and SnTe in their p-type forms have long been considered high- performance thermoelectrics, and both of them largely rely on two valence bands (the first band at L point and the second one along the Sigma line) participating in the transport properties. This work focuses on the thermoelectric transport properties inherent to p-type GeTe, a member of the group IV monotellurides that is relatively less studied. Approximately 50 GeTe samples have been synthesized with different carrier concentrations spanning from 1 to 20 x 10(20) cm (-3), enabling an insightful understanding of the electronic transport and a full carrier concentration optimization for the thermoelectric performance. When all of these three monotellurides (PbTe, SnTe and GeTe) are fully optimized in their p-type forms, GeTe shows the highest thermoelectric figure of merit (zT up to 1.8). This is due to its superior electronic performance, originating from the highly degenerated S band at the band edge in the low-temperature rhombohedral phase and the smallest effective masses for both the L and S bands in the high-temperature cubic phase. The high thermoelectric performance of GeTe that is induced by its unique electronic structure not only provides a reference substance for understanding existing research on GeTe but also opens new possibilities for the further improvement of the thermoelectric performance of this material.