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 Σ 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 × 1020 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 Σ band at the band edge in the low-temperature rhombohedral phase and the smallest effective masses for both the L and Σ 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. A team has uncovered the electronic reasons why germanium tellurides (GeTe) are superior energy harvesters to their periodic counterparts. Efforts to improve GeTe thermoelectric efficiency focus mostly on reducing heat conductivity inside its phase-changing crystal lattice. Yanzhong Pei from Tongji University in Shanghai, China, and colleagues now show how semiconductor band tuning may also improve this compound. They synthesized over 50 samples of GeTe with a broad range of carrier concentrations. The researchers then compared transport measurements and theoretical band calculations with those for the more common lead and tin telluride thermoelectrics. The GeTe samples with the best conversion efficiency had charge carriers with lower effective masses than usual and access to numerous energy bands — a combination that enables more heat to be converted to electricity compared to lead telluride or tin telluride. This work focuses on the thermoelectric properties inherent to p-type GeTe by tuning the carrier concentration. Compared with PbTe and SnTe, GeTe shows the highest Seebeck coefficient (a) and power factor (b) in the important carrier concentration range of 1–3 × 1020 cm−3. Provided all are fully optimized in carrier concentrations, GeTe shows the highest thermoelectric figure of merit, zT at all temperatures (c). This originates from its highest degeneracy of the first valence band (along Σ line) in the low temperature phase, and the smallest effective masses for both L and Σ bands in the high temperature phase (d).