Abstract
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.
Highlights
Owing to the growing energy crisis, thermoelectric conversion as a clean and sustainable solution has attracted increasing attention because it can directly convert waste heat to electricity.[1]
The greatest challenge that prevents its widespread application is its relatively low conversion efficiency, which is evaluated by the TE figure of merit, zT = (S2T)/(ρ(κE+κL)), where S, T, ρ, κE and κL are the Seebeck coefficient, absolute temperature, electrical resistivity, and electronic and lattice contributions to the thermal conductivity, respectively
It is commonly recognized that the Seebeck coefficient, resistivity and electronic thermal conductivity are strongly coupled with each other
Summary
Owing to the growing energy crisis, thermoelectric conversion as a clean and sustainable solution has attracted increasing attention because it can directly convert waste heat to electricity.[1]. In addition to the reported high TE performance of GeTe-based materials, its phase transition deforms the lattice of GeTe from a NaCl-type face-centered cubic (β-GeTe) structure to a rhombohedral (α-GeTe) structure[59,60] at ∼ 700 K or below.[61] The subtle changes of the Seebeck coefficient and resistivity in the vicinity of the phase transition temperature have been barely discussed This phase transition, limited only in GeTe among these three group IV. Since band engineering approaches such as band convergence have recently proven to enhance effectively the TE performance of both PbTe17,21,23,62 and SnTe63,64 in their p-type forms, it is believed that a detailed investigation of the band structure and the inherent transport properties of GeTe in both its low- and high-temperature phases would be helpful for guiding the band/microstructure engineering approaches for further improvements and for evaluating their net contributions. This work demonstrates that the band structure guarantees a high TE performance inherent to GeTe in both the lowand high-temperature phases but it establishes a reference substance for evaluating the net effects of additional approaches such as band engineering and nanostructuring for possible further improvements of this material
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