High-Performance GeTe Thermoelectrics in Both Rhombohedral and Cubic Phases.

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Transport Properties of CdSb Alloys with a Promising Thermoelectric Performance.

Binary group II-V antimonides, especially Zn4Sb3 and ZnSb, have shown great potential for thermoelectric applications because of the intrinsic low lattice thermal conductivity. Another member from this family, CdSb, has also been revealed to show a promising thermoelectric performance, particularly in its single crystal form. This work focuses on the thermoelectric transport properties of polycrystalline CdSb and Cd1-xZnxSb alloys with various doping. It is shown that Ag doping at the cation site enables the highest hole concentration. The obtained broad range of carrier concentrations ensures a systematical assessment on the transport properties of CdSb-based materials and on its potential for thermoelectric applications, according to an effective single parabolic band (SPB) approximation with acoustic phonon scattering. This work not only details the fundamental parameters that determine the thermoelectric performance but also demonstrates CdSb alloys as highly efficient thermoelectrics.

Thermoelectric Properties of SnS with Na-Doping.

Tin sulfide (SnS), a low-cost compound from the IV-VI semiconductors, has attracted particular attention due to its great potential for large-scale thermoelectric applications. However, pristine SnS shows a low carrier concentration, which leads to a low thermoelectric performance. In this work, sodium is utilized to substitute Sn to increase the hole concentration and consequently improve the thermoelectric power factor. The resultant Hall carrier concentration up to ∼1019 cm-3 is the highest concentration reported so far for this compound. This further leads to the highest thermoelectric figure of merit, zT of 0.65, reported so far in polycrystalline SnS. The temperature-dependent Hall mobility shows a transition of carrier-scattering source from a grain boundary potential below 400 K to acoustic phonons at higher temperatures. The electronic transport properties can be well understood by a single parabolic band (SPB) model, enabling a quantitative guidance for maximizing the thermoelectric power factor. Using the experimental lattice thermal conductivity, a maximal zT of 0.8 at 850 K is expected when the carrier concentration is further increased to ∼1 × 1020 cm-3, according to the SPB model. This work not only demonstrates SnS as a promising low-cost thermoelectric material but also details the material parameters that fundamentally determine the thermoelectric properties.