Thermoelectric Materials

The ever-increasing concerns on energy crisis and sustainable resources have spurred a worldwide action in developing alternative energy conversion technologies. Thermoelectric materials, which provide a promising solution for the conversion of heat into electricity and vice versa, have attracted increasing attention in the fields of solid-state physics, chemistry and materials science. From the perspective of historical development, the past two decades have witnessed surged advances in thermoelectrics, featured by the deeper understanding of thermoelectric physics, establishment of novel optimization strategies, discovery of novel thermoelectric materials, success of high-efficiency thermoelectric devices, and practical applications of thermoelectric technologies in multiple areas of public life. To promote the wide applications of thermoelectric technologies, one fundamental issue lies in the improvement of the conversion efficiency of thermoelectric devices, which is directly tied to the dimensionless figure of merit zT of thermoelectric materials. There are two main research directions in the development of thermoelectric materials: one is to deeply understand the transport mechanism of existing good thermoelectric materials and thereby further enhance the zT; the other is to explore new thermoelectric candidates using the established thermoelectric guidelines. In reality, the progress of both directions is strongly related to the advances in solid-state physics. Namely, the discovery of new electronic and phononic structures and the insightful understanding of electron and phonon transport mechanisms in the solid-state materials will bring new opportunities for the thermoelectric field. This special issue of the Annalen der Physik on “Thermoelectric Materials” contains eight original articles and one review article on the dynamic of various thermoelectric studies. The topics range from the understanding of thermoelectric band engineering and scattering mechanism, and the interplay between spintronics and thermoelectrics, to the discovery of new thermoelectric semiconductors. AgSbTe2, as an interesting ternary thermoelectric material, attracts considerable attention with the focus on the origin of its intrinsically low thermal conductivity and high thermoelectric performance. In the first original article, Liu et al.[1] report that AgSbTe2 is thermodynamically unstable and would partially decompose into Ag2Te and Sb2Te3 during thermal cycling. Instead, they find that the compound Ag0.366Sb0.558Te exhibits a single-phase. With the Sn substitution at Sb sites, the electrical transport properties of Ag0.366Sb0.558Te are optimized. A peak zT of ∼1.3 and an average zT of ∼0.9 are achieved, highlighting the potential of Ag0.366(Sb1-xSnx)0.558Te in thermoelectric application. Layer-structured semiconductors have recently attracted increasing attention in the field of thermoelectrics as they generally display low thermal conductivity along the out-of-plane direction. One distinct feature of layer-structured semiconductors is that they usually display strong anisotropy in the transport properties. Yin et al. report a feasible strategy to improve thermoelectric performance by anisotropic tuning in misfit-layered chalcogenide.[2] This strategy, based on in-plane covalent bond and out-of-plane van der Waals bond, induces a higher in-plane electrical transport performance and a lower out-of-plane lattice thermal conductivity, deriving from the natural intercalated structure where the out-of-plane phonon is strongly scattered without influencing the in-plane mobility. The authors show how the in-plane lattice thermal conductivity is reduced by introducing point defects, while the out-of-plane mobility is maintained, thereby leading to a synergistic optimization of anisotropic thermoelectric performance. The present finding opens up a new opportunity for of manipulating thermoelectric performance via anisotropy engineering. Band convergence is one of the most popular band engineering strategies in the past ten years’ thermoelectric research. Tan et al. report a theoretical study on the origin of band convergence in the representative Mg2Si1-xSnx solid solution using the Wannier function analysis. They find that the convergence of conduction bands in Mg2Si1-xSnx is simply driven by the variation of lattice constant, resulting from the different dependence on the bonding length in the heavy and light conduction valleys.[3] Moreover, they predict that Mg2-xSrxSi could be a new material system with band convergence, awaiting experimental confirmation. The interplay of different fields could generate new research directions. The dynamic development in the interdisciplinary fields of thermoelectrics and spintronics is one typical example. In the review article, Hu et al. introduce the fundamental physical concepts that are important to spin-dependent thermoelectric research.[4] Particularly, they highlight some exceptional latest experiments on ferromagnetic and half-metallic Heusler compounds. The potential of using the anomalous Nernst effect to convert heat into electricity is also discussed. This interdisciplinary field may offer new opportunities for discovering novel thermoelectrics. Li et al. investigate the thermoelectric transport properties of 19-electron half-Heusler compound VCoSb,[5] They find that the nominal VCoSb is actually a composite of an off-stoichiometric V0.955CoSb single phase with impurities. Using Ti substitution at the V site, they simultaneously optimize the electrical properties and suppress the thermal conductivity of V0.955CoSb. Consequently, a peak zT of 0.7 at 973 K is reported for V0.855Ti0.1CoSb. This work demonstrates the potential of 19-electron VCoSb-based half-Heusler compounds as thermoelectric materials. Lattice thermal conductivity can be significantly reduced by introducing multiple-scale scattering sources, such as atomic-scale point defects, microscale grain boundaries, and nanoscale precipitates. In the original article, Fang et al. investigate the effect of electron-phonon interaction on phonon transport by taking P-doped single-crystal Si as a case study.[6] They find the rapid reduction in the lattice thermal conductivity in P-doped single-crystal Si can be well explained by considering the roles of both point defect scattering and electron-phonon interaction. This work demonstrates the important role of electron-phonon interaction in reducing the lattice thermal conductivity of heavily doped thermoelectric semiconductors. PbTe is regarded as one of the most promising intermediate-temperature thermoelectric materials. However, the difference between conduction and valence bands leads to a large performance mismatch in p- and n-type PbTe. To match p-type counterpart, Wang et al. report an interesting strategy[7] that can synergistically improve the power factor and reduce the lattice thermal conductivity of n-type PbTe. It is found that the amphoteric Indium exhibits mixed valences (In+ and In3+), which can effectively form defect level to dynamically optimize the power factor in the entire working temperature range. To further reduce its lattice thermal conductivity, Sulfur is introduced to intensify phonon scattering by forming point defects. With the combined roles of Indium doping and Sulfur alloying, both power factor and lattice thermal conductivity are optimized, which finally contributes to a high zT of 1.4 at 773 K. This work indicates that the combination of dynamic doping and point defect scattering is one promising strategy to improve the thermoelectric performance of n-type lead chalcogenides. In the original article by Du et al.[8], they report the thermoelectric transport properties of Se substituted pseudo-binary Ge2Sb2Te5-xSex. They reveal this substitution strategy can optimize the hole concentration and enhance the hole effective mass with the help of the band-structure calculations. Meanwhile, the alloying scattering can reduce the lattice thermal conductivity. Thus, the zT at 703 K increases from 0.24 for Ge2Sb2Te5 to 0.41 for Ge2Sb2Te3.5Se1.5. This work provides an effective way to improve the thermoelectric properties of Ge2Sb2Te5. As a kind of the most promising mediate-temperature oxide thermoelectric materials, oxyselenides (BiCuSeO and Bi2O2Se) have been widely researched in the thermoelectric community. In the last original article, Zhang et al. report a new oxyselenide thermoelectric material Bi6Cu2Se4O6,[9] which can be considered as a composition of 2 BiCuSeO and 2 Bi2O2Se. Bi6Cu2Se4O6 is found to be one n-type thermoelectric oxide. With halogen (Br, Cl) doping at Se site, a maximum zT of 0.15 is reached at 823 K for Bi6Cu2Se3.2Br0.8O6, indicating the potential of Bi6Cu2Se4O6 as an n-type oxide thermoelectric material. Finally, we are very thankful to all the contributors and the editors and staff of Annalen der Physik who make this exciting special issue possible.