Valleytronics in thermoelectric materials

Abstract

The central theme of valleytronics lies in the manipulation of valley degree of freedom for certain materials to fulfill specific application needs. While thermoelectric (TE) materials rely on carriers as working medium to absorb heat and generate power, their performance is intrinsically constrained by the energy valleys to which the carriers reside. Therefore, valleytronics can be extended to the TE field to include strategies for enhancing TE performance by engineering band structures. This review focuses on the recent progress in TE materials from the perspective of valleytronics, which includes three valley parameters (valley degeneracy, valley distortion, and valley anisotropy) and their influencing factors. The underlying physical mechanisms are discussed and related strategies that enable effective tuning of valley structures for better TE performance are presented and highlighted. It is shown that valleytronics could be a powerful tool in searching for promising TE materials, understanding complex mechanisms of carrier transport, and optimizing TE performance.npj Quantum Materials (2018) 3:9 ; doi:10.1038/s41535-018-0083-6 INTRODUCTIONThe demand for renewable energy harvesting has been growing because of the limited fossil fuels and increasing worldwide energy consumption. Thermoelectric (TE) materials, which have the capability of converting heat directly into electricity under a temperature gradient, have been regarded as an alternative option to alleviate energy shortage.1 Though TE devices have already been applied in deep space exploration and solid state cooling, 2,3 the relatively low energy conversion efficiency limits their wide commercialization, 4 which is mainly constrained by the performance of TE materials as characterized by the dimensionless figure of merit, zT = α 2 σT/(κ e + κ L ), where α is the Seebeck coefficient, σ is the electrical conductivity, κ e and κ L are, respectively, the electronic and lattice contributions to the total thermal conductivity κ, and T is the absolute temperature.5 Since all three physical properties (α, σ, and κ) are carrier concentration dependent, zT could reach its maximum value at an optimized carrier concentration. zT max is determined by a combination of intrinsic physical parameters as well as the temperature, all of which integrated into a term called the TE quality factor, B. Higher quality factor B leads to higher zT max at a fixed temperature. With a single parabolic band model in the nondegenerate limit, B could be expressed as:

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

“…References: GeTe, SnTe, PbTe, PbSe, PbS, Bi 2 Te 3 , and Sb 2 Te 3 . Inset is a schematic view of the valley anisotropy in the band structure diagram; Orange ellipsoids are the schematic Fermi surfaces with different wavevector k values; Inset is reproduced with permission . Copyright 2018, Springer Nature; b) Maximum power factor for cubic compounds adopting different chemical bonding mechanisms as a function of the Fermi surface complexity factor, ; c) A 3D map using the basal plane of electrons transferred and electrons shared showing the value of and d) maximum power factor.…”

“…Schematic diagram for the large valley degeneracy is reproduced with permission . Copyright 2011, Macmillan Publishers Limited; Schematic diagram for the band anisotropy is reproduced with permission . Copyright 2018, Springer Nature.…”

Promising and Eco‐Friendly Cu2X‐Based Thermoelectric Materials: Progress and Applications

“…High m * can be achieved through securing either a higher N v or a higher m b *. N v can be increased when multiple bands have similar energy within a few k B T due to either orbital degeneracy (similar extreme energy levels of multi bands) or valley degeneracy (degenerated Brillouin zone of multi carrier pockets due to high symmetry of crystals), while m b * can be increased with flattening band structure . Additionally, resonant levels, which induce a narrow peak in DOS and diffuse conduction electrons in a strongly energy‐dependent manner, are favorable for large S .…”

Anisotropy Control–Induced Unique Anisotropic Thermoelectric Performance in the n‐Type Bi2Te2.7Se0.3 Thin Films

“…The corresponding results might be reflected as weakened phonon–electron scattering represented by lower E def and correspondingly relatively higher µ e along this direction (higher µ e, y / µ e, y ). At a specific tilting angle, the band might have been flattened, leading to enhanced m* and S . Table 1 summarizes the comparison of thermoelectric performance of the n‐type Bi 2 Te 2.7 Se 0.3 films in this work and others, which clearly demonstrates the accurate anisotropy control via external electric field during thermal deposition process could lead to record high zT values in the n‐type Bi 2 Te 2.7 Se 0.3 films.…”