Solution-synthesized nanostructured materials with high thermoelectric performance.

GeTe Thermoelectrics

An insight into β-Zn4Sb3 from its crystal structure, thermoelectric performance, thermal stability and graded material

High thermoelectric performance in multiscale Ag8SnSe6 included n-type bismuth telluride for cooling application

Developing high performance n‐type thermoelectric (TE) materials is fundamentally important for developing high efficiency TE devices. AgBiSe2, which reveals superior n‐type TE performance in a cubic phase, crystallizes in a hexagonal phase at room temperature, and typically, undergoes phase transitions to a cubic phase at a temperature above 580 K. Here, for the first time, through entropy optimization with lead‐selenides (≥9.9 mol%), the high‐temperature cubic phase of AgBiSe2 is stabilized from 300 to 800 K. Furthermore, the AgBiSe2‐PbSe pseudo‐binary diagram is established. The resultant alloys with optimized entropy possess unique local distorted cubic lattices, which contribute low lattice thermal conductivity approaching 0.3 W m−1 K−1 in extended operating temperature range. Consequently, a peak figure of merit zT value of ≈0.8 at 800 K and a record‐high average zT value of 0.42 for n‐type I‐V‐VI2 compounds are attained in pure phase cubic n‐type (AgBiSe2)1−x(PbSe)x solid solutions. These results pave the way for developing new TE materials via entropy engineering.

A powerful economy and a sustainable society are based upon the available energy resources. A nonrenewable energy source is limited in its availability as a result of high consumption rates, output efficiency, and when used at large scale generates a large amount of waste heat and toxic gases. The eco-friendly and long lasting human society has always worked hard to find an alternate source of renewable and clean energy. Material scientists have found possible solutions with the discovery of thermoelectric materials that utilize the waste heat and convert it into useful electricity. This chapter focuses on thermoelectric (TE) materials. The content of this chapter is presented with the assumption that readers have a basic understanding of material’s structural, chemical, and transport properties, as well as the mechanisms responsible for displaying these properties based on computational results. We begin the chapter with an overview of thermoelectric materials and their applications in waste heat energy harvesting. The physical parameters and selection criteria for screening high-performance TE materials are also discussed. To gain a better understanding of TE properties, a brief description of the computational and experimental methods for investigating a material’s crystal structure, electronic and heat transport properties is provided before classifying the category of different materials. With a better understanding of materials properties, a wide range of emergent materials, discovered in recent past years, with high TE performance over a wide temperature range, are covered. We concentrated on the TE properties of chalcogenides, Si–Ge-based alloys, and oxides, in this chapter. For the realization of practical applications, everything from materials to device fabrication and output performance is elaborated. The limitations of conventional approaches and commonly available tools used in TE research, as well as potential solutions, are discussed, paving the way for further investigation of the exceptional properties governed by unusual crystal and electronic structure of materials. Finally, we present a summary of thermoelectric materials as a potential candidate for renewable energy sources in next-generation technological applications.KeywordsThermoelectricityThermoelectric generatorsTransport propertiesThermal diodesEnergy materials

Understanding the full electron–phonon and phonon–phonon interaction effects in semiconductors is crucial for tuning electronic and phonon transport properties with the purpose to the enhancement of thermoelectric performances. In this work, based on the detailed investigations on 2D lead phosphorene (PbP) by using the first‐principle calculations, it is demonstrated that, in , , and PbP, the electron–phonon interactions are significantly stronger than conventional group‐IV monolayers such as silicene by five orders in magnitude, and intravalley scatterings dominate over the intervalley scattering due to less degenerate band extrema near Fermi level. Based on the selection‐rule analysis, it is found that the symmetry breaking in the janus γ phase compared to the higher‐symmetry α phase allows fewer symmetry operations, leading to more allowed scattering channels for electrons and phonons, subsequently suppressing the electron and phonon transport in PbP. Due to the low Debye temperatures, and strong four‐phonon interactions, the lattice thermal conductivities at room temperature in these monolayers are sharply decreased. Finally, it is found that the maximum thermoelectric figure of merits for these three monolayers reach 0.90/0.24/1.25 at room temperature, respectively. This work not only paves the way for the promising applications of PbP monolayers in thermoelectric devices, but also suggests that symmetry breaking and strong high‐order phonon–phonon interactions are crucial to identifying excellent thermoelectric materials.

Thermoelectric effect enables direct conversion between thermal and electrical energy and provides an alternative route for power generation and refrigeration. Hereby it is important to find materials with a high thermoelectric performance. In this sense, in the present work, we study the behavior of the thermoelectric properties of functionalized graphene grain boundaries by employing non-equilibrium Green’s Functions formalism combined with density functional tight-binding theory (NEGF-DFTB) approach. Our results show that H and O ad-atoms affect the phonon and electron transport properties of the grain boundaries but the thermoelectric figure of merit is slightly reduced. However, grain boundaries functionalized with nitrophenyl molecules improve the thermoelectric behavior. We have also found that physisorbed molecules do not alter their transport properties. We expect our results to shed light on the potential of 2D materials for engineering highly efficient nanoscale thermoelectric devices.

Machine learning in thermoelectric materials identification: Feature selection and analysis

Thermoelectric power generation has been recognized as one of the most important technologies, and high-performance thermoelectric materials have long been pursued. However, because of the large number of candidate materials, this quest is extremely challenging, and it has become clear that a firm theoretical concept from the viewpoint of band-structure engineering is needed. In this study, we theoretically demonstrate that pnictogen-dichalcogenide layered compounds, which originally attracted attention as a family of superconductors and have recently been investigated as thermoelectric materials, can exhibit very high thermoelectric performance with elemental substitution. In particular, we clarify a promising guiding principle for materials design and find that LaOAsSe$_2$, a material that has yet to be synthesized, has a powerfactor that is six times as large as that of the known compound LaOBiS$_2$ and can exhibit a very large $ZT$ under some plausible assumptions. This large enhancement of the thermoelectric performance originates from the quasi-one-dimensional gapped Dirac-like band dispersion, which is realized by the square-lattice network. Our study offers one ideal limit of the band structure for thermoelectric materials. Because our target materials have high controllability of constituent elements and feasibility of carrier doping, experimental studies along this line are strongly awaited.

Synergistic effects lead to high thermoelectric performance of iodine doped pseudo-binary layered GeSb2Te4

Semiconductor InSe with wide bandgap and layered crystal structure is expected to be a promising thermoelectric material, and its excellent plasticity brings great potential applications in flexible and wearable thermoelectric devices. To advance its thermoelectric performance, this work systematically investigates the carrier and phonon transport properties in n‐type InSe. It is found that InSe compound presents an exceptional dynamic carrier transport property due to the amphoteric indium (In+ and In3+), which contributes to favorable temperature‐dependent increasing carrier concentration. More importantly, with S alloying in InSe, the carrier concentration can be further enhanced from ∼3.2 × 1013 cm–3 in InSe to ∼4.8 × 1015 cm–3 in InSe0.97S0.03 at 300 K, because S (χP ∼ 2.58) with larger Pauling electronegativity than Se (χP ∼ 2.55) can induce more In3+ state to increase carrier concentration in matrix. This boosted dynamic carrier transport property benefits an obviously enhanced power factor. Additionally, InSe compound presents intrinsically low thermal conductivity ∼1.6 W m–1 K–1 at 300 K due to low‐symmetry crystal structure and strong anharmonicity. This work indicates that the special dynamic carrier transport property and intrinsically low thermal conductivity in InSe make it as a worth‐expecting thermoelectric material.

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Pb-based group-IV chalcogenides including PbTe and PbSe have been extensively studied as high performance thermoelectric materials during the past few decades. However, the toxicity of Pb inhibits their applications in vast fields due to the serious harm to the environment. Recently the Pb-free group-IV chalcogenides have become an extensive research subject as promising thermoelectric materials because of their unique thermal and electronic transport properties as well as the enviromentally friendly advantage. This paper briefly summarizes the recent research advances in Sn-, Ge-, and Sichalcogenides thermoelectrics, showing the unexceptionally high thermoelectric performance in SnSe single crystal, and the significant improvement in thermoelectric performance for those polycrystalline materials by successfully modulating the electronic and thermal transport through using some well-developed strategies including band engineering, nanostructuring and defect engineering. In addition, some important issues for future device applications, including N-type doping and mechanical and chemical stabilities of the new thermoelectrics, are also discussed.

High-performance bulk Bi0.4Sb1.6Te3.0 thermoelectrics prepared from nanocrystal precursor synthesized via chemical precipitation

Low‐dimensional thermoelectric materials systems are proven to possess improved thermoelectric performance, either by enhancing the power factor S2σ through quantum confinement, or decreasing thermal conductivity with numerous interfaces. The 2D tellurium, also called tellurene, is a newly discovered 2D material which showed great potential for thermoelectric applications. In this article, high‐quality tellurene‐like nanosheets are synthesized by the hydrothermal method and assembled into nanostructured bulk materials by low‐temperature hot press, and their thermoelectric performance is investigated. Ultraviolet–ozone treatment is used to remove organic surface ligands. Doping is realized with surface doping with chalcogenidometalates. It is found that the Seebeck coefficient and the thermal conductivity of the nanosheet‐assembled bulk samples increased by ≈20% and decreased by 43% compared to bulk tellurium, respectively. Meanwhile, the carrier mobility is approaching, yet still lower than bulk tellurium. Overall, the best bulk sample possesses a zT of 0.1 at room temperature which is comparable to bulk Te. By further improving the mobility, this solution processable material can provide useful thermoelectric performance for room‐temperature applications.