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Solid-state thermoelectric devices can directly convert electricity into cooling or enable heat pumping through the Peltier effect. The commercialization of thermoelectric cooling technology has been built on the Bi 2 Te 3 alloys, which have had no rival for the past six decades around room temperature. With the discovery and development of more promising materials, it is possible to reshape thermoelectric cooling technology. Here we review the current status of, and future outlook for, thermoelectric cooling materials. Thermoelectric materials can generate electricity from waste heat but can also use electricity for cooling. This Perspective discusses coefficients of performance for these systems and the state-of-the-art for materials, and suggests strategies for the discovery of improved thermoelectric materials.

Widespread application of thermoelectric devices for waste heat recovery requires low-cost high-performance materials. The currently available n-type thermoelectric materials are limited either by their low efficiencies or by being based on expensive, scarce or toxic elements. Here we report a low-cost n-type material, Te-doped Mg 3 Sb 1.5 Bi 0.5 , that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K. Using combined theoretical prediction and experimental validation, we show that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy. This makes Te-doped Mg 3 Sb 1.5 Bi 0.5 a promising candidate for the low- and intermediate-temperature thermoelectric applications. Zintl-phase thermoelectrics are predominantly p-type. Here, Zhang et al . use tellurium to n-dope Mg 3 Sb 1.5 Bi 0.5 and obtain thermoelectric figures of merit up to 1.6 at 700 K. Calculations show that these performances result from a conduction band with sixfold valley degeneracy.

Thermoelectric materials generate electric energy from waste heat, with conversion efficiency governed by the dimensionless figure of merit, ZT. Single-crystal tin selenide (SnSe) was discovered to exhibit a high ZT of roughly 2.2–2.6 at 913 K, but more practical and deployable polycrystal versions of the same compound suffer from much poorer overall ZT, thereby thwarting prospects for cost-effective lead-free thermoelectrics. The poor polycrystal bulk performance is attributed to traces of tin oxides covering the surface of SnSe powders, which increases thermal conductivity, reduces electrical conductivity and thereby reduces ZT. Here, we report that hole-doped SnSe polycrystalline samples with reagents carefully purified and tin oxides removed exhibit an ZT of roughly 3.1 at 783 K. Its lattice thermal conductivity is ultralow at roughly 0.07 W m –1  K –1 at 783 K, lower than the single crystals. The path to ultrahigh thermoelectric performance in polycrystalline samples is the proper removal of the deleterious thermally conductive oxides from the surface of SnSe grains. These results could open an era of high-performance practical thermoelectrics from this high-performance material. SnSe has a very high thermoelectric figure of merit ZT, but uncommonly polycrystalline samples have higher lattice thermal conductivity than single crystals. Here, by controlling Sn reagent purity and removing SnO x impurities, a lower thermal conductivity is achieved, enabling ZT of 3.1 at 783 K.

The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH 3 NH 3 PbI 3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment ( r s  = 0.93) improves significantly on results obtained using a constant relaxation time approximation ( r s  = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power. It is difficult to compute the transport properties of a broad array of complex materials both accurately and inexpensively. Here, the authors develop a computationally efficient method for calculating carrier scattering rates of semiconductors, with good accuracy but low cost.

GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit ( ZT ) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT  > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi 0.07 Ge 0.90 Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials. The intrinsic high-concentration Ge vacancies in GeTe-based thermoelectric materials hinder their performance maximization. Here, the authors find that defect structure engineering strategy is effective for performance enhancement.

Thermoelectric materials can be potentially employed in solid-state devices that harvest waste heat and convert it to electrical power, thereby improving the efficiency of fuel utilization. The spectacular increases in the efficiencies of these materials achieved over the past decade have raised expectations regarding the use of thermoelectric generators in various energy saving and energy management applications, especially at mid to high temperature (400–900 °C). However, several important issues that prevent successful thermoelectric generator commercialization remain unresolved, in good part because of the lack of a research roadmap. Thermoelectric materials can generate energy from a heat differential. This Review provides an overview of mid- to high-temperature thermoelectrics, their application in modules, and the issues that need to be addressed to enable commercial implementation of state-of-the-art materials.

We demonstrate that the thermoelectric properties of p-type chalcogenides can be effectively improved by band convergence and hierarchical structure based on a high-entropy-stabilized matrix. The band convergence is due to the decreased light and heavy band energy offsets by alloying Cd for an enhanced Seebeck coefficient and electric transport property. Moreover, the hierarchical structure manipulated by entropy engineering introduces all-scale scattering sources for heat-carrying phonons resulting in a very low lattice thermal conductivity. Consequently, a peak zT of 2.0 at 900 K for p-type chalcogenides and a high experimental conversion efficiency of 12% at Δ T  = 506 K for the fabricated segmented modules are achieved. This work provides an entropy strategy to form all-scale hierarchical structures employing high-entropy-stabilized matrix. This work will promote real applications of low-cost thermoelectric materials. The synergism of entropy engineering and the typical optimization mechanisms in high-entropy-stabilized chalcogenide is unknown. Here, the authors find high-entropy-stabilized composition works as a promising matrix of applying synergistic effect to realize high thermoelectric performance.

Researches on flexible thermoelectric materials usually focus on conducting polymers and conducting polymer-based composites; however, it is a great challenge to obtain high thermoelectric properties comparable to inorganic counterparts. Here, we report an n-type Ag 2 Se film on flexible nylon membrane with an ultrahigh power factor ~987.4 ± 104.1 μWm −1 K −2 at 300 K and an excellent flexibility (93% of the original electrical conductivity retention after 1000 bending cycles around a 8-mm diameter rod). The flexibility is attributed to a synergetic effect of the nylon membrane and the Ag 2 Se film intertwined with numerous high-aspect-ratio Ag 2 Se grains. A thermoelectric prototype composed of 4-leg of the Ag 2 Se film generates a voltage and a maximum power of 18 mV and 460 nW, respectively, at a temperature difference of 30 K. This work opens opportunities of searching for high performance thermoelectric film for flexible thermoelectric devices. Although flexible thermoelectric materials based on conducting polymers are attractive for energy harvesting, their performance is inferior to their inorganic counterparts. Here, the authors present a facile method to deliver inorganic nanowire films with high power factor and flexibility.

Conjugated polymers promise inherently flexible and low-cost thermoelectrics for powering the Internet of Things from waste heat 1 , 2 . Their valuable applications, however, have been hitherto hindered by the low dimensionless figure of merit (ZT) 3 – 6 . Here we report high-ZT thermoelectric plastics, which were achieved by creating a polymeric multi-heterojunction with periodic dual-heterojunction features, where each period is composed of two polymers with a sub-ten-nanometre layered heterojunction structure and an interpenetrating bulk-heterojunction interface. This geometry produces significantly enhanced interfacial phonon-like scattering while maintaining efficient charge transport. We observed a significant suppression of thermal conductivity by over 60 per cent and an enhanced power factor when compared with individual polymers, resulting in a ZT of up to 1.28 at 368 kelvin. This polymeric thermoelectric performance surpasses that of commercial thermoelectric materials and existing flexible thermoelectric candidates. Importantly, we demonstrated the compatibility of the polymeric multi-heterojunction structure with solution coating techniques for satisfying the demand for large-area plastic thermoelectrics, which paves the way for polymeric multi-heterojunctions towards cost-effective wearable thermoelectric technologies. Thermoelectric plastics with a high figure of merit, suppressed thermal conductivity and an enhanced power factor are realized by combining layered and bulk heterojunctions to create a polymeric multi-heterojunction.

Exploring new near-room-temperature thermoelectric materials is significant for replacing current high-cost Bi 2 Te 3 . This study highlights the potential of Ag 2 Se for wearable thermoelectric electronics, addressing the trade-off between performance and flexibility. A record-high ZT of 1.27 at 363 K is achieved in Ag 2 Se-based thin films with 3.2 at.% Te doping on Se sites, realized by a new concept of doping-induced orientation engineering. We reveal that Te-doping enhances film uniformity and (00 l )-orientation and in turn carrier mobility by reducing the (00 l ) formation energy, confirmed by solid computational and experimental evidence. The doping simultaneously widens the bandgap, resulting in improved Seebeck coefficients and high power factors, and introduces Te Se point defects to effectively reduce the lattice thermal conductivity. A protective organic-polymer-based composite layer enhances film flexibility, and a rationally designed flexible thermoelectric device achieves an output power density of 1.5 mW cm −2 for wearable power generation under a 20 K temperature difference. Flexible Ag 2 Se possesses promising near-room-temperature thermoelectric performance, while trade-off in thermoelectric performance and flexibility enhances its practical utility. Here, the authors fabricate polycrystalline Ag 2 Se-based thin film with a high ZT of 1.27 at 363 K by Te doping.