High Thermoelectric Performance in Bismuth Telluride via Constructing MoSe2-2D Heterojunction

Currently, the only thermoelectric (TE) materials commercially available at room temperature are those based on bismuth telluride. However, their widespread application is limited due to their inferior thermoelectric and mechanical properties. In this study, a strategy of growing a rigid second phase of MoSe2 is employed, in situ within the matrix phase to achieve n-type bismuth telluride-based materials with exceptional mechanical and thermoelectric properties. The in situ grown second phase contributes to both the electronic and lattice thermal conductivities. This is primarily attributed to the strong energy filtering effect, as the second phase forms a semi-common lattice interfacial structure with the matrix phase during growth. Furthermore, for composites containing 2 wt% MoSe2, a maximum zT value of 1.24 at 373 K can be achieved. On this basis, 8-pair TE module is fabricated and 1-pair TE module is optimized using a homemade p-type material. The optimized 1-pair TE module generates a maximum output power of 13.6 µW, which is twice that of the 8-pair TE module and four times more than the 8-pair TE module fabricated by commercial material. This work facilitates the development of the TE module by presenting a novel approach to obtaining bismuth telluride-based thermoelectric materials with superior thermoelectric and mechanical properties.

Hierarchical Low-Temperature n-Type Bi2Te3 with High Thermoelectric Performances

The high performance of a multistage thermoelectric cooler (multi-TEC) used in a wide low-temperature range depends on the optimized thermoelectric (TE) performance of materials during the corresponding working temperature range for each stage. Despite decades of research on the commercial TE materials of Bi2Te3, the main research is still focused on temperatures above 300 K, lacking suitable hierarchical low-temperature n-Bi2Te3 for multistage TEC. In this work, we systematically investigated the influence of doping concentration and matrix material compositions on the TE performance of n-Bi2Te3 below room temperature by the high-energy ball milling and hot deformation. Consequently, two hierarchical n-Bi2Te3 materials with excellent mechanical properties working below 248 and around 298 K, respectively, have been screened out. The Bi2Te2.7Se0.3 + 0.03 wt % TeI4 can be adopted in a low-temperature range that exhibits the high average figure of merit (zTave) of 0.61 within 173-248 K. Meanwhile, the Bi2Te2.7Se0.3 + 0.05 wt % TeI4 sample displays a competitive zTave of 0.85 within 248-298 K, which can be applied above 248 K. The research of hierarchical TE materials provides valuable insights into the high-performance design of multistage TE cooling devices.

Modulation of Vacancy Defects and Texture for High Performance n-Type Bi2 Te3 via High Energy Refinement

The carrier concentration in n-type layered Bi₂ Te₃ -based thermoelectric (TE) material is significantly impacted by the donor-like effect, which would be further intensified by the nonbasal slip during grain refinement of crushing, milling, and deformation, inducing a big challenge to improve its TE performance and mechanical property simultaneously. In this work, high-energy refinement and hot-pressing are used to stabilize the carrier concentration due to the facilitated recovery of cation and anion vacancies. Based on this, combined with SbI₃ doping and hot deformation, the optimized carrier concentration and high texture degree are simultaneously realized. As a result, a peak figure of merit (zT) of 1.14 at 323 K for Bi₂ Te_2.7 Se_0.3  + 0.05 wt.% SbI₃ sample with the high bending strength of 100 Mpa is obtained. Furthermore, a 31-couple thermoelectric cooling device consisted of n-type Bi₂ Te_2.7 Se_0.3  + 0.05 wt.% SbI₃ and commercial p-type Bi_0.5 Sb_1.5 Te₃ legs is fabricated, which generates the large maximum temperature difference (ΔT_max ) of 85 K at a hot-side temperature of 343 K. Thus, the discovery of recovery effect in high energy refinement and hot-pressing has significant implications for improving TE performance and mechanical strength of n-type Bi₂ Te₃ , thereby promoting its applications in harsh conditions.

High-Throughput Screening of Rattling-Induced Ultralow Lattice Thermal Conductivity in Semiconductors

Thermoelectric (TE) materials with rattling model show ultralow lattice thermal conductivity for high-efficient energy conversion between heat and electricity. In this work, by analysis of the key spirit of the rattling model, we propose an efficient empirical descriptor to realize the high-throughput screening of ultralow thermal conductivity in a series of semiconductors. This descriptor extracts the structural information of rattling atoms whose bond lengths with all the nearest neighboring atoms are larger than the sum of corresponding covalent radiuses. We obtain 1171 candidates from the Materials Project (MP) Database that contains more than 100 000 materials. Combining the empirical equation of high-throughput computation with a machine learning algorithm, we compute the approximate lattice thermal conductivities (κ_L) and find the κ_L values of 532 materials are less than 2.0 W m^-1 K^-1 at 300 K, which can be regarded as the criteria of ultralow κ_L in general. In particular, we demonstrate that halide double perovskites structures show ultralow κ_L, which provides valuable references for promising low κ_L materials in future experiments. In order to further verify our computational results, we calculate accurate κ_L for Rb₂SnBr₆ and CsCu₃O₂ as candidates with the low lattice thermal conductivity by solving the phonon Boltzmann transport equation. In particular, we demonstrate that Rb₂SnBr₆ has the lowest κ_L value of 0.1 W m^-1 K^-1 at 300 K of all known thermal conductivity materials with the rattling model so far.

Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted by firstprinciples calculations

Tailoring novel thermoelectric materials (TMs) with a high efficiency is challenging due to a difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose...

Isotropic Thermoelectric Performance of Layer-Structured n-Type Bi2Te2.7Se0.3 by Cu Doping

The lamellar structure of (Bi,Sb)₂(Te,Se)₃ alloys makes it difficult to achieve isotropic thermoelectric properties in the directions along and perpendicular to the c-axis, especially for n-type samples. In this work, by introducing Cu in polycrystalline n-type Cu_xBi₂Te_2.7Se_0.3 and applying the traditional synthesis process of high-energy ball milling and hot pressing, substantial enhancement of the thermoelectric figure of merit zT is obtained in both in-plane and out-of-plane directions. The intercalated Cu not only provides electron transport media for mobility improvement but also reduces the lattice thermal conductivity owing to the strain fluctuation. Typically, the van der Waals gap in the out-of-plane direction leads to relatively slower mobility and lower lattice thermal conductivity. Taking into account the same average density-of-state effective mass (m_avg^* ∼ 1.5m_e) predicted based on a single parabolic model, the obtained quality factor β is comparable in both directions. As a result, a peak zT ∼ 1.05 at 420 K and the average zT approaching to 1.0 in the temperature range 300-500 K are obtained in both directions for the Cu_0. 02Bi₂Te_2.7Se_0.3 sample. The simple synthesis process and isotropic thermoelectric properties in this work make n-type Bi₂Te₃ more convenient for potential production and application.

Amorphous Framework in Electrodeposited CuBiTe Thermoelectric Thin Films with High Room-Temperature Performance

Bismuth telluride-based alloys are the most efficient thermoelectric materials near room temperature and widely used in commercial thermoelectric devices. Nevertheless, their thermoelectric performance needs to be improved further for wide-scale implementation either as a thermoelectric generator or cooler. Here, we propose a simultaneous codeposition of CuBiTe thin films and their phase transition strategy via the traditional electrodeposition process. With just 13 atom % Cu doping, crystalline-to-amorphous phase transformation resulted for the electroplated CuBiTe alloy. A close look at the alloy composition revealed spike-shaped nanocrystalline Bi₂Te₃ embedded in the CuBiTe amorphous matrix. Our result shows an exceptionally high power factor (3.02 mW m^-1 K^-2), which comes from the enhanced Seebeck coefficient (-275 μV K^-1) and high electrical conductivity (3.99 × 10⁴ S m^-1) of CuBiTe films. Therefore, it can be suggested that the adopted strategy to form a unique nanocrystallite-embedded amorphous framework provides a platform to develop next-generation high-performance thermoelectric materials with an extraordinary power factor.

Scattering Mechanisms and Suppression of Bipolar Diffusion Effect in Bi2Te2.85Se0.15Ix Compounds

We investigated the anisotropic thermoelectric properties of the Bi₂Te_2.85Se_0.15I_x (x = 0.0, 0.1, 0.3, 0.5 mol.%) compounds, synthesized by ball-milling and hot-press sintering. The electrical conductivities of the Bi₂Te_2.85Se_0.15I_x were significantly improved by the increase of carrier concentration. The dominant electronic scattering mechanism was changed from the mixed (T ≤ 400 K) and ionization scattering (T ≥ 420 K) for pristine compound (x = 0.0) to the acoustic phonon scattering by the iodine doping. The Hall mobility was also enhanced with the increasing carrier concentration. The enhancement of Hall mobility was caused by the increase of the mean free path of the carrier from 10.8 to 17.7 nm by iodine doping, which was attributed to the reduction of point defects without the meaningful change of bandgap energy. From the electron diffraction patterns, a lattice distortion was observed in the iodine doped compounds. The modulation vector due to lattice distortion increased with increasing iodine concentration, indicating the shorter range lattice distortion in real space for the higher iodine concentration. The bipolar thermal conductivity was suppressed, and the effective masses were increased by iodine doping. It suggests that the iodine doping minimizes the ionization scattering giving rise to the suppression of the bipolar diffusion effect, due to the prohibition of the Bi_Te1 antisite defect, and induces the lattice distortion which decreases lattice thermal conductivity, resulting in the enhancement of thermoelectric performance.

Metal phosphide CuP2 as a promising thermoelectric material: an insight from a first-principles study

We performed first-principles investigation of anharmonic lattice dynamics and thermal transport properties of CuP2, revealing its promising thermoelectric performance.

Thermoelectric Properties and Low-Energy Carrier Filtering by Mo Microparticle Dispersion in an n-Type (CuI)0.003Bi2(Te,Se)3 Bulk Matrix

We investigate the thermoelectric properties of (CuI)_0.003Bi₂Te_2.7Se_0.3/Mo (Mo: 0.0, 0.9, 1.3, 1.8, 3.1, and 4.3 vol %) composites, which were synthesized by extrinsic phase mixing with hot press sintering. From X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) measurements, we confirm that micro-sized Mo particles are dispersed homogeneously in the (CuI)_0.003Bi₂Te_2.7Se_0.3 matrix without doping. While the electrical resistivity of Mo-dispersed (CuI)_0.003Bi₂Te_2.7Se_0.3 composites is not changed significantly, the Seebeck coefficient is significantly increased. Because the work function (5.3 eV) of the (CuI)_0.003Bi₂Te_2.7Se_0.3 compounds, measured by ultraviolet photoelectron spectroscopy (UPS), is larger than that of Mo particles (4.95 eV), we expect the potential barrier near the interfaces between the BTS matrix and Mo particles. The band bending effect and potential barrier can give rise to the low-energy carrier filtering. For a low concentration dispersion of Mo particles (<2 vol %), a decrease of Hall carrier concentration, an increase of Hall mobility, a decrease of effective mass, and an increase of Seebeck coefficient also support the formation of low-energy carrier filtering. The Mo dispersion does not affect the decrease in the lattice thermal conductivity but enhances the power factor significantly, leading to the high ZT value above 1.0 at room temperature, which is a high level in n-type thermoelectric room-temperature applications.

n-Bi2-xSbxTe3: A Promising Alternative to Mainstream Thermoelectric Material n-Bi2Te3-xSex near Room Temperature

For decades, the V₂VI₃ compounds, specifically p-type Bi_2-xSb_xTe₃ and n-type Bi₂Te_3-xSe_x, have remained the cornerstone of commercial thermoelectric solid-state cooling and power generation near room temperature. However, a long-standing problem in V₂VI₃ thermoelectrics is that n-type Bi₂Te_3-xSe_x is inferior in performance to p-type Bi_2-xSb_xTe₃ near room temperature, restricting the device efficiency. In this work, we developed high-performance n-type Bi_2-xSb_xTe₃, a composition long thought to only make good p-type thermoelectrics, to replace the mainstream n-type Bi₂Te_3-xSe_x. The success arises from the synergy of the following mechanisms: (i) the donorlike effect, which produces excessive conduction electrons in Bi₂Te₃, is compensated by the antisite defects regulated by Sb alloying; (ii) the conduction band degeneracy increases from 2 for Bi₂Te₃ and Bi₂Te_3-xSe_x to 6 for Bi_2-xSb_xTe₃, favoring high Seebeck coefficients; and (iii) the larger mass fluctuation yet smaller electronegativity difference and smaller atomic radius difference between Bi and Sb effectively suppresses the lattice thermal conductivity and retains decent carrier mobility. A state-of-the-art zT of 1.0 near room temperature was attained in hot deformed Bi_1.5Sb_0.5Te₃, which is higher than those for most known n-type thermoelectric materials, including commercial Bi₂Te_3-xSe_x ingots and the popular Mg₃Sb₂. Technically, building both the n-leg and p-leg of a thermoelectric module using similar chemical compositions has key advantages in the mechanical strength and the durability of devices. These results attested to the promise of n-type Bi_2-xSb_xTe₃ as a replacement of the mainstream n-type Bi₂Te_3-xSe_x near room temperature.

Possible Charge Density Wave and Enhancement of Thermoelectric Properties at Mild-Temperature Range in n-Type CuI-Doped Bi2Te2.1Se0.9 Compounds

Bi₂Te₃-based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi₂Te_2.1Se_0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI)_xBi₂Te_2.1Se_0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi₂Te₃-based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI)_0.003Bi₂Te_2.1Se_0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZT_avg (1.0) value than those of previously reported ones.

Evolution of the Intrinsic Point Defects in Bismuth Telluride-Based Thermoelectric Materials

In polycrystalline bismuth telluride-based thermoelectric materials, mechanical-deformation-induced donor-like effects can introduce a high concentration of electrons to change the thermoelectric properties through the evolution of intrinsic point defects. However, the evolution law of these point defects during sample preparation remains elusive. Herein, we systematically investigate the evolution of intrinsic point defects in n-type Bi₂Te₃-based materials from the perspective of thermodynamics and kinetics, in combination with positron annihilation measurement. It is found that not only the mechanical deformation but also the sintering temperature is vital to the donor-like effect. The mechanical deformation can promote the formation of cation vacancies and facilitate the donor-like effect, and the sintering process can provide excess energy for Bi antisite atoms to surmount the diffusion potential barrier. This work provides us a better understanding of the evolution law of intrinsic point defects in Bi₂Te₃-based alloys and guides us to control the carrier concentration by manipulating intrinsic point defects.

Thermoelectric power generation: from new materials to devices

Thermoelectric technology offers the opportunity of direct conversion between heat and electricity, and new and exciting materials that can enable this technology to deliver higher efficiencies have been developed in recent years. This mini-review covers the most promising advances in thermoelectric materials as they pertain to their potential in being implemented in devices and modules with an emphasis on thermoelectric power generation. Classified into three groups in terms of their operating temperature, the thermoelectric materials that are most likely to be used in future devices are briefly discussed. We summarize the state-of-the-art thermoelectric modules/devices, among which nanostructured PbTe modules are particularly highlighted. At the end, key issues and the possible strategies that can help thermoelectric power generation technology move forward are considered. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Enhancement of thermoelectric properties over a wide temperature range by lattice disorder and chemical potential tuning in a (CuI)y(Bi2Te3)0.95−x(Bi2Se3)x(Bi2S3)0.05 quaternary system

Bi₂Te₃-based compounds have received attention as thermoelectric materials for room-temperature cooling and waste heat recovery applications. With potential application prospects, quaternary compounds of Bi₂Te₃–Bi₂Se₃–Bi₂S₃ composites can be used for mid-temperature power generation under 500 °C. Herein, we investigated the thermoelectric properties of (CuI)_y(Bi₂Te₃)_0.95−x(Bi₂Se₃)_x(Bi₂S₃)_0.05 (x = 0.05, 0.2; y = 0.0, 0.003) compounds. Through X-ray diffraction and transmission electron microscopy, we confirmed that the lattice disorder in (Bi₂Te₃)_0.95−x(Bi₂Se₃)_x(Bi₂S₃)_0.05 (x = 0.2) was due to multiple element substitutions. Disorder carrier scattering induced the localized nature of electrical resistivity, as confirmed by variable range hopping at low temperature. The temperature-dependent Seebeck coefficient of (Bi₂Te₃)_0.95−x(Bi₂Se₃)_x(Bi₂S₃)_0.05 showed a carrier-type change from p- to n-type behaviour in the intermediate temperature range (525 K for x = 0.05 and 360 K for x = 0.2). Even though strong carrier localization increased electrical resistivity, resulting in degradation of the power factor and thermoelectric performance, when the chemical potential was increased to the conduction band minimum through CuI co-doping into the (CuI)_0.003(Bi₂Te₃)_0.95−x(Bi₂Se₃)_x(Bi₂S₃)_0.05 (x = 0.05, 0.2) compounds, the carriers were delocalized and showed n-type behaviour in the Seebeck coefficient. The temperature-dependent thermal conductivity shows the suppression of bipolar conduction behaviour. The simultaneous effect on carrier optimization through chemical potential tuning and lattice disorder caused a high ZT value of 0.85 at 523 K for CuI-doped (Bi₂Te₃)_0.75(Bi₂Se₃)_0.2(Bi₂S₃)_0.05, which was comparatively high for n-type thermoelectric materials in the mid-temperature range.

Temperature-dependent ZT values of (CuI)_y(Bi₂Te₃)_0.95−x(Bi₂Se₃)_x(Bi₂S₃)_0.05 (x = 0.05, 0.2; y = 0.0, 0.003) compounds compared with other related n-type compounds.

Thermoelectric Bi2Te3−xSex alloys for efficient thermal to electrical energy conversion

Eco-friendly renewable energy conversion methods are constantly investigated.

Anisotropic thermoelectric properties of layered compound SnSe2

Similar to high performance SnSe thermoelectrics, SnSe₂ is also a layered structured semiconductor. However, its anisotropic thermoelectric properties are less experimentally investigated. In this work, Cl-doped SnSe₂ bulk materials are successfully prepared, and their thermal stability and anisotropic transport properties are systematically studied. Unexpectedly, different from the theoretical prediction and other typical layered thermoelectric compounds like Bi₂Te₃, the out-of-plane zT_c value is higher than in-plane zT_a for the same composition. The zT value is significantly enhanced by Cl doping. A maximum zT_c of ∼0.4 at 673 K is achieved in SnSe_1.88Cl_0.12, twice higher than previously reported Cl-doped SnSe₂ synthesized by the solvothermal method.

Enhancing Thermoelectric Performance of n-Type Hot Deformed Bismuth-Telluride-Based Solid Solutions by Nonstoichiometry-Mediated Intrinsic Point Defects

Bismuth-telluride-based solid solutions are the unique thermoelectric (TE) materials near room temperature. Various approaches have been applied to enhance the thermoelectric performance, and much progress has been made in their p-type materials. However, for the n-type counterparts, little breakthrough has been obtained. We herein report on enhancing thermoelectric performance of n-type bismuth-telluride-based alloys by nonstoichiometry to mediate the point defects, combined with one-time hot deformation. The improved power factor of 3.3 × 10^-3 W m^-1 K^-2 and reduced lattice thermal conductivity contribute to a high figure-of-merit, zT, of 1.2 at 450 K for n-type Bi₂Te_2.3Se_0.69 alloys with Se deficiency. The high zT is comparable to that of Bi₂Te_2.3Se_0.7 hot deformed three times, which is a practically complicated process. The results demonstrate that nonstoichiometry can be an effective and simple strategy in mediating intrinsic point defects and enhancing the thermoelectric performance of bismuth-telluride-based alloys.

Simultaneous Enhancement of Electrical Conductivity and Thermopower of Bi₂Te₃ by Multifunctionality of Native Defects

Simultaneous increases in electrical conductivity (up to 200%) and thermopower (up to 70%) are demonstrated by introducing native defects in Bi2 Te3 films, leading to a high power factor of 3.4 × 10(-3) W m(-1) K(-2). The maximum enhancement of the power factor occurs when the native defects act beneficially both as electron donors and energy filters to mobile electrons. They also act as effective phonon scatterers.

One pot green synthesis of graphene–iron oxide nanocomposite (GINC): an efficient material for enhancement of thermoelectric performance

We report for the first time, a green method for graphene–iron oxide nanocomposite (GINC) synthesis by dispersing graphene and nano iron oxide (Fe₂O₃) in ethanolviaultrasonication followed by micro-wave irradiation.