Strong Anisotropy and Bipolar Conduction-Dominated Thermoelectric Transport Properties in the Polycrystalline Topological Phase of ZrTe5.

ZrTe5 has unique features of a temperature-dependent topological electronic structure and anisotropic crystal structure and has obtained intensive attention from the thermoelectric community. This work revealed that the sintered polycrystalline bulk ZrTe5 possesses both (020) and (041) preferred orientations. The transport properties of polycrystalline bulk p-type ZrTe5 exhibits an obvious anisotropic characteristic, that is, the room-temperature resistivity and thermal conductivity, possessing anisotropy ratios of 0.71 and 1.49 perpendicular and parallel to the pressing direction, respectively. The polycrystalline ZrTe5 obtained higher ZT values in the direction perpendicular to the pressing direction, as compared to that in the other direction. The highest ZT value of 0.11 is achieved at 350 K. Depending on the temperature-dependent topological electronic structure, the electronic transport of p-type ZrTe5 is dominated by high-mobility electrons from linear bands and low-mobility holes from the valence band, which, however, are merely influenced by valence band holes at around room temperature. Furthermore, external magnetic fields are detrimental to thermoelectric properties of our ZrTe5, mainly arising from the more prominent negative effects of electrons under fields. This research is instructive to understand the transport features of ZrTe5 and paves the way for further optimizing their ZTs.

Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals.

The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.

Enhancement of Thermoelectric Performance for n-Type PbS through Synergy of Gap State and Fermi Level Pinning.

We report that Ga-doped and Ga-In-codoped n-type PbS samples show excellent thermoelectric performance in the intermediate temperature range. First-principles electronic structure calculations reveal that Ga doping can cause Fermi level pinning in PbS by introducing a gap state between the conduction and valence bands. Furthermore, Ga-In codoping introduces an extra conduction band. These added electronic features lead to high electron mobilities up to µH ∼ 630 cm2 V-1 s-1 for n of 1.67 × 1019 cm-3 and significantly enhanced Seebeck coefficients in PbS. Consequently, we obtained a maximum power factor of ∼32 µW cm-1 K-2 at 300 K for Pb0.9875Ga0.0125S, which is the highest reported for PbS-based systems giving a room-temperature figure of merit, ZT, of ∼0.35 and ∼0.82 at 923 K. For the codoped Pb0.9865Ga0.0125In0.001S, the maximum ZT rises to ∼1.0 at 923 K and achieves a record-high average ZT (ZTavg) of ∼0.74 in the temperature range of 400-923 K.

All-Scale Hierarchically Structured p-Type PbSe Alloys with High Thermoelectric Performance Enabled by Improved Band Degeneracy.

We show an example of hierarchically designing electronic bands of PbSe toward excellent thermoelectric performance. We find that alloying 15 mol % PbTe into PbSe causes a negligible change in the light and heavy valence band energy offsets (Δ EV) of PbSe around room temperature; however, with rising temperature it makes Δ EV decrease at a significantly higher rate than in PbSe. In other words, the temperature-induced valence band convergence of PbSe is accelerated by alloying with PbTe. On this basis, applying 3 mol % Cd substitution on the Pb sites of PbSe0.85Te0.15 decreases Δ EV and enhances the Seebeck coefficient at all temperatures. Excess Cd precipitates out as CdSe1- yTe y, whose valence band aligns with that of the p-type Na-doped PbSe0.85Te0.15 matrix. This enables facile charge transport across the matrix/precipitate interfaces and retains the high carrier mobilities. Meanwhile, compared to PbSe the lattice thermal conductivity of PbSe0.85Te0.15 is significantly decreased to its amorphous limit of 0.5 W m-1 K-1. Consequently, a highest peak ZT of 1.7 at 900 K and a record high average ZT of ∼1 (400-900 K) for a PbSe-based system are achieved in the composition Pb0.95Na0.02Cd0.03Se0.85Te0.15, which are ∼70% and ∼50% higher than those of Pb0.98Na0.02Se control sample, respectively.

High Figure of Merit in Gallium-Doped Nanostructured n-Type PbTe-xGeTe with Midgap States.

PbTe-based thermoelectric materials are some of the most promising for converting heat into electricity, but their n-type versions still lag in performance the p-type ones. Here, we introduce midgap states and nanoscale precipitates using Ga-doping and GeTe-alloying to considerably improve the performance of n-type PbTe. The GeTe alloying significantly enlarges the energy band gap of PbTe and subsequent Ga doping introduces special midgap states that lead to an increased density of states (DOS) effective mass and enhanced Seebeck coefficients. Moreover, the nucleated Ga2Te3 nanoscale precipitates and off-center discordant Ge atoms in the PbTe matrix cause intense phonon scattering, strongly reducing the thermal conductivity (∼0.65 W m-1 K-1 at 623 K). As a result, a high room-temperature thermoelectric figure of merit ZT ∼ 0.59 and a peak ZTmax of ∼1.47 at 673 K were obtained for the Pb0.98Ga0.02Te-5%GeTe. The ZTavg value that is most relevant for devices is ∼1.27 from 400 to 773 K, the highest recorded value for n-type PbTe.

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'.

Ion Beam Induced Artifacts in Lead-Based Chalcogenides.

Metal chalcogenides have attracted great attention because of their broad applications. It has been well acknowledged that microstructure can alter the intrinsic properties and performance of metal chalcogenides. The structure-property-performance relationships can be investigated at atomic scale with scanning transmission and transmission electron microscopy (STEM and TEM). Nevertheless, careful specimen preparation is paramount for accurate analyses and interpretations. In this work, we compare the effects of a variety of well-established TEM specimen preparation methods on the observed microstructure of an ingot stoichiometric lead telluride (PbTe). Most importantly, from aberration corrected STEM and first principles calculations, we discovered that argon (Ar) ion milling can lead to surface irradiation damage in the form of Pb vacancy clusters and self-interstitial atom (SIA) clusters. The SIA clusters appear as orthogonal nanoscale features when characterized along the crystal orientation of the rock salt structured PbTe. This obfuscates the interpretation of the intrinsic microstructure of metal chalcogenides, especially lead chalcogenides. We demonstrate that with sufficiently low energy (300 eV) Ar ion cleaning or appropriate high-temperature annealing, the surface damage layer can be properly cleaned and the orthogonal nanoscale features are significantly reduced. This reveals the materials' intrinsic structure and can be used as the standard protocol for future TEM specimen preparation of lead-based chalcogenide materials.

Rationally Designing High-Performance Bulk Thermoelectric Materials.

There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of the electron and phonon transport through coherent interface, matrix/precipitate electronic bands alignment, and compositionally alloyed nanostructures. Finally, recent discoveries of new compounds with intrinsically low thermal conductivity are summarized, where SnSe, BiCuSeO, MgAgSb, complex copper and bismuth chalcogenides, pnicogen-group chalcogenides with lone-pair electrons, and tetrahedrites are given particular emphasis. Future possible strategies for further enhancing ZT are considered at the end of this review.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity.

We report electron-doped AgBi3S5 as a new high-performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5-0.3 W m-1 K-1 in the temperature range of 300-800 K, which is ascribed to its unusual vibrational properties: "double rattling" phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 × 1018-2 × 1019 cm-3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 me (me is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.

Homologous Series of 2D Chalcogenides Cs-Ag-Bi-Q (Q = S, Se) with Ion-Exchange Properties.

Four new layered chalcogenides Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4, and Cs2Ag2.5Bi8.5Se15 are described. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are isostructural and have a hexagonal P63/mmc space group; their structures consist of [Ag/Bi]2Q3 (Q = S, Se) quintuple layers intercalated with disordered Cs cations. Cs0.6Ag0.8Bi2.2S4 also adopts a structure with the hexagonal P63/mmc space group and its structure has an [Ag/Bi]3S4 layer intercalated with a Cs layer. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be ascribed to a new homologous family Ax[MmS1+m] (m = 1, 2, 3···). Cs2Ag2.5Bi7.5Se15 is orthorhombic with Pnnm space group, and it is a new member of the A2[M5+nSe9+n] homology with n = 6. The Cs ions in Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be exchanged with other cations, such as Ag+, Cd2+, Co2+, Pb2+, and Zn2+ forming new phases with tunable band gaps between 0.66 and 1.20 eV. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 possess extremely low thermal conductivity (<0.6 W·m-1·K-1).

The Two-Dimensional AxCdxBi4-xQ6 (A = K, Rb, Cs; Q = S, Se): Direct Bandgap Semiconductors and Ion-Exchange Materials.

We report the new layered chalcogenides AxCdxBi4-xQ6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). All compounds are isostructural crystallizing in the orthorhombic space group Cmcm, with a = 4.0216(8) Å, b = 6.9537(14) Å, c = 24.203(5) Å for Cs1.43Cd1.43Bi2.57S6 (x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å for Rb1.54Cd1.54Bi2.46S6 (x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å for K1.83Cd1.83Bi2.17S6 (x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å for Cs1.13Cd1.13Bi2.87Se6 (x = 1.13). These structures are intercalated derivatives of the Bi2Se3 structure by way of replacing some Bi3+ atoms with divalent Cd2+ atoms forming negatively charged Bi2Se3-type quintuple [CdxBi2-xSe3]x- layers. The bandgaps of these compounds are between 1.00 eV for Q = Se and 1.37 eV for Q = S. Electronic band structure calculations at the density functional theory (DFT) level indicate Cs1.13Cd1.13Bi2.87Se6 and Cs1.43Cd1.43Bi2.57S6 to be direct band gap semiconductors. Polycrystalline Cs1.43Cd1.43Bi2.57S6 samples show n-type conduction and an extremely low thermal conductivity of 0.33 W·m-1·K-1 at 773 K. The cesium ions between the layers of Cs1.43Cd1.43Bi2.57S6 are mobile and can be topotactically exchanged with Pb2+, Zn2+, Co2+ and Cd2+ in aqueous solution. The intercalation of metal cations presents a direct "soft chemical" route to create new materials.

Distinct Impact of Alkali-Ion Doping on Electrical Transport Properties of Thermoelectric p-Type Polycrystalline SnSe.

Recent findings about ultrahigh thermoelectric performance in SnSe single crystals have stimulated related research on this simple binary compound, which is focused mostly on its polycrystalline counterparts, and particularly on electrical property enhancement by effective doping. This work systematically investigated the thermoelectric properties of polycrystalline SnSe doped with three alkali metals (Li, Na, and K). It is found that Na has the best doping efficiency, leading to an increase in hole concentration from 3.2 × 10(17) to 4.4 × 10(19) cm(-3) at room temperature, accompanied by a drop in Seebeck coefficient from 480 to 142 µV/K. An equivalent single parabolic band model was found adequate to capture the variation tendency of Seebeck coefficient with doping levels within a wide range. A mixed scattering of carriers by acoustic phonons and grain boundaries is suitable for numerically understanding the temperature-dependence of carrier mobility. A maximum ZT of ∼0.8 was achieved in 1% Na- or K-doped SnSe at 800 K. Possible strategies to improve the mobility and ZT of polycrystals were also proposed.

Enhanced Thermoelectric Properties in the Counter-Doped SnTe System with Strained Endotaxial SrTe.

We report enhanced thermoelectric performance in SnTe, where significantly improved electrical transport properties and reduced thermal conductivity were achieved simultaneously. The former was obtained from a larger hole Seebeck coefficient through Fermi level tuning by optimizing the carrier concentration with Ga, In, Bi, and Sb dopants, resulting in a power factor of 21 µW cm(-1) K(-2) and ZT of 0.9 at 823 K in Sn(0.97)Bi(0.03)Te. To reduce the lattice thermal conductivity without deteriorating the hole carrier mobility in Sn(0.97)Bi(0.03)Te, SrTe was chosen as the second phase to create strained endotaxial nanostructures as phonon scattering centers. As a result, the lattice thermal conductivity decreases strongly from ∼2.0 Wm(-1) K(-1) for Sn(0.97)Bi(0.03)Te to ∼1.2 Wm(-1) K(-1) as the SrTe content is increased from 0 to 5.0% at room temperature and from ∼1.1 to ∼0.70 Wm(-1) K(-1) at 823 K. For the Sn(0.97)Bi(0.03)Te-3% SrTe sample, this leads to a ZT of 1.2 at 823 K and a high average ZT (for SnTe) of 0.7 in the temperature range of 300-823 K, suggesting that SnTe is a robust candidate for medium-temperature thermoelectric applications.

Multiple Converged Conduction Bands in K2Bi8Se13: A Promising Thermoelectric Material with Extremely Low Thermal Conductivity.

We report that K2Bi8Se13 exhibits multiple conduction bands that lie close in energy and can be activated through doping, leading to a highly enhanced Seebeck coefficient and a high power factor with elevated temperature. Meanwhile, the large unit cell, complex low symmetry crystal structure, and nondirectional bonding lead to the very low lattice thermal conductivity of K2Bi8Se13, ranging between 0.42 and 0.20 W m-1 K-1 in the temperature interval 300-873 K. Experimentally, we further support the low thermal conductivity of K2Bi8Se13 using phonon velocity measurements; the results show a low average phonon velocity (1605 ms-1), small Young's modulus (37.1 GPa), large Grüneisen parameter (1.71), and low Debye temperature (154 K). A detailed investigation of the microstructure and defects was carried out using electron diffraction and transmission microscopy which reveal the presence of a K2.5Bi8.5Se14 minor phase intergrown along the side of the K2Bi8Se13 phase. The combination of enhanced power factor and low thermal conductivity results in a high ZT value of ∼1.3 at 873 K in electron doped K2Bi8Se13 material.

Concerted Rattling in CsAg5 Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance.

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S(2) σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p-type thermoelectric material, CsAg5 Te3 , is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm(-1) K(-1) ) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state-of-the-art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.

Valence Band Modification and High Thermoelectric Performance in SnTe Heavily Alloyed with MnTe.

We demonstrate a high solubility limit of >9 mol% for MnTe alloying in SnTe. The electrical conductivity of SnTe decreases gradually while the Seebeck coefficient increases remarkably with increasing MnTe content, leading to enhanced power factors. The room-temperature Seebeck coefficients of Mn-doped SnTe are significantly higher than those predicted by theoretical Pisarenko plots for pure SnTe, indicating a modified band structure. The high-temperature Hall data of Sn1-xMnxTe show strong temperature dependence, suggestive of a two-valence-band conduction behavior. Moreover, the peak temperature of the Hall plot of Sn1-xMnxTe shifts toward lower temperature as MnTe content is increased, which is clear evidence of decreased energy separation (band convergence) between the two valence bands. The first-principles electronic structure calculations based on density functional theory also support this point. The higher doping fraction (>9%) of Mn in comparison with ∼3% for Cd and Hg in SnTe gives rise to a much better valence band convergence that is responsible for the observed highest Seebeck coefficient of ∼230 µV/K at 900 K. The high doping fraction of Mn in SnTe also creates stronger point defect scattering, which when combined with ubiquitous endotaxial MnTe nanostructures when the solubility of Mn is exceeded scatters a wide spectrum of phonons for a low lattice thermal conductivity of 0.9 W m(-1) K(-1) at 800 K. The synergistic role that Mn plays in regulating the electron and phonon transport of SnTe yields a high thermoelectric figure of merit of 1.3 at 900 K.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence.

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of ∼1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargement of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of ∼0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.

The new phase [Tl4Sb6Se10][Sn5Sb2Se14]: a naturally formed semiconducting heterostructure with two-dimensional conductance.

We report on a new layered semiconductor Tl8Sn10Sb16Se48 with an indirect band gap of 0.45 eV. The novel structure is made of alternating layers of SnSe2-type [Sn5Sb2Se14] and SnSe-type [Tl4Sb6Se10]. The material exhibits two-dimensional (2D) electron variable range hopping at low temperatures, indicating an absence of interlayer coherency of the electronic state. Theoretical calculations unveil a 2D confinement for electrons in the [Sn5Sb2Se14] sheet and confirm the heterostructure nature. This unique electronic structure is attributed to the weak interlayer coupling and structure distortion in the electron-poor [Tl4Sb6Se10] layer that energetically impedes electron propagation.

High thermoelectric performance of p-type SnTe via a synergistic band engineering and nanostructuring approach.

SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit ZT. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficient, and (b) enlarging the energy band gap. Thus, alloying with Cd atoms enables a form of valence band engineering that improves the high-temperature thermoelectric performance, where p-type samples of SnCd(0.03)Te exhibit ZT values of ~0.96 at 823 K, a 60% improvement over the Cd-free sample. Finally, we introduce endotaxial CdS or ZnS nanoscale precipitates that reduce the lattice thermal conductivity of SnCd(0.03)Te with no effect on the power factor. We report that SnCd(0.03)Te that are endotaxially nanostructured with CdS and ZnS have a maximum ZTs of ~1.3 and ~1.1 at 873 K, respectively. Therefore, SnTe-based materials could be ideal alternatives for p-type lead chalcogenides for high temperature thermoelectric power generation.