Hierarchical Architecturing for Layered Thermoelectric Sulfides and Chalcogenides

All-Scale Hierarchical Structuring, Optimized Carrier Concentration, and Band Manipulation Lead to Ultra-High Thermoelectric Performance in Eco-Friendly MnTe

MnTe emerges as an enormous potential for medium-temperature thermoelectric applications due to its lead-free nature, high content of Mn in the earth's crust, and superior mechanical properties. Here, it is demonstrate that multiple valence band convergence can be realized through Pb and Ag incorporations, producing large Seebeck coefficient. Furthermore, the carrier concentration can be obviously enhance by Pb and Ag codoping, contributing to significant enhancement of power factor. Moreover, microstructural characterizations reveal that PbTe nanorods can be introduced into MnTe matrix by alloying Pb. This can modify the microstructure into all-scale hierarchical architectures (including PbTe nanorods, enhances point-defect scattering, dense dislocations and stacking faults), strongly lowering lattice thermal conductivity to a record low value of 0.376 W m-1 K-1 in MnTe system. As a result, an ultra-high ZT of 1.5 can be achieved in MnTe thermoelectric through all-scale hierarchical structuring, optimized carrier concentration, and valence band convergence, outperforming most of MnTe-based thermoelectric materials.

High thermoelectric performance of two-dimensional SiPGaS/As heterostructures

Thermoelectric technology holds great promise as a green and sustainable energy solution, generating electric power directly from waste heat. Herein, we investigate the thermoelectric properties of SiPGaS/As van der Waals heterostructures by using computations based on density functional theory and semiclassical Boltzmann transport theory. Our results show that both models of SiPGaS/As van der Waals heterostructures have low lattice thermal conductivity at room temperature (300 K). Applying 4% tensile strain to the models leads to a significant enhancement in the figure of merit (ZT), with model-I and model-II exhibiting ZT improvements of up to 24.5% and 14.8%, respectively. Notably, model-II outperforms all previously reported heterostructures in terms of ZT value. Additionally, we find that the maximum thermoelectric conversion efficiency (η) for model-II at 4% tensile strain reaches 23.98% at 700 K. Our predicted ZT_avg > 1 suggests that these materials have practical potential for thermoelectric applications over a wide temperature range. Overall, our findings offer valuable insights for designing better thermoelectric materials.

Lattice Distortions and Multiple Valence Band Convergence Contributing to High Thermoelectric Performance in MnTe

Here, a new route is proposed for the minimization of lattice thermal conductivity in MnTe through considerable increasing phonon scattering by introducing dense lattice distortions. Dense lattice distortions can be induced by Cu and Ag dopants possessing large differences in atom radius with host elements, which causes strong phonon scattering and results in extremely low lattice thermal conductivity. Density functional theory (DFT) calculations reveal that Cu and Ag codoping enables multiple valence band convergence and produces a high density of state values in the electronic structure of MnTe, contributing to the large Seebeck coefficient. Cu and Ag codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, resulting in the significant enhancement of power factor. The maximum power factor reaches 11.36 µW cm^-1 K^-2 in Mn_0.98 Cu_0.04 Ag_0.04 Te. Consequently, an outstanding ZT of 1.3 is achieved for Mn_0.98 Cu_0.04 Ag_0.04 Te by these synergistic effects. This study provides guidelines for developing high-performance thermoelectric materials through the rational design of effective dopants.

First-principles study of the structural, optoelectronic and thermophysical properties of the π-SnSe for thermoelectric applications

Tin selenide (SnSe) has thermoelectric (TE) and photovoltaic (PV) applications due to its exceptional advantages, such as the remarkable figure of merit (ZT ≈ 2.6 at 923 K) and excellent optoelectronic properties. In addition, SnSe is nontoxic, inexpensive, and relatively abundant. These aspects make SnSe of great practical importance for the next generation of thermoelectric devices. Here, we report structural, optoelectronic, thermodynamic, and thermoelectric properties of the recently experimentally identified binary phase of tin monoselenide (π-SnSe) by using the density functional theory (DFT). Our DFT calculations reveal that π-SnSe features an optical bandgap of 1.41 eV and has an exceptionally large lattice constant (12.2 Å, P213). We report several thermodynamic, optical, and thermoelectric properties of this π-SnSe phase for the first time. Our finding shows that the π-SnSe alloy is exceptionally promising for the next generation of photovoltaic and thermoelectric devices at room and high temperatures.

Lattice Strain Leads to High Thermoelectric Performance in Polycrystalline SnSe

Polycrystalline SnSe materials with ZT values comparable to those of SnSe crystals are greatly desired due to facile processing, machinability, and scale-up application. Here manipulating interatomic force by harnessing lattice strains was proposed for achieving significantly reduced lattice thermal conductivity in polycrystalline SnSe. Large static lattice strain created by lattice dislocations and stacking faults causes an effective shortening in phonon relaxation time, resulting in ultralow lattice thermal conductivity. A combination of band convergence and resonance levels induced by Ga incorporation contribute to a sharp increase of Seebeck coefficient and power factor. These lead to a high thermoelectric performance ZT ∼ 2.2, which is a record high ZT reported so far for solution-processed SnSe polycrystals. Besides the high peak ZT, a high average ZT of 0.72 and outstanding thermoelectric conversion efficiency of 12.4% were achieved by adopting nontoxic element doping, highlighting great potential for power generation application at intermediate temperatures. Engineering lattice strain to achieve ultralow lattice thermal conductivity with the aid of band convergence and resonance levels provides a great opportunity for designing prospective thermoelectrics.

Significant improvement in thermoelectric performance of SnSe/SnS via nano-heterostructures

In this work, we study theoretically the electronic and phonon transport properties of heterojunction SnSe/SnS, bilayer SnSe and SnS. The energy filtering effect caused by the nano heterostructure in SnSe/SnS induces an increase in the Seebeck coefficient, causing a large power factor. We calculate the phonon relaxation time and lattice thermal conductivity κL for the three structures; the heterogeneous nanostructure could effectively reduce κL due to the enhanced phonon boundary scattering at interfaces. The average κL notably reduces from around 3.3 (3.2) W m-1 K-1 for bilayer SnSe (SnS) to nearly 2.2 W m-1 K-1 for SnSe/SnS at 300 K. As a result, the average ZT (ZTave in b and c directions) reaches 1.63 with temperature range around 300-800 K, which is improved by 63% (25%) compared with that of bilayer SnSe (SnS). Our theoretical results show that the heterogeneous nanostructure is an innovative approach for improving the Seebeck coefficient and significantly reducing κL, effectively enhancing thermoelectric properties.

Designing disorder into crystalline materials

Crystals are a state of matter characterized by periodic order. Yet, crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies or magnetic structure. Disorder can sometimes be random but, more usually, it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this Review, we survey the core design principles that guide targeted control over correlated disorder. We show how these principles - often informed by long-studied statistical mechanical models - can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics and metal-organic frameworks. We conclude with a forward-looking discussion of the exciting link between disorder and function in responsive media, thermoelectrics and topological phases.

Enhancing the Thermoelectric Properties of Misfit Layered Sulfides (MS)1.2+q (NbS2) n (M = Gd and Dy) through Structural Evolution and Compositional Tuning

The misfit monolayered sulfides, (GdS)_1.20NbS₂, (DyS)_1.22NbS₂, (Gd_0.1Dy_0.9S)_1.21NbS₂, (Gd_0.2Dy_0.8S)_1.21NbS₂, and (Gd_0.5Dy_0.5S)_1.21NbS₂ and the misfit bilayered sulfide (GdS)_0.60NbS₂ were synthesized via sulfurization under flowing CS₂/H₂S gas and consolidated by pressure-assisted sintering. The thermoelectric properties of the monolayered and bilayered sulfides perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction were investigated over a temperature range of 300-873 K. The crystal grains in all the sintered samples were preferentially oriented perpendicular to the pressing direction, which resulted in highly anisotropic electrical and thermal transport properties. All the sintered samples exhibited degenerate n-type semiconductor-like behavior, leading to a large thermoelectric power factor. The misfit layered structure yielded low lattice thermal conductivity. The evolution of the monolayered structures into bilayered structures affected their thermoelectric properties. The thermoelectric figure of merit (ZT) of monolayered (GdS)_1.20NbS₂ was higher than that of bilayered (GdS)_0.60NbS₂ due to the larger power factor and lower lattice thermal conductivity of (GdS)_1.20NbS₂. The lattice thermal conductivity of the monolayered sulfide was lower in (Gd _x Dy_1-x S)_1.2+q NbS₂ solid solutions. The large power factor and low lattice thermal conductivity allowed a ZT value of 0.13 at 873 K in (Gd_0.5Dy_0.5S)_1.21NbS₂ perpendicular to the pressing direction.

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'.

Growth of CaxCoO₂ Thin Films by A Two-Stage Phase Transformation from CaO⁻CoO Thin Films Deposited by Rf-Magnetron Reactive Cosputtering

The layered cobaltates A_xCoO₂ (A: alkali metals and alkaline earth metals) are of interest in the area of energy harvesting and electronic applications, due to their good electronic and thermoelectric properties. However, their future widespread applicability depends on the simplicity and cost of the growth technique. Here, we have investigated the sputtering/annealing technique for the growth of Ca_xCoO₂ (x = 0.33) thin films. In this approach, CaO–CoO film is first deposited by rf-magnetron reactive cosputtering from metallic targets of Ca and Co. Second, the as-deposited film is reactively annealed under O₂ gas flow to form the final phase of Ca_xCoO₂. The advantage of the present technique is that, unlike conventional sputtering from oxide targets, the sputtering is done from the metallic targets of Ca and Co; thus, the deposition rate is high. Furthermore, the composition of the film is controllable by controlling the power at the targets.

Pressure-induced conduction band convergence in the thermoelectric ternary chalcogenide CuBiS2

The electronic and thermoelectric properties of four ternary chalcogenides with space group Pnma, namely, Cu(Sb,Bi)(S,Se)2, are investigated up to 8 GPa hydrostatic pressure using density functional theory combined with semiclassical Boltzmann theory. The effects of the van der Waals interaction are included in all calculations, since these compounds have layered structures. They all have indirect band gaps that decrease monotonically with increasing hydrostatic pressure except for CuBiS2, for which an indirect-indirect band gap transition occurs around 3 GPa, leading to conduction band convergence with a concomitant 20% increase in the Seebeck coefficient. The enhanced Seebeck coefficient results from a complex interplay between multivalley and multiband effects as well as changes of the band effective masses, driven by hydrostatic pressure. Our results suggest that ongoing developments in high-pressure science may open new opportunities for the discovery of efficient thermoelectric materials.

Electrodeposition of p-Type Sb₂Te₃ Films and Micro-Pillar Arrays in a Multi-Channel Glass Template

Antimony telluride (Sb₂Te₃)-based two-dimensional films and micro-pillar arrays are fabricated by electrochemical deposition from electrolytes containing SbO⁺ and HTeO₂⁺ on Si wafer-based Pt electrode and multi-channel glass templates, respectively. The results indicate that the addition of tartaric acid increases the solubility of SbO⁺ in acidic solution. The compositions of deposits depend on the electrolyte concentration, and the micro morphologies rely on the reduction potential. Regarding the electrolyte containing 8 mM of SbO⁺ and 12 mM of HTeO₂⁺, the grain size increases and the density of films decreases as the deposition potential shifts from −100 mV to −400 mV. Sb₂Te₃ film with nominal composition and dense morphology can be obtained by using a deposition potential of −300 mV. However, this condition is not suitable for the deposition of Sb₂Te₃ micro-pillar arrays on the multi-channel glass templates because of its drastic concentration polarization. Nevertheless, it is found that the pulsed voltage deposition is an effective way to solve this problem. A deposition potential of −280 mV and a dissolve potential of 500 mV were selected, and the deposition of micro-pillars in a large aspect ratio and at high density can be realized. The deposition technology can be further applied in the fabrication of micro-TEGs with large output voltage and power.

Up-scaled synthesis process of sulphur-based thermoelectric materials

Up-scaled spark plasma sintering process of sulphur-based TiS₂ and tetrahedrite Cu_10.4Ni_1.6Sb₄S₁₃ thermoelectric materials.

Effect of sulfur substitution on the thermoelectric properties of (SnSe)1.16NbSe2: charge transfer in a misfit layered structure

First time investigation of the thermoelectric properties of misfit layered (SnSe)_1.16NbSe₂ and new insights into the charge transfer tuning in misfit systems.

Enhancement of the thermoelectric properties of MnSb2Se4 through Cu resonant doping

We report the synthesis of Cu substituted Mn_1−xCuxSb₂Se₄ and its interesting resonant doping behavior, leading to zT of 0.64 at 773 K for the Mn_0.75Cu_0.25Sb₂Se₄. Cu-doped MnSb₂Se₄ could be considered as a new platform for power generation.