Realizing N-type SnTe Thermoelectrics with Competitive Performance through Suppressing Sn Vacancies

Defect Engineering Advances Thermoelectric Materials

Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.

All-SnTe-Based Thermoelectric Power Generation Enabled by Stepwise Optimization of n-Type SnTe

The practical application of thermoelectric devices requires both high-performance n-type and p-type materials of the same system to avoid possible mismatches and improve device reliability. Currently, environmentally friendly SnTe thermoelectrics have witnessed extensive efforts to develop promising p-type transport, making it rather urgent to investigate the n-type counterparts with comparable performance. Herein, we develop a stepwise optimization strategy for improving the transport properties of n-type SnTe. First, we improve the n-type dopability of SnTe by PbSe alloying to narrow the band gap and obtain n-type transport in SnTe with halogen doping over the whole temperature range. Then, we introduce additional Pb atoms to compensate for the cationic vacancies in the SnTe-PbSe matrix, further enhancing the electron carrier concentration and electrical performance. Resultantly, the high-ranged thermoelectric performance of n-type SnTe is substantially optimized, achieving a peak ZT of ∼0.75 at 573 K with a high average ZT (ZTave) exceeding 0.5 from 300 to 823 K in the (SnTe0.98I0.02)0.6(Pb1.06Se)0.4 sample. Moreover, based on the performance optimization on n-type SnTe, for the first time, we fabricate an all-SnTe-based seven-pair thermoelectric device. This device can produce a maximum output power of ∼0.2 W and a conversion efficiency of ∼2.7% under a temperature difference of 350 K, demonstrating an important breakthrough for all-SnTe-based thermoelectric devices. Our research further illustrates the effectiveness and application potential of the environmentally friendly SnTe thermoelectrics for mid-temperature power generation.

Effect of Synthesis Factors on Microstructure and Thermoelectric Properties of FeTe2 Prepared by Solid-State Reaction

The alloying compound FeTe2 is a semi-metallic material with low thermal conductivity and has the potential to become a thermoelectric material. Single-phase FeTe2 compounds are synthesized using a two-step sintering method, and the effects of the optimal sintering temperature, holding temperature, and holding time on the thermoelectric properties of the alloy compound FeTe2 are investigated. The phase composition, microstructure, and electrical transport properties of the FeTe2 compound are systematically analyzed. The results show that single-phase FeTe2 compounds can be synthesized within the range of a sintering temperature of 823 K and holding time of 10~60 min, and the thermoelectric properties gradually deteriorate with the prolongation of the holding time. Microstructural analysis reveals that the sample of the alloy compound FeTe2 exhibits a three-dimensional network structure with numerous fine pores, which can impede thermal conduction and thus reduce the overall thermal conductivity of the material. When the sintering temperature is 823 K and the holding time is 30 min, the sample achieves the minimum electrical resistivity of 6.9 mΩ·cm. The maximum Seebeck coefficient of 65.48 μV/K is obtained when the sample is held at 823 K for 10 min; and under this condition, the maximum power factor of 59.54 μW/(m·K2) is achieved. In the whole test temperature range of 323~573 K, when the test temperature of the sample is 375 K, the minimum thermal conductivity is 1.46 W/(m·K), and the maximum ZT is 1.57 × 10-2.

Extremely Low Lattice Thermal Conductivity Leading to Superior Thermoelectric Performance in Cu4TiSe4

Low thermal conductivity is crucial for obtaining a promising thermoelectric (TE) performance in semiconductors. In this work, the TE properties of Cu₄TiS₄ and Cu₄TiSe₄ were theoretically investigated by carrying out first-principles calculations and solving Boltzmann transport equations. The calculated results reveal a lower sound velocity in Cu₄TiSe₄ compared to that in Cu₄TiS₄, which is due to the weaker chemical bonds in the crystal orbital Hamilton population (COHP) and also the larger atomic mass in Cu₄TiSe₄. In addition, the strong lattice anharmonicity in Cu₄TiSe₄ enhances phonon-phonon scattering, which shortens the phonon relaxation time. All of these factors lead to an extremely low lattice thermal conductivity (κ_L) of 0.11 W m^-1 K^-1 at room temperature in Cu₄TiSe₄ compared with that of 0.58 W m^-1 K^-1 in Cu₄TiS₄. Owing to the suitable band gaps of Cu₄TiS₄ and Cu₄TiSe₄, they also exhibit great electrical transport properties. As a result, the optimal ZT values for p (n)-type Cu₄TiSe₄ are up to 2.55 (2.88) and 5.04 (5.68) at 300 and 800 K, respectively. For p (n)-type Cu₄TiS₄, due to its low κ_L, the ZT can also reach high values over 2 at 800 K. The superior thermoelectric performance in Cu₄TiSe₄ demonstrates its great potential for applications in thermoelectric conversion.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Enhancement of the Thermoelectric Performance of Cu2GeSe3 via Isoelectronic (Ag, S)-co-substitution

Recently, ternary Cu-based Cu-IV-Se (IV = Sb, Ge, and Sn) compounds have received extensive attention in the thermoelectric field. Compared with Cu-Sb-Se and Cu-Sn-Se, Cu-Ge-Se compounds have been less studied due to its poor Seebeck coefficient and high thermal conductivity. Here, the Cu₂GeSe₃ material with high electrical conductivity was first prepared, and then, its effective mass was increased by doping with S, which led to the Seebeck coefficient of the doped sample being 1.93 times higher than that of pristine Cu₂GeSe₃ at room temperature. Moreover, alloying Ag at the Cu site in the Cu₂GeSe_2.96S_0.04 sample could further cause a 5.16 times increase in the Seebeck coefficient at room temperature, and the lattice thermal conductivity was remarkably decreased because of the introduction of the dislocations in the Cu₂GeSe₃ sample. Finally, benefitted from the high Seebeck coefficient and low thermal conductivity, a record high ZT = 0.9 at 723 K was obtained for the Cu_1.85Ag_0.15GeSe_2.96S_0.04 sample, which increased 345% in comparison with the pristine Cu₂GeSe₃, and it is among the highest reported values for Cu₂GeSe₃-based thermoelectric.

High-Throughput Screening for Thermoelectric Semiconductors with Desired Conduction Types by Energy Positions of Band Edges

The conduction type of semiconductors is vitally important in many fields (e.g., photovoltaics, transistors, and thermoelectrics), but so far, there is no effective and simple indicator to quickly judge or predict the conduction type of various semiconductors. In this work, based on the relationship between the formation energy of charged defect and the Fermi level, we propose a simple and low-cost strategy for high-throughput screening the potential n-type or p-type semiconductors from the material database by using energy positions of band edges as indicators. As a case study, we validate this strategy in searching potential n-type thermoelectric materials from copper (Cu)-containing metal chalcogenides. A new promising thermoelectric material, CuIn₅Se₈, with potential intrinsic n-type conduction, is successfully screened from 407 Cu-containing metal chalcogenides and validated in the subsequent experiments. Upon doping iodine in CuIn₅Se₈, a peak thermoelectric figure of merit zT of 0.84 is obtained at 850 K. Beyond thermoelectrics, the strategy proposed in this study also sheds light on the new material development with desired conduction types in photovoltaics, transistors, and other fields.

Thermoelectric Performance of the 2D Bi2Si2Te6 Semiconductor

Bi₂Si₂Te₆, a 2D compound, is a direct band gap semiconductor with an optical band gap of ∼0.25 eV, and is a promising thermoelectric material. Single-phase Bi₂Si₂Te₆ is prepared by a scalable ball-milling and annealing process, and the highly densified polycrystalline samples are prepared by spark plasma sintering. Bi₂Si₂Te₆ shows a p-type semiconductor transport behavior and exhibits an intrinsically low lattice thermal conductivity of ∼0.48 W m^-1 K^-1 (cross-plane) at 573 K. The first-principles density functional theory calculations indicate that such low lattice thermal conductivity is derived from the interactions between acoustic phonons and low-lying optical phonons, local vibrations of Bi, the low Debye temperature, and strong anharmonicity result from the unique 2D crystal structure and metavalent bonding of Bi₂Si₂Te₆. The Bi₂Si₂Te₆ exhibits an optimal figure of merit ZT of ∼0.51 at 623 K, which can be further enhanced by the substitution of Bi with Pb. Pb doping leads to a large increase in power factor S²σ, from ∼3.9 μW cm^-1 K^-2 of Bi₂Si₂Te₆ to ∼8.0 μW cm^-1 K^-2 of Bi_1.98Pb_0.02Si₂Te₆ at 773 K, owing to the increase in carrier concentration. Moreover, Pb doping induces a further reduction in the lattice thermal conductivity to ∼0.38 W m^-1 K^-1 (cross-plane) at 623 K in Bi_1.98Pb_0.02Si₂Te₆, due to strengthened point defect (Pb_Bi') scattering. The simultaneous optimization of the power factor and lattice thermal conductivity achieves a peak ZT of ∼0.90 at 723 K and a high average ZT of ∼0.66 at 400-773 K in Bi_1.98Pb_0.02Si₂Te₆.

Contrasting roles of Bi-doping and Bi2Te3 alloying on the thermoelectric performance of SnTe

A step-by-step band convergence strategy contributes to the enhanced thermoelectric figure of merit ZT in SnTe.

Enhanced Thermoelectric Performance by Surface Engineering in SnTe-PbS Nanocomposites

Thermoelectric materials enable the direct conversion between heat and electricity. SnTe is a promising candidate due to its high charge transport performance. Here, we prepared SnTe nanocomposites by employing an aqueous method to synthetize SnTe nanoparticles (NP), followed by a unique surface treatment prior NP consolidation. This synthetic approach allowed optimizing the charge and phonon transport synergistically. The novelty of this strategy was the use of a soluble PbS molecular complex prepared using a thiol-amine solvent mixture that upon blending is adsorbed on the SnTe NP surface. Upon consolidation with spark plasma sintering, SnTe-PbS nanocomposite is formed. The presence of PbS complexes significantly compensates for the Sn vacancy and increases the average grain size of the nanocomposite, thus improving the carrier mobility. Moreover, lattice thermal conductivity is also reduced by the Pb and S-induced mass and strain fluctuation. As a result, an enhanced ZT of ca. 0.8 is reached at 873 K. Our finding provides a novel strategy to conduct rational surface treatment on NP-based thermoelectrics.