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The polycrystalline SnS with a similar layered crystal structure and band structure to SnSe exhibits enormous commercial thermoelectric potential due to its lower cost and environmentally friendly characteristics. However, the wider bandgap of SnS leads to low carrier concentration and inferior electrical transport performance. The two-dimensional interlayer hinders carrier transport, leading to interesting and mysterious anisotropic thermoelectric properties. Herein, we reported the optimized electron–phonon transport in anisotropic polycrystalline SnS by Ag doping and Se alloying, realizing a high quality factor B by multiple strategies of optimizing carrier concentration, modifying band structure, and introducing various defects; further potential performance is predicted by the single parabolic band model. Specifically, Ag-doped SnS not only significantly increases the carrier concentration and weighted mobility μw in both directions but also induces multi-scale precipitates proven by the Debye–Callaway model to suppress phonon transport. Moreover, additional Se alloying optimizes the electronic band structure and increases the Seebeck coefficient, further improving μW and boosting the maximum power factor to ∼3.72 μW cm−1 K−2 at 873 K in the out-of-plane direction. Consequently, the synergistic optimization of carrier and phonon transport achieved a high B of 0.7 and a maximum zTmax of ∼0.8 at 873 K in Ag0.02Sn0.98S0.99Se0.01. Additionally, the high B predicted a high zTmax∼1.5 based on optimized carrier transport characteristics, demonstrating the potential great-performance polycrystalline SnS. This work provides a promising avenue for optimizing the zT of polycrystalline SnS by transport engineering. [ABSTRACT FROM AUTHOR]

Transition metals have excellent valence electrical properties and unique electronic state distribution and are regarded as potential materials for improving thermoelectric performance. However, the impact of transition metals on thermoelectric materials is restricted to the solid solution limit and doping efficiency, reinforcing the shortcomings in systematic research. Here, thermoelectric properties of transition metal (Ti, V, Cr, Zr, Nb, Mo)-doped PbS are compared and analyzed systematically based on the formation enthalpy. The DFT calculation indicates that the doping (except Zr) leads to the bandgap expansion and the density of states distortion near the Fermi level, while the localization property of the latter results in an invalid resonance level. The formation enthalpy dominates the carrier concentration due to the opposite trend of carrier concentration and formation enthalpy. The formation enthalpy of Zr, Ti, and Nb doping is more negative than others, leading to the more significant optimization of carrier concentration. The Moss–Burstein effect promotes the bandgap expansion, leading to weaker bipolar effects for Zr, Ti, and Nb doping. Eventually, the thermoelectric performance for Ti, Zr, and Nb doping is superior to others at high temperature. The Hume-Rothery rule of the formation enthalpy supplementation is more suitable for the doping and alloying in thermoelectricity. Thermodynamic stability analysis based on the formation enthalpy contribute the PbS-based thermoelectric devices evaluation. The present finding demonstrates the significant effect of formation enthalpy on the thermoelectric properties of PbS and provides a useful avenue for the doping modification and thermodynamic stability analysis of other thermoelectric alloy materials. [ABSTRACT FROM AUTHOR]

Breaking the thermoelectric figure of merit zT barrier of SnTe enables it to become a promising alternative to PbTe; however, the inferior and strongly coupled physicochemical properties of pristine SnTe severely restrict the efficient optimization. Herein, we doped trivalent Sb in SnTe and incorporated SnS particles to achieve high quality factor B through a two-step optimization strategy of tuning the valence band structure and intercalating heterostructural precipitates, and well predicted the potential prospects. The high solubility limit of Sb not only reduced the carrier concentration nH but also significantly optimized the valence band structure and improved the Seebeck coefficient, thereby enhancing the weight mobility μw in the all-temperature region. Furthermore, the additional SnS, which tends to exist as precipitates with different micrometer-scale sizes, enhanced low-medium-frequency phonon scattering in a wider frequency range except for point defects scattering, suppressing the lattice thermal conductivity to 0.55 W m−1 K−1. As a result of this synergistic effect, a high B-factor of ∼0.82 greater than triple pure SnTe was obtained in Sn0.91Sb0.09Te-10%SnS, with an enhanced zT of ∼1.15 at 850 K. More importantly, the high B-factor accurately predicted an excellent zT value of ∼1.65 at the optimal Fermi level, which highlights the great potential of Sn1-xSbxTe-y%SnS-based materials. This work provides an effective route for stepwise optimization of electrical and thermal performance from the B-factor perspective and has guiding significance for other thermoelectric materials. [ABSTRACT FROM AUTHOR]

As the pseudo-binary alloys between GeTe and Sb2Te3, GeSbTe-based compounds are promising thermoelectric materials. Although Ge2Sb2Te5 and GeSb2Te4 have widely been studied, the thermoelectric properties of GeSb4Te7 have not been well understood yet. In this work, we design a series of GeSb4-xBixTe7 solid solutions and systematically study the variation in the crystal structure, electrical, and thermal transport properties. Alloying Bi effectively reduces the carrier concentration and, thus, enhances the Seebeck coefficient. Meanwhile, the thermal conductivity is greatly reduced via large mass fluctuations. A maximum zT of 0.69 at 550 K and an average zT value of 0.52 between 300 and 700 K have been achieved for GeSb1.5Bi2.5Te7. These findings will promote the understanding and development of GeSbTe-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

The fact that the thermoelectric performance is far inferior to that of p-type PbTe has inspired many strategies to develop n-type PbTe thermoelectrics. Alloying PbS in n-type PbTe effectively changes the shape of a valley to trigger a heavier conduction band for improving the Seebeck coefficient, while the resulting small orbital overlap inevitably leads to phase separation hindering electron transport. The effect of vacancies on the solubility of sulfur in n-type PbTe is ambiguous; especially, the heterostructure due to phase separation in high-content PbS-alloyed PbTe also requires sufficient modification to optimize the electroacoustic transport. This motivates the current work on the introduction of vacancies by charge-balancing doping via Sb2Te3 and discovers striking new insight that the introduced vacancies can induce a new heterostructure of Pb2Sb2S5 and suppress the aggregation of Sb and PbS in high-solubility n-type PbTe–PbS. The modification of the band structure and optimization of the electron transport give rise to a prominent enhancement in electronic performance. Furthermore, the Debye–Callaway model validates the dramatic contribution of vacancy aggregation and heterostructures to lattice thermal conductivity. As a result, the synergistic modulation of electroacoustic characteristics achieves a significant improvement in both the maximum zT and the near-room-temperature zT. Understanding such unique findings is critical for applicability to other thermoelectric materials. [ABSTRACT FROM AUTHOR]

A decrease in valence band energy offset can considerably improve the thermoelectric performance of SnTe, and alloying CdTe in SnTe has been confirmed to be efficient for inducing band convergence. However, the low solubility of CdTe in SnTe severely limits the further decrease in the energy offset and the reduction of lattice thermal conductivity. Inspired by the high solubility of Sb in SnTe-based thermoelectric materials, the trivalent Sb is introduced into SnTe–CdTe alloys, aiming at manipulating the thermoelectric transport properties. Combined with the valence band model, it is demonstrated that high concentration of Sb in SnTe–CdTe enables a further optimization in valence band structures, resulting in an improvement in density-of-state effective mass, thus significantly reinforces the power factor in the whole temperature range. Meanwhile, we propose the solid solution mode of Sb in SnTe, which always generates vacancies to balance the valence state, and the introduction of vacancies explains the reduced lattice parameters and almost constant carrier concentration. Particularly, the Debye–Callaway model quantitatively compares the contribution of Sb substitutional defects and vacancy defects. As a result, an enhanced zT of ∼1.1 has been achieved for Sn0.83Cd0.05Sb0.12Te at 823 K. This work clearly shows the critical role of Sb for enhancing the thermoelectric performance of SnTe–CdTe materials. [ABSTRACT FROM AUTHOR]

Twin boundaries (TBs) can increase interface scattering to reduce the lattice thermal conductivity. InSb has good electronic transport properties, but its high thermal conductivity constrains its thermoelectric application. In this work, we aim to study the role of TBs on lattice thermal conductivity of InSb. We use non-equilibrium molecular dynamics simulation to investigate how the TB spacing and orientation influence the lattice thermal conductivity of InSb. We find that TBs can hinder the heat flow, leading to decreased temperature gradient and, hence, remarkably reduced lattice thermal conductivity. The relationship between κL and λ can be described by a proportional function. Nanotwinned InSb with TB spacing of 1.1 nm has a lattice thermal conductivity of 12.6 W/m K, a decrease in 22.2% compared with its single crystal (16.2 W/m K). We also find that a minimum lattice thermal conductivity of InSb can be obtained when the TB orientation is perpendicular to the direction of heat flow. These findings provide a theoretical guidance for TBs engineering to reduce the thermal conductivity of thermoelectric materials. [ABSTRACT FROM AUTHOR]

AgCrSe2, which crystallizes in alternative layers of Ag+ and CrSe2− octahedral structure, has shown great potential as good thermoelectric material due to its unique ultralow lattice thermal conductivity. In this work, compound Cr2/3Te is alloyed with the matrix Ag0.97CrSe2 and the carrier concentration ranges within 1018–1020 cm−3, enabling a reliable assessment of transport properties based on single parabolic band model at room temperature. Moreover, homogeneous nanoprecipitate is observed in the matrix for high Cr2/3Te content samples, which leads to the scattering of main heat carrier of long-wavelength phonons, and thus a slight reduction of lattice thermal conductivity (∼0.3 W/m K) compared with intrinsic AgCrSe2. Combined with the optimized carrier concentration and the low lattice thermal conductivity, a figure of merit zT of 0.6 at 650 K is achieved, exceeding other reported AgCrSe2 systems, demonstrating the current Ag0.97CrSe2(Cr2/3Te)x materials as good potential thermoelectrics. [ABSTRACT FROM AUTHOR]

The rich capabilities for both electronic and phonon engineering in SnTe are highly desirable for achieving high thermoelectric performance. Alloying high-solubility MnTe (∼15%) leads to substitutional defects for reducing lattice thermal conductivity and band convergence for enhancing electronic performance, and thus, an improvement of thermoelectric performance of SnTe is realized. However, there is no evidence that the electronic and phonon transport properties are fully optimized in SnTe-15%MnTe thermoelectrics, especially for the near-room-temperature (< 573 K) thermoelectric performance, which still needs a sufficient promotion. Here, the substituted 2% Bi in SnTe-15%MnTe alloys dramatically increases the near-room-temperature zT and peak zT to ∼0.72 (at 550 K) and ∼1.3 (at 850 K), respectively. Combining the experimental evidence and the first-principles calculations, we demonstrate that the prominent enhancement of electronic performance arises from the Bi-doping-driven transport valence band alignment and the carrier concentration optimization. Furthermore, the Debye–Callaway model verifies that the reduction in lattice thermal conductivity is dominated by the Bi substitutional defects. The present findings reveal the importance of transport engineering in achieving high thermoelectric performance particularly near room temperature. [ABSTRACT FROM AUTHOR]

AgCrSe2 materials have shown the potential to be the promising thermoelectric materials due to the intrinsic ultralow lattice thermal conductivity enabled by the fluid-like motion of Ag ions in crystal structures. However, what important role the Ag point defects can play in promoting liquid-like conduction still remains a mystery. Moreover, understanding the electronic transport properties, specifically for clarifying the contribution of various impurity states, is of critical importance for improving the thermoelectric performance of AgCrSe2. Here, the thermoelectric properties of AgCrSe2 with various Ag content have been systematically investigated. It is found that the carrier concentration can be modulated in a broad range by inducing deep level impurity states, enabling a transition from degenerate semiconductor to nondegenerate one and a reliable evaluation on the thermoelectric transport properties. The single parabolic band model gives a good clarification regarding the increased electronic performance. Furthermore, the enhancement of the liquid-like effect triggered by the Ag vacancies contributes to the ultralow lattice thermal conductivity (∼0.25 W m−1 K−1). Eventually, a maximum figure of merit zT of ∼0.6 is realized in Ag0.97CrSe2 at 750 K. The present findings may shed light on the improvement of thermoelectric performance in AgCrSe2 materials. [ABSTRACT FROM AUTHOR]

Numerous endeavors have been made to advance thermoelectric SnTe for potential applications. Effective strategies focus on the manipulation of transport properties, including valence band convergence, resonate state, and defect engineering. It has been demonstrated that alloying trivalent Bi or chalcogenide SnSe alone in SnTe can trigger an inherent enhancement of thermoelectric performance. However, what the critical role in the transport valence band co-doping Bi and Se in SnTe plays is still unclear. Particularly, fully evaluating the effect of band convergence on the carrier concentration-dependent weighted mobility, which dominates the electronic performance, is primary and essential for designing excellent thermoelectric materials. Here, we report that Bi doping in SnTe–SnSe alloys can derive a distinct decrease in the energy offset between the two valence bands, thus improving the density-of-state effective mass by only slightly deteriorating the mobility. The well-established theoretical model reveals that the Bi-doping-induced band convergence and the optimized carrier concentration actually enhance the weighted mobility, contributing to the improvement of electronic performance. Moreover, the Debye–Callaway model demonstrates the origin of the reduced lattice thermal conductivity. The present results confirm the potential of transport engineering in promoting thermoelectric performance. [ABSTRACT FROM AUTHOR]

SnTe-AgSbTe2 alloys have shown promising thermoelectric performance due to phonon-vacancy scattering, while SnTe alloyed with high-solubility monotellurides such as CaTe/MnTe has also shown enhanced thermoelectric performance owing to band convergence. However, to what level the combined effect on thermoelectrics by alloying SnTe with AgSbTe2 and CaTe/MnTe exists, especially for the quantitative relationship between the vacancy concentration and lattice thermal conductivity, still remains a mystery. Here, a spontaneous strategy has shown that combining AgSbTe2 with CaTe/MnTe in SnTe alloys can lead to an inherent valence band convergence and improved electronic performance. In particular, a low lattice thermal conductivity (∼0.4 W/m K), which approaches the amorphous limit of SnTe, is achieved in (Sn0.95Ca0.05Te)0.75(AgSbTe2)0.125 with 3% excess of Sn. The Debye-Callaway model provides a good clarification of underlying physics for the reduced lattice thermal conductivity as well as quantifies the contribution of vacancy scattering. The present finding demonstrates the excellent potential for advancing the thermoelectric performance by SnTe-based alloys. [ABSTRACT FROM AUTHOR]

Pseudo-binary GeSbTe alloys, best known as phase-change materials, are quasi-two-dimensional semiconductors in their stable trigonal phases with high electrical conductivity and low thermal conductivity, but their thermoelectric properties have not been systematically investigated. Here, in this letter, we prepared polycrystalline Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7 bulk materials and studied the thermoelectric transport properties. Large anisotropy in Seebeck coefficient as well as in electrical conductivity is observed, which is not commonly reported in polycrystalline thermoelectric materials. Combining experimental study and theoretical calculations, one can find that this phenomenon is attributed to the asymmetry of a material's band structure. Maximal zT values of 0.46–0.60 are achieved at 750 K, indicating that GeSbTe-based compounds are promising thermoelectric materials for mid-temperature applications. [ABSTRACT FROM AUTHOR]