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.

High Carrier Mobility and Promising Thermoelectric Module Performance of N-Type PbSe Crystals.

The scarcity of Te hampers the widespread use of Bi2Te3-based thermoelectric modules. Here, the thermoelectric module potential of PbSe is investigated by improving its carrier mobility. Initially, large PbSe crystals are grown with the temperature gradient method to mitigate grain boundary effects on carrier transport. Subsequently, light doping with <1mole‰ halogens (Cl/Br/I) increases room-temperature carrier mobility to ~1600 cm2 V-1 s-1, achieved by reducing carrier concentration compared to traditional heavy doping. Crystal growth design and light doping enhance carrier mobility without affecting effective mass, resulting in a high power factor ~40 µW cm-1 K-2 in PbSe-Cl/Br/I crystals at 300 K. Additionally, Cl/Br/I doping reduces thermal conductivity and bipolar diffusion, leading to significantly lower thermal conductivity at high temperature. Enhanced carrier mobility and suppressed bipolar effect boost ZT values across the entire temperature range in n-type PbSe-Cl/Br/I crystals. Specifically, ZT values of PbSe-Br crystal reach ~0.6 at 300 K, ~1.2 at 773 K, and the average ZT (ZTave) reaches ~1.0 at 300-773 K. Ultimately, ~5.8% power generation efficiency in a PbSe single leg with a maximum temperature cooling difference of 40 K with 7-pair modules is achieved. These results indicate the potential for cost-effective and high-performance thermoelectric cooling modules based on PbSe.

Rethinking SnSe Thermoelectrics from Computational Materials Science.

ConspectusThe growing energy crisis and the adverse environmental impacts caused by carbon-based energy consumption have spurred the exploration of green and sustainable energy. Researchers have been devoted to developing thermoelectric technology that could directly and reversibly convert heat into electricity. By virtue of zero emissions, nonmoving parts, precise temperature control, and long service life, thermoelectrics exhibit broad application in power generation and refrigeration. Nevertheless, traditional narrow-bandgap thermoelectrics exhibit high performance within a narrow temperature range, limiting the overall energy conversion. Consequently, a selection rule for exploring advanced thermoelectrics was proposed: materials with wide-bandgap, crystals form, asymmetry, and anisotropic structure. According to the rules, we conducted much research and found some promising materials.As the lead-free, cost-effective, and stable thermoelectric candidates, layered SnSe crystals with wide-bandgap and covalent bonding have gained significant attention due to their ultralow thermal conductivity resulting from strong bonding anharmonicity, via strong polar covalent bonding, because of the electronegativity difference between the Sn and Se atoms. This was proved to be the result from the unique structure of layered SnSe crystals, a distorted rock-salt structure with high and anisotropic Grüneisen parameters. In this Account, we introduce and rethink our recent advancements in developing high-performance thermoelectric SnSe crystals from computational materials science, involving p- and n-type SnSe crystals, respectively. For p-type SnSe crystals, according to the complex valence band structures, we utilized the multiband synglisis via electronic structure calculations and multiband simulations to activate valence bands to participate in electrical transport of in-plane direction, achieving an ultrahigh power factor (PF) of ∼75 µW cm-1 K-2 at room temperature and an average figure-of-merit ZTave of ∼1.9 for Sn0.91Pb0.09Se. Besides, on the basis of defect chemistry, the characteristics of p-type SnSe crystals are determined by intrinsic Sn vacancies. We thus used point-defect calculations to achieve the lattice plainification, and we fixed the lattice intrinsic defects to weaken defect scattering of carriers along the in-plane direction, facilitating further a PF > 100 µW cm-1 K-2 and a ZT of ∼1.5 at room temperature for SnCu0.001Se. For n-type SnSe crystals, inspired by the anisotropic characteristics of the layered materials, we analyzed charge density and proposed the insight of 3D charge and 2D phonon transports and calculated the deformation potential to manipulate layered phonon-electron decoupling to achieve high performance, ultimately Pb-alloyed and Cl-doped SnSe (SnSe-Cl-PbSe) reaching a ZTave of ∼1.7 from 300 to 773 K. In the end, we offer potential perspectives on high-throughput calculations (HTC) and machine learning (ML), combined with our proposed insights, which could be a promising avenue for future thermoelectrics. By virtue of our theoretical and experimental understanding of thermoelectrics, integrating these insights as rules and descriptors for HTC and ML will accelerate the research and development of thermoelectrics. We want to share our recent works and latest perspectives in SnSe thermoelectrics, and we expect to inspire enthusiasm for innovative research on advanced thermoelectric materials and devices.

Large Mobility Enables Higher Thermoelectric Cooling and Power Generation Performance in n-type AgPb18+xSbTe20 Crystals.

The room-temperature thermoelectric performance of materials underpins their thermoelectric cooling ability. Carrier mobility plays a significant role in the electronic transport property of materials, especially near room temperature, which can be optimized by proper composition control and growing crystals. Here, we grow Pb-compensated AgPb18+xSbTe20 crystals using a vertical Bridgman method. A large weighted mobility of ∼410 cm2 V-1 s-1 is achieved in the AgPb18.4SbTe20 crystal, which is almost 4 times higher than that of the polycrystalline counterpart due to the elimination of grain boundaries and Ag-rich dislocations verified by atom probe tomography, highlighting the significant benefit of growing crystals for low-temperature thermoelectrics. Due to the largely promoted weighted mobility, we achieve a high power factor of ∼37.8 µW cm-1 K-2 and a large figure of merit ZT of ∼0.6 in AgPb18.4SbTe20 crystal at 303 K. We further designed a 7-pair thermoelectric module using this n-type crystal and a commercial p-type (Bi, Sb)2Te3-based material. As a result, a high cooling temperature difference (ΔT) of ∼42.7 K and a power generation efficiency of ∼3.7% are achieved, revealing promising thermoelectric applications for PbTe-based materials near room temperature.

NaCdSb: An Orthorhombic Zintl Phase with Exceptional Intrinsic Thermoelectric Performance.

Many Zintl phases are promising thermoelectric materials owning to their features like narrow band gaps, multiband behaviors, ideal charge transport tunnels, and loosely bound cations. Herein we show a new Zintl phase NaCdSb with exceptional intrinsic thermoelectric performance. Pristine NaCdSb exhibits semiconductor behaviors with an experimental hole concentration of 2.9×1018  cm-3 and a calculated band gap of 0.5 eV. As the temperature increases, the hole concentration rises gradually and approaches its optimal one, leading to a high power factor of 11.56 µW cm-1 K-2 at 673 K. The ultralow thermal conductivity is derived from the small phonon group velocity and short phonon lifetime, ascribed to the structural anharmonicity of Cd-Sb bonds. As a consequence, a maximum zT of 1.3 at 673 K has been achieved without any doping optimization or structural modification, demonstrating that NaCdSb is a remarkable thermoelectric compound with great potential for performance improvement.

Mechanically Induced Highly Efficient Hydrogen Evolution from Water over Piezoelectric SnSe nanosheets.

Piezoelectric nanomaterials open new avenues in driving green catalysis processes (e.g., H2 evolution from water) through harvesting mechanical energy, but their catalytic efficiency is still limited. The predicted enormous piezoelectricity for 2D SnSe, together with its high charge mobility and excellent flexibility, renders it an ideal candidate for stimulating piezocatalysis redox reactions. In this work, few-layer piezoelectric SnSe nanosheets (NSs) are utilized for mechanically induced H2 evolution from water. The finite elemental method simulation demonstrates an unprecedent maximal piezoelectric potential of 44.1 V for a single SnSe NS under a pressure of 100 MPa. A record-breaking piezocurrent density of 0.3 mA cm-2 is obtained for SnSe NSs-based electrode under ultrasonic excitation (100 W, 45 kHz), which is about three orders of magnitude greater than that of reported piezocatalysts. Moreover, an exceptional H2 production rate of 948.4 µmol g-1 h-1 is achieved over the SnSe NSs without any cocatalyst, far exceeding most of the reported piezocatalysts and competitive with the current photocatalysis technology. The findings not only enrich the potential piezocatalysis materials, but also provide useful guidance toward high-efficiency mechanically driven chemical reactions such as H2 evolution from water.

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

Due to the intrinsically plentiful Sn vacancies, developing n-type SnTe thermoelectric materials is a big challenge. Herein, n-type SnTe thermoelectric materials with remarkable performance were successfully synthesized through suppressing Sn vacancies, followed by electron-doping. Pb alloying notably depressed the Sn vacancies via populating Sn vacancies in SnTe (supported by transmission electron microscopy), and the electrical transports were shifted from p-type to n-type through introducing electrons using I doping. In the n-type SnTe, we found that the electrical conductivity could be enhanced by increased carrier mobility through sharpening conduction bands after alloying Pb, while the lattice thermal conductivity could be reduced via strong phonon scattering after introducing defects by Pb alloying and I doping. Resulting from these enhancements, the n-type Sn0.6Pb0.4Te0.98I0.02 achieves a notably high ZTmax ∼ 0.8 at 573 K and a remarkable ZTave ∼ 0.51 at 300-823 K, which can match many excellent p-type SnTe. This work indicates that n-type SnTe could be experimentally acquired and is a promising candidate for thermoelectric generation, which will stimulate further research on n-type SnTe thermoelectric materials and even devices on the basis of both n- and p-type SnTe legs.

Ultrahigh Average ZT Realized in p-Type SnSe Crystalline Thermoelectrics through Producing Extrinsic Vacancies.

Crystalline SnSe has been revealed as an efficient thermoelectric candidate with outstanding performance. Herein, record-high thermoelectric performance is achieved among SnSe crystals via simply introducing a small amount of SnSe2 as a kind of extrinsic defect dopant. This excellent performance mainly arises from the largely enhanced power factor by increasing the carrier concentration high as 6.55 × 1019 cm-3, which was surprisingly promoted by introducing extrinsic SnSe2 even though pristine SnSe2 is an n-type conductor. The optimized carrier concentration promotes a deeper Fermi level and activates more valence bands, leading to an extraordinary room-temperature power factor ∼54 µW cm-1 K-2 through enlarging the band effective mass and Seebeck coefficient. As a result, on the basis of simultaneously depressed thermal conductivity induced from both Sn vacancies and SnSe2 microdomains, maximum ZT values ∼0.9-2.2 and excellent average ZT > 1.7 among the working temperature range are achieved in Na doped SnSe crystals with 2% extrinsic SnSe2. Our investigation illustrates new approaches on improving thermoelectric performance through introducing defect dopants, which might be well-implemented in other thermoelectric systems.

Band Sharpening and Band Alignment Enable High Quality Factor to Enhance Thermoelectric Performance in n-Type PbS.

Low-cost and earth-abundant PbS-based thermoelectrics are expected to be an alternative for PbTe, and have attracted extensive attentions from thermoelectric community. Herein, a maximum ZT (ZTmax) ≈ 1.3 at 923 K in n-type PbS is obtained through synergistically optimizing quality factor with Sn alloying and PbTe phase incorporation. It is found that Sn alloying in PbS can sharpen the conduction band shape to balance the contradictory interrelationship between carrier mobility and effective mass, accordingly, a peak power factor of ∼19.8 µWcm-1K-2 is achieved. Besides band sharpening, Sn alloying can also narrow the band gap of PbS so as to make the conduction band position between Pb0.94Sn0.06S and PbTe well aligned, which can benefit high carrier mobility. Therefore, incorporating the PbTe phase into the Pb0.94Sn0.06S matrix can not only favorably maintain the carrier mobility at ∼150 cm2V-1s-1 but also suppress the lattice thermal conductivity to ∼0.61 Wm-1K-1 in Pb0.94Sn0.06S-8%PbTe, which contributes to a largely enhanced quality factor. Consequently, an average ZT (ZTave) ≈ 0.72 in 300-923 K is achieved in Pb0.94Sn0.06S-8%PbTe that outperforms other n-type PbS-based thermoelectric materials.

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.

Realizing High Thermoelectric Performance in p-Type SnSe through Crystal Structure Modification.

The simple binary compound SnSe has been reported as a robust thermoelectric material for energy conversion by showing strong anharmonicity and multiple electronic valence bands. Herein, we report a record-high average ZT value of ∼1.6 at 300-793 K with maximum ZT values ranging from 0.8 at 300 K to 2.1 at 793 K in p-type SnSe crystals. This remarkable thermoelectric performance arises from the enhanced power factor and lowered lattice thermal conductivity through crystal structure modification via Te alloying. Our results elucidate that Te alloying increases the carrier mobility by making the bond lengths more nearly equal and sharpening the valence bands; meanwhile, the Seebeck coefficient remains large due to multiple valence bands. As a result, a record-high power factor of ∼55 µW cm-1 K-2 at 300 K is achieved. Additionally, Te alloying promotes Sn atom displacements, thus leading to a lower lattice thermal conductivity. Our conclusions are well supported by electron localization function calculations, the Callaway model, and structural characterization via aberration-corrected scanning transmission electron microscopy. Our approach of modifying crystal structures could also be applied in other low-symmetry thermoelectric materials and represents a new strategy to enhance thermoelectric performance.

Thermoelectric Material SnPb2Bi2S6: The 4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity.

Although the binary sulfides Bi2S3, PbS, and SnS have attracted extensive interest as thermoelectric materials, no quaternary sulfides containing Sn/Pb/Bi/S elements have been reported. Herein, we report the synthesis of a new quaternary sulfide, SnPb2Bi2S6, which crystallizes in the orthorhombic space group Pnma with unit cell parameters of a = 20.5458(12) Å, b = 4.0925(4) Å, c = 13.3219(10) Å. SnPb2Bi2S6 has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS7 monocapped trigonal prisms. In the lillianite homologous series, SnPb2Bi2S6 can be classified as 4,4L, where the superscripted numbers indicate the maximum numbers of edge-sharing octahedra in the two adjacent NaCl-shaped slabs along the diagonal direction. The obtained SnPb2Bi2S6 phase exhibited good thermal stability up to 1000 K and n-type degenerate semiconducting behavior, with a power factor of 3.7 µW cm-1 K-2 at 773 K. Notably, this compound exhibited a very low lattice thermal conductivity of 0.69-0.92 W m-1 K-1 at 300-1000 K. Theoretical calculations revealed that the low thermal conductivity is caused by the complex crystal structure and the related elastic properties of a low Debye temperature, low phonon velocity, and large Grüneisen parameters. A reasonable figure of merit (ZT) of ∼0.3 was obtained at 770 K.

Effects of temperature and pressure on the optical and vibrational properties of thermoelectric SnSe.

We have conducted a comprehensive investigation of the optical and vibrational properties of the binary semiconductor SnSe as a function of temperature and pressure by means of experimental and ab initio probes. Our high-temperature investigations at ambient pressure have successfully reproduced the progressive enhancement of the free carrier concentration upon approaching the Pnma → Bbmm transition, whereas the pressure-induced Pnma → Bbmm transformation at ambient temperature, accompanied by an electronic semiconductor → semi-metal transition, has been identified for bulk SnSe close to 10 GPa. Modeling of the Raman-active vibrations revealed that three-phonon anharmonic processes dominate the temperature-induced mode frequency evolution. In addition, SnSe was found to exhibit a pressure-induced enhancement of the Born effective charge. Such behavior is quite unique and cannot be rationalized within the proposed effective charge trends of binary materials under pressure.

Approaching Topological Insulating States Leads to High Thermoelectric Performance in n-Type PbTe.

Realizing high thermoelectric performance requires high electrical transport properties and low thermal conductivity, which are essentially determined by balancing the interdependent controversy of carrier mobility, effective mass, and lattice thermal conductivity. Here, we observed an electronic band inversion (approaching topological insulating states) in Sn and Se co-alloyed PbTe, resulting in optimizing effective mass and carrier mobility. The Sn alloying in PbTe(Se) can narrow its band gap due to band inversion and induce a sharper conduction band (equals to lower carrier mass), which further facilitates high carrier mobility, ∼251 cm2 V-1 s-1 in Pb0.89Sn0.11Te0.89Se0.11 at room temperature, thus leading to a high power factor. Meanwhile, we found that the lattice thermal conductivity κl can be reduced from ∼0.77 Wm-1 K-1 in PbTe to ∼0.45 Wm-1 K-1 in (Pb0.91Sn0.09)(Te0.91Se0.09) by producing point defects via Sn and Se co-alloying. Coupling reducing lattice thermal conductivity with integration of optimizing effective mass and carrier mobility by  means of electronic band inversion, we obtained a maximum ZT value ∼1.4 at 773 K in n-type (Pb0.93Sn0.07)(Te0.93Se0.07).

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.

Remarkable Roles of Cu To Synergistically Optimize Phonon and Carrier Transport in n-Type PbTe-Cu2Te.

High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ∼ 1.5 at 723 K and a record high average ZT > 1.0 at 300-873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing Cu2Te inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ∼37 µW cm-1 K-2 at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ∼0.38 Wm-1 K-1 in PbTe-5.5%Cu2Te, approaching the theoretical minimum value of ∼0.36 Wm-1 K-1. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties.

Boosting the Thermoelectric Performance of (Na,K)-Codoped Polycrystalline SnSe by Synergistic Tailoring of the Band Structure and Atomic-Scale Defect Phonon Scattering.

We report the high thermoelectric performance of p-type polycrystalline SnSe obtained by the synergistic tailoring of band structures and atomic-scale defect phonon scattering through (Na,K)-codoping. The energy offsets of multiple valence bands in SnSe are decreased after Na doping and further reduced by (Na,K)-codoping, resulting in an enhancement in the Seebeck coefficient and an increase in the power factor to 492 µW m-1 K-2. The lattice thermal conductivity of polycrystalline SnSe is decreased by the introduction of effective phonon scattering centers, such as point defects and antiphase boundaries. The lattice thermal conductivity of the material is reduced to values as low as 0.29 W m-1 K-1 at 773 K, whereas ZT is increased from 0.3 for 1% Na-doped SnSe to 1.2 for 1% (Na,K)-codoped SnSe.

Unexpected Large Hole Effective Masses in SnSe Revealed by Angle-Resolved Photoemission Spectroscopy.

SnSe has emerged as an efficient thermoelectric material since a high value of the thermoelectric figure of merit (ZT) has been reported recently. Here we show with systematic angle resolved photoemission spectroscopy data that the low-lying electronic structures of undoped and hole-doped SnSe crystals exhibit noticeable temperature variation from 80 to 600 K. In particular, the hole effective masses for the two lowest lying valence band maxima are found to be very large and increase with decreasing temperature. Thermoelectric parameters derived from such hole-mass enhancement agree well with the transport values, indicating comprehensively a reduced impact of multivalley transport to the system's thermoelectric performance.

Mercouri G. Kanatzidis: Excellence and Innovations in Inorganic and Solid-State Chemistry.

Over the last 3-4 decades, solid-state chemistry has emerged as the forefront of materials design and development. The field has revolutionized into a multidisciplinary subject and matured with a scope of new synthetic strategies, new challenges, and opportunities. Understanding the structure is very crucial in the design of appropriate materials for desired applications. Professor Mercouri G. Kanatzidis has encountered both challenges and opportunities during the course of the discovery of many novel materials. Throughout his scientific career, Mercouri and his group discovered several inorganic compounds and pioneered structure-property relationships. We, a few Ph.D. and postdoctoral students, celebrate his 60th birthday by providing a Viewpoint summarizing his contributions to inorganic solid-state chemistry. The topics discussed here are of significant interest to various scientific communities ranging from condensed matter to green energy production.