Realizing high-efficiency thermoelectric module by suppressing donor-like effect and improving preferred orientation in n-type Bi2(Te, Se)3.

Thermoelectric materials have a wide range of application because they can be directly used in refrigeration and power generation. And the Bi2Te3 stand out because of its excellent thermoelectric performance and are used in commercial thermoelectric devices. However, n-type Bi2Te3 has seriously hindered the development of Bi2Te3-based thermoelectric devices due to its weak mechanical properties and inferior thermoelectric performance. Therefore, it is urgent to develop a high-performance n-type Bi2Te3 polycrystalline. In this work, we employed interstitial Cu and the hot deformation process to optimize the thermoelectric properties of Bi2Te2.7Se0.3, and a high-performance thermoelectric module was fabricated based on this material. Our combined theoretical and experimental effort indicates that the interstitial Cu reduce the defect density in the matrix and suppresses the donor-like effect, leading to a lattice plainification effect in the material. In addition, the two-step hot deformation process significantly improves the preferred orientation of the material and boosts the mobility. As a result, a maximum ZT of 1.27 at 373 K and a remarkable high ZTave of 1.22 across the temperature range of 300-425 K are obtained. The thermoelectric generator (TEG, 7-pair) and thermoelectric cooling (TEC, 127-pair) modules were fabricated with our n-type textured Cu0.01Bi2Te2.7Se0.3 coupled with commercial p-type Bi2Te3. The TEC module demonstrates superior cooling efficiency compared with the commercial Bi2Te3 device, achieving a ΔT of 65 and 83.4 K when the hot end temperature at 300 and 350 K, respectively. In addition, the TEG module attains an impressive conversion efficiency of 6.5% at a ΔT of 225 K, which is almost the highest value among the reported Bi2Te3-based TEG modules.

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.

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.

Enhanced Thermoelectric Performance and Low Thermal Conductivity in Cu2GeTe3 with Identified Localized Symmetry Breakdown.

Highly efficient and eco-friendly thermoelectric generators rely on low-cost and nontoxic semiconductors with high symmetry and ultralow lattice thermal conductivity κL. We report the rational synthesis of the novel cubic (Ag, Se)-doped Cu2GeTe3 semiconductors. A localized symmetry breakdown (LSB) was found in the composition of Cu1.9Ag0.1GeTe1.5Se1.5 (i.e., CAGTS15) with an ultralow κL of 0.37 W/mK at 723 K, the lowest value outperforming all Cu2GeCh3 (Ch = S, Se, and Te). A joint investigation of synchrotron X-ray techniques identifies the LSB embedded into the cubic CAGTS15 host matrix. This LSB is an Ångström-scale orthorhombic symmetry unit, characteristic of multiple bond lengths, large anisotropic atomic displacements, and distinct local chemical coordination of anions. Computational results highlight that such an unusual orthorhombic symmetry demonstrates low-frequency phonon modes, which become softer and more predominant with increasing temperatures. This unconventional LSB promotes bond complexity and phonon scattering, highly beneficial for extraordinarily low lattice thermal conductivity.

Fine Tuning of Defects Enables High Carrier Mobility and Enhanced Thermoelectric Performance of n-Type PbTe.

High carrier mobility is critical to improving thermoelectric performance over a broad temperature range. However, traditional doping inevitably deteriorates carrier mobility. Herein, we develop a strategy for fine tuning of defects to improve carrier mobility. To begin, n-type PbTe is created by compensating for the intrinsic Pb vacancy in bare PbTe. Excess Pb2+ reduces vacancy scattering, resulting in a high carrier mobility of ∼3400 cm2 V-1 s-1. Then, excess Ag is introduced to compensate for the remaining intrinsic Pb vacancies. We find that excess Ag exhibits a dynamic doping process with increasing temperatures, increasing both the carrier concentration and carrier mobility throughout a wide temperature range; specifically, an ultrahigh carrier mobility ∼7300 cm2 V-1 s-1 is obtained for Pb1.01Te + 0.002Ag at 300 K. Moreover, the dynamic doping-induced high carrier concentration suppresses the bipolar thermal conductivity at high temperatures. The final step is using iodine to optimize the carrier concentration to ∼1019 cm-3. Ultimately, a maximum ZT value of ∼1.5 and a large average ZT ave value of ∼1.0 at 300-773 K are obtained for Pb1.01Te0.998I0.002 + 0.002Ag. These findings demonstrate that fine tuning of defects with <0.5% impurities can remarkably enhance carrier mobility and improve thermoelectric performance.

Realizing high-ranged thermoelectric performance in PbSnS2 crystals.

Great progress has been achieved in p-type SnS thermoelectric compound recently, while the stagnation of the n-type counterpart hinders the construction of thermoelectric devices. Herein, n-type sulfide PbSnS2 with isostructural to SnS is obtained through Pb alloying and achieves a maximum ZT of ~1.2 and an average ZT of ~0.75 within 300-773 K, which originates from enhanced power factor and intrinsically ultralow thermal conductivity. Combining the optimized carrier concentration by Cl doping and enlarged Seebeck coefficient through activating multiple conduction bands evolutions with temperature, favorable power factors are maintained. Besides, the electron doping stabilizes the phase of PbSnS2 and the complex-crystal-structure induced strong anharmonicity results in ultralow lattice thermal conductivity. Moreover, a maximum power generation efficiency of ~2.7% can be acquired in a single-leg device. Our study develops a n-type sulfide PbSnS2 with high performance, which is a potential candidate to match the excellent p-type SnS.

Multiple valence bands convergence and strong phonon scattering lead to high thermoelectric performance in p-type PbSe.

Thermoelectric generators enable the conversion of waste heat to electricity, which is an effective way to alleviate the global energy crisis. However, the inefficiency of thermoelectric materials is the main obstacle for realizing their widespread applications and thus developing materials with high thermoelectric performance is urgent. Here we show that multiple valence bands and strong phonon scattering can be realized simultaneously in p-type PbSe through the incorporation of AgInSe2. The multiple valleys enable large weighted mobility, indicating enhanced electrical properties. Abundant nano-scale precipitates and dislocations result in strong phonon scattering and thus ultralow lattice thermal conductivity. Consequently, we achieve an exceptional ZT of ~ 1.9 at 873 K in p-type PbSe. This work demonstrates that a combination of band manipulation and microstructure engineering can be realized by tuning the composition, which is expected to be a general strategy for improving the thermoelectric performance in bulk materials.

Long-range ordering and local structural disordering of BiAgSe2 and BiAgSeTe thermoelectrics.

Thermoelectric materials are promising for energy harvesting using waste heat. The thermal management of the thermoelectric materials attract scientific and technological interests. The narrow bandgap semiconductor BiAgSe2 is a good candidate for thermoelectric materials due to its ultralow thermal conductivity. The mother compound BiAgSe2 crystallizes in hexagonal symmetry at room temperature, but experiences structural transitions to cubic phase at high temperature. By contrast, the daughter compound BiAgSeTe exhibits long range ordering and crystallizes into cubic phase at room temperature. Nevertheless, the local structural disorderings due to the Bi3+ and Ag+ anti-site defects, as well as local structural distortions, are ubiquitous in both parent BiAgSe2 and BiAgSeTe. BiAgSeTe exhibits distinct transport properties owing to the disordering-induced drastic changes in the electronic band structure, as well as the scattering dictated by the point defects. It is suggested that BiAgSe2 and BiAgSeTe could be good candidates for phonon glass and crystal glass (PGEC)-type thermoelectrics.

Unveiling the atomic defects and electronic structure of Cu2.2Zn0.8SnSe4-xTex (x = 0 to 0.04) by X-ray absorption fine structure spectroscopy.

The quaternary chalcogenides consisting of earth-abundant elements such as Cu2ZnSnSe4 (CZTSe) have promising electrical and optical properties prompting enormous technological interest. Understanding different types of defects including Cu/Zn ordering is believed to be the key point to tackle technological challenges such as a large open circuit voltage deficit in CZTSe. The Te doped Cu2.2Zn0.8SnSe4-xTex (x = 0.01-0.04) were investigated using X-ray absorption fine structure spectroscopy at the Cu, Zn, and Se K-edges as well as at the Te L3-edge. Cu at the zinc site with anti-site defects and oxygen interstitials are identified. The detailed electronic structure upon Te doping is studied, providing insights into the rich defect chemistry present in this compound.

High thermoelectric performance in low-cost SnS0.91Se0.09 crystals.

Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (µ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (µW cm-1 K-2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZT max) of ~1.6 at 873 K and an average ZT (ZT ave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.

Lattice Dynamics and Thermal Conductivity in Cu2Zn1- xCo xSnSe4.

The quaternary compound Cu2ZnSnSe4 (CZTSe), as a typical candidate for both solar cells and thermoelectrics, is of great interest for energy harvesting applications. Materials with a high thermoelectric efficiency have a relatively low thermal conductivity, which is closely related to their chemical bonding and lattice dynamics. Therefore, it is essential to investigate the lattice dynamics of materials to further improve their thermoelectric efficiency. Here we report a lattice dynamic study in a cobalt-substituted CZTSe system using temperature-dependent X-ray absorption fine structure spectroscopy (TXAFS). The lattice contribution to the thermal conductivity is dominant, and its reduction is mainly ascribed to the increment of point defects after cobalt substitution. Furthermore, a lattice dynamic study shows that the Einstein temperature of atomic pairs is reduced after cobalt substitution, revealing that increasing local structure disorder and weakened bonding for each of the atomic pairs are achieved, which gives us a new perspective for understanding the behavior of lattice thermal conductivity.

Chemical speciation of lead in secondary fly ash using X-ray absorption spectroscopy.

In this study, fly ash samples were collected from bag houses in a Chinese municipal solid waste incinerator (MSWI) and secondary fly ash (SFA) samples were collected from a high-temperature tubular electric furnace by thermal treatment of MSWI fly ash at 1050, 1100, 1150, 1200, and 1250 °C.We determined the speciation and atomic coordinates of lead in SFA using X-ray absorption spectroscopy (XAS) techniques. The results obtained by X-ray absorption near edge structure (XANES) spectra revealed that the mass fraction of PbO in MSWI fly ash was 57.9% (wt %) while PbCl2 and PbS were the dominant species in SFA. Extended X-ray absorption fine structure (EXAFS) data analysis indicated the atomic coordinates of Pb were proportional to the weights of PbCl2 and PbS, in good agreement with the XANES spectra. These findings highlight lead evaporation processes in the MSWI fly ash during heat treatment and provide a method for consistent speciation analysis of environmental samples using XAS.

Nanoscale heterogeneity in thermoelectrics: the occurrence of phase separation in Fe-doped Ca3Co4O9.

The misfit layered cobaltate thermoelectrics are good candidates for high temperature thermoelectric applications. Ca3Co4O9 is a typical compound of this family, which consists of rock salt Ca2CoO3 slabs alternating with hexagonal CoO2 slabs with a large lattice mismatch along the b axis. Each slab is 0.3-0.5 nm thick and shows an inherent structural heterogeneity at the nanoscale. The latter is a key parameter that affects the electrical transport and the heat flow in these misfit structured thermoelectrics. To clarify the physical origin of the thermoelectric performance of iron doped Ca3Co4O9 we combined X-ray near-edge absorption spectroscopy (XANES) and quantum modeling using density functional theory. In contrast to single-site doping, the iron doping first occurs at the Co1 site of the rock salt slab at low doping while at higher doping it prefers the Ca1 site of the rock salt slab. Doping at the Ca1 site modifies the electronic structure tuning the nanoscale structural heterogeneity. This mechanism may open a new route to optimizing the thermoelectric performance of misfit layered thermoelectrics.

Enhanced Thermoelectricity in High-Temperature ß-Phase Copper(I) Selenides Embedded with Cu2Te Nanoclusters.

We report remarkably enhanced thermoelectric performance of Te doped Cu2Se in midtemperature range. Through ball-milling process followed by spark plasma sintering (SPS), nanoscale Cu2Te clusters were embeded in the matrix of Cu2Se, inducing a drastic enhancement of thermoelectric performance by reducing the thermal conductivity without degrading the power factor. A large ZT value of 1.9 was achieved at 873 K for Cu2Se1.9Te0.1, which is about 2 times larger than that of the pure Cu2Se. The nanoscale heat management by Cu2Te nanoclusters in superionic conductors opens up an avenue for thermoelectric materials research.