Modulation Doping Enables Ultrahigh Power Factor and Thermoelectric ZT in n-Type Bi2 Te2.7 Se0.3

Grain Manipulation by Annealing Treatment Realizes High-Performance N-Type Bi2Te2.4Se0.6 Thermoelectric Material and Device

Bi2Te3-based materials play a crucial role in solid cooling and power generation, but the rapidly deteriorated ZT with rising temperatures above 450 K severely limits further applications. Here, this paper reports a novel preparation method of annealing treatment for molten ingot, which can enhance the thermoelectric performance of n-type Bi2Te2.4Se0.6 in a wide temperature range. Instead of conventional halides, copper is adopted to regulate the carrier concentration and grain size to optimal levels. During the process of annealing at 573 K for 4 h, the number of twins significantly increases and the grains of Cu-doped samples become larger and more oriented. These optimizations lead to higher carrier mobility with similar carrier concentration compared with the sample without heat treatment. The synergistic effects of Cu doping and annealing treatment realize a high average ZT of 0.89 within 300-600 K in n-type Cu0.02Bi2Te2.4Se0.6. Combined with p-type (Bi,Sb)2Te3, the fabricated thermoelectric device exhibits a high conversion efficiency of 6.9% at a temperature difference of 300 K. This study suggests that annealing treatment is a simple and effective scheme to promote the applications of n-type Bi2(Te,Se)3 in a wide temperature range.

Synergistic Improvement of BiI3 and In on Thermoelectric Properties of Zone-Melted n-Type Bi2Te2.7Se0.3

Bismuth telluride (Bi2Te3) is the only commercial thermoelectric material so far, and it is also the best thermoelectric material with the best performance at room temperature. However, up to now, the zT value of n-type materials used on a large scale is only about 1.0; this makes the thermoelectric conversion efficiency of thermoelectric devices and thermoelectric applications stagnant. Therefore, under the synergistic action of BiI3 and In, the properties of n-type Bi2Te2.7Se0.3 material are improved. The experiments show that BiI3, which is nontoxic and non-absorbent, can effectively improve the power factor of the material and inhibit the bipolar effect and is an effective dopant. After the inclusion of In, due to the low bond energy of the In-Te bond, it is easy to form the InTe phase in the matrix material and then introduce the second phase, and the presence of the second phase in the material will scatter phonons and reduce the lattice thermal conductivity so that it can reach 0.31 W m-1 K-1 at 350 K. Ultimately, a high maximum zT of 1.20 at 325 K and a remarkable average zT of 1.04 (300-500 K) are attained in the In0.005Bi1.995Te2.7Se0.3 + 0.13 wt % BiI3 sample.

Chiral Twist Interface Modulation Enhances Thermoelectric Properties of Tellurium Crystal

Manipulating the grain boundary and chiral structure of enantiomorphic inorganic thermoelectric materials facilitates a new degree of freedom for enhancing thermoelectric energy conversion. Chiral twist mechanisms evolve by the screw dislocation phenomenon in the nanostructures; however, contributions of such chiral transport have been neglected for bulk crystals. Tellurium (Te) has a chiral trigonal crystal structure, high band degeneracy, and lattice anharmonicity for high thermoelectric performance. Here, Sb-doped Te crystals are grown to minimize the severe grain boundary effects on carrier transport and investigate the interface of chiral Te matrix and embedded achiral Sb2Te3 precipitates, which induce unusual lattice twists. The low grain boundary scattering and conformational grain restructuring provide electrical-favorable semicoherent interfaces. This maintains high electrical conductivity leading to a twofold increase in power factor compared to polycrystal samples. The embedded Sb2Te3 precipitates concurrently enable moderate phonon scattering leading to a remarkable decrease in lattice thermal conductivity and a high dimensionless figure of merit (zT) of 1.1 at 623 K. The crystal growth and chiral atomic reorientation unravel the emerging benefits of interface engineering as a crucial contributor to effectively enhancing carrier transport and minimizing phonon propagation in thermoelectric materials.

Bi2Te3-Based Thermoelectric Modules for Efficient and Reliable Low-Grade Heat Recovery

Bismuth-telluride-based alloy has long been considered as the most promising candidate for low-grade waste heat power generation. However, optimizing the thermoelectric performance of n-type Bi2Te3 is more challenging than that of p-type counterparts due to its greater sensitivity to texture, and thus limits the advancement of thermoelectric modules. Herein, the thermoelectric performance of n-type Bi2Te3 is enhanced by incorporating a small amount of CuGaTe2, resulting in a peak ZT of 1.25 and a distinguished average ZT of 1.02 (300-500 K). The decomposed Cu+ strengthens interlayer interaction, while Ga+ optimizes carrier concentration within an appropriate range. Simultaneously, the emerged numerous defects, such as small-angle grain boundaries, twin boundaries, and dislocations, significantly suppresses the lattice thermal conductivity. Based on the size optimization by finite element modelling, the constructed thermoelectric module yields a high conversion efficiency of 6.9% and output power density of 0.31 W cm-2 under a temperature gradient of 200 K. Even more crucially, the efficiency and output power little loss after subjecting the module to 40 thermal cycles lasting for 6 days. This study demonstrates the efficient and reliable Bi2Te3-based thermoelectric modules for broad applications in low-grade heat harvest.

Enhancement of ZT in Bi0.5Sb1.5Te3 Thin Film through Lattice Orientation Management

Thermoelectric power can convert heat and electricity directly and reversibly. Low-dimensional thermoelectric materials, particularly thin films, have been considered a breakthrough for separating electronic and thermal transport relationships. In this study, a series of Bi0.5Sb1.5Te3 thin films with thicknesses of 0.125, 0.25, 0.5, and 1 μm have been fabricated by RF sputtering for the study of thickness effects on thermoelectric properties. We demonstrated that microstructure (texture) changes highly correlate with the growth thickness in the films, and equilibrium annealing significantly improves the thermoelectric performance, resulting in a remarkable enhancement in the thermoelectric performance. Consequently, the 0.5 μm thin films achieve an exceptional power factor of 18.1 μWcm-1K-2 at 400 K. Furthermore, we utilize a novel method that involves exfoliating a nanosized film and cutting with a focused ion beam, enabling precise in-plane thermal conductivity measurements through the 3ω method. We obtain the in-plane thermal conductivity as low as 0.3 Wm-1K-1, leading to a maximum ZT of 1.86, nearing room temperature. Our results provide significant insights into advanced thin-film thermoelectric design and fabrication, boosting high-performance systems.

Comparison of influence of intercalation and substitution of Cu on electrical and thermoelectric transport properties of InSe alloys

Layered post-transition-metal chalcogenides, such as InSe, In4Se3, SnSe, and SnSe2, have recently been investigated as semiconducting electronic materials and thermoelectric materials owing to their adjustable electrical transport properties either by doping or alloying. Herein, the influence of intercalation doping and substitutional doping of Cu in layered InSe alloys on electrical and thermoelectric transport properties was investigated and compared by synthesizing varied compositions of CuxInSe and In1-yCuySe. It was found that Cu was intercalated in CuxInSe samples (x = 0.01 and 0.02) and behaved as an electron donor, resulting in an increase in the electron concentration and a decrease in the activation energy. Therefore, the power factor of CuxInSe samples was increased compared to that of InSe. In contrast, the substituted Cu in the In site of In1-yCuySe samples (y = 0.01 and 0.02) acted as an acceptor, and the power factor decreased owing to a decrease in the electron concentration and activation energy. Moreover, a decrease in thermal conductivity was seen for CuxInSe and In1-yCuySe samples due to increased phonon scattering after the addition of Cu. Consequently, an enhanced thermoelectric figure of merit (zT) was only observed for intercalated CuxInSe samples due to the increased power factor and decreased thermal conductivity, while substituted In1-yCuySe samples only show degraded zT. A maximum zT value of 0.062 was observed for the CuxInSe (x = 0.02) sample at 700 K, which showed a 77% enhancement compared to that of InSe.

Enhanced Thermoelectric Performance of Mg-Sn Thin Films: Role of Mg9Sn5 Phase and One-Dimensional Electronic Structure

Mg-Sn alloy thin films have garnered significant attention for their outstanding thermoelectric (TE) properties and cost-effective elemental composition, making them potential candidates for wearable energy harvesting devices. While previous studies have explored the properties of these thin films, limited research has been conducted to identify physical factors that can further enhance their performance. In this study, we present a novel approach utilizing a convenient electron beam coevaporation technique to fabricate Mg-Sn alloy thin films. Experimental results revealed that controlling the tin content in the Mg-Sn thin films at 38.9% led to the formation of a mixed-phase structure, comprising Mg2Sn and Mg9Sn5. This dual-phase structure exhibited a notable advantage in enhancing the TE performance. The presence of the Mg9Sn5 phase significantly increased the carrier concentration, while maintaining the original Seebeck coefficient and mobility, thereby improving the conductivity of Mg2Sn. Theoretical calculations indicated that the Mg9Sn5 phase displayed 1D-like characteristics, leading to a highly effective valley degeneracy and consequently a high power factor. Overall, this work introduces a promising approach to fabricate high-performance Mg-Sn alloy thin films through electron beam coevaporation, opening up possibilities for their application in wearable energy harvesting devices.

Hierarchical Phonon Scattering from Nano to Macro Scale in Ag-Nano/TiO2-Micro Particle-Decorated p-type Bismuth Telluride Bulk Composites

We study the thermoelectric properties of a p-type Bi0.4Sb1.6Te3.4 (BST) composite with Ag nanoparticle-decorated TiO2 microparticles (US-Ag/TiO2). The dispersion of US-Ag/TiO2 particles, synthesized by an ultrasonication (US) method, into the matrix effectively decreases lattice and bipolar thermal conductivity, attributed to the scattering centers formed at nano and micro scales. The electron backscattering diffraction (EBSD) measurements revealed smaller grain sizes within the BST composite when paired with the US-Ag/TiO2 particle dispersion. These reduced grain sizes, alongside nanoparticle-decorated microparticles dispersed throughout the matrix, scatter phonons effectively from long- to short-wavelength phonons and subsequently decrease lattice thermal conductivity. While the power factors of the composites are reduced, significant suppression of lattice and bipolar thermal conductivity has led to an increase in the maximum zT value (1.4 at 325 K) for a 0.9 wt % US-Ag/TiO2 particle dispersion within the BST matrix. This particle dispersion in the BST composite consistently demonstrates a high zT value across an extensive temperature spectrum, leading to an exceptionally high average zTavg value (1.38 up to 400 K), which is superior to the other values from reported BST composites. Thus, this research indicates that the dispersion of nanoparticle-decorated microparticles within a thermoelectric material matrix can significantly improve thermoelectric performance, which has promising implications for practical applications in thermoelectric cooling and sustainable and economical energy harvesting technologies.

Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering

Layered Bi-misfit cobaltates, such as Bi₂Sr₂Co₂O_y, are the natural superlattice of an electrically insulating rocksalt (RS) type Bi₂Sr₂O₄ layer and electrically conducting CoO₂ layer, stacked along the crystallographic c-axis. RS and CoO₂ layers are related through charge compensation reactions (or charge transfer). Therefore, thermoelectric transport properties are affected when doping or substitution is carried out in the RS layer. In this work, we have shown improved thermoelectric properties of spark plasma sintered Bi₂Sr_2-xCa_xCo₂O_y alloys (x = 0, 0.3 and 0.5). The substitution of Ca atoms affects the thermal properties by introducing point-defect phonon scattering, while the electronic conductivity and thermopower remain unaltered.

Fine-Tuning Bi2Te3-Copper Selenide Alloys Enables an Efficient n-Type Thermoelectric Conversion

Bismuth tellurides is one of the most promising thermoelectric (TE) material candidates in low-temperature application circumstances, but the n-type thermoelectric property is relatively low compared to the p-type counterpart and still needs to be improved. Herein, we incorporated different copper selenides (CuSe, Cu₃Se₂ and Cu_2-xSe) into a Bi₂Te₃ matrix to create the alloy by grinding and successive sintering to enable higher thermoelectric performance. The results demonstrated that all alloys achieved n-type TE characteristics and Bi₂Te₃-CuSe exhibited the best Seebeck coefficient and power factor among them. Along with the low thermal conductivity, the maximum dimensionless TE figure of merit (ZT) value of 1.64 at 573 K was delivered for Bi₂Te₃-CuSe alloy, which is among the best reported results in the n-type Bi₂Te₃-based TE materials to the best of our knowledge. The improved TE properties should be related to the co-doping process of Se and Cu. Our investigation shows a new method to enhance the performance of n-type TE materials by appropriate co-doping or alloying.