Ultrahigh Power Factor and Ultralow Thermal Conductivity at Room Temperature in PbSe/SnSe Superlattice: Role of Quantum-Well Effect

Reduced dimension is one of the effective strategies to modulate thermoelectric properties. In this work, n-type PbSe/SnSe superlattices with quantum-well (QW) structure are fabricated by pulsed laser deposition. Here, it is demonstrated that the PbSe/SnSe multiple QW (MQW) shows a high power factor of ≈25.7 µW cm^?1 K^?2 at 300 K, four times larger than that of PbSe single layers. In addition, thermal conductivity falls below 0.32 ± 0.06 W m^?1 K^?1 due to the phonon scattering at interface when the PbSe well thickness is confined within the scale of phonon mean free path (1.8 nm). Featured with ultrahigh power factor and ultralow thermal conductivity, ZT at room temperature is significantly increased from 0.14 for PbSe single layer to 1.6 for PbSe/SnSe MQW.     

Single-Crystal SnSe Thermoelectric Fibers via Laser-Induced Directional Crystallization: From 1D Fibers to Multidimensional Fabrics

Single-crystal tin selenide (SnSe), a record holder of high-performance thermoelectric materials, enables high-efficient interconversion between heat and electricity for power generation or refrigeration. However, the rigid bulky SnSe cannot satisfy the applications for flexible and wearable devices. Here, a method is demonstrated to achieve ultralong single-crystal SnSe wire with rock-salt structure and high thermoelectric performance with diameters from micro- to nanoscale. This method starts from thermally drawing SnSe into a flexible fiber-like substrate, which is polycrystalline, highly flexible, ultralong, and mechanically stable. Then a CO₂ laser is employed to recrystallize the SnSe core to single-crystal over the entire fiber. Both theoretical and experimental studies demonstrate that the single-crystal rock-salt SnSe fibers possess high thermoelectric properties, significantly enhancing the ZT value to 2 at 862 K. This simple and low-cost approach offers a promising path to engage the fiber-shaped single-crystal materials in applications from 1D fiber devices to multidimensional wearable fabrics.     

Controllable Colloidal Synthesis of Tin(II) Chalcogenide Nanocrystals and Their Solution‐Processed Flexible Thermoelectric Thin Films

A systematic colloidal synthesis approach to prepare tin(II, IV) chalcogenide nanocrystals with controllable valence and morphology is reported, and the preparation of solution‐processed nanostructured thermoelectric thin films from them is then demonstrated. Triangular SnS nanoplates with a recently‐reported π‐cubic structure, SnSe with various shapes (nanostars and both rectangular and hexagonal nanoplates), SnTe nanorods, and previously reported Sn(IV) chalcogenides, are obtained using different combinations of solvents and ligands with an Sn⁴⁺ precursor. These unique nanostructures and the lattice defects associated with their Sn‐rich composition allow the production of flexible thin films with competitive thermoelectric performance, exhibiting room temperature Seebeck coefficients of 115, 81, and 153 μV K^?1 for SnS, SnSe, and SnTe films, respectively. Interestingly, a p‐type to n‐type transition is observed in SnS and SnSe due to partial anion loss during post‐synthesis annealing at 500 °C. A maximum figure of merit (ZT) value of 0.183 is achieved for an SnTe thin film at 500 K, exceeding ZT values from previous reports on SnTe at this temperature. Thus, a general strategy to prepare tin(II) chalcogenide nanocrystals is provided, and their potential for use in high‐performance flexible thin film thermoelectric generators is demonstrated.     

Design of Domain Structure and Realization of Ultralow Thermal Conductivity for Record‐High Thermoelectric Performance in Chalcopyrite

Chalcopyrite compound CuGaTe₂ is the focus of much research interest due to its high power factor. However, its high intrinsic lattice thermal conductivity seriously impedes the promotion of its thermoelectric performance. Here, it is shown that through alloying of isoelectronic elements In and Ag in CuGaTe₂, a quinary alloy compound system Cu_1?xAg_xGa_0.4In_0.6Te₂ (0 ≤ x ≤ 0.4) with complex nanosized strain domain structure is prepared. Due to strong phonon scattering mainly by this domain structure, thermal conductivity (at 300 K) drops from 6.1 W m^?1 K^?1 for the host compound to 1.5 W m^?1 K^?1 for the sample with x = 0.4. As a result, the optimized chalcopyrite sample Cu_0.7Ag_0.3Ga_0.4In_0.6Te₂ presents an outstanding performance, with record‐high figure of merit (ZT) reaching 1.64 (at 873 K) and average ZT reaching 0.73 (between ≈300 and 873 K), which are ≈37 and ≈35% larger than the corresponding values for pristine CuGaTe₂, respectively, demonstrating that such domain structure arising from isoelectronic multielement alloying in chalcopyrite compound can effectively suppress its thermal conductivity and elevate its thermoelectric performance remarkably.     

Electrical characterization of nanocrystalline SnSe and ZnSe thin films: effect of annealing

In this paper, experimental data on the electrical properties of as deposited and annealed nanocrystalline SnSe and ZnSe thin films are reported. The thin films of SnSe and ZnSe are deposited on glass substrate by chemical bath deposition method. The films are studied before and after thermal annealing at temperatures 473 K for 1 h. This annealing is done in vacuum of 2?×?10^?3 mbar. The various electrical parameters like dark conductivity, photoconductivity, activation energy, photosensitivity and carrier life time have been measured on these films before and after annealing.     

Engineering Multiple Microstructural Defects for Record-Breaking Thermoelectric Properties of Chalcopyrite Cu1-xAgxGaTe2

Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu_1-xAg_xGaTe₂ (0 ≤ x ≤ 0.5) solid solutions through high-ratio alloying and vibratory ball milling, to achieve ultra-low thermal conductivity and record-breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28 W m⁻¹ K⁻¹ at 300 K and 0.40 W m⁻¹ K⁻¹ at 873 K for the Cu_0.5Ag_0.5GaTe₂ are observed, which are ≈79% and ≈58% lower than that of the CuGaTe₂ matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low-temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu_0.7Ag_0.3GaTe₂ exhibits a record high ZT_max of ≈1.73 at 873 K and ZT_ave of ≈0.69 between 300–873 K, which are the highest values of CuGaTe₂-based thermoelectric materials.     

Synthesis and thermoelectric characterization of bulk-type tellurium nanowire/polymer nanocomposites

A simple method to fabricate three-dimensionally (3-D) aligned thermoelectric nanowires attached polymer particle was demonstrated by combination of solution casting of thermoelectric nanostructures (e.g., tellurium nanowires (Te NWs)) on the surface of thermoplastic polymer (e.g., poly(methyl methacrylate (PMMA)) microbeads followed by hot compaction of thermoplastic matrix. The percolation threshold of composite with 3-D assembled Te NWs (i.e., 3.45 vol%) significantly was lower than that of a randomly dispersed Te NWs (i.e., 5.26 vol%), which resulted in an order of magnitude greater thermoelectric figure of merit (ZT of 2.8 × 10^?3) compared to randomly dispersed Te NWs in PMMA matrix (ZT of 6.4 × 10^?4) at room temperature by enhancing the electrical conductivity without increasing thermal conductivity.     

Extraordinary Thermoelectric Performance Realized in n‐Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p‐type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n‐type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record‐high ZT value of ≈1.83 can be achieved at 773 K in n‐type PbTe‐4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state‐of‐the‐art thermoelectric systems.     

Fine-grained polycrystalline MoTe2 with enhanced thermoelectric properties through iodine doping

Consisting of heavy elements and favorable electronic structure, MoTe₂ has great potential as a good thermoelectric material for heat-to-electricity conversion. While some experimental work has been performed on the p-type version, n-type MoTe₂ is theoretically predicted to have a great conversion efficiency and is crucial for eventual device functionality, yet has not been explored. Here, the preparation and thermoelectric properties of n-type iodine-doped nano-polycrystalline MoTe₂ are currently reported. Nano-polycrystalline MoTe_2???xI_x is obtained by ball milling and spark plasma sintering techniques. The composition, morphology and crystal structure of the prepared materials were analyzed by XRD and FESEM, which indicated a homogeneous single phase. The measured transport properties over the temperature range of 298–823 K indicate that iodine doping greatly enhances the carrier concentration and corresponding power factor, and drastically reducing the thermal conductivity. The ECR (Electrical conductivity ratios) carrier scattering analysis demonstrates that dislocation scattering is the main mechanism throughout the experimental temperature range. With the temperature and doping increasing, the thermal conductivity was reduced rapidly, and the minimum value was 1.19 Wm^??1 K^??1 at 673 K. The maximum value of the figure merit ZT?~?0.16 over 673–750 K, which is much higher than other reported values. These excellent properties imply that MoTe₂ will be an efficient candidate for thermoelectric applications.

     

A facile way to optimize thermoelectric properties of SnSe thin films via sonication-assisted liquid-phase exfoliation

In our work, SnSe nanosheets and nanostructured thin films were successfully synthesized via sonication-assisted exfoliation and coating process. The SnSe nanosheets respond to a uniform lateral size, with two to three single layers by 2.82 nm and 280 nm² of average thickness and average area, respectively. The results were confirmed by Scanning Electron Microscope, Transmission Electron Microscope, and Atomic Force Microscope. X-ray diffraction and Raman spectra indicate that the SnSe nanosheets have high crystalline quality along a-axis. The SnSe nanostructured thin films were prepared in various thicknesses from 350 to 650 nm. The highest power factor value is achieved at 450 nm in 375–600 K temperature range. A simple method of fabrication and controllable thermoelectric properties of SnSe nanostructured thin films as well as other two-dimensional (2D) materials are introduced.

     

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃Br . The doped polymer PDPP - EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm⁻¹ but low Seebeck coefficient below 30 µV K⁻¹ and a maximum power factor of 59 ± 10 µW m⁻¹ K⁻². Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃Br into PDPP - EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m⁻¹ K⁻² and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP - EDOT by DPPNMe₃Br is mainly attributed to the increase of energetic disorder for PDPP - EDOT .     

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO₃ thin films. Depending on the substrate, the lattice of epitaxial WO₃ expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, instead of the defect concentration, plays the dominant role in determining the thermal conductivity. In particular, the thermal conductivity increases significantly with proton intercalation, which is contrary to the expectation that point defects typically lower the lattice thermal conductivity. The thermal conductivity can be dynamically varied by a factor of ≈ 1.7 via electrolyte gating, and tuned over a larger range, from 7.8 to 1.1 W m^?1 K^?1, by adjusting the oxygen pressure during film growth. The electrolyte‐gating‐induced changes in thermal conductivity and lattice dimensions are reversible through multiple cycles. These findings not only expand the basic understanding of thermal transport in complex oxides, but also provide a path to dynamically control the thermal conductivity.     

Thermoelectric behavior of aerogels based on graphene and multi-walled carbon nanotube nanocomposites

In this work, we presented that the Seebeck coefficient and electrical conductivity can be increased simultaneously in aerogels based on graphene and multi-walled carbon nanotube (graphene-MWCNT) nanocomposites, and at the same time the thermal conductivity is depressed due to 3D porous skeleton structure. As a result, graphene-MWCNT aerogels possess ultra-low thermal conductivities (∼0.056 W m⁻¹ K⁻¹) and apparent density (∼24 kg m⁻³), thereafter the figure of merit (ZT) of ∼0.001 is achieved. Although the ZT value is too low for practical application as a thermoelectric (TE) material, the unique structure in this project provides a potential way to overcome the challenge in bulk semiconductors that increasing electrical conductivity generally leads to decreased Seebeck coefficient and enhanced thermal conductivity.     

High Thermoelectric Performance in n-Type Perylene Bisimide Induced by the Soret Effect

Low-cost, non-toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n-type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n-type organic thermoelectrics to date. An organic mixed ion–electron n-type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi-frozen ionic carriers yield a large ionic Seebeck coefficient of −3021 μV K⁻¹, while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm⁻¹ at 60% relative humidity. The overall power factor is remarkably high (165 μW m⁻¹ K⁻²), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi-constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.     

Performance of Bi–Te–Sb–Se Thermoelectric Material at Low Temperature

Thermoelectric materials are of interest for applications as heat pump and power generators. The performance of a thermoelectric material, the figure of merit, ZT, is measured. The figure of merit is interrelated to the thermal conductivity, electrical conductivity, and Seebeck coefficient. All of these parameters are functions of temperature. The performance of a Bi–Te–Sb–Se thermoelectric material at low temperature was studied experimentally in this work. Based on the experimental results, the relation between various parameters and temperature, and the figure of merit are reported. The conclusions indicate that this thermoelectric material is not suitable for power generation at low temperature, and only an improvement of production technology or the development of a new production method can improve the electrical power generation performance with this method.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Thermoelectric Effects of Nanogaps between Two Tips

This study designs a microscaled thermoelectric component featuring a nanogap of varying size (133–900 nm) between the tips of the component. Electricity and heat are transmitted between the gap of the tips through the thermionic emission of electrons. Because the gaps exhibit a discontinuous structure, the phonon's contribution to thermal conductivity can be virtually neglected, thereby enhancing the thermoelectric figure of merit (ZT) of the designed thermoelectric component. The experimental results reveal that a narrow tip gap generates stronger thermoelectric effects, with Seebeck voltage and Seebeck coefficient being respectively, one and two orders of magnitude greater than those of the thermoelectric effects of nanowires. The thermoelectric figure of merit without considering the contributions from other heat carriers is higher than the value of thermoelectric devices developed in recent years. For a set of asymmetrical thin film electrodes of differing sizes, the thermoelectric effects generated in the heating process of large thin films are stronger than those of small thin films. Furthermore, adding nanoparticles to the nanogap facilitate the thermionic emission of electrons, in which electrons hop from the hot end to the cold end, thereby intensifying the thermoelectric effects of the nanogap.     

Realizing zT of 2.3 in Ge1−x−ySbxInyTe via Reducing the Phase‐Transition Temperature and Introducing Resonant Energy Doping

GeTe with rhombohedral‐to‐cubic phase transition is a promising lead‐free thermoelectric candidate. Herein, theoretical studies reveal that cubic GeTe has superior thermoelectric behavior, which is linked to (1) the two valence bands to enhance the electronic transport coefficients and (2) stronger enharmonic phonon–phonon interactions to ensure a lower intrinsic thermal conductivity. Experimentally, based on Ge_1?xSb_xTe with optimized carrier concentration, a record‐high figure‐of‐merit of 2.3 is achieved via further doping with In, which induces the distortion of the density of states near the Fermi level. Moreover, Sb and In codoping reduces the phase‐transition temperature to extend the better thermoelectric behavior of cubic GeTe to low temperature. Additionally, electronic microscopy characterization demonstrates grain boundaries, a high‐density of stacking faults, and nanoscale precipitates, which together with the inevitable point defects result in a dramatically decreased thermal conductivity. The fundamental investigation and experimental demonstration provide an important direction for the development of high‐performance Pb‐free thermoelectric materials.     

Thermoelectric properties of p-type semiconductors copper chromium disulfide CuCrS2+x

A series of bulk samples CuCrS_2+x (x = 0, 0.01, 0.02, 0.06, 0.10) were prepared by combining mechanical alloying and spark plasma sintering. The effect of excessive sulfur content on the phase structure, microstructure, and thermoelectric and optical properties was investigated. The excessive sulfur initially entered into the lattice sites and then into the lattice interstices. A direct band gap semiconductor for CuCrS₂ material with an optical band gap of about 2.48 eV was proved. An improved electrical conductivity 2980 S m^?1 at 673 K reached along with an inversely varied Seebeck coefficient as increasing x value, which showed a maximum power factor of 104 μ W m^?1 K^?2 at 673 K for CuCrS_2.01 sample. In addition to the low thermal conductivity between 0.48 and 1.02 W m^?1 K^?1 in the whole temperature range, a peak ZT of 0.15 was achieved at 673 K for CuCrS_2.01 bulk sample, which was 36 % higher than that (0.11) of the CuCrS_2.00.     

Thermoelectric and mechanical properties of the Bi0.5Sb1.5Te3 solid solution prepared by melt spinning

This paper reports the preparation and characterization of pressed microcrystalline materials based on a p-type Bi_0.5Sb_1.5Te₃ solid solution produced from a melt-spun powder. We have examined the effect of melt spinning conditions (temperature, disk rotation rate, and purity of the inert gas in the heat treatment chamber) on the particle size and morphology of the powders and the microstructure and thermoelectric properties of hot-pressed samples and investigated the mechanical properties (compression and bend tests) of materials prepared by various methods. The thermoelectric properties of the materials (thermopower, electrical conductivity, and thermal conductivity) were studied at room temperature and in the range 100–700 K. The highest thermoelectric figure of merit ZT of the materials prepared by pressing the melt-spun powder was 1.3, whereas the ZT of the materials prepared by the other methods did not exceed 1.1. The higher ZT of the materials studied was due to their lower lattice thermal conductivity and slightly higher thermopower.     

Improved thermoelectric properties of La-doped Bi2Sr2Co2O9-layered misfit oxides

Bi₂Sr_2−x La _x Co₂O₉ (x = 0, 0.02, 0.04, and 0.08) polycrystalline-layered misfit oxides have been prepared by solid-state reactions. Electrical property measurements indicated that all the samples are p-type semiconductors. The electrical conductivity decreased and the Seebeck coefficient increased with increasing temperature. The thermal conductivities were very low, only 0.6–0.7 W m⁻¹ K⁻¹ at room temperature. La doping was effective in increasing the Seebeck coefficient, reducing the thermal conductivity, and hence improving the thermoelectric performance. A highest dimensionless figure of merit ZT of 0.147 was obtained for Bi₂Sr_1.96La_0.04Co₂O₉ sample at 737 K, about two times higher than that of the sample without La doping.