Exceptional thermoelectric properties of flexible organic-inorganic hybrids with monodispersed and periodic nanophase

A Strategy for Fabricating Ultra-Flexible Thermoelectric Films Using Ag2Se-Based Ink

Flexible thermoelectric materials have drawn significant attention from researchers due to their potential applications in wearable electronics and the Internet of Things. Despite many reports on these materials, it remains a significant challenge to develop cost-effective methods for large-scale, patterned fabrication of materials that exhibit both excellent thermoelectric performance and remarkable flexibility. In this study, we have developed an Ag2Se-based ink with excellent printability that can be used to fabricate flexible thermoelectric films by screen printing and low-temperature sintering. The printed films exhibit a Seebeck coefficient of -161 μV/K and a power factor of 3250.9 μW/m·K2 at 400 K. Moreover, the films demonstrate remarkable flexibility, showing minimal changes in resistance after being bent 5000 times at a radius of 5 mm. Overall, this research offers a new opportunity for the large-scale patterned production of flexible thermoelectric films.

High Power Factor Flexible Ag2Te Film on Nylon by a Wet Chemical Method for Power Generator

Herein, we develop a facile wet chemical method for the synthesis of Ag2Te powders at room temperature and flexible Ag2Te/nylon thermoelectric (TE) films are prepared by vacuum-assisted filtration of the synthesized Ag2Te powders and then hot pressing. Because of the good crystallinity of Ag2Te grains and continuous grain boundaries, an optimized film exhibits a power factor of 513 μW m-1 K-2 at 300 K, which stands among the highest values reported for Ag2Te-based films to date. In addition, the film also has good flexibility. A four-leg flexible TE device assembled with the film generates a power density of 5.46 W m-2 at a temperature gradient of 31.8 K. This work provides a facile and environmentally friendly method for preparing flexible Ag2Te films.

One Stone, Three Birds: Feasible Tuning of Barrier Heights Induced by Hybridized Interface in Free-Standing PEDOT@Bi2Te3 Thermoelectric Films

Converting low-grade thermal energy into electrical energy is crucial for the development of modern smart wearable energy technologies. The free-standing films of PEDOT@Bi2Te3 prepared by tape-casting hold promise for flexible thermoelectric technology in self-powered sensing applications. Bi2Te3 nanosheets fabricated by the solvothermal method are tightly connected with flat-arranged PEODT molecules, forming an S-Bi bonded interface in the composite materials, and the bandgap is reduced to 1.63 eV. Compared with the PEDOT film, the mobility and carrier concentration of the composite are significantly increased at room temperature, and the conductivity reaches 684 S/cm. Meanwhile, the carrier concentration decreased sharply at 360 K indicating the creation of defect energy levels during the interfacial reaction of the composites, which increased the Seebeck coefficient. The power factor was improved by 68.9% compared to PEDOT. In addition, the introduction of Bi2Te3 nanosheets generated defects and multidimensional interfaces in the composite film, which resulted in weak phonon scattering in the conducting polymer with interfacial scattering. The thermal conductivity of the film is decreased and the ZT value reaches 0.1. The composite film undergoes 1500 bending cycles with a 14% decrease in conductivity and has good flexibility. This self-supporting flexible thermoelectric composite film has provided a research basis for low-grade thermal energy applications.

High-Performance n-Type Organic Thermoelectrics with Exceptional Conductivity by Polymer-Dopant Matching

Achieving high electrical conductivity (σ) and power factor (PF) simultaneously remains a significant challenge for n-type organic themoelectrics (OTEs). Herein, we demonstrate the state-of-the-art OTEs performance through blending a fused bithiophene imide dimer-based polymer f-BTI2g-SVSCN and its selenophene-substituted analogue f-BSeI2g-SVSCN with a julolidine-functionalized benzimidazoline n-dopant JLBI, vis-à-vis when blended with commercially available n-dopants TAM and N-DMBI. The advantages of introducing a more lipophilic julolidine group into the dopant structure of JLBI are evidenced by the enhanced OTEs performance that JLBI-doped films show when compared to those doped with N-DMBI or TAM. In fact, thanks to the enhanced intermolecular interactions and the lower-lying LUMO level enabled by the increase of selenophene content in polymer backbone, JLBI-doped films of f-BSeI2g-SVSCN exhibit a unprecedent σ of 206 S cm-1 and a PF of 114 μW m-1 K-2. Interestingly, σ can be further enhanced up to 326 S cm-1 by using TAM dopant as a consequence of its favorable diffusion behavior into densely packed crystalline domains. These values are the highest to date for solution-processed molecularly n-doped polymers, demonstrating the effectiveness of the polymer-dopant matching approach carried out in this work.

All-Automated Fabrication of Freestanding and Scalable Photo-Thermoelectric Devices with High Performance

Flexible photo-thermoelectric (PTE) devices have great application prospects in the fields of solar energy conversion, ultrabroadband light detection, etc. A suitable manufacturing process to avoid the substrate effects as well as to create a narrow transition area between p-n modules for high-performance freestanding flexible PTE devices is highly desired. Herein, an automated laser fabrication (ALF) method is reported to construct the PTE devices with rylene-diimide-doped n-type single-walled carbon nanotube (SWCNT) films. The wet-compressing approach is developed to improve the thermoelectric power factors and figure of merit (ZT) of the SWCNT hybrid films. Then, the films are cut and patterned automatically to make PTE devices with various structures by the proposed ALF method. The freestanding PTE device with a narrow transition area of ≈2-3 µm between the p and n modules exhibits a high-power density of 0.32 µW cm-2 under the light of 200 mW cm-2, which is among the highest level for freestanding-film-based PTE devices. The results pave the way for the automatic production process of PTE devices for green power generation and ultrabroadband light detection.

High Thermoelectric Performance Related to PVDF Ferroelectric Domains in P-Type Flexible PVDF-Bi0.5Sb1.5Te3 Composite Film

There is increasing demand to power Internet of Things devices using ambient energy sources. Flexible, low-temperature, organic/inorganic thermoelectric devices are a breakthrough next-generation approach to meet this challenge. However, these systems suffer from poor performance and expensive processing preventing wide application of the technology. In this study, by combining a ferroelectric polymer (Polyvinylidene fluoride (PVDF, β phase)) with p-type Bi0.5Sb1.5Te3 (BST) a thermoelectric composite film with maximum is produced power factor. Energy filter from ferroelectric-thermoelectric junction also leads to high Seebeck voltage ≈242 µV K-1. For the first time, compelling evidence is provided that the dipole of a ferroelectric material is helping decouple electron transport related to carrier mobility and the Seebeck coefficient, to provide 5× or more improvement in thermoelectric power factor. The best composition, PVDF/BST film with BST 95 wt.% has a power factor of 712 µW•m-1 K-2. A thermoelectric generator fabricated from a PVDF/BST film demonstrated Pmax T 12.02 µW and Pdensity 40.8 W m-2 under 50 K temperature difference. This development also provides a new insight into a physical technique, applicable to both flexible and non-flexible thermoelectrics, to obtain comprehensive thermoelectric performance.

Fully inkjet-printed Ag2Se flexible thermoelectric devices for sustainable power generation

Flexible thermoelectric devices show great promise as sustainable power units for the exponentially increasing self-powered wearable electronics and ultra-widely distributed wireless sensor networks. While exciting proof-of-concept demonstrations have been reported, their large-scale implementation is impeded by unsatisfactory device performance and costly device fabrication techniques. Here, we develop Ag2Se-based thermoelectric films and flexible devices via inkjet printing. Large-area patterned arrays with microscale resolution are obtained in a dimensionally controlled manner by manipulating ink formulations and tuning printing parameters. Printed Ag2Se-based films exhibit (00 l)-textured feature, and an exceptional power factor (1097 μWm-1K-2 at 377 K) is obtained by engineering the film composition and microstructure. Benefiting from high-resolution device integration, fully inkjet-printed Ag2Se-based flexible devices achieve a record-high normalized power (2 µWK-2cm-2) and superior flexibility. Diverse application scenarios are offered by inkjet-printed devices, such as continuous power generation by harvesting thermal energy from the environment or human bodies. Our strategy demonstrates the potential to revolutionize the design and manufacture of multi-scale and complex flexible thermoelectric devices while reducing costs, enabling them to be integrated into emerging electronic systems as sustainable power sources.

Advanced Thermoelectric Textiles for Power Generation: Principles, Design, and Manufacturing

Self-powered wearable thermoelectric (TE) devices significantly reduce the inconvenience caused to users, especially in daily use of portable devices and monitoring personal health. The textile-based TE devices (TETs) exhibit the excellent flexibility, deformability, and light weight, which fulfill demands of long-term wearing for the human body. In comparison to traditional TE devices with their longstanding research history, TETs are still in an initial stage of growth. In recent years, TETs to provide electricity for low-power wearable electronics have attracted increasing attention. This review summarizes the recent progress of TETs from the points of selecting TE materials, scalable fabrication methods of TE fibers/yarns and TETs, structure design of TETs and reported high-performance TETs. The key points to develop TETs with outstanding TE properties and mechanical performance and better than available optimization strategies are discussed. Furthermore, remaining challenges and perspectives of TETs are also proposed to suggest practical applications for heat harvesting from human body.

Largely Enhanced Thermoelectric Power Factor of Flexible Cu2-xS Film by Doping Mn

Copper-sulfide-based materials have attracted noteworthy attention as thermoelectric materials due to rich elemental reserves, non-toxicity, low thermal conductivity, and adjustable electrical properties. However, research on the flexible thermoelectrics of copper sulfide has not yet been reported. In this work, we developed a facile method to prepare flexible Mn-doped Cu2-xS films on nylon membranes. First, nano to submicron powders with nominal compositions of Cu2-xMnyS (y = 0, 0.01, 0.03, 0.05, 0.07) were synthesized by a hydrothermal method. Then, the powders were vacuum-filtrated on nylon membranes and finally hot-pressed. Phase composition and microstructure analysis revealed that the films contained both Cu2S and Cu1.96S, and the size of the grains was ~20-300 nm. By Mn doping, there was an increase in carrier concentration and mobility, and ultimately, the electrical properties of Cu2-xS were improved. Eventually, the Cu2-xMn0.05S film showed a maximum power factor of 113.3 μW m-1 K-2 and good flexibility at room temperature. Moreover, an assembled four-leg flexible thermoelectric generator produced a maximum power of 249.48 nW (corresponding power density ~1.23 W m-2) at a temperature difference of 30.1 K, and had good potential for powering low-power-consumption wearable electronics.

Staggered-layer-boosted flexible Bi2Te3 films with high thermoelectric performance

Room-temperature bismuth telluride (Bi2Te3) thermoelectrics are promising candidates for low-grade heat harvesting. However, the brittleness and inflexibility of Bi2Te3 are far reaching and bring about lifelong drawbacks. Here we demonstrate good pliability over 1,000 bending cycles and high power factors of 4.2 (p type) and 4.6 (n type) mW m-1 K-2 in Bi2Te3-based films that were exfoliated from corresponding single crystals. This unprecedented bendability was ascribed to the in situ observed staggered-layer structure that was spontaneously formed during the fabrication to promote stress propagation whilst maintaining good electrical conductivity. Unexpectedly, the donor-like staggered layer rarely affected the carrier transport of the films, thus maintaining its superior thermoelectric performance. Our flexible generator showed a high normalized power density of 321 W m-2 with a temperature difference of 60 K. These high performances in supple thermoelectric films not only offer useful paradigms for wearable electronics, but also provide key insights into structure-property manipulation in inorganic semiconductors.

Ambient-Stable n-Type Carbon Nanotube/Organic Small-Molecule Thermoelectrics Enabled by Energy Level Control

The stability of n-type organic and hybrid thermoelectric materials is limited in terms of their practical application to p-n parallel thermoelectric devices. We demonstrate the ambient stability of an n-type single-walled carbon nanotube/organic small-molecule (SWNT/OSM) hybrid by deepening the lowest occupied molecular orbital energy level. This hybrid exhibited the best figure of merit (0.032) among n-type SWNT/OSM hybrid thermoelectrics and an enhanced power factor of 291.0 μW m-1 K-2. Furthermore, we observed that the n-type thermoelectric stability of a hybrid of SWNT and pip containing two N-ethylpiperidinyl groups on both sides of a naphthalenediimide core was retained at 87% over 7 months (220 days) under ambient conditions without encapsulation.

Oxide Materials for Thermoelectric Conversion

Thermoelectric technology has emerged as a prominent area of research in the past few decades for harnessing waste heat and improving the efficiency of next-generation renewable energy technologies. There has been rapid progress in the development of high-performance thermoelectric materials, as measured by the dimensionless figure of merit (ZT = S2 · σ · κ-1). Several heavy-metal-based thermoelectric materials with commercial-level performance (ZT = 1) have so far been proposed. However, the extensive application of these materials still faces challenges due to their low thermal/chemical stability, high toxicity, and limited abundance in the Earth's crust. In contrast, oxide-based thermoelectric materials, such as ZnO, SrTiO3, layered cobalt oxides, etc., have attracted growing interest as they can overcome the limitations of their heavy-metal-based counterparts. In this review, we summarize the recent research progress and introduce improvement strategies in oxide-based thermoelectric materials. This will provide an overview of their development history and design schemes, ultimately aiding in enhancing the overall performance of oxide-based thermoelectric materials.

Nanoengineering Approach toward High Power Factor Ag2Se/Se Composite Films for Flexible Thermoelectric Generators

Herein, a flexible Ag₂Se/Se composite film with a high power factor has been fabricated on a nylon membrane. The film has a high density and contains well-crystallized Ag₂Se grains and embedded Se nanoinclusions, which exhibits not only excellent flexibility but also a comparably large room-temperature power factor and Seebeck coefficient of up to 2023 μW m^-1 K^-2 and -155 μV K^-1, respectively. The high Seebeck coefficient is ascribed to the energy-filtering effect as caused by the Se/Ag₂Se heterointerface. The assembled flexible thermoelectric generator (4-leg) exhibits a maximum output power of 1135 nW and a power density of up to 16.4 W m^-2 when the applied temperature difference is 30 K. This work offers a feasible method to design high-performance and low-cost flexible thermoelectric generators used for wearable electronics.

Spin-Orbit-Coupling-Induced Topological Transition and Anomalously Strong Intervalley Scattering in Two-Dimensional Bismuth Allotropes with Enhanced Thermoelectric Performances

The convergence of multivalley bands is originally believed to be beneficial for thermoelectric performance by enhancing the charge conductivity while preserving the Seebeck coefficients, based on the assumption that electron interband or intervalley scattering effects are totally negligible. In this work, we demonstrate that β-Bi with a buckled honeycomb structure experiences a topological transition from a normal insulator to a Z₂ topological insulator induced by spin-orbit coupling, which subsequently increases the band degeneracy and is probably beneficial for enhancement of the thermoelectric power factor for holes. Therefore, strong intervalley scattering can be observed in both band-convergent β- and aw-Bi monolayers. Compared to β-Bi, aw-Bi with a puckered black-phosphorus-like structure possesses high carrier mobilities with 318 cm²/(V s) for electrons and 568 cm²/(V s) for holes at room temperature. We also unveil extraordinarily strong fourth phonon-phonon interactions in these bismuth monolayers, significantly reducing their lattice thermal conductivities at room temperature, which is generally anomalous in conventional semiconductors. Finally, a high thermoelectric figure of merit (zT) can be achieved in both bismuth monolayers, especially for aw-Bi with an n-type zT value of 2.2 at room temperature. Our results suggest that strong fourth phonon-phonon interactions are crucial to a high thermoelectric performance in these materials, and two-dimensional bismuth is probably a promising thermoelectric material due to its enhanced band convergence induced by the topological transition.

Enhanced Thermoelectric Properties of Coated Vanadium Oxynitride Nanoparticles/PEDOT:PSS Hybrid Films

Thermoelectric (TE) materials transform thermal energy into electricity, which can play an important role for global sustainability. Conducting polymers are suitable for the preparation of flexible TE materials because of their low-cost, lightweight, flexible, and easily synthesized properties. Here, we fabricate organic-inorganic hybrids by combining vanadium oxynitride nanoparticles coated with nitrogen-doped carbon (NC@VNO) and poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS). We find that the electrical conductivity, Seebeck coefficient, and power factor of the NC@VNO/PEDOT:PSS film can be enhanced up to 4158 S/cm, 45.8 μV/K, and 873 μW/mK2 at 380 K, respectively. The large enhancement of the power factor may be due to the facilitation of the interfacial charge transport tunnel between the NC@VNO nanoparticles and PEDOT:PSS. The improvement of the Seebeck coefficient may be due to the energy filter effect as induced by interfacial contact and internal electric field between the NC@VNO nanoparticles and PEDOT:PSS. Our measurement suggests that the high binding energy of pyrrolic-N enhances the Seebeck coefficient, and the high binding energy of oxide-N increases electrical conductivity.

From Brittle to Ductile: A Scalable and Tailorable All-Inorganic Semiconductor Foil through a Rolling Process toward Flexible Thermoelectric Modules

Inorganic thermoelectric (TE) materials with outstanding capacity for energy conversion are expected to be promising eco-friendly and renewable power sources, but they are always intrinsically brittle, restricting their development in flexible TE electronics. Therefore, we have developed a facile manufacturing method of large-scale all-inorganic silver chalcogenide foils and flexible TE generators in this work. A rolling process, as an effective and facile molding technique, is applied in ductile TE materials. The figure-of-merit for flexibility of this free-standing foil is in the range of 0.02-0.13, suggesting the superior flexibility of the all-inorganic TE foils. A high TE figure-of-merit ZT of 0.47 at room temperature is reached for Ag₂S_0.45Se_0.45Te_0.1, which is one of the most promising room-temperature ZTs among flexible TE materials. A proof-of-concept flexible TE generator based on silver chalcogenide foils achieves an open-circuit voltage of 1.19 mV and an output power density of 1.8 mW/m² with a temperature difference of 2.7 °C across the TE leg, showing great potential in heat-to-electricity conversion.

Metal to insulator transition, colossal Seebeck coefficient and large violation of Wiedemann-Franz law in nanoscale granular nickel

We report on the electrical and thermal transport properties of nickel nanoparticles with crystallite size from 23.1 ± 0.3 to 1.3 ± 0.3 nm. These nanoparticles show a systematic metal to insulator transition with the change in the conduction type fromn- to p-type, colossal Seebeck coefficient of 1.87 ± 0.07 mV K^-1, and ultralow thermal conductivity of 0.52 ± 0.05 W m^-1K^-1at 300 K as the crystallite size drops. The electrical resistivity analysis reveals a dramatic change in the electronic excitation spectrum indicating the opening of an energy gap, and cotunneling and Coulomb blockade of the charge carriers. Seebeck coefficient shows transport energy degradation of charge carriers as transport level moves away from the Fermi level with decrease in crystallite size. The Lorenz number rising to about four orders of magnitude in the metallic regimes with decrease in crystallite size, showing a large violation of the Wiedemann-Franz law in these compacted nickel nanoparticles. Such an observation provides the compelling confirmation for unconventional quasiparticle dynamics where the transport of charge and heat is independent of each other. Therefore, such nanoparticles provide an intriguing platform to tune the charge and heat transport, which may be useful for thermoelectrics and heat dissipation in nanocrystal array-based electronics.

Manipulating Single-Walled Carbon Nanotube Arrays for Flexible Photothermoelectric Devices

Flexible photothermoelectric (PTE) devices possess great application prospects in the field of light energy and thermoelectric energy harvesting which are some of the cornerstones of modern green renewable energy power generation. However, the low efficiency of PTE materials and lack of suitable manufacturing processes remain an impediment to restrict its rapid development. Here, we designed a flexible PTE device by printing a highly integrated single-walled carbon nanotubes (SWCNTs) array at intervals that were surface-functionalized with poly(acrylic acid) and poly(ethylene imine) as p-n heterofilms. After the introduction of a mask to give a selective light illumination and taking advantage of the photothermal effect of SWCNTs, a remarkable temperature gradient along the printed SWCNTs and a considerable power density of 1.3 μW/cm² can be achieved. Meanwhile, both experimental data and COMSOL theoretical simulations were adopted to optimize the performance of our device, showing new opportunities for new generation flexible PTE devices.

Synthesis of textured discontinuous-nanoisland Ca3Co4O9 thin films

Controllable engineering of the nanoporosity in layered Ca3Co4O9 remains a challenge. Here, we show the synthesis of discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles with controlled porosity and morphology. These discontinuously dispersed textured Ca3Co4O9 nanoparticles may be a candidate for hybrid thermoelectrics.

Mechanically Flexible Thermoelectric Hybrid Thin Films by Introduction of PEDOT:PSS in Nanoporous Ca3Co4O9

Nanoporous Ca₃Co₄O₉ exhibits high thermoelectric properties and low thermal conductivity and can be made mechanically flexible by nanostructural design. To improve the mechanical flexibility with retained thermoelectric properties near room temperature, however, it is desirable to incorporate an organic filler in this nanoporous inorganic matrix material. Here, double-layer nanoporous Ca₃Co₄O₉/PEDOT:PSS thin films were synthesized by spin-coating PEDOT:PSS into the nanopores. The obtained hybrid films exhibit high Seebeck coefficient (∼+130 μV/K) and thermoelectric power factor (0.75 μW cm^-1 K^-2) at room temperature with no deterioration in electrical properties after cyclic bending tests (98% preservation of electrical conductivity after 1000 cycles bending to a bending radius of 3 mm). Compared with the nanoporous Ca₃Co₄O₉ thin film, the mechanical flexibility of the hybrid film can be effectively improved after hybrid with PEDOT:PSS with only a slight decrease of the thermoelectric properties.

The cross-interface energy-filtering effect at organic/inorganic interfaces balances the trade-off between thermopower and conductivity

The energy-filtering effect has been widely employed to elucidate the enhanced thermoelectric properties of organic/inorganic hybrids. However, the traditional Mott criterion cannot identify the energy-filtering effect of organic/inorganic hybrids due to the limitations of the Hall effect measurement in determining their carrier concentration. In this work, a carrier concentration-independent strategy under the theoretical framework of the Kang-Snyder model is proposed and demonstrated using PANI/MWCNT composites. The result indicates that the energy-filtering effect is triggered on increasing the temperature to 220 K. The energy-filtering effect gives a symmetry-breaking characteristic to the density of states of the charge carriers and leads to a higher thermopower of PANI/MWCNT than that of each constituent. From a morphological perspective, a paracrystalline PANI layer with a thickness of 3 nm is spontaneously assembled on the MWCNT network and serves as a metallic percolation pathway for carriers, resulting in a 5.56-fold increase in conductivity. The cooperative 3D carrier transport mode, including the 1D metallic transport along the paracrystalline PANI and the 2D cross-interface energy-filtering transport, co-determines a 4-fold increase in the power factors of PANI/MWCNT at 300 K. This work provides a physical insight into the improvement of the thermoelectric performance of organic/inorganic hybrids via the energy-filtering effect.

Influence of Thermoelectric Properties and Parasitic Effects on the Electrical Power of Thermoelectric Micro-Generators

Heat recovery systems based on thermoelectric micro-generators (µ-TEGs) can play a significant role in the development of wireless, energetically autonomous electronics. However, to date, the power density recovered for low temperature differences using µ-TEGs is limited to a few micro-watts or less, which is still insufficient to power a wide-range of wireless devices. To develop more efficient µ-TEGs, material, device and system requirements must be considered simultaneously. In this study, an innovative design of an in-plane µ-TEG integrating bismuth telluride forming sinusoidal-shaped trenches is reported. Using 3D numerical modelling, the influence of boundary conditions, parasitic effects (electrical and thermal contact resistances), and transport properties of thermoelectric materials on the output power of these µ-TEGs are investigated in detail for a small temperature difference of 5 K between the hot and cold sources. Compared to wavy-shaped trenches, this novel shape enables enhancing the output power. The results show that either the thermal conductivity or the Seebeck coefficient of the active n- and p-type semiconductors is the key parameter that should be minimized or maximized, depending on the magnitude of the parasitic effects.

Strategies to Improve the Thermoelectric Figure of Merit in Thermoelectric Functional Materials

In recent years, thermoelectric functional materials have been widely concerned in temperature difference power generation, electric refrigeration and integrated circui, and so on. In this paper, the design and research progress of thermoelectric materials around lifting ZT value in recent years are reviewed. Optimizing the carrier concentration to improve the Seebeck coefficient, the steady improvement of carrier mobility and the influence of energy band engineering on thermoelectric performance are discussed. In addition, the impact of lattice thermal conductivity on ZT value is also significant. We discuss the general law that the synergistic effect of different dimensions, scales, and crystal structures can reduce lattice thermal conductivity, and introduce the new application of electro-acoustic decoupling in thermoelectric materials. Finally, the research of thermoelectric materials is summarized and prospected in the hope of providing practical ideas for expanding the application and scale industrialization of thermoelectric devices.

A Bi2Te3-Filled Nickel Foam Film with Exceptional Flexibility and Thermoelectric Performance

The past decades have witnessed surging demand for wearable electronics, for which thermoelectrics (TEs) are considered a promising self-charging technology, as they are capable of converting skin heat into electricity directly. Bi2Te3 is the most-used TE material at room temperature, due to a high zT of ~1. However, it is different to integrate Bi2Te3 for wearable TEs owing to its intrinsic rigidity. Bi2Te3 could be flexible when made thin enough, but this implies a small electrical and thermal load, thus severely restricting the power output. Herein, we developed a Bi2Te3/nickel foam (NiFoam) composite film through solvothermal deposition of Bi2Te3 nanoplates into porous NiFoam. Due to the mesh structure and ductility of Ni Foam, the film, with a thickness of 160 μm, exhibited a high figure of merit for flexibility, 0.016, connoting higher output. Moreover, the film also revealed a high tensile strength of 12.7 ± 0.04 MPa and a maximum elongation rate of 28.8%. In addition, due to the film's high electrical conductivity and enhanced Seebeck coefficient, an outstanding power factor of 850 μW m-1 K-2 was achieved, which is among the highest ever reported. A module fabricated with five such n-type legs integrated electrically in series and thermally in parallel showed an output power of 22.8 nW at a temperature gap of 30 K. This work offered a cost-effective avenue for making highly flexible TE films for power supply of wearable electronics by intercalating TE nanoplates into porous and meshed-structure materials.

Quantum transport and potential of topological states for thermoelectricity in Bi2Te3thin films

This paper reviews recent developments in quantum transport and it presents current efforts to explore the contribution of topological insulator boundary states to thermoelectricity in Bi₂Te₃thin films. Although Bi₂Te₃has been used as a thermoelectric material for many years, it is only recently that thin films of this material have been synthesized as 3D topological insulators with interesting physics and potential applications related to topologically protected surface states. A major bottleneck in Bi₂Te₃thin films has been eliminating its bulk conductivity while increasing its crystal quality. The ability to grow epitaxial films with high crystal quality and to fabricate sophisticated Bi₂Te₃-based devices is attractive for implementing a variety of topological quantum devices and exploring the potential of topological states to improve thermoelectric properties. Special emphasis is laid on preparing low-defect-density Bi₂Te₃epitaxial films, gate-tuning of normal-state transport and Josephson supercurrent in topological insulator/superconductor hybrid devices. Prospective quantum transport experiments on Bi₂Te₃thin-film devices are discussed as well. Finally, an overview of current progress on the contribution of topological insulator boundary states to thermoelectricity is presented. Future explorations to reveal the potential of topological states for improving thermoelectric properties of Bi₂Te₃films and realizing high-performance thermoelectric devices are discussed.

Organic Thermoelectric Materials as the Waste Heat Remedy

The primary reason behind the search for novel organic materials for application in thermoelectric devices is the toxicity of inorganic substances and the difficulties associated with their processing for the production of thin, flexible layers. When Thomas Seebeck described a new phenomenon in Berlin in 1820, nobody could have predicted the future applications of the thermoelectric effect. Now, thermoelectric generators (TEGs) are used in watches, and thermoelectric coolers (TECs) are applied in cars, computers, and various laboratory equipment. Nevertheless, the future of thermoelectric materials lies in organic compounds. This paper discusses the developments made in thermoelectric materials, including small molecules, polymers, molecular junctions, and their applications as TEGs and/or TECs.

Stretchable Thermoelectric-Based Self-Powered Dual-Parameter Sensors with Decoupled Temperature and Strain Sensing

Thermoelectric-based sensors with multifunctional sensing properties that can recognize different stimulations in a self-powered environment by converting low-grade heat into electrical energy have attracted increasing attention. However, the current thermoelectric-based multifunctional sensors are faced with issues such as limited preparation methods, complex structural designs, and hard decoupling, which greatly hinder their further development in the field of wearable electronics. Herein, we have fabricated novel free-standing self-powered temperature-strain sensors based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS)/carbon nanotube (CNT)/waterborne polyurethane (WPU) composite films through a simple drop-casting method. The composite films can maintain stable thermoelectric performance after washing 1000 times and withstand repeated bending and stretching. More importantly, based on the Seebeck effect arising from PEDOT:PSS/CNT composites, the assembled sensor successfully detects temperature changes and strain deformations under a self-powered condition. The decoupling of strain stimulation and temperature stimulation is mainly attributed to the good conductive network inside the composite film and the conductive bridge formed by PEDOT:PSS particles between CNTs when the composite film is stretched. Thus, the designed self-powered sensor with dual-parameter sensing prepared by a simple strategy has shown great potential in wearable electronics.

Thermoelectric characteristics of X[Formula: see text]YH[Formula: see text] monolayers (X=Si, Ge; Y=P, As, Sb, Bi): a first-principles study

Ever since global warming emerged as a serious issue, the development of promising thermoelectric materials has been one of the main hot topics of material science. In this work, we provide an in-depth understanding of the thermoelectric properties of X[Formula: see text]YH[Formula: see text] monolayers (X=Si, Ge; Y=P, As, Sb, Bi) using the density functional theory combined with the Boltzmann transport equation. The results indicate that the monolayers have very low lattice thermal conductivities in the range of 0.09-0.27 Wm[Formula: see text]K[Formula: see text] at room temperature, which are correlated with the atomic masses of primitive cells. Ge[Formula: see text]PH[Formula: see text] and Si[Formula: see text]SbH[Formula: see text] possess the highest mobilities for hole (1894 cm[Formula: see text]V[Formula: see text]s[Formula: see text]) and electron (1629 cm[Formula: see text]V[Formula: see text]s[Formula: see text]), respectively. Si[Formula: see text]BiH[Formula: see text] shows the largest room-temperature figure of merit, [Formula: see text] in the n-type doping ( [Formula: see text] cm[Formula: see text]), which is predicted to reach 3.49 at 800 K. Additionally, Si[Formula: see text]SbH[Formula: see text] and Si[Formula: see text]AsH[Formula: see text] are found to have considerable ZT values above 2 at room temperature. Our findings suggest that the mentioned monolayers are more efficient than the traditional thermoelectric materials such as Bi[Formula: see text]Te[Formula: see text] and stimulate experimental efforts for novel syntheses and applications.

Significantly Enhanced Thermoelectric Properties of Copper Phthalocyanine/Single-Walled Carbon Nanotube Hybrids by Iodine Doping

The copper phthalocyanine/single-walled carbon nanotube (CuPcI/SWCNT) hybrids were fabricated through doping the CuPc/SWCNT mixture using iodine vapor. It was found that both CuPc and SWCNTs were oxidized by iodine vapor resulting in great increase in carrier concentration. Moreover, the strong π-π conjugation interactions between CuPcI- and I-doped SWCNTs make the CuPcI molecules to assemble on the surface of SWCNTs in an ordered face-on packing, which benefits decreasing the carrier transport barrier across the CuPcI/SWCNT interfaces. The combination of iodine bidoping and the ordered face-on packing of CuPcI on the SWCNT surface realizes the synergetic enhancement of carrier concentration and carrier mobility and therefore the great improvement of electrical conductivity. The maximum electrical conductivity (6281 S cm^-1) and thermoelectric power factor (∼304 μW m^-1 K^-2) at room temperature were obtained at a composition of 60 wt % SWCNTs. The power factor value is 3 orders of magnitude higher than the pure CuPcI and 1 order of magnitude higher than SWCNTs. Consequently, the highest ZT value of CuPc/SWCNT hybrids is up to 0.03, which is among the highest value of organic small-molecule complexes.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Nanostructured Inorganic Chalcogenide-Carbon Nanotube Yarn having a High Thermoelectric Power Factor at Low Temperature

As power-conversion devices, flexible thermoelectrics that enable conformal contact with heat sources of arbitrary shape are attractive. However, the low performance of flexible thermoelectric materials, which does not exceed those of brittle inorganic counterparts, hampers their practical applications. Herein, we propose inorganic chalcogenide-nanostructured carbon nanotube (CNT) yarns with outstanding power factor at a low temperature using electrochemical deposition. The inorganic chalcogenide-nanostructured CNT yarns exhibit the power factors of 3425 and 2730 μW/(m·K²) at 298 K for the p- and n-type, respectively, which is higher than those of previously reported flexible TE materials. On the basis of excellent performance and geometry advantage of the nanostructured CNT yarn for modular design, all-CNT based thermoelectric generators have been easily fabricated, showing the maximum power densities of 24 and 380 mW/m² at ΔT = 5 and 20 K, respectively. These results provide a promising strategy for the realization of high-performance flexible thermoelectric materials and devices for flexible/or wearable self-powering systems.

Periodic nanostructures: preparation, properties and applications

Periodic nanostructures, a group of nanomaterials consisting of single or multiple nano units/components periodically arranged into ordered patterns (e.g., vertical and lateral superlattices), have attracted tremendous attention in recent years due to their extraordinary physical and chemical properties that offer a huge potential for a multitude of applications in energy conversion, electronic and optoelectronic applications. Recent advances in the preparation strategies of periodic nanostructures, including self-assembly, epitaxy, and exfoliation, have paved the way to rationally modulate their ferroelectricity, superconductivity, band gap and many other physical and chemical properties. For example, the recent discovery of superconductivity observed in "magic-angle" graphene superlattices has sparked intensive studies in new ways, creating superlattices in twisted 2D materials. Recent development in the various state-of-the-art preparations of periodic nanostructures has created many new ideas and findings, warranting a timely review. In this review, we discuss the current advances of periodic nanostructures, including their preparation strategies, property modulations and various applications.

Thermoelectric Materials for Textile Applications

Since prehistoric times, textiles have served an important role-providing necessary protection and comfort. Recently, the rise of electronic textiles (e-textiles) as part of the larger efforts to develop smart textiles, has paved the way for enhancing textile functionalities including sensing, energy harvesting, and active heating and cooling. Recent attention has focused on the integration of thermoelectric (TE) functionalities into textiles-making fabrics capable of either converting body heating into electricity (Seebeck effect) or conversely using electricity to provide next-to-skin heating/cooling (Peltier effect). Various TE materials have been explored, classified broadly into (i) inorganic, (ii) organic, and (iii) hybrid organic-inorganic. TE figure-of-merit (ZT) is commonly used to correlate Seebeck coefficient, electrical and thermal conductivity. For textiles, it is important to think of appropriate materials not just in terms of ZT, but also whether they are flexible, conformable, and easily processable. Commercial TEs usually compromise rigid, sometimes toxic, inorganic materials such as bismuth and lead. For textiles, organic and hybrid TE materials are more appropriate. Carbon-based TE materials have been especially attractive since graphene and carbon nanotubes have excellent transport properties with easy modifications to create TE materials with high ZT and textile compatibility. This review focuses on flexible TE materials and their integration into textiles.

High Power Factor Ag/Ag2Se Composite Films for Flexible Thermoelectric Generators

Herein, we fabricated an Ag/Ag₂Se composite film on a flexible nylon membrane with a high power factor and excellent flexibility. First, Ag nanoparticles and multiscale Ag₂Se nanostructure composite powders were prepared by wet chemical synthesis using Se nanowires, silver nitrate, and l-ascorbic acid as raw materials, followed by vacuum-assisted filtration of the composite powders on a porous nylon membrane and then hot pressing. The optimized composite film shows a very high power factor of 1860.6 μW m^-1 K^-2 (with a corresponding electrical conductivity of 3958 S cm^-1) at room temperature. The composite film retains 93.3% of the original electrical conductivity after 1000 bending cycles around a rod with a diameter of 8 mm. At a temperature difference of 27 K, an 8-leg thermoelectric prototype device assembled with the optimized composite film generates a maximum power of 7.14 μW with a corresponding power density of 8.74 W m^-2. This work provides a new strategy to synthesize flexible thermoelectric films with both a high power factor and high electrical conductivity.

Highly Flexible, Stretchable, and Self-Powered Strain-Temperature Dual Sensor Based on Free-Standing PEDOT:PSS/Carbon Nanocoils-Poly(vinyl) Alcohol Films

The wearable and self-powered sensors with multiple functions are urgently needed for energy saving devices, economical convenience, and artificial human skins. It is a meaningful idea to convert excess heat sources into power supplies for wearable sensors. In this report, we have fabricated a series of free-standing self-powered temperature-strain dual sensors based on poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS)/carbon nanocoils (CNCs)-poly(vinyl) alcohol composite films by a simple drop casting method. The Seebeck coefficients of the composite films were measured to be 19 μV/K. The sensor, with the addition of CNCs, showed a superior sensing performance to that without CNCs. PEDOT:PSS is used to provide a thermoelectric power to detect temperature changes and strain deformations. The minimum detect limit for the temperature difference was 0.3 K. Under a constant temperature gradient of 30 K, strains from 1 to 10% were detected without any external power supply. The films can be easily made into an array to detect the temperature of the fingers and motions of the wrist by attaching it to the human wrist directly. For the first time, due to the independent action of the thermoelectric material and strain sensing material, the thermoelectric voltage which is generated by a constant temperature difference is maintained under different strains. This kind of free-standing self-powered multifunctional sensors has great application prospects in the fields of healthcare and artificial intelligence in the future.

Interface driven energy-filtering and phonon scattering of polyaniline incorporated ultrathin layered molybdenum disulphide nanosheets for promising thermoelectric performance

The hybrid of organic conducting polymers and inorganic materials with ultralow thermal conductivity, which is a promising strategy for the realization of polymer based effective thermoelectric (TE) applications. In this work, ultrathin layered molybdenum disulphide (MoS₂) nanosheets/PANI nanocomposites are prepared by hydrothermal route. The effect of varying PANI wt% in the nanocomposites and its interface effect on thermoelectric properties are well investigated. The successful incorporation of PANI between the MoS₂ layers confirmed by high resolution transmission electron microscope (HRTEM). The significantly enhanced potential difference of MoS₂/ PANI nanocomposites with increasing PANI content is well clarified by the increased Seebeck value. The variable range hopping property is identified and conductivity is raised up highly due to insertion of PANI in layered van der Waal's gap of MoS₂. The effective interface facilitates charge for fast transport. The reduced thermal conductivity is observed of about 0.248 W*m^-1*K^-1 for 2.5 wt% addition of PANI. The key factor is that the stability of the sample is improved for MoS₂/ PANI nanocomposites than pristine MoS₂. Our work paved a new approach to improve TE performance by preparing TE MoS₂ material through simple chemical route.

A flexible electromagnetic wave-electricity harvester

Developing an ultimate electromagnetic (EM)-absorbing material that can not only dissipate EM energy but also convert the generated heat into electricity is highly desired but remains a significant challenge. Here, we report a hybrid Sn@C composite with a biological cell-like splitting ability to address this challenge. The composite consisting of Sn nanoparticles embedded within porous carbon would split under a cycled annealing treatment, leading to more dispersed nanoparticles with an ultrasmall size. Benefiting from an electron-transmitting but a phonon-blocking structure created by the splitting behavior, an EM wave-electricity device constructed by the optimum Sn@C composite could achieve an efficiency of EM to heat at widely used frequency region and a maximum thermoelectric figure of merit of 0.62 at 473 K, as well as a constant output voltage and power under the condition of microwave radiation. This work provides a promising solution for solving EM interference with self-powered EM devices.

Flexible Thermoelectric Films Based on Bi2Te3 Nanosheets and Carbon Nanotube Network with High n-Type Performance

Flexible thermoelectric materials and devices have gained wide attention due to their capability to stably and directly convert body heat or industrial waste heat into electric energy. Many research and synthetic methods of flexible high-performance p-type thermoelectric materials have made great progress. However, their counterpart flexible n-type organic thermoelectric materials are seldom studied due to the complex synthesis of conductive polymer and poor stability of n-type materials. In this work, bismuth tellurium (Bi₂Te₃) nanosheets are in situ grown on single-walled carbon nanotubes (SWCNTs) assisted by poly(vinylpyrrolidone) (PVP). A series of flexible SWCNTs@Bi₂Te₃ composite films on poly(vinylidene fluoride) (PVDF) membranes are obtained by vacuum-assisted filtration. The high electrical conductivity of 253.9 S/cm, and a corresponding power factor (PF) of 57.8 μW/m·K² is obtained at 386 K for SWCNTs@Bi₂Te₃-0.8 film. Moreover, high electrical conductivity retention of 90% can be maintained after a 300-cycle bending test and no obvious attenuation can be detected after being stored in an Ar atmosphere for 9 months, which exhibits good flexibility and excellent stability of the SWCNTs@Bi₂Te₃ composite films. This work shows a convenient method to fabricate n-type and flexible thermoelectric composite film and further promotes the practical application of n-type flexible thermoelectric materials.

Synthesis of Uniformly Sized Bi0.5Sb1.5Te3.0 Nanoparticles via Mechanochemical Process and Wet-Milling for Reduced Thermal Conductivity

In this study, Bi_0.5Sb_1.5Te_3.0 (BST) nanoparticles (NPs) with high crystallinities were synthesized via a mechanochemical process (MCP). X-ray diffraction (XRD), and Raman and X-ray photoelectron spectroscopy (XPS) spectra of the BST NPs showed that the Bi, Sb, and Te powders successfully formed BiSbTe phase and transmission electron microscopy (TEM) images, verifying the high crystallinity and smaller size, albeit agglomerated. The as-synthesized BST NPs with agglomerated clusters were ground into smaller sizes of approximately 41.8 nm with uniform distribution through a simple wet-milling process during 7 days. The thermal conduction behaviors of bulk alloys fabricated by spark plasma sintering (SPS) of the BST NPs were studied by comparing those of samples fabricated from as-synthesized BST NPs and a BST ingot. The thermal conductivities (κ) of the BST nanocomposites were significantly reduced by introducing BST NPs with smaller grain sizes and finer distributions in the temperature range from 300 to 500 K. The BST nanocomposites fabricated from wet-milled BST NPs offered ultralow κ values of 0.84 W m⁻¹ K⁻¹ at approximately 398 K.

Enhanced-Performance PEDOT:PSS/Cu2Se-Based Composite Films for Wearable Thermoelectric Power Generators

Herein, we report the preparation and thermoelectric (TE) properties of flexible PEDOT:PSS/Cu₂Se-based nanocomposite films on a nylon membrane using facile vacuum filtration and then hot pressing. The main composition of the composite film changed during hot pressing, causing the change of the carrier transport and TE performance intensively. Consequently, the optimized film shows a high power factor of 820 μW/mK² at 400 K, which is 3 times as high as that of the nonhot-pressed one. The film shows excellent flexibility with 85% retention of the power factor after 1000 bending cycles around a 5 mm diameter rod. The outstanding flexibility results from a good combination between the nylon membrane and the Cu₂Se-based nanoporous structured film. By pairing with n-type PEDOT/Ag₂Se/CuAgSe films, a ten-legged flexible TE generator outputs maximum voltage and power of 50 mV and 1.55 μW, respectively, at a temperature difference of 44 K. Our research opens up a promising avenue to design high property flexible TE films for energy conversion.

Polycarbonate/Titania Hybrid Films with Localized Photo-Induced Magnetic-Phase Transition

Materials that exhibit the photo-induced magnetic-phase transition of titania are receiving significant attention because they can be easily switched between diamagnetism and paramagnetism by UV irradiation. However, it is difficult to store photo-induced titanium (Ti³⁺) in air because of its easy oxidation upon oxygen exposure. In this study, titania/polycarbonate hybrid films were prepared using linear 1,6-hexanediol (PHMCD), cyclic 1,4-cyclohexanedimethanol (PCHCD), or their copolymerized carbonate oligomers using the sol-gel method. The oxygen permeability of the hybrid film decreased as the ratio of the ring structure increased by a factor of approximately 32 from PHMCD with only the chain structure to PCHCD with only the ring structure. These hybrid films can generate Ti³⁺ under a UV irradiation of 250 W for 2 h, and the difference in oxygen permeability significantly affected the lifetime of the Ti³⁺ by a factor of up to 120. In addition, the tensile tests and IR measurements demonstrated that UV irradiation had little effect on the mechanical intensity and matrix chemical structure. Moreover, the magnetic susceptibility of Ti³⁺ present in PCHCD was confirmed to be 6.2 (10^-3 emu/g(titania)) under an external magnetic field of 5 T induced using a superconducting quantum interference device.

Preparation and Thermoelectric Properties of Semiconducting Single-Walled Carbon Nanotubes/Regioregular Poly(3-dodecylthiophene) Composite Films

Single-walled carbon nanotubes (SWNTs) have been widely used as leading additives for improving the thermoelectric properties of organic materials, due to their unique structure and excellent electronic transport properties. However, the as-synthesized SWNTs are mixtures (mix-SWNT) of semiconducting (sc-SWNT) and metallic (met-SWNT) carbon nanotubes. The significantly different surface character and transport behavior of sc-SWNT and met-SWNT frequently raise the difficulty of modifying microstructures, and tuning transport properties of SWNTs/organic composites, when using mix-SWNTs as dispersion phase. Herein, we prepared high quality sc-SWNTs/rr-P3DDT composite film by presorting pure sc-SWNT from the raw mix-SWNTs using regioregular poly(3-dodecylthiophene) (rr-P3DDT). Both the smoothness and compactness of sc-SWNTs/rr-P3DDT are great improved, as compared with the mix-SWNTs/rr-P3DDT films, and the sc-SWNTs are well-dispersed and uniformly wrapped by rr-P3DDT with diameter less than 50 nm. The significantly enhanced Seebeck coefficients and power factors are obtained in the sc-SWNT/rr-P3DDT samples. As the result, the maximum power factor of 60 μW/mK² in 50 wt% sc-SWNTs sample is 70% higher than that of mix-SWNTs/P3DDT sample. This work reveals the effectiveness of pure semiconductor SWNTs as fillers to optimize the thermoelectric properties of CNT/polymer nano-composites.

Highly Integrable Thermoelectric Fiber

The integration of thermoelectric (TE) device on fabrics is a challenge. Instead of using adhesive tape to fix traditional filmlike TE devices onto the fabrics, which sacrifices the breathability of fabrics, here we report a flexible TE fiber based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/tellurium nanowires (Te NWs) composites, which can be woven and integrated into the fabric directly. The TE fibers have a tunable content and orientation of Te NWs and also the potential to be scaled up based on the wet-spinning process. The TE fibers exhibit desirable wearable TE performances, including high power factor (78.1 μW m^-1 K^-2), high mass-specific power (9.48 μW g^-1), excellent mechanical flexibility, and superior integrability.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.

Halide Perovskites: Thermal Transport and Prospects for Thermoelectricity

The recent re-emergence of halide perovskites has received escalating interest for optoelectronic applications. In addition to photovoltaics, the multifunctional nature of halide perovskites has led to diverse applications. The ultralow thermal conductivity coupled with decent mobility and charge carrier tunability led to the prediction of halide perovskites as a possible contender for future thermoelectrics. Herein, recent advances in thermal transport of halide perovskites and their potentials and challenges for thermoelectrics are reviewed. An overview of the phonon behavior in halide perovskites, as well as the compositional dependency is analyzed. Understanding thermal transport and knowing the thermal conductivity value is crucial for creating effective heat dissipation schemes and determining other thermal-related properties like thermo-optic coefficients, hot-carrier cooling, and thermoelectric efficiency. Recent works on halide perovskite-based thermoelectrics together with theoretical predictions for their future viability are highlighted. Also, progress on modulating halide perovskite-based thermoelectric properties using light and chemical doping is discussed. Finally, strategies to overcome the limiting factors in halide perovskite thermoelectrics and their prospects are emphasized.

Significantly Enhanced Thermoelectric Properties of Organic-Inorganic Hybrids with a Periodically Ordered Structure

The deficient order in amorphous components severely affects the thermoelectric (TE) properties in polymers. Encouragingly, two-dimensional layered double hydroxides (LDHs) have been regarded as an efficient host material to tune the conformation of guest molecules and construct ordered hybrids. Herein, we report a facile construction of periodically ordered organic-inorganic TE hybrids by alternative stacking of inorganic LDH nanosheets and organic poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) molecules. The ordered structure of PEDOT:PSS-LDH gave rise to the extended molecular configuration of PEDOT:PSS, resulting in the improved carrier mobility in the hybrids. Moreover, the energy filtering was induced by such a periodically ordered structure, which blocked the low-energy carriers preferentially and improved the Seebeck coefficient in the hybrids. Therefore, the power factor of the PEDOT:PSS-LDH hybrid was 120-fold higher than that of pristine PEDOT:PSS. These results not only establish an effective method for the construction of periodically ordered TE materials but also address the significance of an ordered structure of molecules in TE materials.

Ultrahigh Performance of n-Type Ag2Se Films for Flexible Thermoelectric Power Generators

Due to the limited thermoelectric (TE) performance of conducting polymers and rigidity of inorganic materials, it is still a huge challenge to prepare low-cost, highly flexible, and high-performance TE materials. Herein, we fabricated n-type Ag₂Se films using a porous nylon membrane as a flexible substrate by vacuum-assisted filtration, followed by hot pressing. A very high power factor of ∼1882 μW m^-1 K^-2 at room temperature is obtained. The high power factor is mainly the result of the high density of the Ag₂Se film and the tuned grain orientation, which is realized by the synthesis of multisized Ag₂Se nanostructures. The film also exhibits excellent flexibility with 90.7% retention of the power factor after bending around a rod of 4 mm radius for 1000 times. A four-leg TE generator is assembled with the Ag₂Se film, and its maximum output power is up to 3.2 μW at a temperature difference of 30 K, corresponding to the maximum power density of 22.0 W m^-2 and a normalized maximum power density of 408 μW m^-1 K^-2. This work provides an effective route to achieve high-power-factor, high-flexibility, and low-cost TE films.

Application of Organic-Inorganic Hybrids in Chemical Analysis, Bio- and Environmental Monitoring

Organic-inorganic hybrids (OIH) are considered to be a powerful platform for applications in many research and industrial fields. This review highlights the application of OIH for chemical analysis, biosensors, and environmental monitoring. A methodology toward metrological traceability measurement and standardization of OIH and demonstration of the role of mathematical modeling in biosensor design are also presented. The importance of the development of novel types of OIH for biosensing applications is highlighted. Finally, current trends in nanometrology and nanobiosensors are presented.

High-Performance Ag-Modified Bi0.5Sb1.5Te3 Films for the Flexible Thermoelectric Generator

Bi-Sb-Te-based semiconductors possess the best room-temperature thermoelectric performance, but are restricted for application in the wearable field because of their inherent brittleness, rigidity, and nonscalable manufacturing techniques. Therefore, how to obtain thermoelectric materials with excellent thermoelectric properties and flexibility through the batch production process is a serious challenge. Here, we report the fabrication of flexible p-type thermoelectric Ag-modified Bi_0.5Sb_1.5Te₃ films on flexible substrates using a facile approach. Their optimized power factors are ∼12.4 and ∼14.0 μW cm^-1 K^-2 at 300 and 420 K, respectively. These high-power factors mainly originate from the optimized carrier transport of the composite system, through which a high level of electrical conductivity is achieved, whereas a remarkably improved Seebeck coefficient is simultaneously obtained. Bending tests demonstrate the excellent flexibility and mechanical durability of the composite films, and their power factors decrease by only about 10% after bending for 650 cycles with a bending radius of 5 mm. A flexible thermoelectric module is designed and constructed using the optimized composite films and displays a power density of ∼1.4 mW cm^-2 at a relatively small ΔT of 60 K. This work demonstrates the potential of inorganic thermoelectric materials to be made on flexible/wearable substrates for energy harvesting and management devices.

Fiber-Based Energy Conversion Devices for Human-Body Energy Harvesting

Following the rapid development of lightweight and flexible smart electronic products, providing energy for these electronics has become a hot research topic. The human body produces considerable mechanical and thermal energy during daily activities, which could be used to power most wearable electronics. In this context, fiber-based energy conversion devices (FBECD) are proposed as candidates for effective conversion of human-body energy into electricity for powering wearable electronics. Herein, functional materials, fiber fabrication techniques, and device design strategies for different classes of FBECD based on piezoelectricity, triboelectricity, electrostaticity, and thermoelectricity are comprehensively reviewed. An overview of fiber-based self-powered systems and sensors according to their superior flexibility and cost-effectiveness is also presented. Finally, the challenges and opportunities in the field of fiber-based energy conversion are discussed.