Highly robust and flexible n-type thermoelectric film based on Ag2Te nanoshuttle/polyvinylidene fluoride hybrids

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.

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.

Chemically Transformed Ag2 Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications

Flexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2 Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2 Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt-grown Ag2 Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n-type) of Ag2 Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc ) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Synthesis and DC Electrical Conductivity of Nanocomposites Based on Poly(1-vinyl-1,2,4-triazole) and Thermoelectric Tellurium Nanoparticles

In this work, the structural characteristics and DC electrical conductivity of firstly synthesized organic-inorganic nanocomposites of thermoelectric Te⁰ nanoparticles (1.4, 2.8, 4.3 wt%) and poly(1-vinyl-1,2,4-triazole) (PVT) were analyzed. The composites were characterized by high-resolution transmission electron microscopy, X-ray diffractometry, UV-Vis spectroscopy, and dynamic light scattering analysis. The study results showed that the nanocomposite nanoparticles distributed in the polymer matrix had a shape close to spherical and an average size of 4-18 nm. The average size of the nanoparticles was determined using the Brus model relation. The optical band gap applied in the model was determined on the basis of UV-Vis data by the Tauc method and the 10% absorption method. The values obtained varied between 2.9 and 5.1 nm. These values are in good agreement with the values of the nanoparticle size, which are typical for their fractions presented in the nanocomposite. The characteristic sizes of the nanoparticles in the fractions obtained from the Pesika size distribution data were 4.6, 4.9, and 5.0 nm for the nanocomposites with percentages of 1.4, 2.8, and 4.3%, respectively. The DC electrical conductivity of the nanocomposites was measured by a two-probe method in the temperature range of 25-80 °C. It was found that the formation of an inorganic nanophase in the PVT polymer as well as an increase in the average size of nanoparticles led to an increase in the DC conductivity over the entire temperature range. The results revealed that the DC electrical conductivity of nanocomposites with a Tellurium content of 2.8, 4.3 wt% at 80 °C becomes higher than the conventional boundary of 10^-10 S/cm separating dielectrics and semiconductors.

Thermoelectric Silver-Based Chalcogenides

Heat is abundantly available from various sources including solar irradiation, geothermal energy, industrial processes, automobile exhausts, and from the human body and other living beings. However, these heat sources are often overlooked despite their abundance, and their potential applications remain underdeveloped. In recent years, important progress has been made in the development of high-performance thermoelectric materials, which have been extensively studied at medium and high temperatures, but less so at near room temperature. Silver-based chalcogenides have gained much attention as near room temperature thermoelectric materials, and they are anticipated to catalyze tremendous growth in energy harvesting for advancing internet of things appliances, self-powered wearable medical systems, and self-powered wearable intelligent devices. This review encompasses the recent advancements of thermoelectric silver-based chalcogenides including binary and multinary compounds, as well as their hybrids and composites. Emphasis is placed on strategic approaches which improve the value of the figure of merit for better thermoelectric performance at near room temperature via engineering material size, shape, composition, bandgap, etc. This review also describes the potential of thermoelectric materials for applications including self-powering wearable devices created by different approaches. Lastly, the underlying challenges and perspectives on the future development of thermoelectric materials are discussed.

Hydrogel-based printing strategy for high-performance flexible thermoelectric generators

Flexible thermoelectric (TE) devices can utilize the slight temperature difference between curved surfaces and surroundings to generate TE potential, presenting great potential in microelectronic energy supply and wearable sensing. Printing method has been employed to fabricate high-performance flexible TE films by means of excellent capability of assembling nanomaterials, but the decrease in the electrical conductivity caused by organic matters in the thermoelectric pastes will significantly reduce the thermoelectric performance. Herein, we report a hydrogel-based printing strategy to deposit flexible TE generators on various flexible substrates. The hydrogel network formed by physical crosslinking and molecular chain entanglement at 0.498 wt% carboxylated cellulose nanofibers can effectively limit the fluidity of 1D nanorod dispersion, which produces only <5% decline in electrical conductivity and Seebeck coefficient compared to the pure inorganic nanorod films. The device with 72 couples constructed by printing presents a high power density of 1.278 W m^-2 under a temperature difference of 50 K. The advantages of hydrogel-based printing can broaden application prospects in the field of wearable electronics.

Researching progress on bio-reactive electrogenic materials with electrophysiological activity for enhanced bone regeneration

Bone tissues are dynamically reconstructed during the entire life cycle phase, which is an exquisitely regulated process controlled by intracellular and intercellular signals transmitted through physicochemical and biochemical stimulation. Recently, the role of electrical activity in promoting bone regeneration has attracted great attention, making the design, fabrication, and selection of bioelectric bio-reactive materials a focus. Under specific conditions, piezoelectric, photoelectric, magnetoelectric, acoustoelectric, and thermoelectric materials can generate bioelectric signals similar to those of natural tissues and stimulate osteogenesis-related signaling pathways to enhance the regeneration of bone defects, which can be used for designing novel smart biological materials for engineering tissue regeneration. However, literature summarizing studies relevant to bioelectric materials for bone regeneration is rare to our knowledge. Consequently, this review is mainly focused on the biological mechanism of electrical stimulation in the regeneration of bone defects, the current state and future prospects of piezoelectric materials, and other bioelectric active materials suitable for bone tissue engineering in recent studies, aiming to provide a theoretical basis for novel clinical treatment strategies for bone defects.

Recent Advances and Prospects of Small Molecular Organic Thermoelectric Materials

Thermoelectric (TE) materials possess unique energy conversion capabilities between heat and electrical energy. Small organic semiconductors have aroused widespread attention for the fabrication of TE devices due to their advantages of low toxicity, large area, light weight, and easy fabrication. However, the low TE properties hinder their large-scale commercial application. Herein, the basic knowledge about TE materials, including parameters affecting the TE performance and the remaining challenges of the organic thermoelectric (OTE) materials, are initially summarized in detail. Second, the optimization strategies of power factor, including the selection and design of dopants and structural modification of the dope-host are introduced. Third, some achievements of p- and n-type small molecular OTE materials are highlighted to briefly provide their future developing trend; finally, insights on the future development of OTE materials are also provided in this study.

Highly Stretchable Carbon Nanotubes/Polymer Thermoelectric Fibers

Thermoelectric (TE) technology provides a new way to directly harvest and convert the heat continuously released from the human body. The greatest challenge for TE materials applied in wearable TE generators is compatible with the constantly changing morphology of the human body while offering a continuous and stable power output. Here, a stretchable carboxylic single-walled carbon nanotube (SWNT)-based TE fiber is prepared by an improved wet-spinning method. The stable Seebeck coefficient of the annealed carboxylic SWNT-based TE fiber is 44 μV/K even under the tensile strain of ∼30%. Experimental results show that the fiber can continue to generate constant TE potential when it is changed to various shapes. The new stretchable TE fiber has a larger Seebeck coefficient and more stretchability than existing TE fibers based on the Seebeck effect, opening a path to using the technology for a variety of practical applications.

Synergistically enhanced thermoelectric performance by optimizing the composite ratio between hydrothermal Sb2Se3 and self-assembled β-Cu2Se nanowires

Sb₂Se₃ and β-Cu₂Se nanowires were synthesized via hydrothermal reaction and a water-evaporation induced self-assembly method, respectively, and a 70%-Sb₂Se₃ and 30%-β-Cu₂Se disk pellet shows enhanced thermoelectric performance.

Biomimetic Approach to Facilitate the High Filler Content in Free-Standing and Flexible Thermoelectric Polymer Composite Films Based on PVDF and Ag2Se Nanowires

A high filler content is often needed in polymer composite-based thermoelectric (TE) films to improve their performance. Nevertheless, this often leads to poor processability and poor mechanical performance. Herein, a biomimetic approach is adopted to facilitate the filler content up to 90.5 wt % in free-standing and flexible n-type PVDF/Ag₂Se TE films, where PVDF dendricolloids are a solution mixed with Ag₂Se nanowires (NWs), followed by filtration. These soft dendric nanoparticles within PVDF dendricolloids have high adhesivity and strong network-building ability, which allows the formation of "grapevine-grape"-like networks with soft dendritic particles and inorganic TE fillers as "grapevine" and "manicure finger grapes", respectively. The maximum power factor of 189.02 μW m^-1 K^-2 is achieved for a PVDF/Ag₂Se mass ratio of 1:9.5 at 300 K. Meanwhile, excellent flexibility with only 15.8% decrease in electrical conductivity after 1000 bending cycles was observed. These properties at such a high filler content are attributed to the long-range grapevine-like network of soft PVDF dendritic particles and entanglement between numerous Ag₂Se NWs. This work carves a path to fabricate high-performance free-standing flexible n-type TE composite films as well as other functional polymer composites requiring high inorganic filler loading.

Semiconductor glass with superior flexibility and high room temperature thermoelectric performance

Most crystalline inorganic materials, except for metals and some layer materials, exhibit bad flexibility because of strong ionic or covalent bonds, while amorphous materials usually display poor electrical properties due to structural disorders. Here, we report the simultaneous realization of extraordinary room temperature flexibility and thermoelectric performance in Ag₂Te_1-x S _x -based materials through amorphization. The coexistence of amorphous main phase and crystallites results in exceptional flexibility and ultralow lattice thermal conductivity. Furthermore, the flexible Ag₂Te_0.6S_0.4 glass exhibits a degenerate semiconductor behavior with a room temperature Hall mobility of ~750 cm² V^-1 s^-1 at a carrier concentration of 8.6 × 10¹⁸ cm^-3, which is at least an order of magnitude higher than other amorphous materials, leading to a thermoelectric power factor also an order of magnitude higher than the best amorphous thermoelectric materials known. The in-plane prototype uni-leg thermoelectric generator made from this material demonstrates its potential for flexible thermoelectric device.

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.

Room-Temperature Welding of Silver Telluride Nanowires for High-Performance Thermoelectric Film

Flexible thermoelectric materials that can harvest waste heat energy have attracted great attention because of the rapid progress of flexible electronics. Ag₂Te nanowires (Ag₂Te NWs) are considered as promising thermoelectric materials to fabricate flexible thermoelectric film and device because of their high Seebeck coefficient, but poor contact between the Ag₂Te NWs results in low electrical conductivity. Generally, hot or cold pressing can increase the electrical conductivity between the Ag₂Te NWs. However, these process tend to destroy the initial morphology of the Ag₂Te NWs and/or cause only physical contact between the Ag₂Te NWs. Herein, we report an approach to the room-temperature welding of Ag₂Te NWs to enhance their contacts by facile combination of vacuum filtration and drop-coating methods. The obtained Ag₂Te NWs film exhibits excellent Seebeck coefficient of -99.48 μV/K and high electrical conductivity of 15 335.05 S/m at room temperature, which gives the power factor of 151.76 μW m^-1 K^-2. Surprisingly, an optimal Seebeck coefficient of -154.96 μV/K and electrical conductivity of 14 982.42 S/m can be obtained at 420 K, giving a power factor of 359.76 μW m^-1 K^-2. Moreover, the electrical resistance of the Ag₂Te NWs film was only 1.3 times of the initial electrical resistance after 1000 bending cycles, indicating good flexibility of the film. A finger-touch test is conducted by using the Ag₂Te NWs film as thermoelectric power generator, which achieves a stable output voltage of about 0.52 mV, suggesting its great potential applications in self-powered flexible electronic devices.

Integration of multi-scale defects for optimizing thermoelectric properties of n-type Cu1-xCdxFeS2 (x = 0-0.1)

The performance of thermoelectric (TE) materials is strongly influenced by multi-scale defects. Some defects can improve the TE performance but some are unfavorable. Therefore, the multi-scale defects need to be integrated rationally to enhance the TE properties. Here, the defects including atomic-scale point defects, high-density grain boundaries and nano-precipitates were integrated into CuFeS₂, an n-type and Earth-abundant TE material. Primitively, a Cd dopant with high scattering factor was introduced to form point defects in Cu_1-xCd_xFeS₂ (x = 0-0.1) according to the calculated scattering parameters. Furthermore, the processes of quenching, annealing, high-energy ball milling (QAH) and sintering were carried out to integrate the multi-scale defects into Cu_1-xCd_xFeS₂. The results suggested that point defects and antisite defects were achieved and the unfavorable Cd'_Fe defects were suppressed effectively, leading to a higher electrical conductivity. Moreover, the CdS nano-precipitates played a vital role in carrier filtering to increase the Seebeck coefficient. Meanwhile, the high-density grain boundaries suppressed the lattice thermal conductivity. As a result, a peak ZT value of 0.39 at 723 K was obtained in Cu_0.92Cd_0.08FeS₂, which is the highest value reported so far in the CuFeS₂ family.

High Performance and Flexible Polyvinylpyrrolidone/Ag/Ag2Te Ternary Composite Film for Thermoelectric Power Generator

In this work, polyvinylpyrrolidone (PVP) coated Ag-rich Ag₂Te nanowires (NWs) were synthesized by a wet chemical method using PVP coated Te NWs as templates, and a flexible PVP/Ag/Ag₂Te ternary composite film on a nylon membrane was prepared by vacuum assisted filtration, followed by heat treatment. TEM and STEM observations of the focused ion beam prepared sample reveal that the composite film shows a porous network-like structure and that the Ag and Ag₂Te exist as nanoparticles and NWs, respectively, both bonded with PVP. The Ag nanoparticles are formed by separation of the Ag-rich Ag₂Te NWs during the heat treatment. For the composite film starting from a Ag/Te initial molar ratio of 6:1, a high power factor of 216.5 μW/mK² is achieved at 300 K, and it increases to 370.1 μW/mK² at 393 K. Bending tests demonstrate excellent flexibility of the hybrid film. A thermoelectric (TE) prototype composed of five legs of the hybrid film is assembled, and a maximum output power of 469 nW is obtained at a temperature gradient of 39.6 K, corresponding to a maximum power density of 341 μW/cm². This work provides an effective route to a composite film with high TE performance and excellent flexibility for wearable TE generators.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.

Recent Advances in Organic Thermoelectric Materials: Principle Mechanisms and Emerging Carbon-Based Green Energy Materials

Thermoelectric devices have recently attracted considerable interest owing to their unique ability of converting heat to electrical energy in an environmentally efficient manner. These devices are promising as alternative power generators for harvesting electrical energy compared to conventional batteries. Inorganic crystalline semiconductors have dominated the thermoelectric material fields; however, their application has been restricted by their intrinsic high toxicity, fragility, and high cost. In contrast, organic thermoelectric materials with low cost, low thermal conductivity, easy processing, and good flexibility are more suitable for fabricating thermoelectric devices. In this review, we briefly introduce the parameters affecting the thermoelectric performance and summarize the most recently developed carbon-material-based organic thermoelectric composites along with their preparation technologies, thermoelectric performance, and future applications. In addition, the p- and n-type carbon nanotube conversion and existing challenges are discussed. This review can help researchers in elucidating the recent studies on carbon-based organic thermoelectric materials, thus inspiring them to develop more efficient thermoelectric devices.