Flexible Thermoelectric Materials and Generators: Challenges and Innovations

Sandwich Engineering Advances Ductile Thermoelectrics

Flexible thermoelectrics offer the possibility of utilizing human body heat to generate electricity, enabling self-powered wearable electronics. Ductile and plastic semiconductors are promising materials for flexible thermoelectrics due to their inherent ductility and tunable electrical properties. However, balancing ductility with high thermoelectric performance remains challenging, especially for n-type materials. Here, a novel n-type ductile Ag2(S, Se)-Ag2Se sandwich-like thermoelectric film is designed with different functional layers, where the Ag2(S, Se) core layer provides ductile deformation ability and low thermal conductivity, while epitaxially grown highly oriented Ag2Se shell layers ensure superior electrical transport performance. This architecture achieves a record high figure-of-merit near room-temperature range, 0.91 at 323 K, among n-type ductile semiconductors while preserving excellent flexibility. Additionally, a flexible in-plane device fabricated from this material delivers an exceptional power density of 26.5 W m-2 at a temperature difference of 50 K, demonstrating its great application potential for wearable electronics. Importantly, such novel sandwich engineering can pave the way to alleviate the compromise between thermoelectric performance and ductility in inorganic semiconductors.

Defect Engineering for Flexible n-Type Mo2TiC2Tx o-MXene Thermoelectric Efficiency Enhancement

With the rapid advancement of wearable electronics, the demand for efficient portable power supplies has become increasingly urgent. Thermoelectric materials, which can directly convert heat, such as body heat, into electricity, offer a promising avenue for sustainable energy supplementation. However, achieving a high thermoelectric performance in flexible materials suitable for body heat harvesting remains a significant challenge. Here, we introduce a strategy for synergistically tuning surface oxygen defects and optimizing microstructures in low-dimensional semiconductor materials, resulting in flexible, ammoniated dual-transition metal carbide o-MXene N-Mo2TiC2Tx with enhanced properties. Theoretical and experimental analyses reveal that high-temperature ammoniation produces a low-oxygen-functionalized surface, reduces interlayer spacing, and minimizes defect density, thereby significantly increasing the electrical conductivity. Nitrogen atoms incorporated at the nanosheet terminals further increase the electron density near the Fermi level, resulting in an enhanced Seebeck coefficient. Consequently, N-Mo2TiC2Tx films treated at 900 K achieve an electrical conductivity of 1.03 × 104 S m-1, a Seebeck coefficient of -27.8 μV K-1, and a power factor of 7.99 μW m-1 K-2 at room temperature, nearly 1.2-fold higher than that of untreated materials, while retaining excellent flexibility. Moreover, a wearable thermoelectric generator constructed from these N-Mo2TiC2Tx films produces a voltage of 1.4 mV under a temperature gradient of approximately 12 K between human skin and ambient air, underscoring its excellent capacity for harvesting low-grade thermal energy. These findings establish a paradigm for the development of flexible, high-performance thermoelectric materials, paving the way for next-generation wearable and industrial energy applications.

Multiple Phase Transition Induced Enhancement of Low-Temperature Thermoelectric Power in Ductile AgCuS-Based Thin Films

The increasing demand for energy autonomy in microscale and wearable electronics has intensified research interest in thermoelectric thin-film-based power generators. However, the development of such devices is challenging due to the intrinsic brittleness of inorganic materials and the poor performance of thin films. Recently, Ag2S-based compounds have emerged as ductile thermoelectric semiconductors. Nonetheless, the thermoelectric performance of their thin films remains constrained, especially at low temperatures. Herein, we present a solution-processed fabrication of a high-performance AgCuS/Cu2S composite thin film operable below 100 °C. These composite thin films underwent multiple phase transitions below 100 °C, notably increasing the thermoelectric power factors. Furthermore, the films exhibited significant intrinsic stretchability up to a strain of 16.1% owing to their intrinsic ductility. Wrinkled thin-film-based devices demonstrated enhanced power generation owing to multiple phase transitions and retained properties under 30% stretching, highlighting the potential of these films as viable energy harvesters for emerging electronic systems.

Iodide Ion-Assisted Silver-Telluride-Based Nanowires: Morphology Optimization and Efficient Doping for Improved Thermoelectric Application

Silver-telluride-based nanowires (STNWs) are promising thermoelectric (TE) materials for room temperature (RT) applications, can be utilized to fabricate flexible TE composites or inks to facilitate TE conversion in various situations. However, current research on doping design and morphology optimization of STNWs is still limited. Such strategies are expected to enhance the TE performance and flexibility, thereby improving the compatibility of STNWs for TE applications. Herein, we report a simple yet efficient iodide ion-assisted strategy for doping design and morphology optimization, significantly enhancing both the TE performance and flexibility of the STNW films. Iodide ions serve dual roles of regulating the chemical transformation process and acting as a effective dopant, ultimately achieving a 1.97-fold increment in the power factor of the STNW films at RT, as well as promoting the formation of a structure of bundles composed of morphology-optimized STNWs, which significantly improves the flexibility of STNW films. On this basis, flexible TE composite films and TE devices are fabricated and systematically evaluated, demonstrating the excellent potential of STNWs for practical applications. This study paves the way for the development of STNW-based TE materials and provides insights for synthesis of other nanostructured TE materials.

Recent Progress in Polymer Gel-Based Ionic Thermoelectric Devices: Materials, Methods, and Perspectives

Polymer gel-based ionic thermoelectric (i-TE) devices, including thermally chargeable capacitors and thermogalvanic cells, represent an innovative approach to sustainable energy harvesting by converting waste heat into electricity. This review provides a comprehensive overview of recent advancements in gel-based i-TE materials, focusing on their ionic Seebeck coefficients, the mechanisms underlying the thermodiffusion and thermogalvanic effects, and the various strategies employed to enhance their performance. Gel-based i-TE materials show great promise due to their flexibility, low cost, and suitability for flexible and wearable devices. However, challenges such as improving the ionic conductivity and stability of redox couples remain. Future directions include enhancing the efficiency of ionic-electronic coupling and developing more robust electrode materials to optimize the energy conversion efficiency in real-world applications.

Removal of protonic doping from PEDOT:PSS by weak base for improving aluminum solid electrolytic capacitor performance

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising conductive polymer material, but its acidic nature leads to device deterioration and poor long-term durability. It is usual practice to neutralise PEDOT:PSS solutions with bases, resulting in a decrease of conductivity. The causes of the decline in conductivity remain disputed and uncertain. In this study, three bases (sodium hydroxide, ammonium hydroxide, and imidazole) were employed to treat PEDOT:PSS to investigate their effects on doping level, composition, and structure. The results show that bases may dedope PEDOT:PSS by removing the proton doping from the PEDOT chain. Meanwhile, insulating PSS is removed, and the aggregated structure is optimised to ensure that conductivity is not significantly reduced. The PEDOT:PSS treated with the weak base imidazole showed the least loss of conductivity due to only a modest decrease in doping level and the greatest loss of insulating PSS. Aluminium electrolytic capacitors made using imidazole pH-adjusted solutions have a capacitance ratio of up to 98.28% and an equivalent series resistance of 26 mΩ. These findings provide a valuable reference for developing high electrical characteristics and neutral PEDOT:PSS materials.

Highly Deformable Van der Waals Chalcogenides with Superior Thermoelectric Performance from Interpretable Machine Learning

Van der Waals (vdW) chalcogenide-based flexible thermoelectric devices hold great promise for wearable electronics. However, intrinsic vdW chalcogenides that combine high flexibility with superior thermoelectric figures of merit (ZT) remain extremely rare. Consequently, there is an urgent need to develop methods capable of high-throughput screening to identify potential vdW chalcogenides with both robust flexibility and favorable ZT value. In this study, over 1000 vdW chalcogenides are high-throughput screened for their flexibility and ZT values. Flexibility is evaluated using the previously developed deformability factor, while ZT values are predicted using a machine learning model. Several candidates with large deformability and high ZT are successfully identified. Among these, NbSe2Br2 emerges as the top-performing material. Further first-principles calculations reveal that it achieves a maximum ZT value of 1.35 at 1000 K, the highest reported so far among flexible inorganic thermoelectric materials. Its power factor value of 8.1 µW cm-1K-2 at 300 K also surpasses most organic and inorganic flexible thermoelectric materials. The high ZTmax is mainly contributed by the extremely low thermal conductivity and the high Seebeck coefficient along the out-of-plane direction at high temperatures. The study offers new material options for the development and application of flexible thermoelectric devices based on layered chalcogenides.

Flexible thermoelectric generators from spray-printed PEDOT:PSS/Bi0.5Sb1.5Te3 composites

Energy harvesting technologies play a pivotal role in powering the next generation of wearable and portable devices, where thin-film thermoelectric generators (TEGs) offer a compact and flexible solution. In this study, flexible thin films of poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS)/Bi0.5Sb1.5Te3 composite on flexible polymeric substrates were initially developed using a spray printing technique. The effect of substrate temperature during printing was assessed on the microstructural and thermoelectric properties, yielding a maximum power factor at a substrate temperature of 110 °C. Additionally, the printed films demonstrated excellent flexibility and mechanical/electrical stability during 1000 cycles of bending, confirming their suitability for wearable electronic applications. Subsequently, a flexible thin-film TEG containing 40 thermoelectric legs was fabricated by spray printing of the composite ink for the first time. Finally, the electrical performance of the flexible thin-film TEG was thoroughly assessed under various temperature gradients, exhibiting maximum open circuit voltage of 52 mV at a temperature difference of 50 °C. This study establishes a foundation for the facile fabrication of flexible TEGs using organic/inorganic composite inks. Further enhancement of the thermoelectric performance can be envisaged through post-processing chemical treatments to optimize charge carrier concentration in the printed TEGs.

Harnessing Thermoelectric Power in Self-Healing Wearables: A Review

Wearable thermoelectric generators are sustainable devices that generate electricity from body heat to provide a continuous power supply for electronic devices. In healthcare, they are particularly valuable for powering wireless devices that transmit vital health signals, where maintaining an uninterrupted power source is a significant challenge. However, these generators are prone to failure over time or due to mechanical damage caused by mechanical stress or environmental factors, which can lead to the loss of critical healthcare data. To address these issues, the integration of self-healing capabilities alongside flexibility and longevity is essential for their reliable operation. To our knowledge, this review is one of the first to look in depth at self-healing materials specifically designed for wearable thermoelectric generators. It explores the latest innovations and applications in this field highlighting how these materials can improve the reliability and lifetime of such systems.

Substrate-Free Inorganic-Based Films for Thermoelectric Applications

The development of highly integrated electronic components and the Internet of Things demands efficient thermal management and uninterrupted energy harvesting, which provides exciting opportunities for thermoelectric (TE) technology since it allows direct conversion between electricity and thermal energy. The improved output performance of TE devices has traditionally been driven by advancements in inorganic materials. Recently, there has been growing interest in studying substrate-free inorganic-based TE thin films because they provide improved adherence to curved surfaces and offer a more compact size compared to the corresponding rigid form of these materials. This review begins by summarizing various methods for fabricating freestanding inorganic-based TE films, including leveraging the intrinsic plasticity of certain materials, exfoliating layered-structure materials, using sacrificial substrates, and creating composites with flexible components such as polymers and carbon-based materials. A key challenge in achieving high device performance is determining how to maintain the favorable TE properties of inorganic materials. This can be addressed through strategies such as high inorganic content loading, multicomponent engineering, and interfacial structure design. The review also discusses the applications of substrate-free inorganic-based TE devices in both power generation and solid-state cooling. Finally, it outlines current challenges and proposes potential research directions to further advance the field.

Constructing a Conductive Micro/Nano Network Within Interpenetrating Phase Composites for Triboelectric Nanogenerators

Interpenetrating phase composites (IPCs) are gaining popularity because of their unique topological characteristics, which include their light weight and ability to conduct electricity. Herein, a novel metal‒polymer IPC composed mainly of a 3D nickel foam(NF) at the microscale and a polytetrafluoroethylene (PTFE) matrix are developed. Moreover, a nanonetwork composed of carbon nanotubes (CNTs) bridging the nickel skeleton and PTFE matrix is created within the IPC via a simple impregnation method. The construction of a micro/nano hybrid network not only enhances the tribological performance and friction heat dissipation but also increases the triboelectric output response, which is 30 and 7.5 times greater than that of pristine PTFE and binary NF-PTFE IPC, respectively. The fabricated IPC endures long-term tests after 36 000 cycles without notable wear scarring, exhibiting great potential for future optimization of triboelectric nanogenerator(TENG) durability and potential application in a variety of fields, such as wear-resisting materials, high-performance energy absorption and heat management.

Barrier Energy Engineering Enables Efficient Carrier Transport of Single-Walled Carbon Nanotube-Small Organic Molecules Hybrid Thermoelectrics

Understanding the inherent charge carrier transport mechanism within carbon nanotube-organic hybrid thermoelectric (TE) materials is crucial for enhancing their TE performance. Although various carbon nanotube-organic hybrid TE materials have been developed, the influence of the barrier energy on the TE transport mechanism within these hybrids remains elusive. Our study focuses on the engineering of barrier energy between single-walled carbon nanotubes (SWCNTs) and small organic molecules (SOMs) by modulating the mesomeric effects of terminal functional groups on T-shaped SOMs. The minimization of barrier energy in an SWCNTs-BTBIN hybrid to 0.04 eV resulted in a semiconducting-dominant transport character, facilitating the energy filtering of SWCNTs-BTBIN. Consequently, SWCNTs-BTBIN achieved a higher Seebeck coefficient (65 μV K-1) than other hybrids (39-54 μV K-1) having steeper barrier energies (0.30 and 0.19 eV), enabling the highest power factor (798 μW m-1 K-2) and ZT value up to 0.035 at room temperature among SWCNTs-BTBI hybrid series. A TE module consisting of SWCNTs-BTBIN incorporating five-leg TE elements produced an output power exceeding 0.82 μW, suggesting that integrating barrier energy engineering with analyses of TE transport mechanisms can significantly advance the design of high-performance nanocarbon-based organic hybrid TEs.

Helical dislocation-driven plasticity and flexible high-performance thermoelectric generator in α-Mg3Bi2 single crystals

Inorganic plastic semiconductors play a crucial role in the realm of flexible electronics. In this study, we present a cost-effective plastic thermoelectric semimetal magnesium bismuthide (α-Mg3Bi2), exhibiting remarkable thermoelectric performance. Bulk single-crystalline α-Mg3Bi2 exhibits considerable plastic deformation at room temperature, allowing for the fabrication of intricate shapes such as the letters "SUSTECH" and a flexible chain. Transmission electron microscopy, time-of-flight neutron diffraction, and chemical bonding theoretic analyses elucidate that the plasticity of α-Mg3Bi2 stems from the helical dislocation-driven interlayer slip, small-sized Mg atoms induced weak interlayer Mg-Bi bonds, and low modulus of intralayer Mg2Bi22- networks. Moreover, we achieve a power factor value of up to 26.2 µW cm-1 K-2 along the c-axis at room temperature in an n-type α-Mg3Bi2 crystal. Our out-of-plane flexible thermoelectric generator exhibit a normalized power density of 8.1 μW cm-2 K-2 with a temperature difference of 7.3 K. This high-performance plastic thermoelectric semimetal promises to advance the field of flexible and deformable electronics.

Two-Step Design Rule for Simultaneously High Conductivity and Seebeck Coefficient in Conjugated Polymer-Based Thermoelectrics

The trade-off between enhancing conductivity (σ) through doping while concurrently observing a reduction in the Seebeck coefficient (S) presents a key challenge in organic thermoelectrics. Here, a two-step structural design strategy is developed, where the first step enhances the backbone planarity which enhances the conductivity by an improved ordering of conjugated polymers (CPs). The second step, which is fluorination of the backbone, improves the Seebeck coefficient by the controlled induction of energetic disorder, stemming from the fluorine's disruption of the homogeneous electrostatic potential across the CP backbone. This strategy is applied to two series of donor-acceptor (D-A) types of CPs based on the BDT donor unit and BDD and TT as acceptor units, respectively. A maximum power factor (PF) over 155 (142.7 ± 12.7) µW m-1 K-2, coupled with S ≈202 (194.6 ± 7.6) µV K-1 is achieved, leading to up to 32-fold enhancement of the PF compared to the initial non-planar and non-fluorinated polymer. This study provides valuable conceptual insights for designing CPs with high conductivity and Seebeck coefficient.

Fiber-Shaped Temperature Sensor with High Seebeck Coefficient for Human Health and Safety Monitoring

Wearable thermoelectric (TE)-based temperature sensors capable of detecting and transmitting temperature data from the human body and environment show promise in intelligent medical systems, human-machine interfaces, and electronic skins. However, it has remained a challenge to fabricate the flexible temperature sensors with superior sensing performance, primarily due to the low Seebeck coefficient of the TE materials. Here, we report an inorganic amorphous TE material, Ge5As55Te40, with a high Seebeck coefficient of 1050 μV/K, which is around 3 times higher than the organic TE materials and 2 times higher than the inorganic crystal TE materials. Due to the strong anticrystallization ability, the amorphous state of Ge5As55Te40 can be well maintained during the thermal fiber-drawing process. The resulting TE fibers demonstrate superior temperature sensing properties, encompassing a broad working range (25-115 °C), a precise temperature resolution of 0.1 K, and a rapid response time of 5 s. Importantly, the TE properties of the fiber show high stability after repeated temperature variations between 5 and 10 K. Moreover, the fibers can be integrated into a mask and a wearable fabric for monitoring human respiratory rate and providing early warning for fire source proximity. These results highlight that the TE fiber with both natural flexibility and superior temperature sensing performance may find potential applications in wearable systems.

Flame-Resistant Inorganic Films by Self-Assembly of Clay Nanotubes and their Conversion to Geopolymer for CO2 Capture

Self-assembling of very long natural clay nanotubes represents a powerful strategy to fabricate thermo-stable inorganic thin films suitable for environmental applications. In this work, self-standing films with variable thicknesses (from 60 to 300 µm) are prepared by the entanglement of 20-30 µm length Patch halloysite clay nanotubes (PT_Hal), which interconnect into fibrosus structures. The thickness of the films is crucial to confer specific properties like transparency, mechanical resistance, and water uptake. Despite its completely inorganic composition, the thickest nanoclay film possesses elasticity comparable with polymeric materials as evidenced by its Young's modulus (ca. 1710 MPa). All PT_Hal-based films are fire resistant and stable under high temperature conditions preventing flame propagation. After their direct flame exposure, produced films do not show neither deterioration effects nor macroscopic alterations. PT_Hal films are employed as precursors for the development of functional materials by alkaline activation and thermal treatment, which generate highly porous geopolymers or ceramics with a compact morphology. Due to its high porosity, geopolymer can be promising for CO2 capture. As compared to the corresponding inorganic film, the CO2 adsorption efficiency is doubled for the halloysite geopolymeric materials highlighting their potential use as a sorbent.

Advanced Dielectric Materials for Triboelectric Nanogenerators: Principles, Methods, and Applications

Triboelectric nanogenerator (TENG) manifests distinct advantages such as multiple structural selectivity, diverse selection of materials, environmental adaptability, low cost, and remarkable conversion efficiency, which becomes a promising technology for micro-nano energy harvesting and self-powered sensing. Tribo-dielectric materials are the fundamental and core components for high-performance TENGs. In particular, the charge generation, dissipation, storage, migration of the dielectrics, and dynamic equilibrium behaviors determine the overall performance. Herein, a comprehensive summary is presented to elucidate the dielectric charge transport mechanism and tribo-dielectric material modification principle toward high-performance TENGs. The contact electrification and charge transport mechanism of dielectric materials is started first, followed by introducing the basic principle and dielectric materials of TENGs. Subsequently, modification mechanisms and strategies for high-performance tribo-dielectric materials are highlighted regarding physical/chemical, surface/bulk, dielectric coupling, and structure optimization. Furthermore, representative applications of dielectric materials based TENGs as power sources, self-powered sensors are demonstrated. The existing challenges and promising potential opportunities for advanced tribo-dielectric materials are outlined, guiding the design, fabrication, and applications of tribo-dielectric materials.

Flexible All-Inorganic Thermoelectric Yarns

Achieving both formability and functionality in thermoelectric materials remains a significant challenge due to their inherent brittleness. Previous approaches, such as polymer infiltration, often compromise thermoelectric efficiency, underscoring the need for flexible, all-inorganic alternatives. This study demonstrates that the extreme brittleness of thermoelectric bismuth telluride (Bi2Te3) bulk compounds can be overcome by harnessing the nanoscale flexibility of Bi2Te3 nanoribbons and twisting them into a yarn structure. The resulting Bi2Te3 yarn, with a Seebeck coefficient of -126.6 µV K-1, exhibits remarkable deformability, enduring extreme bending curvatures (down to 0.5 mm-1) and tensile strains of ≈5% through over 1000 cycles without significant resistance change. This breakthrough allows the yarn to be seamlessly integrated into various applications-wound around metallic pipes, embedded within life jackets, or woven into garments-demonstrating unprecedented adaptability and durability. Moreover, a simple 4-pair thermoelectric generator comprising Bi2Te3 yarns and metallic wires generates a maximum output voltage of 67.4 mV, substantiating the effectiveness of thermoelectric yarns in waste heat harvesting. These advances not only challenge the traditional limitations posed by the brittleness of thermoelectric materials but also open new avenues for their application in wearable and structural electronics.

Achieving High Thermoelectric Performance in Bi2(Te, Se)3 Thin Film with (00l)-Oriented by Incorporating Tellurization and Selenization Processes

Flexible thermoelectric (TE) generators have received great attention as a sustainable and reliable option to convert heat from the human body and other ambient sources into electricity. This study provides a synthesis route that involves thermally induced diffusion to introduce Te and Se into Bi, fabricating an n-type Bi-Te-Se flexible thin film on a flexible substrate. This specific synthesis alters the crystal orientation (00l) of the thin film, improving in-plane electrical transportation and optimizing carrier concentration. Consequently, BixTeySe0.42 enhanced both the Seebeck coefficient and electrical conductivity, achieving a power factor of 17.1 μW cm-1 K-2 at room temperature. The TE device assembled with p-type Sb2Te3 exhibited exceptional flexibility with only a 26.2% change in resistance after 1000 times of bending at a radius of about 6 mm. The resistance change was further reduced to 7.5% after the application of a vinyl laurate coating. The fabricated TE device generated an ultrahigh output power of 792 nW with a temperature difference of 30 K.

Improved electrical and thermoelectric properties of electrodeposited Bi1-x Sb x nanowire networks by thermal annealing

Arrays of thermoelectric nanowires embedded in organic films are attracting increasing interest to fabricate flexible thermoelectric devices with adjustable dimensions and complex shapes, useful for sustainable power sources of portable electronic devices and wireless sensor networks. Here, we report the electrochemical synthesis of interconnected bismuth-antimony (Bi1-x Sb x ) nanowires (with 0.06 < x < 0.15) within the branched cylindrical nanopores of polycarbonate membranes. The influence of temperature and magnetic field on the electrical and thermoelectric properties was studied by considering electric and thermal currents flowing in the plane of the films. We show that short annealing times with temperature up to 250 °C of the nanowire-based nanocomposite lead to a large increase in the thermoelectric power, reaching values up to -80 μV K-1 at room temperature, which are comparable to those of bulk Bi-Sb alloys. In addition, we report Hall effect measurements on crossed nanowires, made possible for the first time by the remarkable electrical connectivity of the nanowire network. These measurements, combined with variations in temperature and under the magnetic field of the electrical resistance, indicate that the interconnected networks of Bi1-x Sb x nanowires after thermal annealing behave like n-type, narrow band gap semiconductors. Overall, the electrical and thermoelectric properties near the ambient temperature of the heat-treated Bi1-x Sb x nanowire networks are similar to those of bulk polycrystalline Bi-Sb alloys, which are well-known thermoelectric materials exhibiting optimal performance near and below room temperature.

Citric Acid and Polyvinyl Alcohol Induced PEDOT: PSS with Enhanced Electrical Conductivity and Stretchability for Eco-Friendly, Self-Healable, Wearable Organic Thermoelectrics

Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) is a promising material for organic thermoelectric (TE) applications. However, it is challenging to achieve PEDOT: PSS composites with stretchable, self-healable, and high TE performance. Furthermore, some existing self-healing TE materials employ toxic reagents, posing risks to human health and the environment. In this study, a novel intrinsically self-healable and wearable composite is developed by incorporating environmentally friendly, highly biocompatible, and biodegradable materials of polyvinyl alcohol (PVA) and citric acid (CA) into PEDOT: PSS. This results in the formation of double hydrogen bonding networks among CA, PVA, and PEDOT: PSS, inducing microstructure alignment and leading to simultaneous enhancements in both TE performance and stretchability. The resulting composites exhibit a high electrical conductivity and power factor of 259.3 ± 11.7 S·cm-1, 6.9 ± 0.4 µW·m-1·K-2, along with a tensile strain up to 68%. Furthermore, the composites display impressive self-healing ability, with 84% recovery in electrical conductivity and an 85% recovery in tensile strain. Additionally, the temperature and strain sensors based on the PEDOT: PSS/PVA/CA are prepared, which exhibit high resolution suitable for human-machine interaction and wearable devices. This work provides a reliable and robust solution for the development of environmentally friendly, self-healing and wearable TE thermoelectrics.

Hybrid Integration of Wearable Devices for Physiological Monitoring

Wearable devices can provide timely, user-friendly, non- or minimally invasive, and continuous monitoring of human health. Recently, multidisciplinary scientific communities have made significant progress regarding fully integrated wearable devices such as sweat wearable sensors, saliva sensors, and wound sensors. However, the translation of these wearables into markets has been slow due to several reasons associated with the poor system-level performance of integrated wearables. The wearability consideration for wearable devices compromises many properties of the wearables. Besides, the limited power capacity of wearables hinders continuous monitoring for extended duration. Furthermore, peak-power operations for intensive computations can quickly create thermal issues in the compact form factor that interfere with wearability and sensor operations. Moreover, wearable devices are constantly subjected to environmental, mechanical, chemical, and electrical interferences and variables that can invalidate the collected data. This generates the need for sophisticated data analytics to contextually identify, include, and exclude data points per multisensor fusion to enable accurate data interpretation. This review synthesizes the challenges surrounding the wearable device integration from three aspects in terms of hardware, energy, and data, focuses on a discussion about hybrid integration of wearable devices, and seeks to provide comprehensive guidance for designing fully functional and stable wearable devices.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Ductile P-Type AgCu(Se,S,Te) Thermoelectric Materials

Ductile inorganic thermoelectric (TE) materials open a new approach to develop high-performance flexible TE devices. N-type Ag2(S,Se,Te) and p-type AgCu(Se,S,Te) pseudoternary solid solutions are two typical categories of ductile inorganic TE materials reported so far. Comparing with the Ag2(S,Se,Te) pseudoternary solid solutions, the phase composition, crystal structure, and physical properties of AgCu(Se,S,Te) pseudoternary solid solutions are more complex, but their relationships are still ambiguous now. In this work, via systematically investigating the phase composition, crystal structure, mechanical, and TE properties of about 60 AgCu(Se,S,Te) pseudoternary solid solutions, the comprehensive composition-structure-property phase diagrams of the AgCuSe-AgCuS-AgCuTe pseudoternary system is constructed. By mapping the complex phases, the "ductile-brittle" and "n-p" transition boundaries are determined and the composition ranges with high TE performance and inherent ductility are illustrated. On this basis, high performance p-type ductile TE materials are obtained, with a maximum zT of 0.81 at 340 K. Finally, flexible in-plane TE devices are prepared by using the AgCu(Se,S,Te)-based ductile TE materials, showing high output performance that is superior to those of organic and inorganic-organic hybrid flexible devices.

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.

Seebeck Coefficient of Ionic Conductors from Bayesian Regression Analysis

We propose a novel approach to evaluating the ionic Seebeck coefficient in electrolytes from relatively short equilibrium molecular dynamics simulations, based on the Green-Kubo theory of linear response and Bayesian regression analysis. By exploiting the probability distribution of the off-diagonal elements of a Wishart matrix, we develop a consistent and unbiased estimator for the Seebeck coefficient, whose statistical uncertainty can be arbitrarily reduced in the long-time limit. We assess the efficacy of our method by benchmarking it against extensive equilibrium molecular dynamics simulations conducted on molten CsF using empirical force fields. We then employ this procedure to calculate the Seebeck coefficient of molten NaCl, KCl, and LiCl using neural network force fields trained on ab initio data over a range of pressure-temperature conditions.

Progress on Material Design and Device Fabrication via Coupling Photothermal Effect with Thermoelectric Effect

Recovery and utilization of low-grade thermal energy is a topic of universal importance in today's society. Photothermal conversion materials can convert light energy into heat energy, which can now be used in cancer treatment, seawater purification, etc., while thermoelectric materials can convert heat energy into electricity, which can now be used in flexible electronics, localized cooling, and sensors. Photothermoelectrics based on the photothermal effect and the Seebeck effect provide suitable solutions for the development of clean energy and energy harvesting. The aim of this paper is to provide an overview of recent developments in photothermal, thermoelectric, and, most importantly, photothermal-thermoelectric coupling materials. First, the research progress and applications of photothermal and thermoelectric materials are introduced, respectively. After that, the classification of different application areas of materials coupling photothermal effect with thermoelectric effect, such as sensors, thermoelectric batteries, wearable devices, and multi-effect devices, is reviewed. Meanwhile, the potential applications and challenges to be overcome for future development are presented, which are of great reference value in waste heat recovery as well as solar energy resource utilization and are of great significance for the sustainable development of society. Finally, the challenges of photothermoelectric materials as well as their future development are summarized.

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.

Ultrafast Response and Threshold Adjustable Intelligent Thermoelectric Systems for Next-Generation Self-Powered Remote IoT Fire Warning

Fire warning is vital to human life, economy and ecology. However, the development of effective warning systems faces great challenges of fast response, adjustable threshold and remote detecting. Here, we propose an intelligent self-powered remote IoT fire warning system, by employing single-walled carbon nanotube/titanium carbide thermoelectric composite films. The flexible films, prepared by a convenient solution mixing, display p-type characteristic with excellent high-temperature stability, flame retardancy and TE (power factor of 239.7 ± 15.8 μW m-1 K-2) performances. The comprehensive morphology and structural analyses shed light on the underlying mechanisms. And the assembled TE devices (TEDs) exhibit fast fire warning with adjustable warning threshold voltages (1-10 mV). Excitingly, an ultrafast fire warning response time of ~ 0.1 s at 1 mV threshold voltage is achieved, rivaling many state-of-the-art systems. Furthermore, TE fire warning systems reveal outstanding stability after 50 repeated cycles and desired durability even undergoing 180 days of air exposure. Finally, a TED-based wireless intelligent fire warning system has been developed by coupling an amplifier, analog-to-digital converter and Bluetooth module. By combining TE characteristics, high-temperature stability and flame retardancy with wireless IoT signal transmission, TE-based hybrid system developed here is promising for next-generation self-powered remote IoT fire warning applications.

High performance magnesium-based plastic semiconductors for flexible thermoelectrics

Low-cost thermoelectric materials with simultaneous high performance and superior plasticity at room temperature are urgently demanded due to the lack of ever-lasting power supply for flexible electronics. However, the inherent brittleness in conventional thermoelectric semiconductors and the inferior thermoelectric performance in plastic organics/inorganics severely limit such applications. Here, we report low-cost inorganic polycrystalline Mg3Sb0.5Bi1.498Te0.002, which demonstrates a remarkable combination of large strain (~ 43%) and high figure of merit zT (~ 0.72) at room temperature, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). By revealing the inherent high plasticity in Mg3Sb2 and Mg3Bi2, capable of sustaining over 30% compressive strain in polycrystalline form, and the remarkable deformability of single-crystalline Mg3Bi2 under bending, cutting, and twisting, we optimize the Bi contents in Mg3Sb2-xBix (x = 0 to 1) to simultaneously boost its room-temperature thermoelectric performance and plasticity. The exceptional plasticity of Mg3Sb2-xBix is further revealed to be brought by the presence of a dense dislocation network and the persistent Mg-Sb/Bi bonds during slipping. Leveraging its high plasticity and strength, polycrystalline Mg3Sb2-xBix can be easily processed into micro-scale dimensions. As a result, we successfully fabricate both in-plane and out-of-plane flexible Mg3Sb2-xBix thermoelectric modules, demonstrating promising power density. The inherent remarkable plasticity and high thermoelectric performance of Mg3Sb2-xBix hold the potential for significant advancements in flexible electronics and also inspire further exploration of plastic inorganic semiconductors.

Greatly Enhanced Thermoelectric Performance of Flexible Cu2-xS Composite Film on Nylon by Se Doping

In this work, flexible Cu2-xS films on nylon membranes are prepared by combining a simple hydrothermal synthesis and vacuum filtration followed by hot pressing. The films consist of Cu2S and Cu1.96S two phases with grain sizes from nano to submicron. Doping Se on the S site not only increases the Cu1.96S content in the Cu2-xS to increase carrier concentration but also modifies electronic structure, thereby greatly improves the electrical properties of the Cu2-xS. Specifically, an optimal composite film with a nominal composition of Cu2-xS0.98Se0.02 exhibits a high power factor of ~150.1 μW m-1 K-2 at 300 K, which increases by ~138% compared to that of the pristine Cu2-xS film. Meanwhile, the composite film shows outstanding flexibility (~97.2% of the original electrical conductivity is maintained after 1500 bending cycles with a bending radius of 4 mm). A four-leg flexible thermoelectric (TE) generator assembled with the optimal film generates a maximum power of 329.6 nW (corresponding power density of 1.70 W m-2) at a temperature difference of 31.1 K. This work provides a simple route to the preparation of high TE performance Cu2-xS-based films.

Largely Enhanced Thermoelectric Performance of Flexible Ag2Se Film by Cationic Doping and Dual-Phase Engineering

Recent studies have shown that silver selenide is a promising thermoelectric material at room temperature. Herein, flexible films with a nominal composition of (Ag1-xCux)2Se are prepared by a simple and efficient one-pot method combined with vacuum-assisted filtration and hot pressing. The thermoelectric properties of the films are regulated by both cationic doping and a dual-phase strategy via a wet chemical method. As the x increases, not only Cu is doped into the Ag2Se, but different new phases (CuAgSe and/or CuSe2) also appear. The (Ag1-xCux)2Se film with x = 0.02 composed of Cu-doped Ag2Se and CuAgSe shows a high PF of ∼2540 μW m-1 K-2 (ZT ∼ 0.90) and outstanding flexibility at room temperature. The high thermoelectric properties of the film are due to the effect of Cu doping and the CuAgSe phase, including the increase in electrical conductivity caused by doping, the enhanced phonon scattering at the Ag2Se/CuAgSe interface, and the interaction between the energy filtering effect and the doping effect. In addition to the high output performance (PDmax = 28.08 W m-2, ΔT = 32.2 K), the flexible device assembled with the (Ag0.98Cu0.02)2Se film also has potential applications as a temperature sensor.

A Waterproof Flexible Paper-Based Thermoelectric Generator for Humidity and Underwater Environments

A thermoelectric generator (TEG) is one of the important energy harvesting sources for wearable electronic devices, which converts waste heat into electrical energy without any external stimuli, such as light or mechanical motion. However, the poor flexibility of traditional TEGs (e.g., Si-based TE devices) causes the limitations in practical applications. Flexible paper substrates are becoming increasingly attractive in wearable electronic technology owing to their usability, environmental friendliness (disposable, biodegradable, and renewable materials), and foldability. The high water-absorbing quality of paper restricts its scope of application due to water failure. Therefore, we propose a high-performance flexible waterproof paper-based thermoelectric generator (WPTEG). A modification method that infiltrates TE materials into cellulose paper through vacuum filtration is used to prepare the TE modules. By connecting the TE-modified paper with Al tape, as well as a superhydrophobic layer encapsulation, the WPTEG is fabricated. The WPTEG with three P-N modules can generate an output voltage of up to 235 mV at a temperature difference of 50 K, which can provide power to portable electronic devices such as diodes, clocks, and calculators in hot water. With the waterproof property, the WPTEG paves the way for achieving multi-scenario applications in humid environments on human skin.

Stretchable and Flexible Painted Thermoelectric Generators on Japanese Paper Using Inks Dispersed with P- and N-Type Single-Walled Carbon Nanotubes

As power sources for Internet-of-Things sensors, thermoelectric generators must exhibit compactness, flexibility, and low manufacturing costs. Stretchable and flexible painted thermoelectric generators were fabricated on Japanese paper using inks with dispersed p- and n-type single-walled carbon nanotubes (SWCNTs). The p- and n-type SWCNT inks were dispersed using the anionic surfactant of sodium dodecylbenzene sulfonate and the cationic surfactant of dimethyldioctadecylammonium chloride, respectively. The bundle diameters of the p- and n-type SWCNT layers painted on Japanese paper differed significantly; however, the crystallinities of both types of layers were almost the same. The thermoelectric properties of both types of layers exhibited mostly the same values at 30 °C; however, the properties, particularly the electrical conductivity, of the n-type layer increased linearly, and of the p-type layer decreased as the temperature increased. The p- and n-type SWCNT inks were used to paint striped patterns on Japanese paper. By folding at the boundaries of the patterns, painted generators can shrink and expand, even on curved surfaces. The painted generator (length: 145 mm, height: 13 mm) exhibited an output voltage of 10.4 mV and a maximum power of 0.21 μW with a temperature difference of 64 K at 120 °C on the hot side.

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.

Spin coated ultrathin PEDOT:PSS/SWCNT film with high electronic conductivity

Conductive Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been extensively used as non-metallic electrodes. However, the relatively low electrical conductivity of pristine PEDOT:PSS film restricts its further application. Although doping high content conductive filler or increasing the film thickness are effective for enhancing the electrical property, the transparency is sacrificed, which limits the application of PEDOT:PSS films. In this study, preparing PEDOT:PSS composite film with highly conductive and transparent property was the primary purpose. To achieve this goal, single-walled carbon nanotubes (SWCNTs) and dimethyl sulfoxide (DMSO) was chosen to composite with PEDOT:PSS. The spin-coated SWCNT/PEDOT:PSS composite film exhibited excellent electrical conductivity and transparency. The electrical conductivity of composite film with desired transmittance property (78%) reached the highest value (1060.96 S cm-1) at the SWCNTs content was 6 wt%. Under the modification process applied in this work, the non-conductive PSS was partially removed by incorporated DMSO and SWCNTs. Then, the molecular chains of PEDOT stretched and adsorbed onto the surface of SWCNTs, forming a highly efficient three-dimensional conductive structure, which contributed to the enhancement of electrical conductivity and transparency. Additionally, the spin-coating process allowed for the reduction of film thickness, ensuring better transparency. This research contributed to expanding the further applications of PEDOT:PSS films in high-performance transparent film electrodes.

High-Performance Polyaniline-Coated Carbon Nanotube Yarns for Wearable Thermoelectric Generators

Carbon nanotubes/polyaniline (CNTs/PANI) composites have attracted significant attention in thermoelectric (TE) conversion due to their excellent stability and easy synthesis. However, their TE performance is far from practical demands, and few flexible yarns/fibers have been developed for wearable electronics. Herein, we developed flexible CNTs/PANI yarns with outstanding TE properties via facile soaking of CNT yarns in a PANI solution, in which the PANI layer was coated on the CNT surface and served as a bridge to interconnect adjacent CNT filaments. With optimizing PANI concentration, immersing duration, and doping level of PANI, the power factor reached 1294 μW m-1 K-2 with a high electrical conductivity of 3651 S cm-1, which is superior to that of most of the reported CNTs/PANI composites and organic yarns. Combining outstanding TE performance with excellent bending stability, a highly integrated and flexible TE generator was assembled consisting of 40 pairs of interval p-n segments, which generate a high power of 377 nW at a temperature gradient of 10 K along the out-of-plane direction. These results indicate the promising application of CNTs/PANI yarns in wearable energy harvesting.

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.

Biomimetic Exogenous "Tissue Batteries" as Artificial Power Sources for Implantable Bioelectronic Devices Manufacturing

Implantable bioelectronic devices (IBDs) have gained attention for their capacity to conformably detect physiological and pathological signals and further provide internal therapy. However, traditional power sources integrated into these IBDs possess intricate limitations such as bulkiness, rigidity, and biotoxicity. Recently, artificial "tissue batteries" (ATBs) have diffusely developed as artificial power sources for IBDs manufacturing, enabling comprehensive biological-activity monitoring, diagnosis, and therapy. ATBs are on-demand and designed to accommodate the soft and confining curved placement space of organisms, minimizing interface discrepancies, and providing ample power for clinical applications. This review presents the near-term advancements in ATBs, with a focus on their miniaturization, flexibility, biodegradability, and power density. Furthermore, it delves into material-screening, structural-design, and energy density across three distinct categories of TBs, distinguished by power supply strategies. These types encompass innovative energy storage devices (chemical batteries and supercapacitors), power conversion devices that harness power from human-body (biofuel cells, thermoelectric nanogenerators, bio-potential devices, piezoelectric harvesters, and triboelectric devices), and energy transfer devices that receive and utilize external energy (radiofrequency-ultrasound energy harvesters, ultrasound-induced energy harvesters, and photovoltaic devices). Ultimately, future challenges and prospects emphasize ATBs with the indispensability of bio-safety, flexibility, and high-volume energy density as crucial components in long-term implantable bioelectronic devices.

Interface Engineering for High-Performance Thermoelectric Carbon Nanotube Films

Carbon nanotubes (CNTs) stand out for their exceptional electrical, thermal, and mechanical attributes, making them highly promising materials for cutting-edge, lightweight, and flexible thermoelectric applications. However, realizing the full potential of advanced thermoelectric CNTs requires precise management of their electrical and thermal characteristics. This study, through interface optimization, demonstrates the feasibility of reducing the thermal conductivity while preserving robust electrical conductivity in single-walled CNT films. Our findings reveal that blending two functionalized CNTs offers a versatile method of tailoring the structural and electronic properties of CNT films. Moreover, the modified interface exerts a substantial influence over thermal and electrical transfer, effectively suppressing heat dissipation and facilitating thermoelectric power generation within CNT films. As a result, we have successfully produced both p- and n-type thermoelectric CNTs, attaining impressive power factors of 507 and 171 μW/mK2 at room temperature, respectively. These results provide valuable insights into the fabrication of high-performance thermoelectric CNT films.

Application and Development of Smart Thermally Conductive Fiber Materials

In recent years, with the rapid advancement in various high-tech technologies, efficient heat dissipation has become a key issue restricting the further development of high-power-density electronic devices and components. Concurrently, the demand for thermal comfort has increased; making effective personal thermal management a current research hotspot. There is a growing demand for thermally conductive materials that are diversified and specific. Therefore, smart thermally conductive fiber materials characterized by their high thermal conductivity and smart response properties have gained increasing attention. This review provides a comprehensive overview of emerging materials and approaches in the development of smart thermally conductive fiber materials. It categorizes them into composite thermally conductive fibers filled with high thermal conductivity fillers, electrically heated thermally conductive fiber materials, thermally radiative thermally conductive fiber materials, and phase change thermally conductive fiber materials. Finally, the challenges and opportunities faced by smart thermally conductive fiber materials are discussed and prospects for their future development are presented.

Modulation of the morphotropic phase boundary for high-performance ductile thermoelectric materials

The flexible thermoelectric technique, which can convert heat from the human body to electricity via the Seebeck effect, is expected to provide a peerless solution for the power supply of wearables. The recent discovery of ductile semiconductors has opened a new avenue for flexible thermoelectric technology, but their power factor and figure-of-merit values are still much lower than those of classic thermoelectric materials. Herein, we demonstrate the presence of morphotropic phase boundary in Ag2Se-Ag2S pseudobinary compounds. The morphotropic phase boundary can be freely tuned by adjusting the material thermal treatment processes. High-performance ductile thermoelectric materials with excellent power factor (22 μWcm-1 K-2) and figure-of-merit (0.61) values are realized near the morphotropic phase boundary at 300 K. These materials perform better than all existing ductile inorganic semiconductors and organic materials. Furthermore, the in-plane flexible thermoelectric device based on these high-performance thermoelectric materials demonstrates a normalized maximum power density reaching 0.26 Wm-1 under a temperature gradient of 20 K, which is at least two orders of magnitude higher than those of flexible organic thermoelectric devices. This work can greatly accelerate the development of flexible thermoelectric technology.

High-Performance Paper-Based Thermoelectric Generator from Cu2SnS3 Nanocubes and Bulk-Traced Bismuth

Flexible paper-based thermoelectric generators (PTEGs) have drawn significant interest in recent years due to their various advantages such as flexibility, adaptability, environment friendliness, low cost, and easy fabrication process. However, the reported PTEG's output performance still lags behind the performance of other flexible devices as it is not so easy to obtain a compact film on a paper-based substrate with desirable power output with the standard thermoelectric (TE) materials that have been previously utilized. In this direction, Cu2SnS3 (CTS), an earth-abundant, ternary sulfide, can be a good choice p-type semiconductor, when paired with a suitable n-type TE material. In this article, CTS nanocubes are synthesized via a simple hot injection method and a thick film device on emery paper was prepared and optimized. Furthermore, a flexible, 20-pair PTEG is fabricated with p-type CTS legs and traced and pressed n-type bismuth legs assembled using Kapton tape that produced a significantly high output power of 2.18 μW (output power density ∼0.85 nW cm-2 K-1) for a temperature gradient of ΔT = 80 K. The TE properties are also supported by finite element simulation. The bending test conducted for the PTEG suggests device stability for up to 800 cycles with <0.05% change in the internal resistance. A proof-of-concept field-based demonstration for energy harvesting from waste heat of a motorbike exhaust is shown recovering an output power of ∼42 nW for ΔT = 20 K, corroborating the experimental and theoretical results.

Tunable Thermoelectric Performance of the Nanocomposites Formed by Diketopyrrolopyrrole/Isoindigo-Based Donor-Acceptor Random Conjugated Copolymers and Carbon Nanotubes

This paper presents the development of thermoelectric properties in nanocomposites comprising donor-acceptor random conjugated copolymers and single-walled carbon nanotubes (SWCNTs). The composition of the conjugated polymers, specifically the ratio of diketopyrrolopyrrole (DPP) to isoindigo (IID), is manipulated to design a series of random conjugated copolymers (DPP0, DPP5, DPP10, DPP30, DPP50, DPP90, DPP95, and DPP100). The objective is to improve the dispersion of SWCNTs into smaller bundles, leading to enhanced thermoelectric properties of the polymer/SWCNT nanocomposite. This dispersion strategy promotes an interconnected conducting network, which plays a critical role in optimizing the thermoelectric performance. Accordingly, the effects of morphologies on the thermoelectric properties of the nanocomposites are systematically investigated. The DPP95/SWCNT nanocomposite exhibits the strongest interaction, resulting in the highest power factor (PF) of 711.1 μW m-1 K-2, derived from the high electrical conductivity of 1690 S cm-1 and Seebeck coefficient of 64.8 μV K-1. The prototype flexible thermoelectric generators assembled with a DPP95/SWCNT film achieve a maximum power output of 20.4 μW m-2 at a temperature difference of 29.3 K. These findings highlight the potential of manipulating the composition of random conjugated copolymers and incorporating SWCNTs to efficiently harvest low-grade waste heat in wearable thermoelectric devices.

Densification Induced Decoupling of Electrical and Thermal Properties in Free-Standing MWCNT Films for Ultrahigh p- and n-Type Power Factors and Enhanced ZT

Generating sufficient power from waste heat is one of the most important things for thermoelectric (TE) techniques in numerous practical applications. The output power density of an organic thermoelectric generator (OTEG) is proportional to the power factors (PFs) and the electrical conductivities of organic materials. However, it is still challenging to have high PFs over 1 mW m-1  K-2 in free-standing films together with high electrical conductivities over 1000 S cm-1 . Herein, densifying multi-walled carbon nanotube (MWCNT) films would increase their electrical conductivity dramatically up to over 10 000 S cm-1 with maintained high Seebeck coefficients >60 µV K-1 , thus leading to ultrahigh PFs of 7.25 and 4.34 mW m-1  K-2 for p- and n-type MWCNT films, respectively. In addition, it is interesting to notice that the electrical properties increase faster than the thermal conductivities, resulting in enhanced ZT of 3.6 times in MWCNT films. An OTEG made of compressed MWCNT films is fabricated to demonstrate the heat-to-electricity conversion ability, which exhibits a high areal output power of ∼12 times higher than that made of pristine MWCNT films. This work demonstrates an effective way to high-performance nanowire/nanoparticle-based TE materials such as printable TE materials comprised of nanowires/nanoparticles.

Flexible Carbon Nanotube-Epitaxially Grown Nanocrystals for Micro-Thermoelectric Modules

Flexible thermoelectric materials have attracted increasing interest because of their potential use in thermal energy harvesting and high-spatial-resolution thermal management. However, a high-performance flexible micro-thermoelectric device (TED) compatible with the microelectronics fabrication process has not yet been developed. Here a universal epitaxial growth strategy is reported guided by 1D van der Waals-coupling, to fabricate freestanding and flexible hybrids comprised of single-wall carbon nanotubes and ordered (Bi,Sb)2 Te3 nanocrystals. High power factors ranging from ≈1680 to ≈1020 µW m-1 K-2 in the temperature range of 300-480 K, combined with a low thermal conductivity yield a high average figure of merit of ≈0.81. The fabricated flexible micro-TED module consisting of two p-n couples of freestanding thermoelectric hybrids has an unprecedented open circuit voltage of ≈22.7 mV and a power density of ≈0.36 W cm-2 under ≈30 K temperature difference, and a net cooling temperature of ≈22.4 K and a heat absorption density of ≈92.5 W cm-2 .

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.

Evolution of Thermoelectric Generators: From Application to Hybridization

Considerable thermal energy is emitted into the environment from human activities and equipment operation in the course of daily production. Accordingly, the use of thermoelectric generators (TEGs) can attract wide interest, and it shows high potential in reducing energy waste and increasing energy recovery rates. Notably, TEGs have aroused rising attention and been significantly boosted over the past few years, as the energy crisis has worsened. The reason for their progress is that thermoelectric generators can be easily attached to the surface of a heat source, converting heat energy directly into electricity in a stable and continuous manner. In this review, applications in wearable devices, and everyday life are reviewed according to the type of structure of TEGs. Meanwhile, the latest progress of TEGs' hybridization with triboelectric nanogenerator (TENG), piezoelectric nanogenerator (PENG), and photovoltaic effect is introduced. Moreover, prospects and suggestions for subsequent research work are proposed. This review suggests that hybridization of energy harvesting, and flexible high-temperature thermoelectric generators are the future trends.

Carbon Nanotube-Polyurethane Composite Sheets for Flexible Thermoelectric Materials

Integration of single-wall carbon nanotubes (SWCNTs) in the form of fabriclike sheets or other preformed assemblies (films, fibers, etc.) simplifies their handling and allows for composites with higher nanotube contents, which is needed to better exploit their outstanding properties and achieve multifunctional materials with improved performance. Here, we show the development of p-type SWCNT-thermoplastic polyurethane (TPU) fabric materials with a wide range of SWCNT contents (from 5 to 90 wt %) by employing a one-step filtration method using a suspension of SWCNTs in a TPU solvent/nonsolvent mixture. The mechanical and thermoelectric (TE) properties of these SWCNT-TPU nanocomposites were tailored by varying the SWCNT/TPU wt % ratio, achieving significant advantages relative to the pristine SWCNT buckypaper (BP) sheets in terms of strength and stretchability. In particular, the SWCNT-TPU nanocomposite with a 50/50 wt % ratio composition (equivalent to 15 vol % of SWCNTs) shows a power factor (PF) of 57 μW m-1 K-2, slightly higher compared to the PF of the SWCNT BP prepared under the same conditions (54 μW m-1 K-2), while its mechanical properties significantly increased (e.g., ∼7-, 25-, and 250-fold improvements in stiffness, strength, and tensile toughness, respectively). These results represent a significant step toward the development of easy-to-process self-supporting and stretchable materials with robust mechanical properties for flexible thermoelectric devices.

High-Performance Thermoelectric Flexible Ag2Se-Based Films with Wave-Shaped Buckling via a Thermal Diffusion Method

Herein, an n-type Ag2Se thermoelectric flexible thin film has been fabricated on a polyimide (PI) substrate via a novel thermal diffusion method, and the thermoelectric performance is well-optimized by adjusting the pressure and temperature of thermal diffusion. All of the Ag2Se films are beneficial to grow (013) preferred orientations, which is conducive to performing a high Seebeck coefficient. By increasing the thermal diffusion temperature, the electrical conductivity can be rationally regulated while maintaining the independence of the Seebeck coefficient, which is mainly attributed to the increased electric mobility. As a result, the fabricated Ag2Se thin film achieves a high power factor of 18.25 μW cm-1 K-2 at room temperature and a maximum value of 21.7 μW cm-1 K-2 at 393 K. Additionally, the thermal diffusion method has resulted in a wave-shaped buckling, which is further verified as a promising structure to realize a larger temperature difference by the simulation results of finite element analysis (FEA). Additionally, this unique surface morphology of the Ag2Se thin film also exhibits outstanding mechanical properties, for which the elasticity modulus is only 0.42 GPa. Finally, a flexible round-shaped module assembled with Sb2Te3 has demonstrated an output power of 166 nW at a temperature difference of 50 K. This work not only introduces a new method of preparing Ag2Se thin films but also offers a convincing strategy of optimizing the microstructure to enhance low-grade heat utilization efficiency.