Flexible Thermoelectric Devices Research Articles

Layer-by-Layer Flexible Organic Thermoelectric Devices based on PEDOT:PSS and PBFDO

Recent advances in n-type conducting polymers are beginning to rival those of p-type materials. Notably, the n-type conducting polymer poly(benzodifurandione) (PBFDO) demonstrates a notable Seebeck coefficient along with exceptionally high electrical conductivity, positioning it as a promising n-type thermoelectric material with substantial research potential. Despite its promise, the exploration of PBFDO’s thermoelectric properties and the development of related thermoelectric devices have been limited. In this study, we introduce a flexible thermoelectric device that utilizes a combination of the p-type polymer poly(3,4ethylenedioxythiophene):polystyrene sulfonate and the n-type polymer PBFDO, using a straightforward print-and-fold technique. This approach enabled the production of flexible devices with thermoelectric generators whose properties were assessed. The polymer films and the resultant devices demonstrated commendable performance stability even after being subjected to 1,000 bending cycles at a 90° angle. Our findings corroborate the potential of PBFDO as a viable material for flexible thermoelectric applications, a development that is eagerly anticipated in the field.

Extraordinary self-powered Y-shaped flexible film thermoelectric device for wearables

Herein, by using the ductile thermoelectric material Ag2Se0.67S0.33, we have successfully developed Y-shaped flexible film thermoelectric devices with excellent flexibility and record-high output power density when worn on the human body.

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.

High-performance flexible thermoelectric devices with a copper foam heatsink for personal thermal management

A Bi2Te3-based thermoelectric device with high thermoelectric performance and flexibility for human body waste heat recovery and temperature management.

Kirigami-inspired organic and inorganic film-based flexible thermoelectric devices with built-in heat sink

Thermoelectric (TE) devices can convert heat to electricity directly, which offers a unique opportunity to realize waste heat recovery. However, conventional TE devices inevitably use heat sinks, which are bulky, rigid and heavy, limiting practical applications. Herein, we propose a fully integrated film-based TE device with intrinsically built-in fins as heat sink in a hexagonal honeycomb device structure, that simultaneously achieves high TE performance and conformability, as confirmed by experiments and modelling. A flexible Kapton substrate with copper electrodes, integrating either carbon nanotube (CNT) veils or bismuth telluride (Bi2Te3) TE ‘legs’, both of n- and p-type, achieved a remarkable specific power of 185.4 nW K−2 for a Bi2Te3-based device and 53.1 nW K−2 for a CNT-based device, thanks to the heat dissipation effect granted by the built-in fins. Besides, the addition of oriented polymer films interconnects, contracting when above their glass transition temperature, allowed a single substrate two-dimensional (2D) TE device to self-fold into a three-dimensional (3D) hexagonal honeycomb structure, with built-in fins, contactlessly and autonomously. The demonstrated shape-programmed kirigami-inspired scalable TE device paves the way for realising self-powered applications comprising hundreds of TE legs with both inorganic (e.g., Bi2Te3) and organic (e.g. CNT veils) TE materials and integrated heat sinks.

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.

Electronic skin with biomimetic structures realizes excellent isothermal regulation

Simulating the various functions of the human body in electronic skin to give it a higher level of bionics is the basis for realizing intelligent interaction. Isothermal regulation is the key to realizing a healthy life of the body, yet only simple heating or cooling is achieved in electronic skin. Here, we propose a novel thermoregulating electronic skin (thermo-e-skin) with biomimetic structures, which mimics the human body thermoregulation mechanism by integrating a flexible thermoelectric device with a hydrogel composite material. Through the delicate balance of heat generation and dissipation, the thermo-e-skin maintained a stable surface temperature of 35 °C across a wide range of ambient temperatures from 10 °C to 45 °C for extended durations (e.g., 4.2 h at an ambient temperature of 40 °C). Additionally, the thermo-e-skin, equipped with water regeneration capabilities, exhibits exceptional cyclic performance, further demonstrating its practical potential for soft electronics, bionics, and prosthetics.

High Thermoelectric Power Factors in Plastic/Ductile Bulk SnSe2 -Based Crystals.

The recently discovered plastic/ductile inorganic thermoelectric (TE) materials open a new avenue for the fabrication of high-efficiently flexible TE devices, which can utilize the small temperature difference between human body and environment to generate electricity. However, the maximum power factor (PF) of current plastic/ductile TE materials is usually around or less than 10 µW cm-1 K-2 , much lower than the classic brittle TE materials. In this work, a record-high PF of 18.0 µW cm-1 K-2 at 375 K in plastic/ductile bulk SnSe2 -based crystals is reported, superior to all the plastic inorganic TE materials and flexible organic TE materials reported before. The origin of such high PF is from the modulation of material's stacking forms and polymorph crystal structures via simultaneously doping Cl/Br at Se-site and intercalating Cu inside the van der Waals gap, leading to the significantly enhanced carrier concentrations and mobilities. An in-plane fully flexible TE device made of the plastic/ductile SnSe2 -based crystals is successfully developed to show a record-high normalized maximum power density to 0.18W m-1 under a temperature difference of 30 K. This work indicates that the plastic/ductile material can realize high TE power factor to achieve large output electric power density in flexible TE technology.

Advances in flexible inorganic thermoelectrics

AbstractSolid‐state bismuth telluride‐based thermoelectric devices enable the generation of electricity from temperature differences and have been commercially applied in various fields. However, in many scenarios, the surface of the heat source is not flat. Therefore, it is crucial to develop flexible thermoelectric materials and devices to efficiently utilize heat sources and expand their applications. Compared with organic thermoelectric materials and devices, inorganic flexible thermoelectric materials and devices have much higher thermoelectric performance and stability. Considering the rapid development in this research field, we carefully summarize the design principles, structures, and thermoelectric properties of inorganic flexible materials and their devices reported in the recent 3 years, including sulfides, selenides, tellurides, and composite materials designed based on these inorganics. The structural designs of flexible thermoelectric devices based on micro‐sized bulk materials are also carefully summarized. Additionally, we overview the mechanical stability and methods for reducing internal resistance for designs of inorganic flexible thermoelectric devices. In the end, we provide outlooks on future research directions for inorganic flexible thermoelectric materials and devices. This review will help guide thermoelectric researchers, beginners, and students.

Improved thermoelectric power factor of multilayered poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and Cu2Se thin films

Cu2Se is a promising thermoelectric material due to its superionic behavior with superior transport properties. Here we report a facile method of spin coating for the fabrication of multilayers of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and Cu2Se thermoelectric materials for mid-temperature range applications. The synthesis of multilayered thin films with a unique organic-inorganic hybrid framework could be used in flexible thermoelectric devices. The detailed investigation of PEDOT:PSS and Cu2Se multilayered thin films reveals interaction between organic and inorganic material as inferred by AFM and FTIR. The electronic transport properties were investigated for all specimens, with the highest PF of 20.1 µW/m. K2 at 450 K was achieved from the bilayer stacking of Cu2Se and PEDOT:PSS, which is about 82 % higher as compared to that of pure PEDOT:PSS at the same temperature. These improved transport properties are the combined effect of energy filtering and interface effects. The proposed strategy opens up an avenue for research on chalcogenide based composite materials with an organic framework for flexible thermoelectric device applications.

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.

Insight into Scattering Mechanisms and Transport Properties of AgCuS for Flexible Thermoelectric Applications.

The development of flexible thermoelectric devices requires materials possessing ductility and high thermoelectric performance at room temperature. However, only a few existing materials meet both criteria. In this study, the ductile properties, electronic structure, and transport properties of the low-temperature phase α-AgCuS were elucidated using first-principles calculations combined with Boltzmann transport theory. With a layered zigzag structure similar to the well-known ductile semiconductor Ag2S, AgCuS is determined to have good metal-like ductility. Through consideration of various intrinsic scattering mechanisms, we found that electron-polar optical phonon interactions have the most significant impact on the transport behavior of AgCuS. The predominance of this type of interaction is also disclosed by the covalent-ionic bonding nature of the Ag-S and Cu-S bonds. Therefore, weakening this interaction via doping or alloying could optimize the thermoelectric performance of the system. At room temperature, a maximum dimensionless figure of merit ZT of up to 0.592 could be achieved under a tuning of hole concentration to 2 × 1019 cm-3, suggesting that α-AgCuS could be a promising p-type candidate for flexible thermoelectric applications.

Effective Thermoelectric Switch of Hollow Weakly-Coupled Molecular Junction Based on Twist Angle Effect with Boron-Doping

ABSTRACT Rational design and adjustment of flexible thermoelectric devices are key points for sustainable and effective thermoelectric conversion, which remains a fundamental challenge due to inherent high thermal conductivity and uncontrolled carrier concentration induced by non-uniform dispersion. Under ingenious combination of weakly-coupled hollow interface and nanotube structure, thermoelectric performance of a dumbbell-like molecular junction comprised of a phenyl-terminated polyyne as central molecule and two semi-infinite 1D single-walled carbon nanotube (SWCNT) as electrodes has been investigated at certain twisted angles (θ). The results indicate that molecule twisting can be reviewed as an effective thermoelectric switch to coordinatingly control electronic and phononic transmission properties simultaneously. Resonance of molecular discrete state and electrode continuous state leads to low thermal conductance, which is sensitively affected by twist angle. Meanwhile, cyclic transformation between p-type and n-type flexible thermoelectrics can be realized by manipulating twist angle in a certain period of rotation. Thermoelectric performance of such a molecular junction can be further improved by boron atom doping at head-to-tail positions, and an excellent figure of merit (ZT = 1.75) is observed near Fermi level under 25° twisted angle. This result inspires an effective strategy to modulate and control thermoelectric conversion, which will greatly broaden applications in thermoelectric twistronics.

Mechanical and thermoelectric properties in Te-rich Ag2(Te,S) meta-phases

Ductile Ag2(Te,S) pseudobinary compounds have attracted great attention in thermoelectric community since they can be fabricated into high-performance flexible and hetero-shaped thermoelectric devices. However, in spite of the numerous studies, the ‘brittle–ductile’ transition boundary in Ag2(Te,S) is still unclear. In this work, a series of Te-rich Ag2(Te,S) pseudobinary compounds have been prepared. The structure characterizations confirm they belong to the new-concept of meta-phase. The systematically investigation on the mechanical properties demonstrate that the ‘brittle–ductile’ transition boundary appears around x = 0.1. Unexpected good ductility is observed in the Te-rich Ag2Te1–xSx crystalizing in the Ag2Te room-temperature monoclinic structure and high-temperature cubic structure, which are thought to be brittle before. Likewise, Ag content is found to be a very critical parameter determining the ductility of Te-rich Ag2Te1–xSx. Very slight Ag-deficiency can greatly deteriorate the ductility. The thermoelectric properties of these ductile Te-rich Ag2Te1–xSx pseudobinary compounds are investigated. A maximum thermoelectric figure-of-merit of 0.6 is obtained for Ag2Te0.9S0.1 at 600 K. This work sheds light on the future investigation of Ag2(Te,S) pseudobinary compounds.

Printable graphite-based thermoelectric foam for flexible thermoelectric devices

Flexible thermoelectric materials are primarily composed of organic thermoelectric materials, which suffer from issues such as instability and poor conductivity. To overcome these limitations, this study aims to develop a cost-effective thermoelectric material by utilizing graphite and hydroxypropyl methylcellulose as raw materials. Through a mechanical foaming process, a graphite-based foam with a porous structure is fabricated. The obtained foam exhibits a Seebeck coefficient of approximately 32 μV K−1 and a power factor of around 0.013 μW m−1 K2. Following an analysis of the microstructural morphology, the foam samples are subjected to compression treatment to convert the 3D foam into a porous film and thereby enhance the power generation efficiency of the thermoelectric material. Notably, the Seebeck coefficient of the compressed sample is 41 μV K−1, and its power factor is approximately 6.7 μW m−1 K2. Furthermore, owing to the high plasticity of the foam slurry prior to molding, it can be used to fabricate thermoelectric devices through printing techniques. By employing this approach, a thermoelectric device consisting of nine individual p-type graphite thermoelectric units is printed on the A4 paper. The device produces a temperature difference of 32 K and a thermoelectric potential of 10 mV on a 373 K heating plate. Since both the sample and the paper are flexible, the device can be folded to reduce its size, which broadens the range of potential applications of these thermoelectric materials.

Flexible free-standing BixSey@C/PEDOT:PSS thermoelectric composite film with high-performance electromagnetic interference shielding

Due to the component advantages and structural features, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) based inorganic–organic composite films shows huge application potential in various fields. However, there are few reports on the bi-functions of thermoelectric (TE) and electromagnetic interference (EMI) shielding for the PEDOT:PSS-based functional films. Herein, metal–organic frameworks (MOFs)-derived BixSey@C sheets with average size about ∼ 7.32 μm are obtained via the solvothermal reaction and selenization pyrolysis treatment, and then flexible free-standing BixSey@C/PEDOT: PSS (BSCP) composite films are obtained by a feasible vacuum filtration combined with cold pressing treatment, using different contents of BixSey@C content as inorganic component. Micrometer BixSey@C sheets are uniformly dispersed in the PEDOT:PSS matrix constructing bi-functional composite films with unique layered structure. When the MOFs-derived BixSey@C content is 70%, BSCP-70 composite film gets a power factor of 11.23 μW m−1 K−2 at 340 K. Meanwhile, the BSCP-70 composite film reaches the highest average total EMI shielding effectiveness (SET) value of 45.0 dB at only 41.9 μm thickness at X-band. Based on multi-physical properties of MOF-derived BixSey@C and PEDOT:PSS, flexible BixSey@C/PEDOT:PSS composite films display good application potentiality in the flexible TE devices and EMI shielding protection.

Advances in Flexible Thermoelectric Materials and Devices Fabricated by Magnetron Sputtering

Due to the direct conversion between thermal and electrical energy, thermoelectric materials and their devices exhibit great potential for power generation and refrigeration. With the rapid development of personal wearable electronics, the design of flexible inorganic thermoelectric materials and devices receives increasing attention. As one of the most mature thin‐film fabrication techniques, magnetron sputtering plays a key role in the fabrication of inorganic thermoelectric thin films and devices, but its progress is still not timely and comprehensively reviewed. Herein, recent advances in magnetron sputtering‐fabricated thermoelectric materials and devices are studied, including their thermoelectric properties, mechanical properties, and device design routes. The differences in the properties of thermoelectric materials under different sputtering conditions, as well as their underlying mechanisms, are carefully discussed. In the end, it is pointed out the challenges and future directions for magnetron sputtering‐prepared inorganic thermoelectric thin‐film materials and devices for practical applications. This review can serve as a useful reference to guide the design of inorganic thermoelectric materials and devices prepared by magnetron‐sputtering‐based deposition techniques.

Ag2Se Nanorod Arrays with Ultrahigh Room Temperature Thermoelectric Performance and Superior Mechanical Properties.

Ag2Se is an intriguing material for room-temperature energy harvesting. Herein, we report the fabrication of Ag2Se nanorod arrays by glancing angle deposition technique (GLAD) followed by simple selenization in a two-zone furnace. Ag2Se planar films of different thickness were also prepared. The unique tilted Ag2Se nanorod arrays show excellent zT = 1.14 ± 0.09 and a power factor of 3229.21 ± 149.01 μW/m-K2, respectively, at 300 K. The superior thermoelectric performance of Ag2Se nanorod arrays compared to planar Ag2Se films could be ascribed to the unique nanocolumnar architecture that not only facilitates efficient electron transport but also significantly scatters phonons at the interfaces. Furthermore, the nanoindentation measurements were performed to explore mechanical properties of the as-prepared films. The Ag2Se nanorod arrays showed hardness values of 116.51 ± 4.25 MPa and elastic modulus of 10,966.01 ± 529.61 MPa, which are lowered by 51.8 and 45.6%, compared to Ag2Se films, respectively. The synergetic dependence between the tilt structure and thermoelectric properties accompanied with the simultaneous improvement in mechanical properties opens a new avenue for the practical applications of Ag2Se in next-generation flexible thermoelectric devices.

A hybrid ceramic-based flexible thermoelectric nanogenerator with enhanced thermopower for human energy harvesting

The growing demand for power sources to run portable devices has led to the development of wearable energy-harvesting technology. The integration of wearable thermoelectric generators (WTEGs) into fabrics that can incessantly convert body heat into electricity has been researched extensively. Yet, commercial fabric-based wearable thermoelectric generators have not become conscious. In this study, a fabric-based flexible wearable thermoelectric device was designed using Cobalt doped ZnO nanorods via hydrothermal method as the n-type and Bi2Te3 nanostructures via dip coating method as the p-type and strips of each were connected to form TE legs. The structural and morphological analyses confirmed the formation of Co: ZnO and Bi2Te3 on the fabrics. The electrical conductivity and average Seebeck coefficient of Co:ZnO coated cellulose fabric were 4.80 S cm−1 and 390.5 µV/K. The device was fabricated and showed a device output voltage oscillating from ∼0.3 mV to ∼0.7 mV with a different human body temperature varying from 308 K to 309.4 K. The experimental data showed the output generated from the flexible device to be more suitable for energy harvesting from the human body and can be used in Biomedical sensing applications.

A Photo‐Thermoelectric Twist to Wireless Energy Transfer: Radial Flexible Thermoelectric Device Powered by a High‐Power Laser Beam

AbstractSystems for wireless energy transmission (WET) are gaining prominence nowadays. This work presents a WET system based on the photo‐thermoelectric effect. With an incident laser beam at λ = 1450 nm, a temperature gradient is generated in the radial flexible thermoelectric (TE) device, with a carbon‐based light collector in its center to enhance the photoheating. The three‐part prototype presents a unique approach by using a radial TE device with one simple manufacturing process ‐ screen‐printing. A TE ink with a polymeric matrix of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate and doped‐Poly(vinyl alcohol) with Sb‐Bi‐Te microparticles is developed (S∽33 µVK−1 and s∽10.31 Sm−1), presenting mechanical and electrical stability. Regarding the device, a full electrical analysis is performed, and the influence of the light collector is investigated using thermal tests, spectrophotometry, and numerical simulations. A maximum output voltage (Vout) of ∽16 mV and maximum power density of ∽25 µWm−2 are achieved with Plaser = 2 W. Moreover, the device's viability under extreme conditions is explored. At T∽180 K, a 25% increase in Vout compared to room‐temperature conditions is achieved, and at low pressures (∽10‒6 Torr), an increase of 230% is obtained. Overall, this prototype allows the supply of energy at long distances and remote places, especially for space exploration.