Self‐Healing and Stretchable 3D‐Printed Organic Thermoelectrics

With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy‐harvesting solutions that are compatible with such wearables. However, many of the proposed energy‐harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self‐healability is essential to realize self‐powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self‐healing and stretchable properties produced via 3D‐printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self‐healable and solution‐processable composite, 3D‐printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self‐healing. This approach represents a significant step in achieving damage‐free and truly wearable 3D‐printed organic thermoelectrics.     

Doping Dependent In‐Plane and Cross‐Plane Thermoelectric Performance of Thin n‐Type Polymer P(NDI2OD‐T2) Films

Thermoelectric generators pose a promising approach in renewable energies as they can convert waste heat into electricity. In order to build high efficiency devices, suitable thermoelectric materials, both n‐ and p‐type, are needed. Here, the n‐type high‐mobility polymer poly[N,N′‐bis(2‐octyldodecyl)naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene) (P(NDI2OD‐T2)) is focused upon. Via solution doping with 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)‐N,N‐diphenylaniline (N‐DPBI), a maximum power factor of (1.84 ± 0.13) µW K^?2 m^?1 is achieved in an in‐plane geometry for 5 wt% dopant concentration. Additionally, UV–vis spectroscopy and grazing‐incidence wide‐angle X‐ray scattering are applied to elucidate the mechanisms of the doping process and to explain the discrepancy in thermoelectric performance depending on the charge carriers being either transported in‐plane or cross‐plane. Morphological changes are found such that the crystallites, built‐up by extended polymer chains interacting via lamellar and π–π stacking, re‐arrange from face‐ to edge‐on orientation upon doping. At high doping concentrations, dopant molecules disturb the crystallinity of the polymer, hindering charge transport and leading to a decreased power factor at high dopant concentrations. These observations explain why an intermediate doping concentration of N‐DPBI leads to an optimized thermoelectric performance of P(NDI2OD‐T2) in an in‐plane geometry as compared to the cross‐plane case.     

Self‐Healable and Stretchable Organic Thermoelectric Materials: Electrically Percolated Polymer Nanowires Embedded in Thermoplastic Elastomer Matrix

Self‐healable and stretchable energy‐harvesting materials can provide a new avenue for the realization of self‐powered wearable electronics, including electronic skins, whose main materials are required to be robust to and stable under external damage and severe mechanical stresses. However, thermoelectric (TE) materials showing both self‐healing properties and stretchability have not yet been demonstrated despite their great potential to harvest thermal energy in the human body. As most existing TE materials are either mechanically brittle or unrecoverable after being subjected to damage, a novel approach is necessary for designing such materials. Herein, self‐healable and stretchable TE materials based on all‐organic composite system wherein polymer semiconductor nanowires are p‐doped with a molecular dopant and embedded in a thermoplastic elastomer matrix are reported. The polymer nanowires are electrically percolated in the matrix, and the resulting composite materials exhibit good TE performance. The composites also exhibit both excellent self‐healing properties under mild heat and pressure conditions and good stretchability. It is believed that this work can be a cornerstone for the design of self‐healable and stretchable energy‐harvesting materials as it provides useful guidelines for imparting electrical conductivity to insulating thermoplastic elastomers, which typically possess versatile and useful mechanical properties.     

Simple Salt‐Coordinated n‐Type Nanocarbon Materials Stable in Air

After more than three decades of molecular and carbon‐based electronics, the creation of air‐ and thermally stable n‐type materials remains a challenge in the development of future p/n junction devices such as solar cells and thermoelectric modules. Here a series of ordinary salts are reported such as sodium chloride (NaCl), sodium hydroxide (NaOH), and potassium hydroxide (KOH) with crown ethers as new doping reagents for converting single‐walled carbon nanotubes to stable n‐type materials. Thermoelectric analyses reveal that these new n‐type single‐walled carbon nanotubes display remarkable air stability even at 100 °C for more than 1 month. Their thermoelectric properties with a dimensionless figure‐of‐merit (ZT) of 0.1 make these new n‐type single‐walled carbon nanotubes a most promising candidate for future n‐type carbon‐based thermoelectric materials.     

Highly Stretchable,Resilient, Adhesive,and Self-Healing Ionic Hydrogels for Thermoelectric Application

Harvesting low-grade waste heat from the natural environment with thermoelectric materials is considered as a promising solution for the sustainable energy supply for wearable electronic devices. For practical applications, it is desirable to endow the thermoelectric materials with excellent mechanical and self-healing properties, which remains a great challenge. Herein, the design and characterization of a series of high-performance ionic hydrogels for soft thermoelectric generator applications are reported. Composed of a physically cross-linked network of polyacrylic acid (PAA) and polyethylene glycol (PEO) doped with sodium chloride, the resulting PAA-PEO-NaCl ionic hydrogels demonstrates impressive mechanical strength (breaking stress >1.3 MPa), stretchability (>1100%), and toughness (up to 7.34 MJ m⁻³). Moreover, the reversible hydrogen bonding interaction and chain entanglement render the ionic hydrogels with excellent mechanical resilience, adhesion properties, and self-healing properties. At ambient conditions, the electrochemical and thermoelectric performance of the ionic hydrogels can be restored immediately from physical damage such as cutting, and the mechanical healing can be completely restored within 24 h. At the optimized composition, the Seebeck coefficient of the ionic hydrogels can reach 3.26 mV K⁻¹ with a low thermal conductivity of 0.321 W m⁻¹ K⁻¹. Considering the excellent mechanical properties and thermoelectric performance, it is believed that the ionic hydrogels are widely applicable in ionic thermoelectric capacitors to convert low-grade heat into electricity for soft electronic devices.     

Energy Filtering of Charge Carriers: Current Trends,Challenges, and Prospects for Thermoelectric Materials

Exhaustive attempts are made in recent decades to improve the performance of thermoelectric materials that are utilized for waste heat‐to‐electricity conversion. Energy filtering of charge carriers is directed toward enhancing the material thermopower. This paper focuses on the theoretical concepts, experimental evidence, and the authors' view of energy filtering in the context of thermoelectric materials. Recent studies suggest that not all materials experience this effect with the same intensity. Although this effect theoretically demonstrates improvement of the thermopower, applying it poses certain constraints, which demands further research. Predicated on data documented in literature, the unusual dependence of the thermopower and conductivity upon charge carrier concentrations can be altered through the energy filtering approach. Upon surmounting the physical constraints discussed in this article, thermoelectric materials research may gain a new direction to enhance the power factor and thermoelectric figure of merit.     

热释电红外检测器研究探讨

热释电材料在吸收红外辐射后其温度会发生变化,从而产生极化.将其连接在电路中,则会有与温度变化率成比例的电信号产生.热释电红外检测器就是利用材料的热释电效应在红外光谱范围内检测物体吸收到的辐射能量的.根据形态,热释电材料分为体材料和薄膜材料.相比之下,后者具有体积比热小、灵敏度高、成本低和易于集成化的特性,因而其制备、性能和应用已成为热释电材料的研究方向.目前热释电红外检测器的研究已经取得了实用性的成果,如辐射计、测速计、测温计等实用设备或仪器的制作生产.随着研究的深入,具有广阔市场前景的热释电红外检测器将得到更为广泛的应用.     

Recent Advances in Liquid‐Like Thermoelectric Materials

Thermoelectric technology has attracted great attention due to its ability to recover and convert waste heat into readily available electric energy. Among the various candidate materials, liquid‐like compounds have received tremendous research interest on account of their intrinsically ultralow lattice thermal conductivity, tunable electrical properties, and high thermoelectric performance. Despite their complex phase transitions and diverse crystal structures, liquid‐like materials have two independent sublattices in common: one rigid sublattice formed by immobile ions for the free transport of electrons and one liquid‐like sublattice consisting of highly mobile ions to interrupt the thermal transports. This review first outlines the common structural features of liquid‐like thermoelectrics, along with their unusual electron and phonon transport behaviors that well satisfy the concept of “phonon‐liquid electron‐crystal.” Next, some commonly adopted strategies for further improving their thermoelectric performance are highlighted. The main progress achieved in the typical liquid‐like TE materials is then summarized, with an emphasis on their diverse crystal structures, common characteristics, and unique transport properties. The recent understandings on the stability issue of liquid‐like TE materials are also introduced. Finally, an outlook is given for the liquid‐like materials with the aim to boost further development in this exciting scientific subfield.     

Unveiling the Correlation between the Crystalline Structure of M‐Filled CoSb3 (M = Y,K, Sr) Skutterudites and Their Thermoelectric Transport Properties

Skutterudite‐type pnictides based on CoSb₃ are promising semiconductor materials for thermoelectric applications. An exhaustive structural characterization by synchrotron X‐ray powder diffraction of different M‐filled CoSb₃ (M = Y, K, Sr, La, Ce, Yb) skutterudites, with a panoply of M atoms with very different chemical nature, allows to better understand the effects of filling from a crystallo‐chemical point of view. These analyses focus on the correlation of chemical and structural features with the enhanced thermoelectric properties displayed by certain families of filled‐CoSb₃ skutterudites. These are mainly determined by Sb positional parameters, yielding Oftedal plots that depend on the filling fraction, ionic state, and atomic radius of the filler. Together with the distortion of [Sb₄] rings and [CoSb₆] octahedra present in the skutterudite structure, these results are linked to the band‐convergence concept and its influence on the thermoelectric transport properties. Here, the structural changes observed in the different chemical compositions are relevant to understand the improved thermoelectric performance of single partially filled n‐type skutterudites.     

Monolithic Dual‐Material 3D Printing of Ionic Skins with Long‐Term Performance Stability

Artificial “ionic skin” is of great interest for mimicking the functionality of human skin, such as subtle pressure sensing. However, the development of ionic skin is hindered by the strict requirements of device integration and the need for devices with satisfactory performance. Here, a dual‐material printing strategy for ionic skin fabrication to eliminate signal drift and performance degradation during long‐term use is proposed, while endowing the ionic skins with high sensitivity by 3D printing of ionic hydrogel electrodes with microstructures. The ionic skins are fabricated by alternative digital light processing 3D printing of two photocurable precursors: hydrogel and water‐dilutable polyurethane acrylate (WPUA), in which the ionically conductive hydrogel layers serve as soft, transparent electrodes and the electrically insulated WPUA as flexible, transparent dielectric layers. This novel dual‐material printing strategy enables strong chemical bonding between the hydrogel and the WPUA, endowing the device with designed characteristics. The resulting device has high sensitivity, minimal hysteresis, a response time in the millisecond range, and excellent repetition durability for pressure sensing. The results demonstrate the potential of the dual‐material 3D printing strategy as a pathway to realize highly stable and high‐performance ionic skin fabrication to monitor human physiological signals and human–machine interactions.     

Modulated Thermoelectric Properties of Organic Semiconductors Using Field‐Effect Transistors

Organic thermoelectric materials, which can transform heat flow into electricity, have great potential for flexible, ultra‐low‐cost and large‐area thermoelectric applications. Despite rapid developments of organic thermoelectric materials, exploration and investigation of promising organic thermoelectric semiconductors still remain as a challenge. Here, the thermoelectric properties of several p‐ and n‐type organic semiconductors are investigated and studied, in particular, how the electric field modulations of the Seebeck coefficient in organic field‐effect transistors (OFETs) compare with the Seebeck coefficient in chemically doped films. The extracted relationship between the Seebeck coefficient (S) and electrical conductivity (σ) from the field‐effect transistor (FET) geometry is in good agreement with that of chemically doped films, enabling the investigation of the trade‐off relationship among σ, S, carrier concentration, and charging level. The results make OFETs an effective candidate for the thermoelectric studies of organic semiconductors.     

n‐Type Quinoidal Oligothiophene‐Based Semiconductors for Thin‐Film Transistors and Thermoelectrics

Organic electronic devices have gained immense popularity in the last 30 years owing to their increasing performance. Organic thin‐film transistors (OTFTs) are one of the basic organic electronic devices with potential industrial applications. Another class of devices called organic thermoelectric (OTE) materials can directly transform waste heat into usable electrical power without causing any pollution. p‐Type transistors outperform n‐type transistors because the latter requires a lower orbital energy level for efficient electron injection and stable electron transport under ambient conditions. Aromatic building blocks can be utilized in constructing n‐type semiconductors. Quinoidal compounds are another promising platform for optoelectronic applications because of their unique properties. Since their discovery in 1970s, quinoidal oligothiophene‐based n‐type semiconductors have drawn considerable attention as candidates for high‐performance n‐type semiconductors in OTFTs and OTEs. Herein, the development history of quinoidal oligothiophene‐based semiconductors is summarized, with a focus on the molecular design and the influence of structural modification on molecular packing and thus the device performance of the corresponding quinoidal oligothiophene‐based semiconductors. Insights on the potential of quinoidal oligothiophenes for high‐performance n‐type OTFTs and OTEs are also provided.     

Highly Bendable Ionic Soft Actuator Based on Nitrogen‐Enriched 3D Hetero‐Nanostructure Electrode

Electrically responsive ionic soft actuators that can exhibit large bending strain under low electrical input power are promising candidates for future soft electronics and wearable devices. However, some drawbacks such as low blocking force, slow response time, and poor durability should be overcome for practical engineering applications. Herein, this study reports defect‐engineered 3D graphitic carbon nitride (GCN) and nitrogen‐doped graphene (NG) hetero‐nanostructure that were developed by one‐pot hydrothermal method in order to design functionally antagonistic hybrid electrodes for superior ionic soft actuators. While NG facilitates rapid electron transfer in 3D networked nanoarchitectures, the enriched‐nitrogen content in GCN provides good wettability and mechanical resiliency with poly(3,4 ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The 3D hybrid nanostructures generate unimpeded ion channels and sufficient contact area with the electrolyte membrane to provide higher capacitance and mechanical integrity, which are critical prerequisites for high‐performance actuation. The developed soft actuator based on the nitrogen‐enriched 3D hetero‐nanostructure is found to exhibit large bending strain (0.52%), wide frequency response, 5 h durability (93% retention), 2.4 times higher bending displacement, and twofold higher electromechanical efficiency compared to PEDOT:PSS under ±0.5 V input voltage. Such 3D functionally antagonistic hybrid electrodes offer hitherto unavailable opportunities in developing ultralow voltage‐driven ionic actuators for the next‐generation soft electronics.     

High Energy Density All‐Solid‐State Batteries: A Challenging Concept Towards 3D Integration

Rechargeable all‐solid‐state batteries will play a key role in many autonomous devices. Planar solid‐state thin film batteries are rapidly emerging but reveal several drawbacks, such as a relatively low energy density and the use of highly reactive metallic lithium. In order to overcome these limitations a new 3D‐integrated all‐solid‐state battery concept with significantly increased surface area is presented. By depositing the active battery materials into high‐aspect ratio structures etched in, for example silicon, 3D‐integrated all‐solid‐state batteries are calculated to reach a much higher energy density. Additionally, by adopting novel high‐energy dense Li‐intercalation materials the use of metallic Lithium can be avoided. Sputtered Ta, TaN and TiN films have been investigated as potential Li‐diffusion barrier materials. TiN combines a very low response towards ionic Lithium and a high electronic conductivity. Additionally, thin film poly‐Si anodes have been electrochemically characterized with respect to their thermodynamic and kinetic Li‐intercalation properties and cycle life. The Butler‐Vollmer relationship was successfully applied, indicating favorable electrochemical charge transfer kinetics and solid‐state diffusion. Advantageously, these new Li‐intercalation anode materials were found to combine an extremely high energy density with fast rate capability, enabling future 3D‐integrated all‐solid‐state batteries.     

Enhancement of Thermoelectric Figure‐of‐Merit by a Bulk Nanostructuring Approach

Recently a significant figure‐of‐merit (ZT) improvement in the most‐studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high‐energy ball milling and a direct‐current‐induced hot‐press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.     

Responsive Macroscopic Materials From Self‐Assembled Cross‐Linked SiO2‐PNIPAAm Core/Shell Structures

A way to obtain macroscopic responsive materials from silicon‐oxide polymer core/shell microstructures is presented. The microparticles are composed of a 60 nm SiO₂‐core with a random copolymer corona of the temperature responsive poly‐N‐isopropylacrylamide (PNIPAAm) and the UV‐cross‐linkable 2‐(dimethyl maleinimido)‐N‐ethyl‐acrylamide. The particles shrink upon heating and form a stable gel in both water and tetrahydrofuran (THF) at 3–5 wt% particle content. Cross‐linking the aqueous gel results in shrinkage when the temperature is increased above the lower critical solution temperature and it regains its original size upon cooling. By freeze drying with subsequent UV irradiation, thin stable layers are prepared. Stable fibers are produced by extruding a THF gel into water and subsequent UV irradiation, harnessing the cononsolvency effect of PNIPAAm in water/THF mixtures. The temperature responsiveness translates to the macroscopic materials as both films and fibers show the same collapsing behavior as the microcore/shell particle. The collapse and re‐swelling of the materials is related to the expelling and re‐uptake of water, which is used to incorporate gold nanoparticles into the materials by a simple heating/cooling cycle. This allows for future applications, as various functional particles (antibacterial, fluorescence, catalysis, etc.) can easily be incorporated in these systems.     

Methods for Enhancing the Thermal Durability of High-Temperature Thermoelectric Materials

Thermoelectric materials, for example skutterudites and magnesium silicides, are being investigated as promising materials for medium-to-high-temperature waste heat recovery in transport and in industry. A crucial aspect of the success of a thermoelectric material is its stability over time when exposed to rapid heating and cooling. In this work different aspects of the degradation of these thermoelectric materials at high temperature were examined. Initial thermal durability was studied, and several candidate coatings were evaluated to enhance durability by protecting the materials from oxidation and sublimation during thermal cycles in air for up to 500 h and up to 873 K. The samples were characterized by SEM and EDS. The results showed it is possible to reduce degradation of the thermoelectric material without compromising overall thermoelectric efficiency.     

Simulation and Optimization for System Integration of a Solar Thermoelectric Device

We established a three-dimensional (3D) finite-element model of a solar thermoelectric device (STED) based on high-performance thermoelectric materials and carried out performance analysis under different operating conditions. The effects of input energy, natural convection, radiation, contact resistance, and electric load on STED performance are taken into account. It is shown that heat losses and contact resistance have a significant impact on conversion efficiency. If contact resistance and all heat losses are neglected, the total STED efficiency can reach 9.95%.     

Green Biocomposites for Thermoelectric Wearable Applications

The materials commonly used to fabricate thermoelectric devices are tellurium, lead, and germanium. These materials ensure the best thermoelectric performance, but exhibit drawbacks in terms of availability, sustainability, cost, and manufacturing complexity. Moreover, they do not guarantee a safe and cheap implementation in wearable thermoelectric applications. Here, p‐Type and n‐type flexible thermoelectric textiles are produced with sustainable and low‐cost materials through green and scalable processes. Cotton is functionalized with inks made with biopolyester and carbon nanomaterials. Depending on the nanofiller, i.e., graphene nanoplatelets, carbon nanotubes, or carbon nanofibers, positive or negative Seebeck coefficient values are obtained, resulting in a remarkable electrical conductivity value of 55 S cm^?1 using carbon nanotubes. The best bending and washing stability are registered for the carbon nanofiber‐based biocomposites, which increase their electrical resistance by 5 times after repeated bending cycles and only by 30% after washing. Finally, in‐plane flexible thermoelectric generators coupling the best p‐ and n‐type materials are fabricated and analysed, resulting in an output voltage of ≈1.65 mV and a maximum output power of ≈1.0 nW by connecting only 2 p/n thermocouples at a temperature difference of 70 °C.     

Nanoscale Organic Thermoelectric Materials: Measurement,Theoretical Models,and Optimization Strategies

The demands for waste heat energy recovery from industrial production, solar energy, and electronic devices have resulted in increasing attention being focused on thermoelectric materials. Over the past two decades, significant progress is achieved in inorganic thermoelectric materials. In addition, with the proliferation of wireless mobile devices, economical, efficient, lightweight, and bio‐friendly organic thermoelectric (OTE) materials have gradually become promising candidates for thermoelectric devices used in room‐temperature environments. With the development of experimental measurement techniques, the manufacturing for nanoscale thermoelectric devices has become possible. A large number of studies have demonstrated the excellent performance of nanoscale thermoelectric devices, and further improvement of their thermoelectric conversion efficiency is expected to have a significant impact on global energy consumption. Here, the development of experimental measurement methods, theoretical models, and performance modulation for nanoscale OTE materials are summarized. Suggestions and prospects for the future development of these devices are also provided.