High-performance and compact-designed flexible thermoelectric modules enabled by a reticulate carbon nanotube architecture

High-Power-Density and Excellent-Flexibility Thermoelectric Generator Based on All-SWCNTs/PVP Composites

Flexible polymer/single-wall carbon nanotube (SWCNT) composites are a vital component for wearable/portable electronics, but the development of their n-type counterpart is laggard. Furthermore, little attention is paid to the interaction between SWCNT and polymers, especially the unconjugated polymers, as well as the conversion mechanism of conduction characteristics. Here, the n-type flexible SWCNTs/Polyvinyl Pyrrolidone (PVP) films are successfully fabricated, where the oxygen atoms in PVP interacted with SWCNT via hydrogen bonds, which can lower the energy barrier of electron tunneling, providing the pathway for the electron transfer. Furthermore, with the increasing synthesis temperature, the hydrogen bonds strengthened and the thermal activation energy further improved, both of which enhanced the electron-donating ability of PVP, resulting in a high-power-factor value of 260 µW m-1 K-2. Based on the optimized SWCNTs/PVP films, a thermoelectric module is assembled, which achieved a power density of 400 µW cm-2 at a temperature difference of 56 K, coupled with excellent flexibility, showing a less than 1% variation of resistance after 5000 bending cycles. It shows the highest output-performance and the best flexibility among the reported SWCNT-based thermoelectric modules. This work provides significant insights into the interaction mechanism and performance optimization of hybrid thermoelectric composites, based on SWCNTs/unconjugated polymers.

Upscaling Thermoelectrics: Micron-Thick, Half-a-Meter-Long Carbon Nanotube Films with Monolithic Integration of p- and n-Legs

In order for organic thermoelectrics to successfully establish their own niche as energy-harvesting materials, they must reach several crucial milestones, including high performance, long-term stability, and scalability. Performance and stability are currently being actively studied, whereas demonstrations of large-scale compatibility are far more limited and for carbon nanotubes (CNTs) are still missing. The scalability challenge includes material-related economic considerations as well as the availability of fast deposition methods that produce large-scale films that simultaneously satisfy the thickness constraints required for thermoelectric modules. Here we report on true solutions of CNTs that form gels upon air exposure, which can then be dried into micron-thick films. The CNT ink can be extruded using a slot-shaped nozzle into a continuous film (more than half a meter in the present paper) and patterned into alternating n- and p-type components, which are then folded to obtain the finished thermoelectric module. Starting from a given n-type film, differentiation between the n and p components is achieved by a simple postprocessing step that involves a partial oxidation reaction and neutralization of the dopant. The presented method allows the thermoelectric legs to seamlessly interconnect along the continuous film, thus avoiding the need for metal electrodes, and, most importantly, it is compatible with large-scale printing processes. The resulting thermoelectric legs retain 80% of their power factor after 100 days in air and about 30% after 300 days. Using the proposed methodology, we fabricate two thermoelectric modules of 4 and 10 legs that can produce maximum power outputs of 1 and 2.4 μW, respectively, at a temperature difference ΔT of 46 K.

Boosting thermoelectric performance of single-walled carbon nanotubes-based films through rational triple treatments

Single-walled carbon nanotubes (SWCNTs)-based thermoelectric materials, valued for their flexibility, lightweight, and cost-effectiveness, show promise for wearable thermoelectric devices. However, their thermoelectric performance requires significant enhancement for practical applications. To achieve this goal, in this work, we introduce rational "triple treatments" to improve the overall performance of flexible SWCNT-based films, achieving a high power factor of 20.29 µW cm-1 K-2 at room temperature. Ultrasonic dispersion enhances the conductivity, NaBH4 treatment reduces defects and enhances the Seebeck coefficient, and cold pressing significantly densifies the SWCNT films while preserving the high Seebeck coefficient. Also, bending tests confirm structural stability and exceptional flexibility, and a six-legged flexible device demonstrates a maximum power density of 2996 μW cm-2 at a 40 K temperature difference, showing great application potential. This advancement positions SWCNT films as promising flexible thermoelectric materials, providing insights into high-performance carbon-based thermoelectrics.

Developing Air-Stable n-Type SWCNT-Based Composites with High Thermoelectric and Robust Mechanical Properties for Wearable Electronics

Flexible organic thermoelectric generators are gaining prominence in wearable electronics, leveraging body heat as an energy source. Their advancement is hindered by the scarcity of air-stable n-type organic materials with robust mechanical properties. This study introduces two new polymers (HDCN4 and HDCN8), created through polycondensation of paraformaldehyde and diamine-terminated poly(ethylene glycol) (PEGDA) with molecular weights of 4000 and 8000 g/mol into single-walled carbon nanotubes (SWCNTs). The resulting HDCN4/SWCNT and HDCN8/SWCNT composites show impressive power factors of 225.9 and 108.2 μW m-1 K-2, respectively, and maintain over 90% in air for over four months without encapsulation. The HDCN4/SWCNT composite also demonstrates significant tensile strength (33.2 MPa) and flexibility (up to 10% strain), which is currently the best mechanically n-type thermoelectric material with such a high power factor reported in the literature. A thermoelectric device based on HDCN4/SWCNT generates 4.2 μW of power with a 50 K temperature difference. Additionally, when used in wearable temperature sensors, these devices exhibit high mechanical reliability and a temperature resolution of 0.1 K. This research presents a viable method to produce air-stable n-type thermoelectric materials with excellent performance and mechanical properties.

N-DMBI Doping of Carbon Nanotube Yarns for Achieving High n-Type Thermoelectric Power Factor and Figure of Merit

The application of carbon nanotube (CNT) yarns as thermoelectric materials for harvesting energy from low-grade waste heat including that generated by the human body, is attracting considerable attention. However, the lack of efficient n-type CNT yarns hinders their practical implementation in thermoelectric devices. This study reports efficient n-doping of CNT yarns, employing 4-(1, 3-dimethyl-2, 3-dihydro-1H-benzimidazole-2-yl) phenyl) dimethylamine (N-DMBI) in alternative to conventional n-dopants, with o-dichlorobenzene emerging as the optimal solvent. The small molecular size of N-DMBI enables highly efficient doping within a remarkably short duration (10 s) while ensuring prolonged stability in air and at high temperature (150 °C). Furthermore, Joule annealing of the yarns significantly improves the n-doping efficiency. Consequently, thermoelectric power factors (PFs) of 2800, 2390, and 1534 µW m-1  K-2 are achieved at 200, 150, and 30 °C, respectively. The intercalation of N-DMBI molecules significantly suppresses the thermal conductivity, resulting in the high figure of merit (ZT) of 1.69×10-2 at 100 °C. Additionally, a π-type thermoelectric module is successfully demonstrated incorporating both p- and n-doped CNT yarns. This study offers an efficient doping strategy for achieving CNT yarns with high thermoelectric performance, contributing to the realization of lightweight and mechanically flexible CNT-based thermoelectric devices.

Synergistic Interactions in Sequential Process Doping of Polymer/Single-Walled Carbon Nanotube Nanocomposites for Enhanced n-Type Thermoelectric Performance

This study focuses on the fabrication of nanocomposite thermoelectric devices by blending either a naphthalene-diimide (NDI)-based conjugated polymer (NDI-T1 or NDI-T2), or an isoindigo (IID)-based conjugated polymer (IID-T2), with single-walled carbon nanotubes (SWCNTs). This is followed by sequential process doping method with the small molecule 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) to provide the nanocomposite with n-type thermoelectric properties. Experiments in which the concentrations of the N-DMBI dopant are varied demonstrate the successful conversion of all three polymer/SWCNT nanocomposites from p-type to n-type behavior. Comprehensive spectroscopic, microstructural, and morphological analyses of the pristine polymers and the various N-DMBI-doped polymer/SWCNT nanocomposites are performed in order to gain insights into the effects of various interactions between the polymers and SWCNTs on the doping outcomes. Among the obtained nanocomposites, the NDI-T1/SWCNT exhibits the highest n-type Seebeck coefficient and power factor of -57.7 µV K-1 and 240.6 µW m-1 K-2 , respectively. However, because the undoped NDI-T2/SWCNT exhibits a slightly higher p-type performance, an integral p-n thermoelectric generator is fabricated using the doped and undoped NDI-T2/SWCNT nanocomposite. This device is shown to provide an output power of 27.2 nW at a temperature difference of 20 K.

Power Generation by Thermal Evaporation Based on a Button Supercapacitor

Thermal evaporation generators exhibit remarkable output performance, sustainability, and economy and, as a result, have attracted considerable interest as a prospective energy-converting technology for harvesting renewable energy. Here, we investigate power generation induced by water evaporation within a button supercapacitor with a simple sandwich structure. For conventional water evaporation devices, the thermodiffusion direction of hydrated ions driven by the Soret effect is opposite to the migration direction of hydrated ions driven by the streaming potential effect during thermal evaporation, which could reduce the output performance of the device. By tuning the thermodiffusion direction to be consistent with the thermal evaporation direction, our button supercapacitor achieves enhanced output performance as high as 674.4 mV, 70.7 mA, and 4.68 mW cm-2 due to the synergistic mechanism of the streaming potential effect and the Soret effect. Moreover, the system could effectively achieve in situ energy generation and storage owing to the device's ability to act as a supercapacitor. Our findings supply a feasible strategy for the synergistic integration of waste energy sources (low-grade waste heat, etc.) to generate electricity.

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

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

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.

Solvent-assisted synthesis of Ag2Se and Ag2S nanoparticles on carbon fabric for enhanced thermoelectric performance

The challenge of developing low-cost, highly flexible, and high-performance thermoelectric (TE) materials persists due to the low thermoelectric efficiency of conducting polymers and the inflexibility of inorganic materials. In this study, we successfully integrated Ag2Se and Ag2S with highly conductive carbon fabric (CF) to produce a flexible thermoelectric material. A facile one-step solvothermal method was employed to synthesize the Ag2Se-CF and Ag2S-CF, which were then subjected to X-ray analysis to confine the phase formation of Ag2Se and Ag2S on the carbon fabric. The analysis revealed that Ag2Se and Ag2S nanoparticles were tightly packed on the surface of carbon fabric, and compositional analysis confirmed the interaction between the material and carbon fabric. The thermoelectric properties of Ag2Se-CF and Ag2S-CF were significantly altered due to carrier concentration and mobility variations, resulting in a low power factor of 6.7 μW/mK2 for Ag2Se-CF and a high-power factor of 24 μW/mK2 at 373 K for Ag2S-CF. The growth of Ag2Se-CF and Ag2S-CF on carbon fabric led to an enhancement in their thermoelectric properties. Further, TE legs were fabricated using the Ag2Se-CF (p-type) and Ag2S-CF (n-type), and the fabricated legs exhibited an output voltage of ∼20 mV to ∼86.65 mV at a temperature gradient (ΔT) of 3-8 K. This work represents a cutting-edge approach to the fabrication of high-performance, wearable thermoelectric devices.

N-type silver ammonia-polyethyleneimine/single-walled carbon nanotube composite films with enhanced thermoelectric properties

Carbon nanotubes and their composite thermoelectric (TE) materials have significant advantages in supplying power to flexible electronics due to their high electrical conductivity, excellent flexibility, and facile preparation technology. In this work, stable n-type silver ammonia-polyethyleneimine/single-walled carbon nanotube ([Ag(NH3)2]+-PEI/SWCNT) composite films were facilely prepared by solution blending and vacuum-filtration methods. The results demonstrate that light silver ammonia doping optimizes the carrier concentration and carrier mobility of the composite film, and a maximum power factor (PF) of [Ag(NH3)2]+-PEI/SWCNT of 91.9 μW m-1 K-2 was obtained, which is higher than that of PEI/SWCNT (70.0 μW m-1 K-2). Furthermore, when the composite films were reduced by the NaBH4 solution, the Seebeck coefficient and the PF value were further increased to -45.5 μV K-1 and 115.8 μW m-1 K-2, respectively. For demonstration, a maximum output voltage of 13.8 mV and output power of 492 nW were achieved using a three p-n junction-based TE device constructed by [Ag(NH3)2]+-PEI/SWCNT at a temperature difference of 50 K. Thus, this study provides a metal complex ion doping strategy to improve thermoelectrical properties and air stability of the PEI/SWCNT composite films, which have potential applications in flexible electronics.

Inter-Shell Sliding in Individual Few-Walled Carbon Nanotubes for Flexible Electronics

Few-walled carbon nanotube (FWCNT) is composed of a few coaxial shells of CNTs with different diameters. The shells in one tube can slide relatively to each other under external forces, potentially leading to regulated electrical properties, which are never explored due to experimental difficulties. In this work, the electromechanical response induced by inter-shell sliding of individual CNTs is studied and revealed the linear electrical current variation for the first time. Based on centimeter-long FWCNTs grown through chemical vapor deposition, controllable and reversible inter-shell sliding is realized while simultaneously recording the electrical current. Reversible and linear current variation with inter-shell sliding is observed, which is consistent with the proposed inter-shell tunneling model. Further, a silk fibroin-assisted transfer technique is developed for long CNTs and realized the fabrication of FWCNT-based flexible devices. Tensile stress can be applied on the FWCNTs@silk film encapsulated in elastic silicone to induce inter-shell sliding and thus controls electrical current, which is demonstrated to serve as a new human-machine interface with high reliability. Besides, it is foreseen that the electromechanical behaviors induced by inter-layer sliding in 1D nanotubes may also be extended to 2D layered materials, shedding new light on the fabrication of novel electronics.

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 .

Molecular Engineering for Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/π-Conjugated Organic Small Molecule Hybrids

Hybridizing single-walled carbon nanotubes (SWCNTs) with π-conjugated organic small molecules (π-OSMs) offers a promising approach for producing high-performance thermoelectric (TE) materials through the facile optimization of the molecular geometry and energy levels of π-OSMs. Designing a twisted molecular structure for the π-OSM with the highest occupied molecular orbital energy level comparable to the valence band of SWCNTs enables effective energy filtering between the two materials. The SWCNTs/twisted π-OSM hybrid exhibits a high Seebeck coefficient of 110.4 ± 2.6 µV K-1 , leading to a significantly improved power factor of 2,136 µW m-1 K-2 , which is 2.6 times higher than that of SWCNTs. Moreover, a maximum figure of merit over 0.13 at room temperature is achieved via the efficient TE transport of the SWCNTs/twisted π-OSM hybrid. The study highlights the promising potential of optimizing molecular engineering of π-OSMs for hybridization with SWCNTs to create next-generation, efficient TE materials.

Toward Self-Powered Sensing and Thermal Energy Harvesting in High-Performance Composites via Self-Folded Carbon Nanotube Honeycomb Structures

The development of high-performance self-powered sensors in advanced composites addresses the increasing demands of various fields such as aerospace, wearable electronics, healthcare devices, and the Internet-of-Things. Among different energy sources, the thermoelectric (TE) effect which converts ambient temperature gradients to electric energy is of particular interest. However, challenges remain on how to increase the power output as well as how to harvest thermal energy at the out-of-plane direction in high-performance fiber-reinforced composite laminates, greatly limiting the pace of advance in this evolving field. Herein, we utilize a temperature-induced self-folding process together with continuous carbon nanotube veils to overcome these two challenges simultaneously, achieving a high TE output (21 mV and 812 nW at a temperature difference of 17 °C only) in structural composites with the capability to harvest the thermal energy from out-of-plane direction. Real-time self-powered deformation and damage sensing is achieved in fabricated composite laminates based on a thermal gradient of 17 °C only, without the need of any external power supply, opening up new areas of autonomous self-powered sensing in high-performance applications based on TE materials.

Optimization of thermoelectric properties of carbon nanotube veils by defect engineering

Carbon nanotubes (CNTs), with their combination of excellent electrical conductivity, Seebeck coefficient, mechanical robustness and environmental stability are highly desired as thermoelectric (TE) materials for a wide range of fields including Internet of Things, health monitoring and environmental remediation solutions. However, their high thermal conductivity (κ) is an obstacle to practical TE applications. Herein, we present a novel method to reduce the κ of CNT veils, by introducing defects, while preserving their Seebeck coefficient and electrical conductivity. Solid-state drawing of a CNT veil embedded within two polycarbonate films generates CNT veil fragments of reducing size with increasing draw ratio. A successive heat treatment, at above the polycarbonate glass-to-rubber transition temperature, spontaneously reconnects the CNT veils fragments electrically but not thermally. Stretching to a draw ratio of 1.5 and heat repairing at 170 °C leads to a dramatic 3.5-fold decrease in κ (from 46 to 13 W m-1 K-1), in contrast with a decrease in electrical conductivity of only 26% and an increase in Seebeck coefficient of 10%. To clarify the mechanism of reduction in thermal conductivity, a large-scale mesoscopic simulation of CNT veils under uniaxial stretching has also been used. This work shows that defect engineering can be a valuable strategy to optimize TE properties of CNT veils and, potentially, other thermoelectric materials.

Advancing Thermoelectric Materials: A Comprehensive Review Exploring the Significance of One-Dimensional Nano Structuring

Amidst the global challenges posed by pollution, escalating energy expenses, and the imminent threat of global warming, the pursuit of sustainable energy solutions has become increasingly imperative. Thermoelectricity, a promising form of green energy, can harness waste heat and directly convert it into electricity. This technology has captivated attention for centuries due to its environmentally friendly characteristics, mechanical stability, versatility in size and substrate, and absence of moving components. Its applications span diverse domains, encompassing heat recovery, cooling, sensing, and operating at low and high temperatures. However, developing thermoelectric materials with high-performance efficiency faces obstacles such as high cost, toxicity, and reliance on rare-earth elements. To address these challenges, this comprehensive review encompasses pivotal aspects of thermoelectricity, including its historical context, fundamental operating principles, cutting-edge materials, and innovative strategies. In particular, the potential of one-dimensional nanostructuring is explored as a promising avenue for advancing thermoelectric technology. The concept of one-dimensional nanostructuring is extensively examined, encompassing various configurations and their impact on the thermoelectric properties of materials. The profound influence of one-dimensional nanostructuring on thermoelectric parameters is also thoroughly discussed. The review also provides a comprehensive overview of large-scale synthesis methods for one-dimensional thermoelectric materials, delving into the measurement of thermoelectric properties specific to such materials. Finally, the review concludes by outlining prospects and identifying potential directions for further advancements in the field.

High-Performance MoS2/SWCNT Composite Films for a Flexible Thermoelectric Power Generator

Single-walled carbon nanotube (SWCNT)-based thermoelectric materials have been extensively studied in the field of flexible wearable devices due to their high flexibility and excellent electrical conductivity (σ). However, poor Seebeck coefficient (S) and high thermal conductivity limit their thermoelectric application. In this work, free-standing MoS₂/SWCNT composite films with improved thermoelectric performance were fabricated by doping SWCNTs with MoS₂ nanosheets. The results demonstrated that the energy filtering effect at the MoS₂/SWCNT interface increased the S of composites. In addition, the σ of composites was also improved due to the reason that S-π interaction between MoS₂ and SWCNTs made good contact between MoS₂ and SWCNTs and improved carrier transport. Finally, the obtained MoS₂/SWCNT showed a maximum power factor of 131.9 ± 4.5 μW m^-1 K^-2 at room temperature with a σ of 680 ± 6.7 S cm^-1 and an S of 44.0 ± 1.7 μV K^-1 at a MoS₂/SWCNT mass ratio of 15:100. As a demonstration, a thermoelectric device composed of three pairs of p-n junctions was prepared, which exhibited a maximum output power of 0.43 μW at a temperature gradient of 50 K. Therefore, this work offers a simple method of enhancing the thermoelectric properties of SWCNT-based materials.

Simultaneously enhanced tenacity, rupture work, and thermal conductivity of carbon nanotube fibers by raising effective tube portion

Although individual carbon nanotubes (CNTs) are superior to polymer chains, the mechanical and thermal properties of CNT fibers (CNTFs) remain inferior to synthetic fibers because of the failure of embedding CNTs effectively in superstructures. Conventional techniques resulted in a mild improvement of target properties while degrading others. Here, a double-drawing technique is developed to rearrange the constituent CNTs. Consequently, the mechanical and thermal properties of the resulting CNTFs can simultaneously reach their highest performances with specific strength ~3.30 N tex-1 (4.60 GPa), work of rupture ~70 J g-1, and thermal conductivity ~354 W m-1 K-1 despite starting from low-crystallinity materials (IG:ID ~ 5). The processed CNTFs are more versatile than comparable carbon fiber, Zylon and Dyneema. On the basis of evidence of load transfer efficiency on individual CNTs measured with in situ stretching Raman, we find that the main contributors to property enhancements are the increasing of the effective tube contribution.

Ultra-long air-stability of n-type carbon nanotube films with low thermal conductivity and all-carbon thermoelectric generators

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

Enhancement Effect of the C60 Derivative on the Thermoelectric Properties of n-Type Single-Walled Carbon Nanotube-Based Films

Obtaining air-stable and high-performance flexible n-type single-walled carbon nanotube (SWCNT)-based thermoelectric films used in wearable electronic devices is a challenge. In this work, the microstructure and thermoelectric properties of n-type SWCNT-based films have been optimized via doping C₆₀ and its derivative into polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) films. The result demonstrated that the dispersity of triethylene glycol-modified C₆₀ (TEG-C₆₀) was better in PEI/SWCNT films than that of pure C₆₀. Among the prepared composite films, TEG-C₆₀-doped PEI/SWCNT (TEG-C₆₀/PEI/SWCNT) films exhibited the highest TE performance, achieving a peak electrical conductivity of 923 S cm^-1 with a Seebeck coefficient of -42 μV K^-1 at a TEG-C₆₀/SWCNT mass ratio of 1:100. Compared to that of PEI/SWCNT, the power factor was increased significantly from 40 to 162 μW m^-1 K^-2 after the addition of TEG-C₆₀, which was higher than that of films after the addition of C₆₀. In addition, the n-type doped SWCNT films had good air stability at high temperatures, and the Seebeck coefficients of C₆₀/PEI/SWCNT and TEG-C₆₀/PEI/SWCNT at 120 °C were still negative and remained at 92% and 85%, respectively, after 20 days. Furthermore, a flexible TE device consisting of five pairs of p-n junctions was assembled using the optimum hybrid film, which generated a maximum output power of 3.6 μW at a temperature gradient of 50.2 K. Therefore, this study provides a facile way to enhance the thermoelectric properties of n-type carbon nanotube-based materials, which have potential application in flexible power generators.

Manipulating Single-Walled Carbon Nanotube Arrays for Flexible Photothermoelectric Devices

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

Direct Wet-Spun Single-Walled Carbon Nanotubes-Based p-n Segmented Filaments toward Wearable Thermoelectric Textiles

Three-dimensional thermoelectric (TE) textiles (TETs) fabricated with TE filaments (TEFs) possess merits over other types such as thickness-direction thermal energy harvesting and excellent conformability with dynamic body curves, revealing the prospect of generating electricity for on-body application. Nonetheless, there is still a lack of a costless but scalable method to automatically and seamlessly produce in-series interconnected p-n segmented TEFs with high TE properties via conventional fiber spinning processes. Here, we developed an alternate wet-spinning strategy to continuously manufacture single-walled carbon nanotube-based p-n segmented TEFs at large scale. The TEF with high electrical conductivity (400-800 S cm^-1) displays a low contact resistivity of 189.8 μΩ cm² between the segments and interelectrode, showing 2 orders of magnitude smaller than that reported in the literature. More importantly, the power factors of p-type and n-type segments are 26.25 and 17.14 μW m^-1 K^-2, respectively, which are 3 and 4 orders of magnitude higher than those of advanced studies. We finally embroidered it into spacer fabric to fabricate a wearable TET, demonstrating an output power density of 501 nW m^-2 at ΔT = 27.7 K. The methodology can inspire the development of fiber-based electronics such as wearable TEs and diodes and so forth.

High-Performance Ultrabroadband Photodetector Based on Photothermoelectric Effect

Ultrabroadband photodetectors (PDs) working in the frequency range from the UV to THz regions of the spectrum play a crucial role in integrated multifunction photoelectric detection. Even so, a shortage of high-performance PDs has seriously restricted the overall development of this field. The present work demonstrates a high-performance, ultrabroadband PD with a composite nanostructure comprising a suspended carbon nanotube (CNT) film on which titanium and palladium are deposited. The application of titanium and palladium to the CNT film in this device provides n-doping and p-doping, respectively, and the deposited metal nanoparticles also ensure enhanced thermal localization. This device exhibits short response time, high responsivity, large linear dynamic range, and small noise equivalent power over the ultrabroadband spectrum based on a strong photothermoelectric effect. Numerical simulation results also confirm the effective doping and enhanced thermal localization in this PD resulting from the deposited metals. A theoretical analysis shows that the thermal conductivity of the composite film is no longer independent of the temperature over a wide temperature range. This work provides a simple but novel strategy for the design of high-performance ultrabroadband PDs.

Bicyclic-ring base doping induces n-type conduction in carbon nanotubes with outstanding thermal stability in air

The preparation of air and thermally stable n-type carbon nanotubes is desirable for their further implementation in electronic and energy devices that rely on both p- and n-type material. Here, a series of guanidine and amidine bases with bicyclic-ring structures are used as n-doping reagents. Aided by their rigid alkyl functionality and stable conjugate acid structure, these organic superbases can easily reduce carbon nanotubes. n-Type nanotubes doped with guanidine bases show excellent thermal stability in air, lasting for more than 6 months at 100 °C. As an example of energy device, a thermoelectric p/n junction module is constructed with a power output of ca. 4.7 μW from a temperature difference of 40 °C.

Recent Advances in Materials for Wearable Thermoelectric Generators and Biosensing Devices

Recently, self-powered health monitoring systems using a wearable thermoelectric generator (WTEG) have been rapidly developed since no battery is needed for continuous signal monitoring, and there is no need to worry about battery leakage. However, the existing materials and devices have limitations in rigid form factors and small-scale manufacturing. Moreover, the conventional bulky WTEG is not compatible with soft and deformable tissues, including human skins or internal organs. These limitations restrict the WTEG from stabilizing the thermoelectric gradient that is necessary to harvest the maximum body heat and generate valuable electrical energy. This paper summarizes recent advances in soft, flexible materials and device designs to overcome the existing challenges. Specifically, we discuss various organic and inorganic thermoelectric materials with their properties for manufacturing flexible devices. In addition, this review discusses energy budgets required for effective integration of WTEGs with wearable biomedical systems, which is the main contribution of this article compared to previous articles. Lastly, the key challenges of the existing WTEGs are discussed, followed by describing future perspectives for self-powered health monitoring systems.

Structural Design of Nanowire Wearable Stretchable Thermoelectric Generator

Wearable thermoelectric generators as renewable energy conversion technologies have witnessed rapid development in the past decade. Herein, we design a nanowire (NW) film wavy structure which possesses an excellent temperature gradient ratio for stretchable thermoelectric generators. Taking advantage of the photothermal effect of Te NWs as the hot side and p-n NWs heterofilms (n-type Ag₂Te and p-type Cu_1.75Te NWs) as thermoelectric materials, a considerable output voltage can be achieved under light irradiation. Besides the electricity output, the wearable device can also make our skin warm and comfortable in cold weather. Meanwhile, we combine thermoelectric generators with passive radiative cooling technology to reduce insolation of the human body and improve the performance of the device under intense solar irradiation in hot weather. Interestingly, it can also offer continuous green energy to realize various signal perceptions, suggesting a robust strategy for electricity output and self-powered wearable electronics.

High-Power-Density Wearable Thermoelectric Generators for Human Body Heat Harvesting

Wearable thermoelectrics has attracted significant interest in recent years. Among them, rigid-structure thermoelectric generators (TEGs) were seldomly employed for wearable applications, although those exhibit significant advantages of high device output performance and impact resistance. Here, we report a type of rigid wearable TEGs (w-TEGs) without ceramic substrates made using a simple cutting-and-bonding method. Owing to the small contact area, the w-TEGs comprising 48-n/p-pairs can be well attached to the human body. The lack of ceramic substrates leaves more space in the height direction, which benefits the wearability in practical applications and high power density. We demonstrated that increasing the height of w-TEGs from 1.38 to 3.14 mm significantly improves the power density by a factor of 10. As a result, the maximum power densities of 7.9 μW cm^-2 and 43.6 μW cm^-2 for the w-TEGs were realized under the breezeless condition and a wind speed for normal walking, respectively. This work provides a feasible design solution for rigid-structure free-substrate w-TEGs with very high power density, which will speed up the research of wearable thermoelectrics.

Multifunctional Superelastic Graphene-Based Thermoelectric Sponges for Wearable and Thermal Management Devices

Power generation through harvesting human thermal energy provides an ideal strategy for self-powered wearable design. However, existing thermoelectric fibers, films, and blocks have small power generation capacity and poor flexibility, which hinders the development of self-powered wearable electronics. Here, we report a multifunctional superelastic graphene-based thermoelectric (TE) sponge for wearable electronics and thermal management. The sponge has a high Seebeck coefficient of 49.2 μV/K and a large compressive strain of 98%. After 10 000 cyclic compressions at 30% strain, the sponge shows excellent mechanical and TE stability. A wearable sponge array TE device was designed to drive medical equipment for monitoring physiological signals by harvesting human thermal energy. Furthermore, a 4 × 4 array TE device placed on the surface of a normal working Central Processing Unit (CPU) can generate a stable voltage and reduce the CPU temperature by 8 K, providing a feasible strategy for simultaneous power generation and thermal management.

Stable Doping of Single-Walled Carbon Nanotubes for Flexible Transparent Conductive Films

Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O₂ and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

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

Wearable Thermoelectric Materials and Devices for Self-Powered Electronic Systems

The emergence of artificial intelligence and the Internet of Things has led to a growing demand for wearable and maintenance-free power sources. The continual push toward lower operating voltages and power consumption in modern integrated circuits has made the development of devices powered by body heat finally feasible. In this context, thermoelectric (TE) materials have emerged as promising candidates for the effective conversion of body heat into electricity to power wearable devices without being limited by environmental conditions. Driven by rapid advances in processing technology and the performance of TE materials over the past two decades, wearable thermoelectric generators (WTEGs) have gradually become more flexible and stretchable so that they can be used on complex and dynamic surfaces. In this review, the functional materials, processing techniques, and strategies for the device design of different types of WTEGs are comprehensively covered. Wearable self-powered systems based on WTEGs are summarized, including multi-function TE modules, hybrid energy harvesting, and all-in-one energy devices. Challenges in organic TE materials, interfacial engineering, and assessments of device performance are discussed, and suggestions for future developments in the area are provided. This review will promote the rapid implementation of wearable TE materials and devices in self-powered electronic systems.

Integrated, Highly Flexible, and Tailorable Thermoelectric Type Temperature Detectors Based on a Continuous Carbon Nanotube Fiber

As possible alternatives to traditional thermoelectric (TE) materials, carbon nanomaterials and their hybrid materials have great potential in the future application of flexible and lightweight temperature detection. In this work, an integrated, highly flexible, and tailorable TE temperature detector with high performance has been fabricated based on a continuous single-walled carbon nanotube (SWCNT) fiber. The detector consists of more than one pairs of thermocouples composed of p-type SWCNT fiber and n-type SWCNT hybrid fiber in situ doped by polyethylenimine. Due to the node contact mechanism of the detection, the sensitivity of the detector can be improved with the increase of the number of p-n thermocouples, independent of the length of the thermocouple. The temperature detection process of the detector has been studied in detail. In particular, the integrated and flexible detector can be divided into several sub-detectors easily by cutting, illustrating the prospect of large-scale preparation of this kind of novel temperature detectors. Its high flexibility ensures the detector to maintain excellent detection performance after 15 000 bending circles. Furthermore, the as-designed TE type temperature detector demonstrates a great application promise for real-time temperature detection and temperature change sensing even in complex surface and harsh environment.

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

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

Oxygen-Rich Polymer Polyethylene Glycol-Functionalized Single-Walled Carbon Nanotubes Toward Air-Stable n-Type Thermoelectric Materials

It is crucial for thermoelectric (TE) devices to obtain both p-type and n-type materials and control charge carrier density. However, n-type thermoelectric materials are quite deficient and have lower thermoelectric properties. We report one oxygen-rich polymer named polyethylene glycol (PEG) for converting p-type single-walled carbon nanotubes (SWCNTs) to air-stable n-type thermoelectric materials. When pristine SWCNTs were doped with 2 mg·mL^-1 PEG in an ethanol solution, the optimal Seebeck coefficient of PEG/SWCNT composites reached -50.8 μV·K^-1. The result of ultraviolet photoelectron spectroscopy demonstrated that the lone pair of oxygen atoms in the PEG chain has electron transferability to SWCNTs. According to the hard and soft acid and base theory, sodium hydroxide (NaOH) was further introduced to improve air stability and thermoelectric performance of doped SWCNTs. As a result, PEG/NaOH/SWCNT composites achieved the highest power factor of 173.8 μW·m^-1·K^-2 at 300 K. Meanwhile, their final changes in electrical conductivity and the Seebeck coefficient are less than 8% in the investigation of air stability over two months. Inspired by this finding, we fabricated the TE generator composed of the pristine p-type SWCNTs and n-type PEG/NaOH/SWCNT composites. The maximum output power of this robust TE device reached 5.3 μW at a temperature gradient of 76 K, which is superior to many reported TE devices. Moreover, the experimental procedure is attractive as a sustainable process for materials preparation. Our study has indicated that the oxygen-rich polymer-functionalized SWCNTs have huge potential for developing air-stable n-type carbon-based thermoelectric materials.

Leaf-Inspired Flexible Thermoelectric Generators with High Temperature Difference Utilization Ratio and Output Power in Ambient Air

The inherently small temperature difference in air environment restricts the applications of thermoelectric generation in the field of Internet of Things and wearable electronics. Here, a leaf-inspired flexible thermoelectric generator (leaf-TEG) that makes maximum use of temperature difference by vertically aligning poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and constantan thin films is demonstrated. Analytical formulae of the performance scales, i.e., temperature difference utilization ratio (φth) and maximum output power (Pmax), are derived to optimize the leaf-TEG dimensions. In an air duct (substrate: 36 °C, air: 6 °C, air flowing: 1 m s-1), the 10-leaf-TEG shows a φth of 73% and Pmax of 0.38 µW per leaf. A proof-of-concept wearable 100-leaf-TEG (60 cm2) generates 11 µW on an arm at room temperature. Furthermore, the leaf-TEG is flexible and durable that is confirmed by bending and brushing over 1000 times. The proposed leaf-TEG is very appropriate for air convection scenarios with limited temperature differences.

Application-Driven Carbon Nanotube Functional Materials

Carbon nanotube functional materials (CNTFMs) represent an important research field in transforming nanoscience and nanotechnology into practical applications, with potential impact in a wide realm of science, technology, and engineering. In this review, we combine the state-of-the-art research activities of CNTFMs with the application prospect, to highlight critical issues and identify future challenges. We focus on macroscopic long fibers, thin films, and bulk sponges which are typical CNTFMs in different dimensions with distinct characteristics, and also cover a variety of derived composite/hierarchical materials. Critical issues related to their structures, properties, and applications as robust conductive skeletons or high-performance flexible electrodes in mechanical and electronic devices, advanced energy conversion and storage systems, and environmental areas have been discussed specifically. Finally, possible solutions and directions are proposed for overcoming current obstacles and promoting future efforts in the field.

Thermoelectric Materials for Textile Applications

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

Progress in micro/nano sensors and nanoenergy for future AIoT-based smart home applications

Self-sustainable sensing systems composed of micro/nano sensors and nano-energy harvesters contribute significantly to developing the internet of things (IoT) systems. As one of the most promising IoT applications, smart home relies on implementing wireless sensor networks with miniaturized and multi-functional sensors, and distributed, reliable, and sustainable power sources, namely energy harvesters with a variety of conversion mechanisms. To extend the capabilities of IoT in the smart home, a technology fusion of IoT and artificial intelligence (AI), called the artificial intelligence of things (AIoT), enables the detection, analysis, and decision-making functions with the aids of machine learning assisted algorithms to form a smart home based intelligent system. In this review, we introduce the conventional rigid microelectromechanical system (MEMS) based micro/nano sensors and energy harvesters, followed by presenting the advances in the wearable counterparts for better human interactions. We then discuss the viable integration approaches for micro/nano sensors and energy harvesters to form self-sustainable IoT systems. Whereafter, we emphasize the recent development of AIoT based systems and the corresponding applications enabled by the machine learning algorithms. Smart home based healthcare technology enabled by the integrated multi-functional sensing platform and bioelectronic medicine is also presented as an important future direction, as well as wearable photonics sensing system as a complement to the wearable electronics sensing system.

Novel butterfly-shaped organic semiconductor and single-walled carbon nanotube composites for high performance thermoelectric generators

Herein, a series of novel butterfly-shaped small-molecule organic semiconductors (OSCs) have been designed, synthesized and complexed with single-walled carbon nanotubes (SWCNTs) as p-type thermoelectric materials. The butterfly-shaped molecules exhibit curved molecular structures, which tune their frontier molecular orbitals and increase their interactions with SWCNTs. A systematic study shows that the composites based on butterfly-shaped OSCs exhibit significantly improved thermoelectric performances compared with that of the composite based on the analoguous planar OSC. The enhanced thermoelectric performances are attributable to the higher activation energy, improved doping level and charge transport process between the organic molecules and SWCNTs. The butterfly-shaped OSC and SWCNT composite opens up a new avenue for the design of thermoelectric materials and devices.

High-Power All-Carbon Fully Printed and Wearable SWCNT-Based Organic Thermoelectric Generator

In this study, we introduce the fabrication process of a highly efficient fully printed all-carbon organic thermoelectric generator (OTEG) free of metallic junctions with outstanding flexibility and exceptional power output, which can be conveniently and rapidly prepared through ink dispensing/printing processes of aqueous and low-cost CNT inks with a mask-assisted specified circuit architecture. The optimal p-type and n-type films produced exhibit ultrahigh power factors (PFs) of 308 and 258 μW/mK², respectively, at ΔΤ = 150 K (T_HOT = 175 °C) and outstanding stability in air without encapsulation, providing the OTEG device the ability to operate at high temperatures up to 200 °C at ambient conditions (1 atm, relative humidity: 50 ± 5% RH). We have successfully designed and fabricated the flexible thermoelectric (TE) modules with superior TE properties of p-type and n-type SWCNT films resulting in exceptionally high performance. The novel design OTEG exhibits outstanding flexibility and stability with attained TE values among the highest ever reported in the field of organic thermoelectrics, that is, open-circuit voltage V_OC = 1.05 V and short-circuit current I_SC = 1.30 mA at ΔT = 150 K (T_HOT = 175 °C) with an internal resistance of R_TEG = 806 Ω, generating a 342 μW power output. It is also worth noting the remarkable PFs of 145 and 127 μW/mK² for the p-type and n-type films, respectively, at room temperature. The fabricated device is highly scalable, providing opportunities for printable large-scale manufacturing/industrial production of highly efficient flexible OTEGs.

Optimized Thermoelectric Performance of Carbon Nanoparticle-Carbon Nanotube Heterostructures by Tuning Interface Barrier Energy

Herein, thermoelectric carbon nanoparticle (CNP)-carbon nanotube (CNT) heterostructures are introduced as a promising flexible thermoelectric material. The optimal barrier energy between the CNP and CNT increases the Seebeck coefficient (S) of the heterostructures through the energy filtering effect. For optimized thermoelectric performance, the CNP-CNT barrier energy can be effectively tuned by controlling the work function of the CNPs. The optimized p-type CNP-CNT heterostructures exhibited S and power factor (PF) of 50.6 ± 1.4 μV K^-1 and 400 ± 26 μW m^-1 K^-2, respectively. The n-type CNP-CNT heterostructures, optimized for another work function of the CNPs, exhibited S and PF of up to -37.5 ± 3.4 μV K^-1 and 214 ± 42 μW m^-1 K^-2, respectively. The energy harvesting capability of a thermoelectric generator prepared using p- and n-type CNP-CNT heterostructures with optimized barrier energies is demonstrated. The thermoelectric generator with 10 p-type and 9 n-type thermoelectric elements exhibited a maximum output power of 0.12 μW from a ΔT of 5 K. This work shows a facile strategy for synthesizing thermoelectric CNP-CNT heterostructures with optimized energy filtering effects. Application to the thermoelectric device on a paper substrate is also discussed.

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

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

Molecular Doping of a Naphthalene Diimide-Bithiophene Copolymer and SWCNTs for n-Type Thermoelectric Composites

Molecular doping is a powerful tool to tune the thermoelectric (TE) properties of solution-processed semiconductors. In this work, we prepared a binary composite and effectively doped both of its constituents, that is, naphthalene diimide-bithiophene copolymers (PNDI2OD-T2) and single-walled carbon nanotubes (SWCNTs), by a 1H-benzimidazole derivative (N-DMBI). The doped composites show an n-type character and an in-plane TE figure of merit (ZT), exceeding the values obtained with the doped polymers. The use of SWCNTs consistently results in a higher σ with a maximum value above 10² S/cm, resulting in the highest power factor of 18.1 μW/mK² for an SWCNT loading of 45.5 wt %. Furthermore, an SWCNT content up to 9 wt % does not compromise the low thermal conductivity of the polymer matrices, leading to a ZT value of 0.0045. The n-type composites show good solution processability and relatively stable Seebeck coefficients upon air exposure for 8 months.

Conductive PEDOT: PSS-Based Organic/Inorganic Flexible Thermoelectric Films and Power Generators

We present a simple thermoelectric device that consists of a conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based inorganic/organic thermoelectric film with high thermoelectric performance. The PEDOT:PSS-coated Se NWs were first chemically synthesized in situ, and then mixed with an Ag precursor solution to produce the PEDOT:PSS-coated Ag₂Se NWs. The PEDOT:PSS matrix was then treated with dimethyl sulfoxide (DMSO) prior to the production of flexible PEDOT:PSS-coated Ag₂Se NW/PEDOT:PSS composite films with various weight fractions of Ag₂Se via a simple drop-casting method. The thermoelectric properties (Seebeck coefficient, electrical conductivity, and power factor) of the composite films were then analyzed. The composite film with 50 wt.% NWs exhibited the highest power factor of 327.15 μW/m·K² at room temperature. The excellent flexibility of this composite film was verified by bending tests, in which the thermoelectric properties were reduced by only ~5.9% after 1000 bending cycles. Finally, a simple thermoelectric device consisting of five strips of the proposed composite film was constructed and was shown to generate a voltage of 7.6 mV when the temperature difference was 20 K. Thus, the present study demonstrates that that the combination of a chalcogenide and a conductive composite film can produce a high-performance flexible thermoelectric composite film.

Effective Doping of Single-Walled Carbon Nanotubes with Polyethyleneimine

More and more electrically conducting materials are required to sustain the technological progress of civilization. Faced with the performance limits of classical materials, the R&D community has put efforts into developing nanomaterials, which can offer sufficiently high operational parameters. In this work, single-walled carbon nanotubes (SWCNTs) were doped with polyethyleneimine (PEI) to create such material. The results show that it is most fruitful to combine these components at the synthesis stage of an SWCNT network from their dispersion. In this case, the electrical conductivity of the material is boosted from 249 ± 21 S/cm to 1301 ± 56 S/cm straightforwardly and effectively.

Fabrication of PEDOT:PSS/Ag2Se Nanowires for Polymer-Based Thermoelectric Applications

Flexible Ag₂Se NW/PEDOT:PSS thermoelectric composite films with different Ag₂Se contents (10, 20, 30, 50, 70, and 80 wt.%) are fabricated. The Ag₂Se nanowires are first fabricated with solution mixing. After that, Ag₂Se NW/PEDOT:PSS composite film was fabricated using a simple drop-casting method. To evaluate the potential applications of the Ag₂Se NW/PEDOT:PSS composite, their thermoelectric properties are analyzed according to their Ag₂Se contents, and strategies for maximizing the thermoelectric power factor are discussed. The maximum room-temperature power factor of composite film (178.59 μW/m·K²) is obtained with 80 wt.% Ag₂Se nanowires. In addition, the composite film shows outstanding durability after 1000 repeat bending cycles. This work provides an important strategy for the fabrication of high-performance flexible thermoelectric composite films, which can be extended to other inorganic/organic composites and will certainly promote their development and thermoelectric applications.

Printable, Highly Sensitive Flexible Temperature Sensors for Human Body Temperature Monitoring: A Review

In recent years, the development and research of flexible sensors have gradually deepened, and the performance of wearable, flexible devices for monitoring body temperature has also improved. For the human body, body temperature changes reflect much information about human health, and abnormal body temperature changes usually indicate poor health. Although body temperature is independent of the environment, the body surface temperature is easily affected by the surrounding environment, bringing challenges to body temperature monitoring equipment. To achieve real-time and sensitive detection of various parts temperature of the human body, researchers have developed many different types of high-sensitivity flexible temperature sensors, perfecting the function of electronic skin, and also proposed many practical applications. This article reviews the current research status of highly sensitive patterned flexible temperature sensors used to monitor body temperature changes. First, commonly used substrates and active materials for flexible temperature sensors have been summarized. Second, patterned fabricating methods and processes of flexible temperature sensors are introduced. Then, flexible temperature sensing performance are comprehensively discussed, including temperature measurement range, sensitivity, response time, temperature resolution. Finally, the application of flexible temperature sensors based on highly delicate patterning are demonstrated, and the future challenges of flexible temperature sensors have prospected.

Thermoelectric Energy Harvesters: A Review of Recent Developments in Materials and Devices for Different Potential Applications

The thermoelectric effect encompasses three different effects, i.e. Seebeck effect, Peltier effect, and Thomson effect, which are considered as thermally activated materials that alter directions in smart materials. It is currently considered one of the most challenging green energy harvesting mechanisms among researchers. The ability to utilize waste thermal energy that is generated by different applications promotes the use of thermoelectric harvesters across a wide range of applications. This review illustrates the different attempts to fabricate efficient, robust and sustainable thermoelectric harvesters, considering the material selection, characterization, device fabrication and potential applications. Thermoelectric harvesters with a wide range of output power generated reaching the milliwatt range have been considered in this work, with a special focus on the main advantages and disadvantages in these devices. Additionally, this review presents various studies reported in the literature on the design and fabrication of thermoelectric harvesters and highlights their potential applications. In order to increase the efficiency of equipment and processes, the generation of thermoelectricity via thermoelectric materials is achieved through the harvesting of residual energy. The review discusses the main challenges in the fabrication process associated with thermoelectric harvester implementation, as well as the considerable advantages of the proposed devices. The use of thermoelectric harvesters in a wide range of applications where waste thermal energy is used and the impact of the thermoelectric harvesters is also highlighted in this review.