Organic thermoelectric materials and devices based on p- and n-type poly(metal 1,1,2,2-ethenetetrathiolate)s

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

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

Preparation and characterization of conjugated PVA/PANi blend films doped with functionalized graphene for thermoelectric applications

Flexible nanocomposite thick films consisting of PVA0.7PANi0.3 polymer blend doped with different concentrations of nanoplatelets functionalized Graphene (NPFGx) (where x = 0, 5, 10, 15, 20, and 25 wt.%) were fabricated using the solution cast technique. Scanning electron microscopy (SEM), X-ray diffractometer (XRD), energy dispersive spectroscopy analysis (EDX), and Fourier-transform infrared spectra (FT-IR) were used to study the structure of the samples. The results showed that the ordered structure, its orientation, the PANis' well dispersion, and the electrostatic forces play a significant role in enhancing the interfaces between the polymer blend and the NPFG. Thermogravimetric analyses (TGA) and Thermoelectrical analyses (TE) showed that the PVA-PANi conducts a promised conjugated blend for thermoelectric applications. The introduction of the NPFG contents into the blend increased the TE measurements as the DC electrical conductivity ≈ 0.0114 (S cm-1), power factor ≈ 3.93 × 10-3 (W m-1 K-2), and Z.T. ≈ 8.4 × 10-7, for the 25 wt.% NPFG nanocomposite film. The effect of the polymers' phonon contribution in the thermal conductivity controlling and enhancing the thermal stability of the prepared nanocomposite films.

Enhanced n-Type Thermoelectric Properties and Structure Evolution of Carbonized Metal-Coordination Polydopamine

Carbonized polydopamine (cPDA) exhibits a nitrogenous graphite-like structure with n-type semiconductor property. However, the low electrical conductivity and Seebeck coefficient of cPDA cannot meet the needs of flexible thermoelectric devices. In this study, a series of metal ions were coordinated with cPDA to enhance n-type thermoelectric properties. At 300 K, all metal-coordination cPDA (metal-cPDA) samples obtain lower thermal conductivity compared to cPDA. Mn-cPDA exhibits the greatest Seebeck coefficient of -25.94 μV K-1, which is almost six times higher than cPDA. Fe-cPDA shows the best electrical conductivity of 2.45 × 105 S m-1. An optimal power factor (PF) value of 11.93 μW m-1 K-2 is achieved in Ca-cPDA with the enhanced electrical conductivity of 9.5 × 104 S m-1 and Seebeck coefficient of -11.24 μV K-1. Using Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), we revealed the structural characterization of metal-cPDA. Our results indictate that the different metal ions (Mn2+, Zn2+, Mg2+, Al3+, Ca2+, and Fe3+) exert varying influences on the growth of graphite-like structure within metal-cPDA, which lead to the evolution of electrical conductivity. We observe that the carrier density and carrier mobility depend on both the degree of graphitization and the metal-ion coordination, which work together on electrical conductivity and Seebeck coefficient. These findings and understanding of the thermoelectric properties of PDA-based materials will help to realize high-performance n-type thermoelectric materials for flexible electronic device applications.

High-Performance Thermoelectric Fibers from Metal-Backboned Polymers for Body-Temperature Wearable Power Devices

Metal-backboned polymers (MBPs), with a unique backbone consisting of bonded metal atoms, are promising for optic, electric, magnetic, and thermoelectric fields. However, the application of MBP remains relatively understudied. Here, we develop a shear-induced orientation method to construct a flexible nickel-backboned polymer/carbon nanotube (NBP/CNT) thermoelectric composite fiber. It demonstrated a power factor of 719.48 μW ⋅m-1 K-2, which is ca. 3.5 times as high as the bare CNT fiber. Remarkably, with the regulation of carrier mobility and carrier concentration of NBP, the composite fiber further showed simultaneous increases in electrical conductivity and Seebeck coefficient in comparison to the bare CNT fiber. The NBP/CNT fiber can be integrated into fabrics to harvest thermal energy of human body to generate an output voltage of 3.09 mV at a temperature difference of 8 K. This research opens a new avenue for the development of MBPs in power supply.

Self-Assembled Conjugated Coordination Polymer Nanorings: Role of Morphology and Redox Sites for the Alkaline Electrocatalytic Oxygen Evolution Reaction

Electrocatalytic water splitting provides a sustainable method for storing intermittent energies, such as solar energy and wind, in the form of hydrogen fuel. However, the oxygen evolution reaction (OER), constituting the other half-cell reaction, is often considered the bottleneck in overall water splitting due to its slow kinetics. Therefore, it is crucial to develop efficient, cost-effective, and robust OER catalysts to enhance the water-splitting process. Transition-metal-based coordination polymers (CPs) serve as promising electrocatalysts due to their diverse chemical architectures paired with redox-active metal centers. Despite their potential, the rational use of CPs has faced obstacles including a lack of insights into their catalytic mechanisms, low conductivity, and morphology issues. Consequently, achieving success in this field requires the rational design of ligands and topological networks with the desired electronic structure. This study delves into the design and synthesis of three novel conjugated coordination polymers (CCPs) by leveraging the full conjugation of terpyridine-attached flexible tetraphenylethylene units as electron-rich linkers with various redox-active metal centers [Co(II), Ni(II), and Zn(II)]. The self-assembly process is tuned for each CCP, resulting in two distinct morphologies: nanosheets and nanorings. The electrocatalytic OER performance efficiency is then correlated with factors such as the nanostructure morphology and redox-active metal centers in alkaline electrolytes. Notably, among the three morphologies studied, nanorings for each CCP exhibit a superior OER activity. Co(II)-integrated CCPs demonstrate a higher activity between the redox-active metal centers. Specifically, the Co(II) nanoring morphology displays exceptional catalytic activity for OER, with a lower overpotential of 347 mV at a current density of 10 mA cm-2 and small Tafel slopes of 115 mV dec-1. The long-term durability is demonstrated for at least 24 h at 1.57 V vs RHE during water splitting. This is presumably the first proof that links the importance of nanostructure morphologies to redox-active metal centers in improving the OER activity, and it may have implications for other transdisciplinary energy-related applications.

Controlling the thermoelectric properties of organo-metallic coordination polymers through backbone geometry

Poly(nickel-benzene-1,2,4,5-tetrakis(thiolate)) (Ni-btt), an organometallic coordination polymer (OMCP) characterized by the coordination between benzene-1,2,4,5-tetrakis(thiolate) (btt) and Ni2+ ions, has been recognized as a promising p-type thermoelectric material. In this study, we employed a constitutional isomer based on benzene-1,2,3,4-tetrakis(thiolate) (ibtt) to generate the corresponding isomeric polymer, poly(nickel-benzene-1,2,3,4-tetrakis(thiolate)) (Ni-ibtt). Comparative analysis of Ni-ibtt and Ni-btt reveals several common infrared (IR) and Raman features attributed to their similar square-planar nickel-sulfur (Ni-S) coordination. Nevertheless, these two polymer isomers exhibit substantially different backbone geometries. Ni-btt possesses a linear backbone, whereas Ni-ibtt exhibits a more undulating, zig-zag-like structure. Consequently, Ni-ibtt demonstrates slightly higher solubility and an increased bandgap in comparison to Ni-btt. The most noteworthy dissimilarity, however, manifests in their thermoelectric properties. While Ni-btt exhibits p-type behavior, Ni-ibtt demonstrates n-type carrier characteristics. This intriguing divergence prompted further investigation into the influence of OMCP backbone geometry on the electronic structure and, particularly, the thermoelectric properties of these materials.

Thermoelectric Power of a Single van der Waals Interface between Carbon Nanotubes

Control of van der Waals interfaces is crucial for fabrication of nanomaterial-based high-performance thermoelectric devices because such interfaces significantly affect the overall thermoelectric performances of the device due to their relatively high thermal resistance. Such interfaces could induce different thermoelectric power from the bulk, i.e., interfacial thermoelectric power. However, from a macroscopic point of view, a correct evaluation of the interfacial thermoelectric power is difficult owing to various interface configurations. Therefore, the study of the thermoelectric properties at a single interface is crucial to address this problem. Herein, we used in situ transmission electron microscopy and nanomanipulation to investigate the thermoelectric properties of carbon nanotubes and their interfaces. The thermoelectric power of the bridged carbon nanotubes was individually measured. The existence of the interfacial thermoelectric power was determined by systematically changing the contact size between the two parallel nanotubes. The effect of interfacial thermoelectric power was qualitatively supported by Green's function calculations. When the contact length between two parallel nanotubes was less than approximately 100 nm, the experimental results and theoretical calculations indicated that the interface significantly contributed to the total thermoelectric power. However, when the contact length was longer than approximately 200 nm, the total thermoelectric power converged to the value of a single nanotube. The findings herein provide a basis for investigating thermoelectric devices with controlled van der Waals interfaces and contribute to thermal management in nanoscale devices and electronics.

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

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

Ambient-Stable n-Type Carbon Nanotube/Organic Small-Molecule Thermoelectrics Enabled by Energy Level Control

The stability of n-type organic and hybrid thermoelectric materials is limited in terms of their practical application to p-n parallel thermoelectric devices. We demonstrate the ambient stability of an n-type single-walled carbon nanotube/organic small-molecule (SWNT/OSM) hybrid by deepening the lowest occupied molecular orbital energy level. This hybrid exhibited the best figure of merit (0.032) among n-type SWNT/OSM hybrid thermoelectrics and an enhanced power factor of 291.0 μW m-1 K-2. Furthermore, we observed that the n-type thermoelectric stability of a hybrid of SWNT and pip containing two N-ethylpiperidinyl groups on both sides of a naphthalenediimide core was retained at 87% over 7 months (220 days) under ambient conditions without encapsulation.

Chemical Doping of Organic and Coordination Polymers for Thermoelectric and Spintronic Applications: A Theoretical Understanding

ConspectusThe controlled doping of organic semiconductors (OSCs) is crucial not only for improving the performance of electronic and optoelectronic devices but also for enabling efficient thermoelectric conversion and spintronic applications. The mechanism of doping for OSCs is fundamentally different from that of their inorganic counterparts. In particular, the interplay between dopants and host materials is complicated considering the low dielectric constant, strong lattice-charge interaction, and flexible nature of materials. Recent experimental breakthroughs in the molecular design of dopants and the precise doping with high spatial resolution call for more profound understandings as to how the dopant interacts with the charge introduced to OSCs and how the admixture of dopants alters the electronic properties of host materials before one can exploit controllable doping to realize desired functionalities.By employing state-of-the-art computational tools, we revealed the effects of doping in representative and emerging organic and coordination polymers aiming toward thermoelectric and spintronic applications. We showed that dopants and hosts should be taken as an integrated system, and the type of charge-transfer interaction between them is the key for spin polarization. First, we found doping-induced modifications to the electronic band in a potassium-doped coordination polymer, an n-type thermoelectric material. The charge localization due to the Coulomb interaction between the completely ionized dopant and the injected charge on the polymer backbone and also the polaron band formation at low doping levels are responsible for the nonmonotonic temperature dependence of the conductivity and Seebeck coefficient observed in recent experiments. The mechanistic insights gained from these results have provided important guidelines on how to control the doping level and working temperature to achieve a high thermoelectric conversion efficiency. Next, we demonstrated that the ionized dopants scatter charge carriers via screened Coulomb interactions, and it may become a dominant scattering mechanism in doped polymers. After incorporating the ionized dopant scattering mechanism in PEDOT:Tos, a p-type thermoelectric polymer, we were able to reproduce the measured Seebeck coefficient-electrical conductivity relationship spanning a wide range of doping levels, highlighting the importance of ionized dopant scattering in charge transport.In the two cases described above, charge injection is enabled by integral charge transfer between the dopant and host polymers. In a third example, we showed that a novel type of stacked two-dimensional polymer, conjugated covalent organic frameworks (COFs) with closed-shell electronic structures, can be spin polarized by iodine doping via fractional charge transfer even at high doping levels. We then manifested that magnetization can be attained in nonmagnetic materials lacking metal d electrons and further designed two new COFs with tunable spintronic structure and magnetic interactions after the iodine doping. These findings have suggested a practical route to enable spin polarization in nonradical materials by chemical doping via orbital hybridization, which holds great promise for flexible spintronic applications.

Efficient and air-stable n-type doping in organic semiconductors

Chemical doping of organic semiconductors (OSCs) enables feasible tuning of carrier concentration, charge mobility, and energy levels, which is critical for the applications of OSCs in organic electronic devices. However, in comparison with p-type doping, n-type doping has lagged far behind. The achievement of efficient and air-stable n-type doping in OSCs would help to significantly improve electron transport and device performance, and endow new functionalities, which are, therefore, gaining increasing attention currently. In this review, the issue of doping efficiency and doping air stability in n-type doped OSCs was carefully addressed. We first clarified the main factors that influenced chemical doping efficiency in n-type OSCs and then explain the origin of instability in n-type doped films under ambient conditions. Doping microstructure, charge transfer, and dissociation efficiency were found to determine the overall doping efficiency, which could be precisely tuned by molecular design and post treatments. To further enhance the air stability of n-doped OSCs, design strategies such as tuning the lowest unoccupied molecular orbital (LUMO) energy level, charge delocalization, intermolecular stacking, in situ n-doping, and self-encapsulations are discussed. Moreover, the applications of n-type doping in advanced organic electronics, such as solar cells, light-emitting diodes, field-effect transistors, and thermoelectrics are being introduced. Finally, an outlook is provided on novel doping ways and material systems that are aimed at stable and efficient n-type doped OSCs.

Linear Conjugated Coordination Polymers for Electrocatalytic Oxygen Evolution Reaction

Conjugated coordination polymers (CCPs) have attracted extensive attention for various applications related to energy storage and conversion in the past few years, despite that there are many CCPs with unclear chemical states and structures. Here, linear CCPs (LCCPs), with metal-O₄ active sites grown on carbon paper (CP) for oxygen evolution reaction (OER), are presented. The LCCPs with high crystallinity and simple structures exhibit the order of electrocatalytic activity of Co-O₄  > Ni-O₄  > Fe-O₄ in terms of the metal-O₄ centers. The Co-based LCCP shows higher OER performance (263 mV at 10 mA cm^-2 ) and better durability (90 h at 30 mA cm^-2 ) than commercial IrO₂ /CP. The structures and chemical states of LCCPs are carefully investigated, and density functional theory is used to reveal the mechanism of OER at the central metal site. This investigation into LCCPs provides new sights for a better understanding of CCPs and expands the applications of LCCPs with metal-O₄ sites.

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃ Br. The doped polymer PDPP-EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm^-1 but low Seebeck coefficient below 30 µV K^-1 and a maximum power factor of 59 ± 10 µW m^-1 K^-2 . Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃ Br into PDPP-EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m^-1 K^-2 and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP-EDOT by DPPNMe₃ Br is mainly attributed to the increase of energetic disorder for PDPP-EDOT.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Stamp-Like Energy Harvester and Programmable Information Encrypted Display Based on Fully Printable Thermoelectric Devices

Thermoelectric (TE) devices exhibit considerable application potential in Internet of Things and personal health monitoring systems. However, TE self-powered devices are expensive and their fabrication process is complex. Therefore, large-scale preparation of the TE devices remains challenging. In this work, simple screen-printing technology is used to fabricate a user-friendly and high-performance paper-based TE device, which can be used in both stamp-like paper-based TE generators and infrared displays. When used as a paper-based TE generator, an output power of 940.8 µW is achieved with a temperature difference of 40 K. The programmable infrared pattern based on the TE array display could be used to realize encryption and anti-counterfeiting properties. Moreover, a visual extraction algorithm is used to develop a mobile application for processing and decoding the infrared quick response code information. These findings offer an exciting approach to using paper-based TEGs in applications such as energy harvesting devices, optical encryption, anti-counterfeiting, and dynamic infrared display.

Physics-guided co-designing flexible thermoelectrics with techno-economic sustainability for low-grade heat harvesting

Flexible thermoelectric harvesting of omnipresent spatial thermodynamic energy, though promising in low-grade waste heat recovery (<100°C), is still far from industrialization because of its unequivocal cost-ineffectiveness caused by low thermoelectric efficiency and power-cost coupled device topology. Here, we demonstrate unconventional upcycling of low-grade heat via physics-guided rationalized flexible thermoelectrics, without increasing total heat input or tailoring material properties, into electricity with a power-cost ratio (W/US$) enhancement of 25.3% compared to conventional counterparts. The reduced material usage (44%) contributes to device power-cost "decoupling," leading to geometry-dependent optimal electrical matching for output maximization. This offers an energy consumption reduction (19.3%), electricity savings (0.24 kWh W^-1), and CO₂ emission reduction (0.17 kg W^-1) for large-scale industrial production, fundamentally reshaping the R&D route of flexible thermoelectrics for techno-economic sustainable heat harvesting. Our findings highlight a facile yet cost-effective strategy not only for low-grade heat harvesting but also for electronic co-design in heat management/recovery frontiers.

Triggering ZT to 0.40 by Engineering Orientation in One Polymeric Semiconductor

Breaking the thermoelectric (TE) trade-off relationship is an important task for maximizing the TE performance of polymeric semiconductors. Existing efforts have focused on designing high-mobility semiconductors and achieving ordered molecular doping, ignoring the critical role of the molecular orientation during TE conversion. Herein, the achievement of ZT to 0.40 is reported by fine-tuning the molecular orientation of one diketopyrrolopyrrole (DPP)-based polymer (DPP-BTz). Films with bimodal molecular orientation yield superior doping efficiency by increasing the lamellar spacing and achieve increased splitting between the Fermi energy and the transport energy to enhance the thermopower. These factors contribute to the simultaneous improvement in the Seebeck coefficient and electrical conductivity in an unexpected manner. Importantly, the bimodal film exhibits a maximum power factor of up to 346 µW m^-1 K^-2 , >400% higher than that of unimodal films. These results demonstrate the great potential of molecular orientation engineering in polymeric semiconductors for developing state-of-the-art organic TE (OTE) materials.

Ag2 Q-Based (Q = S, Se, Te) Silver Chalcogenide Thermoelectric Materials

Thermoelectric technology provides a promising solution to sustainable energy utilization and scalable power supply. Recently, Ag₂ Q-based (Q = S, Se, Te) silver chalcogenides have come forth as potential thermoelectric materials that are endowed with complex crystal structures, high carrier mobility coupled with low lattice thermal conductivity, and even exceptional plasticity. This review presents the latest advances in this material family, from binary compounds to ternary and quaternary alloys, covering the understanding of multi-scale structures and peculiar properties, the optimization of thermoelectric performance, and the rational design of new materials. The "composition-phase structure-thermoelectric/mechanical properties" correlation is emphasized. Flexible and hetero-shaped thermoelectric prototypes based on Ag₂ Q materials are also demonstrated. Several key problems and challenges are put forward concerning further understanding and optimization of Ag₂ Q-based thermoelectric chalcogenides.

Thermoelectric Silver-Based Chalcogenides

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

Presynthetic Redox Gated Metal-to-Insulator Transition and Photothermoelectric Properties in Nickel Tetrathiafulvalene-Tetrathiolate Coordination Polymers

Photothermoelectric (PTE) materials are promising candidates for solar energy harvesting and photodetection applications, especially for near-infrared (NIR) wavelengths. Although the processability and tunability of organic materials are highly advantageous, examples of organic PTE materials are comparatively rare and their PTE performance is typically limited by poor photothermal (PT) conversion. Here, we report the use of redox-active Sn complexes of tetrathiafulvalene-tetrathiolate (TTFtt) as transmetalating agents for the synthesis of presynthetically redox tuned NiTTFtt materials. Unlike the neutral material NiTTFtt, which exhibits n-type glassy-metallic conductivity, the reduced materials Li_1.2Ni_0.4[NiTTFtt] and [Li(THF)_1.5]_1.2Ni_0.4[NiTTFtt] (THF = tetrahydrofuran) display physical characteristics more consistent with p-type semiconductors. The broad spectral absorption and electrically conducting nature of these TTFtt-based materials enable highly efficient NIR-thermal conversion and good PTE performance. Furthermore, in contrast to conventional PTE composites, these NiTTFtt coordination polymers are notable as single-component PTE materials. The presynthetically tuned metal-to-insulator transition in these NiTTFtt systems directly modulates their PT and PTE properties.

Plastic/Ductile Bulk 2D van der Waals Single-Crystalline SnSe2 for Flexible Thermoelectrics

The recently discovered ductile/plastic inorganic semiconductors pave a new avenue toward flexible thermoelectrics. However, the power factors of current ductile/plastic inorganic semiconductors are usually low (below 5 µW cm^-1  K^-2 ) as compared with classic brittle inorganic thermoelectric materials, which greatly limit the electrical output power for flexible thermoelectrics. Here, large plasticity and high power factor in bulk two-dimensional van der Waals (2D vdW) single-crystalline SnSe₂ are reported. SnSe₂ crystals exhibit large plastic strains at room temperature and they can be morphed into various shapes without cracking, which is well captured by the inherent large deformability factor. As a semiconductor, the electrical transport properties of SnSe₂ can be readily tuned in a wide range by doping a tiny amount of halogen elements. A high power factor of 10.8 µW cm^-1  K^-2 at 375 K along the in-plane direction is achieved in plastic single-crystalline Br-doped SnSe₂ , which is the highest value among the reported flexible inorganic and organic thermoelectric materials. Combining the good plasticity, excellent power factors, as well as low-cost and nontoxic elements, bulk 2D vdW single-crystalline SnSe₂ shows great promise to achieve high power density for flexible thermoelectrics.

A Highly Conductive n-Type Coordination Complex with Thieno[3,2-b]thiophene Units: Facile Synthesis, Orientation, and Thermoelectric Properties

An organometallic nickel complex containing thieno[3,2-b]thiophene units was designed and synthesized. Composite films of the resulting nickel complex and polyvinylidene difluoride, which can be fabricated via a simple solution process under atmospheric conditions, exhibit remarkably high n-type conductivity (>200 S cm^-1). Moreover, the thermoelectric power factor of the n-type composite film was proven to be air stable. A grazing-incidence wide-angle X-ray diffraction analysis indicated a significant impact of introducing the thieno[3,2-b]thiophene core into the backbone of the nickel complex on the orientation within the composite films.

Flexible thermoelectrics based on ductile semiconductors

Flexible thermoelectrics provide a different solution for developing portable and sustainable flexible power supplies. The discovery of silver sulfide-based ductile semiconductors has driven a shift in the potential for flexible thermoelectrics, but the lack of good p-type ductile thermoelectric materials has restricted the reality of fabricating conventional cross-plane π-shaped flexible devices. We report a series of high-performance p-type ductile thermoelectric materials based on the composition-performance phase diagram in AgCu(Se,S,Te) pseudoternary solid solutions, with high figure-of-merit values (0.45 at 300 kelvin and 0.68 at 340 kelvin) compared with other flexible thermoelectric materials. We further demonstrate thin and flexible π-shaped devices with a maximum normalized power density that reaches 30 μW cm^-2 K^-2. This output is promising for the use of flexible thermoelectrics in wearable electronics.

Thermal management and control of wearable devices

The emergence of wearable devices over the recent decades has motivated numerous studies aimed at developing flexible or stretchable materials and structures for their electronic or optoelectronic functionalities. Like in conventional devices, electronic and optoelectronic components in wearable devices must be kept within certain temperature ranges to ensure reliability, performance, and/or functionality. But this must be accomplished without requiring any bulky heat sinks or other heat transfer augmentation elements. At the same time, the proximity of wearable devices to the human skin poses additional requirements of thermal comfort and safety. A growing body of literature is now focusing on the thermal management or control of wearable devices and related development of new materials and structures. The present article aims to provide a broad overview of such materials and structures and offer suggestions for future research directions.

Facile Fabrication of N-Type Flexible CoSb3-xTex Skutterudite/PEDOT:PSS Hybrid Thermoelectric Films

Alongiside the growing demand for wearable and implantable electronics, the development of flexible thermoelectric (FTE) materials holds great promise and has recently become a highly necessitated and efficient method for converting heat to electricity. Conductive polymers were widely used in previous research; however, n-type polymers suffer from instability compared to the p-type polymers, which results in a deficiency in the n-type TE leg for FTE devices. The development of the n-type FTE is still at a relatively early stage with limited applicable materials, insufficient conversion efficiency, and issues such as an undesirably high cost or toxic element consumption. In this work, as a prototype, a flexible n-type rare-earth free skutterudite (CoSb₃)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) binary thermoelectric film was fabricated based on ball-milled skutterudite via a facile top-down method, which is promising to be widely applicable to the hybridization of conventional bulk TE materials. The polymers bridge the separated thermoelectric particles and provide a conducting pathway for carriers, leading to an enhancement in electrical conductivity and a competitive Seebeck coefficient. The current work proposes a rational design towards FTE devices and provides a perspective for the exploration of conventional thermoelectric materials for wearable electronics.

Organic Thermoelectric Materials as the Waste Heat Remedy

The primary reason behind the search for novel organic materials for application in thermoelectric devices is the toxicity of inorganic substances and the difficulties associated with their processing for the production of thin, flexible layers. When Thomas Seebeck described a new phenomenon in Berlin in 1820, nobody could have predicted the future applications of the thermoelectric effect. Now, thermoelectric generators (TEGs) are used in watches, and thermoelectric coolers (TECs) are applied in cars, computers, and various laboratory equipment. Nevertheless, the future of thermoelectric materials lies in organic compounds. This paper discusses the developments made in thermoelectric materials, including small molecules, polymers, molecular junctions, and their applications as TEGs and/or TECs.

Realizing High Thermoelectric Performance of Bi-Sb-Te-Based Printed Films through Grain Interface Modification by an In Situ-Grown β-Cu2-δSe Phase

It has been a substantial challenge to develop a printed thermoelectric (TE) material with a figure-of-merit ZT > 1. In this work, high ZT p-type Bi_0.5Sb_1.5Te₃-based printable TE materials have been advanced by interface modification of the TE grains with a nonstoichiometric β-Cu_2-δSe-based inorganic binder (IB) through a facile printing-sintering process. As a result, a very high TE power factor of ∼17.5 μW cm^-1 K^-2 for a p-type printed material is attained in the optimized compounds at room temperature (RT). In addition, a high ZT of ∼1.2 at RT and of ∼1.55 at 360 K is realized using thermal conductivity (κ) of a pellet made of the prepared printable material containing 10 wt % of IB. Using the same material for p-type TE legs and silver paste for n-type TE legs, a printed TE generator (print-TEG) of four thermocouples has been fabricated for demonstration. An open-circuit voltage (V_OC) of 14 mV and a maximum power output (P_max) of 1.7 μW are achieved for ΔT = 40 K for the print-TEG.

Conductive One-Dimensional Coordination Polymers with Tunable Selectivity for the Oxygen Reduction Reaction

Conductive materials involving nonprecious metal coordination complexes as electrocatalysts for the oxygen reduction reaction (ORR) have received increasing attention in recent years. Herein, we reported efficient ORR electrocatalysts containing M-S2N2 sites with tunable selectivity based on simple one-dimensional (1D) coordination polymers (CPs). The 1D CPs were synthesized from M(OAc)2 and 2,5-diamino-1,4-benzenedithiol (DABDT) by a solvent thermal method. Due to their good electrical conductivities (10-6-10-2 S cm-1), the 1D CPs could be used as ORR catalysts in low catalytic amounts without the addition of carbon materials. Cobalt-based CPs showed a well-organized structure of nanosheets with Co-S2N2 sites exposed and exhibited remarkable electrocatalytic ORR activity (Eonset = 0.93 V vs reversible hydrogen electrode (RHE), E1/2 = 0.82 V, n = 3.85, JL = 5.22 mA cm-2, Tafel slope of 63 mV dec-1) in alkaline media. However, nickel-based CPs favored a 2e- ORR process with ∼87% H2O2 selectivity and an Eonset of 0.78 V. This work provides new opportunities for the construction of ORR catalysts based on conductive nonprecious metal CPs.

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.

Crystalline Domain Formation to Enable High-Performance Polymer Thermoelectrics Inspired by Thermocleavable Materials

Improving the electrical conductivity is an important role in realizing high thermoelectric performance of solution-processable polymers. Herein, a simple and robust approach to boost the mobility and doping efficiency of a diketopyrrolopyrrole-based copolymer with the introduction of thermocleavable side chains (PDPPS-X, where X is the molar ratio of the thermocleavable side chains and alkyl chains) is first provided. Notably, the incorporated thermocleavable groups can be effectively removed after thermal treatment and therefore contribute to the crystalline domain formation via hydrogen-bonded networks, which is critical for conductivity enhancements. Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns give a clear indication that the thermal treatment of PDPPS-5 can greatly improve the structural arrangement, resulting in a significantly enhanced hole mobility (5.4 times that of PDPPS-0 without thermocleavable chains). Compared to PDPPS-0, a larger Fermi level shift is observed after doping PDPPS-5 with FeCl₃, reflecting a better doping efficiency. Consequently, remarkably improved conductivity and power factor are achieved by PDPPS-5 after doping with 0.03 M FeCl₃ at room temperature, which are about 2.2 and 3.5 times higher than that of PDPPS-0 at the same testing condition, respectively. Moreover, PDPPS-5 achieved a maximum power factor of 57.5 μW m^-1 K^-2 at 404 K.

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.

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.

Effect of the Linking Group on the Thermoelectric Properties of Poly(Schiff Base)s and Their Metallopolymers

As polymer-based thermoelectric (TE) materials possess attractive features such as light weight, flexibility, low toxicity and ease of processibility, an increasing number of conducting polymers and their composites with high TE performances have been developed in recent years. Up to date, however, the research focusing on the structure-performance relationship remains rare. In this paper, two series of poly(Schiff base)s with either C=C or C≡C linker and their metallopolymers were synthesized and doped with single-walled carbon nanotubes to evaluate how the linking groups affected the TE properties of the resulting composites. Apart from the effect exerted by the morphology, experimental results suggested that the linkers played a key role in determining the band gaps, preferred molecular conformation and extent of conjugation of the polymers, which became key factors that influenced the TE properties of the resulting materials. Additionally, upon coordination with transition metal ions, the TE properties could be tuned readily.

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.

Energy Harvesting and Storage with Soft and Stretchable Materials

This review highlights various modes of converting ambient sources of energy into electricity using soft and stretchable materials. These mechanical properties are useful for emerging classes of stretchable electronics, e-skins, bio-integrated wearables, and soft robotics. The ability to harness energy from the environment allows these types of devices to be tetherless, thereby leading to a greater range of motion (in the case of robotics), better compliance (in the case of wearables and e-skins), and increased application space (in the case of electronics). A variety of energy sources are available including mechanical (vibrations, human motion, wind/fluid motion), electromagnetic (radio frequency (RF), solar), and thermodynamic (chemical or thermal energy). This review briefly summarizes harvesting mechanisms and focuses on the materials' strategies to render such devices into soft or stretchable embodiments.

Synthesis of Bis-Terpyridine-Based Metallopolymers and the Thermoelectric Properties of Their Single Walled Carbon Nanotube Composites

Although the organic and the conventional inorganic thermoelectric (TE) materials have been extensively developed in recent years, the number of cases involving conducting metallopolymers is still quite limited. In view of the versatile coordination capability of the terpyridine fraction and the electron-rich nature of the 3,4-ethylenedioxythiophene moiety, a bis-terpyridine-featured ligand was designed, and a series of metallopolymers were then synthesized. Upon the addition of single-walled carbon nanotube (SWCNT), the TE properties of the resulting metallopolymer-SWCNT composite films were investigated. It was found that metal centres played an important role in affecting the morphology of the thin films, which was a key factor that determined the TE performances of the composites. Additionally, the energy levels of the metallopolymers were feasibly tuned by selecting different metal centres. With the combined effects of a uniform and condensed surface and an optimized band structure, the highest power factor was achieved by the Cu(II)-containing metallopolymer-SWCNT composite at the doping ratio of 75%, which reached 38.3 μW·m-1·K-2.

Polymer-Inorganic Thermoelectric Nanomaterials: Electrical Properties, Interfacial Chemistry Engineering, and Devices

Though solar cells are one of the promising technologies to address the energy crisis, this technology is still far from commercialization. Thermoelectric materials offer a novel opportunity to convert energy between thermal and electrical aspects, which show the feasibility to improve the performance of solar cells via heat management and light harvesting. Polymer–inorganic thermoelectric nanocomposites consisting of inorganic nanomaterials and functional organic polymers represent one kind of advanced hybrid nanomaterials with tunable optical and electrical characteristics and fascinating interfacial and surface chemistry. During the past decades, they have attracted extensive research interest due to their diverse composition, easy synthesis, and large surface area. Such advanced nanomaterials not only inherit low thermal conductivity from polymers and high Seebeck coefficient, and high electrical conductivity from inorganic materials, but also benefit from the additional interface between each component. In this review, we provide an overview of interfacial chemistry engineering and electrical feature of various polymer–inorganic thermoelectric hybrid nanomaterials, including synthetic methods, properties, and applications in thermoelectric devices. In addition, the prospect and challenges of polymer–inorganic nanocomposites are discussed in the field of thermoelectric energy.

Recent development and applications of electrical conductive MOFs

Metal-organic frameworks (MOFs) have emerged as attractive materials for energy and environmental-related applications owing to their structural, chemical and functional diversity over the last two decades. It is known that the poor carrier mobility and low electrical conductivity of ordinary MOFs severely limit their utility in practical applications. In the past 10 years, several MOF materials with high carrier mobility and outstanding electrical conductivity have received a worldwide upsurge of research interest and many techniques and strategies have been used to synthesize such MOFs. In this critical review, we provide an overview of the significant advances in the development of conductive MOFs reported until now. Their theoretical and synthetic design strategies, conductive mechanisms, electrical transport measurements, and applications are systematically summarized and discussed. In addition, we will also give some discussions on challenges and perspectives in this exciting field.

Copper Tetracyanoquinodimethane: From Micro/Nanostructures to Applications

Copper tetracyanoquinodimethane (CuTCNQ) has been investigated around 40 years as a representative bistable material. Meanwhile, micro/nanostructures of CuTCNQ is considered as the prototype of molecular electronics, which have attracted the world's attention and shown great potential applications in nanoelectronics. In this review, methods for synthesis of CuTCNQ micro/nanostructures are first summarized briefly. Then, the strategies for controlling morphologies and sizes of CuTCNQ micro/nanostructures are highlighted. Afterwards, the devices based on these micro/nanostructures are reviewed. Finally, an outlook of future research directions and challenges in this area is presented.