Organic Thermoelectric Materials Research Articles

DEVELOPMENT OF HIGH-PERFORMANCE THERMOELECTRIC POLYMERS VIA DOPING OR DEDOPING ENGINEERING.

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scare/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

Thermoelectric generator and temperature sensor based on polyamide doped n-type single-walled nanotubes toward self-powered wearable electronics

Due to its ability to convert body heat into electricity, organic thermoelectric material is considered a promising and smart maintenance-free power source to charge wearable electronics. However, developing flexible n-type organic thermoelectric materials and wearable p/n junction thermoelectric devices remains challenging. In this work, two insulated polyamides (PA6 and PA66) that have been widely used as fiber materials are employed as novel dopants for converting p-type single-walled carbon nanotubes (SWCNTs) to n-type thermoelectric materials. Because of the electron transferability of the amide group, polyamide-doped SWCNTs exhibit excellent thermopower values as large as −56.0 μV K−1 for PA66, and −54.5 μV K−1 for PA6. Thermoelectric devices with five p/n junctions connected in series are fabricated. The testing device produces a thermoelectric voltage of 43.1 mV and generates 1.85 μW thermoelectric power under temperature gradients of approximately 80 K. Furthermore, they display charming capability for temperature recognition and monitoring human activities as sensors. These promising results suggest that the flexible polyamide-doped SWCNT composites herein have high application potential as wearable thermoelectric electronics.

Organic Thermoelectric Devices for Energy Harvesting and Sensing Applications

AbstractThe rise of artificial intelligence and the Internet of Things have spurred an increasing demand for wearable, sustainable, and maintenance‐free power sources. Organic thermoelectric (OTE) devices are emerged as promising candidates because they are capable of converting heat energy into electricity directly without the need of moving parts, capable of flexible and seamless integration with multifunctional miniaturized electronics. In addition, OTE devices can perform straightforward as various self‐powered sensors, boosting their applications in the field of intelligent interactions. This review focuses on recent advances in OTE materials and devices for applications in energy harvesting and sensing. The basic knowledge and key parameters of OTE materials and devices are presented, followed by detailed introduction of recent progress of OTE generators, sensors, and other OTE‐integrated devices. Next, several aspects of optimizing OTE devices toward multifunctional applications are highlighted. Finally, an overview of the current challenges and future research directions of OTE‐based devices is addressed. It is hoped that this review can pave the way for speeding up a bright future for the development and practical applications of OTE devices.

Advances in Nanoparticle-Enhanced Thermoelectric Materials from Synthesis to Energy Harvesting: A Review.

This comprehensive review analysis examines the domain of composite thermoelectric materials that integrate nanoparticles, providing a critical assessment of their methods for improving thermoelectric properties and the procedures used for their fabrication. This study examines several approaches to enhance power factor and lattice thermal conductivity, emphasizing the influence of secondary phases and structural alterations. This study investigates the impact of synthesis methods on the electrical characteristics of materials, with a particular focus on novel techniques such as electrodeposition onto carbon nanotubes. The acquired insights provide useful guidance for the creation of new thermoelectric materials. The review also compares and contrasts organic and inorganic thermoelectric materials, with a particular focus on the potential of inorganic materials in the context of waste heat recovery and power production within industries. This analysis highlights the role of inorganic materials in improving energy efficiency and promoting environmental sustainability.

A Rolled Organic Thermoelectric Generator with High Thermocouple Density

AbstractThe surge in the number of distributed microelectronics and sensors requires versatile, scalable, and affordable power sources. Heat‐harvesting organic thermoelectric generators (TEGs) are regarded as potential key components of the future energy landscape. Recent advances in the performance of organic thermoelectric materials have made practical applications of organic TEGs more feasible than ever before, yet the challenges of designing and fabricating organic TEGs suitable for real scenarios are scarcely addressed. Specifically, small sensors and wearables demand for micro‐thermoelectric generators (µTEGs) with high power density architectures and small form factors, while typical demonstrations of organic TEGs are characterized by < 10 thermocouples (TCs) per cm2. This work presents a rolled, organic µTEG architecture combining large‐area, solution‐based deposition techniques, such as inkjet and spray‐coating, and an ultrathin parylene substrate to achieve a thermocouple density of 1842 TCs cm−2. Such demonstrative µTEG reaches a thermoelectric conversion performance of 0.15 µW cm−2 at ΔT = 50 K. Such power output is well in line with finite element method simulations, which highlight the benefit of the architecture and show that remarkable power densities, in the mW cm−2 range at ΔT = 10 K, are realistically achievable with geometrical improvements and already ongoing advancements in organic thermoelectric inks.

A Novel N-Type Molecular Dopant With a Closed-Shell Electronic Structure Applicable to the Vacuum-Deposition Process.

Rational design, synthesis, and characterization of a new efficient versatile n-type dopant with a closed-shell electronic structure are described. By employing the tetraphenyl-dipyranylidene (DP0) framework with two 7π-electron systems modified with N,N-dimethylamino groups as the strong electron-donating substituent, 2,2',6,6'-tetrakis[4-(dimethylamino)phenyl]-4,4'-dipyranylidene (DP7), a closed-shell molecule with an extremely high-lying energy level of the highest occupied molecular orbital, close to 4.0eV below the vacuum level, is successfully developed. Thanks to its thermal stability, DP7 is applicable to vacuum deposition, which allows utilization of DP7 in bulk doping for the development of n-type organic thermoelectric materials and contact doping for reducing contact resistance in n-type organic field-effect transistors. As vacuum-deposition processable n-type dopants are very limited, DP7 stands out as a useful n-type dopant, particularly for the latter purpose.

A novel solution-based approach for PANI growth on PEDOT:PSS film to fabricate PEDOT:PSS/PANI-H2SO4 bilayer and its application in thermoelectric energy harvesting

The construction of bilayer or multilayer structures has been recognized as one of the most promising strategies for exploring high-performance organic thermoelectric materials. In this study, a novel poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate)/polyaniline (PEDOT:PSS/PANI) bilayer film was successfully fabricated through a facile solution method, wherein PANI was initially dissolved in concentrated sulfuric acid (H2SO4) to form a homogenous solution which was then applied to the PEDOT:PSS film. Benefiting from the introduction of PANI, the Seebeck coefficient of the PEDOT:PSS film was improved from 16.34 to 23.17 μV K−1, while the electrical conductivity experienced a remarkable enhancement, reaching 723.34 S cm−1. Accordingly, the resulting PEDOT:PSS/PANI-H2SO4 bilayer film demonstrated a high power factor (PF) of 38.83 μW m−1 K−2, which further increased to 44.38 μW m−1 K−2 at 380 K. Hence, this technique offers a facile method to fabricate large-scale PEDOT:PSS/PANI-H2SO4 bilayer films, expanding application fields of PANI via solution-based preparation.

Effect of Linker Group on Thermoelectric Properties of Dimer Zinc Porphyrin-Based Molecular Junctions

The discovery of high-efficiency organic thermoelectric materials has the potential to pave the path for various energy-harvesting technologies and the development of flexible, transparent thin-film materials for Peltier coolers. Our study involves a comparison of the electrical and thermoelectric characteristics of four zinc porphyrin configurations. The results show that changing the linker between zinc diner units has an essential effect on the electrical conductance and thermopower values of these structures. Consequently, the thermoelectric figure of merit ZT will change which means the thermoelectric efficiency change.

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

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

Base-Assisted Polymerizations of Elemental Sulfur and Alkynones for Temperature-Controlled Synthesis of Polythiophenes or Poly(1,4-dithiin)s.

With the increasing demand for functional polythiophenes in extensive applications such as organic solar cells, electronic skins, thermoelectric materials, and field effect transistors, efficient and economic synthetic approaches for polythiophenes are urgently required. In this work, KOH-assisted polymerizations of elemental sulfur and alkynones were developed to directly afford polythiophenes with various backbones, regioselective structures, and high molecular weights (Mns up to 20700 g/mol) in high yields (up to 97%) at 80 °C in 30 min. When the same polymerization was conducted at room temperature, stable and unique poly(1,4-dithiin)s (Mns up to 21800 g/mol) could be rapidly obtained in high yields (up to 87%) in 10 min. The temperature-controlled KOH-assisted polymerizations of sulfur and alkynones possessed high efficiency, mild conditions, and simple operation, which had provided an economic, efficient, and convenient approach for the direct conversion from elemental sulfur to functional polythiophenes and poly(1,4-dithiin)s with the in situ constructed aromatic or nonaromatic heterocycles embedded in the polymer backbones, demonstrating great synthetic simplicity, high efficiency, good selectivity, and robustness. It is anticipated to accelerate the development of semiconducting polymer materials and their applications.

High-Performance and Ecofriendly Organic Thermoelectrics Enabled by N-Type Polythiophene Derivatives with Doping-Induced Molecular Order.

The ability of n-type polymer thermoelectric materials to tolerate high doping loading limits further development of n-type polymer conductivity. Herein, two alcohol-soluble n-type polythiophene derivatives that are n-PT3 and n-PT4 are reported. Due to the ability of two polymers to tolerate doping loading more significantly than 100mol%, both achieve electrical conductivity >100Scm-1 . Moreover, the conductivity of both polythiophenes remains almost constant at high doping concentrations with excellent doping tunability, which may be related to their ability to overcome charging-induced backbone torsion and morphology change caused by saturated doping. The characterizations reveal that n-PT4 has a high doping level and carrier concentration (>3.10×1020 cm-3 ), and the carrier concentration continues to increase as the doping concentration increases. In addition, doping leads to improved crystal structure of n-PT4, and the crystallinity does not decrease significantly with increasing doping concentration; even the carrier mobility increases with it. The synergistic effect of these two leads to both n-PT3 and n-PT4 achieving a breakthrough of 100 in conductivity and power factor. The DMlmC-doped n-PT4 achieves a power factor of over 150µWm-1 K-2 . These values are among the highest for n-type organic thermoelectric materials.

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

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

Advances in flexible inorganic thermoelectrics

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

Ternary system based on polyaniline, graphene, and acrylic matrix applied to thermoelectric systems

AbstractThermoelectric (TE) materials have attracted attention for offering a green option for power generation, due to their ability to convert thermal energy into electricity. In recent years, a promising way to achieve efficiency in TE properties has been proposed based on composites of conjugated polymers, such as polyaniline (PANI), and carbon nanomaterials such as graphene (GR). Since polyaniline and GR composites are promising fillers for organic thermoelectric materials (OTE), we expanded their investigations for a ternary system (TS), providing materials with multiple functionalities, and high performance. In this research work, a TS based on an acrylic matrix (ACR), GR, and PANI was successfully prepared through the combination of in situ polymerization of aniline in contact with GR and mechanical mixture of the resulting hybrid with an ACR. Structural and morphological characterization confirmed that GR affected PANI morphology and crystallinity. The band gap determination by Tauc's relation indicated the occurrence of π‐π interaction between the chains and an increase of the electrical conductivity of the composites allowed to infer a synergistic effect. The measured Seebeck coefficient reached a maximum value of −17.02 μVK−1 and the highest power factor obtained was 4.94 μWm−1 K−2 for the ACR/PANI sample, indicating a material with promising thermoelectric properties.

Large Mechanosensitive Thermoelectric Enhancement in Metallo-Organic Magnetic Molecules.

Organic materials are promising candidates for thermoelectric cooling and energy harvesting at room temperature. However, their electrical conductance (G) and Seebeck coefficient (S) need to be improved to make them technologically competitive. Therefore, radically new strategies need to be developed to tune their thermoelectric properties. Here, we demonstrate that G and S can be tuned mechanically in paramagnetic metallocenes, and their thermoelectric properties can be significantly enhanced by the application of mechanical forces. With a 2% junction compression, the full thermoelectric figure of merit is enhanced by more than 200 times. We demonstrate that this is because spin transport resonances in paramagnetic metallocenes are strongly sensitive to the interaction between organic ligands and the metal center, which is not the case in their diamagnetic analogue. These results open a new avenue for the development of organic thermoelectric materials for cooling future quantum computers and generating electricity from low-grade energy sources.

Mixed Ionic-Electronic Conducting Hydrogels with Carboxylated Carbon Nanotubes for High Performance Wearable Thermoelectric Harvesters.

Mixed ionic-electronic conducting (MIEC) thermoelectric (TE) materials offer higher ionic conductivity and ionic Seebeck coefficient compared to those of purely ionic-conducting TE materials. These characteristics make them suitable for direct use in thermoelectric generators (TEGs) as the charge carriers can be effectively transported from one electrode to the other via the external circuit. In the present study, MIEC hydrogels are fabricated via the chemical cross-linking of polyacrylamide (PAAM) and polydopamine (PDA) to form a double network. In addition, electrically conducting carboxylated carbon nanotubes (CNT-COOH) are dispersed evenly within the hydrogel via sonication and interaction with the PDA. Moreover, the electrical properties of the hydrogel are further improved via the in situ polymerization of polyaniline (PANI). The presence of CNT-COOH facilitates the ionic conductivity and enhances the ionic Seebeck coefficient via ionic-electronic interactions between sodium ions and carboxyl groups on CNT-COOH, which can be observed in X-ray photoelectron spectroscopy results, thereby promoting the charge transport properties. As a result, the optimum device exhibits a remarkable ionic conductivity of 175.3 mS cm-1 and a high ionic Seebeck coefficient of 18.6 mV K-1, giving an ionic power factor (PFi) of 6.06 mW m-1 K-2 with a correspondingly impressive ionic figure of merit (ZTi) of 2.65. These values represent significant achievements within the field of gel-state organic TE materials. Finally, a wearable module is fabricated by embedding the PAAM/PDA/CNT-COOH/PANI hydrogel into a poly(dimethylsiloxane) mold. This configuration yields a high power density of 171.4 mW m-2, thus highlighting the considerable potential for manufacturing TEGs for wearable devices capable of harnessing waste heat.

Electrochemically tuning the thermoelectric performance of flexible polythiophene films by ionic liquid

As one of the new generations of green energy materials, thermoelectric materials enable the direct conversion of heat and electric energy, thus alleviating the pressure of energy crisis and environmental pollution. Organic thermoelectric (TE) materials can be used as candidate materials for flexible wearable electronic devices due to their low toxicity, low cost, and lightweight, among others. In this work, free-standing polythiophene (PTh) film was obtained by electrochemical deposition using boron trifluoride diethyl etherate (BFEE) as the electrolyte, and the prepared films were processed electrochemically in [EMIM]BF4 ionic liquid. The UV–Visible absorption spectroscopy of PTh films at different constant potentials from -0.2 to 1.0 V illustrated that electrochemical treatment could modulate the doping states of the PTh film. The power factor and electrical conductivity of the PTh film treated at 0.4 V reached the maximum values of 2.91 S cm−1 and 2.59 μW m−1 K−2, respectively, an improvement of 315% and 130% compared with the pristine film. The results indicate that electrochemically doped is an effective method for modulating the thermoelectric performance of polymer films, providing enlightenment for obtaining high-performance organic thermoelectric material henceforth.

The thermoelectric power investigation of the ϒ2-graphyne molecule device

Graphyne, is an interesting carbon nanostructure, with unique properties such as high carrier mobility, thermal stability, and electronic band characteristics shaped by sp and sp2 hybridizations. Due to its intrinsic porous nature, graphyne exhibits higher phonon scattering, and lower thermal conductance compared to graphene, making it an excellent nominee to be investigated as an organic nanoscale thermoelectric material. In this study, we delve into the thermoelectric power properties of the ϒ2-Graphyne structures using the modeling and simulated approaches. Our modeling studies focus on unit cell base vector variation at temperatures higher and lower than the Fermi temperature, additionally, the hopping energy variation is calculated in the same temperatures. We have employed the semi-classical Boltzmann transport equation to analyze these effects, considering the relaxation time factor, as a way of exploring the phonon scatterings, and the changing system behavior. It is concluded structural variation leads to a change in the scattering mechanisms. ATK Quantum Wise simulations on three groups of ϒ2-Graphyne structures are carried out. In the first two groups, the influence of the electrode length and the asymmetry are investigated, but for the last group, only the main region variation is considered. Generally, the number of the unit cells in the main region is changed from one group to the other. The density of states, the transmission spectrum, and electrical conductance values are calculated moreover, the effect of the temperature on thermoelectric power for all groups of devices is simulated. The figure of merit (Z) values at room temperature for all the structures is determined and the highest value of ZT is for the device with one unit cell and asymmetric electrodes are reported.

Disorder‐Controlled Efficient Doping of Conjugated Polymers for High‐Performance Organic Thermoelectrics

AbstractMolecular doping of conjugated polymers (CPs) is critical to improve their electrical properties in organic thermoelectric materials, which are promising candidates for low‐temperature energy harvesting and self‐powered sensors. Because excessive dopants are used in the doping process, securing highly crystalline CPs by suppressing dopant‐induced disorder is crucial. Recently, tris(pentafluorophenyl)borane (BCF) has attracted interest as a strong Brønsted acid dopant that enables efficient integer charge transfer by forming a complex with water. However, the crystalline ordering of BCF‐doped CPs has not been properly controlled; the resultant thermoelectric performance is therefore still lower than other p‐doped CPs prepared by conventional redox doping. Here, an efficient doping strategy is proposed with BCF to attain highly doped CPs with dramatically suppressed structural and energetic disorder. In a non‐polar aliphatic solvent, hexane, BCF can effectively diffuse into poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) via sequential doping, leading to remarkable electrical conductivity and power factor of 230 S cm−1 and 140 µW m−1 K−2, respectively. In addition, sequentially doped films exhibit highly delocalized transport characteristics owing to the low degree of charge carrier localization within their high solid‐state ordering. The results provide an approach for disorder‐tolerant doping strategies with a systematic investigation of structure–property relationships in highly doped CPs.

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.