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Tuning thermoelectric performance with N-annulated perylene-based small molecules and single-walled carbon nanotube nanocomposite films
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Despite notable progress in thermoelectric (TE) materials and devices, developing TE aerogels with high-temperature resistance, superior TE performance and excellent elasticity to enable self-powered high-temperature monitoring/warning in industrial and wearable applications remains a great challenge. Herein, a highly elastic, flame-retardant and high-temperature-resistant TE aerogel, made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/single-walled carbon nanotube (PEDOT:PSS/SWCNT) composites, has been fabricated, displaying attractive compression-induced power factor enhancement. The as-fabricated sensors with the aerogel can achieve accurately pressure stimuli detection and wide temperature range monitoring. Subsequently, a flexible TE generator is assembled, consisting of 25 aerogels connected in series, capable of delivering a maximum output power of 400 μW when subjected to a temperature difference of 300 K. This demonstrates its outstanding high-temperature heat harvesting capability and promising application prospects for real-time temperature monitoring on industrial high-temperature pipelines. Moreover, the designed self-powered wearable sensing glove can realize precise wide-range temperature detection, high-temperature warning and accurate recognition of human hand gestures. The aerogel-based intelligent wearable sensing system developed for firefighters demonstrates the desired self-powered and highly sensitive high-temperature fire warning capability. Benefitting from these desirable properties, the elastic and high-temperature-resistant aerogels present various promising applications including self-powered high-temperature monitoring, industrial overheat warning, waste heat energy recycling and even wearable healthcare.
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.
In recent years, many conjugated polymer thermoelectric (TE) materials have been reported, but the high performance TE composites prepared by ionic functional group side chain organic small molecules (OSM) and single-walled carbon nanotubes (SWCNT) are rarely reported. In this study, three kinds of OSM were designed and synthesized, which were respectively complexed with SWCNT as P-type TE materials. The strong interaction between OSM and SWCNT improves the doping level and the excellent charge transfer facilitates the transport of carriers, thus the synergistic effect of OSMs and SWCNT improves the TE performance of the composite films. By changing the mass ratio of SWCNT in the composite film, TE performance can be effectively improved and adjusted. The maximum power factor of the composite film at room temperature is 330.2 μW m⁻¹ K⁻². The development of OSM/SWCNT composite films with different conjugated main chains and the same side chain ionic functional groups provides a new approach for the design of organic TE materials with high TE properties.
Graphical abstract
Smart wearable electronics have attracted increasing attention because of their great prospects in motion monitoring. To date, a specific design of thermoelectric wearable devices aiming at precision monitoring is still challenging, although thermoelectric materials and devices have witnessed significant developments. In this work, intercalated composites of reduced graphene oxide/reduced poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (rGO/rPEDOT:PSS) are reported through the insertion of PEDOT:PSS into 2D graphene oxide layers followed by precise optimization of oxidation‐level of composite, exhibiting an outstanding thermoelectric performance along with excellent thermoelectric stability and mechanical flexibility. Furthermore, a novel‐concept, simple‐structural, self‐powered thermoelectric wearable sensor is demonstrated with rGO/rPEDOT:PSS composite as the active sensing component of the thermoelectric device. Combined with the excellent thermoelectric performance, ingenious device design, and optimized algorithm, the thermoelectric wearable device achieves precision recognition of various hand motions. This study provides a new way to design wearable sensors toward the precision recognition of human motions.
Thanks to the combined efforts of scientists in several research fields, the preceding decade has witnessed considerable progress in the use of conjugated polymers as emerging thermoelectric materials leading to significant improvements in performance and demonstration of a number of diverse applications. Despite these recent advances, systematic assessments of the impact of molecular design on thermoelectric properties are scarce. Although several reviews marginally highlight the role of chemical structure, the understanding of structure-performance relationships is still fragmented. An in-depth understanding of the relationship between molecular structure and thermoelectric properties will enable the rational design of next-generation thermoelectric polymers. To this end, this review showcases the state-of-the-art thermoelectric polymers, discusses structure-performance relationships, suggests strategies for improving thermoelectric performance that go beyond molecular design, and highlights some of the most impressive applications of thermoelectric polymers.
Thermoelectric (TE) materials possess unique energy conversion capabilities between heat and electrical energy. Small organic semiconductors have aroused widespread attention for the fabrication of TE devices due to their advantages of low toxicity, large area, light weight, and easy fabrication. However, the low TE properties hinder their large‐scale commercial application. Herein, the basic knowledge about TE materials, including parameters affecting the TE performance and the remaining challenges of the organic thermoelectric (OTE) materials, are initially summarized in detail. Second, the optimization strategies of power factor, including the selection and design of dopants and structural modification of the dope‐host are introduced. Third, some achievements of p‐ and n‐type small molecular OTE materials are highlighted to briefly provide their future developing trend; finally, insights on the future development of OTE materials are also provided in this study.
Significant progress has been achieved for flexible polymer thermoelectric (TE) composites in the last decade due to their potential application in wearable devices and sensors. In sharp contrast to the exceptional increase in TE studies at room temperature, the mechanical performance of polymer TE composites has received relatively less attention despite the significance of the application of TE composites in high-temperature environments. The TE and mechanical performances of flexible poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/single-walled carbon nanotube (PEDOT:PSS/SWCNT) composite films with an ionic liquid (IL) (referred to as "PEDOT:PSS/SWCNT-IL") at high temperatures are studied in the present work. The fabricated composite films show increasing TE performance with increasing temperature and SWCNT content. The maximum value of the power factor reaches 301.35 μW m-1 K-2 at 470 K for the PEDOT:PSS/SWCNT-IL composite. Furthermore, the addition of the IL improves the elongation at break of the composites compared to the IL-free composites. This work promotes the advancement of flexible polymer TE composites and widens their potential applications at different temperature ranges.
Four donor-acceptor salicylaldimines with benzoheterocyclic substituents at nitrogen imine were prepared and characterised by means of optical spectroscopy. Their luminescent properties were compared with two isoelec-tronic carbocyclic analogues. In contrast to the typical salicylaldimines, our reported benzoheterocyclic compounds exhibit solely local emissive behaviour in all media whereas for the carbocyclic analogues we expectedly observe dominant ESIPT emission in the majority of instances. Low temperature (5 K-liquid helium) fluores-cence measurements revealed the effect of the restriction of the molecular rotation which was manifested by significant increment of the fluorescence intensity. The comprehensive experimental and ab initio study reveal effect of solvent polarity, viscosity and temperature on the complex photophysics of heterocyclic and carbocyclic substituted salicylaldimines with several degrees of freedom for molecular transformations. This work adds considerable insight in emission lifetime data which greatly facilitates understanding of the fluorescence mechanism of the molecules with environmentally dependent restrictions.
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.
Organic charge transfer complexes (CTCs) or electron donor–acceptor complexes have been intensively studied as organic semiconductors or organic conductors in organic electronics. Herein, the composite of CTCs and single-walled carbon nanotubes (SWCNTs) is studied as both p-type and n-type thermoelectric materials for flexible thermoelectric generators. CTCs are formed by [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives with different side chains as electron donors and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as electron acceptors. The thermoelectric properties of the composites as well as the effect of side chains in BTBT derivatives are investigated. It is found that the PhBTBT–F4TCNQ/SWCNT composite film shows the highest p-type power factor of 244.3 µW m⁻¹ K⁻²; while the C8BTBT–F4TCNQ/SWCNT composite film exhibits the highest n-type power factor of 105.1 µW m⁻¹ K⁻². The thermoelectric module based on five p–n junctions of C8BTBT–F4TCNQ/SWCNT exhibits the highest output voltage of 13.1 mV and output power of 340 nW under a 38 K temperature gradient. This device performance is mainly generated from the moderate carrier concentrations and low film defects in the n-type C8BTBT–F4TCNQ/SWCNT composite film.
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.
In the recent decade, polymer thermoelectric (TE) composite has witnessed explosive achievements to address energy generation and utilization. Besides the significant progress in enhancement of TE performance, the high mechanical property has received increasing attention, being important for practical applications in complex environments. However, the mechanical performance has always been improved at the sacrifice of TE performance, and vice versa, which poses a great challenge. Here, ionic liquid (IL)‐assisted fabrication of flexible films of polymer TE composites with simultaneously high TE and mechanical performances based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polyvinyl alcohol (PVA), and single‐walled carbon nanotubes (SWCNTs) are reported. The resultant composite shows a high TE performance with a power factor of 106.1 ± 8.2 µW m−1 K−2 at room temperature, and strong mechanical robustness with a tensile modulus of 4.2 ± 0.5 GPa and fracture strength of 136.5 ± 10.6 MPa. It is the most mechanically robust TE composite known with such a high power factor in the available literature. The present study provides a promising way to help address the longstanding and intractable issue of inferior mechanical performance of TE composites without compromising TE performance. Ionic liquid‐assisted fabricated flexible films of polymer composites show a high thermoelectric performance (power factor of 106.1 ± 8.2 µW m−1 K−2) and strong mechanical robustness (tensile modulus of 4.2 ± 0.5 GPa, fracture strength of 133.4 ± 10.6 MPa). This study provides a promising way to help address the intractable issue of inferior mechanical performances without compromising thermoelectric performance.
Thermoelectric materials allow direct conversion of waste heat energy into electrical energy, thus contributing to solving energy related issues. Polymer‐based materials have been considered for use in heat conversion in the temperature range from 20 to 200 °C, within which conventional materials are not efficient enough, whereas polymers due to their good electronic transport properties, easy processability, non‐toxicity, flexibility, abundance, and simplicity of adjustment, are considered as promising materials. Due to the large variety of available polymers and the almost unlimited combinations of possible modifications, the field of polymer‐based thermoelectrics is very rapidly developing, already reaching efficiency values close to those of inorganic systems. In the current progress report, the most recent advances in the field are discussed. New approaches to improve thermoelectric performance are described, with a focus on revising the mechanisms to improve the thermoelectric properties of the three most investigated polymer matrixes: poly(3,4‐ethylenedioxythiophene) polystyrene sulfonat, poly(3‐hexylthiophene‐2,5‐diyl), and polyaniline, alongside the three main paths of optimizing properties: incorporation of carbon‐based material and inorganic substances, and treatment with chemical agents. The most promising research in the field is highlighted and thoroughly analyzed. The path toward a lab‐to‐fab transition for thermoelectric polymers is suggested in perspective. This progress report covers the latest work in the field of polymer‐based thermoelectric materials with low operating temperatures. Emphasis is given to flexible, high‐yield and low‐cost thermoelectrics. The reader is provided with the background behind flexible thermoelectrics, the main strategies to improve thermoelectric efficiency, through suitable tuning of the mechanisms of charge/heat transport, and the most promising future research directions.
The urgent need for consistent, reliable, ecofriendly, and stable power sources drives the development of new green energy materials. Thermoelectric (TE) materials receive increasing attention due to their unique capability of realizing the direct energy conversion between heat and electricity, showing diverse applications in harvesting waste heat and low‐grade heat. Carbon materials such as carbon nanotubes (CNTs) and graphene have experienced a rapid development as TE materials because of their intrinsic ultrahigh electrical conductivity and light weight. Besides, polymer‐based carbon composites are particularly fascinating as the combination of the merits of polymers and filler materials leads to high TE performance and superior flexibility. Herein, the recent TE advances are systematically summarized in the studied popularity of carbon materials (ie, CNTs and graphene) and the category of polymers. The conducting polymer‐based carbon materials are particularly highlighted. Finally, the remaining challenges and some tentative suggestions possibly guiding future developments are proposed, which may pave a way for a bright future of carbon and carbon composites in the energy market. The urgent need for consistent, reliable, ecofriendly, and stable power sources drives the development of thermoelectric (TE) materials due to their unique capability to directly convert temperature gradient into electricity. Particularly, carbon and carbon composites have experienced rapid development as TE materials. This review summarized their recent advances, and proposed some the remaining challenges and some tentative suggestions possibly guiding future developments in this topic.
Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m−2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m−2. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties. The use of rigid linkers to control the relative position and interaction of donor and acceptor units in exciplex emitters leads to the realization of organic light-emitting devices with enhanced external quantum efficiency.
The booming of flexible electronics calls for consistent and reliable power supply. Compared to conventional energy storage systems which require frequent recharge, replacement, and maintenance, thermoelectric (TE) devices that directly convert heat into electricity exhibit desirable features including no noise, no vibration, no gas emission, and long lifetime, thus favoring the miniaturization and integrity of current flexible electronics. TE polymer composites are especially attractive as they combine the merits of both polymers and filler materials, which results in high TE performance as well as superior flexibility, thereby demonstrating high potential for flexible TE devices. Herein, some fundamental basics regarding polymer composites are presented and then recent progresses of various polymer composites based on the category of filler materials are summarized so that research efforts and results can be better comprehended. Subsequently, effective strategies to further strengthen the TE performance of polymer composites are emphasized. The following section introduces recent design prototypes of flexible TE devices based on polymer composites with rational discussion on the characteristics of each design. Finally, the remaining challenges with future research perspectives are proposed in conclusion.
Although numerous thermoelectric materials based on single-walled carbon nanotubes (SWNTs) and organic semiconductors have been reported during the past decade, the correlation between energy levels of organic semiconductors and thermoelectric performances of their hybrids is still ambiguous. In this study, we demonstrate that simultaneous modulation of the bandgap and highest occupied molecular orbital levels in organic small molecules (OSMs) largely improves the Seebeck coefficient and thus maximizes the figure of merit (ZT) of SWNT/OSM hybrids. SWNT/CzS with an enlarged bandgap and reduced barrier energy exhibited a synergistic increment in the Seebeck coefficient (108.7 μV K-1) and power factor (337.2 μW m-1 K-2), with the best ZT of 0.058 at room temperature among dopant-free carbon nanotube-hybridized thermoelectrics. The efficient charge carrier transport and reduced thermal conductivity of SWNT/CzS provided enhanced thermoelectric performance. Our strategy based on energy level modulation could be broadly applied for performance enhancement of organic and hybrid thermoelectric materials.
Although thermoelectric composites of carbon nanotubes (CNTs) with organic polymers or small organic molecules have witnessed explosive achievements, judicious molecular design still remains a great challenge. Here, p-type doping using spiro-bifluorene derivatives is proposed to significantly improve single-walled CNT (SWCNT) thermoelectric performance. The composite shows an increase of ∼108% in room-temperature power factor relative to the pristine SWCNT (112.6 ± 4.3 μW m⁻¹ K⁻²), with the maximum of 239.2 ± 4.2 μW m⁻¹ K⁻². The corresponding mechanism of effective p-type doping and strong interfacial interaction are elucidated by calculated energy levels, SWCNT surface coating and photophysical spectroscopic measurements. Finally, the thermoelectric device connected in series display an open-circuit voltage and an output power of 9.77 mV and 57.3 nW, respectively at a temperature gradient of 30 K. The results will benefit the exploitation of novel CNT-based TE composites and their applications.
Recently, organic thermoelectric (TE) materials have been intensively studied because of their great potential for application in flexible/wearable TE generators for power generation at low temperatures. However, reports on the...
The molecular adsorption of dopants on the surface of carbon nanotubes (CNTs) is directly related to the thermoelectric (TE) properties of CNTs by tuning carrier densities and mobilities. Although imidazolium-based ionic liquids have shown great potential as dopants owing to their feasibility in various interactions with CNTs, the effects of doping on the TE performance of CNTs have not been fully explored. Herein, the doping mechanisms of CNTs with 1-hexyl-3-methylimidazolium tetrafluoroborate are investigated under different surface states of CNTs controlled by adjusting the amount of anionic surfactants. For the negatively charged CNT surfaces by the anionic surfactants, imidazolium cations are preferentially adsorbed through the strong interactions with the anionic head groups of the surfactants, leading to p-doping. When the CNT surfaces become relatively neutral, more tetrafluoroborate anions can easily access the CNT surface, thereby inducing n-doping. These doping mechanisms can be selectively controlled using a simple solution process. The optimized CNT films exhibit significantly enhanced p-type TE power factors of up to 762 μW m⁻¹ K⁻². The solution-processed easy-to-cut CNT films are integrated with the n-type CNT films doped with poly(ethyleneimine) to fabricate high-performance flexible TE generators with a maximum power of 6.75 μW and voltage of 334 mV.
Organic thermoelectric materials have attracted tremendous attention in virtue of their unique advantages including low thermal conductivity, low cost, easy processability and mechanical flexibility. Herein, a series of novel Spiro molecules are designed to be composited with single-walled carbon nanotubes (SWCNT) as p-type thermoelectric materials. An optimum power factor of 218.6 μW m⁻¹ K⁻² is acquired from the SFX-2/SWCNT composite at room temperature. The corresponding flexible thermoelectric generator containing 10 p-type legs exhibits an output power of 1.0 μW under a temperature gradient of 60 K. The high thermoelectric performance is attributable to the three dimensional cross-conjugated architecture of the Spiro molecules, which promotes interactions with SWCNT leading to high carrier concentration and moderate carrier mobility in the composite film.
Herein, three hybrids of metal Phthalocyanine complexes (MPc, M=Ni, Cu, Co) with single-walled carbon nanotubes (SWCNTs) were prepared by ball milling combined with cold pressing method and their thermoelectric (TE) properties were studied. The electrical conductivity of the MPc/SWCNT hybrids remarkably increased with increasing SWCNT content and was higher than the values calculated based on the mixture rule, whereas the Seebeck coefficient slightly decreased in the whole range. Moreover, the NiPc/SWCNT hybrids showed higher electrical conductivity than those of CuPc/SWCNT and CoPc/SWCNT hybrids. It is demonstrated by Raman analyses and energy level measurement that strong donor-acceptor interactions occur between MPc and SWCNTs. Such interactions may promote the carrier transport at the interface, and therefore increase the carrier mobility resulting in the enhancement of electrical conductivity greatly overstepping the mixture rule. Furthermore, among the three hybrids, the interface energy barrier of NiPc/SWNT hybrids is the lowest, which also contributes to the high electrical conductivity. Finally, the maximum electrical conductivity and thermoelectric power factor of NiPc/SWCNT hybrids are up to 540 S cm⁻¹ and 120 μWm⁻¹ K⁻² respectively, which is 30–50% higher than those of CuPc/SWCNT and CoPc/SWCNT hybrids and among the best level of metal-organic small molecules based TE materials.
Molecular doping is vital to control the charge transport in organic/inorganic semiconductors, which provides an important technique to optimize the thermoelectric (TE) performance. Here, two organic borates with designed molecular structures, Ph3C⁺[B(C6F5)4]⁻ and Ph2I⁺[B(C6F5)4]⁻, were employed as p-type dopants to enhance the TE performance of single-walled carbon nanotubes (SWCNTs). The electrons are transferred from SWCNTs to the electron-deficient cations of the borates, resulting in charge-transfer complexes stabilized by [B(C6F5)4]⁻. Specifically, compared to the pristine SWCNTs, the borate-doped SWCNT films reveal enhanced power factors of 135.5 ± 8.4 μW m⁻¹ K⁻² for SWCNT/Ph3C⁺[B(C6F5)4]⁻, and 156.6 ± 7.2 μW m⁻¹ K⁻² for SWCNT/Ph2I⁺[B(C6F5)4]⁻. Moreover, the p-doped SWCNTs display good air and thermal stabilities due to the chemical inertness and thermal stability of the organic [B(C6F5)4]⁻ anions. In addition, by immersion in organic solvent, the TE performance can be further increased. This work provides a novel strategy to achieve stable p-doped SWCNT films with excellent TE performance via rational molecular design of organic borate dopants.
Twisted small organic molecules (SOMs) enhance the thermoelectric performance of single-walled carbon nanotubes (SWCNTs)/SOM hybrid films by dramatically increasing the Seebeck coefficient and minimising the inevitable reduction in electrical conductivity....
Organic/single-walled carbon nanotube (SWCNT) composites as thermoelectric (TE) materials have been developed rapidly in recent years; however, research on n-type organic/SWCNTs is relatively lagging, and their performance needs to be further improved. In this work, three low-cost organometallic complexes (OMCs) with reducing abilities, including ferrocene (FeCp2), FeCp2 with an acetenyl group (EtFc) to extend the conjugation degree, and FeCp2 with a (dimethylamino)methyl group (FcMA) to increase the electron-donating ability, were developed to enhance the TE properties of n-type SWCNT-based composites. In addition, the effect of solvents was also studied, and it was determined that a synergistic effect of N-methyl pyrrolidone (NMP) and ferrocene derivatives occurred in enhancing the n-type properties. Among the three OMCs, SWCNT/FcMA prepared from NMP displayed the highest TE performance, indicating that the ferrocene and (dimethylamino)methyl group can also form synergistic effects. When the mass ratio of the SWCNTs and FcMA was 1:7, the highest power factor of 567.54 ± 27.18 μW m⁻¹ K⁻² at room temperature was achieved, which is among the state-of-the-art n-type SWCNT/organic molecule-based TE materials in literature. A combination of field emission scanning electron microscopy and photoelectron spectroscopy indicated that the electrons could be transferred from NMP, FeCp2 and its derivatives to SWCNTs, leading to an upshift in the Fermi level and an increase in conductivity. Moreover, a TE device containing five p (SWCNTs) − n (SWCNT/FcMA) junctions was assembled, and an open-circuit voltage of 22.7 mV with an output power of 0.75 µW at a temperature difference of 54.1 K was achieved. These results showed that OMCs possess promising applications in future n-type carbon nanotube-based TE materials.
Poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is perhaps the most successful polymer material for thermoelectric (TE) applications. So far, treatments by high-boiling solvents, acid or base, or mixing with the carbon nanotube (CNT) are the main ways to improve its TE performance. Herein, we report the synergistically boosting TE properties of PEDOT:PSS/single-walled CNT (SWCNT) composites by the ionic liquid (IL). The composites are prepared by physically mixing the SWCNT dispersion containing the IL with PEDOT:PSS solution and subsequent vacuum filtration. The IL additive has two major functions, that is, inducing the phase separation of PEDOT:PSS and a linear quinoid conformation of PEDOT and promoting the SWCNT dispersion. The maximum power factor at room temperature reaches 182.7 ± 9.2 μW m-1 K-2 (the corresponding electrical conductivity and Seebeck coefficient are 1602.6 ± 103.0 S cm-1 and 33.4 ± 0.4 μV K-1, respectively) for the free-standing flexible film of the PEDOT:PSS/SWCNT composites with the IL, which is much higher than those of the pristine PEDOT:PSS, the IL-free PEDOT:PSS/SWCNT, and the SWCNT films. The high TE performance of composites can be ascribed to synergistic roles of the ion-exchange effect and promotion of SWCNT dispersion by the IL. This work demonstrates the dual roles for the IL in regulating each component of the PEDOT:PSS/SWCNT composite that synergistically boosts the TE performance.
With the ever-growing development of multifunctional and miniature electronics, the exploring of high-power microwatt-milliwatt self-charging technology is highly essential. Flexible thermoelectric materials and devices, utilizing small temperature difference to generate electricity, exhibit great potentials to provide the continuous power supply for wearable and implantable electronics. In this review, we summarize the recent progress of flexible thermoelectric materials, including conducting polymers, organic/inorganic hybrid composites, and fully inorganic materials. The strategies and approaches for enhancing the thermoelectric properties of different flexible materials are detailed overviewed. Besides, we highlight the advanced strategies for the design of mechanical robust flexible thermoelectric devices. In the end, we point out the challenges and outlook for the future development of flexible thermoelectric materials and devices.
Herein, a series of novel butterfly-shaped small-molecule organic semiconductors (OSCs) have been designed, synthesized and complexed with single-walled carbon nanotube (SWCNT) as p-type thermoelectric materials. The butterfly-shaped molecules exhibit curved molecular structures, which tune their frontier molecular orbitals and increase their interactions with SWCNT. A systematic study shows that the composites based on butterfly-shaped OSCs exhibit significant improved thermoelectric performances compared with that of the composit based on the analogue of 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 SWCNT. The buttefly-shaped OSC and SWCNT composite opens up a new avenue for the design of thermoelectric materials and devices.
Organic conjugated small molecules (OCSMs)-based single-walled carbon nanotube (SWCNT) composites have received great attention as potential thermoelectric (TE) materials. The studies show that both π-πand cation-πinteractions between OCSMs and SWCNTs obviously improve the TE performance. However, up to now, few OCSMs containing both cation groups and πstructures have been combined with SWCNTs as TE materials. In this work, an organic conjugated small molecule with a cation group, (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP), was applied to combine with SWCNTs for the first time due to π-πand cation-πinteractions between PyBOP and SWCNTs, which are beneficial to improving the electrical conductivity and thermoelectric (TE) performance. The highest electrical conductivity of 3731.5 S cm-1 was achieved for SWCNT/PyBOP, which was 6.59 times larger than that of the pristine SWCNTs, and also one of the highest values for OCSM-based SWCNT composites reported up to date. Among all of the composites, SWCNT/PyBOP displays the maximum power factor of 279.5 μW m-1 K-2 at 403 K. The results demonstrate that the electrical conductivity of SWCNT composites can be dramatically improved by tuning the structure of the OCSMs, which points out a new direction for the rapid development of high-performance TE composite materials.
Single-walled carbon nanotube (SWCNTs-P)-small organic molecule hybrid materials are promising candidates for achieving high thermoelectric (TE) performance. In this study, we synthesized rod-coil amphiphilic molecules, that is, tri(ethylene oxide) chain-attached bis(bithiophenyl)-terphenyl derivatives (1 and 2). Supramolecular functionalization of SWCNTs-P with 1 or 2 induced charge-transfer interactions between them. Improved TE properties of the supramolecular hybrids (SWCNTs-1 and SWCNTs-2) are attributed to increased charge-carrier concentration (electrical conductivity), interfacial phonon scattering (thermal conductivity), and energy difference between the transport and Fermi levels (ETr - EF; Seebeck coefficient). SWCNTs-2 exhibited a ZT of 0.42 × 10-2 at 300 K, which is 350% larger than that of SWCNTs-P. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped SWCNTs-2 showed the highest ZT value of 1.96 × 10-2 at 300 K among SWCNTs-P/small organic molecule hybrids known until now. These results demonstrated that the supramolecular functionalization of SWCNTs-P with small organic molecules could be useful for enhancement of TE performance and applications in wearable/flexible thermoelectrics.
Small organic molecules are promising as the next generation of thermoelectric materials due to their unique advantages, such as low cost, high mechanical flexibility, low thermal conductivity, and low toxicity. However, their low electrical conductivities seriously limit the realization of high power factors. Herein, naphthalene diimide derivatives (NDI-1 and NDI-2) carrying a radical substituent of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) are designed and complexed with single-walled carbon nanotubes (SWCNTs) as both p-type and n-type thermoelectric composites. The introduction of the radical substituent remarkably improved the electrical conductivity by almost fifty percent compared to that of the NDI derivative without the radical substituent (NDI-0). The obtained radical-containing composites display greatly enhanced TE performance with highest power factors of 277.1 ± 5.4 μW m⁻¹ K⁻² for the p-type composite and 79.6 ± 1.7 μW m⁻¹ K⁻² for the n-type composite, respectively. Furthermore, the thermoelectric module based on NDI-1/SWCNT composite films consisting of five p–n junctions reaches a large output power of 2.81 μW under a 65 K temperature gradient. The enhanced electrical conductivities and TE performances of radical-containing NDI/SWCNT composite films are attributable to the improved doping level and charge transport process between the radical molecules and SWCNTs. This design strategy of introducing radical moieties into organic thermoelectric materials might be beneficial to the future application for high-performance p-type and n-type thermoelectric materials and devices.
The SWNT/ dmBT hybrid with a low barrier of 0.06 eV between SWNT and dmBT showed a maximum Seebeck coefficient of 78.5 μV K ⁻¹ and a power factor of 183.9 μW m ⁻¹ K ⁻¹ , 1.5 and 4.6 times higher compared to the SWNT/ dCNBT with a high barrier of 0.64 eV.
Recently, single-walled carbon nanotubes (SWCNTs)-based thermoelectric (TE) materials have received much interest owing to their advantages, such as high electrical conductivity and flexibility, as well as their ability to be easily tuned to exhibit p-type or n-type characteristics by the addition of redox reagents. Compared with the numerous p-type composites, less n-type agents have been reported. Borane-nitrogen derivatives (BNs) have been widely used as reducing agents; however, little attention has been paid on their TE performance. As the reducing ability of BNs can be tuned by the basicity and steric hindrance of the nitrogen containing derivatives, herein, pyridineborane (PYB), morpholineborane (MPB) and N,N-diethylanilineborane (DEANB) were investigated as n-type dopants of SWCNTs by simply dispersing each of them with SWCNTs in various solvents. The results suggested the structures of the borane complexes and their reducing ability affect the TE properties greatly. Among them, SWCNT/PYB displayed the best performance at all the investigated temperatures, with power factors ranging from 193.6 μW m⁻¹ K⁻² to 223.8 μW m⁻¹ K⁻², which are the highest PF values reported to date based on boron containing SWCNT composites. In addition, the TE devices containing five p-n junctions were also combined. SWCNT/PYB exhibited a higher open-circuit voltage (28.8 mV) and output power (1.15 μW) than SWCNT/NaBH4 (23.7 mV, 0.79 μW) at the same conditions. Our results suggest that organic boron compounds can be developed as good agents for boosting the performance of n-type TE materials.
Constructing hybrid composites with organic and inorganic materials at different length scales provides unconventional opportunities in the field of thermoelectric materials, which are classified as hybrid crystal, superlattice, and nanocomposite. A variety of new techniques have been proposed to fabricate hybrid thermoelectric materials with homogeneous microstructures and intimate interfaces, which are critical for good thermoelectric performance. The combination of organic and inorganic materials at the nano or atomic scale can cause strong perturbation in the structural, electron, and phonon characteristics, providing new mechanisms to decouple electrical and thermal transport properties that are not attainable in the pure organic or inorganic counterparts. Because of their increasing thermoelectric performance, compositional diversity, mechanical flexibility, and ease of fabrication, hybrid materials have become the most promising candidates for flexible energy harvesting and solid-state cooling.
Expected final online publication date for the Annual Review of Materials Research, Volume 50 is July 1, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Copper phthalocyanine (CuPc)/single-walled carbon nanotube (SWCNT) hybrids were prepared by high energy ball milling combined with cold pressing method. The electrical conductivity of the hybrids increased with the increasing SWCNT content, whereas the Seebeck coefficient decreased. Moreover, the electrical conductivity of CuPc/SWCNT hybrids were much higher than the values calculated based on the general parallel and series connected two-component mixture model. SEM, TEM, XPS, Raman and XRD analyses indicated that there existed a large number of CuPc-SWCNT nano-interfaces in the hybrids. The strong donor-acceptor and π-π conjugation interactions between CuPc molecules and SWCNTs can induce CuPc to highly ordered face-on packing on the surface of SWCNTs, which contributed to the efficient interface charge transport between CuPc and the surface of SWCNTs, and therefore both the carrier concentration and carrier mobility of CuPc/SWCNT hybrids were significantly increased. Finally, the maximum thermoelectric (TE) power factor at room temperature reached 70.1 μW m⁻¹ K⁻². The optimal TE property of the CuPc/SWCNT hybrids is remarkably higher than those of either individual component of the hybrid, and is among the highest values of metal-organic small molecules based TE materials. This study may provide a new route to enhance the TE properties of metal-organic small molecules materials.
N-annulated perylene based materials show outstanding and tunable optical and physical properties, making them suitable to be charge transport materials for optoelectronic applications. However, this type of materials has so far not been well studied in solar cells. Here, we develop a new hole transport material (HTM), namely S5, based on perylene building block terms, for organic-inorganic hybrid perovskite solar cells (PSCs). We have systematically studied the influences of the film thickness of S5 on their photovoltaic performance, and a low concentration of S5 with a thinner HTM film is favorable for obtaining higher solar cell efficiency. S5 shows excellent energy alignment with perovskite as well as high-quality thin film formation, and the PSCs based on S5 as HTMs show remarkable power conversion efficiency (PCE) of 14.90% with a much higher short-circuit photocurrent than that for conventional HTM spiro-OMeTAD (PCE = 13.01%). We conclude that the superior photocurrent for S5 is mainly attributed to the enhanced interfacial hole transfer kinetics as well as the high hole conductivity. In addition, we have investigated the stability of N-annulated perylene derivative as HTMs in PSCs devices, showing that the unencapsulated devices based on S5 demonstrate outstanding stability by remaining 85% of initial PCEs in ambient condition with a relative humidity of ~30–45% for 500 h, while for devices with spiro-OMeTAD the cell efficiency degrade to 57% of initial performance at the same conditions.
Precisely controlling the doping profile of organic semiconductors is critical for optimizing the Fermi levels (EFs) of the materials and thus models their thermoelectric (TE) performance. Herein, four new 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT)-cored small-molecular semiconductors with gradient changes of lowest unoccupied molecular orbital (LUMO) levels and synchronously decreased energy gaps are elaborately designed to adapt to single-walled carbon nanotube (SWNT)-incorporating TE composites. The doping levels can be manipulated by either two different n-doping mechanisms or the variation of IDT hosts. A mild doping process with a weak n-dopant results in synchronous improvements in the Seebeck coefficient and conductivity (σ), featuring a high power factor (PF) of 212.8 μW m⁻¹ K⁻² (from IDTT-CN/SWNT = 1:1). On the other hand, the hydride-transfer-induced n-doping mechanism achieved by using a powerful n-dopant leads to significantly different n-doping behaviors among these IDTs, and a remarkably high σ of over 2500 S cm⁻¹ is achieved by Q-IDT-CN/SWNT (1:1).
Three bodipy-based (BDP = 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) donor-acceptor dyads were designed and synthesized, and their ground-state and photophysical properties were systematically characterized. The electronic coupling between the BDP chromophore and an electron-donating carbazole (Carb) moiety was tuned by attachment via the meso and the beta positions on the BDP core, and through the use of various chemical linkers (phenyl and alkynyl) to afford mesoBDP-Carb, mesoBDP-phen-Carb, and betaBDP-alk-Carb. meso-Substituted dyads were found to retain ground-state absorption features of the unsubstituted BDP. However, variation of the linkage between the donor and acceptor moieties modulated the photophysical behavior of excited-state deactivation by controlling the rate of photoinduced internal charge transfer (ICT). The beta-substituted dyad dramatically tuned (red shifted) the absorption spectrum, while retaining desired features of the BDP, specifically stability and high extinction coefficients, however the ICT kinetics were accelerated compared to the meso-substituted dyads. Density functional theory (DFT) and time-dependent DFT (TDDFT) were carried out on the six potential dyads formed between BDP and Carb (attachment using the beta and meso positions for all three connections: direct, phenyl and alkynyl) to support the experimental observations. DFT and TDDFT showed molecular orbital density spread across the HOMO level only when attachment occurred through the beta position of BDP. In the meso-substituted BDP-Carb dyads, the molecular orbitals resembled those of the unsubstituted BDP. This work reveals several possible synthetic paradigms to tune photophysical properties with directed synthetic modifications and provides a mechanistic understanding of the ground- and excited- state behavior in these small-molecule donor-acceptor dyads.