Flexible Thermoelectric Devices Research Articles

Boosting the Thermoelectric Properties of PEDOT:PSS via Low‐Impact Deposition of Tin Oxide Nanoparticles

AbstractPoly(3,4‐ethylenedioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS) exhibits valuable characteristics concerning stability, green‐processing, flexibility, high electrical conductivity, and ease of property modulation, qualifying it as one of the most promising p‐type organic conductors for thermoelectric (TE) applications. While blending with inorganic counterparts is considered a good strategy to further improve polymeric TE properties, only a few attempts succeed so far due to inhomogeneous embedding and the non‐ideal organic‐inorganic contact. Here a new strategy to include nanoparticles (NPs) without any ligand termination inside PEDOT:PSS thin films is proposed. Spark discharge‐generated tin oxide NPs (SnOx‐NPs) are “gently” and homogenously deposited through low‐energy diffusion mode. Strong interaction between naked SnOx‐NPs and PSS chains occurs in the topmost layer, causing a structural reorganization towards an improved PEDOT chains crystalline packing at the bottom, providing a positive contribution to the electrical conductivity. Meanwhile, dedoping and energy filtering effect introduced by the SnOx‐NPs cause dramatic Seebeck coefficient enhancement. The optimized power factor of 116 μWm−1 K−2 achieved is more than six times higher than the value found for the film without NPs. This easy and efficient strategy promises well for future mass production of flexible TE devices and the mechanism revealed may inspire future research on TEs and flexible electronics.

CoSb3-Based Thin-Film Thermoelectric Devices with High Performance Via Electrode Optimization

Flexible thin-film thermoelectric devices have been extensively studied for powering wearable electronics, particularly as a power source for self-powered sensors or temperature detection. Skutteru...

Recent advances in flexible thermoelectrics

Recently, flexible thermoelectric (TE) materials and devices have attracted extensive attention due to their capability to convert heat into electricity directly and their conformal contact with arbitrarily shaped heat sources, demonstrating great promise for application in self-powered portable/wearable low power consuming electronics. Here, we review the state of the art in the development of flexible TEs, including TE modules and materials themselves. The remaining challenges that limit the practical application of flexible TE devices are discussed, and possible solutions and suggestions to guide future development are also provided in this perspective.

Highly Stretchable PU Ionogels with Self-Healing Capability for a Flexible Thermoelectric Generator.

With the development of thermoelectric (TE) generator, the flexible, stretchable, self-healable, and wearable TE devices have aroused great interest. Therefore, we designed a self-healable and stretchable polyurethane (PU) ionogel, composed of polyurethane main chains with double bonds in the side, cross-linkers (BDB) and nonconjugated ionic liquids (EMIM:DCA). The PU ionogels with 30 wt % ILs have a high mechanical stretchability (300%), good tensile strength (1.61 MPa), and suitable Young's modulus (0.79 MPa). The proposed materials also exhibited an excellent ionic figure of merit (ZTi) of 0.99 ± 0.3, as well as rapid self-healability in the absence of any external stimuli. The thermoelectric capability of PU ionogels kept stable under the severe condition (50% strain) and during self-healing process, which is rarely reported in recent studies. Furthermore, a stretchable and self-healable ionic thermoelectric capacitor device is also fabricated by the PU ionogels, which can efficiently convert heat into electricity.

Transformative Electronics: Design Strategy for Transformative Electronic System toward Rapid, Bidirectional Stiffness Tuning using Graphene and Flexible Thermoelectric Device Interfaces (Adv. Mater. 10/2021)

In article number 2007239, Byung Jin Cho, Jae-Woong Jeong, and co-workers present a design strategy for transformative electronics capable of rapid bidirectional stiffness tuning. Transformative electronics are built on a gallium platform interfaced with graphene and a flexible thermoelectric device (f-TED). The graphene works as a catalyst to mitigate the supercooling phenomenon in the liquid-to-solid transition of gallium, and the f-TED offers active temperature control to help accelerate the rigid–soft transformation of the overall system.

Design Strategy for Transformative Electronic System toward Rapid, Bidirectional Stiffness Tuning using Graphene and Flexible Thermoelectric Device Interfaces.

Electronics with tunable shape and stiffness can be applied in broad range of applications because their tunability allows their use in either rigid handheld form or soft wearable form, depending on needs. Previous research has enabled such reconfigurable electronics by integrating a thermally tunable gallium-based platform with flexible/stretchable electronics. However, supercooling phenomenon caused in the freezing process of gallium impedes reliable and rapid bidirectional rigid-soft conversion, limiting the full potential of this type of "transformative" electronics. Here, materials and electronics design strategies are reported to develop a transformative system with a gallium platform capable of fast reversible mechanical switching. In this electronic system, graphene is used as a catalyst to accelerate the heterogeneous nucleation of gallium to mitigate the degree of supercooling. Additionally, a flexible thermoelectric device is integrated as a means to provide active temperature control to further reduce the time for the solid-liquid transition of gallium. Analytical and experimental results establish the fundamentals for the design and optimized operation of transformative electronics for accelerated bidirectional transformation. Proof-of-concept demonstration of a reconfigurable system, which can convert between rigid handheld electronics and a flexible wearable biosensor, demonstrates the potential of this design approach for highly versatile electronics that can support multiple applications.

Design of flexible inorganic thermoelectric devices for decrease of heat loss

Thermoelectric (TE) devices can realize the conversion of heat energy and electrical power based on Seebeck effect and Peltier effect. Among them, flexible TE devices have received more attention recently due to their better attachment to various heat sources and aimed components with arbitrary shapes. To improve the performance of flexible TE devices for various application scenarios, large efforts have been made to design the leg patterns, the electrical and thermal contact issues, and the substrate and encapsulation materials for the decrease of heat loss. This paper is to review the advancements about the design of flexible inorganic TE devices over the last decade. Firstly, the design of flexible thin-film TE devices based on the direction of temperature gradient, including the patterns of TE legs, the fabrication methods, and the flexible substrate materials is summarized. Secondly, the design of wearable TE devices that contains common architecture of the module, the substrates and encapsulations, the electrical and thermal contact, and some thin-film based wearable devices with curving TE legs is demonstrated. Thirdly, the characterizations of the flexibility of TE devices and the current applications are outlined. Moreover, some views about the future development for TE devices are proposed.

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

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

Printed flexible thermoelectric materials and devices

Combined with printing technologies, thermoelectric materials receive considerable attention owing to their superiority in flexibility and mass production. this review focuses on printed flexible thermoelectric devices and its applications.

Progress of hybrid nanocomposite materials for thermoelectric applications

Hybrid nanocomposite materials are widely being investigated due to their superior thermoelectric and mechanical properties. Due to their eco-friendly behaviour, and low cost processing, these can be utilized in flexible thermoelectric devices.

Flexible Diodes/Transistors Based on Tunable p-n-Type Semiconductivity in Graphene/Mn-Co-Ni-O Nanocomposites.

We report a novel Mn-Co-Ni-O (MCN) nanocomposite in which the p-type semiconductivity of Mn-Co-Ni-O can be manipulated by addition of graphene. With an increase of graphene content, the semiconductivity of the nanocomposite can be tuned from p-type through electrically neutral to n-type. The very low effective mass of electrons in graphene facilitates electron tunneling into the MCN, neutralizing holes in the MCN nanoparticles. XPS analysis shows that the multivalent manganese ions in the MCN nanoparticles are chemically reduced by the graphene electrons to lower-valent states. Unlike traditional semiconductor devices, electrons are excited from the filled graphite band into the empty band at the Dirac points from where they move freely in the graphene and tunnel into the MCN. The new composite film demonstrates inherent flexibility, high mobility, short carrier lifetime, and high carrier concentration. This work is useful not only in manufacturing flexible transistors, FETs, and thermosensitive and thermoelectric devices with unique properties but also in providing a new method for future development of 2D-based semiconductors.

Vertically Aligned Se Nanowires-PEDOT:PSS Hybrid Composites on Flexible PDMS Substrate and Their Thermoelectric Properties

The best thermoelectric materials should have low thermal conductivity (κ) and high Seebeck coefficient (S) and electrical conductivity (σ). However, the three parameters, which constitute ZT (ZT = S2σT/κ), are interdependent on each other. Advances in high-ZT thermoelectric materials have been mostly achieved by optimizing material composition and modifying the nanostructure of inorganic materials. However, the inorganic thermoelectric materials are generally less abundant, expensive, and toxic. Recently, researches have been focused on polymer-based thermoelectric materials in order to overcome these limitations of inorganic materials. In particular, conducting polymers have attracted great attention as promising thermoelectric materials because of their unique advantages such as low thermal conductivity, flexibility, low cost, and low toxicity. However, the typical ZT value of the conducting polymer-based materials are on the order of 10−2 ~ 10−3 and are much lower than those of inorganic materials. It is essential to enhance the Seebeck coefficient without decreasing the electrical conductivity. The poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising materials. Water soluble PEDOT:PSS is already widely used for thermoelectrics as a base material for composition between inorganic and organic materials. It can be also modified by a secondary doping according to adding high boiling point cosolvents to increase its conductivity more than 1000 S/cm. Among the cosolvents, ethylene glycol (EG), which is miscible with water and has a boiling point of 189 °C, is one of the most commonly used cosolvents for this purpose.Interestingly, the one-dimensionally aligned inorganic structures can also allow for more effective thermal and electrical energy transfer as forming the hybrid microstructures with the conducting PEDOT:PSS, because the number of inorganic-organic interface grain boundaries that cause energy loss is lower than that in the case of randomly dispersed structures. Moreover, the aligned structures transfer the produced energy to the neighboring structures more effectively. These characteristics of the aligned structures can prevent the cancellation of charge carrier movements among the core materials and can produce the relative enhancement of the energy generation.In this study, we have demonstrated the developed thermoelectric properties of ethylene glycol (EG)-doped poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS)/Se nanowires hybrid composites film on the flexible PDMS substrate (PEDOT:PSS-Se/PDMS). The amount of EG added to the PEDOT:PSS solutions was optimized by measuring the σ , using a conventional four-probe method. As the EG concentration increased from 0 to 10%, σ increased to ~ 1000 S/cm. We have demonstrated in tuning the barrier energy difference of between PEDOT:PSS and Se via the EG doping. The S can be maximized up to 121.6 μV/K through optimization of PEDOT:PSS-Se/PDMS barrier energy according to so-called energy filtering effect. The power factor of the fabricated EG 8%-doped PEDOT:PSS-Se/PDMS composite film was 1.59 mW/mK2 at 30 °C. The electrical conductivity can be also enhanced simultaneously owing to the EG-induced π - π stacking among PEDOT chains. This work provides a smart approach to design and modulate the thermoelectric properties of conducting polymer/inorganic nanostructure composites. This study also shows that flexible thermoelectric devices based on cheap conducting polymers have great potential in wearable electronics. Figure 1

Variable Rigidity Module with a Flexible Thermoelectric Device for Bidirectional Temperature Control.

Dynamic stiffness tuning is a promising approach for shape reconfigurable systems that must adapt their flexibility in response to changing operational requirements. Among stiffness tuning technologies, phase change materials are particularly promising because they are size scalable and can be powered using portable electronics. However, the long transition time required for phase change is a great limitation for most applications. In this study, we address this by introducing a rapidly responsive variable rigidity module with a low melting point material and flexible thermoelectric device (f-TED). The f-TED can conduct bidirectional temperature control; thereby, both heating and cooling were accomplished in a single device. By performing local cooling, the phase transition time from liquid to solid is reduced by 77%. The module in its rigid state shows 14.7 × higher bending stiffness than in the soft state. The results can contribute to greatly widening the application of phase transition materials for variable rigidity.

Spin Caloritronics in 3D Interconnected Nanowire Networks.

Recently, interconnected nanowire networks have been found suitable as flexible macroscopic spin caloritronic devices. The 3D nanowire networks are fabricated by direct electrodeposition in track-etched polymer templates with crossed nano-channels. This technique allows the fabrication of crossed nanowires consisting of both homogeneous ferromagnetic metals and multilayer stack with successive layers of ferromagnetic and non-magnetic metals, with controlled morphology and material composition. The networks exhibit extremely high, magnetically modulated thermoelectric power factors. Moreover, large spin-dependent Seebeck coefficients were directly extracted from experimental measurements on multilayer nanowire networks. This work provides a simple and cost-effective way to fabricate large-scale flexible and shapeable thermoelectric devices exploiting the spin degree of freedom.

Growth mechanisms and related thermoelectric properties of innovative hybrid networks fabricated by direct deposition of Bi2Se3 and Sb2Te3 on multiwalled carbon nanotubes

Flexible thermoelectric generators are an emerging trend in the field of waste heat conversion, as well as wearable and autonomous devices. However, the energy conversion efficiency of the state-of-the-art flexible thermoelectric devices is too low for their wide application and commercialization. In this work, n- and p-type multiwalled carbon nanotube (MWCNT)-thermoelectric material hybrid networks that may become a promising building block for the fabrication of flexible thermoelectric devices are presented. The hybrid networks were fabricated by direct deposition of thermoelectric material (Bi2Se3, Sb2Te3) on the MWCNT networks using physical vapor deposition technique. Growth mechanisms of Bi2Se3 and Sb2Te3 on MWCNTs were investigated. The Seebeck coefficient and charge transport properties of MWCNT-Bi2Se3 and MWCNT-Sb2Te3 hybrid networks were studied as function of MWCNT wt% in the networks. Variable-range hopping models were applied for the interpretation of conductance mechanisms in the hybrid networks. The Seebeck coefficients of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with low MWCNT wt% were found to be comparable with the Seebeck coefficients of pure inorganic Sb2Te3 and Bi2Se3 thin films. At the same time, flexibility tests of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with MWCNT 50% showed no significant increase in the resistance when they were bent to a radius of 5 mm. This makes hybrid networks, presented in this work, the perspective for applications in flexible thermoelectrics as thermoelectric coatings for flexible substrates and fillers for polymers-based composites.

High-Performance n-Type Carbon Nanotubes Doped by Oxidation of Neighboring Sb2Te3 for a Flexible Thermoelectric Generator.

Flexible thermoelectric devices can be potentially used for flexible cooling and energy harvesting from various heat sources such as the human body. However, the development of flexible thermoelectric materials with excellent thermoelectric performance is still very challenging. In this study, a simple solution process is proposed for the preparation of flexible inorganic/carbon nanotube hybrid films with record power factors among those of the reported flexible n-type thermoelectric materials. The hybrid films fabricated by bar-coating a carbon nanotube-dispersed Sb2Te3 solution exhibit n-type power factors of up to 2440 ± 267 μV m-1 K-2 at room temperature. The dissolved Sb2Te3 recrystallizes on the carbon nanotube surfaces and form hybrid solids. The ultrahigh power factor may be originated from the effective n-doping of carbon nanotubes by the oxidation of neighboring Sb2Te3. Using the thermoelectric hybrid film, a multilayer stacked thermoelectric generator is fabricated. The flexible device with a thermal contact area of 3.8 cm2 exhibits an output power of up to 11.3 μW at a vertical ΔT of 7.5 K. This study paves the way for the realization of flexible thermoelectric devices with various device geometries.

Acido-treatment of PEDOT:PSS/Carbon Dots (CDots) nano-composite films for high thermoelectric power factor performance and generator

Thermoelectric materials are considered a main key for sustainable development because they have the ability to convert a low-quality heat into electrical energy. Among them, polymer nano-composite materials have captured attention owing to their unique features such as resilience, low or no toxicity, and abundance. In this work, the thermoelectric parameters of carbon dots/poly(3,4-ethylene-dioxythiophene): poly (styrenesulphonate) (CDots/PEDOT:PSS) nano-composite films have been significantly enhanced by using Sulphuric (H2SO4) acids treatment approach. The electrical conductivity and power factor have remarkably increased, while Seebeck coefficient has decreased after the acid treatment when compared to the as prepared CDots/PEDOT:PSS. The optimal electrical conductivity of CDots/PEDOT:PSS films is 1753.92 ± 50 S/cm and the power factor is 148.27 ± 20 μW/mK2 for 10 M of H2SO4 acid at room temperature. The improvement in the thermoelectric performance is ascribed to the impact of high carrier mobility and carrier concentration by the H2SO4 treated nano-composite. A flexible thermoelectric device containing 5 legs is engineered and can generate an output voltage of (Voutput = 4.3 mV) at a ΔT = 18 K. Finally, the obtained data confirm that the resultant acid treated CDots/PEDOT:PSS nano-composite represents a great chance for future industrial applications in smart thermoelectric power devices.

Free‐Standing Single‐Walled Carbon Nanotube/SnSe Nanosheet/Poly(3,4‐Ethylenedioxythiophene):Poly(4‐Styrenesulfonate) Nanocomposite Films for Flexible Thermoelectric Power Generators

Flexible single‐walled carbon nanotube (SWCNT)/SnSe nanosheet (NS)/poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) nanocomposite films are first successfully prepared by a facile vacuum filtration method on nylon membrane. An optimized power factor of 109.9 μWm−1 K−2 for the as‐prepared free‐standing nanocomposite film with 60 wt% SWCNTs is obtained at the temperature of 380 K, and the electrical conductivity only decreases by 1% after 1000 bending cycles (the bending radius is ≈4.3 mm), indicating excellent flexibility. A flexible thermoelectric generator consisting of 8‐unit free‐standing composite film strips (25 mm × 4 mm for each strip) is assembled on a polyimide substrate, which can generate an open‐circuit voltage of 20.6 mV and a maximum output power of 1.16 μW at a temperature gradient of 74.4 K. This work offers quite an effective strategy for preparation of high‐performance flexible free‐standing thermoelectric composite films, and the as‐prepared films can avoid excess heat loss induced by the substrate‐adherent layer of thin films, when compared with the traditional thermoelectric composite films prepared on different kinds of substrates, e.g., glass substrate. Therefore, the composite films have a great potential to be used for the flexible thermoelectric devices.

Thermal display glove for interacting with virtual reality

Thermal perception is essential for the survival and daily activities of people. Thus, it is desirable to realize thermal feedback stimulation for improving the sense of realism in virtual reality (VR) for users. For thermal stimulus, conventional systems utilize liquid circulation with bulky external sources or thermoelectric devices (TEDs) on rigid structures. However, these systems are difficult to apply to compact wearable gear used for complex hand motions to interact with VR. Furthermore, generating a rapid temperature difference, especially cooling, in response to a thermal stimulus in real-time is challenging for the conventional systems. To overcome this challenge and enhance wearability, we developed an untethered real-time thermal display glove. This glove comprised piezoelectric sensors enabling hand motion sensing and flexible TEDs for bidirectional thermal stimulus on skin. The customized flexible TEDs can decrease the temperature by 10 °C at room temperature in less than 0.5 s. Moreover, they have sufficiently high durability to withstand over 5,000 bends and high flexibility under a bending radius of 20 mm. In a user test with 20 subjects, the correlation between thermal perception and the displayed object’s color was verified, and a survey result showed that the thermal display glove provided realistic and immersive experiences to users when interacting with VR.

Air-Stable Organic Radicals: New-Generation Materials for Flexible Electronics?

In the last few years, air-stable organic radicals and radical polymers have attracted tremendous attention due to their outstanding performance in flexible electronic devices, including transistors, batteries, light-emitting diodes, thermoelectric and photothermal conversion devices, and among many others. The main issue of radicals from laboratory studies to real-world applications is that the number of known air-stable radicals is very limited, and the radicals that have been used as materials are even less. Here, the known and newly developed air-stable organic radicals are summarized, generalizing the way of observing air-stable radicals. The special electric and photophysical properties of organic radicals and radical polymers are interpreted, which give radicals a wide scope for various of potential applications. Finally, the exciting applications of radicals that have been achieved in flexible electronic devices are summarized. The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.