Symposium EN13 : Flexible and Miniaturized Thermoelectric Devices Based on Organic Semiconductors and Hybrid Materials

Recent advances in ubiquitous low-power electronics call for the development of light-weight and flexible energy sources. The textile format is highly attractive for unobtrusive energy harvesting. In my talk, I will present some of our recent work on thermoelectric textiles. I will discuss how n- and p-type yarns can be prepared by coating or dyeing of natural materials such as silk, as well as by melt or wet spinning of conjugated polymers. Some of these fibers and yarns feature a high degree of ambient stability as well as the ability to withstand both machine washing and dry cleaning. Embroidery of base fabrics with such conducting yarns readily permits the fabrication of thermoelectric modules.

Carbon nanotube (CNT) yarns have attracted considerable attention as they are not only flexible, robust and lightweight but also can generate high Seebeck coefficient; S and high electrical conductivity; σ, they realize remarkably high thermoelectric generators (TEGs). One promising application is a small wearable electronics which can generate electricity from heat gradients between the human body and atmosphere in order to harvest waste heat. One of the critical challenges is the tuning of electrical and thermal transport since they interrelate against the high performance of thermoelectric properties. Although the thermal and electrical properties of CNT yarns are likely to enhance their properties by the post-high temperature annealing process, the phenomena underlying the improvement of properties have not explicated yet. The interface between the components governs the properties of the TEGs made by CNT yarns, but the debates have not expanded to characterization and modification of their complicated interface. Then the thermoelectric properties were characterized for the DWCNT yarns without treatment, with Joule heating, with n-type polyethylenimine (PEI) doped, and with PEI doped after Joule heating.
High quality dense and tall vertically aligned CNT forest on Si was grown rapidly by a water vapor free thermal chemical vapor deposition using Fe catalyst, and it consists of more than 80% of double-walled CNTs (DWCNTs). DWCNT yarns were fabricated from CNT forest by a dry spinning process. We doped PEI to realize n-type DWCNT yarn. Electrical Joule heating was done on the DWCNT yarns with and without PEI dope in the vacuum at the temperature of 1800–2000 K for 1 min.
Pristine DWCNT yarn is a p-type semiconductor which exhibits a positive Seebeck coefficient of +25 μV/K. The Joule heating process did not change the polarity of the DWCNT yarn; in contrast, the Seebeck coefficient was enhanced to be +100 μV/K. In case of n-typed DWCNT yarn, after Joule heating, PEI was effectively doped in the DWCNT yarn, and the Seebeck coefficient was enhanced to be –100 μV/K (quite high power factor PE=1000 μW/mK²). To the best of our knowledge, these positive and negative of 100 μV/K are the highest Seebeck coefficients ever observed in CNT yarn and are close to those of inorganic thermoelectric materials. Raman characterization of the joule-heated CNT yarn, we observed the G* peak at a wavenumber of 2400 cm^–1, originated from the double resonance process in graphene or graphite, indicating the presence of graphene or graphite. The computation analysis reviled that the residual amorphous carbon in the interface between CNTs changes into the graphene-like structure. Our results indicate that the improvement of TE properties is due to graphene layers.
Our results provide a guiding principle for designing DWCNT yarns and applying them for thermoelectric devices.

The structural and design versatility of Metal-Organic Frameworks (MOFs) is stimulating considerable activity to develop them for a wide range of applications for which nanoporous materials are extensively used, such as chemical seperations and catalysis. However, MOFs also represent a unique opportunity to solve a vexing problem faced in the field of thermoelectrics; namely, the difficulty of uncoupling thermal and electrical transport so that high Seebeck coefficient and power factor can be achieved. In principle, the extremely low density of MOFs, many of which are comprised primarily of empty space, and the poor phonon coupling across the discontinuous covalent vs. coordination bonding in their structures, should inhibit thermal transport. Recent research suggests that electrically conducting MOFs are feasible, although so far the conductivities achieved are very low compared with the best thermoelectric materials. In most cases, electronic coupling in MOFs is weak due to the ionic nature of the bonding, leading to low band dispersion and insulating behavior. Nevertheless, the number and variety of electrically conducting Metal-Organic Frameworks (MOFs) continues to grow and both 2- and 3-dimensional MOFs with conductivities as high as 160 S/cm are known.
This presentation will discuss mechanisms of electronic charge conduction in MOFs and the related class of materials known as Covalent Organic Frameworks (COFs). We will describe theoretical investigations we performed to probe thermoelectronic transport properties and compare these predictions with charge transport and spectroscopic data to gain insight into the intrinsic conductivity mechanisms and their dependence on MOF structure, chemical composition, and morphology. Our studies provide guidance needed to design MOFs suitable for thermoelectric and other design applications. In addition, through a combination of experiments with MOFs infiltrated with guest molecules under highly controlled conditions, ab initio simulations, and band transport theory, we assessed the influence of structural and interface defects on the electronic structure of 2D and 3D MOFs. The results show that interface defects can introduce a transport barrier in 2D MOFs by breaking the π-conjugation, and/or by decreasing the dispersion of the electronic bands near the Fermi level. Both defect types cause a small band gap to form (in the range of 15-200 meV), which is consistent with the experimentally inferred hopping barrier. In 3D MOFs, the guest molecule, which can be considered a defect in much the same way that dopants are in conventional semiconductors can enhance electrical conductivity but also lead to phase transformations that change the majority charge carrier. These results provide strong evidence supporting the ongoing speculation that the charge transport properties of MOFs are strongly influenced by defects, in much the same way as conventional inorganic and organic conducting materials.

We report thermal conductivity measurements of methylammonium lead iodide (MAPbI₃) based Ruddlesden-Popper Perovskites which exhibit signatures of coherent phonon transport. The hybrid organic-inorganic halide perovskite, MAPbI₃, has exhibited exceptional power conversion efficiency when used in solar cell devices. However, this material is unstable in oxygen and moisture-rich environments, particularly at high temperatures which are practically unavoidable given its ultra-low thermal conductivity. A two-dimensional, layered Ruddlesden-Popper phase similar to MAPbI₃, (BA)₂(MA)_n-1Pb_nI_3n+1 (where n denotes the number of perovskite layers per two organic layers), has shown improved stability compared to its three-dimensional counterpart due to the inclusion of hydrophobic spacer cations, n-butylamine.

In order to learn about heat dissipation and phonon properties in the Ruddlesden-Popper phase, we measured the thermal conductivity of (BA)₂(MA)_n-1Pb_nI_3n+1 for n = 1-6 using a non-contact pump-probe technique called frequency-domain thermoreflectance. We find that the thermal conductivity initially decreases at a function of n for n = 1-3 then levels off for n = 4-6. Decreasing thermal conductivity with increased interface spacing is a signature of coherent phonon transport, and opposite to our expectation for diffuse phonon transport. Our results agree with the Simkin-Mahan model which was formulated to describe the wave nature of phonons in superlattices [1].

While Ruddlesden-Popper organic perovskites can be synthesized by solution processes, coherent phonon transport has previously only been observed in superlattices of GaAs and AlAs [2], and SrTiO₃/CaTiO₃ and SrTiO₃/BaTiO₃ [3] grown painstakingly by MBE processes. Our work suggests that low n (BA)₂(MA)_n-1Pb_nI_3n+1 are both more chemically stable and thermally conductive than MAPbI₃. These properties may make the Ruddlesden-Popper phases more desirable for photovoltaics, while the demonstration of phonon coherence may open a pathway to additional applications in phonon optics and thermoelectricity.

References:

Simkin, M. V., & Mahan, G. D. (2000). Minimum thermal conductivity of superlattices. Physical Review Letters, 84(5), 927.
Luckyanova, M. N., Garg, J., Esfarjani, K., Jandl, A., Bulsara, M. T., Schmidt, A. J., ... & Fitzgerald, E. A. (2012). Coherent phonon heat conduction in superlattices. Science, 338(6109), 936-939.
Ravichandran, J., Yadav, A. K., Cheaito, R., Rossen, P. B., Soukiassian, A., Suresha, S. J., ... & Lichtenberger, A. W. (2014). Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices. Nature materials, 13(2), 168.

Voltage generation in a conductor can be resulted from electron or/and ion transport due to a temperature difference. When a thermoelectric voltage is induced by electron, it is typically much smaller than that of ion transport, particularly for organic thermoelectric materials. Here this study shows how to enlarge the small thermopower of carbon nanotubes (CNTs) using a polymer by controlling electronic transport at the junctions of CNTs with an organic electrochemical transistor configuration where poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) channel was disposed between two separated CNT films. The PEDOT:PSS channel could create energy barriers for abating the contribution of metallic CNT (m-CNT) to thermopower as well as injecting holes to CNT films so that charge imbalance upon imposing temperature gradients can be enlarged. The thermopower was raised up to ~150 mV/K and a remarkably high PF was obtained up to ~1.3×10³ μW/m-K², which is ~460% improvement compared with that of pristine CNT. This study provides not only better understanding of thermoelectric behaviors for organic thermoelectric materials, but also a practical method for suppressing the electronic transport from m-CNT, which would be widely applicable for other organic materials for thermoelectrics and beyond. On the other hand, with the extremely large voltage based on thermally induced ion transport, novel simultaneously harvesting and storing electrical devices using graphene oxides and polystryrene sulfonic acid have been developed without losing the benefit of solid-state non-moving part devices like conventional thermoelectrics. We called the device thermally chargeable supercapacitor (TCSC), and arrays of TCSC have been batch-fabricated, and connected together to raise the output voltage up to 2.1 V, suggesting excellent suitability for roll-to-roll mass manufacturing and practical implementation of thermal energy harvesting. The outcomes suggest that it is feasible to supply power to various distributed electronic systems whenever and wherever a temperature gradient is present.

Low-grade thermal energy (<100 °C) is abundantly available in the form of waste heat or in the environment. Current technologies using liquid-based thermo-electrochemical cells (TECs) is both cost-effective and scalable for low-grade heat harvesting, and their temperature coefficient (mV/K) is one order of magnitude higher than that of solid-state thermoelectrics. The research on TECs has mainly focused on the exploit of thermal gradient or thermal cycle, but the potential of these approaches has been limited by the poor energy conversion efficiency or the need of external electricity. We invent a new asymmetric thermoelectrochemical cell (a-TEC) for low-grade-heat-to-electricity conversion under an isothermal condition without the aid of the thermal gradient across two electrodes or the thermal cycle. The a-TEC consists of graphene oxide (GO)/platinum nanoparticles (PtNPs) cathode and polyaniline (PANI) anode and an aqueous Fe²⁺/Fe³⁺ electrolyte, which can be thermally charged in the open circuit condition. Under isothermal operation, the pouch cell configuration of a-TEC with a short distance between two electrodes can be employed for improving electrolyte conductance and rapid heating. Notably, the thermal voltage is generated based on thermo-pseudocapacitive reaction at the GO-electrolyte interface, demonstrating a very high temperature coefficient of 5.0 mV/K and the a-TEC exhibits the energy conversion efficiency of 5.19% at 70 ^oC (39.6% of Carnot efficiency). The great applicability of this new thermo-electrochemical system has been demostrated on supplying power for an electrochromic smart window by immerimg a-TECs in a hot water and lightening up an organic light emitting diode by placing a-TECs on a running compressor.

As conducting polymers are intrinsically semi-metallic, the charge carriers inducing a thermal voltage can be tuned from electronic (hole) to ionic by doping. In particular, controlling the ionic doping level is crucial for maximizing the power factor of thermoelectric materials, as it optimizes the free ion concentration and carrier mobility. Due to the contribution from both electronic and ionic carriers, the thermopower in a mixed ionic electronic conduction (MIEC) system is synergetic and can produce a stable thermal capacity under constant temperature gradient. A robust thermoelectric harvester was explored from an ion doped mixed ionic conductive polymer film to give a high a Seebeck coefficient of over 16 mV K−1 and a thermal voltage of 80 mV for a temperature gradient of 5 K. The thermal charging on the MIEC film device afforded high thermal voltage and current output, reproducibly, to show cumulative thermoelectric nature. Environmentally sustainable thermoelectric harvesting will be demonstrated by introducing a self-humidifying bilayer system.

Three-dimensional scaffolds comprising different ratio of graphene nanosheets and multi-walled carbon nanotubes (GA-MWCNT) macro-assemblies. The resulting hydrothermally synthesized hydrogels are freeze-dried and thermally reduced to yield graphene and graphene-carbon nanotube aerogels with ultralow densities and tunable mesoscopic pore sizes. These ‘all carbon’ aerogels prepared as monolithic solids from suspensions of few-layer graphene oxide nanosheets and small diameter multiwalled carbon nanotubes in which organic wet chemistry is used to cross-link the individual sheets and with carbon nanotubes. In contrast to methods that utilize physical cross-links between graphene oxide nanosheets, this approach with polymeric linkers and organic functionalization provides covalent carbon bonding among the graphene sheets and molecular attachment with carbon nanotubes, respectively, thus facilitating rapid and facile electron transport. As a result they are expected to exhibit improved electrical conductivities, moderate thermal conductivity, highly interconnected multiplexed topology with large internal surface areas thus promoting enhanced surface ion adsorption which makes these mesoporous materials viable candidates for use in harvesting thermo-electrochemical energy and energy storage technologies. Thermoelectric property measurements for both the carbon nanotube aerogels by themselves and for hybrids with graphene nanosheets revealed promising and unprecedented (p-type and n-type) thermopower values especially with electrolyte with an upper bound to 3.2 mV/K. We used complementary analytical techniques including electron microscopy, temperature dependent electrical property, and Raman spectroscopy while evaluating performance to establish microstructure-processing-property-performance correlations. Supported in parts by KSEF-RDE Grant, KY NASA EPSCoR and KY NSF EPSCoR Grants.

This talk will discuss our recent work to simulate electron and phonon thermoelectric transport in complex materials and across interfaces based on the density-functional theory and atomic Green’s function. One example of the complex material is Mg₂Sb₃, which has been recently reported to have high figure of merit. We compare the charge defect scattering due to different dopants using first principle calculation and find weak impact of defect on mobility. Instead, the intrinsic carrier-phonon scattering is the major scattering mechanism. Only specific polar optical phonon modes make dominant contribution to the scattering rates, which we attribute to the atomic structure and electrostatic interactions in Mg₂Sb₃. For transport across interfaces, an important question to ask is if electron and phonon scatterings conserve lateral momentum, i.e., if they experience specular or diffuse scattering, in the presence of interfacial atomic mixing. We employ atomic Green’s function to simulate electron and phonon transmission across interfaces for SiGe interface with mode-by-mode resolution. We find the angular dependent interfacial transport for electron and phonon are behaving differently, which provides new opportunities for optimizing thermoelectric performance. This work is supported by DARPA MATRIX program (Grant No. HR0011-16-2-0041).

Thermoelectric materials (TEs) could play an important role in future energy management
through environmentally sound cooling and power generation, e.g., converting waste heat into
electricity. Efficient TEs inhibit the propagation of heat (low thermal conductivity, κ) but
conduct electricity well (high power factor, PF). Although κ in a given material can be reduced
via alloying and nanostructuring, identifying materials with intrinsically low κ is still needed.
It has already been known that soft phonon modes due to weakly bonded atoms and s² lone-
pair electron are common to materials with low-κ. Here, we predict a series of new materials
which are weakly bonded systems with same constituent elements but different stoichiometry
either with s² lone-pair electron or high mass density. Due to giant phonon anharmonicity and
low phonon group velocities, they offer extremely low κ (0.3-0.6 W/mK) at 300K approaching
to those found in the amorphous/disordered regime. In addition to low-κ, high Seebeck
coefficients and electrical conductivities in these materials may provide a new opportunity for
designing high-efficiency thermoelectrics at room temperature.

For the next revolution in thermoelectrics, development of new physics as well as new materials needs to go hand-in-hand. In this talk, I will introduce new perspectives such as correlated electron physics, wave effects in phonons as well as opportunities for new inorganic-organic (hybrid) materials¹. Then, I will introduce specifically the case of CuTe:PEDOT thin films and using this as a test case, describe the design principles for creating the next generation of hybrid materials². In addition, I will describe how data-driven approaches can augment our knowledge: used in conjunction with the right material and transport descriptors, these can prove prescient in predicting new TE materials. I will end with a specific example of how we’ve used a Crystal Graph Convolutional Neural Network (CG-CNN) with training data from Materials Project to learn about TE properties. In addition, we use high-throughput, high-fidelity DFT calculations with the Electron Phonon Averaging (EPA) approximation to calculate carrier relaxation times, we propose that the transport effective mass remains an effective single descriptor that can guide inorganic TE material screening.

¹ JJ Urban, AK Menon, Z Tian, A Jain, K Hippalgaonkar, Journal of Applied Physics 125, 180902 (2019)
² P Kumar, EW Zaia, E Yildirim, DVM Repaka, SW Yang, JJ Urban, K Hippalgaonkar, Nature Communications 9, 5347 (2019)

With the maturity of sophisticated, high throughput computational tools and the advent of machine learning applications in materials science, identifying the appropriate fundamental level descriptors has never been more relevant and pressing in data-driven materials discovery domain. In this work, we first present the inertial effective mass as an important descriptor in thermoelectric transport. The conclusion was obtained by analyzing data from 1617 compounds of all crystal stuctures mined from the materialsproject.org. Data was analyzed using polycrystalline averaging Seebeck, effective mass, as well as electrical conductivity. In addition, constant relaxation time approximation was used in all data analysis, which is known not to necessarily reflect experimentally measured compounds.
Subsequently, we present a state-of-the-art approach of using electron-phonon averaging (EPA) to obtain contribution of all possible scattering mechanisms to the relaxation time and challenge the initial assumption of constant relaxation time. More interestingly, we sought to evaluate the veracity of insights and conclusions drawn under constant relaxation time approximation from the EPA point of view and indeed confirm that effective mass is an important descriptor. Further insights gained from analysis of scattering times provide us additional insight towards prediction of new thermoelectric compounds.

The two-dimensional layered materials with interlayer van der Waals (vdWs) bonding such as graphene have attracted tremendous interests in scientific community. Recently, layered materials composed of transition metal trichalcogenides with strong in-plane anisotropy, which show unusual properties based on theory, have been successfully synthesized so that their properties can be explored through experiment. For instance, due to theoretically calculated ZT value (3.1) of monolayer titanium trisulfide along y direction at moderate carrier concentration, layered titanium trisulfide nanoribbons with intralayer covalent bonding and interlayer vdWs bonding burst extensive research, which inspires us to investigate the lattice thermal conductivity of the nanoribbon and the size effects on the thermal conductivity because lowering the thermal conductivity can increase the ZT value significantly. In order to better understand the thermal transport in the titanium trisulfide nanoribbon, the theoretical calculation combined with the experiment are adopted. Through solving the phonon Boltzmann transport equation (implemented in ShengBTE package) with the first-principles second- and third-order force constant (implemented in VASP package), the thermal properties of titanium trisulfide are obtained. The calculation results reveal that the thermal conductivity monotonously decreases with the number of titanium trisulfide layers increasing from one to three and displays a strong in-plane anisotropy as the temperature range from 40 to 500 K. For example, the thermal conductivity along y direction is 2.7 times larger than x direction at room temperature. Compared to layered graphite, due to the weaker interlayer coupling strength, the out-of-plane thermal conductivity (around 1.1 W/m-K) at the room temperature is 5 times lower.
Based on the results of theoretical calculation, we synthesized the titanium trisulfide nanoribbons with different thicknesses and measured the thermal conductivity using a suspended micro-thermometry from 20 to 300 K. The experimental results show that the in-plane thermal conductivity of titanium trisulfide nanoribbons decreases monotonously as the thickness increases, which is same as the theoretical calculation. Interestingly, the thermal conductivity keeps decreasing as the thickness is smaller than 272 nm, which is quite different from multi-layer graphene. In detail, the thermal conductivity decreases rapidly as the thickness is below 60 nm and becomes slowly beyond 150 nm. Due to the computational cost, 4 layers or thicker titanium trisulfide cannot be calculated. The giant large layer thickness dependent in-plane thermal conductivity is surprising and confusing. We speculated that this phenomenon may stem from weak interlayer coupling strength and unique atomic structure, which needs further investigation.

Thermoelectric semiconductors directly convert heat into electricity. These solid-state devices have been use reliably in space for over 40 years without maintenance. Temperature gradients and heat flow are omnipresent in natural and human-made settings and offer the opportunity to scavenge energy from the environment recovering waste heat from industry or replacing the need for batteries in remote sensor networks or mobile devices. Particularly attractive is the ability to generate electricity from body heat that could power medical devices or implants, personal wireless networks or other consumer devices. This talk will discuss the design principles for thermoelectric generators using a generalized electrical transport model combined with an effective thermal conductivity approach [1, 2]. Such design principles provide good estimates of the power that could be produced and the size and complexity of the thermoelectric generator that would be required.
In addition these design principles can guide the search and optimization new thermoelectric materials. In organic and polymer semiconductors the optimum doping is identified and the peak zT for a given class of materials can be determined.
Also, a complete system design shows the misconconception that power factor rather than zT should be optimized for a TE generator. The power factor misconception also leads to misleading strategies for optimization and discovery of new TE materials.
The materials requirements for flexibility, leading to a flexibility figure-of-merit will be discussed.
[1] L. Baranowski, GJS, Eric S. Toberer “Effective Thermal Conductivity in Thermoelectric Materials” J. Applied Physics113, 204904 (2013)
[2] S. Kang, GJS “Charge Transport Model for Conducting Polymers” Nature Materials 16, 252 (2017)

Nanopatterned holey silicon materials follow the concept of “phonon glass and electron crystal” and possess potential for thermoelectric device applications as it is also compatible with the well-developed Si-based industry. We fabricate a boron-implanted holey Si thin-film device, and study the thermoelectric transport properties. A novel method to measure in-plane thermal conductivity of thin film materials using thermal reflectance imaging is successfully conducted. Subsequently, we fabricate a hybrid F4TCNQ-silicon, which utilizes a non-destructive doping mechanism by charge transfer at the heterointerface. We conduct transport measurements and validate that organic
F4TCNQ molecules effectively p-type dope the Si surface, resulting in power factor enhancement. Outcomes of this study serve as proof of concept in designing 3D hybrid structures with closely packed interfaces towards an efficient thermoelectric device, using organic-inorganic transfer doping mechanism.

Hybrid materials consisting of inorganic nanostructures embedded in conducting polymer matrices have emerged as promising systems for room temperature thermoelectric applications. They are attractive due to their intrinsic low thermal conductivities, the ability to engineer interfaces for energy filtering effects and phonon scattering, and their ability to take advantage of high-throughput and solution processable manufacturing. Most polymer and hybrid materials reported in the literature have been p-type, owing to difficulties in n-type doping of conducting polymers in conjunction with the nature of the applied nanocrystals. This has resulted in a strong drive to develop new n-type materials, since both are necessary for module development. Here we explore our recent developments in the synthesis chalcogenide nanowires encapsulated in poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) that are used as templates for the synthesis of Ag_2-xE (where E=Te, Se) via topotactic chemical transformation processes. This synthetic method allows us to engineer the composition of our hybrids, whereby we are able to directly influence the thermoelectric properties, including the production of both p-type and n-type materials from the same parent material. We detail the structural and morphological development of our materials with changing stoichiometry during aqueous based synthesis via X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) and relate this to their thermoelectric performance. We take the example of one of our n-type Ag₂Te/PEDOT:PSS hybrid materials and further manipulate its performance via post-deposition (de)doping schemes, whereby the PEDOT:PSS is reduced by treatment with Tetrakis(dimethylamino)ethylene (TDAE). This process is followed by UV-Vis spectroscopy and electron paramagnetic resonance spectroscopy (EPR) to detail how this (de)doping process effects the charge carrying species. Control of the competing charge carrier concentration allows further improvement in the n-type hybrid device performance.

Porous modification is general approach to endowing the rigid inorganic thermoelectric (TE) materials with considerable flexibility, however, by which the TE performances are severely sacrificed. Thus, there remains a struggling against the trade-off between the TE properties and flexibility. Herein, we develop a novel strategy to combine the Bi₂Te₃ thick film with the ubiquitous cellulose-fiber (CF) print-paper via unbalanced magnetron sputtering technique (ACS Appl. Mater. Interfaces 10, 1743, 2018). The Bi₂Te₃/CF TE composites with tailorable shapes and dimensions were successfully obtained by our approach, which have reasonable internal resistance as components of TE devices with in-plane configurations. Owing to the hierarchical nano-micro porous microstructures and the excellent fraction resistance of the Bi₂Te₃/CFs constructions, the prepared TE composites with Bi₂Te₃ nominal deposition thickness of tens of micrometers exhibit mechanically reliable flexibility, of which the bending deformation radius could be as small as a few millimeters. Meanwhile, the thermal conductivity was remarkably reduced, due to the phonon-nanopore scattering effects. Enhanced Seebeck coefficients were observed comparing with the dense films and the power factors of ~250 to 400 μW/mK² were obtained for the composites from room temperature (RT) to 473 K, which can be further improved by optimizing the carrier concentrations. As a result, the TE figure of merit, ZT, is as high as ~0.38 at 473K.
Moreover, Fig. 1 depicts the photograph, structure, and working principle diagram of the flexible TE device consisting of 12 pairs of p- and n-type legs integrated on the double sides of CF paper sheet employing a laser beam micro-cutting system, which displays great potential as flexible TE device for thermal energy harvesting. When the light bulb works for about a few minutes, the temperature of the central side rises quickly, the temperature difference between the center and periphery is about 50 K from the infrared image, and the TE device generates an output voltage of 0.144 V. This kind of flexible TE device can realize the collection and recovery of heat energy in daily life to power flexible electronics, such as wearable devices and environmental monitors, which also promotes the development of paper-based and thin-film electronics.

Since the 1960’s, NASA has implemented Radioisotope Thermoelectric Generators (RTGs) to supply energy for many of its satellites and space probes. Similar generators for industrial and automotive waste heat recovery have been proposed and many new thermoelectric generator materials have been investigated. Nonetheless, mechanical integrity for the full operational life of the thermoelectric modules, which can extend to decades, has not been given much consideration in such applications. Among many contributors, clamping forces, vibrational stresses, and thermally-induced mismatch stresses may combine to give stress levels high enough to deform the thermoelectric module by creep, thus diminishing its useful lifetime. To date, few thermoelectric materials have been tested for creep, including Bi₂Te₃, PbTe, Mg_1.96Al_0.04Si_0.97Bi_0.03, and TAGS-85.

In the present talk, we show the case of the compressive creep deformation behavior of two thermoelectric materials; half-Heusler n-type Hf_0.3Zr_0.7NiSn_0.98Sb_0.02 and n-type skutterudite (Yb-CoSb₃) alloys, at 500-705 ^oC. When subjected to uniaxial compressive stresses at 600^oC, the n-type half-Heusler alloy Hf_0.3Zr_0.7NiSn_0.98Sb_0.02 exhibits Newtonian flow, consistent with diffusional creep of its fine-grain (1-7 μm) microstructure achieved via spark-plasma sintering of powders. In addition to its promising thermoelectric performance at high temperatures, this alloy sustains very high compressive stresses at 600^oC (from 21 to 359 MPa, for ~ 23 days) without macroscopic failure. However, the brittle nature of the alloy leads to the formation of numerous cracks at such high stresses, which in turn deteriorate the thermoelectric performance. A more realistic creep stress range (15-46 MPa, for ~ 4 days) preserves the high thermoelectric figure of merit zT. Among thermoelectric materials mechanically creep-tested to date, the ZrNiSn-based Half Heusler alloy has the highest creep resistance. On the other side, the n-type skutterudite (Yb-CoSb₃) alloy showed acceptable creep resistance under protected environment.

Given their high melting temperature, stiffness, and creep resistance, half-Heusler alloys appear uniquely suited for long-term thermoelectric applications where high stresses and temperatures are present.

Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions as well as terrestrial applications where unattended operation in remote locations is required. The discovery of new, more efficient materials, and the development of practical, robust elements and device technologies are the key to improving existing space power system performance and versatility and expanding the use of thermoelectrics into efficient, cost-effective terrestrial applications using medium to high grade heat sources.
We present an overview of NASA-funded collaborative research efforts to identify advanced bulk thermoelectric materials, capable of quadrupling current state-of-practice average ZT values over the available operating temperature range of 1275 K to 475 K, through the exploration of structurally complex compounds allowing for a wide range of chemical tuning and the possibility of forming stable nano- and micro-scale composites. Materials- and device-level experimental performance validation accomplished to date, technical challenges, progress and plans for technology infusion into future thermoelectric power systems are discussed.

Energy harvesting and thermal management are required for applications in the internet-of-things, autarkic sensors, or highly integrated electronic devices. Thermoelectric generators and coolers are promising technologies for localized energy harvesting and thermal management. These devices are curretnly well optimized for near-room temperature operation at the macroscopic scale. However, the high integration density of today’s most significant applications requires an increasing degree of miniaturization. Using finite element calculations, we gained insight on the design guidelines for micro thermoelectric devices with realistic material properties, and with concurring size and geometry constraints [1]. Understanding the interplay between thermal and electrical heat fluxes at the micro scale allowed our group to fabricate micro thermoelectric devices for thermal management, efficient enough that can be integrated in electronic packages [2]. Our relevant findings about the design of micro thermoelectric devices will be presented.

[1] D. A. Lara Ramos et al. Adv. Sustainable Syst. 1800093 (2019)
[2] G. Li et al. Nature Electronics1, 555 (2018).

The detection of temperature and dynamics of working fluids, including water, is important for experimental and industrial applications, such as detection of precise changes in fluid environment, monitoring of drainage or cooling water at factory facilities, micro / nano flow analysis. However, due to the spatial limitations or complex geometry of fluid channel interfaces, it is important to accurately detect multiple parameters that are optimally integrated into the operating platform. Also, many fluid sensors necessarily disrupt the natural flow within the platform, and even though the installation of the sensing device should not affect the intrinsic properties of the target element in fluid applications. In this work, we develop a flexible, attachable dual-output sensor for fluid temperature and transfer dynamics based on structural design of thermoelectric materials (SDTM). The SDTM flexible substrates using PET was developed to detect real-time changes in temperature and peak voltage reflecting fluid dynamics. Simple sputtering deposition of Bi2Te3 through a patterning mask has allowed the fabrication of dual-power sensors which do not affect the fluid flow shape. When the working fluid contacts the surface, thermoelectric structure pattern induces continuous double thermal wave and time interval without disturbing the natural flow of working fluid. The raw voltage signal induced by thermal gradient wave provides the magnitude of the first peak voltage and the duration between the two peaks, which show the real time temperature and the moving velocity of the working fluid. Furthermore, as a demonstration of an expandable platform using SDTM, a scalable sensor array comprising multiple SDTMs was fabricated as a large-area device for sensing a fluid temperature flow dynamic sensing device. Its performance with respect to sensing the fluctuation of working fluid temperature and kinematics was verified using a 4 × 4 SDTM array. As a result, the new methodology using SDTM can contribute to the development of entirely new technologies for next-generation sensors that require advanced features such as multi-element detection and a variety of integrated flexible and removable features.

Bi₂Te₃-based p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se₃ have been the only materials used for thermoelectric cooling for decades. Even though the progress on advancing the thermoelectric figure-of-merit (ZT) has been significant especially the materials with peak ZT at high temperatures, materials with high enough ZT around room temperature are very rare. Up to now, in addition to Bi₂Te₃-based ones, the only reported is p-type MgAgSb with ZT of ~0.8 at room temperature. There is no report on any n-type material exhibiting ZT similar to that of the n-type Bi₂Te_2.7Se₃. In this talk, I will present a new n-type material that has a ZT of ~0.7 at room temperature, which is comparable to that of n-type Bi₂Te_2.7Se₃. The cooling performance of a unicouple consisting of the new n-type material and the p-type Bi_0.5Sb_1.5Te₃ is also in par with the commercial legs consisting of the p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se₃.

Thermoelectric effects play a significant role in phase change memory (PCM) and Ovonic threshold switch (OTS) devices used as access devices in PCM cells [1]. Typical PCM cells are two-terminal nanometer-scale resistive memory devices which can be reversibly switched between a low-resistance crystalline and a high-resistance amorphous state via nanosecond electrical pulses. Amorphization in PCM devices is achieved by self-heating the phase change material, typically a chalcogenide, close to its melting temperature, followed by a sudden quench. Crystallization is achieved by self-heating the phase change material above its glass-transition temperature. OTS devices typically use amorphous chalcogenides that do not crystallize during normal device operation.
The local current densities in PCM and OTS devices can reach 10⁸ A / cm², giving rise to local temperatures in excess of 900 K and thermal gradients as high as 50 K / nm; hence, Peltier effects at material interfaces and Thomson heating within the active area are substantial. Accurate modeling of thermoelectric effects requires knowledge of temperature dependent electrical resistivity and Seebeck coefficients of these materials. These parameters can be measured at low temperatures on as-deposited amorphous films [2]. However, melt-quenched amorphous materials’ parameters tend to differ from as-deposited films, and PCM materials rapidly crystallize at higher temperatures. High-speed metastable electrical resistivity measurements can be performed on nanoscale devices using electrical pulses to uniformly amorphize devices up to approximately 200 nm in diameter [3], but larger devices tend to form current filaments and hence do not amorphize uniformly. On the other hand, measurement of the Seebeck coefficient (S), which is vital to understanding thermoelectric effects, is very difficult at small scales.
In this work, we model the Seebeck coefficient for metastable amorphous Ge₂Sb₂Te₅ (aGST) based on high-speed experimental results [3] and an energy band diagram proposed by Muneer et. al. [4] from 300-850 K [3], [5], and we analyze thermoelectric effects in PCM cells using finite element phase change device simulations [6]–[9]. We calculate the electron and hole Seebeck contributions S_e and S_h in metastable aGST with the band diagram in [4] and find that S_h is similar in both magnitude and slope to S measurements on as-deposited aGST thin films in 300-400 K range [5], [10], consistent with the unipolar conduction assumed when deriving the band gap in [4]. We use S_h as the Seebeck coefficient in metastable aGST and simulate reset and set operations in a PCM double mushroom cell and find that the Seebeck differential between crystalline and amorphous GST results in significant heating/cooling at amorphous-crystalline junctions during both crystallization (set) and melting (reset).
[1] A. Faraclas et al., IEEE Electron Device Lett., 32, 12, pp. 1737–1739, Dec. (2011) DOI: 10.1109/LED.2011.2168374.
[2] L. Adnane et al., J. Appl. Phys., 122, 12, p. 125104, Sep. (2017) DOI: 10.1063/1.4996218.
[3] F. Dirisaglik et al., Nanoscale, 7, 40, pp. 16625–16630, Oct. (2015) DOI: 10.1039/C5NR05512A.
[4] S. Muneer et al., AIP Adv., 8, 6, (2018) DOI: 10.1063/1.5035085.
[5] L. Adnane et al., APS Meet., (2012).
[6] Z. Woods and A. Gokirmak, IEEE Trans. Electron Devices, 64, 11, pp. 4466–4471, Nov. (2017) DOI: 10.1109/TED.2017.2745506.
[7] Z. Woods et al., IEEE Trans. Electron Devices, 64, 11, pp. 4472–4478, Nov. (2017) DOI: 10.1109/TED.2017.2745500.
[8] J. Scoggin et al., Appl. Phys. Lett., 112, 19, p. 193502, (2018) DOI: 10.1063/1.5025331.
[9] J. Scoggin et al., Appl. Phys. Lett., 114, 4, pp. 1–6, Oct. (2019) DOI: 10.1063/1.5067397.
[10] S. A. Baily et al., Solid State Commun., 139, 4, pp. 161–164, (2006) DOI: 10.1016/j.ssc.2006.05.031.

Flexible materials with high thermoelectric performance have attracted growing interest recently. By intercalating organic molecules into the van der Waals gap of varieties of inorganic two dimensional layered compounds, we developed a large family of thermoelectric materials with excellent mechanical flexibility. The inorganic and organic monolayers are alternatively stacked to form an inorganic/organic superlattice, in which the high electronic transport properties of the inorganic component has been maintained and the thermal conductivity was dramatically suppressed by the organic components, finally resulting in boosted ZT value. We have demonstrated this idea in several two dimensional host materials, including TiS₂, Bi₂Se₃ and TaS₂, etc. The abundant choice of the organic molecules also brings new opportunities to optimize the thermoelectric performance, such as the dielectric screening effect and the quantum confinement effect. We finally developed a solution-processed strategy to fabricate large area flexible thermoelectric foil based on this inorganic/organic superlattice, which enables easy integration into energy-harvesting electronic devices.

Recently, our group developed a Bi–Te-based TEG device by using conventional semiconductor packaging technology. In that device, Ni/Au was used as the diffusion barrier, and Sn–Ag-based solder was used as the bonding material. Although Bi–Te-based TE materials can be used up to approximately 300°C, the maximum operating temperature of the device was limited to approximately 150°C owing to the low melting point of the Sn–Ag-based solder (~150°C) that bonds Bi–Te-based TE materials and Cu electrodes. Therefore, in order to apply higher temperature differences to the device, in this study, we attempted to develop a bonding interface resistant to heat up to 250°C. Ni/Au was retained as the diffusion barrier layer, but Ag paste was chosen as the bonding material, instead of the Sn–Ag-based solder, because it has many advantages such as high-temperature stability (melting point ~ 960°C), printability, low electrical resistivity, high thermal conductivity (150 Wm^-1K^-1), and a temperature requirement of less than 300°C for sintering-bonding. The Ni/Au diffusion barrier layer was prepared using two different deposition processes, sputtering and electroplating, and the element diffusion behaviors of these two bonding interfaces at 250°C were investigated.
In the bonded sample with sputtered Ni/Au, the Cu electrode diffused violently into the chip and formed a Cu–Te-rich phase. On the other hand, in the bonded sample with electroplated Ni/Au, the diffusion of Cu was blocked, but a NiTe phase formed on the chip side. The above results demonstrate that, with sputtered Ni/Au, grain-boundary diffusion is predominant and Cu electrodes diffused along the grain boundaries, while with electroplated Ni/Au, self-diffusion is predominant because of the instability of the monocrystalline–amorphous phase. These results suggest that the highly crystalline Ni layer, which has dense grain boundaries and no pores, is a highly effective diffusion barrier.

Conducting polymers are investigated regarding their potential application in hybrid thermoelectric thin films. Key parameters for the conversion efficiency of heat into electricity are addressed and correlated with the morphology and with optical properties. Hybrid thermoelectric films based on PEDOT:PSS are nanostructured with inorganic nanoparticles. It reduces the thermal conductivity of the thin hybrid films as probed with infrared thermography and explained with resonant x-ray scattering. The intrinsically high charge carrier concentration in PEDOT:PSS is systematically reduced with the help of inorganic salts with acido-basic and redox properties in order to increase the Seebeck coefficient of the films. The impact on electronic, compositional and conformational properties is observed with various spectroscopic techniques. Ionic liquids are used for post-treatment of PEDOT:PSS thin films, in order to simultaneously increase Seebeck coefficients and electrical conductivities. The combined effect on electronic and structural properties is proven with the help of spectroscopy and x-ray scattering experiments.

The thermoelectric performance of organic/inorganic hybrid composites is strongly governed by interfacial interactions where the energetic mismatches provide effective charge carrier scattering for high Seebeck coefficient. In addition, the conformation of polymer chain at the interfaces could engineer electronic structure for a favorable hopping mechanism. Herein, we report on the template-assisted fabrication of PEDOT:PSS/Ge₂Sb₂Te₅ (GST) nanowires hybrid composites for improvement in electrical conductivity and Seebeck coefficient. The GST nanowires array fabricated by a solvent-assisted nanotransfer printing technique was participated as a template to align the PEDOT:PSS. Significantly enhanced thermoelectric performance of PEDOT:PSS/GST nanowires hybrid composites compared to their individual counterparts is because of energy filtering effect at the GST nanowire/PEDOT:PSS interfaces.
The alignment effect of PEDOT:PSS chains on the electrical conductivity was elucidated by measurement in parallel and perpendicular to the GST nanowires. The best power factor of 1560 µWm^–1K^–2 was achieved in parallel-direction owing to not only the high Seebeck coefficient but also the confomationally ordered PEDOT:PSS interfacial layer at the GST nanowires template.

Thermoelectric generators (TEGs) transform heat to electricity without any movable parts. These devices will play an important role in energy harvesting for wearables, autonomous sensor nodes, and the Internet-of-Things (IoT). Conjugated polymers as well as printable inorganic nanomaterials offer the unique advantage of being processable on printing machines. They have recently made strong progress in their thermoelectric properties. This opens a pathway for the fabrication of powerful thermoelectric generators with unprecedented low costs for mass applications. We have developed novel printable PEDOT formulations and novel printable inorganic materials for a device layout which allows for a roll-to-roll printing process on ultrathin plastic foils. The TEGs are then subsequently fabricated by an automated folding process which allows to adapt the geometry of the devices such that the desired thermal resistance is matched to the specific thermal boundary condition. Using this approach in combination with designed low power electronics forms the basis for several wireless sensor nodes. The talk will cover our recent developments on printable organic and inorganic nanomaterials.

Printable thermoelectric generators (TEGs) are attractive to researchers because of their potentially higher power density, scalability and lower cost than rigid conventional TEGs. This additive manufacturing of TEGs requires active thermoelectric (TE) particles to be dispersed in a polymeric binder to synthesize printable slurry inks, and printed films to be subsequently subjected to a long and high temperature curing for grain coalescence and enhancing electrical conductivity. A large quantity of polymeric binder in composite TE films results in a sizable loss of the electrical conductivity and TE performance. Moreover, an energy intensive long and high-temperature curing process is another challenge for printable TEGs.
We report a novel approach eliminating the long and high temperature curing requirements for the printable thermoelectric generators but attain enhanced electrical conductivity. This work presents the feasibility of using a small amount of naturally occuring chitosan as a binder for p-type thermoelectric composite films. Less than 10^-3 weight ratio of chitosan was sufficient to hold the thermoelectric particles together in the form of composite films. Samples of various weight ratios such as 1:2000, 1:5000, and 1:7000 between binder and TE materials were prepared and tested. Various particle sizes of the TE materials were also explored to observe their effect on electrical conductivity. These various samples were drop casted and cured at relatively lower temperature (150°C) for shorter time (5 minutes). These cured films were further subjected to uniaxial pressure for densification of the films by reduction of pores, voids and also merge of small particles between big particles to give bulk like structure. Analysis of the obtained samples indicated that a wide range sizes of particles with chitosan binder in composite films under the application of mechanical pressure lead to plate structure which resulted in improved electrical conductivity of the composite films.
The highest power factor achieved for best performing p-type Sb₂Te₃ composite films is 1236±110 µW/mK², and the figure of merit (ZT) is 1.236 at room temperature (300K). The highest ZT obtained for p-type Bi_0.5Sb_1.5Te₃ film is 0.838 at room temperature.

Wearable thermoelectric generators are a promising energy source for powering activity trackers and portable health monitors. However, known iterations of wearable generators have large form factors, contain expensive or toxic materials with low elemental abundance, and quickly reach thermal equilibrium with a human body, meaning that thermoelectric power can only be generated over a short period of wear. Here, we create an all-fabric thermopile by vapor printing persistently p-doped poly(3,4-ethylenedioxythiophene) (PEDOT-Cl) onto commercial cotton and integrate this thermopile into a specially-designed, wearable band that generates thermovoltages >20 mV when worn on the hand. We show that the reactive vapor coating process creates mechanically-rugged fabric thermopiles that yield notably-high thermoelectric power factors at low temperature differentials, as compared to solution-processed counterparts. Further we describe best practices for naturally integrating thermopiles into garments, which allow for significant temperature gradients to be maintained across the thermopile despite continuous wear.

As the demand of wearable devices increases, flexible thermoelectric devices are attracting attention as alternative power sources of them. Because wearable devices are worn on the human body, it is possible to create electricity using thermoelectric devices from body heat without the need for other heat sources. In addition, flexible thermoelectric devices have been studied due to their potential for using arbitrary heat source shapes without an additional heat exchanger.
The biggest issue to make a flexible thermoelectric device is that the thermoelectric device has flexibility so that it can maintain its performance even when bent during use. Unlike conventional thermoelectric devices, materials, fabrication processes, and device design have to be changed in order to impart flexibility to thermoelectric devices. Existing rigid ceramic substrates cannot be used. The use of bulk thermoelectric materials limits the flexibility of thermoelectric devices. The joint between the electrodes and the thermoelectric legs in a thermoelectric device reduces its flexibility. If necessary, the structure of the thermoelectric device may need to be changed.
Flexible thermoelectric devices are subjected to cyclic loading during continuous use. The repetitive load generates cracks inside the thermoelectric device, thereby increasing the internal resistance of the device and eventually causing failure of the thermoelectric device. Therefore, in order to develop and commercialize a flexible thermoelectric device, it is essential to establish a method of evaluating the reliability of the thermoelectric devices under cyclic loading.
In this study, the current state of flexible thermoelectric devices research is reviewed in terms of materials, devices and applications. A method for evaluating the reliability of a flexible thermoelectric device is presented.

Emerging energy generators has expanded the potential of electronics in the way that they can operate wireless-interconnected Internet of Things (IoT) devices without the assistance of additional power sources or batteries. Among promising candidates, thermoelectric generators (TEGs) have offered attractive opportunities for realizing self-powered electronics due to their energy conversion ability from waste heat to electricity. For the realization of high-performance TEGs, the maximized temperature difference across the thermoelectric (TE) layers is the most important factor in given temperature boundaries and TE material properties, therefore, efficient heat collection and heat transferring to TE materials is necessary to achieve high energy conversion efficiency. In this context, the demands for compliant TEGs which can collect thermal energy from arbitrary-shaped heat sources minimizing parasitic heat loss have increased, which is not allowed to conventional rigid TEGs. In this regard, there have been many efforts to integrate high-performance cuboid-shape inorganic Bi₂Te₃ –based TE rods into soft platforms.
As a representative approach to realize rod-based compliant TEGs, the infiltration of soft medium and flexible interconnection have been introduced to provide acceptable mechanical tolerance despite brittle and rigid nature of the inorganic alloys. However, unexpected lower TE performances and imperfect conformal contacts have been reported due to parasitic heat loss in hundreds μm-thick polymer substrates and metal contacts, respectively. Therefore, optimized module designs with the minimization of parasitic heat loss and fully soft interconnections have to be developed toward highly efficient compliant TEGs.
In this presentation, we will report a new strategy to improve heat collection from arbitrary shaped-heat sources via the soft heat collector embedded in ultra-thin polymer supporting layers. The selectively patterned and aligned thermally conductive soft composites are lined up with Bi₂Te₃ bulk legs; thus the significantly improved heat transfer ability can be delivered toward in the out-of-plane direction with the minimization of parasitic thermal energy loss, allowing conformal thermal contacts simultaneously. These results indicate that the heat transfer ability of soft heat collectors is much superior to that of the previously used compliant substrates, such as PDMS, pure and engineered Ecoflex, even comparable with that of thermo-pads. In addition, the embedded soft interconnection and infiltrated PDMS offer the opportunity to realize further compliant TEGs. The excellent adhesion property between the Bi₂Te₃ legs and soft interconnection via the optimized highly conductive epoxy layer also improves the mechanical reliability preventing delamination issues. Finally, we will show, for the first time, facilely customized large-are TEGs with high yields fabricated by fully automatic additive manufacturing.

Organic thermoelectric (TE) devices have attract attention as a low thermal energy-harvesting device. Organic materials can be synthesized and processed by tuning the electronic structures and molecular ordering in a solid state. They can be also fabricated with solution-based deposition techniques, such as inkjet printing, spray printing, and spin-coating. These properties are highly related to their electrical properties which are important for practical implementation of organic TE devices and modules. In this presentation, we will present the advances on improving the electrical properties of various p-type polymers, i.e., >70 S/cm for polypyrroles and >4000 S/cm for poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), by tailoring the molecular ordering and by changing the carrier concentration via acid-based doping. The Seebeck coefficients and power factors were also measured and optimized along with the modification. The film properties were systematically investigated to reveal the origin of the enhancement. Also, we will briefly discuss on the performance of TE modules.

In this work, we report the design and manufacturing of textile shaped thermoelectric generators (T-TEGs). Carbon nanotube yarns were selected as the base materials to creat segmentally-impregnated yarns with p-type materials (PEDOT: PSS) and n-type (PEI ), which was assembled into a spacer fabric by an industrial knitting machine. The effect of n-type PEI doping and fabric structure on the thermoelectric performance of T-TEGs will be carefully investigated. Moreover, the potential application of the T-TEG will be demonstrated.

The mechanical flexibility, low temperature processing, potential printability, capability of blending to form composites, and use of common elements are attractive features for the use of polymers in thermoelectrics. This presentation focuses on polymers blended with dopants and polymer additives to tune the charge carrier density and Fermi energy-transport energy difference while maintaining charge carrier mobility. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecyl thioquaterthiophene) (PQT12 and PQTS12, respectively), were used initially. Cyclic voltammetry and current-voltage measurements indicated that the introduction of sulfurs into the side chains induces traps in films containing PQTS12. Doping the polymer with sulfur in side chains (PQTS12) with the strong oxidant nitrosonium tetrafluroborate (NOBF4), we obtained an especially high conductivity up to 350 S cm^-1. Furthermore, the high conductivity is stable in air without extrinsic ion contributions. The thermoelectric power factor compared favorably with prior reports for p-type polymers that were made by the alternative process of immersion of polymer films into dopant solutions, and fit the established models and newly performed simulations for thermoelectric polymers. Additional data obtained from thiophene copolymers containing the ethylenedioxithiophene and thieno[3,2-b]thiophene subunits supported these conclusions. Blend microstructure, assessed using grazing incidence X-ray scattering, and mobility evaluated in field-effect transistors, was not adversely affected by the blending. This structural preservation is a key aspect of designing additional polymer compositions with high power factor, including n-type polymers. The value of using such polymers as matrices of composites with inorganic thermoelectric particles will be considered.

Thermoelectric (TE) energy conversion is an attractive and environmentally friendly way to recover energy from industrial waste heat or natural heat because of its potential for improving the energy efficiency. As TE materials, organic materials have unique advantages, such as cost effectiveness, low intrinsic thermal conductivity, high flexibility, and amenability to large area applications. With increasing attention on flexible or wearable power-conversion devices, intensive research efforts have been devoted to flexible organic TE modules to replace the brittle inorganic ones.
First, a highly integrated and flexible TE module with a novel device architecture based on a carbon nanotube (CNT) web is proposed. The pristine CNT web shows superior electrical conductivity of 998.3 S cm^-1, owing to the increased longitudinal carrier mobility derived from the highly aligned structure. To realize optimal TE property, the pristine CNT web is alternately doped with p- and n-type carriers using FeCl₃ and benzyl viologen, respectively, via a brush-casting method. Brush-casting is the simple doping process that enables large-scale and continuous fabrication of flexible TE modules by allowing precise doping of the localized area without a shadow mask. Flexible TE modules were then fabricated by repeated brushing and folding of the CNT webs. Owing to the synergic effect of the highly integrated high-performance TE material (highly aligned CNT web) and the facile doping process (brush-casting), flexible TE modules consisting of 120 p-n couples over an area of 8 cm² show a maximum power output of 5.3 μW for a temperature difference of 11.7 K.
Second, a rapid solvent evaporation method based on the triple point of a processing solvent to prepare CNT foam with a porous structure for TE power generators is presented. The rapid solvent evaporation process allows the preparation of CNT foam with various sizes and shapes. Furthermore, the density and porosity of the CNT foam are precisely and easily controlled by manipulating the initial CNT/solvent ratio. The obtained highly porous CNT foam with porosity exceeding 90% exhibits a low thermal conductivity of 0.17 W m⁻¹ K⁻¹ with increased phonon scattering, which is 100 times lower than that of a CNT film with a densely packed network. The aforementioned structural and thermal properties of the CNT foam are advantageous to develop a sufficient temperature gradient between the hot and cold parts to enhance TE output characteristics. To improve the electrical conductivity and Seebeck coefficient further, p- and n-molecular dopants are easily introduced into the CNT foam, and the optimized condition is investigated based on the TE properties. Finally, optimized p- and n-doped CNT foams are used to fabricate a vertical and flexible TE power generator with a combination of series and parallel mixed circuits. The maximum output power and output power per weight of the TE generator reach 1.5 μW and 82 μW g⁻¹, respectively, at a temperature difference of 13.9 K.

High electrical conductivity and high Seebeck coefficient are the two important prerequisites for achieving high power factor in organic thermoelectric (TE) materials. However, these two properties are quite often in conflict. In this work, we demonstrate that incorporating CNT in a conducting polymer PEDOT:PSS could facilitate the formation of stable and effective conductive channels, which provides an effective approach to optimize the TE parameters with simultaneously enhanced electrical conductivity and Seebeck coefficient against the initial organic TE materials. With further tailoring charge concentration of the SWNT/PEDOT:PSS composite by base treatment, the TE performance could be improved. Nanocomposite of 60 wt% SWNT and PEDOT:PSS exhibits high TE power factor of ∼526 μW m⁻¹ K⁻² with Seebeck coefficient of 55.6 μV K⁻¹ and electrical conductivity of 1701 S cm⁻¹, which is by far one of the highest power factors among the reported organic TE nanocomposites. The enhancement in both Seebeck coefficient and electrical conductivity could be attributed to the carrier concentration optimization as CNT conductive networks retains the ability for charge transfer, which is revealed by Raman, XPS, UV-Vis and Hall effect measurements. Considering thermal conductivity around 0.4–0.6 W/m K, the highest estimated ZT value of our TE nanocomposite can approach 0.39, demonstrating the feasibility of this strategy to enhance TE performance of organic composite materials.

As emerging flexible thermoelectric (TE) materials, polymer composites with embedded carbon nanotube (CNT) networks have shown promising properties such as high electrical conductivity through CNT networks, low thermal conductivity by polymer matrix, and high mechanical flexibility provided by both components. However, the thermoelectric transport in CNT networks has not been fully understood to achieve their optimal performance in thermoelectric energy conversion. In this talk, we present a combined experimental and theoretical study on carbon nanotube networks embedded in polydimethylsiloxane (PDMS) elastomers. PDMS offers excellent CNT dispersion within its large thickness (> 1 mm) and volume, along with many other advantages such as low-cost, light-weight, bio-compatibility, and high flexibility, thus making it suited for wearable TE application. Since PDMS is electrically insulating, carrier transport occurs only through the CNT networks, which makes the composites an excellent material system for studying carrier transport in CNT networks with no parasitic transport paths. We synthesize and characterize CNT:PDMS composites over a wide range of CNT content, and analyze the thermoelectric properties using Landauer theory. We find that the simultaneous increase in Seebeck coefficient and electrical conductivity with increasing CNT content up to ~40 % CNTs can be attributed to the tunneling transport at CNT junctions with the junction distance decreasing with increasing CNT content. Beyond 40 % CNTs, both properties are reduced and saturated due to the reduced PDMS content and increased material non-uniformity, which effectively replaced the PDMS gap with an air gap at the junction, thus imposing a higher potential barrier to reduce the properties. We find that a figure of merit zT greater than 10 at room temperature is possible by doping optimization with appropriate junction parameters in semiconducting single-walled CNT networks. Our results and analysis provide important insights into material optimization for hybrid thermoelectric composites based on CNT networks and many other nanoscale fillers.

Research into flexible thermoelectric materials has greatly expanded in recent years as interest in waste heat recovery and wearable electronics has increased. 2D graphene is a potentially high-performance, stable material for thermoelectric applications, but its practicality is somewhat limited by traditional synthesis routes. For graphene produced through oxidative top-down methods, such as Hummer’s Method, numerous defects and oxygenous groups remain in the basal plane of the flakes. These oxygenous groups not only adversely affect electrical conductivity, but also create uncontrollable doping in the graphene flakes because certain oxygenous groups p-dope graphene while others n-dope graphene. This leads to a low overall Seebeck coefficient (typically <20 µV/K) and reduces the effectiveness of any post-synthesis doping strategies. As an alternative, we introduce a synthesis route of non-oxidized graphene flakes (NOGF) through an intercalation method and demonstrate their superior properties as a thermoelectric material. These non-oxidized graphene flakes (NOGF) show an extremely low defect ratio (Id/Ig < 0.1) and minimal oxidative damage, along with a high aspect ratio. By changing the adsorbed surfactant during graphene synthesis, both n and p type graphene can be synthesized. The n-type NOGF showed electrical conductivity, Seebeck coefficient, and power factor of 3300 S/cm, -45 µV/K, and 670 µW/mK², respectively, demonstrating performance that is vastly superior to oxidized graphene flakes in thermoelectric applications. In addition, preliminary results for p-type NOGF revealed values of 650 S/cm, 52 µV/K, and 180 µW/mK², which may improve with future optimization. The films were also stable under bending conditions (<3% resistance change after 1000 bending cycles at a bending radius of 3 mm), and a full thermoelectric device based on n-type and p-type NOGF generated stable power output over a wide temperature range (RT to 300 °C). These results demonstrate the potential of NOGF for highly robust, flexible, and high-performance thermoelectric devices.

The thermoelectric properties of materials are strongly dependent on their carrier concentration. The effects of electrical doping of semiconducting polymers are complex due to the interplay between transport properties and microstructure. We will discuss our recent work in understanding the mechanisms of electrical doping of semiconducting polymers and the impact of processing on electrical properties. The thermopower of doped polymers depends on changes in the electronic density of states (DOS) upon doping. We have used electrochemical transistors to control the carrier concentration in semiconducting polymers to study their thermoelectric properties. In-situ X-ray scattering revealed the changes in microstructure of poly(3-hexylthiophene) during gating. In P3HT, and other polymers, a sharp change in the structure of crystalline domains occurs at high carrier concentration. Our results show that chemical doping leads to a broadening of the DOS making it more difficult to predict electronic properties as a function of carrier concentration. Recent models to explain the connection between electrical conductivity and thermopower in disordered systems will be discussed. A power law form for the electronic density of states of doped semiconducting polymers near the Fermi level provides a reasonable fit to the dependence of transport and thermopower on carrier concentration. The implications of this behavior for improvement of the thermoelectric properties of polymers will be discussed.

Despite their significant impact on electronic applications, charge transport in conducting polymers is still debated due to disorder (paracrystallinity). It has been reported that charge transport in polymers is correlated with paracrystallinity [1]. The shape of the electronic density of states (DOS) determines transport properties in solids. In this work, we establish the missing link to understanding charge transport by connecting paracrystallinity to transport properties of conducting polymers. First, we confirm that the DOS in conducting polymers is represented by a Gaussian distribution by performing tight-binding model calculations supported by density functional theory and Molecular dynamics simulations. We find that the DOS broadening (w) increases exponentially with paracrystallinity. Second, by using Gaussian DOS in the Boltzmann Transport Equation, we find that charge transport can be fully described by the energy dependent scattering parameter (r) and the ratio of the total number of energy states to broadening, N_t/w.

For a wide variety of conducting polymers, we find that r affects all transport properties, while N_t/w affects only electrical conductivity. For example, by fitting literature data of electrical conductivity and thermopower of conducting polymers, we show that PEDOT:Tos (w~0.2 eV) is less disordered compared to PEDOT:PSS (w~0.8 eV). Furthermore, due to their distinct monomer structure, PEDOT:PSS and PEDOT:Tos, for instance, exhibit r=-0.5, while P3HT and PBTTT exhibit r=1.5. Moreover, we show that charge mobility in conducting polymers is predominantly determined by r. For example, our calculated Hall mobility is consistent with the measured values reported for PBTTT and PEDOT:PSS [2], demonstrating different values of r. Finally, we provide a design principle for future organic electronics, where r=1.5 polymers are exciting as they can exhibit higher mobilities and are also well suited for thermoelectrics.

Noriega et al. (Nature Materials, 12, 1038 (2013))
Kang et al. (Nature Materials, 15, 896 (2016))

While highly conductive polymers are now commonplace, they generally demonstrate lower thermopower at a given conductivity than inorganic counterparts. Ion conducting materials have previously been demonstrated to have very large Seebeck coefficients, but ions generally have much lower mobility than electronic charge carriers. We have recently shown that doping with protic ionic liquids and other proton conductors can be used to control the thermoelectric power factor. In this talk, I will discuss how electrochemical transistor geometries can be used to understand the scaling of thermopower with carrier concentration and to begin to untangle the intertwined ion/electron effects in mixed conductor thermoelectrics. Further I will discuss how the presence of ionic charges can be used to control the mesoscopic self-assembly of conducting polymers.

Intrinsically conductive polymers have promising thermoelectric application due to their high mechanical flexiblity, high electrical conductivity while low thermal conductivity. But their Seebeck coefficient is usually lower by inorganic thermoelectric materials by 1-2 orders in magnitude. It is important to significantly improve the Seebeck coefficient of thermoelectric polymers. Here, I will present some methods by our lab to significantly enhance the Seebeck coefficient and thus the overall thermoelectric properties of PEDOT:PSS by using ionic liquids. These methods can enhance the figure of merit (ZT) to ~0.75 at room temperature, comparable to inorganic thermoelectric materials.

This talk will discuss the thermoelectric properties of poly(3-alkylchalcogenophene) thin films as a function of heteroatom (sulfur, selenium, tellurium), and how these properties change with ferric chloride dopant concentration. Using UV-Vis-NIR spectroscopy, electrical conductivity, and Seebeck coefficient measurements, we observed that as the heteroatom is substituted from S to Se to Te, the susceptibility to doping increases. The increased doping susceptibility leads to poly(3-alkyltellurophenes) requiring less dopant to achieve an optimal thermoelectric power factor (ca. 10 µW/m-K²) in comparison to the sulfur and selenium derivatives. Poly(3-alkylchalcogenophene) films’ electrical conductivities and Seebeck coefficients at each dopant concentration was also measured as a function of temperature and compared with relevant charge transport. We observed that the electrical conductivity increases with increasing temperature, but the Seebeck coefficient is effectively temperature-independent; this is similar to the findings of Crispin, Emin, and coworkers who studied NOPF₆ doped P3HT charge transport using a model similar to the Mott Polaron Model. We therefore analyzed our temperature-dependent charge transport data in the context of the Mott Polaron model and found that as the heteroatom is substituted from S to Se to Te the activation energy required for charge transport decreases. Overall, this talk will cover a systematic study of charge transport in doped poly(chalcogenophenes) and will demonstrate that tuning the heteroatom may lead to optimized thermoelectric performance.

Thermoelectricity can play an important role in power generation in an environment-friendly way by using waste heat. State-of-the-art materials used for thermoelectric generators (TEG) often contain elements that are toxic and heavy, and suffer from chemical instability or difficulties during processing. Contrarily, our work focuses on the development of a TEG based on foils of non-toxic copper-nickel alloys and polymer Poly(3,4- ethylenedioxythiophene)-Poly(styrenesulfonate) (PEDOT:PSS). The composition of the n-type copper-nickel alloy is optimized to maximize the power factor to 5,5 × 10^-3 W/mK² and a Seebeck coefficient of -48 × 10 ^-6 V/K. The TEGs were fabricated through depositing thin layers of PEDOT:PSS on the optimized CuNi foils by dispense printing. The electrical insulation between the different layers is realized by atomic layer deposition (ALD) of insulating Al₂O₃ film. The resulting metal-polymer sandwiches were mechanically pressed for close contact.

Thermoelectrics have attracted rapid growing interest for sustainable energy harvesting technology to power flexible electronics, such as wearable devices and environmental monitors. The inorganic thermoelectric (TE) semiconductors are still the most possible candidates for this technique due to their best efficiencies, although the pristine materials cannot be directly used as their intrinsic brittleness and rigidity. Therefore, great efforts from various interdisciplinary fields have been dedicated to searching solutions to improve the flexibility of conventional inorganic TE materials. Herein, we present a novel approach to fabricate flexible TE hybrids through depositing Bi₂Te₃-based alloys on the free-standing transparent single-walled carbon nanotube (SWCNT) scaffold (Nature Materials, 18, 62-68, 2019). The nanocomposite reveals well-ordered and porous microstructures, which consists of (000l) textured Bi₂Te₃ nanograins grown on the SWCNTs with good adhesion and perfect alignment along the Bi₂Te₃ <-12-10> and SWCNT axes (Fig.1). The freestanding Bi₂Te₃-SWCNT hybrid exhibits remarkable mechanically reliable flexibility over hundreds bending circles, of which the bending deformation radius could be as high as a few millimeters. Large power factors of ~1600 to 1100 μW/mK² are obtained for the Bi₂Te₃-SWCNT hybrids room temperature to 473 K. Owing to the high density of defects, such as Bi₂Te₃-SWCNT interfaces, stacking faults and nanopores, the in-plane thermal conductivity is as low as 0.53 W/mK. Such high-power factor and low thermal conductivity give rise to a ZT of ~ 0.89 at room temperature. Moreover, flexible n-type Bi₂Se₃-SWCNT and p-type Sb₂Te₃-SWCNT TE hybrids with similar crystal structure were also fabricated, indicating that our approach opens a new way to fabricate free-standing flexible TE materials with high performance.

Thermoelectric generators (TEGs) are solid-state devices that can directly convert temperature differences into electrical voltages via Seebeck effect, which neither requires moving parts nor consumes liquid or gas media, implying favorable qualities, such as high reliability and eco-friendliness. Given the ubiquity of heat and the versatility of electricity, thermoelectrics have long been recognized as a highly promising transformative technology to address our global energy needs. At present conventional inorganic TE materials have achieved tremendous advances, which however have drawbacks such as high cost of production, scarcity of materials, toxicity as well as limited scope of application. To overcome these intrinsic issues, organic thermoelectric materials as alternatives has emerged since they are based on abundant elements, together with low cost, light weight, mechanically flexibility and low-temperature solution processing over a large area. Moreover, organic thermoelectric devices (OTEs) can employ low quality heat. All of these imply that OTE materials have wide application prospects, especially in the development of personal, portable, wearable and flexible heating devices and near-room temperature energy generation. However, for the lacking of high performance OTE materials and proper device architecture design, the efficiency of thermoelectrics is still lower than other energy conversion technologies. It remains full challenges before fully realizing their potential. In 2016, research on poly(Ni-ethylenetetrathiolate) brought our attention to conjugated coordination polymers, which provided the best thermoelectric performance among n-type OTE materials, with an optimum power factor over 453 μW/mK², benefit from the well-ordered arrangement and stronger π-π interaction of molecular. In metal-organic conjugated coordination polymers, high electrical conductivity can be achieved, whether 1D or 2D coordination polymers (CPs). It is an essential advantage for developing efficient OTE materials. By the targeted design with proper metal and ligand and precisely controlling the oxidation or reduction, the Fermi level, band gap and density of state etc. can be modified, which will significantly affect the thermoelectric performance of polymers. Therefore, it is reasonable to believe the research on TE properties of 2D coordination polymers will be greatly accelerated while providing high-efficient materials for OTEs design. We aim to explore a feasible strategy for developing high-performance 2D coordination nanosheets polymers, reveal the internal transport mechanisms and obtain a better-performing OTE device based on the 2D materials we obtained. It will open up new possibilities for the development of organic thermoelectrics.

Energy filtering phenomenon has been evidenced in hybrid organic-inorganic composites. It has been reported that appropriate barrier on the organic-inorganic surface is beneficial for the enhancement of thermoelectric performance. However, in the hybrid organic-organic system, the existence of energy filtering phenomenon has not been strongly confirmed in organic system, nor it is clear that if it shares same rules as the inorganic thermoelectric system. In this work, we develop a novel organic thermoelectric composite composed of poly(3,4-ethylenedioxythiophene) nanowires (PEDOT) and polypyrrole nanowires (PPy) at different contents for the optimization of ZT and power factor. The optimized thermoelectric performance is achieved at 0.5 wt.% PPy. By varying the treatment time of PPy with ferric chloride (FeCl₃), the degree of oxidation of polypyrrole is tuned, which further influences the value of energetic barrier. The relationship between thermoelectric performance and the barrier has been verified. To the best of our knowledge, we are the first to quantitatively investigate the effect of energy filtering on thermoelectric performance of organic composite. By comparing experimental data with the series-parallel model, it is found that energy filtering is possibly an important factor the enhancement of Seebeck coefficient and power factors due to that energy filtering can selectively scatter low-energy carriers.

To improve thermoelectric output, we positively deviated the thermoelectric trade-off rule of electrical conductivity and the Seebeck coefficient by modifying the electrostatic interaction between conductive polymers and dopant molecules, i.e., poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), by addition of alkylsulfonate anions. We analyzed the conformational change of PEDOT:PSS by the added anions from a solution level to a film morphological level, and verified that the intervention of added anions brought about molecular ordering in PEDOT domain by pulling down existing tight-bounded state between PEDOT and PSS. Furthermore, different lengths of alkyl chains of added anions controlled the oxidation levels of PEDOT backbone by changing the chain-chain interaction between PEDOT and PSS to different extents. As a result, we found that these changes could augment both electrical conductivity and the Seebeck coefficient of PEDOT:PSS simultaneously, which produces a remarkable power factor of 715.2 uW/mK².

The addition of nanomaterials into the polymer matrix has been shown increased thermoelectric properties due to the internal organization of the polymer molecules at the nanomaterial template.¹ The preparation methods, the concentration of the nano-inclusions, nature of the nanotemplates (oxidised or reduced) defects, and impurities in the nanotemplates affect the organization of the molecules at the surface of the nanotemplates and eventually influences the thermoelectric properties. The internal organization of the molecules at the interface plays a major role in the enhancement in electrical conductivity and Seebeck coefficient eventually affects the power factor of the thermoelectric material.² The effect of the interface at the polymer and nanomaterial on the power factor or the conversion efficiency of the thermoelectric material is still not well understood. The conducting polymer polyaniline (PANI) has many attractive features such as inexpensive, environmental stability, easy synthesis protocol, easy processability, and tunable electrical conductivity etc. over the other conducting polymers.^{3, 4} The SnS is more earth-abundant material and easy to synthesize, moreover, it has lower thermal conductivity, high Seebeck coefficient and it has good thermoelectric properties. We mainly focused on the synthesis of polymer nanocomposite by mixing the SnS nanoparticles into the polyaniline matrix and studied their thermoelectric properties.
References
1. C. Gao, and G. Chen. Compos. Sci. Technol. 124, 52-70 (2016)
2. Q. Wang, Q. Yao, J. Chang, and L. Chen. J. Mater. Chem. 22, 17612 (2012)
3. Conductive polymers as a new type of thermoelectric material. Proceedings of the Macromolecular Symposia, (2002);
4. N. Dubey, and M. Leclerc. J. Polym. Sci., Part B: Polym. Phys. 49, 467-475 (2011)

In this paper, we proposed the fabricated process of a flexible thermoelectric generator (TEG) module that can use body heat and evaluated its electrical performances. The metal/ceramic materials have been widely used for substrate of TEG but there is a difficulty of conformal contact because of their rigidity, therefore, flexibility of the module is required in order to maximize the use of curved heat sources such as the human body. For this reason, we suggest a manufacturing method of flexible thermoelectric devices based on inorganic thermoelectric material. As a thermoelectric element for fabrication of TEG, we used BiTe-based material which is most widely used for TEG system. It has much better performance than other organic materials such as carbon nanotube and PEDOT:PSS used for flexible TEG.
In previous researches, metal electrodes were formed by a deposition process followed by the printing of thermoelectric material. And a flexible device was fabricated by laser-lifting method with filling of flexible polymer. In this research, we have proposed a novel fabrication process that using the flexible silver electrodes. The silver electrodes were printed on the sacrificial layer which can be removed by an etching process to obtain a flexible thermoelectric device. Through this method, flexible TEG can be easily fabricated without complex laser process or organic materials or by using sacrificial layers such as PVA and PVP that can be etched by water.
We also have studied the performance changes of the TEG by various polymer materials that have different thermal conductivity. As a supporting layer which have flexibility, the filler material is needed after the etching of the sacrificial layer. Also, because external pollution can make degradation of TE in the actual usage, passivation is required. But high thermal conductivity of polymer filler reduces temperature different between hot and cold side that concludes the output power of the TEG. We compared the electrical performance of the TEG modules by different filler material, and it was confirmed that the porous polyurethane foam can be good candidate of wearable TEG system.
Though metal fin is most widely used structure for the heat sink, but it is not suitable for wearable system because of the rigidity and bulky structure. To overcome those limitation, inspired by nature, we introduce an artificial perspiration membrane that automatically regulates the evaporation by programming a deformation of thermo-responsive hydrogels. The thermo-responsive hydrogel is patterned with pinwheel-like shape and supported by backbone through copolymerizing at the interface to control the evaporation area depending on the temperature. To prevent the unintended evaporation through the hydrogel, the stretchable rubber is selectively coated, which layer leaves the water path for swelling of the hydrogel. Our heat sink can absorb the water and its latent heat can decrease the temperature of the cold side during drying process. Compared to metal fin heatsink, it was confirmed that the power of TEG module increased more than 2.5 times.

Wearable flexible organic thermoelectric materials harvesting low-grade heat especially human body’s heat have drawn a lot of attention because of their lightweight, flexible, nontoxic and easy-processing et al. In this work, we report organic-based T-TEGs by assembling segmentally-impregnated carbon nanotube yarn with PEDOT: PSS (p-type) and PEI (n-type) into a spacer fabric. The T-TEG shows multi-functionality with superior electrical power output, ultrahigh sensitivity to tactile touch and excellent wearing performance. Finally, this kind of CNT yarn based thermoelectric fabric can be continuously and automatically produced in bulk using traditional knitting machine. In addition, we successfully demonstrate an all-in-one T-TEG based self-charging power package for wearable electronics, which has never been succeeded in previous studies. This package can power a series of essential electronics in extreme environment when the battery cannot work even damaged (T<-20 degrees Celsius). All these results make the T-TEG outperforming all reported flexible organic-based TEGs. The developed strategy can inspire the manufacturing of cost-effective smart T-TEGs using well-established textile industrial processes.

Thermoelectric (TE) composites have turned out to be a promising way to enhance the ZT value of organic TE materials. However, a comprehensive study on TE charge transport in composite material has been scarce. Here we demonstrate that designed physical and energy structures in TE composite materials cause all three critical TE parameters to contribute to the improvement of the ZT value. Adding the carbon quantum dot (CQD) to poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) provided additional charge transport pathways, which also acted as an energy-selective mobility enhancer. These contributed to increase both the electrical conductivity and the Seebeck coefficient, simultaneously. Addition of the CQD also reduced the thermal conductivity by increasing the interfaces in the film. With an optimized interfacial potential gap between CQD and PEDOT:PSS, the composite film showed significant increases in the power factor and the ZT value.

Nanocarbon materials are expected to be potential candidates for nontoxic and flexible thermoelectric materials with high power, which are suitable for wearable power generation technology [2][3]. Carbon Nanotube (CNT) thin films being the network consisting of randomly distributed numerous CNTs is one of nanocarbon materials. CNT films require high electrical conductance and Seebeck coefficient to enhance the performance of the films. In order to increase the conductance and Seebeck coefficient of CNT films, we have to understand the relation between the CNT network structure and its electrical and thermal transport properties.
To understand the relation between the CNT network structure and its thermoelectric performance, we theoretically investigate the effect of several parameters such as nanotube length and nanotube density on thermoelectric performance using the thermal circuit theory, electrical circuit theory, and the percolation theory. First, we generated a two-dimensional random network by changing the nanotube density and the nanotube length and calculated the temperature distribution by solving thermal circuit equations for the CNT networks. Then, we calculated the voltage of CNT own and contact between CNTs generated by temperature difference using the temperature distribution. Finally, we calculated the Seebeck coefficient and electrical conductivity by solving electrical circuit equations for the CNT networks. The obtained results in this study are summarized as follows.

1. The electrical conductance of the film increases with the nanotube length.
2. The Seebeck coefficient of the film is almost dependent on the contact between CNTs in any nanotube length from 0.5µm to 1.5µm.

References
[1] Masaaki Tsukuda, Keisuke Ishizeki, Kengo Takashima and Takahiro Yamamoto, Appl. Phys. Express 12 055006-1 - 055006-4(2019).
[2] T. Izawa, K. Takashima, S. Konabe and T. Yamamoto, Synthetic Metals 225, 98-102 (2017).
[3] J. Bahk, H. Fang, K. Yazawa and Ali Shakouri, Journal of Materials Chemistry C, 3, 10362-10374 (2015).

Thermoelectric generators that can directly convert wasted heat into electricity have gained much attention as a sustainable energy generator. Specially, for the last decade, organic-based thermoelectric materials have attracted interests due to their inherent flexibility that offer great opportunities to realize geometrically versatile energy harvesting systems with high degree of design freedom. However, there are still challenges on the low output power of organic thermoelectrics compared to that of rigid inorganic based counterparts. The practical strategy for enhancing the output power is to increase the number of organic thermoelectric legs; therefore, emerging solution processing, especially additive printing technologies, have been widely used due to its large-area processability in ambient. In addition, the efficient reduction of internal resistance can pave a key pathway for further improved output power of large-area organic thermoelectric applications.
In this presentation, we will present all-printed organic thermoelectric modules using the combination of spray printing and inkjet printing. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and nanoparticle-type Ag inks were used for the thermoelectric and highly conductive metallic layers formation, respectively. Especially, spray printing enabled us to make few-μm thick organic films onto large area flexible substrates, resulting in the higher output power compared to that of counterparts fabricated by using conventional spin coating. To reduce the internal resistance of organic thermoelectrics, functionalized interlayers are introduced between the PEDOT:PSS legs and Ag electrodes using inkjet-printing that significantly reduce the contact resistance by modulating the dominant carrier injection mechanism. With these efforts, the interface-engineered organic thermoelectrics exhibit the two times higher output power comparing to the pristine counterparts. These results will provide a practical solution to realize high-performance printed organic thermoelectric modules.

Single-walled carbon nanotube (SWNT) has been regarded as one of the nanomaterials for future electronic devices because of their outstanding mechanical and electrical properties. For these reasons, researches for implementation of their electronic applications such as field-effect transistors and thermoelectric devices have been substantially promoted. Therefore, various techniques to isolate the electronically-pure SWNTs have been developed for the utilization in electronic devices. In particular, conjugated polymer wrapping of SWNT has specially aroused great attraction as a method for isolation of SWNTs due to their advantages of high selectivity toward semiconducting (sc-) SWNT and simple polymer sorting process. Various wrapping polymers such as polyflourene and polythiophene derivatives have been studied for the selective dispersion of sc-SWNT, but the selection rule between the main chain of polymer and specific sc-SWNT still remain question. In this research, we introduce the pyrene moiety to control the conformation over the main-chain on polymer. New pyrene-based conjugated polymers were carefully designed and synthesized via suzuki polymerization. Their dispersion selectivity and diameter of sc-SWNTs enriched by pyrene-based conjugated polymers were characterized by various measurements such as UV-vis absorption spectra, Raman spectroscopy and Photoluminescence analysis.

Nowadays, as one of environmental problems, it is focused that much consumed energy is wasted as heat. Therefore, to realize a "low-carbon society," the way to utilize the wasted heat is required. As one of candidates for the solution of this problem, the thermoelectric power generating technologies are focused. Carbon nanotubes (CNTs) are expected to be new thermoelectric materials because they have high electrical conductivity and thermoelectromotive force, but there are the following issues in using them as thermoelectric materials. First, low thermal conductivity is required for efficient thermoelectric generation, but CNTs have high thermal conductivity. Second, CNTs are generally produced as powders and are difficult to handle. To solve these problems, we have proposed using carbon-nanotube-composite threads (CNTCTs). CNTCTs are composite materials produced by coating cotton thread fibers with CNTs. It is expected that CNT can be easily handled by combining it with thread, and cotton thread with low thermal conductivity also prevents heat diffusion. In previous study, we examined methods to fix CNTs on cotton thread surface efficiently, and succeeded in producing CNTCTs that can be used as thermoelectric materials. In this study, we examined a method of coating two types of CNT on thread with a pattern suitable for thermoelectric power generation in order to fabricate thermoelectric elements using CNTCTs. CNTCTs are produced by traditional dyeing methods. In concrete, we prepare CNT dispersion by mixing CNT (Sigma-Aldrich, (6,5)-chirality) and sodium dodecyl sulfate as dispersant in pure water and ultrasonication for them, firstly. After that, the cotton thread is dipped in the CNT dispersion, taken out, dried, and washed with pure water. By performing the above operation, CNTCTs in which the same kind of CNT uniformly covers the thread surface are obtained. However, if CNTCTs in which two types of CNTs having different Seebeck coefficients cover the thread surface at equal intervals can be obtained, the thermoelectric generation element can be easily manufactured. This is because when the thread is sewed on a cloth, it becomes a structure like a π-type thermoelectric generator. So, in addition to the operation described above, we carry out the process called "fusenori" in this study. Fusenori is a Japanese traditional dyeing method to partially prevent the dye from being fixed to the thread. In concrete, we prepare a rubber paste by dissolving latex in tetrachloroethylene, and apply the rubber paste at intervals of 1 cm to cotton thread before the above-mentioned dyeing operation. Because tetrachloroethylene is volatile, the latex covers the thread surface when it is dried. The latex is hydrophobic and keeps covering the thread surface without melting out of the CNT dispersion. Therefore, it can be expected to prevent CNT from being fixed on the surface of the cotton thread in the part coated with rubber paste. After the dyeing operation is performed on the thread, the latex is removed from the thread surface by washing with tetrachloroethylene heated to 80°C. The resistance value and Raman spectrum of the produced CNTCTs were measured, and it was confirmed that the CNTs were hardly fixed at the place where the rubber paste was applied. After removing the latex, we apply a rubber paste to the portion where the CNTs are fixed, and perform the above-mentioned dyeing operation using CNT (Nanocyl, multi-walled) whose Seebeck coefficient is different from that used previously. The CNTCTs obtained by the above operation were sewed on a cloth, and it was confirmed that an electromotive force was generated when only one side of the cloth was heated. From the above results, it was confirmed that CNTCTs whose sueface is covered two different types of CNTs at intervals of 1 cm by fusenori, and a thermoelectric element could be produced simply by sewing it.

Fluorinated polyimides gate dielectric based on 6FDA-BisAAF-PI and 6FDA-TFDB-PI were synthesized to use as a gate dielectric to fabricate high performance organic field-effect transistors (OFETs). The fluorine substituted into polyimide chain was improved the efficiency of solution-process to make uniform film. These fluorinated polyimide make it possible to fabricate transparent devices. Especially, OFETs based on 6FDA-TFDB-PI which has low surface energy have high-performance because defect density formed by grain boundary is lowest. The OFETs based on 6FDA-TFDB-PI gate dielectric have high carrier mobility (1.83 cm²V^-1s^-1 for the p-type pentacene FETs and 0.56 cm²V^-1s^-1 for the n-type PTCDI-C₈ FETs) and a high on/off ratio exceeding 10⁶. And, we check that the devices have excellent electrical stability from measured bias stress experiment. The 6FDA-TFDB-PI film was used to fabricate complementary inverters that have high gain value. Lastly, the logic gate circuit (NAND, NOR) were successfully fabricated using 6FDA-TFDB-PI gate dielectric.

Heavily-doped conjugated polymers are among the most promising organic materials for thermoelectric applications due to their outstanding charge transport properties and low thermal conductivity. These properties are well-known to be dependent on the structural arrangement of the polymer, disregarding the state and position of the dopants. In this study, we utilize Kelvin Probe Force Microscopy to track iodine dopants across P3HT and PDPP4T films and show that their distribution within the polymer film alters the distribution of the density of states. As a consequence, it impacts the shape of the Seebeck coefficient—electrical conductivity curve and thus its thermoelectric properties. Our study shows that it is necessary to control the distribution of dopants within the films for optimal properties in thermoelectric and other organic electronic applications.

We investigated method to improve properties of thermally evaporated WO_3-x/Ag/WO_3-x (WAW) multilayer using the deposition rate of WO_3-x layer and Ag layer as parameters, in order to fabricate flexible and transparent thin film heaters (TFHs). In this work, we demonstrate increase of grain size and reduce interface roughness and enhance electrical and optical properties of WAW multilayer by simply control of deposition rate. And we analyzed properties of WAW multilayer in aspects of electrical, optical, morphological, and mechanical properties. Especially, unlike conventional research which were just focused on and thickness control of each layer to optimize oxide/metal/oxide multilayer, our work presents you how deposition rates affect to properties of WAW multilayer and suggest another way to optimize OMO multilayer. For evaluation, we measured electrical properties and optical properties of WAW multilayers by hall measurement and UV-Vis measurement to optimized performance of WAW multilayer. When WAW multilayer deposited at optimized deposition rate of 2.5 Å/s, 10 Å/s, 2.5 Å/s, showed low sheet resistance of 3.78 ohm/square and high transmittance of 92.16 % at 550 nm wavelength. Morphological properties of surfaces, interfaces are investigated by FE-SEM, XRD, and mechanical stability and flexibility were investigated bending test, rolling test, twisting test. In mechanical deformation test, after 10000 times of inner and outer bending tests, there were no sheet resistance change in WAW multilayer compare to WAW multilayer before the test. Furthermore, we fabricated the WAW based TFHs to investigate heating profile of WAW multilayer based TFHs. Compare to typical ITO based TFHs, WAW based TFHs show low DC voltage to achieve 120 °C due to its low sheet resistance. Moreover, WAW based TFHs show higher convective heat transfer coefficient than ITO based TFHs. Therefore, WAW based TFHs are promising as flexible and transparent highly efficient convection TFHs.

To examine our hypothesis of sticky thermoelectric (TE) materials to ease the fabrication processes of flexible TE modules at reasonable cost, we investigated hybrids of inorganic TE powders and low-volatizable solvents as a series of sticky TE materials. Through our previous study, we have specified that common electric conductive adhesives (ECAs) do not have enough electric conductivity and mechanical strength to hold the structure of flexible TE modules when rolled. As a possible research direction, we have hypothesized sticky TE materials, which can adhere to electrodes pressure-sensitively without ECAs and transform the shape to cancel the mechanical stress when bent. In this study, we prepared p-type and n-type sticky TE materials from antimony and bismuth and examined electric contacts and measured thermoelectric voltages. This presentation discusses the outcomes in details.

Flexible organic thermoelectric devices base on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and Polyimide (PI) were fabricated and investigated in this work. PI was selected as a substrate because of its excellent mechanical property to secure from repeated bending cycles of flexible device. However, due to hydrophobicity of PI, there was limited research on utilizing PEDOT: PSS and PI for flexible devices. A facile method, oxygen plasma treatment was adopted on PI’s surface. The effect of the surface treatment with oxygen plasma on the synthesized PI substrate was significant. The polar component of surface free energy of PI was increased from 2.8 mJ/m2 to 31.8mJ/m2. Therefore, PEDOT: PSS could be stably coated on the PI substrate. The power factor of PEDOT:PSS on the PI substrate was increased from 25.86μW mK-2 to 43.78μW mK-2. Also, as a result of 10k times of bending test, the electrical performance consistency and the mechanical stability of the fabricated devices were confirmed. This verified fabricated flexible organic thermoelectric devices based on PEDOT: PSS and PI are suitable for the various applications

This research was supported by the Mid-career Researcher Program through the National Research Foundation of South Korea (NRF) funded by the Science and Engineering (NRF-2017R1A2B4012051).

Single-walled carbon nanotubes (SWNTs) have remarkable electrical conductivity, (σ), Seebeck coefficient (S), light weight, mechanical toughness and flexibility which makes them ideal candidates for thermoelectric (TE) applications. Among SWNTs, semiconducting SWNTs (s-SWNTs) have been found to have larger S than metallic SWNTs (m-SWNTs) both theoretically and experimentally.^1,2 However, SWNTs are synthesized as a mixture of s-SWNTs and m-SWNTs, which limits the maximum possible S for SWNTs. Therefore, it is of utmost importance to extract s-SWNTs having high purity as a TE material. Among the various extraction methods,^3,4,5,6 Flavin derivative (dmc12) extraction method reported by Kato et. al. is profitable due to its many advantages, namely, high yield of s-SWNTs, easy operation of the extraction method, low damage to the s-SWNTs and easy removal and recycling of dmc12 .^7,8 In this study, we investigated the effect of doping s-SWNTs extracted by dmc12 having different bundle size of s-SWNTs.
Purified SWNTs (eDips, Meijo Nano Carbon) were used to extract s-SWNTs and the extracted s-SWNTs were collected by vacuum filtration. The addition of CH₂Cl₂ to remove dmc12 before and after the filtration provided bundled and isolated s-SWNT sheets, respectively. The bundled sheet represented thicker bundles of s-SWNTs in the sheet while the isolated sheet comprised of thinner bundles. The sheets were doped at various concentrations with triethyloxonium hexachloroantimonate (OA)⁹ and 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole (o-MeO-DMBI)¹⁰ as p- and n-dopant, respectively. The highest obtained S for p- and n-doped s-SWNT sheets were 83.3 μVK^-1 and -89.5 μVK^-1, respectively. Both values are higher than the values attained previously for doped-unsorted SWNT sheets (p-doped: 54 μVK^-1, n-doped: -49 μVK^-1).¹⁰ Moreover, it was found that the isolated s-SWNT sheet showed larger S value than the bundled sheet in the n-doped region. We consider doping is more effective for isolated than bundled sheets as the power factor of 126.3 μWm^-1K^-2 was achieved for isolated sheets.

References-
[1] Avery et. al., Nat. Energy 2016, 1, 1-9. [2] Nakai et. al., Appl. Phys. Express 2014, 7, 025103. [3] Ghosh et. al., Nat. Nanotechnol. 2010, 5, 443-450. [4] Huang et. al., Anal. Chem. 2005, 77(19), 6225-6228. [5] Tanaka et. al., Nano. Lett. 2009, 9(4), 1497-1500. [6] Nish et. al., Nat Nanotechnol. 2007, 2, 640-646. [7] Kato et. al., Chem. Lett. 2015, 44, 566-567. [8] Huang et. al., Chem. Commun. 2019, 55, 2636-2639. [9] Chandra et. al., Chem. Mater. 2010, 22, 5179–5183. [10] Nakashima et. al., Adv. Energy Mater. (submitted).

The low dielectric constant of organic materials and inhomogeneous distribution of molecular dopants in a conductive matrix impose difficulties in populating free charge carriers in organic materials, leading to practical challenges in developing highly conductive organic materials. We designed a novel conjugated polymer (PIDF-BT) having long side chains and a strong electron-donating moiety in a way that the PIDF-BT strongly interact with molecular dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethan (F4-TCNQ), and the doping efficiency was further enhanced through the management of the diffusion degree of F4-TCNQ into CP matrix. With these strategies, dramatic enhancement of electrical conductivity over 200 S/cm, was achieved from PIDF-BT, which is much larger than that obtained from the conventional blending approach (5 S/cm). Furthermore, the doping level was scalable with the exposure duration in the F4-TCNQ solution as well as the solution’s concentration. The diffusion degree of F4-TCNQ was systematically correlated with physical properties (glass transition temperature and crystallinity) of PIDF-BT through the characterization of electrical conductivity according to the thermal annealing temperature of the pre-deposited PIDF-BT films. Finally, characteristic connections between charge carrier density, mobility and thermoelectric effects were extracted with doping level-controlled PIDF-BT films prepared under different thermal annealing conditions.

We propose a unique thermoelectric power generation material based on a carbon-nanotube(CNT) composite paper that a composite material of the CNT and a paper, i.e., “thermoelectric power generating paper.”
Recently, thermoelectric power generation is attracted attention because it can effectively use heat that is often discarded. However, there is a problem that many of the currently used thermoelectric materials are rare metals. For this, we focus on CNTs because CNTs have been confirmed to have high electrical conductivity and thermal conductivity. Moreover, showing a high Seebeck coefficient has been found in recent studies. However, the CNT is difficult to handle because it exists in powder form. Also, having high thermal conductivity is a disadvantage for the thermoelectric generation. Therefore, we are developing a thermoelectric power generating device as the CNT composite paper by combining the CNT with the paper that is easy to handle and has the thermal insulation.
In this study, we are aiming to improve the performance of thermoelectric power generating paper. In the previous work, we studied the suitable amount ratio of the CNTs and pulps to make the CNT composite paper. Because if there was little pulp or more CNTs, it was unable to maintain the temperature difference between both ends. For obtaining low thermal conductivity, we found the ratio. However, the following problems remained. They are that the resistance value of the CNT composite paper was high and enough current could not be obtained.
In this report, we conducted following things to improve the performance of our thermoelectric power generating paper. First, we tried to modify the making method of the CNT composite paper. We here use the “kamisuki” that is Japanese washi paper making method. In “kamisuki,” we prepare a pulp dispersion and a CNT dispersion. Then, we mix the two dispersions and remove moisture by using a fine net. Finally, we dry it and finalize making. Previously, we used “silicone case method” not “kamisuki.” In “silicone case method,” we use the silicone case to evaporate the moisture. Therefore, the CNT composite paper contains many dispersants. Because dispersants are insulator, the resistance value of the CNT composite paper become high, i.e., the dispersant remains on the paper. In contrast, in “kamisuki,” the dispersant is expected to flow with water. So, we change the production method of CNT composite paper “Silicone case method” to “kamisuki.” As a result, we succeeded in lowering the resistance value by about 13%.
Second, the semiconducting CNT usually behaves p-type property in the atmosphere. For efficient thermoelectric generation, not only p-type but n-type semiconducting CNTs are required and they must be connected alternately. So, we applied a doping way to our CNT composite paper for obtaining n-type property. As results, we found temperature dependence of the doping, i.e., there was suitable temperature condition for the doping. Moreover, we also found sustainability of the n-doped CNT composite paper.
We believe our CNT composite paper will be used as flexible thermoelectric power generating elements in near future.

Heavy heteroatom substitution of the backbone is an effective strategy to improve charge delocalization and increase ionised potential of organic polymers. This facilitates the doped states of those materials more conductive without sacrificing the seebeck coefficient a lot at the fixed doping level. Here, we report the effect of the single selenophene replacement on the thermoelectric transport mechanism of polythiophene poymers at different doping levels. By feat of a novel doping method, we achieve the maximum conductivity of ~700S/cm for doped selenophene polymer, which is 3 times higher than its counterpart thiophene polymer with the metallic-like seebeck coefficients of ~25 uV/K for both. Interestingly, temperature dependence of conductivity for the highly doped selenophene shows upturn behaviour (metal-semiconductor) while the seebeck coefficients show linear dependence versus temperature. This indicates we approach a regime where the conductivity is decoupled with the seebeck coefficient. By introducing heavy atom into polymer backbones, it is potentially possible to improve the thermoelectric performance via conductivity boost.

For a decade, π–conjugated semiconducting polymers have attracted great attention for thermoelectric materials because of their easy tunability, solution processability, and mechanical flexibility. Molecular doping is widely used to improve the thermoelectric properties of the polymer films, because it can easily control the electrical conductivity (σ) by changing the carrier concentration (n). Among various molecular doping methods, solution mixing is typically performed due to its simplicity. However, it is not easy to heavily dope the polymer film due to solubility limit. In addition, the presence of a significant amount of dopant molecules in the doped polymer films can distort the film morphology, limiting the thermoelectric properties. Therefore, it is important to select appropriate doping methods to enhance the thermoelectric properties. Previously, we reported the organic thermoelectric device using p-type conjugated polymer poly[(4,4’-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b’]dithiophene)-alt (benzo[c][1,2,5]thiadiazole)] (PCPDTSBT), showing the electrical conductivity of 7.47 S cm⁻¹ using BCF as a dopant. In this study, we changed the dopant to FeCl₃, and introduced a overcoating method to dope the PCPDTSBT film, demonstrating high electrical conductivity up to 300 S cm⁻¹, which is ~40 times higher than that of the BCF-doped film. The resulting power factors of FeCl₃-doped PCPDTSBT film was 29.75 μW/mK², which is ~4 times higher than that of the BCF-doped film. The reason of rapid increase in σ is systemically investigated using various analysis methods like UV absorption, Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) and impedance spectroscopy. Kang-Snyder model is also used to investigate the charge transport in the PCPDTSBT film.

Driven by the prospects of organic thermoelectric(OTE) devices, which have the potential for energy generation, have gathered considerable attention worldwide. We compared various properties of poly[(4,4’-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b’]dithiophene)-alt (benzo[c][1,2,5]thiadiazole)] (PCPDTSBT) and its analogue with oligoethylene glycol (OEG) side chains in place of alkyl ones (PCPDTSBT-A), before and after doping. Incorporation of polar OEG side chains improves intermolecular ordering and exhibits ‘self-doping’ with enhancement in doping efficiency, thereafter resulting in higher electrical conductivity and power factor of PCPDTSBT-A surpassing those of PCPDTSBT. OEG chains improve the miscibility of dopant solution and polymer film during sequential doping which was performed by casting dopant solution (by varying the concentration of F₄TCNQ in acetonitrile) onto the film. This study emphasizes the importance of the polar side chain engineering to modulate doping efficiency, inter-chain packing, crystalline morphology and miscibility whose outcome is revamped electrical and thermoelectrical properties.

Our current energy production results in a tremendous amount of waste heat. In an effort to more efficiently utilize dissipated heat, multilayer thin films, comprised of poly(diallyldimethylammonium chloride) (PDDA), graphene, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and double-walled carbon nanotubes (DWNT), were prepared using layer-by-layer assembly followed by heating in an inert atmosphere to degrade film constituents to varying degrees. PEDOT:PSS was used to stabilize graphene and DWNT. A 20 QL thin film, that is 21 nm thick, heated to 425 °C for 60 minutes exhibits a simultaneous increase in electrical conductivity and Seebeck coefficient as compared to an unheated film. This behavior is not commonly observed in bulk thermoelectric materials. The power factor of this film is 153 uW m^-1 K^-2, which is an order of magnitude larger than the unheated control. This marked improvement in thermoelectric performance is the result of degrading the insulating PDDA:PSS complex from the film, while maintaining the highly ordered conductive network formed during layer-by-layer deposition. This strategy of thermally degrading non-conductive material required for film fabrication can be used to prepare numerous high performing thermoelectric materials.

PEDOT:PSS is considered as the most promising p-type material for organic thermoelectrics due to its high electrical conductivity, high power factor, stability and solution processability from an aqueous dispersion. However, the neat PEDOT:PSS lacks flexibility and stretchability. Here, we demonstrate mechanically robust thick free-standing films of PEDOT:PSS blended with waterborne polyurethane (PU). As the blend is prepared from water dispersions, it is solution processable and convenient for large area coating and printing. We explored the mechanical properties, nanostructure and thermoelectric performance of the blend with different PU loading. Films with <50% PU are mechanically robust with high electrical conductivity while those with >50% PU are stretchable but show low conductivity. At 50% PU loading, the blend film maintained a power factor of ~10 µW m^-1 K^-2 with excellent flexibility and even twistability. The blend film with 75% PU loading withstands 1000 stretching cycles at 10% strain with no loss in power factor.

The aim of this study is increasing power factor and decreasing thermal conductivity simultaneously through the production of hollow spherical Ca₃Co₄O₉ and air composite materials. This structure comprises of hollow spherical Ca₃Co₄O₉ particles and porosities dispersed in the matrix. This means that air having very low thermal conductivity is trapped in the structure, so thermal conductivity of the materials can be decreased. Also, high amount of porosity provides increase in Seebeck coefficient. Ca₃Co₄O₉ is environment friendly, nontoxic, humidity resistant at high temperatures, oxidation resistant, abundant, chemically and thermally stable in air and light. Also, Ca₃Co₄O₉ is favored due to its natural layered structure composing of conduction and insulation layers alternately which provide electronic transport and phonon scattering layers separately. Therefore, natural superlattice structure of Ca₃Co₄O₉ brings about high thermopower and low electrical resistivity at the same time.
Ultrasonic spray pyrolysis (USP) method is used to obtain hollow spherical particles. Many parameters have an effect on forming hollow spherical structure such as frequency of ultrasonic generator, the concentration of initial solution, the temperature of reactor and the flow rate of the carrier gas which influence the morphology, dispersity, structure and the phase composition of the end product. High evaporation rate and high mass transfer are required for getting hollow spherical structure. Three different solutions were prepared using deionized water, dimethylformamide and methanol. These solvents were chosen according to their capability of producing an aerosol. Being nontoxic and eco-friendly, deionized water was determined as the solvent in USP technique.
The production of high porosity structure was achieved by using pressure-less sintering method. Furthermore, the surfaces of hollow spherical particles comprising of nanoscale constituents are about 2D, so this leads to quantum confinement effect and energy filtering which increase the power factor and phonon scattering at interfaces resulting in a decrease in lattice thermal conductivity. The temperature dependence of Seebeck coefficient and electrical resistivity were measured using a lab-made system in the temperature interval from 300 K to 900 K. Seebeck coefficients of the samples having 2.3 mm, 1.5 mm and 500 nm diameter at 900 K are 204.8 mV/K, 216.9 mV/K and 228.4 mV/K, respectively. Electrical resistivities of the samples having 2.3 mm, 1.5 mm and 500 nm diameter at 900 K were measured as 18.6 mW.cm, 12.4 mW.cm and 8.1 mW.cm, respectively. Power factors of the samples having 2.3 mm, 1.5 mm and 500 nm diameter at 900 K are 0.23 mW/mK², 0.38 mW/mK² and 0.64 mW/mK², respectively. It was shown that, by reducing the particle size to submicron range, Seebeck coefficient and power factor values were successfully increased together with a decrease in electrical resistivity using our air composite approach employed in this work.

Poly 3,4-ethylenedioxythiophene (PEDOT) is a conducting polymer that has thermoelectric properties that has been studied extensively. The majority of the studies, however, were focused on the thermoelectric properties of PEDOT:PSS. PEDOT:tosulate could exhibit up to two times higher thermoelectric efficiency than PEDOT:PSS but a limited number of studies have been devoted to the optimization of the performance of PEDOT:tosulate films. After a systematic study of the parameters that could affect the efficiency of the films of PEDOT:tosylate (e.g., oxidative status, introduction of defects in the film, secondary dopants, improving thermal conductivity) we have concluded that the incorporation of defects/pores in the PEDOT/tosulate films increase the thermoelectric efficiency of the films significantly (from a ZT value of 0.39 to 1.07). A proof-of-concept, flexible thermoelectric device that contain strips of porous PEDOT/tosulate films as p-type elements and strips of bismuth films as n-type elements was prepared and its performance in realistic conditions was evaluated.

Thermoelectric materials that have a large Seebeck coefficient and electrical conductivity are required to effectively convert waste heat into electricity, but their interdependence makes simultaneous improvement a significant challenge. To address this problem, multilayers of poly(diallyldimethylammonium chloride) (PDDA) and double-walled carbon nanotubes (DWNT), stabilized by KBr-doped poly(3,4-ethylenedixoythiophene):poly(styrene sulfonate) (PEDOT:PSS) were deposited using layer-by-layer assembly (LbL). Doping PEDOT:PSS with KBr before LbL assembly results in a simultaneous increase in the Seebeck coefficient and electrical conductivity. A maximum power factor of 534534 μW m-1 K-2 was achieved after doping with 3 mmol KBr, which is a six-fold improvement compared to the undoped control. This improvement in thermoelectric properties is attributed to charge screening effects between PEDOT and PSS from the added KBr, which leads to a denser deposition of DWNT. This facile salt-doping strategy can be used to prepare high performance PEDOT:PSS-based thermoelectric materials for lightweight, low temperature applications.

Here we will show the use of pro-quinoid moieties like benzobistriazole and benzobisthiadiazole in conjugated polymers as an effective strategy towards high performance solution process thermoelectrics. The pro-quinoid character of these moieties allow efficient doping and the formation of highly delocalized polarons even by sequential solution-phase doping. This led to high electrical conductivities, and enhancement in Seebeck coefficient due to carrier-induced softening could be observed. As a result, power factor approaching 70 μW m^-1 K^-2 was achieved. These results are among the highest for conjugated polymer thermoelectric using solution-phase doping.

Organic semiconductors (OSC) hold tremendous potential to address the demand for cheap and sustainable thermoelectric (TE) materials. OSCs offer a major advantage with their inherently low thermal conductivity, but have to be heavily doped to improve the electrical conductivity which has the undesirable effect of significantly reducing their Seebeck coefficient. Hence efficient design of organic TEs requires deep understanding of the role of doping and charge transport dynamics on the Seebeck vs. electrical conductivity trends. In this work, we investigate the TE properties of disordered OSCs, and establish the effect of disorder and correlation in energy and position on the Seebeck coefficient, electrical conductivity, and the Lorenz number. We find that the electronic and morphological properties of the polymer alone cannot account for the experimentally observed Seebeck and conductivity trends. Importantly, it is the doping and the clustering of dopants within the polymer films that dictates the slope of the Seebeck vs. conductivity plots. Our charge transport model is based on electron hopping between localized sites with a modified Gaussian disorder model to account for the impact of doping and clustering of dopants on the energies. We iteratively solve the non-linear Pauli’s master equation to compute the time-averaged occupational probabilities of the sites from which relevant transport quantities are calculated. We measure the Seebeck coefficient and electrical conductivity across a broad range of dopings by a de-doping technique where the sample is measured continuously while gradually de-doping over time. We then fit experimentally determined data of iodine-doped P3HT and PDPP4T to our hopping simulations, which revealed that the shape of the DOS is responsible for the observed Seebeck and conductivity trends. The physical distribution of dopant molecules within the sample affects the carrier DOS (fortified by Kelvin probe force microscopy), with dopant clustering dramatically increasing the energetic disorder resulting in a heavy-tailed distribution [3], whereas homogenous doping maintains a Gaussian DOS. This ‘change in shape’ of the DOS results in a qualitative change of the Seebeck vs. conductivity curve. A Gaussian distribution leads to a sharp drop off in Seebeck at high conductivity values, whereas, a heavy-tailed distribution leads to a gradual decrease in Seebeck with increasing conductivity and a flatter curve. This can be correlated to the value of the Kang-Snyder transport parameter ‘s’ [4]. Seebeck vs. conductivity curve from a Gaussian DOS is best fit by a lower value of s (mostly<1), whereas, one from a heavy-tailed DOS distribution can only be partially fit with higher s values, indicating the limitations of a band model in predicting transport in highly disordered systems. Our results highlight the importance of tuning the energetic disorder using spatial distribution of dopants to obtain the highest power factor for TE applications. We conclude that it is not just how much the semiconductor is doped; thermoelectric performance is affected by how the semiconductor has been doped across all carrier concentrations.

REFERENCES: [1] C. J. Boyle, M. Upadhyaya, P. Wang, L. Renna, M. Lu-Díaz, S. P. Jeong, N. Hight-Huf, Lj. Korugic-Karasz, M. Barnes, Z. Aksamija, and D. Venkataraman, accepted in Nat. Comm. (2019). [2] M. Upadhyaya, C. J. Boyle, D. Venkataraman, and Z. Aksamija, Scientific Reports 9, 5820 (2019). [3] V. I. Arkhipov, P. Heremans, E. V. Emelianova, and H. Bässler, Phys. Rev. B 71, 045214 (2005). [4] S. D. Kang & G. J. Snyder, Nat. Mater. 16, 252–257 (2017). [5] H. Abdalla, G. Zuo, and M. Kemerink, Phys. Rev. B 96, 241202 (2017).

Conjugated polymers can be used in mechanically flexible and low cost thermoelectric (TE) devices, but their thermoelectric performance must be improved to make them commercially viable. The performance of thermoelectric materials depends on the electrical conductivity, Seebeck coefficient and thermal conductivity. However, higher electrical conductivity is typically accompanied by a decrease in the Seebeck coefficient. Blending two or more π-conjugated polymers together provides a means of manipulating charge transport properties and potentially improving the performance of organic thermoelectrics by minimizing this tradeoff between electrical conductivity and the Seebeck coefficient. By modifying the model introduced by Bässler and Arkhipov, we model how the electronic properties of the individual polymers influence the Seebeck coefficient, electrical conductivity, and power factor in the polymer blends. These calculations show that gains in the power factor should be attainable when the homogenous blend of two polymers have a small (e.g., 0.1-0.2 eV) offset in their density of states (DOS) distributions and the polymer with the higher energy DOS has a wider DOS distribution and a larger localization length, where the larger localization length translates to higher charge-carrier mobility. Experimentally, power factors in an appropriate polymer blend are demonstrated to exceed the power factors of the individual polymers by nearly two-fold. With the combined theoretical and experimental approach presented, this work provides insight into designing higher performing organic thermoelectric materials based on π-conjugated polymer blends.

Conductive polymers are promising thermoelectric materials, particularly for use in devices where mechanical flexibility or low-cost are primary considerations. Currently, the record power factors of P-type polymer thermoelectric materials are approximately 20 times greater than that of record N-type polymers. We have recently discovered that heavy P-doping of donor-acceptor based conjugated polymers can change the sign of the Seebeck coefficient from positive to negative, which for some polymers can lead to near record N-type power factors. Moderate P-doping of a DPP-containing polymer leads to a P-type power factor of 24.5 µW m^-1 K^-2, while further increases in doping concentration lead to an N-type power factor of 9.2 µW m^-1 K^-2, where 9.2 µW m^-1 K^-2 is a near the record power factor for an N-type donor-acceptor (D-A) conjugated polymer. Additional investigation into the origin of the sign change through a combination of ultraviolet and inverse photoelectron spectroscopy measurements shows that the density of unoccupied and occupied states converges upon heavy doping; thereby giving rise to either positive or negative Seebeck coefficients based on the relative charge-carrier mobilities and density of states near the Fermi energy for electrons and holes. AC Hall effect measurements of heavily doped polymers confirm that the dominant band-like carriers are indeed electrons. This work provides fundamental insight into the thermoelectric behavior of heavily doped conjugated polymers and shows a promising route to developing P- and N-type polymer thermoelectrics based on the same polymer-dopant combination.

Chemical doping in a conjugated polymer always involves redox reaction between a host polymer and guest dopant. Even though physics and chemistry in material science have been evolving rapidly, an efficiency of chemical doping is limited primarily by the redox potential of employed materials, which has been well described by Marcus theory. In contrast, we have successfully overcome this limitation by introducing anion exchange [1]. A thin film of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) is doped by exposing it to solution of tetracyano-2,3,5,6-tetrafluoroquinodimethane (F4TCNQ). When a salt containing bis(trifluoromethane)sulfonamide (TFSI) anion is introduced to this system, the additional TFSI anion is instantaneously exchanged with the F4TCNQ radical anion that forms the intermediate ion-pair [PBTTT⁺ F4TCNQ^●-]. Spontaneous anion exchange (from F4TCNQ^●- to TFSI^-) is confirmed comprehensively by UV-Vis-NIR, FTIR and ESR spectroscopies, where a lower limit of exchange efficiency is determined to be 98 %. Surprisingly, Hall measurements suggest that the efficient anion exchange increases the doping level up to almost one hole per monomer unit, which is three times higher than that of conventional F4TCNQ doping [2]. Promotion of chemical doping is also demonstrated with a weak acceptor tetracyanoquinodimethane (TCNQ). Even though the LUMO level of TCNQ (−4.5 eV) does not exceed the HOMO band edge of PBTTT (−4.8 eV), UV-Vis-NIR confirms a reasonably high doping level achieved by introducing anion exchange.
To examine the observed increase in doping level, one needs to consider equilibrium of doping reaction, where the forward reaction is charge transfer, and the reverse reaction is back-charge transfer. In a standard binary system, charge transfer takes place between neutral PBTTT and F4TCNQ, while back-charge transfer makes ionized [PBTTT⁺ F4TCNQ^●-] back to the neutral state. In a steady state of equilibrium, these charge transfer and back-charge transfer occur at the same rate, where the doping level would not increase any more. Introduction of efficient anion exchange to this system converts [PBTTT⁺ F4TCNQ^●-] to [PBTTT⁺ TFSI^-], which results in suppression of back-charge transfer. Note that [PBTTT⁺ TFSI^-] does not contribute to back-charge transfer because TFSI^- is a stable closed-shell anion. Thus, efficient anion exchange biases the charge transfer equilibrium by suppressing back-charge transfer, so that the doping level increases.
Anion exchange doping provides a platform to explore material science in conjugated materials. In terms of charge transport property, the high doping level of PBTTT thin film results in Hall mobility of 2.0 cm² V^-1 s^-1 at room temperature and weak temperature dependence of it, which manifest that this system is close to onset metallicity. Another perspective would be creation of functional host-guest structures. One example here is that thermal stability is improved remarkably when a closed shell anion is introduced into polymeric semiconductors as a dopant. We believe that this study will be a benchmark for storage, transport and conversion of functional molecules within a solid-state of conjugated materials, which are the essential phenomena in broad fields of material science, e.g. catalytic activity, molecular recognition, and energy conversion.
[1] Y. Yamashita, J. Takeya, S. Watanabe et al., under review.
[2] K. Kang, S. Watanabe, H. Sirringhaus et al., Nat. Mater. 15, 896 (2016).

Carbon nanotubes (CNTs) are expected as a high-performance thermoelectric material because of their one-dimensional nanostructure and their excellent electrical and mechanical properties. However, the thermoelectric performance is not as good as expected when they are assembled into a macroscale film. The main reason is the poor interconnections among CNTs in the film especially when the length of the CNTs is in micrometer scale. In this work, we used the special multiwalled CNTs, which have length in 1-2 mm and diameter less than 10 nm, and we employ polyacrylonitrile (PAN) as precursor to create crosslinks among CNTs after carbonization. The CNT films have improved conductivity and they are elastic and robust. For enhancing the thermoelectric performance, we further infiltrated PAN into the CNT network and investigated the effects of the PAN contents and its different status after stabilization and carbonization through thermal treatments. The results show that a maximum power factor of 13 µW/mK² was obtained from the CNT/PAN composite when the PAN content was 48% and thermally treated at 300 degree in air for 2 hours. A model was built to explain the phenomena.

Silicon-based thermoelectric materials would be of great significance due to the huge dependence of electronic industry on silicon. Bulk silicon is not a good thermoelectric material due to its very high thermal conductivity, thereby limiting its thermoelectric efficiency. Nanostructuring and alloying are alternative solutions to reduce thermal conductivity, but the techniques involved are complex and costly. Recently, a silicon-based chalcogenide Si₂Te₃ has been experimentally synthesized. Si₂Te₃ exhibits layered structure, in which Te atoms form hexagonal sub-lattice and Si atoms can occupy any of the octahedral voids. Due to uncertainty in Si positions, previously unknown ground state structure of Si₂Te₃ was obtained using the Wyckoff positions of space group P-31c. The minimum energy configuration exhibits combination of desirable electronic and transport properties. In particular, n-doped Si₂Te₃ has an unprecedented figure of merit of 1.86 at 1000 K, which is comparable to some of the best state-of-the-art thermoelectric materials. Hence, n-doped Si₂Te₃ can be a long-sought silicon-based thermoelectric material which could be integrated to the existing electronic devices.