Stretchable and Flexible Painted Thermoelectric Generators on Japanese Paper Using Inks Dispersed with P- and N-Type Single-Walled Carbon Nanotubes.

As power sources for Internet-of-Things sensors, thermoelectric generators must exhibit compactness, flexibility, and low manufacturing costs. Stretchable and flexible painted thermoelectric generators were fabricated on Japanese paper using inks with dispersed p- and n-type single-walled carbon nanotubes (SWCNTs). The p- and n-type SWCNT inks were dispersed using the anionic surfactant of sodium dodecylbenzene sulfonate and the cationic surfactant of dimethyldioctadecylammonium chloride, respectively. The bundle diameters of the p- and n-type SWCNT layers painted on Japanese paper differed significantly; however, the crystallinities of both types of layers were almost the same. The thermoelectric properties of both types of layers exhibited mostly the same values at 30 °C; however, the properties, particularly the electrical conductivity, of the n-type layer increased linearly, and of the p-type layer decreased as the temperature increased. The p- and n-type SWCNT inks were used to paint striped patterns on Japanese paper. By folding at the boundaries of the patterns, painted generators can shrink and expand, even on curved surfaces. The painted generator (length: 145 mm, height: 13 mm) exhibited an output voltage of 10.4 mV and a maximum power of 0.21 µW with a temperature difference of 64 K at 120 °C on the hot side.

Self-powered broadband photo-detection and persistent energy generation with junction-free strained Bi2Te3 thin films.

We experimentally demonstrate efficient broadband self-powered photo-detection and power generation in thin films of polycrystalline bismuth telluride (Bi2Te3) semiconductors under inhomogeneous strain. The developed simple, junction-free, lightweight, and flexible photo-detectors are composed of a thin active layer and Ohmic contacts on a flexible plastic substrate, and can operate at room temperature and without application of an external bias voltage. We attribute the observed phenomena to the generation of an electric field due to a spontaneous polarization produced by strain gradient, which can separate both photo-generated and thermally-generated charge carriers in bulk of the semiconductor material, without a semiconductor junction. We show that the developed photo-detectors can generate electric power during both the daytime and the nighttime, by either harnessing solar and thermal radiation or by emitting thermal radiation into the cold sky. To the best of our knowledge, this is the first demonstration of the power generation in a simple junction-free device under negative illumination, which exhibits higher voltage than the previously used expensive commercial HgCdTe photo-diode. Significant improvements in the photo-detector performance are expected if the low-charge-mobility polycrystalline active layer is replaced with high-quality single-crystal material. The technology is not limited to Bi2Te3 as the active material, and offers many potential applications in night vision, wearable sensors, long-range LIDAR, and daytime/nighttime energy generation technologies.

Thermoelectric properties of nanocrystalline Sb2Te3 thin films: experimental evaluation and first-principles calculation, addressing effect of crystal grain size.

The effect of crystal grain size on the thermoelectric properties of nanocrystalline antimony telluride (Sb2Te3) thin films was investigated by experiments and first-principles studies using a developed relaxation time approximation. The Sb2Te3 thin films were deposited on glass substrates using radio-frequency magnetron sputtering. To change the crystal grain size of the Sb2Te3 thin films, thermal annealing was performed at different temperatures. The crystal grain size, lattice parameter, and crystal orientation of the thin films were estimated using XRD patterns. The carrier concentration and in-plane thermoelectric properties of the thin films were measured at room temperature. A theoretical analysis was performed using a first-principles study based on density functional theory. The electronic band structures of Sb2Te3 were calculated using different lattice parameters, and the thermoelectric properties were predicted based on the semi-classical Boltzmann transport equation in the relaxation time approximation. In particular, we introduced the effect of carrier scattering at the grain boundaries into the relaxation time approximation by estimating the group velocities from the electronic band structures. Finally, the experimentally measured thermoelectric properties were compared with those obtained by calculation. As a result, the calculated thermoelectric properties were found to be in good agreement with the experimental results. Therefore, we can conclude that introducing the effect of carrier scattering at the grain boundaries into the relaxation time approximation contributes to enhance the accuracy of a first-principles calculation relating to nanocrystalline materials.

Structural, optical, and transport properties of nanocrystalline bismuth telluride thin films treated with homogeneous electron beam irradiation and thermal annealing.

We investigated the effects of homogeneous electron beam (EB) irradiation and thermal annealing treatments on the structural, optical, and transport properties of bismuth telluride thin films. Bismuth telluride thin films were prepared by an RF magnetron sputtering method at room temperature. After deposition, the films were treated with homogeneous EB irradiation, thermal annealing, or a combination of both the treatments (two-step treatment). We employed Williamson-Hall analysis for separating the strain contribution from the crystallite domain contribution in the x-ray diffraction data of the films. We found that strain was induced in the thin films by EB irradiation and was relieved by thermal annealing. The crystal orientation along c-axis was significantly enhanced by the two-step treatment. Scanning electron microscopy indicated the melting and aggregation of nano-sized grains on the film surface by the two-step treatment. Optical analysis indicated that the interband transition of all the thin films was possibly of the indirect type, and that thermal annealing and two-step treatment methods increased the band gap of the films due to relaxation of the strain. Thermoelectric performance was significantly improved by the two-step treatment. The power factor reached a value of 17.2 µW (cm(-1) K(-2)), approximately 10 times higher than that of the as-deposited thin films. We conclude that improving the crystal orientation and relaxing the strain resulted in enhanced thermoelectric performance.

Sb2Te3 nanoparticle-containing single-walled carbon nanotube films coated with Sb2Te3 electrodeposited layers for thermoelectric applications.

Single-walled carbon nanotubes (SWCNTs) are promising thermoelectric materials owing to their flexibility and excellent durability when exposed to heat and chemicals. Thus, they are expected to be used in power supplies for various sensors. However, their thermoelectric performances are inferior to those of inorganic thermoelectric materials. To improve the thermoelectric performance while maintaining the excellent characteristics of SWCNTs, a novel approach to form inorganic thermoelectric layers on the SWCNT bundle surfaces using electrodeposition is proposed. We synthesized Sb2Te3 nanoparticle-containing SWCNT films and coated them with electrodeposited Sb2Te3 layers. The Sb2Te3 nanoparticles were synthesized via a spontaneous redox reaction, which were then added to a SWCNT dispersion solution, and films were produced via vacuum filtration. At higher nanoparticle contents in the films, the Sb2Te3 electrodeposited layers completely covered the SWCNT bundles owing to the increase in the concentration of precursor ions near the SWCNT bundle surface, which in turn was the result of melted nanoparticles. The thermoelectric performance improved, and the maximum power factor at approximately 25 °C was 59.5 µW/(m K2), which was 4.7 times higher than that of the normal SWCNT film. These findings provide valuable insights for designing and fabricating high-performance flexible thermoelectric materials.

High thermoelectric performance of flexible nanocomposite films based on Bi2Te3 nanoplates and carbon nanotubes selected using ultracentrifugation.

Thermoelectric generators with flexibility and high performance near 300 K have the potential to be employed in self-supporting power supplies for Internet of Things (IoT) devices. Bismuth telluride (Bi2Te3) exhibits high thermoelectric performance, and single-walled carbon nanotubes (SWCNTs) show excellent flexibility. Therefore, composites of Bi2Te3 and SWCNTs should exhibit an optimal structure and high performance. In this study, flexible nanocomposite films based on Bi2Te3 nanoplates and SWCNTs were prepared by drop casting on a flexible sheet, followed by thermal annealing. Bi2Te3 nanoplates were synthesized using the solvothermal method, and SWCNTs were synthesized using the super-growth method. To improve the thermoelectric properties of the SWCNTs, ultracentrifugation with a surfactant was performed to selectively obtain suitable SWCNTs. This process selects thin and long SWCNTs but does not consider the crystallinity, chirality distribution, and diameters. A film consisting of Bi2Te3 nanoplates and the thin and long SWCNTs exhibited high electrical conductivity, which was six times higher than that of a film with SWCNTs obtained without ultracentrifugation; this is because the SWCNTs uniformly connected the surrounding nanoplates. The power factor was 6.3 µW/(cm K2), revealing that this is one of the best-performing flexible nanocomposite films. The findings of this study can support the application of flexible nanocomposite films in thermoelectric generators to provide self-supporting power supplies for IoT devices.

Determination of Seebeck coefficient originating from phonon-drag effect using Si single crystals at different carrier densities.

The phonon-drag effect is useful for improving the thermoelectric performance, especially the Seebeck coefficient. Therefore, the phonon and electron transport properties of Si single crystals at different carrier densities were investigated, and the relationship between these properties and the phonon-drag effect was clarified. Phonon transport properties were determined using nanoindentation and spot-periodic heating radiation thermometry. The electron transport properties were determined based on the electrical conductivity of Si. The diffusive Seebeck coefficient derived from the electron transport properties was in good agreement with previous reports. However, the value of the phonon-drag Seebeck coefficient derived from the phonon transport properties is very low. This phenomenon suggests that phonons with a normal mean free path (MFP) do not contribute to the increase in the Seebeck coefficient; however, phonons with a long MFP and low frequency increase the Seebeck coefficient via the phonon-drag effect. Moreover, the phonon-drag effect was sufficiently pronounced even at 300 K and in the heavily doped region. These features are key in designing thermoelectric materials with enhanced performance derived from the phonon-drag effect.

Ultra-long air-stability of n-type carbon nanotube films with low thermal conductivity and all-carbon thermoelectric generators.

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

Heat source free water floating carbon nanotube thermoelectric generators.

Thermoelectric generators (TEGs) produce electric power from environmental heat energy and are expected to play a key role in powering the Internet of things. However, they require a heat source to create a stable and irreversible temperature gradient. Overcoming these restrictions will allow the use of TEGs to proliferate. Therefore, we propose heat source-free water-floating carbon nanotube (CNT) TEGs. Output voltage and power are generated by the temperature gradient in the CNT films in which water pumping via capillary action leads to evaporation-induced cooling in selected areas. Furthermore, the output voltage and power increase when the films are exposed to sunlight and wind flow. These water-floating CNT TEGs demonstrate a pathway for developing wireless monitoring systems for water environments.

Origin of n type properties in single wall carbon nanotube films with anionic surfactants investigated by experimental and theoretical analyses.

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.

Flexible thermoelectric films formed using integrated nanocomposites with single-wall carbon nanotubes and Bi2Te3 nanoplates via solvothermal synthesis.

Single-wall carbon nanotubes (SWCNTs) and Bi2Te3 nanoplates are very promising thermoelectric materials for energy harvesting. When these two materials are combined, the resulting nanocomposites exhibit high thermoelectric performance and excellent flexibility. However, simple mixing of these materials is not effective in realizing high performance. Therefore, we fabricated integrated nanocomposites by adding SWCNTs during solvothermal synthesis for the crystallization of Bi2Te3 nanoplates and prepared flexible integrated nanocomposite films by drop-casting. The integrated nanocomposite films exhibited high electrical conductivity and an n-type Seebeck coefficient owing to the low contact resistance between the nanoplates and SWCNTs. The maximum power factor was 1.38 µW/(cm K2), which was 23 times higher than that of a simple nanocomposite film formed by mixing SWCNTs during drop-casting, but excluding solvothermal synthesis. Moreover, the integrated nanocomposite films maintained their thermoelectric properties through 500 bending cycles.

Facile preparation of air-stable n-type thermoelectric single-wall carbon nanotube films with anionic surfactants.

Thermoelectric generators based on single-wall carbon nanotubes (SWCNTs) have great potential for use in wearable and skin electronics because of their lightweight and mechanically soft structure. However, the fabrication of air-stable n-type thermoelectric SWCNTs using conventional processes is challenging. Herein, we propose a facile process for fabricating air-stable n-type SWCNT films with anionic surfactants via drop casting followed by heat treatment. We examined different surfactants (Sodium Dodecyl Sulfate, Sodium Dodecylbenzene Sulfonate, and Sodium Cholate) and heat-treatment temperatures. The optimal SWCNT film maintained the n-type Seebeck coefficient for 35 days. Moreover, to further extend the n-type Seebeck coefficient maintenance, we periodically reheated the SWCNT film with a surfactant that had returned to the p-type Seebeck coefficient. The reheated film recovered the n-type Seebeck coefficient, and the effect of the reheating treatment lasted for several reheating cycles. Finally, we elucidated a simple mechanism for realizing an air-stable n-type Seebeck coefficient based on spectroscopic analyses of the SWCNT films.

Solvothermal synthesis of n-type Bi2(SexTe1-x)3 nanoplates for high-performance thermoelectric thin films on flexible substrates.

To improve thermoelectric performance of materials, the utilization of low-dimensional materials with a multi-alloy system is a promising approach. We report on the enhanced thermoelectric properties of n-type Bi2(SexTe1-x)3 nanoplates using solvothermal synthesis by tuning the composition of selenium (Se). Variation of the Se composition within nanoplates is demonstrated using X-ray diffraction and electron probe microanalysis. The calculated lattice parameters closely followed Vegard's law. However, when the Se composition was extremely high, an impurity phase was observed. At a reduced Se composition, regular-hexagonal-shaped nanoplates with a size of approximately 500 nm were produced. When the Se composition was increased, the shape distribution became random with sizes more than 5 µm. To measure the thermoelectric properties, nanoplate thin films (NPTs) were formed on a flexible substrate using drop-casting, followed by thermal annealing. The resulting NPTs sufficiently adhered to the substrate during the bending condition. The electrical conductivity of the NPTs increased with an increase in the Se composition, but it rapidly decreased at an extremely high Se composition because of the presence of the impurity phase. As a result, the Bi2(SexTe1-x)3 NPTs exhibited the highest power factor of 4.1 µW/(cm∙K2) at a Se composition of x = 0.75. Therefore, it was demonstrated that the thermoelectric performance of Bi2(SexTe1-x)3 nanoplates can be improved by tuning the Se composition.

Growth of single-crystalline Bi2Te3 hexagonal nanoplates with and without single nanopores during temperature-controlled solvothermal synthesis.

Bismuth telluride (Bi2Te3) is a promising thermoelectric material for applications near room temperature. To increase the thermoelectric performance of this material, its dimensions and thermal transport should be decreased. Two-dimensional nanoplates with nanopores are an ideal structure because thermal transport is disrupted by nanopores. We prepared Bi2Te3 nanoplates with single nanopores by a solvothermal synthesis and investigated their structural and crystallographic properties. The nanoplates synthesized at a lower reaction temperature (190 °C) developed single nanopores (approximately 20 nm in diameter), whereas the nanoplates synthesized at a higher reaction temperature (200 °C) did not have nanopores. A crystal growth mechanism is proposed based on the experimental observations.

Effects of different electrolytes and film thicknesses on structural and thermoelectric properties of electropolymerized poly(3,4-ethylenedioxythiophene) films.

The effects of the type of electrolyte and film thickness on the structural and thermoelectric properties of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films on indium-tin-oxide (ITO) substrates prepared using electropolymerization were investigated. Two electrolytes were prepared using two different solvents: a water/methanol solvent (protic solvent) and acetonitrile (aprotic solvent) with 3,4-ethylenedioxythiophene (EDOT) and LiCF3SO3, typically included in electrolytes as dopants. The electrochemical properties of the two electrolytes were analyzed; it was found that the polymerization process for EDOT on an ITO substrate varied based on the electrolyte used. When the electropolymerization time was increased, the surface morphology of the PEDOT films prepared using the water/methanol solvent appeared to contain grains approximately 100 nm in size whereas the PEDOT films prepared using acetonitrile appeared to contain aggregated grains connected by polymeric networks. Even though there were differences in the surface morphology and chemical bonds determined using Fourier-transform infrared spectroscopy/attenuated total reflectance analysis, the thermoelectric properties were strongly dependent on the film thickness and were only weakly dependent on the type of electrolyte used. The highest power factor was 41.3 µW (m-1 K-2) for a PEDOT film with a thickness of 0.5 µm prepared using the water/methanol solvent electrolyte.