Highly Flexible and Durable Thermoelectric Power Generator Using CNT/PDMS Foam by Rapid Solvent Evaporation.

Using a simple, rapid solvent evaporation process, the authors produced 3D carbon nanotube (CNT)/polydimethylsiloxane (PDMS) foams with both high thermoelectric (TE) and good mechanical performance and used them to fabricate highly flexible and durable TE generators. The numerous pores and interfaces in the CNT/PDMS foams, which have porosities exceeding 87%, afford very low thermal conductivity of 0.13 W m-1 K-1 . The foam has a zT value of 6.6 × 10-3 , which is twice as high as that of pristine CNT foam. Importantly, the CNT/PDMS foam exhibits good tensile strength of 0.78 MPa, elongation greater than 20%, and excellent resilience even at compressive strain of 80%. This foam is used to fabricate a highly flexible TE foam generator that exhibits a moderate output power of 1.9 µW generated from the large temperature gradient of 18.1 K produced by applied heat. The authors also demonstrate a practical TE foam generator that produces sustainable output power of 3.1 µW under a compressive strain of 80% and 38.2 nW under the continuous vibrational stress produced by a car engine. The TE foam generator also exhibits excellent stability and durability under cyclic bending and harsh vibrational stress.

Microstructure and Thermoelectric Characterization of Composite Nanofiber Webs Derived from Polyacrylonitrile and Sodium Cobalt Oxide Precursors.

We report the microstructure and thermoelectric properties of composite nanofiber webs, which were fabricated by dual-electrospinning of polyacrylonitrile (PAN) and sodium cobalt oxide (NaCo2O4) precursor solutions with different input compositions and following heat-treatment at 600-900 °C for simultaneous carbonation and calcination. The SEM and EDS mapping images revealed that PAN-derived carbon nanofibers (CNFs) and NaCo2O4-based ceramic nanofibers coexisted in the composite nanofiber webs and that their relative contents could be controlled by the input compositions. The Seebeck coefficient increased from ~26.77 to ~73.28 µV/K and from ~14.83 to ~40.56 µV/K with increasing the relative content of NaCo2O4 nanofibers in the composite nanofiber webs fabricated at 700 and 800 °C, respectively. On the other hand, the electrical conductivity of the composite nanofiber webs increased with the decrement of the relative content of NaCo2O4 nanofibers as well as the increment of the heat-treatment temperature. Owing to the opposite contributions of NaCo2O4 nanofibers and CNFs to the Seebeck coefficient, electrical conductivity and thermal conductivity, a maximum power factor of ~5.79 µW/mK2 and a figure of merit of ~0.01 were attained for CNF/NaCo2O4-based composite nanofiber webs fabricated at 45 wt% input composition of NaCo2O4 and at heat-treatment of 700 °C.

Proton Conducting Perhydropolysilazane-Derived Gate Dielectric for Solution-Processed Metal Oxide-Based Thin-Film Transistors.

Perhydropolysilazane (PHPS), an inorganic polymer composed of Si-N and Si-H, has attracted much attention as a precursor for gate dielectrics of thin-film transistors (TFTs) due to its facile processing even at a relatively low temperature. However, an in-depth understanding of the tunable dielectric behavior of PHPS-derived dielectrics and their effects on TFT device performance is still lacking. In this study, the PHPS-derived dielectric films formed at different annealing temperatures have been used as the gate dielectric layer for solution-processed indium zinc oxide (IZO) TFTs. Notably, the IZO TFTs fabricated on PHPS annealed at 350 °C exhibit mobility as high as 118 cm2 V-1 s-1, which is about 50 times the IZO TFTs made on typical SiO2 dielectrics. The outstanding electrical performance is possible because of the exceptional capacitance of PHPS-derived dielectric caused by the limited hydrolysis reaction of PHPS at a low processing temperature (<400 °C). According to our analysis, the exceptional dielectric behavior is originated from the electric double layer formed by mobile of protons in the low temperature-annealed PHPS dielectrics. Furthermore, proton conduction through the PHPS dielectric occurs through a three-dimensional pathway by a hopping mechanism, which allows uniform polarization of the dielectric even at room temperature, leading to amplified performance of the IZO TFTs.

Thermoelectric fibers from well-dispersed carbon nanotube/poly(vinyliedene fluoride) pastes for fiber-based thermoelectric generators.

High-performance thermoelectric composite fibers were prepared via simple wet-spinning of single-walled carbon nanotube (SWCNT)/poly(vinylidene fluoride) (PVDF) pastes using a common solvent/coagulation system. By improving the content and dispersion state of SWCNTs in the composite fibers, the thermoelectric performance could be effectively enhanced. With n-type doping of SWCNTs using polyethylenimine, high-performance n-type SWCNT/PVDF composite fibers could be prepared. The power factors of the p- and n-type SWCNT/PVDF composite fibers with the SWCNT content of 50 wt% were 378 ± 56 and 289 ± 98 µW m-1 K-2, respectively. The electric power generation capability of an organic thermoelectric generator with the p- and n-type composite fibers was confirmed.

High Thermoelectric Power Factor of a Diketopyrrolopyrrole-Based Low Bandgap Polymer via Finely Tuned Doping Engineering.

We studied the thermoelectric properties of a diketopyrrolopyrrole-based semiconductor (PDPP3T) via a precisely tuned doping process using Iron (III) chloride. In particular, the doping states of PDPP3T film were linearly controlled depending on the dopant concentration. The outstanding Seebeck coefficient of PDPP3T assisted the excellent power factors (PFs) over 200 µW m-1K-2 at the broad range of doping concentration (3-8 mM) and the maximum PF reached up to 276 µW m-1K-2, which is much higher than that of poly(3-hexylthiophene), 56 µW m-1K-2. The high-mobility of PDPP3T was beneficial to enhance the electrical conductivity and the low level of total dopant volume was important to maintain high Seebeck coefficients. In addition, the low bandgap PDPP3T polymer effiectively shifted its absorption into near infra-red area and became more colorless after doping, which is great advantage to realize transparent electronic devices. Our results give importance guidance to develop thermoelectric semiconducting polymers and we suggest that the use of low bandgap and high-mobility polymers, and the accurate control of the doping levels are key factors for obtaining the high thermoelectric PF.

Foldable Thermoelectric Materials: Improvement of the Thermoelectric Performance of Directly Spun CNT Webs by Individual Control of Electrical and Thermal Conductivity.

We suggest the fabrication of foldable thermoelectric (TE) materials by embedding conducting polymers into Au-doped CNT webs. The CNT bundles, which are interconnected by a direct spinning method to form 3D networks without interfacial contact resistance, provide both high electrical conductivity and high carrier mobility. The ZT value of the spun CNT web is significantly enhanced through two simple processes. Decorating the porous CNT webs with Au nanoparticles increases the electrical conductivity, resulting in an optimal ZT of 0.163, which represents a more than 2-fold improvement compared to the ZT of pristine CNT webs (0.079). After decoration, polyaniline (PANI) is integrated into the Au-doped CNT webs both to improve the Seebeck coefficient by an energy-filtering effect and to decrease the thermal conductivity by the phonon-scattering effect. This leads to a ZT of 0.203, which is one of the highest ZT values reported for organic TE materials. Moreover, Au-doped CNT/PANI web is ultralightweight, free-standing, thermally stable, and mechanically robust, which makes it a viable candidate for a hybrid TE conversion device for wearable electronics. When a 20 K temperature gradient is applied to the TE module consisting of seven p-n couples, 1.74 µW of power is generated.

Facile Preparation of Highly Conductive Metal Oxides by Self-Combustion for Solution-Processed Thermoelectric Generators.

Highly conductive indium zinc oxide (IZO) thin films were successfully fabricated via a self-combustion reaction for application in solution-processed thermoelectric devices. Self-combustion efficiently facilitates the conversion of soluble precursors into metal oxides by lowering the required annealing temperature of oxide films, which leads to considerable enhancement of the electrical conductivity of IZO thin films. Such enhanced electrical conductivity induced by exothermic heat from a combustion reaction consequently yields high performance IZO thermoelectric films. In addition, the effect of the composition ratio of In to Zn precursors on the electrical and thermoelectric properties of the IZO thin films was investigated. IZO thin films with a composition ratio of In:Zn = 6:2 at the low annealing temperature of 350 °C showed an enhanced electrical conductivity, Seebeck coefficient, and power factor of 327 S cm(-1), 50.6 µV K(-1), and 83.8 µW m(-1) K(-2), respectively. Moreover, the IZO thin film prepared at an even lower temperature of 300 °C retained a large power factor of 78.7 µW m(-1) K(-2) with an electrical conductivity of 168 S cm(-1). Using the combustive IZO precursor, a thermoelectric generator consisting of 15 legs was fabricated by a printing process. The thermoelectric array generated a thermoelectric voltage of 4.95 mV at a low temperature difference (5 °C). We suggest that the highly conductive IZO thin films by self-combustion may be utilized for fabricating n-type flexible printed thermoelectric devices.

Enhancement of Thermoelectric Properties of PEDOT:PSS and Tellurium-PEDOT:PSS Hybrid Composites by Simple Chemical Treatment.

The thermoelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) and tellurium- PEDOT: PSS (Te- PEDOT: PSS) hybrid composites were enhanced via simple chemical treatment. The performance of thermoelectric materials is determined by their electrical conductivity, thermal conductivity, and Seebeck coefficient. Significant enhancement of the electrical conductivity of PEDOT: PSS and Te- PEDOT: PSS hybrid composites from 787.99 and 11.01 to 4839.92 and 334.68 S cm(-1), respectively was achieved by simple chemical treatment with H2SO4. The power factor of the developed materials could be effectively tuned over a very wide range depending on the concentration of the H2SO4 solution used in the chemical treatment. The power factors of the developed thermoelectric materials were optimized to 51.85 and 284 µW m(-1) K(-2), respectively, which represent an increase of four orders of magnitude relative to the corresponding parameters of the untreated thermoelectric materials. Using the Te- PEDOT: PSS hybrid composites, a flexible thermoelectric generator that could be embedded in textiles was fabricated by a printing process. This thermoelectric array generates a thermoelectric voltage of 2 mV using human body heat.

Solution synthesis of telluride-based nano-barbell structures coated with PEDOT:PSS for spray-printed thermoelectric generators.

Solution-processable telluride-based heterostructures coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (Te-Bi2Te3/PEDOT:PSS) were synthesized through a solution-phase reaction at low temperatures. The water-based synthesis yielded PEDOT: PSS-coated Te-Bi2Te3 nano-barbell structures with a high Seebeck coefficient that can be stably dispersed in water. These hybrid solutions were deposited onto a substrate by the spray-printing method to prepare thermoelectric generators. The thermoelectric properties of the Te-Bi2Te3/PEDOT:PSS hybrid films were significantly enhanced by a simple acid treatment due to the increased electrical conductivity, and the power factor of those materials can be effectively tuned over a wide range depending on the acid concentration of the treatment. The power factors of the synthesized Te-Bi2Te3/PEDOT:PSS hybrids were optimized to 60.05 µW m(-1) K(-2) with a Seebeck coefficient of 93.63 µV K(-1) and an electrical conductivity of 69.99 S cm(-1). The flexible thermoelectric generator fabricated by spray-printing Te-Bi2Te3/PEDOT:PSS hybrid solutions showed an open-circuit voltage of 1.54 mV with six legs at ΔT = 10 °C. This approach presents the potential for realizing printing-processable hybrid thermoelectric materials for application in flexible thermoelectric generators.

Enhanced thermoelectric performance of bar-coated SWCNT/P3HT thin films.

The influence of processing conditions, such as ink concentration and coating method, on the thermoelectric properties of SWCNT/P3HT nanocomposite films was investigated systematically. Using simple wire-bar-coating, SWCNT/P3HT nanocomposite films with high thermoelectric performance could be obtained without additional P3HT doping. The wire-bar-coated SWCNT/P3HT nanocomposite films exhibited power factors of up to 105 µW m(-1) K(-2) at room temperature. The SWCNT bundles with diameters in the range of 6-23 nm formed an interconnected network in the wire-bar-coated nanocomposite films. Network formation in these nanocomposite films was expected to be strongly related to the development of electrical pathways due to inter-SWCNT bundle connections. This study suggests that the thermoelectric performance of SWCNT/P3HT nanocomposite films could be optimized by controlling their processing conditions and morphology.

Ambient atmosphere-processable, printable Cu electrodes for flexible device applications: structural welding on a millisecond timescale of surface oxide-free Cu nanoparticles.

Recently, various functional devices based on printing technologies have been of paramount interest, owing to their characteristic processing advantages along with excellent device performance. In particular, printable metallic electrodes have drawn attention in a variety of optoelectronic applications; however, research into printable metallic nanoparticles has been limited mainly to the case of an environmentally stable Ag phase. Despite its earth-abundance and highly conductive nature, the Cu phase, to date, has not been exploited as an ambient atmosphere-processable, printable material due to its critical oxidation problem in air. In this study, we demonstrate a facile route for generating highly conductive, flexible Cu electrodes in air by introducing the well-optimized photonic sintering at a time frame of 10(-3) s, at which the photon energy, rather than conventional thermal energy, is instantly provided. It is elucidated here how the surface oxide-free, printed Cu particulate films undergo chemical structural/microstructural evolution depending on the instantly irradiated photon energy, and a successful demonstration is provided of large-area, flexible, printed Cu conductors on various substrates, including polyimide (PI), polyethersulfone (PES), polyethylene terephthalate (PET), and paper. The applicability of the resulting printed Cu electrodes is evaluated via implementation into both flexible capacitor devices and indium-gallium-zinc oxide (IGZO) flexible thin-film transistors.

Resist-free antireflective nanostructured film fabricated by thermal-NIL.

Resist-free antireflective (AR) nanostructured films are directly fabricated on polycarbonate (PC) film using thermal-nanoimprint lithography (T-NIL) and the moth-eye shape of AR nanostructure is elaborately optimized with different oxygen reactive ion etching conditions. Anodic aluminum oxide (AAO) templates are directly used as master molds of T-NIL for preparation of AR nanostructures on PC film without an additional T-NIL resist. AR nanostructures are well arranged with a period of about 200 nm and diameter of about 150 nm, which corresponds to those of the AAO template mold. The moth-eye AR nanostructures exhibit the average reflectance of 2% in wavelength range from 400 to 800 nm. From the results, highly enhanced AR properties with simple direct imprinting on PC film demonstrate the potential for panel application in the field of flat display, touch screen, and solar cells.

Method for synthesis of tetrabenzoporphyrin precursor for use in organic electronic devices.

We developed a new synthetic method for bicyclo[2.2.2]octadiene (BCOD)-fused porphyrin (1), a tetrabenzoporphyrin (TBP, 2) precursor that is well-known as a good material for use in organic electronic devices. The newly developed method synthesizes the BCOD-fused pyrrole intermediate (which is the most important intermediate in synthesizing BCOD-fused porphyrin) in a simpler and easier manner than other existing methods, and thus, the new method can efficiently synthesize the TBP precursor.