Song Yun Cho

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.

Chemically exfoliated transition metal dichalcogenide nanosheet-based wearable thermoelectric generators

To utilize human heat energy as a permanent power source, we demonstrate, for the first time, an intrinsically highly foldable and stretchable thermoelectric generator that is based upon chemically exfoliated 1T-transition metal dichalcogenide (TMDC) nanosheets (NSs) for self-powered wearable electronics. The power factors of WS2 (n-type) and NbSe2 (p-type) NS films were evaluated to be 5–7 μ K−2 m−1 and 26–34 μW K−2 m−1, respectively, near room temperature. With these films, parallel-connected thermoelectric generators that were fabricated were able to constantly produce up to 38 nW of output power at Δ60 K. The thermoelectric device stably sustained its performance, even after 100 bending cycles and after 100 stretching cycles (50% strain). By direct observation, we found that the film is highly stretched by partial tearing and folding but still maintains an electrical percolation pathway. The morphology then is quickly recovered by a plug-in contact between the torn parts as the external strain is released. Finally, we demonstrate the electric power generation from a prototype wearable thermoelectric generator that was woven into a wristband fitted on a real human body.

Inkjet-printed organic thin film transistors based on TIPS pentacene with insulating polymers

The blending of the crystalline organic semiconductor, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), with amorphous polymers exhibits not only excellent solution processability, but also superior performance characteristics in organic thin film transistors (OTFTs). To understand the inkjet-printing behavior of TIPS pentacene/polymer blends, we use amorphous polycarbonate (APC), which is structurally beneficial to the facile phase separation of TIPS pentacene crystals due to the strong segregation strength estimated by the Flory–Huggins interaction parameter. The various inkjet-printing behaviors of TIPS pentacene/APC inks, which depend on the TIPS pentacene/APC compositions, ink viscosities, and different solvent mixtures, are investigated. These behaviors can ultimately determine the phase separation, morphology, shape, and orientation of the TIPS pentacene crystals in OTFT films. Flory–Huggins phase separation theory is applied, and various analytical methods, such as polarized optical microscopy, 3D surface profile, and time-of-flight secondary ion mass spectroscopy (TOF-SIMS), are utilized to explain these relationships. By controlling these inkjet-printing conditions, it is possible to easily regulate the optimal inkjet-printing process for TIPS-pentacene/polymer systems, which can derive the desirable stripe-shaped and vertically phase-separated TIPS pentacene crystals with the proper orientation and enhanced surface morphology. The resultant inkjet-printed films from the TIPS pentacene with APC show excellent device stability and an average mobility of 0.53 cm2 V−1 s−1. Furthermore, the inkjet-printed flexible OTFT array with an average mobility of 0.27 cm2 V−1 s−1 sustains the application of TIPS pentacene/APC in the field of flexible printed electronics.

Effective doping by spin-coating and enhanced thermoelectric power factors in SWCNT/P3HT hybrid films

This study investigates a coating method for doping a single-walled carbon nanotube (SWCNT)/poly(3-hexylthiophene) (P3HT) hybrid film. In the hybrid film, P3HT chains were very highly doped by simple spin-coating of a FeCl3/nitromethane solution. Hybrid films doped by spin-coating exhibited power factors of 267 ± 38 µW m−1 K−2, which is an improvement over that of hybrid films doped by conventional immersion, 103 ± 24 µW m−1 K−2. The excellent thermoelectric performance is originated from the dramatically increased electrical conductivity by the sufficient doping of P3HT in the hybrid films.

Spray-printed CNT/P3HT organic thermoelectric films and power generators

This study demonstrates the fabrication of high-performance thermoelectric carbon nanotube/poly(3-hexylthiophene) (CNT/P3HT) nanocomposite films and flexible CNT/P3HT organic thermoelectric generators (OTEGs) by spray-printing. The spray-printed few-walled CNT/P3HT nanocomposite films exhibited excellent thermoelectric properties. The Seebeck coefficient, electrical conductivity, and power factor of the nanocomposite films were 97 ± 11 μV K−1, 345 ± 88 S cm−1, and 325 ± 101 μW m−1 K−2, respectively, at room temperature. We fabricated flexible OTEGs solely from p-type CNT/P3HT nanocomposite patterns spray-printed on a polyimide substrate, and confirmed their electric power generation capabilities.

Two-component solution processing of oxide semiconductors for thin-film transistors via self-combustion reaction

Indium zinc oxide (IZO) thin films were fabricated via self-combustion of In and Zn salts coordinated with fuel and oxidizer ligands. The intense heat generated from the exothermic reaction compensated for the energy required for oxide formation and reduced the temperature required to anneal the oxide films. Thermal analysis of the fuel and oxidizer precursors confirmed the generation of exothermic heat at a relatively low annealing temperature. With the aid of the internal energy that evolved as heat from the combustion reaction, the formation of the metal–oxygen–metal lattice and the removal of organic ligands could be easily accomplished with lower amounts of externally supplied energy. IZO thin-film transistors (TFTs), obtained from this combustive In–Zn pair at a low annealing temperature of 350 °C, showed a significantly enhanced field-effect mobility of 13.8 cm2 V−1 s−1 and a high on/off current ratio of 1.06 × 108. Inkjet printing of the combustive precursors yielded TFTs with a high field-effect mobility of 5.3 cm2 V−1 s−1 and an on/off ratio of 106. The high performance, good device uniformity, and high yield of TFTs fabricated by our self-combustion method demonstrate the potential of the proposed system to facilitate the processing of flexible printed electronics.

Soluble oxide gate dielectrics prepared using the self-combustion reaction for high-performance thin-film transistors

We report the fabrication of high-performance metal oxide thin-film transistors (TFTs) with AlOx gate dielectrics using combustion chemistry in a solution process to provide energy to convert oxide precursors into oxides at low temperatures. Our self-combustion system utilizing two Al precursors as a fuel and an oxidizer is systematically compared with conventional combustive Al precursors with urea in terms of combustion efficiency and dielectric properties. AlOx gate dielectric layers are spin-coated from a solution of combustive AlOx precursors in 2-methoxyethanol and annealed at 250 °C. In this process, organic compounds can be introduced to improve the resulting layer morphology. The thermal behaviors of the self-combustive AlOx precursors are investigated and compared with those of noncombustive AlOx precursors and combustive urea–AlOx precursors to evaluate the generation of exothermic heat at a relatively low annealing temperature. The AlOx dielectrics prepared from self-combustive precursors have uniform and smooth surfaces, low leakage current densities, and high dielectric constants above 8.7. They also exhibit excellent insulating properties and no breakdown at high electric fields. Furthermore, the ZnO TFTs prepared to confirm the operation of the AlOx gate dielectrics show a good mobility of 24.7 cm2 V−1 s−1 and an on/off ratio of 105. It is believed that the AlOx dielectrics prepared using the self-combustion reaction at a low temperature can form a good interface facilitating the growth of desirable ZnO crystal structures, which leads to a considerable improvement in ZnO TFT performance.

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.

Improving the thermoelectric power factor of CNT/PEDOT:PSS nanocomposite films by ethylene glycol treatment

This study investigates a post treatment method for improving the thermoelectric properties of carbon nanotube (CNT)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) nanocomposite films. Ethylene glycol (EG) treatment is performed by immersing the as-prepared CNT/PEDOT:PSS nanocomposite films in EG for 1 h followed by annealing at 140 °C for 10 min. Non-complexed PSS chains could be effectively removed using the EG treatment, resulting in a decrease in inter-bundle distances in the nanocomposite films. This drastically increases the electrical conductivity of the PEDOT:PSS matrix. EG-treated nanocomposite films with 20 wt% CNTs exhibited the highest power factor of 151 ± 34 μW m−1 K−2 among all the prepared samples. Our results indicate that the EG treatment is a promising method for fabricating cost-effective, high-performance thermoelectric CNT/PEDOT:PSS nanocomposite films. We fabricated an organic thermoelectric generator using EG-treated CNT/PEDOT:PSS nanocomposite patterns and evaluated its electric power generation capabilities.

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.