Wonseok Cho

Effects of one- and two-dimensional carbon hybridization of PEDOT: PSS on the power factor of polymer thermoelectric energy conversion devices

We investigated the thermoelectric properties of polymer composites based on a conducting polymer and carbon materials with various dimensionalities. PEDOT:PSS as a conducting polymer matrix was successfully hybridized with graphene sheets and multi-walled carbon nanotubes through in situ polymerization of 3,4-ethlyenedioxythiophene monomers in an aqueous solution in the presence of the carbon materials dispersed by using a polymeric dispersant. The hybrid structures of PEDOT:PSS, graphene, and carbon nanotubes in the composite showed an electrical conductivity, Seebeck coefficient, and power factor of 689 S cm−1, 23.2 μV K−1, and 37.08 μW mK−2, respectively, values that are much higher than those of pristine PEDOT:PSS, PEDOT:PSS/graphene, or PEDOT:PSS/carbon-nanotube composites. The thermoelectric figure of merit increased from 0.017 in the pristine PEDOT:PSS to 0.031 in the composite, corresponding to an 80% enhancement. We believe that the enhanced thermoelectric performance comes from the synergic effects of multi-component systems with excellent electrical bridging and electronic coupling between PEDOT and carbon materials.

Aqueous chemical synthesis of tellurium nanowires using a polymeric template for thermoelectric materials

We report the simple synthesis of tellurium nanowires (TeNWs) by a one-pot scale-up hydrothermal process. A clean wet-chemical method, employing telluric acid (Te(OH)6) as a source of tellurium, ascorbic acid as a weak reducing agent, and a linear polymer as a template, has been developed for the synthesis of TeNWs with a diameter of 30–140 nm and a length of several micrometers at 105 °C. A possible explanation for the one-dimensional growth of TeNWs is the linear polymer template. The effects of the concentration of the polymeric template on the nanowire morphology were investigated. Evaluation of the thermoelectric properties indicated a high Seebeck coefficient and a power factor of about 568 μV K−1 and 8.44 μW mK−2, respectively, of the optimized TeNW films, and these values were about 80 times larger than those of the TeNW films formed without a polymeric template. We expect this simple process to be widely applicable for large-scale production of one-dimensional inorganic nanomaterials for energy harvesting and electronic devices.

Photothermal ablation of cancer cells using self-doped polyaniline nanoparticles.

Water-stable confined self-doping polyaniline nanocomplexes are successfully fabricated by nano-assembly using lauric acid both as a stabilizer and as a localized dopant. In particular, the colloidal stability of the polyaniline nanocomplexes in neutral pH and the photothermal potential by near-infrared light irradiation are characterized. We demonstrate that confined self-doping polyaniline nanocomplexes as a photothermal nanoagent are preserved in the doped state even at a neutral pH. Finally, confined self-doping polyaniline nanocomplexes aided by lauric acid are successfully applied for the photothermal ablation of cancer cells.

Synthesis of conductive and transparent PEDOT:P(SS-co-PEGMA) with excellent water-, weather-, and chemical-stabilities for organic solar cells

The water-, weather- and chemical-resistant conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)-co-poly(ethylene glycol methacrylate) (PEDOT:P(SS-co-PEGMA)) copolymer was successfully synthesized with thermally curable P(SS-co-PEGMA) copolymers. The PSS and P(SS-co-PEGMA) copolymers were synthesized by solution polymerization and PEDOT:PSS and PEDOT:P(SS-co-PEGMA) were synthesized by Fe+-catalyzed oxidative polymerization. PSS and P(SS-co-PEGMA) were characterized by Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). The electrical properties of the conductive PEDOT:P(SS-co-PEGMA) thin films were characterized in two parts; first, the mechanism and characterization of the conductivity change, and second, the characterization of the water-, chemical-, and weather-stability of the films. The conductivity and transmittance, respectively, of PEDOT:P(SS-co-PEGMA) at 550 nm under optimized conditions were maintained at the levels found in PEDOT:PSS, 160.3 S cm−1 and 86.7%. The introduction of PEGMA to the PSS copolymer improved the mechanical properties and weather stability. The PEDOT:P(SS-co-PEGMA) was highly stable to chemical solvents and independent of the type of solvents used for stability analysis. The conductivity in the weather stability test of PEDOT:PSS decreased by 44.9%, on the other hand, the conductivity of PEDOT:P(SS-co-PEGMA) was decreased by only 22.2%. The PEDOT:PSS and PEDOT:P(SS-co-PEGMA) copolymers were used as buffer layers in organic solar cells (OSC) and showed as high efficiency as conventional PEDOT:PSS materials. The decrease of OSC efficiency with PEDOT:P(SS-co-PEGMA) was 30% less than the OSCs with the commercial and reference PEDOT:PSS buffer layers.

Multi-purpose overcoating layers based on PVA/silane hybrid composites for highly transparent, flexible, and durable AgNW/PEDOT:PSS films

Flexible hybrid overcoating layers with antireflective properties are fabricated by a catalyzed two-step sol–gel process using poly(vinyl alcohol) (PVA) and silane precursors, followed by simple bar coating. Hydrolysis of alkoxysilanes followed by the condensation reaction forms chemical bonds between Si–OH and C–OH groups. A glass substrate with the hybrid overcoating layer shows higher transmittance than a bare glass substrate because the overcoating provides an antireflective layer with an intermediate refractive index. Haziness and surface roughness of the hybrid overcoating decrease with increasing PVA content. Further, complexation of PVA and silanes yields greater mechanical flexibility. Various silanes functionalized with methyl, epoxy, amino, and phenyl groups are also incorporated into the hybrid overcoating to adjust the hydrophobicity, transparency, and protective properties. A methyltrimethoxysilane (MTMS)-based hybrid overcoating shows the best optical transparency and water repellency. Application of a PVA/MTMS hybrid overcoating over silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conducting films dramatically improves the long-term stability of the sheet resistance through enhanced resistance to moisture penetration. Consequently, highly stable and durable AgNW/PEDOT:PSS conducting films (sheet resistance, 20 Ω sq−1; transmittance, 95.4%) are fabricated by passivating them on both sides with the hybrid overcoating.

A gas sensor using double split-ring resonator coated with conducting polymer at microwave frequncies

In this research, a gas sensor using double split-ring resonator (DSRR) incorporated with conducting polymer (CP) is proposed at microwave frequencies (Ku-band). The DSRR fabricated on printed circuit board (PCB) is excited by a high-impedance microstrip line, and the CP is coated inside of an inner circle of the DSRR. Electrical characteristics of the CP can be deviated by an interaction between CP and a target gas, and then this deviation of electrical characteristic is demonstrated by S21 frequency response of the DSRR. To examine the performance of the proposed sensor, 100 ppm ethanol (C2H5OH) gas is exposed at room temperature. According to the measured result, the S21 resonance frequency of the DSRR is shifted by 220 MHz and simultaneously, the resonance amplitude is changed by 0.79 dB level. It is clearly found that the DSRR with CP material can be a good candidate for a sensitive gas sensor operating at microwave frequencies.

Large-scalable RTCVD Graphene/PEDOT:PSS hybrid conductive film for application in transparent and flexible thermoelectric nanogenerators

Poly(3,4-ethyldioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), as an thermoelectric(TE) material, exhibits a high electrical conductivity and ZT value (10−1–100). Nevertheless, a low thermovoltage of the organic thermoelectric materials must be overcome, in comparison to that of semi metals. Recently, to address these challenges, several researchers have investigated PEDOT:PSS/carbon material composites. Herein, a transparent and flexible hybrid film made up of rapid thermal chemical vapor deposition (RTCVD) graphene and PEDOT:PSS results in enhanced TE performance. The PEDOT:PSS was synthesized by oxidative polymerization, and the hybrid process of the graphene film and PEDOT:PSS film was conducted using the layer-by-layer method. The results of AFM and Raman spectroscopy revealed that the synergistic effect through composite films improved the electrical properties. The maximum electrical conductivity and power factor of the RTCVD graphene/PEDOT:PSS (RCG/P) hybrid film were 1096 S cm−1 and 57.9 μW m−1 K−2, respectively. In addition, the RCG/P hybrid film exhibited excellent mechanical flexibility and stability.

Modification of heat storage ability and adhesive properties of core/shell structured phase change material nanocapsules

Phase change material-polystyrene (PCM-PSt) nanocapsules were prepared via a modified resin-fortified miniemulsion (RFME) polymerization process using an alkali soluble resin (ASR). Poly(styrene-co-acrylic acid) (SAA), which is a functional amphiphilic polymer, was used as the surfactant for the resin-fortified emulsion polymerization. A co-surfactant and a crosslinker were adopted to improve the PCM encapsulation efficiency. The average particle size and heat capacity of the optimized PCM-PSt nanocapsules were about ∼280 nm as measured by dynamic light scattering (DLS) and ∼110 J/g as measured by differential scanning calorimetry (DSC), respectively. The morphology and the inner structure of the nanocapsules were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The synthesized nanocapsules showed good adhesive and thermal storage properties, and were amenable for processing by dip-coating methods.

Influence of residual sodium ions on the structure and properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising conducting polymer in terms of its applicability to transparent and flexible electronic devices. Generally, a negatively charged PSS chain can interact with alkali metal cations like sodium and potassium. During polymerization, these ions, especially sodium ions, remain in an aqueous state and affect particle formation. This paper describes the effect of residual sodium ions on the synthesis of PEDOT:PSS and its electrical and optical properties. Removing the sodium ions weakens the coulombic interaction between the PEDOT and PSS chains, which leads to a linear conformation. This conformational change enhances the electrical conductivity and work function. Furthermore, transmittance in the visible region increased remarkably because the intrinsic electrical properties of the PEDOT:PSS particles were improved. Moreover, the colloidal stability was enhanced because the particle coagulation caused by residual sodium ions was reduced. In summary, we determined that sodium ions in PEDOT:PSS have a considerable influence on its electrical and optical properties and colloidal stability for practical applications.

Electrical characteristics of heterogeneous polymer layers in PEDOT:PSS films

(3,4-Ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a representative conducting polymer, is environment-friendly and offers easy processing and flexibility owing to its hydro-dispersive properties. In this study, we investigated the electrical properties of PEDOT:PSS films with H2SO4/DMSO post-treatment. PEDOT:PSS is an electrolyte complex that has been attracting attention as a next-generation transparent electrode. The PEDOT:PSS films are composed of two heterogeneous phases: a PSS-rich layer and a PEDOT-rich layer. The PSS-rich layer is observed to be a homogeneous phase that does not influence the electrical properties. Thus, the PSS-rich layer, which occupies a large area of the PEDOT:PSS films, was removed to obtain films having high electrical properties. Both the layers exhibit a homogeneous phase regardless of their electrical properties. In particular, the PSS-rich layer without the PEDOT chains does not affect the electrical properties, since it does not contribute to hole transport. The heterogeneity of the PEDOT:PSS films with good electrical properties has been revealed by eliminating the unnecessary PSS-rich layers. The highest electrical conductivity obtained was 2239 S cm−1, which is about 4.1 times higher than that of the pristine PEDOT:PSS films, for the films involving 15 M H2SO4/DMSO post-treatment.