Haekyoung Kim

Al2O3/TiO2 nanolaminate thin film encapsulation for organic thin film transistors via plasma-enhanced atomic layer deposition.

Organic electronic devices require a passivation layer that protects the active layers from moisture and oxygen because most organic materials are very sensitive to such gases. Passivation films for the encapsulation of organic electronic devices need excellent stability and mechanical properties. Although Al2O3 films obtained with plasma enhanced atomic layer deposition (PEALD) have been tested as passivation layers because of their excellent gas barrier properties, amorphous Al2O3 films are significantly corroded by water. In this study, we examined the deformation of PEALD Al2O3 films when immersed in water and attempted to fabricate a corrosion-resistant passivation film by using a PEALD-based Al2O3/TiO2 nanolamination (NL) technique. Our Al2O3/TiO2 NL films were found to exhibit excellent water anticorrosion and low gas permeation and require only low-temperature processing (<100 °C). Organic thin film transistors with excellent air-stability (52 days under high humidity (a relative humidity of 90% and a temperature of 38 °C)) were fabricated.

The Origin of Excellent Gate‐Bias Stress Stability in Organic Field‐Effect Transistors Employing Fluorinated‐Polymer Gate Dielectrics

Tuning of the energetic barriers to charge transfer at the semiconductor/dielectric interface in organic field-effect transistors (OFETs) is achieved by varying the dielectric functionality. Based on this, the correlation between the magnitude of the energy barrier and the gate-bias stress stability of the OFETs is demonstrated, and the origin of the excellent device stability of OFETs employing fluorinated dielectrics is revealed.

National-wide data on the treatment of BPH in Korea

The healthcare system in Korea provides coverage to all the people who are residing in Korea, so the data of the Korea healthcare system are national-wide and relatively accurate. We obtained the recent 5-year data (2004–2008) on the treatment of BPH from the national health insurance system. We tried to determine the trends or changes of BPH treatments in Korea. Over 3.8 million men visited clinics and were prescribed one or more BPH medications, and more than 44 000 men underwent surgical treatment during 2004–2008. Compared with the year 2004, two times the patients were prescribed BPH medications in 2008. With respect to the surgical treatment, the number of cases was increased 1.6 times in 2006 compared with the previous years. The most commonly used surgical option was TURP before 2006, but laser therapy was carried out as much as TURP in 2006 and in the following years. The relative risk of laser therapy in the 50 s is 1.53 (95% CI is 1.47–1.59). In conclusion, our national-wide data for the Korean BPH patients show that these patients’ medical treatment increased during the 5 years from 2004 to 2008. Laser treatment had increased and it might replace TURP in several years.

Fluorinated Polyimide Gate Dielectrics for the Advancing the Electrical Stability of Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) that operated with good electrical stability were prepared by synthesizing fluorinated polyimide (PI) gate dielectrics based on 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI. 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI contain 6 and 18 fluorine atoms per repeat unit, respectively. These fluorinated polymers provided smooth surface topographies and surface energies that decreased as the number of fluorine atoms in the polymer backbone increased. These properties led to a better crystalline morphology in the semiconductor film grown over their surfaces. The number of fluorine atoms in the PI backbone increased, the field-effect mobility improved, and the threshold voltage shifted toward positive values (from -0.38 to +2.21 V) in the OFETs with pentacene and triethylsilylethynyl anthradithiophene. In addition, the highly fluorinated polyimide dielectric showed negligible hysteresis and a notable gate bias stability under both a N2 environment and ambient air.

3D Hollow Framework Silver Nanowire Electrodes for High-Performance Bottom-Contact Organic Transistors

We successfully fabricated high performance bottom-contact organic field-effect transistors (OFETs) using silver nanowire (AgNW) network electrodes by spray deposition. The synthesized AgNWs have the dimensions of 40-80 nm in diameter and 30-80 μm in length and are randomly distributed and interconnected to form a 3D hollow framework. The AgNWs networks, deposited by spray coating, yield an average optical transmittance of up to 88% and a sheet resistance as low as 10 ohm/sq. For using AgNWs as source/drain electrodes of OFETs with a bottom-contact configuration, the large contact resistance at the AgNWs/organic channel remains a critical issue for charge injection. To enhance charge injection, we fabricate semiconductor crystals on the AgNW using an adsorbed residual poly(N-vinylpyrrolidone) layer. The resulting bottom-contact OFETs exhibit high mobility up to 1.02 cm(2)/(V s) and are similar to that of the top-contact Au electrodes OFETs with low contact resistance. A morphological study shows that the pentacene crystals coalesced to form continuous morphology on the nanowires and are highly interconnected with those on the channel. These features contribute to efficient charge injection and encourage the improvement of the bottom-contact device performance. Furthermore, the large contact area of individual AgNWs spreading out to the channel at the edge of the electrode also improves device performance.

Electrostatic-Force-Assisted Dispensing Printing of Electrochromic Gels for Low-Voltage Displays

In this study, low-voltage, printed, ion gel-based electrochromic devices (ECDs) were successfully fabricated. While conventional dispensing printing provides irregularly printed electrochromic (EC) gels, we improved the adhesion between the printed gel and the substrate by applying an external voltage. This is called electrostatic-force-assisted dispensing printing. As a result, we obtained well-defined, printed, EC gels on substrates such as indium tin oxide-coated glass. We fabricated a gel-based ECD by simply sandwiching the printed EC gel between two transparent electrodes. The resulting ECD, which required a low coloration voltage (∼0.6 V), exhibited a high coloration efficiency (η) of 161 cm2/C and a large transmittance contrast (∼82%) between the bleached and colored states at -0.7 V. In addition, electrostatic-force-assisted dispensing printing was utilized to fabricate directly patterned ECDs.

Low-voltage, simple WO3-based electrochromic devices by directly incorporating an anodic species into the electrolyte

Low-voltage tungsten trioxide (WO3)-based electrochromic devices (ECDs) are successfully demonstrated. In contrast to conventional ECDs based on metal oxides, we directly introduce ferrocene (Fc) as an anodic species into the electrolyte layer. Accordingly, ECDs in this work do not require secondary electrochromic materials on the counter electrode, and thus device configuration and the fabrication process are simplified. The coloration and bleaching behaviors of the device occur in the voltage range of 0 V to −1.5 V with a maximum transmittance contrast (ΔTmax) of ∼52% at 700 nm. In addition, we systematically investigated a correlation between device performance and Fc concentration in the electrolyte layer. The optimized Fc concentration is 0.05 M, at which the coloration efficiency (η) was ∼34.3 cm2 C−1. Furthermore, adding Fc to the electrolyte results in better cyclic (coloration/bleaching) stability.

Hybrid membranes with low permeability for vanadium redox flow batteries using in situ sol-gel process

Vanadium redox flow batteries (VRFBs) have been researched as large energy storage systems due to their long cycle life, high energy efficiency, low cost, and flexible design. However, cation exchange membranes are permeable to the vanadium ions in aqueous acidic electrolyte, and vanadium ions crossover reduces the efficiency and capacity of VRFBs. To improve membrane selectivity, proton conducting inorganic materials are proposed for the modification of conventional membranes, e.g., Nafion. Clusters inside Nafion membrane are filled with inorganic materials using in situ sol-gel processes, and this results in homogeneous distribution of inorganic materials. Hybrid membranes with Nafion 115 (coded as HN115) exhibit comparable ionic conductivity and a 70% reduced permeability to vanadium ions compared with pristine Nafion 115 (coded as N115). The columbic and energy efficiencies of VRFBs with HN115 at 20mA·cm−2 exhibit higher values of 95% and 80% in their columbic and energy efficiencies, respectively; VRFBs with N115 exhibit 78% and 70%, respectively. The capacity performance is also improved when HN115 is used in VRFBs. The VRFBs with hybrid membranes (lower permeable membrane) show higher columbic efficiency than the VRFB with N115. HN115 exhibit similar columbic efficiency values of 95% over entire current ranges, which are almost unrelated to the current density. However, N115 shows a fluctuating and lower columbic efficiency of 75%, 88%, 93% at 20mA·cm−2, 40mA·cm−2, 80mA·cm−2, respectively. VRFB with N115 (high conductive membrane) exhibits lower voltage drops for discharging and higher energy efficiency at high current ranges. With these results, it is proposed that the energy efficiencies of VRFBs are compromised with membrane conductivity and permeability. The columbic efficiencies are more contributed by membrane permeability. The permeability properties are more dominant in low current density and the ionic conductivity is more effective in high current ranges. To obtain higher performance of VRFBs, the membrane design for selectivity should be considered according to the operation conditions.

Enhancement of electrical conductivity of silver nanowires-networked films via the addition of Cs-added TiO2

This study proposes a novel method of improving the electrical conductivity of silver nanowires (NWs)-networked films for the application of transparent conductive electrodes. We applied Cs-added TiO2 (TiO2:Cs) nanoparticles onto Ag NWs, which caused the NWs to be neatly welded together through local melting at the junctions, according to our transmission and scanning electron microscopy analyses. Systematic comparison of the sheet resistance of the samples reveals that these welded NWs yielded a significant improvement in conductivity. OLED devices, fabricated by using the NW film planarized via embedding the wires into PMMA, demonstrated device performance was comparable with the reference sample with indium tin oxide electrode.

Critical Issues in the Commercialization of DMFC and Role of Membranes

Mobile telecommunication devices in the next generation require a new concept of quick charging and a long-lasting mobile energy source. The direct methanol fuel cell (DMFC) is becoming attractive, but there are critical issues involved in its commercialization with regard to the core technologies of catalyst, membrane, membrane electrode assembly (MEA), stack, and system. More importantly, the main role of the proton-conducting membrane is enhancing the energy and power density and affecting the other components in DMFC systems. Functions, current status, and technical approaches are discussed in terms of protonic conductivity, methanol permeability, water permeability, life cycle, and processing cost as well as interaction with other compartments. Materials such as perfluorinated and partially fluorinated membranes, hydrocarbon membranes, composite membranes, and other modified ionomers have been studied in connection with technology roadmap of membrane and mobile DMFC systems. These would explain the critical issues of DMFC and the role of membranes for commercialization.