Hwa-Min Kim

Reversible Electrical and Optical Behaviors due to Excess Oxygen in RF-Sputtered In2O3–ZnO Films

Thin films are prepared by rf-magnetron sputtering using a In2O3–ZnO powder target with a Zn content [Zn/(Zn+In)] of 33%. Their electrical and optical properties are strongly dependent on the oxygen concentration on the grain boundary surface of the films during heat treatment. The oxygen adsorption begins to occur around 400°C in air and the excess oxygen is almost desorbed around 400°C in vacuum. Therefore, the electrical and optical behaviors can be reversibly controlled by the desorption of the oxygen corresponding to the heat treatment environment.

A Stable Super-Hydrophobic and Self-Cleaning Al Surface Formed by Using Roughness Combined with Hydrophobic Coatings

A stable super-hydrophobic and self-cleaning surface with a high contact angle above 150° and the contact angle hysteresis below 3° were produced by using roughness combined with hydrophobic coatings, which were also confirmed to exhibit extreme water repellence and self-cleaning properties which facilitate removal of contaminant particles. To fabricate micro-patterened aluminium surface, the aluminium was etched with hydrochloric acid. A very thin polytetrafluoroethylene (PTFE) film was coated on the etched aluminium surface in order to achieve the super-hydrophobic surface. Roughness factors on the aluminium surfaces coated with PTFE were calculated as a function of etching time, based on the homogeneous interface (Wensel model). A critical roughness factor on the super-hydrophobic surface of the aluminium coated with PTFE was found to be 1.51 which can be formed on the surface of the aluminium etched over 10 min with hydrochloric acid of 7 wt.%.

Enhanced Hydrophilic Property of TiO 2 Thin Film Deposited on Glass Etched with O 2 Plasma

TiO2 films were deposited on glass substrates with and without O2 plasma etching by using the RF-magnetron sputtering method. We focused on the effect of surface structure on the photoinduced hydrophilic properties of TiO2 films, fabricated on different surface conditions according to the presence or absence of the O2 plasma treatment on glass substrates. The wettability and photoinduced hydrophilic properties of the TiO2 films were investigated according to the changes in water contact angles under UV light irradiations with a very low intensity of 0.1 mW/cm 2 . The photoinduced hydrophilic properties on the TiO2 formed above the plasma treated glass were also superior to those on the TiO2 formed above the bare glass. This enhanced TiO2 film has been used practically for self cleaning and anti-fogging glasses.

Theoretical Comparison of Electronic and Lattice Thermal Conductivities in n-type (Bi1-xSbx)2Te3 Solid Solutions

We have theoretically investigated the electronic and lattice thermal conductivities for various carrier concentrations in the n-type (Bi1-xSbx)2Te3 solid solutions using the two-band conduction model with a mixed scattering mechanism. For operation of thermoelectric cooling at low temperatures, the carrier concentration must be controlled so that the electronic thermal conductivity makes the total thermal conductivity have the minimum at an applicable temperature. Under this condition, the electronic thermal conductivity is over 25% of the lattice thermal conductivity and results in the gradual increase of the thermal conductivity. These effects are almost independent of the composition x under the same carrier concentration.

Surface-Modified TiO2 Photoelectrode for More Efficient Dye-Sensitized Solar Cells

The time dependence in the surface treatment of TiO2 photoelectrode on the photovoltaic properties of dye-sensitized solar cells (DSSCs) was studided. Nanoporous TiO2 electrodes were modified with aqueous sodium sulfate (Na2SO4) solution (0.05 M) by a dip coating process at varied dipping time, and the resulting electrodes were applied to DSSCs. Fill factor values were significantly decreased with increasing dipping time, and short circuit current values were ranged from 18.11 to 21.53 mA/cm2. However, there were no meaningful changes in open circuit voltage. The power conversion efficiency (PCE) of 6.59 ∼ 9.35% was obtained from the DSSCs with Na2SO4-modified TiO2 photoelectrode. Even small differences in dipping time, PCEs were significantly changed.

Preparation of Transparent Metal Films, Titanium-Doped Zinc-Oxide Films (TiO2)2 (ZnO)98 on PEN by Using a RF-Magnetron Sputtering Method

In this work, (TiO2)x (ZnO)100-x (TZO) films were prepared on glass substrate at room temperature by RF-magnetron sputtering. The TZO film with x = 2 wt.% shows a very low resistivity of 4.7 × 10−4 Ω·cm which is comparable to that of ITO films and a high transmittance over 85% in the visible range. In particular, TiO2 (2wt.%)-doped ZnO (TZO) films with thicknesses ranging from 100 ∼ 500 nm were also prepared on Polyethylene naphthalate (PEN) substrates under various RF-powers. Their electrical property were investigated as a function of RF-power. This property was found to be closely related with the crystallization and density of TZO films. It was also noted that vaporization of the water and other adsorbed particles, such as organic solvents contained in most plastic substrate affect the properties of the TCO films.

Electrical Stabilities of In2O3-ZnO-SnO2 (IZTO) Films Against the External Stresses

ABSTRACT The (In2O3)77.34(ZnO)x(SnO2)22.66-x (denote to IZTO-2) films with x = 13, 15, 17, 19 wt.% were prepared on PET film with a 100-nm-thick SiO2 buffer layer by using RF-magnetron sputtering method, and their electrical stabilities were tested under various external stresses such as moist heat, temperature fluctuations and bending of substrate films. The lowest resistivity of 4.8 × 10−4 Ω•cm was observed in the IZTO-2 film of x = 17 wt.% which also had such good properties as high electrical stability, surface uniformity, and high conductivity as compared with those of another (In2O3)90(ZnO)7(SnO2)3 (IZTO-1) film. These results suggest that the optimum starting composition for IZTO film with high quality as a flexible transparent conducting oxide(TCO) film is In2O3 : ZnO : SnO2 = 77.34 : 17 : 5.66 wt.%. The electrical stability of the optimum IZTO-2 film was greatly improved because the SiO2 buffer layer introduced between IZTO thin film and PET substrate acted as a blocking barrier to water vapour and organic solvents diffusing from the PET film and also as a buffer layer withstanding the bending damage.

Increased Power Conversion Efficiency of Dye-Sensitized Solar Cells with Surface-Modified Photoelectrode

A nanoporous TiO2 film was modified with an aqueous potassium chloride (KCl), and the resulting film was applied to the photoelectrode of dye-sensitizes solar cell (DSSC). The DSSC with KCl-modified photoelctrode showed an increase in short circuit current (Jsc) and open circuit voltage (Voc), resulting in a power conversion efficiency (PCE) of 7.04%, compared to that (6.36%) of reference device with bare TiO2. The suppression of charge recombination between photo-injected electrons and triiodide (I3−) ions was found to increase both Jsc and Voc of the device with KCl-modified photoelctrode.

Preparation of Transparent Metal Films, Gallium-Doped Zinc-Oxide (Ga2O3)x (ZnO)100-x Films by Using Facing Target Sputtering System

(Ga2O3)x(ZnO)100-x, x = 3, 5, 7, 9 wt.%, (GZO) films were prepared at room temperature by using a conventional rf-magnetron sputtering method. Their electrical resistivity was investigated as a function of the Ga2O3 content. The GZO film with x = 7 wt.%, shows the lowest resistivity of 1.5×10−3Ω. cm. This GZO films were also prepared at various substrate temperatures from room temperature to 400°C, Their electrical resistivity was found to be improved as the substrate temperature was increased, A very low resistivity of 4.5×10−4Ω.cm was obtained in the film prepared at the substrate temperature of 300°C. In addition, we found that the GZO films prepared by using facing target sputtering (FTS) system showed a dramatically improved conductivity as compared with that of the film prepared by the conventional sputtering system. In particular, we also found that the lowest resistivity of 2.8×10−4Ω.cm that is almost comparable with that of ITO film was obtained in the GZO films prepared at the substrate temperature of 300°C by using the facing target sputtering (FTS) system.

Homogeneous Al2O3 Multilayered Films Deposited on Poly(ethylene terephthalate) by Facing-Target Sputtering System for Thin-Film Passivation of Organic Light-Emitting Diodes

Homogeneous multilayered barrier films were fabricated by means of reactive and nonreactive processes using Al2O3 with the facing-target sputtering (FTS) system. The multilayered films showed 60% improved barrier performance and their fabrication was 30% faster than that of single Al2O3 layers of the same thickness. The water vapor transmission rate was increased up to the order of 10-4 gm-2d-1 from a three-pair system of reactive and nonreactive sputtered Al2O3 bilayers.