Young-Baek Kim

Transparent conductive ZnInSnO–Ag–ZnInSnO multilayer films for polymer dispersed liquid-crystal based smart windows

Multilayer transparent films with electrical resistances lower than those in conventionally used transparent conductive electrodes were prepared at room temperature on glass substrates in an RF/DC magnetron sputtering system. The multilayer structure of the films consisted of three layers, ZnInSnO (ZITO)–Ag–ZITO. The optical and electrical properties of the multilayer structures were investigated with respect to the thickness of each ZITO–Ag–ZITO layer. Transparent conductive films with a sheet resistance of 9.4 Ω/square and an average transmittance of 92% at 550 nm were obtained at the following thicknesses of the glass substrate: ZITO (100 nm)–Ag (8 nm)–ZITO (42 nm). The surface roughness (RRMS) of the obtained ZITO–Ag–ZITO multilayer films was below 0.8 nm. Overall, the properties of the ZITO–Ag–ZITO multilayer films were comparable or superior to those of other multilayers such as InSnO (ITO)–Ag–ITO and InZnO (IZO)–Ag–IZO. The deposited ZITO single layer and ZITO–Ag–ZITO multilayer films were used in the fabrication of polymer-dispersed liquid-crystal (PDLC)-based smart windows. The ZITO–Ag–ZITO multilayer-based smart windows exhibited a lower operating voltage (16 V) and a higher cutoff rate of infrared light than ITO or ZITO-based smart windows 20–26 V. However, they showed a lower PDLC-ON transmittance than ITO-based smart windows.

Fabrication of Micro-sized Al/Cu Clad by Repeated Hydrostatic Extrusion and Its Mechanical Properties

Al/Cu clads were fabricated by hydrostatic extrusion at 523 K, and their microstructure and mechanical properties were evaluated. The cell size of the Al/Cu clads decreases from 12 mm to 600 µm after three passes of extrusion. Intermetallic compounds (IMCs) appeared at the interface of Al/Cu clad during annealing at 683 K, and their sizes increased as a function of the annealing process. X-ray microanalysis and scanning electron microscope studies showed that the intermetallic species were CuAl2, CuAl, and Cu3Al2, and the thickness of the IMC layers increased at different rates depending on the annealing time. The formation and growth of IMCs resulted in an increase of yield strength and a decrease in elongation of the clads under tensile testing.

Characteristics of CIGS thin films prepared by RF sputtering employing CIGS single target

CuIn1-xGaxSe2 (CIGS) films were prepared by a RF sputtering system using a CIGS single target having a composition of CuIn0.75Ga0.25Se2. X-ray diffraction measurements confirmed that CIGS films grown on Mo-coated soda-lime glass at 350 °C exhibited only (112) diffraction, while CIGS films annealed at 550 °C and for 30 min in rapid thermal annealing (RTA) chamber showed (112), (220), (312), and other diffraction peaks of the chalcopyrite structure. The CIGS films annealed showed much higher intensity of (112) diffraction than that of the films grown at 350 °C, demonstrating improvement of crystal-quality of the films. However, no peaks originated from other phase were observed. The average composition of the CIGS films determined by energy dispersive x-ray spectrometer (EDX) was in good agreement with that of the target. Furthermore, secondary ion mass spectrometry (SIMS) analysis revealed that RTA treatment for Mo layer prior to CIGS film deposition suppresses the inter-diffusion of In, Ga, and Mo at the interface. These results demonstrate that RF sputtering of CIGS single target can be a promising method to fabricate high-quality CIGS films and heat treatment of Mo layer is indispensible to control the interface of CIGS/Mo.

Organic light-emitting devices based on solution-processable small molecular emissive layers doped with interface-engineering additives

In this study, we investigate small molecular organic light-emitting diodes (SM-OLEDs) consisting of emission layers (EMLs) fabricated using a solution-coating process of self-metered horizontal dip- (H-dip-) coating. The EML used was composed of a co-mixed small molecular host matrix of hole-transporting 4,4′,4′′-tris(9-carbazolyl)-triphenylamine (TcTa) and electron-transporting 2,7-bis (diphenyl phosphoryl)-9,9′-spirobifluorene (SPPO13) doped with blue-, green-, and/or red-emitting phosphorescent iridium complexes. To improve the electron-injecting and hole-blocking properties at the cathode interface and to enhance the film-forming capabilities, an interface-engineering additive of poly(oxyethylene tridecyl ether) (PTE) was mixed with the small molecular EMLs. Using PTE additives was shown to reduce dramatically the formation of film defects such as nano-pinholes in the EMLs, resulting in thin and homogeneous PTE-mixed EMLs with smooth surface morphologies, even when using a single H-dip-coating process. The use of simple H-dip-coated EMLs mixed with PTEs in SM-OLEDs resulted in good device performance, with maximum luminance levels of 29 200 cd m−2, 115 000 cd m−2, and 16 400 cd m−2, with corresponding peak current efficiencies of 18.8 cd A−1, 31.2 cd A−1, and 10.0 cd A−1, for blue, green, and red SM-OLEDs, respectively. Furthermore, we demonstrated the feasibility of fabricating large-area and high-performance solution-processable SM-OLEDs using H-dip-coated EMLs doped with PTEs. These results clearly indicate that H-dip-coated small molecular EMLs mixed with PTE can be used to yield simple, bright, and efficient solution-processable SM-OLEDs.

Electrical and optical properties of compositionally spread MoInSnO thin films deposited by combinatorial RF magnetron cosputtering

Using a combinatorial cosputtering method, we have deposited MoInSnO transparent conducting oxide (TCO) films at various gas ratios, constituent compositions, and film thicknesses to control the transparency window in the infrared and ultraviolet wavelength regions. The MoInSnO thin films showed a minimum sheet resistance of 8.25 Ω/ and a transmittance of 90% (at 550 nm) when deposited at a substrate temperature of 350 °C, a film thickness of 298 nm, and an elemental composition ratio of 2.8/84.2/13.0 at. % (Mo/In/Sn, at. %). The carrier concentration of a MoInSnO film deposited in an Ar gas atmosphere was higher than that of an InSnO (ITO) film. The MoInSnO film also showed a shift of the transparency window to the short-wavelength region in the infrared range compared with the ITO film. However, the film resistivity increased and the transparency window shifted to the long-wavelength region with increasing O2 gas ratio. The absorption edge of the MoInSnO film in the ultraviolet region shifted to the short-wavelength region with increasing Mo content in the film.

Characteristics of Sputtered Cr Thin Films and Application as a Working Electrode in Transparent Conductive Oxide-Less Dye-Sensitized Solar Cells

Cr metal electrode was suggested as the working electrode material to fabricate DSSCs without the TCO, and thin films were fabricated by an unbalanced magnetron sputtering system. The surface morphologies show uniform and smooth surfaces regardless of various film thicknesses, and the small crystallites of various sizes were showed with the vertical direction on the surface of Cr thin films with the increase of film thickness. And also, the root mean square (RMS) surface roughness value of Cr thin films increased, and the sheet resistance is decreased with the increase of film thickness. The maximum cell efficiency of the TCO-less DSSC was observed when a Cr working electrode with a thickness of 80 nm was applied to the TCO-less DSSC. Consequently, these results are related to the result of the optimization of conduction characteristics, transmission properties and surface properties of Cr thin films.

A Simple Separation Method of the Protein and Polystyrene Bead-Labeled Protein for Enhancing the Performance of Fluorescent Sensor

Dielectrophoresis- (DEP-) based separation method between a protein, amyloid beta 42, and polystyrene (PS) beads in different microholes was demonstrated for enhancement of performance for bead-based fluorescent sensor. An intensity of ∇|E|2 was relative to a diameter of a microhole, and the diameters of two microholes for separation between the protein and PS beads were simulated to 3 μm and 15 μm, respectively. The microholes were fabricated by microelectromechanical systems (MEMS). The separation between the protein and the PS beads was demonstrated by comparing the average intensity of fluorescence (AIF) by each molecule. Relative AIF was measured in various applying voltage and time conditions, and the conditions for allocating the PS beads into 15 μm hole were optimized at 80 mV and 15 min, respectively. In the optimized condition, the relative AIF was observed approximately 4.908 ± 0.299. Finally, in 3 μm and 15 μm hole, the AIFs were approximately 3.143 and −1.346 by 2 nm of protein and about −2.515 and 4.211 by 30 nm of the PS beads, respectively. The results showed that 2 nm of the protein and 30 nm of PS beads were separated by DEP force in each microhole effectively, and that our method is applicable as a new method to verify an efficiency of the labeling for bead-based fluorescent sensor ∇|E|2.

Physical stability against hydrogen plasma for ZnInSnO thin films deposited by combinatorial RF magnetron sputtering

We investigated the physical stability of ZnInSnO thin films against hydrogen plasma using a plasma-enhanced chemical vapor deposition (PECVD) system. The transmittance and resistivity characteristics of the entire ZnInSnO films showed very little degradation after the hydrogen plasma treatment. However, the deposited films with a zinc content of ≤8.8 at. % [Zn/(In + Sn + Zn), at. %] showed some optical and electrical property degradation. Within this compositional range, the resistivity of the films treated with hydrogen plasma increased compared with that of the as-deposited films. For the film with a zinc content of 7.6 at. %, the transmittance decreased by 21% compared with that of the as-deposited ZnInSnO film (at a standard optical wavelength of 2000 nm). The figure of merit of the deposited ZnInSnO thin films with a zinc content of >8.8 at. % was physically stable against hydrogen plasma. We found that the deposited ZnInSnO thin films with a zinc content above 8.8 at. % have high physical stability against hydrogen plasma.

Conductive properties of transparent Ni–In–Zn–O films deposited by combinatorial RF magnetron cosputtering

The electrical, optical, and structural properties of cosputtered Ni–In–Zn–O films deposited on a flexible poly(ether sulfone) (PES) substrate were investigated by a combinatorial technique. The X-ray diffraction results showed that amorphous Ni–In–Zn–O films were deposited regardless of the Ni content [Ni/(Ni + In + Zn), at. %] in the range of 8.5–45.6 at. %. The surface of the amorphous Ni–In–Zn–O films was quite smooth. The obtained surface roughness (RRMS) values ranged from 0.7 to 1.5 nm. A high resistivity of 5.2 × 10−4 Ω cm and an average transmittance of 88% in the visible wavelength range were obtained for a Ni–In–Zn–O film with an elemental composition ratio of 10.5/73.8/15.7 at. % (Ni/In/Zn). The In content could be reduced by as much as −10 at. % compared with that of commercial indium tin oxide (ITO) while retaining a similar resistivity of −10−4 Ω cm. The measured work functions ranged from 4.84 to 5.02 eV, which are higher than those for ITO films. These findings indicated that Ni–In–Zn–O films grown by RF magnetron sputtering are promising for use as flexible transparent conducting electrodes owing to their low resistivity, high optical transmittance, and high work function.