Kwang-Hyuk Choi

Highly flexible and transparent InZnSnOx∕Ag∕InZnSnOx multilayer electrode for flexible organic light emitting diodes

By inserting a very thin layer of Ag between two layers of amorphous indium zinc tin oxide (IZTO), we fabricated a highly flexible, low resistance, and highly transparent IZTO-Ag-IZTO multilayer electrode on a polyethylene terephthalate (PET) substrate. Due to surface plasmon resonance (SPR) effects and the ductility of the Ag layer, the IZTO-Ag-IZTO electrode exhibited a low sheet resistance of 4.99Ω∕sq. and a high transparency of 86% as well as superior flexibility despite the very thin thickness of the IZTO layer (30nm). It was found that the transition of the Ag layer from distinct islands to a continuous film occurred at a critical thickness (∼14nm). Continuity of the Ag film is very important for SPR in IZTO-Ag-IZTO electrode. The current density-voltage-luminance characteristics of flexible organic light emitting diodes (OLEDs) fabricated on IZTO-Ag-IZTO/PET were better than those of flexible OLEDs fabricated on an ITO/PET substrate due to the low sheet resistance and high work function of the IZTO.

Comparative Investigation of Transparent ITO/Ag/ITO and ITO/Cu/ITO Electrodes Grown by Dual-Target DC Sputtering for Organic Photovoltaics

We investigated the characteristics of transparent indium tin oxide (ITO)/Ag/ITO (IAI) and ITO/Cu/ITO (ICI) multilayer electrodes grown by continuous dual-target dc sputtering for bulk heterojunction organic solar cells (OSCs). The IAI and ICI multilayer electrodes show a significant reduction in their sheet resistance and resistivity with increasing thickness of the Ag and Cu layers, respectively, despite the very small thickness of ITO (80 nm). However, the IAI electrode exhibits a much higher optical transmittance in the visible wavelength region under optimized conditions due to the more effective surface plasmon resonance of the Ag layer than that of the Cu layer. The Auger electron spectroscopy depth profile results for the IAI and ICI electrodes show that there is no severe interfacial reaction between the metal (Ag or Cu) layers and the ITO layers due to the high formation enthalpy of the Ag-O and Cu-O phases at room temperature. Moreover, the OSC fabricated on the IAI electrode shows a higher power conversion efficiency (3.26%) than the OSC prepared on the ICI electrode (2.78%) due to its high optical transmittance in the region of 400-600 nm corresponding to the absorption wavelength range of the organic active layer. This indicates that the IAI multilayer electrode is a promising transparent conducting electrode for OSCs or flexible OSCs due to its very low resistivity and high optical transmittance in the 400-600 nm range.

Room temperature flexible and transparent ITO/Ag/ITO electrode grown on flexile PES substrate by continuous roll-to-roll sputtering for flexible organic photovoltaics

We prepared a flexible and transparent indium tin oxide (ITO)/Ag/ITO electrode on a polyethersulfone (PES) substrate using a specially designed roll-to-roll sputtering system at room temperature for use in flexible organic solar cells (OSCs). By the continuous roll-to-roll sputtering of the ITO and Ag targets, we fabricated a flexible ITO/Ag/ITO electrode with a sheet resistance (Rsh) of 4.28 Ω/square, optical transmittance (T) of 89.28% and figure of merit value of 75.03 × 10−3 Ω−1 at the optimized Ag thickness of 12 nm (dc power of 200 W) without breaking the vacuum, even though it was deposited at room temperature. The bending test results demonstrated that the ITO/Ag/ITO electrode has superior flexibility to the single ITO electrode, due to the high failure strain of the ductile Ag layer. Moreover, the flexible OSC fabricated on the optimized flexible ITO/Ag/ITO electrode showed an identical efficiency of 3.73% to the OSC fabricated on the commercial crystalline ITO electrode, due to its low sheet resistance and high transparency, which are well matched with the organic active layer. This indicates that the flexible ITO/Ag/ITO multilayer electrode grown by the continuous roll-to-roll sputtering technique is a promising flexible and transparent electrode for use in cost-efficient flexible OSCs.

Correlation between Ti source/drain contact and performance of InGaZnO-based thin film transistors

Ti contact properties and their electrical contribution to an amorphous InGaZnO (a-IGZO) semiconductor-based thin film transistor (TFT) were investigated in terms of chemical, structural, and electrical considerations. TFT device parameters were quantitatively studied by a transmission line method. By comparing various a-IGZO TFT parameters with those of different Ag and Ti source/drain electrodes, Ti S/D contact with an a-IGZO channel was found to lead to a negative shift in VT (−Δ 0.52 V). This resulted in higher saturation mobility (8.48 cm2/Vs) of a-IGZO TFTs due to effective interfacial reaction between Ti and an a-IGZO semiconducting layer. Based on transmission electron microcopy, x-ray photoelectron depth profile analyses, and numerical calculation of TFT parameters, we suggest a possible Ti contact mechanism on semiconducting a-IGZO channel layers for TFTs.

Flexible Al-doped ZnO films grown on PET substrates using linear facing target sputtering for flexible OLEDs

We report the characteristics of flexible Al-doped zinc oxide (AZO) films prepared by a plasma damage-free linear facing target sputtering (LFTS) system on PET substrates for use as a flexible transparent conducting electrode in flexible organic light-emitting diodes (OLEDs). The electrical, optical and structural properties of LFTS-grown flexible AZO electrodes were investigated as a function of dc power. We obtained a flexible AZO film with a sheet resistance of 39 Ω/ and an average transmittance of 84.86% in the visible range although it was sputtered at room temperature without activation of the Al dopant. Due to the effective confinement of the high-density plasma between the facing AZO targets, the AZO film was deposited on the PET substrate without plasma damage and substrate heating caused by bombardment of energy particles. Moreover, the flexible OLED fabricated on the AZO/PET substrate showed performance similar to the OLED fabricated on a ITO/PET substrate in spite of a lower work function. This indicates that LFTS is a promising plasma damage-free and low-temperature sputtering technique for deposition of flexible and indium-free AZO electrodes for use in cost-efficient flexible OLEDs.

Characteristics of Flexible ITO Electrodes Grown by Continuous Facing Target Roll-to-Roll Sputtering for Flexible Organic Solar Cells

The preparation and characteristics of flexible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates are described for use in flexible organic solar cells and prepared using a specially designed facing target roll-to-roll sputtering (FTRS) system. Due to the effective confinement of the high-density plasma between two facing ITO targets, we can deposit the ITO electrode continuously on the PET substrate at a substrate temperature below 50°C without the need of a substrate cooling system, which is used in conventional roll-to-roll sputtering systems. In spite of a low substrate temperature, the FTRS-grown flexible ITO electrode showed a sheet resistance of 42.2 Ω/square, a resistivity of 8.44 × 10 -4 Ω cm, and a transmittance of 85.41% in the 500-550 nm wavelength range with superior flexibility. Furthermore, the flexible organic solar cell fabricated on the FTRS-grown flexible ITO electrode at optimized conditions exhibited a power-conversion efficiency of 2.43%, which is similar to an organic solar cell fabricated on a reference dc sputter-grown ITO electrode. This indicates that the FTRS technique is a promising continuous-sputtering process for preparing flexible ITO electrodes and can substitute for conventional roll-to-roll sputtering systems for mass production of flexible solar cells.

Antireflective ZnSnO/Ag bilayer-based transparent source and drain electrodes for transparent thin film transistors

We report on antireflective ZnSnO (ZTO)/Ag bilayer and ZTO/Ag/ZTO trilayer source/drain (S/D) electrodes for highly transparent ZTO channel-based thin film transistors (TFTs). Although both bilayer and trilayer films have a similar sheet resistance (3–5 Ω/sq), the ZTO/Ag bilayer is a more effective transparent S/D electrode for ZTO channel layer than the ZTO/Ag/ZTO trilayer S/D electrode, due to the direct contact of the Ag layer on the ZTO channel layer and a desirable oxide-metal-oxide multilayer structure for antireflection effects. ZTO channel-based all-transparent TFTs with ZTO/Ag bilayer S/D electrodes exhibited a saturation mobility of 4.54 cm2/Vs and a switching value (1.31 = V/decade), comparable to those of a ZTO channel-based TFT with metallic Ag S/D electrodes. This indicates that the antireflective ZTO/Ag bilayer is a promising transparent S/D electrode for use in all-transparent TFTs as a substitute for conventional opaque metal S/D electrodes.

Development of a Micro Quad-Rotor UAV for Monitoring an Indoor Environment

The purpose of this paper is to develop a micro UAV (Unmanned Aerial Vehicle) as the platform for monitoring an indoor environment, more specifically, a quad-rotor aircraft that has strong advantages of its small size, stable hovering and precise flight. To begin with, we analyze the dynamics of a quad-rotor aircraft, and introduce control strategies based on the PD control. Then, the developed micro quad-rotor UAV, equipped with micro controllers, various sensors, and a wireless camera, is presented and is compared with the DraganFlyer commercial quad-rotor aircraft. Finally, experimental results are also provided so as to illustrate stable flight performances.

d-orbital ordering of oxygen-deficient amorphous and anatase TiO2−x channels for high mobility thin film transistors

Semiconducting behavior in oxygen-deficient amorphous and anatase TiO2−x films prepared by direct current magnetron sputtering was investigated for high mobility oxide thin film transistors (OxTFTs). Rapid thermal annealing (RTA) was found to induce transition from an amorphous to anatase phase in oxygen-deficient TiO2−x channels and was significantly influenced by DC power during sputtering. Compared to the low field effect mobility (μFE: 0.57 cm2/V s) of OxTFTs with d-orbital ordered amorphous TiO2−x channels, OxTFTs with anatase TiO2−x channels exhibited a higher mobility (μFE: 1.02 cm2/V s) due to an increased amount of free carriers and effective d-orbital ordering. In addition, the oxygen ambient annealing time during the RTA process had a critical effect on Von shift and shallow/deep trap states of anatase TiO2−x-based TFTs.

Atmospheric Plasma Treatment of Flexible IZO Electrode Grown on PET Substrate for Flexible Organic Solar Cells

The atmospheric plasma (AP) surface treatment of a flexible indium zinc oxide (IZO) electrode grown on a poly(ethylene terephthalate) (PET) substrate was investigated as an alternative to oxygen vacuum plasma treatment for low cost organic solar cells. In this study, the surface properties of the flexible IZO electrode were modified by a simple AP treatment without changing their sheet resistance and optical transmittance. X-ray photoemission spectroscopy showed that the atmospheric air plasma treatment led to a decrease in the carbon content and an increase in the oxygen content at the surface of the flexible IZO electrode, indicating that the organic contamination on the flexible IZO electrode was effectively removed by the AP treatment.