Joo Hyon Noh

Transparent organic light-emitting diodes consisting of a metal oxide multilayer cathode

The authors have developed a semitransparent, multilayered cathode of indium tin oxide (ITO)/Ag/tungsten oxide (WO3) for transparent organic light-emitting diodes. The device showed a weak negative differential resistance (NDR), until the operating voltage of 8V was reached. NDR was due to the resonant tunneling by both the quantum barrier and quantum well. The silver oxide (Ag2O) on the Ag metal was confirmed by x-ray photoelectron spectroscopy, and the energy levels of Ag2O were quantized due to the quantum size effect and this produced the resonant tunneling channels. The device using ITO∕Ag∕WO3 with a LiF∕Al bilayer was superior to those devices which only used ITO or WO3, mainly because the out coupling was enhanced by employing a WO3 material, which is much more transparent than ITO.

Superhydrophobic modification of gate dielectrics for densely packed pentacene thin film transistors

Pentacene organic thin film transistors (OTFTs) with low-k and high-k hybrid gate dielectrics by CF4 plasma treatment exhibited excellent device performance with field effect mobilities (maximum 1.41cm2∕Vs), a low threshold voltage of +1V, and on/off current ratios of 105 at −5V gate bias. After CF4 plasma treatment, fluorine atoms diffuse into the interior low-k polymer and eliminate ionic impurities which reduce the leakage current density and overall pentacene initial growth on the superhydrophobic surface is significantly improved. It seems apparent that proper surface treatment is desirable for higher quality pentacene film and improving the performance of OTFTs.

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

Increase in indium diffusion by tetrafluoromethane plasma treatment and its effects on the device performance of polymer light-emitting diodes

The effects of tetrafluoromethane (CF4) plasma treatment of indium-tin-oxide (ITO) anode on indium diffusion into a poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate) (PEDOT:PSS) layer were studied. Auger electron spectroscopy (AES) depth profile showed that 0.2at.% indium was present in the PEDOT:PSS layer when ITO was not plasma treated. The plasma treatment of ITO increased the indium concentration to ∼6at.%. The increase in indium can be explained by an oxygen deficiency in the CF4 plasma treated ITO. The presence of indium in the PEDOT:PSS layer showed a correlation with performance degradation of polymer light-emitting diodes.

Low-voltage-driven bottom-gate amorphous indium-gallium-zinc-oxide thin-film transistors with high dielectric constant oxide/polymer double-layer dielectric

In this study, we report on the fabrication of a low-voltage-driven bottom-gate amorphous indium–gallium–zinc-oxide (a-IGZO) thin-film transistor with a high-k oxide/polymer double-layer dielectric on a glass substrate. The hybrid double-layer dielectric was formed through a sequential process of e-beam evaporation of 300-nm-thick amorphous high-k oxide (SiO2–CeO2 mixture) and spin coating of poly(4-vinylphenol) (PVP). The channel layer was deposited by rf magnetron cosputtering with InGaZO4 and ZnO dual targets at room temperature in argon and oxygen mixed-gas atmosphere. The room-temperature-deposited a-IGZO channel with the hybrid double-layer dielectric exhibits the operating characteristics of a threshold voltage of 1.7 V, drain-source current on/off modulation ratio (Ion/Ioff) of ~2.9×104, and field effect mobility of 0.97 cm2 V-1 s-1 at 5 V.

Polymeric tandem organic light-emitting diodes using a self-organized interfacial layer

The authors have demonstrated efficient polymeric tandem organic light-emitting diodes (OLEDs) with a self-organized interfacial layer, which was formed by differences in chemical surface energy. Hydrophilic poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS) was spin coated onto the hydrophobic poly(9,9-dyoctilfluorene) (PFO) surface and a PEDOT:PSS bubble or dome was built as an interfacial layer. The barrier heights of PEDOT:PSS and PFO in the two-unit tandem OLED induced a charge accumulation at the interface in the heterojunction and thereby created exciton recombination at a much higher level than in the one-unit reference. This effect was confirmed in both the hole only and the electron only devices.

Characterization of CF4 Plasma-Treated Indium–Tin-Oxide Surfaces Used in Organic Light-Emitting Diodes by X-ray Photoemission Spectroscopy

The CF4 plasma treatment of indium–tin-oxide (ITO) greatly improves the performance of organic light-emitting diodes (OLEDs). The effects of plasma treatment on the chemical composition change and device performance were investigated by X-ray photoemission spectroscopy (XPS). The present results suggest that fluorine directly bonded to indium or tin on the ITO surface and markedly enhanced device performance.

High-performance and low-voltage pentacene thin film transistors fabricated on the flexible substrate

We report on the fabrication of low operating voltage pentacene thin film transistors with PVP (poly-4-vinylphenol)/CeO2–SiO2/PVP triple dielectric layers on a flexible substrate. Our triple dielectric layers exhibited a markedly reduced leakage current characteristic and a relatively high capacitance. The field effect mobility, on/off current ratio and subthreshold slope obtained from pentacene thin film transistors were 0.67 cm2 V s−1, 105 and 0.36 V dec−1, respectively. It was demonstrated that the spin-coating polymer layer can be used to planarize the surface irregularities and to improve the dielectric and device properties.

Formation of stable direct current microhollow cathode discharge by venturi gas flow system for remote plasma source in atmosphere

We introduce a microhollow cathode configuration with venturi gas flow to ambient air in order to obtain glow discharge at atmospheric pressure. Stable microhollow cathode discharge was formed in a 200μm diameter at 9mA and the optimum value of gas velocity×diameter for hollow cathode effect was obtained in our system. In order to confirm hollow cathode effect, we measured the enhancement of E∕N strength for 200μm (0.31m2∕s) and 500μm (0.78m2∕s) air discharge at 8mA under the velocity of 156m∕s. As a result, an increase of 46.7% in E∕N strength of the discharge of 200μm hole was obtained compare to that of 500μm.

Low leakage current gate dielectrics prepared by ion beam assisted deposition for organic thin film transistors

This communication reports on the fabrication of low operating voltage pentacene thin-film transistors with high-k gate dielectrics by ion beam assisted deposition (IBAD). These densely packed dielectric layers by IBAD show a much lower level of leakage current than those created by e-beam evaporation. These results, from the fact that those thin films deposited with low adatom mobility, have an open structure, consisting of spherical grains with pores in between, that acts as a significant path for leakage current. By contrast, our results demonstrate the potential to limit this leakage. The field effect mobility, on/off current ratio, and subthreshold slope obtained from pentacene thin-film transistors (TFTs) were 1.14 cm2/V s, 105, and 0.41 V/dec, respectively. Thus, the high-k gate dielectrics obtained by IBAD show promise in realizing low leakage current, low voltage, and high mobility pentacene TFTs.