Dong-Sik Zang

Weak-microcavity organic light-emitting diodes with improved light out-coupling

We propose and demonstrate weak-microcavity organic light-emitting diode (OLED) displays with improved light-extraction and viewing-angle characteristics. A single pair of low- and high-index layers is inserted between indium tin oxide (ITO) and a glass substrate. The electroluminescent (EL) efficiencies of discrete red, green, and blue weak-microcavity OLEDs are enhanced by 56%, 107%, and 26%, respectively, with improved color purity. Moreover, full-color passive-matrix bottom-emitting OLED displays are fabricated by employing low-index layers of two thicknesses. As a display, the EL efficiency of white color was 27% higher than that of a conventional OLED display.

Optically bifacial thin-film wire-grid polarizers with nano-patterns of a graded metal-dielectric composite layer

We report on the concept of a thin film wire-grid polarizer (WGP) with optically dual characteristics by introducing a nano-patterned graded metal-dielectric composite-material layer. The Ti-SiO(2) composite layer with a depth profile of a gradually-varied composition ratio shows an absorptive feature due to the elimination of an optical interface between a metal and a glass substrate, while the metal side of the WGP gives a reflective character. The unprecedented optically-bifacial thin-film WGP with the 144 nm-period straight-line patterns of a 100 nm-thick Ti-SiO(2) composite layer and a 185 nm-thick Al layer shows the exceptionally low reflectance below 15 % from the absorptive side and the high polarization extinction ratio (PER) of over 500 at 550 nm, which is acceptable for use as various display applications such as AMOLEDs and LCDs.

Design of fine phosphor system for the improvement in the luminescent properties of the phosphor layer in the plasma display panel: Theoretical and experimental analysis

Improvement in the luminescent properties of plasma display panels via phosphor size control was theoretically and experimentally investigated. From theoretical analyses of photon extraction and plasma efficiency, fine phosphor system was designed, which was compared with experimental data. The denser microstructure of finer phosphor-based layer promoted photon extraction efficiency by higher reflectivity. Also, the finer phosphor increased vacuum-ultraviolet discharge space and corresponding plasma efficiency via decrease in layer thickness. Based on the results, the phosphor size control improved the panel efficiency by the synergistic effect of improvements in photon extraction and plasma efficiency.

36.2: Distinguished Paper: New Polycrystalline MgO for a Thermally Stable and Fast Delay Time in AC PDP

We have extensively studied polycrystalline MgO sources for achieving a thermally stable and fast delay time in AC PDP. The key idea was to improve MgO film properties by doping p-type, equivalent, and n-type elements to the MgO matrix. Their behaviors in a gas discharge were tested. Most of them showed severe discharge jitters. By doping Ca, Al, Si, and a particular n-type element, the discharge instability was greatly reduced, so that the elements were studied in more details. We found that doping of the last element was highly effective in lowering the delay time, improving the thermal stability, and lowering the firing voltage. We proved that the improvement is largely due to the electron emission enhanced by defects in MgO band gap and lattice distortion which is induced by dopants.

32.1: Correlation between Wall Charge of a MgO Film and Discharge Delay Time

We prepared MgO films using e-beam deposition and polycrystalline MgO pellets and oxygen flow rates of 50 sccm and 100 sccm. These films were examined in gas discharge tests at different temperatures. The dependence of the discharge delay time on film temperature was smaller for the film grown at 100 sccm O2 flow compared to the film grown at 50 sccm. Their wall charges were also measured. This revealed that MgO films grown at 100 sccm had more wall charges. The analysis showed a difference in surface roughness. MgO films with a larger surface roughness had more wall charge.

Field Emission and Lifetime Characteristics of Titanium-coated Carbon Nanotubes

We investigated the effect of titanium (Ti)-coated carbon nanotubes (CNTs) theoretically and experimentally. We found that adsorption of single Ti atom lowers the work function of CNTs by density functional calculations. Also, Ti-coated CNTs showed the largest increase in local density of states around Fermi level under electric fields. Coating of Ti metal on CNTs was carried out by electroless plating method. Ti-coated CNTs were mixed with conductive pastes, and then screen-printed. The measurement of field emission carried out using a diode structure showed that the electron emission of Ti-coated CNT films uniformly had a field of 3.7 V/mum at a current density of 100 mu/cm2 (1/500 duty), as compare to bare CNT films showed a field of 5.5 V/mum. Furthermore, the lifetime of our CNT samples was about a few times as much as that of the pristine ones. Therefore, Ti-coated CNTs improved the characteristics of CNT-based field emission emitters

19.4: Luminescence Characteristics of YAl3B4O12: Tb3+ Phosphor for PDP Application

Photoluminescence(PL) characteristics under vacuum ultraviolet(VUV) region of Tb3+ doped yttrium aluminium borate phosphor were investigated for Plasma Display panel(PDP) application. The single-phase green emitting Y1−xTbxAl3B4O12(YAB: Tb3+) powder phosphors were prepared with various Tb3+ ion concentration. As a result, the intensity around 147nm of excitation spectrum in YAB: Tb3+ decrease slightly with increase of Tb3+ ion concentration due to concentration quenching behavior, but on the contrary, the intensity of 172nm increase with increase of Tb3+ ion concentration due to attributing the 4f8 4f7 5d1 transition of Tb3+ [1]. However in the result of gas discharge chamber test which has the same gas mixture as real PDP, relative PL luminance of YAB: Tb3+ increase with increase of Tb3+ ion concentration due to effect of increasing absorption at 172nm. Besides, YAB: Tb3+ with high Tb3+ ion concentration (x=0.7) has higher VUV luminance saturation property than low Tb3+ ion concentration (x=0.4) since high Tb3+ ion concentration phosphor has more activators than lower one which can react with VUV photon in unit volume.

Ion-induced electron emission from different crystalline phases of ZrO2

A detailed study of ZrO2 as a candidate material for a protective layer of plasma display panels has been performed. The ion-induced electron emission coefficient γ and the firing voltage were measured for cubic, tetragonal, and monoclinic ZrO2 films. Different crystalline phases of ZrO2 showed a very small difference in the coefficient γ and the firing voltage. Compared to MgO, the coefficient γ was smaller and, as a result, the firing voltage was higher for ZrO2.

Weak-microcavity organic light-emitting diodes with improved light-extraction and wide viewing-angle

We propose and demonstrate weak-microcavity organic light-emitting diode (OLED) displays that deliver both a high light-extraction efficiency and wide viewing-angle characteristics. A single pair of low- and high-index layers is inserted between indium tin oxide (ITO) and a glass substrate. The electroluminescent (EL) efficiencies of discrete red, green, and blue weak-microcavity OLEDs (WMOLEDs) are enhanced by 56%, 107%, and 26%, respectively with minimal changes viewing angle and EL spectra characteristics. The color purity is also improved for all three colors. Moreover, we fabricated full-color 128×160 passive-matrix bottom-emitting WMOLED displays to prove their manufacturability. This design is realized by simple one-step 20-nm etching of the low-index layer of red/green subpixels. The EL efficiency of white color in the WMOLED display is 27% higher than that of a conventional OLED display.

Thermal Annealing Effect on the Field Emission Characteristics of Double-Walled Carbon Nanotubes

The effect of thermal annealing on the field emission properties of double-walled carbon nanotubes (DWCNT) grown by chemical vapour deposition is studied. The properties of the DWCNTs were examined using SEM, TEM, Raman and TGA analysis. It was found that high-temperature annealing enhances the field emission performance of DWCNTs