Jongwoon Park

Investigation on a short circuit of large-area OLED lighting panels

A short circuit often arises from large-area organic light-emitting device (OLED) lighting panels due to particles (i.e. dust, organic or metal debris) or the spike-like surface of the indium?tin?oxide (ITO) anode. On the emergence of a short circuit, an instant current crowding occurs, thereby reducing substantially the resistance of the panels and causing a failure of a dimming control. In this paper, we investigate the effect of the surface morphology of ITO on the resistance and dimmability of the panels. We have demonstrated that the peak-to-valley roughness of ITO should be much less than 20?nm or the resistance of the panels should be much higher than 1?M? in order to avoid an unwanted short-circuit phenomenon and thus achieve the high-yield fabrication of OLED lighting panels. It is also addressed that much care is taken to ensure a dimming control of OLED lighting panels with a larger active area because the resistance of those panels varies depending more sensitively on the surface roughness of ITO.

Design of moiré-free metal meshes using ray tracing for touch screen panels

Abstract Using a ray tracing technique, we investigate the dependence of the moire effect on the crossing angles between touch screen panels (TSPs) and display panels, the metal grid structures (spacing and width), and metal grid shapes (non-uniform and random grids). Of those design parameters, adjusting the crossing angle (∼45°), reducing the grid width (∼1xa0μm), and employing a random grid in the shape of irregular hexagon are found to suppress the moire phenomenon to a great extent. We also provide the simulation scheme that can capture the moire patterns observed experimentally and useful design guidelines for metal grids.

Thermal property of transparent silver nanowire films

Through a comparison with transparent polymer composite films, we investigate the thermal property of transparent silver nanowire (AgNW) films that may be employed for heat sink in transparent electronic devices. To fabricate transparent polymer composite films and enhance their thermal property, poly(methyl methacrylate) (PMMA) solution featuring high transparency (?90%) and thermal emissivity (0.9) is mixed with thermal conductive fillers such as aluminum nitride (AlN) and silicon carbide (SiC). It is observed that the thermal emissivity of the AgNW films is decreased as the sheet resistance is reduced. However, we have found that the AgNW film shows the most excellent heat dissipation property (53.7??C) while maintaining relatively higher transparency (77.1% at 520?nm), followed by the PMMA:SiC and then PMMA:AlN films.

Angular Dependence of Moiré Fringes Induced by Metal Grids for Touch Screen Panels

By way of experiments and ray tracing simulations, we investigate the angular dependence of moiré fringes induced by metal grids of various shapes (e.g., rectangular, square, hexagonal, and random metal grids) for touch screen panels (TSPs). To quantify the moiré phenomenon, we have utilized the standard deviation (SD) calculated from the simulated ray distribution. The SD value is significantly decreased (i.e., the moiré phenomenon is substantially suppressed) with the rectangular and square metal grids when the crossing angle between the metal grid and a black matrix (BM) of display panels lies within the range between 20 ° and 60 °. For the hexagonal metal grid, there appears a sharp dip corresponding to the minimum SD value every 30 ° rotation angle. However, the SD value is observed to be very high near the crossing angles at which a sharp dip occurs. Though no angular dependence arises and no moiré patterns appear with the random metal grid, yet the SD value is even higher due to point defects, compared with the rectangular and square metal grids. These results are in qualitatively good agreement with the experiment results observed with the naked eye.

Suppression of Moiré Phenomenon Induced by Metal Grids for Touch Screen Panels

With attempt to suppress the Moiré phenomenon, we conduct ray tracing simulations with various metal grids having different degree of randomness (e.g., rectangular, width-altered rectangular, line-shifted rectangular, wavy square, and random metal grids) in the presence of twisted nematic (TN) or in-plane switching (IPS) mode black matrix (BM). We analyze their angular dependence in terms of the standard deviation (SD) calculated from the simulated ray distribution. It is observed that metal grids with a lower degree of randomness exhibit a stronger angular dependence, yet lower SD value over the broader range of crossing angles. Though metal grids with a higher degree of randomness show a weaker angular dependence, yet they bring in higher fluctuation in the SD value due most likely to point defects. The wavy square metal grid results in periodic point defects, whereas the random metal grid gives rise to aperiodic point defects. Through a comparison between simulation and experiment results, we have found that moiré patterns are hardly seen to the naked eye when the SD value is less than 10, which provides useful design guidelines of metal grids.

Integration of organic LEDs with inorganic LEDs for a hybrid lighting system

We demonstrate that a surface-emitting hybrid light source can be realized by a combination of organic and inorganic light-emitting devices (LEDs). To this end, a blue inorganic LED bar is deployed at one side of a transparent light guide plate (LGP), and a yellow organic LED (OLED) is in contact with the rear surface of the LGP. In such a configuration, it is found that the overall luminance is almost equivalent to the sum of the luminances measured from each light source, and the overall luminance uniformity is determined mainly by the luminance uniformity of the OLED panel at high luminances. We have achieved a white color showing the Commission Internationale dEclairage (CIE) chromaticity coordinates of (x = 0.34, y = 0.33), the power efficiency of 9.3 lm/W, the luminance uniformity of 63% at the luminance of 3100 cd m?2, the color rendering index as high as 89.3, and the correlated color temperature finely tunable within the range between 3000 and 8000?K. Such a system facilitates color tuning by adjusting their luminous intensities and hence the implementation of the emotional lighting system.

Photolithography-free fabrication of organic light-emitting diodes for lighting applications

We investigate the photolithography-free fabrication of organic light-emitting diodes (OLEDs) for lighting applications with an attempt to embed the deposition and patterning process of an indium–tin–oxide (ITO) anode and insulating layer into an in-line-type organic evaporation system. This scheme inevitably brings in leakage current induced by the spike-like surface of ITO. To suppress it, we cover the ITO edges with three different insulation materials (i.e. sputter-deposited inorganic Al2O3xa0thin film, monomer (polymer) thin film deposited by organic acrylate evaporation or thermally evaporated organic insulation layer (tris-(8-hydroxyquinoline) aluminum (Alq3))). Although small-molecule organic insulation materials that can be thermally evaporated are the most suitable for such a cost-effective fabrication process, yet their insulation capability is low due to the carrier transporting property. In this paper, we demonstrate that it can be boosted to a great extent with an increase of their thickness. It is likely that pinholes existing on the Al2O3xa0thin film act as leak channels, degrading the device performance. We also verify that the insulation capability of polymer fabricated by organic acrylate evaporation is just comparable with that of polyimide (PI) insulator patterned using a standard photolithography process.

Simulation and analysis on visibility enhancement for laser beam projected on display panels using black matrix with scattering particles

Abstract With an attempt to enhance the visibility of laser beam projected on a display panel, we investigate the single-layered black matrix (BM) structure where light scattering particles are embedded in BM and the double-layered BM structure in which the scattering layer is separated from BM. By a ray tracing tool, we perform simulations, and then calculate the scattered ray distribution and detected power for those BM structures. It is observed from the results for both structures that the visibility is enhanced with increasing scattering particle density, but lowered with increasing oblique incidence of the laser beam. It is also demonstrated that the double-layered BM structure shows higher visibility than the single-layered BM structure. However, there exists a trade-off between the visibility of the projected laser beam on display panels and color mixing among pixels on the selection of those BM structures. Although color mixing is still unavoidable, our study shows that the scattered power into the active pixel area (pixel aperture) is as low as 1% of the incident laser power.

Investigation of starburst phenomenon using ray tracing for touch screen panels

Abstract We investigate the effect of the metal pattern shape (i.e., square, hexagonal, and random grids) on the starburst phenomenon of touch screen panels (TSPs) based on opaque metallic grids. It is demonstrated that a standalone random metal grid can suppress the starburst phenomenon to a great extent. By way of ray tracing simulation, we have found that specular reflection of light on the gentle slope of the patterned edges of metallic grids contributes to the generation of starburst patterns. It is also addressed that employing a light-absorbing material and increasing metal grid spacing can reduce the intensity of starburst patterns.