Byung Wok Park

Fast switching of long-pitch cholesteric liquid crystal device

We propose a long-pitch cholesteric liquid crystal (ChLC) device capable of operation in both the dynamic mode and the memory mode. Fast switching between the homeotropic state and the focal conic state allows the display of moving pictures at a low operating voltage. In addition, we can write text messages on the proposed ChLC device by applying an external pressure locally to switch it from the focal conic state to the planar state.

Optical compensation methods for the elimination of off-axis light leakage in an in-plane-switching liquid crystal display

Optical compensation to improve the wide-viewing-angle characteristics in a liquid crystal display (LCD) is essential for high-quality dark images, especially from the bisector direction of the crossed polarizers. This paper reviews optical compensation schemes for the elimination of off-axis light leakage in an in-plane-switching LCD. A uniaxial film with normal dispersion commonly increases the dark-state light leakage, but it can be used in the opposite manner to remove the dark-state light leakage at off-axis viewing angles. Using symmetrical rotation of the polarization state on the Poincaré sphere, the dark-state light leakage over the entire viewing cone for white light can be effectively eliminated.

Formation of liquid crystal multi-domains with different threshold voltages by varying the surface anchoring energy

We propose methods to obtain multi-domains with different threshold voltages for vertically aligned liquid crystal displays without additional transistors or capacitors. We exposed an ultraviolet light through a photomask and spatially varied the surface anchoring energy by partially decomposing the polyimide or inducing the adsorption of reactive monomers. The threshold voltage was reduced with smaller surface anchoring, and the reduction of the threshold could be qualitatively interpreted using a model describing a weak anchoring condition.

Vertical alignment of liquid crystals without alignment layers

We propose a method for vertical alignment (VA) of liquid crystals (LCs) without alignment layers by mixing only a very low concentration (0.05 wt%) of an organic compound with LCs. The electro-optic characteristics of a VA cell fabricated by the proposed method were almost the same as those of a conventional VA cell with VA layers. Moreover, we used the proposed method to realise a polymer-stabilised VA device with a rapid response time without alignment layers. The proposed method can simplify the fabrication process and reduce the manufacturing cost of LC display devices by removing the need for alignment layers and the alignment process such as coating of polyimide layers and the UV exposure for the photo-alignment. We believe that the proposed method can play an essential role in the fabrication of plastic flexible LC display devices requiring the low-temperature process.

Achromatic optical compensation using dispersion of uniaxial films for elimination of off-axis light leakage in a liquid crystal cell

We propose an achromatic optical-compensation method using uniaxial films to eliminate the off-axis light leakage at the dark state in a homogeneously aligned liquid crystal cell. Three uniaxial films with different dispersion characteristics are used so that they can compensate each other to achieve achromatic effective phase retardation at off-axis. The retardation values are optimized with the aid of the Poincaré sphere and through numerical research. A contrast ratio of higher than 2000∶1 is predicted over the entire ±60° viewing cone for a homogeneously aligned LC cell with zero pretilt angle.

Formation of Polymer Networks for Fast In-Plane Switching of Liquid Crystals at Low Temperatures

We formed a polymer structure to enable fast in-plane switching of liquid crystals at low temperatures. The problem of the inevitable slow response at low temperatures was reduced by the formation of in-cell polymer networks in in-plane switching (IPS) cells. The electro-optic characteristics of polymer-networked IPS cells were measured at temperatures ranging from -10 to 20 °C. The turn-on and turn-off times of an IPS cell were reduced by 44.5 and 47.2% at -10 °C by the formation of polymer networks. We believe that the proposed technology can be applied to emerging display devices such as mobile phones and automotive displays that may be used at low temperatures.

Pixel electrode structure for high transmittance in a multi-domain vertical alignment liquid crystal display device

We propose a pixel electrode structure for the multi-domain vertical alignment liquid crystal mode with high optical efficiency. We form a micro-slit electrode structure on the top substrate to reduce the width of disclination lines at domain boundaries. We insert additional electrodes on the bottom substrate to improve the transmittance at the edges of each pixel. We confirm that using both structures, the transmittance can be increased from 22.9% to 27.3% with no change in the operating voltage.

High-transmittance Multi-domain Vertical Alignment Liquid Crystal Device with Protrusion Structure

We propose a high-transmittance multi-domain vertical alignment liquid crystal device with a protrusion structure. Disclination lines, which inevitably appear at the boundaries of domains in a multi-domain structure, can be reduced by adding a protrusion structure on the top substrate. The transmittance was improved by 11% using the proposed structure with no change of either the dark state or the operating voltage.

Fast Switching of Vertically Aligned Liquid Crystals by Low-Temperature Curing of the Polymer Structure

We proposed a method for fast turn-off switching of a vertically-aligned liquid crystal cell by low-temperature curing of the polymer structure. We confirmed that the turn-off times of the fabricated cells were reduced significantly as the curing temperature was lowered to

Long memory retention time and high contrast ratio in a tristate liquid crystal display device.

-20^{\circ}C