Advanced Viewing-Angle and Coloration Technologies for High-Performance Liquid Crystal Displays

Abstract

In the past decades, liquid crystal display (LCDs) have been extensively studied for use in flat panel displays (FPDs) because of light weight panel, thin thickness and low power consumption. Due to their high contrast, uniform brightness, and rapid switching times compared to conventional cathode ray tubes, LCDs have achieved a significant position in the FPD industry. However, according to the development of other FPD technologies including organic light-emitting didoes, the enhancement of LCD performances such as wide-viewing angle and vivid coloration has been demanded. In this thesis, novel image generation technologies for high performance LCD are proposed. At first, for the reduction of the gamma distortion, wide-viewing technology was demonstrated by sophisticated multi-domain configuration in which liquid crystal (LC) molecules are aligned along different polar directions at middle gray scale. In order to realize the multi-domain LC cell, a new alignment layer which can control polar LC alignment properties including the pretilt angle and the anchoring energy was developed. Uncured hydrophobic oligomers in a cross-linked polymer mold were transferred onto a substrate to change the surface wettability of the substrate. The thermal-transfer printing, which is a contact-transfer printing method including an additional heat treatment process, accelerated the molecular diffusion of oligomers from the polymer mold onto the substrate, resulting in the increase of the amount of transferred oligomer. Consequently, the surface energy of the substrate was manipulated according to the heat treatment temperature during the thermal-transfer printing. Then, the LC alignment properties in both azimuthal and polar directions were measured through LC cell fabrication and electro-optical characterization. It was found that the polar pretilt angle and polar anchoring energy were changed in a wide-range, resulting from the adjustment of the substrate surface energy. Based on the results described above, the reduction of the gamma distortion through the multi-domain configuration in the vertical alignment (VA) mode was demonstrated. The oligomer layer was formed onto a conventional VA alignment layer to control the polar alignment properties. The ultra-violet ozone (UVO) treatment for the enhancement of the oligomer transfer onto hydrophobic substrates was added in the fabrication process. The surface energy and anchoring energy of the stacked alignment layers depending on the UVO treatment was examined. The anchoring disparity, which is the anchoring energy difference between sub-domains as a result of the oligomer layer patterning, induced the threshold voltage difference in the sub-domains and constructed the multi-domain at applied voltages. The experimental measurements for the assessment of the gamma distortion were performed in a conventional patterned VA mode cell and proposed multi-domain cell. Next, for the expansion of the color space, a new type of coloration technique based on the photo-luminescent emission of the quantum dots (QDs) in an organic polymer matrix was described. Recently, QD patterning technologies based on the polymer matrix have been extensively studied to accurately express the primary color in pixel units, but suffered from the non-uniform light emission owing to uneven distribution of the QDs in the polymer matrix. Reactive mesogen (RM), which is photo-curable, transparent, and soluble in organic solvents, was used for the fabrication of uniformly distributed QDs in the RM matrix. The emission characteristics of QD-RM composites depending on the concentration ratios of QD and RM were examined. In addition, color-separated patterns of QD-RM composites on a single substrate through conventional photo-lithography processes were demonstrated. Finally, a novel QD emissive LCD based on the color-separated pattern for high color purity was proposed. The proposed LCD consists of modulation part and emission part. LC cells acts as a role of electrically tunable lightwave retarder depending on the applied voltage. The transmitted light through the modulation part emits the photo-luminescent light from the color-separated QD pattern. The color purity and color gamut of the QD emissive LCD was measured in comparison with previous LCDs such as the color filter LCD and QD compensated LCD. In conclusion, the applicability of proposed image generation technologies to the FPD industry was explored. The new concepts and experimental results will lead to the development of the advanced LCD performance.