Hyang Yul Kim

Effect of magnitude of dielectric anisotropy of a liquid crystal on light efficiency in the fringe‐field switching nematic liquid crystal cell

The light efficiency of most well‐known nematic liquid crystal (LC) modes, such as twisted nematic and in‐plane switching, depends only on the cell retardation value, irrespective of the cell gap and dielectric anisotropy of the LC. Interestingly, the light efficiency of a homogenously aligned nematic LC cell driven by a fringe electric field, termed the fringe‐field switching (FFS) mode, is found to be dependent on the magnitude of dielectric anisotropy, such that an LC with high dielectric anisotropy results in lower light efficiency than that of an LC with low dielectric anisotropy.

Cell gap‐dependent transmission characteristics of a fringe‐electric field‐driven homogeneously aligned liquid crystal cell, for a liquid crystal with negative dielectric anisotropy

Transmittance characteristics were studied as a function of cell gap for a homogeneously aligned liquid crystal (LC) cell driven by a fringe‐electric field—named fringe‐field switching (FFS) mode. The light efficiency of a conventional LC cell using in‐plane switching and twisted nematic modes, where the LC director is determined by competition between elastic energy and electrical energy, does not depend on cell gap as long as the cell retardation value remains the same; i.e. only dielectric torque contributes to the deformation of the LC director. However, the transmittance of the FFS mode is dependent on the cell gap such that it decreases as the cell gap decreases, although the cell retardation value remains the same. This unusual behaviour (unlike that of conventional LC cells) arises because in the device the elastic and dielectric torques have the role of determining the LC director, such that the driving voltage giving rise to maximum transmittance becomes strongly dependent on the electrode position when the cell gap is as small as 2 µm. In addition, the LCs at the centre of the pixel and common electrodes are not sufficiently twisted because of a competition between the two elastic forces, which tries to twist the LCs in plane and hold them in their initial state by surface anchoring.

Analysis of Optimal Phase Retardation of a Fringe Field-Driven Homogeneously Aligned Nematic Liquid Crystal Cell

We studied the optimal cell retardation value that shows the maximal transmittance in a fringe-field-driven homogeneously aligned (HA) nematic liquid crystal (LC) cell. When the LC with positive dielectric anisotropy is used, the transmittance is much higher than that of an in-plane-field-driven HA cell. The unexpected electrooptic behaviors are caused because the transmittance of light from a deformed LC in a white state cannot be explained using only the uniaxial medium model that describes an in-plane-field-driven HA cell.

Dual Domain Effects on a Homogeneously Aligned Nematic Liquid Crystal Cell Driven by a Fringe-Field

The fringe-field switching (FFS) mode is known to exhibit both a wide viewing angle and high transmittance, owing to the approximated in-plane rotation of the liquid crystal (LC) director. However, in the bright state, the device shows bluish and yellowish color parallel and perpendicular to the LC director at off-normal directions since the LC director rotates only in one direction. The degree of color shift becomes even stronger when the retardation value of the cell is high. This problem was greatly improved using a wedge shape of only pixel electrodes. These wedged shaped pixel electrodes allow two different field directions to exist in a pixel, enabling the LC director to rotate in two opposite directions. Consequently, owing to the dual domain effect with unidirectional rubbing, the color shift dependence on the viewing angle is greatly reduced.

Dynamic Stability of the Fringe-Field Switching Liquid Crystal Cell Depending on Dielectric Anisotropy of a Liquid Crystal

A voltage-dependent transmittance curve of the fringe-field switching (FFS) mode was studied with varying dielectric anisotropy of a liquid crystal (LC). The results show that for the LC with positive dielectric anisotropy the disclination lines existing between domains extend into the active region with increasing voltage over the operating voltage, whereas their width becomes smaller with increasing voltage for the LC with negative dielectric anisotropy. This indicates that the LC with negative dielectric anisotropy shows strong dynamic stability in the FFS mode, which can be a great advantage when fabricating microdisplays.

A Novel Fringe Field Switching Mode with 3-partition Pixel Slit

We proposed a novel fringe field switching (FFS) liquid crystal (LC) mode with a 3-partition pixel slit. In this mode, a pixel branch is partitioned into 3 areas, namely, two edges and a center, where the edge slit angle is larger than the center slit angle. Thus the reverse twist region in the pixel edge is reduced and the LC dynamics in this region becomes very stable. Also, when an external pressure is applied to the panel at the operating voltage, the disclination line is barely extended into the active area. Consequently, this mode exhibits a high external pressure resistance with a wide viewing angle and is attractive for pen-based touch panels such as personal digital assistants (PDAs) and tablet personal computers (PCs).

Analysis of cell gap-dependent driving voltage in a fringe field-driven homogeneously aligned nematic liquid crystal display

We have studied cell gap-dependent driving voltage characteristics in a homogeneously aligned nematic liquid crystal (LC) cell driven by a fringe electric field, termed the fringe field switching (FFS) mode. The results show that for the FFS mode using a LC with positive dielectric anisotropy, the operating voltage decreases as the cell gap decreases, whereas it increases with a decreasing cell gap when using a LC with negative dielectric anisotropy. The difference between LCs is explained by simulation and experiment.

Wide-Viewing-Angle Hybrid Aligned Nematic Liquid Crystal Cell Controlled by Complex Electric Field

We have developed a hybrid aligned nematic liquid crystal (LC) cell driven by a complex electric field. In the device, the pixel electrode exists on the bottom substrate and the counter electrodes exist on the top and bottom substrates such that with a bias voltage both vertical and horizontal fields are generated. The LC molecules are hybrid aligned with homogeneous alignment on the bottom substrate where the alignment direction is coincident with one of the transmission axes of the crossed polarizers. Therefore, the cell appears to be black in the absence of an electric field. When a voltage is applied to obtain a white state, both vertical and horizontal fields enable the LC molecules to rotate with lowered tilt angles than those in the dark state. The device shows a much wider viewing angle than that of the twisted nematic mode, high light efficiency and low driving voltage in electro-optic characteristics.

P-164: Pressure-Resistant Characteristic of Fringe-Field Switching (FFS) Mode Depending on the Distance Between Pixel Electrodes

We have investigated pressure-resistant characteristic of the FFS mode depending on the distance between slits of pixel electrode by applying external pressure to the panel. The panel with pixel electrode with long distance shows stronger pressure resistance than that with short distance, which is much advantageous for pen-based touch panel display.

Dynamic Stability of Liquid Crystal Depending on Shape of Pixel Edge in the Fringe Field Switching Mode

We studied the voltage–dependent liquid crystal (LC) dynamic stability corresponding to the pixel edge shape in the fringe field switching (FFS) mode. LC dynamics is very unstable near the edge of the pixel slit, where there is a horizontally different field direction compared with the active region, particularly when the slit angle decreases to 3°. Actually, there are strong field competitions near the edge of the pixel slit due to the patterned pixel shape. Also, a dark disclination line (D/L) at the domain boundary is generated with increasing operation voltage and the D/L extends into the active area at a high applied voltage. It is possible to control LC dynamics near the pixel edge by using different pixel edge shapes. In this paper, we propose an advanced edge shape. This shape has no reverse twist region, unlike the conventional structure, and therefore, LC dynamics is very stable near the edge of the pixel slit. This result indicates that a pixel edge shape with no reverse twist is very important in the design of a high-image-quality FFS mode.