Gi-Dong Lee

Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen

In the patterned vertical alignment (PVA) cell in which multidomains are formed from the perfect vertical alignment through an oblique field only, the formation of disclinations between liquid crystal (LC) molecules is inevitable in the presence of an electric field, which lowers transmittance and the response time. In the proposed PVA device, the pretilt angle is formed in four different directions through the polymerization of an UV curable reactive mesogen monomer at the surface. In this way, the reorientation of LC responding to an electric field is well defined, and thus the device shows reduced threshold voltage and much improved response time in all gray scales.

Nontwist quarter-wave liquid-crystal cell for a high-contrast reflective display

We propose an optical configuration of a nontwist quarter-wave liquid-crystal cell for a high-contrast reflective display that can be applied to most nontwist display modes. By fabricating a homogeneous cell with the proposed configuration, we have demonstrated a high contrast ratio of 50:1 without using a wideband retardation film.

Alignment of liquid crystal on a polyimide surface exposed to an Ar ion beam

In this paper properties of nematic liquid crystals aligned on polyimide surfaces exposed to a low-energy Ar ion beam are investigated. We studied how the pretilt angle of a liquid crystal cell is affected by ion-beam conditions, such as the energy of the incident ions, the angle of incidence, and the exposure time. X-ray photoelectron spectroscopy data indicate that ion-beam exposure changes the chemical bonding states of the polyimide surface, which may be one of the main causes of anisotropic liquid crystal alignment. We also found that twisted-nematic cells fabricated by the ion-beam alignment method have the voltage holding time enough for application to thin-film-transistor liquid crystal displays.

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.

Viewing angle switching of vertical alignment liquid crystal displays by controlling birefringence of homogenously aligned liquid crystal layer

Viewing angle switching from a wide viewing angle to a narrow viewing angle has been studied. Conventional multidomain vertical alignment (VA) mode offers the advantages of a high contrast ratio (CR) not only in the front view but also in the wide viewing directions only if compensation films such as a negative C plate and a positive A plate are used. The positive A plate can be replaced by a homogeneous aligned (HA) liquid crystal layer, and the retardation of the HA layer at the off axis can be controlled by applying an electric field while keeping the retardation value at zero in the normal direction. Consequently, the viewing angle range of a VA device can be controlled from a wide viewing mode (over 170° in terms of CR=10) to a narrow viewing angle mode (approximately 60° in terms of CR=2) in the horizontal direction while keeping a high image quality at the normal direction.

Configuration Optimization of a Reflective Bistable-Twisted-Nematic Cell for High-Contrast Operation

In this study, the configuration of a reflective bistable-twisted-nematic (BTN) liquid-crystal cell is optimized for high contrast and high brightness operation. We searched for the optimum optical parameters of a reflective BTN cell by calculating its optical performances at three wavelengths; red, green, and blue. By studying the effect of each optical parameter on the optical performances, we found that the angle of the polarizer is more important than any other optical parameter in the design of a reflective BTN cell. We fabricated a reflective BTN cell with a wide-band retardation film, whose measured contrast ratio is 10.6:1.

A single-gap transflective fringe field switching display using a liquid crystal with positive dielectric anisotropy

There is considerable difficulty in fabricating a reflector with embossing in an array substrate using a conventional single gap transflective fringe-field switching nematic liquid-crystal display. In order to solve this problem, we propose a new structure, which consists of a reflector on a colour filter substrate. The newly proposed structure with a complex field direction has problems such that the voltage-dependent transmittance and reflectance curves do not match each other, which necessitate a dual driving circuit. This paper reports the optimized electrode structure and calculated electro-optical results realizing a single gamma curve and high light efficiency.

Study on colour characteristics depending on orientation of liquid crystal in the in‐plane switching mode

The degree of colour shift depending on viewing direction in the in‐plane switching (IPS) mode has been investigated. The IPS cell with pure twist deformation exhibits a stronger colour shift than that of the IPS cell with tilt as well as twist deformation, although the former has better luminance uniformity in the bright state than the latter. Furthermore, the IPS cell with multi‐directional LC orientation, with tilt as well as twist deformation, shows the least colour shift.

Homeotropically aligned nematic liquid crystal device locked by a polymer wall with wide viewing angle

We fabricated a homeotropically aligned nematic liquid crystal display (LCD), where the chiral-doped LC with negative dielectric anisotropy is locked by a polymer wall under crossed polarizers and whose on and off states are controlled by a vertical electric field. In the absence of an electric field, the rubbing-free device appears to be dark. In the presence of the field, the homeotropically aligned LCs tilt down, giving rise to brightness but four brush schlieren textures appear with point singularity S=+1. This indicates that the mid-directors have radial alignment inside the polymer wall in the voltage-on state. Consequently, the device shows excellent viewing angle characteristics. The electro-optic characteristics of one prototype with excellent viewing angles are reported herein.

Viewing angle switching of patterned vertical alignment liquid crystal display

Viewing angle control of a patterned vertical alignment (PVA) liquid crystal display using only one panel is investigated. In conventional PVA modes, a vertically aligned liquid crystal (LC) director tilts down in four directions making 45° with respect to crossed polarizers to exhibit a wide viewing angle. In the viewing angle control device, one pixel was divided into two sub-pixels such that the LC director in the main pixel is controlled to be tilted down in multiple directions making an angle with the polarizer, playing the role of main display with the wide viewing angle, while the LC director in the sub-pixel is controlled to be tilted down to the polarizer axis, playing the role of sub-pixel to the viewing angle control for the narrow viewing angle. Using sub-pixel control, light leakage or any type of information such as characters and image can be generated in oblique viewing directions without distorting the image quality in the normal direction, which will prevent others from peeping at the displayed image by overlapping the displayed image with the made image.