Tae-Hoon Choi

Elimination of image flicker in fringe-field switching liquid crystal display driven with low frequency electric field

Recently, a low frequency driving of a fringe-field switching-liquid crystal display (FFS-LCD) draws much attention to minimize the power consumption. In the low frequency driving of FFS-LCD, an image flickering effect occurs when the sign of the electric field is reversed. We suggested a method to eliminate the image flickering effect by doping small amount of bent-core liquid crystal (BLC) molecules. The BLC molecules have an opposite sign of flexoelectric polarization and reduce the flexoelectric polarization of the host liquid crystal. By adding 2.0 wt% of BLC, the total transmittance during a positive and a negative electric field could be balanced and the image flickering effect was not observed by eyes.

Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure.

Fast grey-to-grey switching of a homogeneously aligned liquid crystal device

We propose a method for fast grey-to-grey (GTG) switching of a homogeneously aligned liquid crystal (LC) cell. Homogeneously aligned LCs are instantly vertically aligned by applying a vertical electric field for a short time before switching to a target grey level. Once the LCs are vertically aligned, we apply an in-plane voltage higher than the data voltage that corresponds to a target grey level, while maintaining the vertical electric field. Then, at the time the vertical field is removed, we apply a data voltage required for switching to the target grey level. We experimentally demonstrated GTG switching of a homogeneously aligned LC cell, which is 11 times faster than a fringe-field switching cell.

Fast in-plane switching of negative liquid crystals using crossed patterned electrodes

We demonstrated fast in-plane switching (IPS) of negative liquid crystals by using crossed patterned electrodes. Switching to the bright state can be achieved by applying an in-plane field using the bottom patterned electrodes, whereas switching to the dark state can be obtained by applying an in-plane field using the top patterned electrodes. We obtained, experimentally, turn-off switching that was four times faster than that of the conventional IPS mode.

Design of an electrode structure for the elimination of the off-axis gamma shift in a multi-domain vertical-alignment liquid crystal cell

Various types of electrode structures were investigated to obtain the threshold voltage difference that allows for the elimination of the off-axis gamma shift in a multi-domain vertical-alignment liquid crystal cell. The threshold voltage difference can be obtained simply by changing the width of the electrodes or the distance between the electrodes in a two-terminal cell. It can also be obtained by applying a different bias voltage to a three-terminal (3T) cell. The advantages and disadvantages of three types of cell structures were compared. By optimizing the electrode structure of a 3T cell with the top electrode grounded, a 2.9 V threshold voltage difference was experimentally obtained.

A Low Voltage Liquid Crystal Phase Grating with Switchable Diffraction Angles

We demonstrate a simple yet high performance phase grating with switchable diffraction angles using a fringe field switching (FFS) liquid crystal (LC) cell. The LC rubbing angle is parallel to the FFS electrodes (i.e. α = 0°), leading to symmetric LC director distribution in a voltage-on state. Such a grating exhibits three unique features: 1) Two grating periods can be formed by controlling the applied voltage, resulting in switchable diffraction angles. In our design, the 1st diffraction order occurs at 4.3°, while the 2nd order appears at 8.6°. 2) The required voltage to achieve peak diffraction efficiency (η~32%) for the 1st order is only 4.4 V at λ = 633 nm as compared to 70 V for a conventional FFS-based phase grating in which α ≈ 7°, while the 2nd order (η~27%) is 15 V. 3). The measured rise and decay time for the 1st order is 7.62 ms and 6.75 ms, and for the 2nd order is 0.75 ms and 3.87 ms, respectively. To understand the physical mechanisms, we also perform device simulations. Good agreement between experiment and simulation is obtained.

Interdigitated pixel electrodes with alternating tilts for fast fringe-field switching of liquid crystals

We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

Electro-optical characteristics of an in-plane-switching liquid crystal cell with zero rubbing angle: dependence on the electrode structure.

When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with zero rubbing angle, virtual walls are built such that the switching speed can be increased several-fold. In this study, we investigate the dependence on the interdigitated electrode structure of the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that when the rubbing angle is zero, the single-layered IPS electrode structure provides a higher transmittance than the double-layered FFS electrode structure because of the reduced width of dead zones at domain boundaries between interdigitated electrodes. Single-layered IPS electrodes not only minimize the transmittance decrease but also provide a shorter response time than double-layered FFS electrodes, although the operating voltage is higher and fabrication requires a more precise rubbing process. The transmittance decrease due to the zero rubbing angle in an IPS cell can be minimized using optimization of the electrode structure while retaining a short response time.

Superfast Low-Temperature Switching of Nematic Liquid Crystals Using Quasi-Impulsive Driving and Overdrive

In this paper, we report on the low-temperature, high-transmittance operation of a nematic liquid crystal (LC) cell. We investigated the electro-optic characteristics of a homogeneously-aligned LC cell over a wide temperature range. By applying high vertical and in-plane trigger pulse voltages between frames to an LC cell, the response time of the LC cell at -20 °C was decreased by 12.5 times compared to that of a conventional fringe-field switching (FFS) cell. In addition to providing a fast response, the LC cell exhibited the same high transmittance as an FFS cell over a wide temperature range.

Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field

We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs.