Jong-Min Baek

Effect of electrode spacing on image flicker in fringe-field-switching liquid crystal display

ABSTRACT Low-frequency driving of a liquid crystal display (LCD) panel to minimise power consumption has drawn much attention recently. In case an LCD panel is driven by a low-frequency fringe field, image flickering phenomenon is detected by the naked human eye when the sign of the applied electric field is reversed. We investigated the effect of electrode spacing on the image flickering phenomenon induced by the flexoelectric effect in a fringe-field-switching (FFS) liquid crystal cell. We found that the image flicker in an FFS cell under low-frequency driving can be eliminated in a simple manner by changing the electrode spacing. GRAPHICAL ABSTRACT

Optical and electrical switching of cholesteric liquid crystals containing azo dye

We propose an optically and electrically switchable cholesteric liquid crystal (ChLC) cell doped with a push–pull azo dye. When the proposed ChLC cell is exposed to UV light, it is switched from the focal-conic to isotropic state by a cholesteric-isotropic phase transition through a trans–cis photo-isomerization of push–pull azo dye molecules. With removal of UV light, the ChLC cell will rapidly relax back to its initial state without exposure to a second light source that has a longer wavelength. We confirmed that the proposed optically and electrically switchable ChLC cell can be used as a smart window that can be switched on by applying sunlight or an electric field.

Sunlight-switchable light shutter fabricated using liquid crystals doped with push-pull azobenzene

We propose a sunlight-switchable light shutter using liquid crystal/polymer composite doped with push-pull azobenzene. The proposed light shutter is switchable between the translucent and transparent states by application of an electric field or by UV irradiation. Switching by UV irradiation is based on the change of the liquid crystal (LC) clearing point by the photo-isomerization effect of push-pull azobenzene. Under sunlight, the light shutter can be switched from the translucent to the transparent state by the nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of the push-pull azobenzene molecules. When the amount of sunlight is low because of cloud cover or when there is no sunlight at sunset, the light shutter rapidly relaxes from its transparent state back to its initial translucent state by the isotropic-nematic phase transition induced by cis-trans back-isomerization of the push-pull azobenzene molecules.

Fast Control of Haze Value Using Electrically Switchable Diffraction in a Fringe-Field Switching Liquid Crystal Device

In this paper, we present a diffractive liquid crystal (LC) device capable of rapid switching between the transparent and translucent states for window display applications. In contrast to previously reported LC light shutters based on light scattering, the proposed LC device relies on diffraction of white incident light by an electric field-induced periodic continuous LC profile. It can be switched between the transparent and translucent states without a complicated driving scheme or a polymer structure. This device exhibits outstanding features for window display applications, such as a high transparency and a wide viewing angle in the transparent state, a low operating voltage, and a short response time.

Dependence of image flicker on dielectric anisotropy of liquid crystal in a fringe field switching liquid crystal cell

Two types of image flicker, which are caused by the flexoelectric effect of liquid crystals (LCs), are observed when a fringe-field switching (FFS) LC cell is driven by a low frequency electric field. Static image flicker, observed because of the transmittance difference between neighboring frames, has been reported previously. On the other hand, research on dynamic image flicker has been minimal until now. Dynamic image flicker is noticeable because of the brief transmittance drop when the sign of the applied voltage is reversed. We investigated the dependence of the image flicker in an FFS LC cell on dielectric anisotropy of the LCs in terms of both the static and dynamic flicker. Experimental results show that small dielectric anisotropy of the LC can help suppress not only the static but also dynamic flicker for positive LCs. We found that both the static and dynamic flicker in negative LCs is less evident than in positive LCs.

Fabrication of an initially-focal-conic cholesteric liquid crystal cell without polymer stabilization

Abstract A dye-doped cholesteric liquid crystal (LC) cell with the focal-conic initial state is demonstrated. Although dye-doped LC/polymer composites have been widely used for LC light shutters, dye-doped LC cells with polymer structures suffer from the degradation of dichroic dyes during the UV curing process. To avoid this problem, we propose a dye-doped cholesteric LC cell using vertical alignment layers instead of polymer structures. We have shown that the focal-conic initial state can be obtained by homeotropic anchoring without the polymer structure. The proposed device not only provides the black color but can also hide objects behind a display panel in the focal-conic initial state without the degradation of dichroic dyes.

Smart window using a thermally and optically switchable liquid crystal cell

Light shutter technologies that can control optical transparency have been studied extensively for developing curtain-free smart windows. We introduce thermally and optically switchable light shutters using LCs doped with push-pull azobenzene, which is known to speed up thermal relaxation. The liquid crystal light shutter can be switched between translucent and transparent states or transparent and opaque states by phase transition through changing temperature or photo-isomerization of doped azobenzene. The liquid crystal light shutter can be used for privacy windows with an initial translucent state or energy-saving windows with an initial transparent state.

Cholesteric liquid crystal cell with the focal-conic initial state

Dye-doped liquid crystal (LC) light shutters with polymer structures suffer from the degradation of dichroic dyes during the UV curing process. To avoid this problem, we propose a dye-doped cholesteric LC light shutter using vertical alignment layers instead of polymer structures for the focal-conic initial state.

Smart windows using polymer-networked liquid crystals doped with push-pull azobenzene

We propose a smart window using polymer-networked liquid crystals doped with push-pull azobenzene. Azobenzene is used to provide phase transition from the nematic to isotropic state through the trans-cis isomerization of azobenzene. When exposed to sunlight, the device switches from the opaque nematic phase to the transparent isotropic phase. Switching from the transparent to opaque state can be obtained through rapid cis-trans isomerization of push-pull azobenzene without sunlight exposure. The proposed device can reduce the transmittance of the incident sunlight during daytime, whereas it can scatter the incident light during the night for privacy.

Elimination of image flicker in a fringe-field switching liquid crystal cell

Recently, low-frequency driving of a display panel to reduce the power consumption has drawn much attention, especially in mobile devices. In case a liquid crystal display panel is driven by a fringe-field at a low frequency, the image flickering phenomenon can be observed when the sign of the applied electric field is reversed. Image flicker can be eliminated simply by applying a bias voltage to a liquid crystal cell so that the transmittance during the positive frame is the same as that during the negative frame. However, it may be difficult to employ this technique for practical applications because it requires a bias voltage that is dependent on the gray level. In this talk, we introduce methods to eliminate the image flicker by controlling the material parameters of liquid crystals, such as the flexoelectric anisotropy and the dielectric anisotropy. Methods to eliminate image flicker without controlling the material parameters, such as driving by a bipolar wave and optimization of the electrode spacing, are also introduced.