Transmittance-control devices, such as a suspended particle device, electrochromic device, and dye-doped liquid crystal (LC) device, have been studied for a smart window, eyewear, and automotive applications. These devices require a high transmittance difference between the transparent and opaque states. Among the dye-doped LC devices, a dye-doped chiral-nematic LC (CNLC) cell has been widely used for transmittance-control devices. However, the colors of cells are different between the homogeneously aligned and CNLC cell. In this study, we demonstrated a systematic approach to find optimum dye concentrations for black color in a dye-doped CNLC cell. We took its transmission spectrum into account in the numerical calculation to realize the black color in a dye-doped CNLC cell. Through the iterative method, we could optimize the concentration of each single dye for realizing the black color. We confirmed that a dye-doped CNLC cell designed by considering transmission spectrum of it could provide the black color in the CIE 1931 color space.
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.
We introduce a method for achieving a short response time in homogeneously aligned liquid crystal cells by twodimensional confinement of LCs with virtual walls. When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with interdigitated electrodes parallel to the LC alignment direction, virtual walls are built so that the switching speed can be increased several-fold. We also introduce an interdigitated pixel electrode structure with alternating tilts for a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, this 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.
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.
Recently, photo-alignment technology has been the focus of research efforts because lowering the pre-tilt angle is essential for complete elimination of the off-axis light leakage. However, even though photo-alignment can provide zero pre-tilt angle, it has not yet been widely applied in mass production because of its weak surface anchoring, high curing energy, and strong image sticking. In this paper, we demonstrate that the zero pre-tilt angle can be obtained by employing the field-induced UV-alignment method. We have shown electro-optical characteristics and parameters related to the image quality of a fringe-field switching cell fabricated using the proposed method as functions of the monomer concentration and the UV irradiation time.
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.
Among various liquid crystal display modes, the in-plane switching mode exhibits the widest viewing angle because the liquid crystals are homogeneously-aligned initially and rotate within a plane parallel to the substrates when an in-plane field is applied. However, further improvement is still needed for viewing high-quality dark images from the bisector direction of the crossed polarizers. Several compensation schemes have been proposed to eliminate the off-axis light leakage in a homogeneously-aligned liquid crystal cell. Although a 100:1 iso-contrast contour at an wavelength of 550 nm can cover the entire viewing cone, light leakage at other wavelengths still remains very severe. In this paper we introduce achromatic optical compensation methods using uniaxial films to eliminate the off-axis light leakage at the dark state in homogeneously-aligned liquid crystal cell.Uniaxial films with different dispersion characteristics are used so that they can compensate one another to achieve achromatic optical compensation. The retardation values are optimized through numerical research with the aid of the Poincaré sphere.
We propose a method for fast bistable switching of chiral-nematic liquid crystals. Fast switching from the focal conic to the planar state can be achieved by applying an in-plane electric field for a short period of time. The in-plane field induces a transient state, which relaxes rapidly to the initial planar state. We demonstrate that the switching time from the focal conic to the planar state could be reduced from 150 to 5 ms by applying an in-plane field instead of a vertical field. We achieved a total response time of less than 10 ms. The proposed device is applicable to a reflective display and to other optical switching devices requiring both fast response time and low power consumption.