Polymer-stabilized Vertical Alignment Research Articles

Recent progress in liquid crystal devices and materials of TFT-LCDs

Thin-film-transistor-liquid crystal displays (TFT-LCDs) still dominate most of the display markets due to a big leap in image quality while keeping low cost and superior reliability, and its evolution is continuing to compete with emerging emissive displays. In this paper, recent progress in liquid crystal technologies mainly focusing on fringe-field switching (FFS) and polymer-stabilized vertical alignment (PS-VA) modes and materials for improving electro-optic performances is reviewed.

34‐5: Invited Paper: C‐PS‐VA — Innovative LC Materials Enabling Super‐High Transmittance for High‐Resolution Displays

In recent years, and for the next generation of TVs and monitors, increasing resolution is a key trend. One challenge is keeping the transmittance while pixel density increases. Chiral Polymer Stabilized Vertical Alignment (C‐PS‐VA) technology uses LC mixtures with higher birefringence and a chiral dopant and increases transmittance by more than 10% compared to conventional Polymer Stabilized Vertical Alignment (PS‐VA). This paper summarizes the recent developments of liquid crystal mixtures for C‐PS‐VA.

P‐159: The Transmittance Improved of Polymer Stabilized Vertical Alignment Liquid Crystal Displays by Chiral Doping

High transmittance is desired for high‐resolution liquid crystal display (LCD). In this work, the transmittance of Polymer Stabilized Vertical Alignment (PSVA) LCD mode has been greatly improved by chiral doping, and the mechanism of such chiral PSVA mode is illustrated for the first time. Compared with normal PSVA mode, the present chiral PSVA mode exhibited transmittance higher than 10%, accompanishing with comparably high contrast ratio and wide viewing angle. This novel chiral PSVA technology can be used in various LCD applications.

P‐158: Generation of Pretilt Angle and an Alignment Layer Utilizing Binary Mixture in PI‐less Vertical Alignment Mode

We demonstrate new approach of polyimide (PI)‐less polymer stabilized vertical alignment (PS‐VA) mode in which binary mixture of single kind of reactive mesogen and LC induces a VA with temperature treatment. The induced VA is fixed at first UV exposure and then a pretilt angle is generated with second UV exposure. The proposed device with two‐step UV exposure exhibits excellent electro‐optics such as fast response time and enhanced on‐state transmittance. This approach fixes a vertical alignment physically unlike chemical bonding in conventional PI‐less VA mode, giving rise to high reliability.

Microscopic-Stress-Induced retardation between amorphous silicon nitride and amorphous indium zinc oxide having different stresses

It is very important to increase the contrast ratio in liquid crystal display (LCD) TV using polymer-stabilized vertical alignment (PS-VA) mode. The luminance of the black state is greatly reduced by changing the parameters of the polarizer, the color filter, and the liquid crystal. However, these have the side effect of reducing the characteristics such as the transmittance, the response time, and the viewing angle. In this paper, the luminance of the black state can be reduced by lowering the stress induced retardation at the edge of pixel electrode without changing the materials in the PS-VA mode. It has been shown for the first time that the stress difference in the heterogeneous layers such as amorphous indium zinc oxide (a-IZO) and amorphous silicon nitride (a-SiNx) can optically create birefringence in the microscopic region. By controlling the stress difference in the heterogeneous membrane, the microscopic-stress-induced retardation (MSIR) could be lowered and the contrast ratio of the PS-VA mode could be increased up to 10%.

Reduction of residual DC voltage via RC matching in LCD

To reduce the residual direct-current (DC) voltage in liquid-crystal display (LCD) using the polymer-stabilized vertical-alignment (PS-VA) mode, the electrical time constant of two or more serially connected dielectric materials, such as liquid crystals (LCs) and silicon nitride (SiNx), is investigated. PSpice simulation of the alternating-current (AC) voltage changes in the two dielectric materials shows mismatch of the time constant, and a resistance cross capacitance (RC) between the two layers is found to be the source of the residual DC voltage. The optimization of the stoichiometry of SiNx is effective for matching the time constant of LCs because it is easy to control the time constant by changing the gas ratio, e.g., SiH4, N2, and NH3, when SiNx is deposited. We suggest that image sticking in the LCD panel can be minimized via RC optimization between the SiNx and LCs by adjusting the stoichiometry of SiNx, which means that the chronic problem of off-axis gamma distortion in the VA mode for large sized, high-resolution TV up to Quad Ultra High Definition (QUHD, 15360 × 8640) can be solved.

Mobile ion induced electric field effect in polymer-stabilized vertical alignment liquid crystal display

Mechanisms of radical polymerization on the surface of the alignment layer in polymer stabilized vertical alignment (PS-VA) liquid crystal display (LCD)are suggested. When DC voltage is applied between top and bottom electrode, it is observed that the liquid crystal molecules distort alignment directions, which deteriorate luminance uniformity during UV irradiation process. It is revealed that the liquid crystal movement is attributed to mobile ions, (K+, OH−), smearing out from the interface at the lower layer of the pixel electrode. The mobile ions are generated from the developer remains after color filter formation process. To minimize the influence of the mobile ions, the thickness of the insulating SiNx layer under the pixel electrode and the color filter is increased, which contribute to luminance reduction rate from 20% to less than 1%.

P‐139: The Investigation of Field Sequence Color Liquid Crystal Display with New Polymer Stabilized Vertical Alignment Mode

In this paper, a field‐sequential‐color liquid crystal display (FSC LCD) based on new polymer stabilized vertical alignment (N‐PSVA) liquid crystal display mode and photo alignment technology have been developed. An excellent response time of the device can be obtained by the developed of N‐PSVA LC and the optimizing of process, with the operating voltage keep in 10V. By this work, the 32 inch large size FSC TV panels were presented

Transition of vertically aligned liquid crystal driven by fan-shaped electric field

Interdigital electrodes are implemented in many commercial and novel liquid crystal devices to align molecules. Although many empirical principles and patents apply to electrode design, only a few numerical simulations of alignment have been conducted. Why and how the molecules align in an ordered manner has never been adequately explained. Hence, this investigation addresses the Fréedericksz transition of vertically aligned liquid crystal that is driven by fishbone electrodes, and thereafter identifies the mechanism of liquid crystal alignment. Theoretical calculations suggest that the periodic deformation that is caused by the fan-shaped fringe field minimizes the free energy in the liquid crystal cell, and the optimal alignment can be obtained when the cell parameters satisfy the relation p/2d=k11/k33, where p is the spatial period of the strips of the electrode; d denotes the cell gap; and k11 and k33 are the splay and bend elastic constants of the liquid crystal, respectively. Polymer-stabilized vertical alignment test cells with various p values and spacings between the electrodes were fabricated, and the process of liquid crystal alignment was observed under an optical microscope. The degree of alignment was evaluated by measuring the transmittance of the test cell. The experimental results were consistent with the theoretical predictions. The principle of design, p/2d=k11/k33, greatly improves the uniformity and stability of the aligned liquid crystal. The methods that are presented here can be further applied to cholesteric liquid crystal and other self-assembled soft materials.

Reduction of Gamma Distortion in Oblique Viewing Directions in Polymer-stabilized Vertical Alignment Liquid Crystal Mode

Reduction of Gamma Distortion in Oblique Viewing Directions in Polymer-stabilized Vertical Alignment Liquid Crystal Mode

Transmittance Improvement with Reversed Fishbone-Shape Electrode in Vertical Alignment Liquid Crystal Display

A polymer-stabilized vertical alignment (PS-VA) mode with fishbone-shaped pixel electrode structure is mainly used in large-sized liquid crystal displays (LCDs) owing to its advantages such as wide viewing angle, good transmittance and fast response time. One drawback of this mode is a main bone electrode, which crosses in the center of a pixel. It causes the transmittance to decrease badly because LCs cannot be reoriented in this region, and thus, it is particularly unfavorable in an ultra-high-definition LCD. Here, we propose an innovative structure with the main bone electrode relocated to the edge area in a pixel, and investigate how this reverse directed fishbone-shaped pixel electrode structure affects electro-optic characteristics. The proposed structure shows enhanced electro-optic performance, such as the higher transmittance and the faster response time than the conventional VA mode with fishbone-shaped pixel electrode structure.

P-133: A Novel Approach to Suppress the Gamma Distortion in Oblique Viewing Angles for Polymer-stabilized Vertical Alignment Mode

P-133: A Novel Approach to Suppress the Gamma Distortion in Oblique Viewing Angles for Polymer-stabilized Vertical Alignment Mode

Polymer stabilized vertical alignment liquid crystal display: effect of monomer structures and their stabilizing characteristics

A polymer-stabilized vertical alignment (PS-VA) mode using a new type of photoreactive monomer for polymer stabilization of the liquid crystal (LC) director was developed. Conventional reactive mesogens having a higher molecular weight than those of the host LC tend to aggregate and form large-sized polymer grains when exposed to ultraviolet (UV) light, subsequently deteriorating the quality of the dark state. To address these problems, bis(4-hydroxyphenyl) diacrylates were synthesized with four different linking groups as stabilizing monomers (SMs) which have molecular weights similar to that of the host LC. Their stabilizing characteristics with respect to the molecular size and polarity of SMs were evaluated by examining the electro-optic characteristics of LC cells after UV irradiation. The results showed that the SM containing a small linking group in size between biphenyls with high polarity was favored to achieve excellent polymer stabilization. The SM containing an ether linkage showed excellent electro-optic characteristics with no large-sized polymer grains even in the absence of a photo-initiator. Consequently, we anticipate that SMs, polar and smaller in size, can improve the electro-optic characteristics in PS-VA mode.

27.2: The Fabrication of Novel PSVA Pixel Structure by GTM Technology

The novel polymer‐stabilized vertical alignment (PSVA) pixel structure was fabricated by gray tone mask (GTM) technology. With the GTM technology, one mask is reduced and the process is simplified. The influence of process parameters on groove structure have been researched in detail.

Phase separation of monomer in liquid crystal mixtures and surface morphology in polymer-stabilized vertical alignment liquid crystal displays

The polymer-stabilized vertically aligned (PS-VA) liquid crystal display (LCD) driving mode has high potential for manufacturing low power consuming displays due to the higher transmittance and fast response as compared with the existing patterned vertically aligned and multi-domain vertically aligned modes. In this paper we have investigated the reaction mechanisms of monomer–liquid crystal blends to form a surface pre-tilt angle of liquid crystal in vertical alignment LCD associated with a fishbone electrode structure. The observed sizes of polymer bumps formed on the substrates were found to be dependent on the exposed UV wavelength and were almost equal in both top and bottom substrates. When a large UV wavelength was used, the phase separation mechanism of monomer in PS-VA mode was found nearly isotropic rather than anisotropic in contrast to the previous studies.

69.1: The Method and Mechanism of Optimizing Optical Performances of Polymer‐Stabilized Liquid Crystals Panels

AbstractThe method modifying optical performances of polymer stabilized vertical alignment (PSVA) LCDs is discussed in this paper. The mechanism and relationship between UV curing conditions and optical properties is also proposed. A PSVA panel with high contrast ratio and fast response time could be achieved by controlling the curing conditions during fabricating process.

Switching of Polymer-Stabilized Vertical Alignment Liquid Crystal Pi Cell-Curing Voltage and Driving Scheme Effects

We investigate the electrooptical properties of polymer-stabilized vertically aligned (PSVA) pi cells. Curing a cell with a curing voltage causes the polymerized cell to have a low threshold voltage and a fast field-on response time, because the curing voltage gives the polymerized cell in-cell polymer structures, inhibiting the flow of the liquid crystal (LC) domains and also providing the LCs a sustained pretilt angle. Additionally, under high-voltage operation, the field-on and field-off response times of the PSVA pi cell operated under the step voltage driving scheme are found to be less than those of the cell operated under the conventional bias voltage driving scheme, because the electric torques exerted on the LCs in the step voltage driving scheme are higher than those exerted on the LCs in the bias voltage driving scheme.

Switching of polymer-stabilized vertical alignment liquid crystal cell

This work investigates the switching characteristics of the polymer-stabilized vertical alignment (VA) liquid crystal (LC) cell. The experimental results reveal that the fall time of the cell declines as the monomer concentration increases because the vertically-aligned polymer networks accelerate the relaxation of the LC molecules. Furthermore, the formed polymer networks impede the growth and annihilation of LC defects, suppressing the optical bounce in the time dependent transmittance curve of the cell when the voltage is applied to the cell, substantially reducing the rise time of the cell. A step-voltage driving scheme is demonstrated to eliminate completely the optical bounce and hence improve further the rise time of the VA LC cell. The rise times of the pristine and the polymer-stabilized VA LC cells under the step-voltage driving scheme are less than 50% of those under the conventional driving scheme.