Vertical Alignment Mode 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.

Rigid-flex stamped indium tin oxide film for convertible self-alignment

The alignment layer process, which is a key element for liquid crystal (LC) displays, was investigated for efficiency improvement and for suggesting a new mode. To produce alignment films that can be used in both twisted-nematic (TN) and vertical alignment (VA) configurations, InO solution is doped with SnO. Then, the pattern was transferred by attaching a one-dimensional soft-imprint mold, and pattern and orientation analyses were performed in accordance with the SnO ratio. The horizontal orientation performance improved as the SnO ratio increased, and the best orientation performance was observed for a film doped with 70% SnO. Further, the thermal stability improved and anchoring energy increased, which was measured to be very close to that of the rubbed polyimide film. The low transmittance of InO was overcome by doping with SnO, which has high transmittance. In the film doped with 80% SnO, the LC molecules were aligned in a vertical direction and were suitable for VA mode operation. By simple adjustment of the doping ratio, differences are observed with respect to the existing films in that the performance of the alignment layer is improved and the pretilt angles of the LC molecules can be changed. These convertible characteristics are expected to play a revolutionary role in the development of new LC driving modes.

Temperature-switched orientation properties through physicochemically constructed aluminum zirconium oxide films

In general, it has one of the characteristics of homeotropic alignment and homogeneous alignment depending on the material and manufacturing process of the liquid crystal alignment layer. However, in this study, we propose a new type of alignment film that has the characteristics of changing the liquid crystal alignment direction according to the curing temperature. The AlZrO alignment film produced by the solution process and brush coating method showed homeotropic alignment characteristics when cured at low temperatures and homogeneous alignment characteristics when cured at high temperatures. These switching characteristics according to the curing temperature showed stable LC alignment, respectively, and the threshold voltages were 0.81 V and 0.857 V in vertical alignment (VA) mode and twisted nematic (TN) mode, respectively, demonstrating excellent electro-optical properties. The mechanism of these switching characteristics was analyzed through surface modification analysis. This study introduces new insights into liquid crystal alignment films that satisfy all of the high-efficiency fabrication process, various application possibilities due to their switching properties, and excellent electro-optical properties.

Self aligned liquid crystals on sol-gel-processed zirconium oxide surface via nanoimprinting lithography

In this paper, a nanopattern was effectively formed on a zirconium oxide (ZrO) film, and a uniform liquid crystal (LC) alignment layer was developed through it. One-dimensional (1D) nanopatterns from polydimethylsiloxane (PDMS) molds were transferred to ZrO films via nanoimprinting lithography (NIL). The pattern height of the PDMS mold was 30 nm, and the ZrO film was cured at 100 °C and 200 °C. The transfer of nanopatterns was analyzed by observation of surface modification through atomic force microscopy. The ZrO solution injected between the molds in a sol state by capillary force did not sufficiently solidify at 100 °C, but at 200 °C, sufficient pattern height and period were secured. Using X-ray photoelectron spectroscopy, it was confirmed that ZrO used as an alignment layer was well deposited on the substrate surface. An increase in the contact angle with pattern formation was also confirmed, indicating a decrease in surface energy and hydrophobicity of the surface. A polarized light microscope and pretilt angle were analyzed using a liquid crystal cell manufactured with an antiparallel structure, and the excellent alignment characteristics of the 200 °C cell were confirmed. The vertical alignment mode was confirmed in the voltage-transmittance curve, and a good threshold voltage was observed. Through these analysis methods, the nanopattern was successfully transferred to the ZrO film through NIL, and excellent performance as a liquid crystal alignment layer was confirmed.

Wide-temperature-range liquid-crystal display with liquid-crystal material of negative dielectric anisotropy

A vertical alignment mode liquid-crystal display (VA-LCD) with a wide nematic temperature range was developed in this work. For development of this LC cell, a negative dielectric anisotropy liquid-crystal material (nega-LCM) with wide nematic temperature range from −40 to +90 °C (nega-WNT-LCM) was selected. The fabricated VA mode LC cell showed twice faster response time at −20 °C compared with the conventional negative-type LCM, and significantly high contrast ratio (2910:1). In addition, the VA mode LC cell with the nega-WNT-LCM would be used in a severe environment with remarkably high- and low-temperature region. Thus, the developed VA mode LC cell would be useful for outdoor-use applications, such as the automotive application.

Rational Analysis of Self-Alignment Force of Reactive Additive Monomer with Multiple Methods

Analysis self-alignment force (SAF) of reactive self-assambly additive monomer is highly valuable in self-alignment vertical alignment mode(SAVA), because this force directly influences performance of optoelectronics device. In this work, SAF has been studied by multiple methods, such as atomic force microscopy, mueller matrix imaging polarimeter. Reactive additive monomer and LC molecular were immobilized on the surface of an AFM probe and ITO, respectively. Due to the complexity of the additive monomer-hydroxy, additive monomer-LC moluclar interaction, parallel experiments were designed to discriminate specific interactions. The difference of pretilt angle between TFT side and CF side also reveals the difference of SAF at various structure cell.The additive with strong SAF showed well alignment state after forming into optoelectronice device.

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.

P‐157: Rational Analysis of Self‐Alignment Force of Reactive Additive Monomer with Multiple Methods

Analysis self‐alignment force (SAF) of reactive self‐assembly additive monomer is highly valuable in self‐alignment vertical alignment mode(SAVA), because this force directly influence performance of optoelectronics device. In this work, SAF has been studied by multiple methods, such as atomic force microscopy, mueller matrix imaging polarimeter. Reactive additive monomer and LC molecular were immobilized on the surface of an AFM probe and ITO, respectively. Due to the complexity of the additive monomer‐hydroxy, additive monomer‐LC moluclar interaction, parallel experiments were designed to discriminate specific interactions. The difference of pretilt angle between TFT side and CF side also reveals the difference of SAF at various structure cell. The additive with strong SAF showed well alignment state after forming into optoelectronic device.

P‐11.14: The Controllable of Viewing angle of Polymer Stabilized Vertical Alignment Liquid crystal Displays by adjusting parameter of liquid crystal

The viewing angle of the Polymer Stabilized Vertical Alignment Mode(PSVA) was studied detailly and a new way to optimize the viewing angle of PSVA LCD is developed. An excellent viewing angle can be obtained by optimizing the parameters of the Liquid crystal and curing procss, such as birefringence, dielectric et.al. By this work, it will offer a new direction for future PSVA and improve its competitiveness in high‐end market compared with the IPS mode.

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%.

Fast switching of nematic liquid crystals using a new alkyl-free bifunctional mesogenic monomer

The vertical alignment mode is widely applied in liquid crystal (LC) devices but exhibits a slow response time. To induce fast switching of LCs, bifunctional reactive monomers (RMs) were mixed with polyimide (PI). Two types of RMs were used: RMs with alkyl spacers and alkyl-free RMs. The mixture of RMs and PI was treated with ultraviolet light under an electric field to induce the pre-tilted LC alignment. The newly fabricated alignment layers exhibited low threshold voltages and fast response times compared to pure PI. Furthermore, the alkyl-free monomers induced a faster switching of the LCs than RMs with alkyl spacers.

Fabrication of vertical alignment liquid crystal cell without forming a conventional polyimide-alignment layer

ABSTRACTA novel preparation method of the vertical alignment polymer film (VAPF) was proposed by polymerisation of the monomers, 2,3-difluoro-4ʹ-[2-(diacryloyl-oxy-hexyl)methyl ester-hexadecyl-oxy]-biphenyl (DFDAMHB) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment (VA), the ultraviolet exposure was carried out to the LC cell below the nematic to isotropic transition temperature of the LC material. The VA mode LC cell with the VAPF produced from the monomers DFDAMHB and DMAPhen exhibited enough level of electro-optical and response properties, high voltage holding ratio and low residual direct current voltage.

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

Micropatterned Vertical Alignment Liquid Crystal Mode With Dual Threshold Voltages for Improved Off-Axis Gamma Distortion

We proposed an eight-domain vertical alignment (VA) liquid crystal (LC) mode, operated by dual pixel domains of fishbone-structured electrodes for dual threshold (F-DT) voltages in the VA mode, which can improve off-axis gamma distortions without additional electronic circuits and fabrication procedures compared with the conventional multidomain VA modes. In the proposed F-DT VA mode, different threshold voltage levels can be obtained for the main- and subpixel regions by utilizing different field-induced LC reorientations determined by the pixel electrode structures, whose properties are especially enhanced in low-voltage regimes. When the relative area ratio ( ${R} _{A}={A}_{\text {main}}/{A}_{\text {sub}}$ ) between the main- and subpixel domains was ${R} _{A}=\text {5}$ , these dual threshold properties of the F-DT VA mode improved the off-axis gamma distortion index (GDI) value up to 34.04% at the oblique viewing condition of $\theta = {60}^{\circ }$ , compared to the conventional patterned VA (PVA) modes, while maintaining the maximum transmittance level. The proposed pixel electrode design scheme could be applied to further improve the GDI value simply by controlling ${R}_{A}$ , where the GDI of the F-DT VA mode was improved by 39.17% and 17.05% at ${R} _{A}= {2}$ , compared to the conventional PVA and micropatterned VA modes, respectively.

Electrically tunable liquid crystal lens with suppressed axial chromatic aberration

We demonstrate an electrically tunable dual-switching liquid crystal lens with suppressed axial chromatic aberration. A liquid crystal (LC) switch twisted vertical alignment (TVA) mode is utilized in the lens. By using a chiral nematic LC with a proper cell thickness-to-helical pitch ratio (d/p) and a thick cell gap, the lens provides a low director rotation rate, and the focal length dispersion at different wavelengths is reduced. The image quality of the LC lenses with different d/p ratios is investigated. The axial chromatic aberration can be reduced up to 30% when the d/p ratio is 0.5 ± 0.05. Comparing with a traditional liquid crystal lens without a twisted-nematic structure (i.e., d/p = 0), the contrast ratio of the image obtained from a lens with a twist structure (d/p = 0.5 ± 0.05) increases by 15%. The static voltage response from a LC cell shows that the suppressed chromatic aberration results from the suppressed wavelength dispersion of phase retardation-voltage relation of the TVA mode.

Fabrication of vertically aligned liquid crystal cell without using a conventional alignment layer

ABSTRACTWe propose a novel method to fabricate a vertical alignment (VA) of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the vertical alignment polymer layer (VAPL) by polymerisation of a monomer or mixed monomers including in the LC layer above TNI of the LC material. The VA mode LC cell with the VAPL (VAPL-LC cell) produced from the mixed monomers of acrylic acid 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester and 1,2-bis-(4-methacryloxy-phenyl)-2,2-dimethoxy-ethane-1-one exhibited enough level of alignment state and electro-optical property with high voltage holding ratio. We can expect that the VAPL-LC cell is useful for next-generation displays such as flexible liquid crystal displays because the method does not need the process including high temperature over 200°C.

Analytical equation for the motion picture response time of display devices

Motion picture response time (MPRT) affects the image blurs of thin-film transistor (TFT) liquid crystal displays and organic light emitting diode (OLED) displays. We derive an analytical equation to correlate MPRT with the liquid crystal (LC)/OLED response time and TFT frame rate. Good agreement between our physical model and experimental results is obtained. Based on our model, we find that if the LC's response time is 2 ms or less, then its MPRT is nearly the same as that of OLED, even if OLED's response time is assumed to be 0. To achieve MPRT comparable to OLEDs, we developed an ultra-low viscosity LC mixture for the vertical alignment mode operation. The measured average gray-to-gray response time is 0.93 ms, and its MPRT at 120 Hz is 6.88 ms. In comparison, OLED's MPRT is 6.67 ms. To further shorten MPRT, we could either increase the frame rate or reduce the backlight duty ratio. Pros and cons of these approaches are discussed.

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.

New three-gamma-curves-integrated VA LCD design for excellent image quality and enhanced transmittance

ABSTRACTCompared with the in-plane switching (IPS) mode of the liquid crystal display (LCD), the vertical-alignment (VA) LCD mode provides an excellent contrast ratio and a fast response time, but it still has a relative weakness: image quality deterioration as a function of the viewing direction. In the VA mode, the pixel division method, in which two different gamma curves from the divided pixels are integrated, is generally used to improve the off-axis image quality. Reported herein is the new VA mode for enhancing the off-axis image without decreasing the transmittance. The pixels divided into 12 domains instead of 8 using double electrode layers produce three different gamma curves from the high, middle, and low pixels, which are combined and enhance the image quality. In this pixel structure, the middle pixels are formed by two electrode layers and produce the third gamma curve. The new design of the VA mode shows the best off-axis image quality in the VA mode, which is the most comparable with that in the IPS mode, even with the organic light-emitting diode, and will provide a solution for the heavy competition in the ultrahigh-definition (UHD) and large-LCD-TV-panel market in the very near future.

23-3: Splay and Bend of Liquid Crystal in Vertical Alignment Mode

The process of liquid crystal alignment in Polymer Stabilized Alignment Mode (PSA) is demonstrated in this work. The splay and bend deformation of liquid crystal in PSA was analyzed, and the principle to control the sequence of splay and bend deformation was discovered.