Vertical Alignment Mode Research Articles

P‐22: A Single Cell‐Gap Transflective LCD with Active Level‐Shifting Technology

AbstractA novel single cell‐gap transflective liquid crystal display (LCD) with active level—shifting technology has been developed. Pixel structure design enables different voltages to be independently applied to transmissive and reflective sub‐pixels. The prototype 2.2‐inch vertical alignment (VA) mode transflective LCD achieves a 17% increase in aperture ratio compared to the conventional dual cell‐gap approach. The reflective mode has 15:1 contrast ratio and 3.6% reflectance, and the transmissive mode accomplishes much higher contrast ratio of 500:1, compared to 200:1 for the dual cell‐gap transflective LCD.

Comparison of Friction Characteristics on TN and VA Mode Alignment Films with Friction Force Microscopy

Using frictional force microscopy (FFM), the friction surface characteristics were compared between twisted nematic (TN) mode and vertical alignment (VA) mode alignment films (AFs). The friction asymmetry was detected depending on temperature conditions on TN mode AF, but not on VA mode AF. The difference between two modes was explained by leaning intermolecular repulsion caused by the pre-tilt angle uniformity and the density of side chain. No level difference according to temperature conditions appeared when the pre-tilt angle were measured after liquid crystal (LC) injection.

Control of liquid crystal director near signal lines and reduction of load of signal lines by optimized pattern of common electrode in the patterned vertical alignment mode

Abstract This study examined the electro-optical characteristics and stability of a liquid crystal director depending on the electrode patterning of a common electrode on the top substrate in patterned vertical alignment (PVA) mode. The new type of common electrode pattern suggests that the dark state of the device is improved by reducing the interfering electrical field caused by the signal lines, which can help to improve the aperture ratio and also the load-on signal lines were reduced, resulting in a decrease in power consumption.

28.2: The Molecular Orientation of Polyimide Alignment Layer used in TFT‐LCDs with NEXAFS Study

AbstractAromatic polyimide thin films are used as the alignment film in thin film transistor‐liquid crystal displays (TFT‐LCD's). The films play an important role in the initial alignment of the liquid crystal materials in LCD panels. Therefore, any structural or morphological deformation may lead to defect in the physical interactions thus malfunction of the display eventually. for example, an LCD is known to bear image sticking problem when it is assembled following 3 days or longer stagnancy in the process after main curing of the layer. In this paper, the molecular orientation of polyimide surfaces with respect to the stagnancy is investigated by means of surface sensitive Near Edge X‐ray Absorption Fine Structure (NEXAFS) spectroscopy. NEXAFS measurements clearly reveal preferred in‐plane and out‐of‐plane orientation of phenyl and C=O groups at the surface of the polyimide thin film in regard to the stagnancy after main curing. with longer stagnancy phenyl and C=O groups preferred in‐plane orientation. The preferred in‐plane orientation within the film can accelerate the image sticking problem in LCD's operating in PVA mode. Based on the result, vertical alignment of polyimide pendant group is found helpful for the better alignment of LC molecules in LCD's operating in the patterned vertical alignment (PVA) mode.

P‐199: Achromatic Reduction of Off‐Axis Light Leakage in LCDs by Self‐Compensated Phase Retardation (SPR) Film

AbstractWavelength dispersion of refractive indices of compensation films causes the problems of light leakage and color shift at oblique incident angles in the black state of liquid crystal displays. We propose a self‐compensated phase retardation (SPR) film reducing the light leakage stemming from the wavelength dispersion of compensation films. Numerical calculations show that SPR films suppress the leaked light significantly more than the state‐of‐art compensation scheme used in vertical alignment mode liquid crystal displays.

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

AbstractOn p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode).We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.

Mesomorphic, dielectric, and optical properties of fluorosubstituted biphenyls, terphenyls, and quaterphenyls

AbstractCompounds with moderate and large negative dielectric anisotropy (Δɛ) are very attractive liquid crystal (LC) for vertical alignment mode (VA). Materials with such properties can be achieved by lateral substitution of a polar group into a mesogenic molecule. We synthesized some new LC materials with a negative value of Δɛ, a moderately high birefringence (Δn), and a low viscosity. The mesomorphic and physical behaviour of the novel biphenyls, terphenyls and quaterphenyls fluorosubstituted in the rigid core and also with fluorinated alkyl and alkoxy chains are investigated. The prepared series of four LC compounds are promising for new LC mixtures for various applications. Examples of nematic mixtures with Δɛ∼−3.25 will be presented.

Optimization of LCD Pixel Structure for Continuous Pinwheel Alignment (CPA) Mode

In this article, we report our theoretical study on the electro-optical properties of continuous pinwheel alignment (CPA) mode. We performed three-dimensional finite element method (3D-FEM) simulations on various CPA electrode patterns and compared our simulation results with the conventional vertical alignment (VA) mode. Each CPA pixel pattern was carefully examined with respect to the transmission, dynamic response, and contrast ratio as well as viewing characteristics in an effort to optimize the electrode pattern. Our numerical study revealed that the optimum dimension for the thickness of the liquid crystal (▵d) should be chosen as 2.9 μm where CPA mode can exhibit superior performance on VA mode in terms of transmission as well as viewing characteristics. Further, our numerical simulation demonstrated that the transmittance of CPA mode can be improved over VA mode by 81% while the viewing angle characteristics can be improved by 22%.

Stabilization of the liquid crystal director in the patterned vertical alignment mode through formation of pretilt angle by reactive mesogen

In the patterned vertical alignment (PVA) cell in which multidomains are formed from the perfect vertical alignment through an oblique field only, the formation of disclinations between liquid crystal (LC) molecules is inevitable in the presence of an electric field, which lowers transmittance and the response time. In the proposed PVA device, the pretilt angle is formed in four different directions through the polymerization of an UV curable reactive mesogen monomer at the surface. In this way, the reorientation of LC responding to an electric field is well defined, and thus the device shows reduced threshold voltage and much improved response time in all gray scales.

P‐205L: Late‐News Poster: The Study of the Correlation between Surface Anchoring Energy and Activation Energy on Patterned Vertical Alignment (PVA) Mode

AbstractThe adsorption and desorption of the ions are closely related to the image sticking of liquid crystal (LC) cells. We made an effort to measure the fixed quantity of ion concentration by measuring the residual DC voltage in LC cell and the surface anchoring energy in PVA cell. To understand the mechanism of image sticking, we studied these adsorption and desorption activation energy and found that the desorption activation energy increases as the surface anchoring energy does, and this relationship is rather universal. This observation led us to a conclusion that smaller anchoring energy is required to reduce the image sticking of LC cells.

P‐143: Wide Viewing Liquid Crystal Display based on a Multi‐Patterned Electrode

AbstractWe proposed the wide‐viewing‐angle liquid crystal display (WVALCD) of the patterned vertical alignment (PVA) mode by adopting differential geometry of the electrode pattern. Due to the patterned electrode structure, we can obtain 8‐domain LC alignment in a simple fabrication process while the conventional PVA mode has only 4‐domain structure. This multi‐patterned electrode technique enlarges the viewing angle of LCD about 12% compared to the conventional PVA mode.

P‐144: Control of Liquid Crystal Director through Defined Pretilt Angle in Vertical Alignment Mode Surrounded by Polymer Wall

AbstractWe proposed vertical alignment liquid crystal devices (VALCDs) in which the vertically aligned LC is surrounded by polymer wall, named locked‐super homeotropic (LSH) device. Although the device has advantages of wide‐viewing angle, spacer‐free and simple fabrication, it exhibited relatively slow rising response time due to undefined azimuthal tilting direction with bias voltage. To resolve the problem, we fixed pretilt angle of VA using the polymerization of reactive mesogen (RM) with bias voltage. Conclusively, dynamic stability of LC directors is improved, exhibiting a fast response time.

P‐148: Viewing Angle Switching in Vertical Alignment Liquid Crystal Display by Optimizing Pixel Structure and Controlling LC Orientation

AbstractViewing angle control of a vertical alignment (VA) liquid crystal display using only one panel has been investigated. In conventional VA modes, LC director tilts down in four directions making 45° with respect to crossed polarizers to give rise to wide viewing angle. By dividing pixel into two regions or RGBW structure and controlling the LC director to tilt down to the polarizer axes, the image quality in oblique directions can be controlled without distorting the image quality at normal direction. Also using light leakage in oblique directions, any type of information such as characters and image can be generated. This generates extra image over a main image in oblique viewing directions, that is, the original image is overlapped with made image when the voltage is controlled to the extra pixel for viewing angle control.

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.

Electrode structure for high transmittance and aperture ratio in TFT-LCD

Abstract In this paper, we propose a new cell structure having a high aperture ratio and transmittance with chevron-shaped electrode. As an exemplary cell, we selected patterned vertical alignment (PVA) mode. Optical transmittance and aperture ratio were calculated by 3D-FEM numerical solver and extended Jones method solver. The simulation results exhibited 4.8% improvement in aperture ratio and 11.3% improvement in optical transmission. Furthermore, in the view of aperture ratio, the aperture ratio of the proposed cell design is 4.8% higher than that of the conventional cell design.

Viewing angle switching of vertical alignment liquid crystal displays by controlling birefringence of homogenously aligned liquid crystal layer

Viewing angle switching from a wide viewing angle to a narrow viewing angle has been studied. Conventional multidomain vertical alignment (VA) mode offers the advantages of a high contrast ratio (CR) not only in the front view but also in the wide viewing directions only if compensation films such as a negative C plate and a positive A plate are used. The positive A plate can be replaced by a homogeneous aligned (HA) liquid crystal layer, and the retardation of the HA layer at the off axis can be controlled by applying an electric field while keeping the retardation value at zero in the normal direction. Consequently, the viewing angle range of a VA device can be controlled from a wide viewing mode (over 170° in terms of CR=10) to a narrow viewing angle mode (approximately 60° in terms of CR=2) in the horizontal direction while keeping a high image quality at the normal direction.

A new electrode structure for color‐shift reduction in PVA LCD

We introduce a new electrode structure for the patterned vertical alignment (PVA) mode, which has a lower color‐shift at large viewing angle than the one‐domain VA mode. By manipulating the period of electrode structure, we can make more multi‐domains and use the existing fabrication processes without having to use additional materials.

Effects of Vertical Alignment of Liquid Crystals on Rubbed Polymer Surfaces Using Transparent Electrodes

We investigated the liquid crystal (LC) aligning capabilities and electrooptical characteristics of transparent Al-doped ZnO (ZnO:Al) electrodes substituting indium tin oxide (ITO) electrodes. Experimental results show that a uniform vertical LC alignment and a large pretilt angle were achieved using the ZnO:Al electrodes. The vertical alignment mode based on the ZnO:Al electrodes showed appropriate electrooptical characteristics and a high transparency in comparison with that based on the ITO electrodes. These results indicate that the transparent ZnO:Al electrodes of liquid crystal displays could substitute the ITO electrodes.

15.1: A Novel VA Architecture for Improved Response Time

We designed a novel mountain-shaped cell architecture in order to obtain less than 15 ms of tr+tf response time without an overdriving circuit in vertical alignment mode. This structure significantly reduces the tr+tf time of PVA mode by eliminating abnormal motion in the turn-on process, which is detrimental to rapid switching.

15.2: Distinguished Paper: A New Pixel Design and a Novel Driving Scheme for Multi-Domain Vertically-Aligned LCDs

A cost-effective wide-viewing-angle LCD mode, named Biased Vertical Alignment (BVA) mode, has been successfully developed by using a novel driving scheme and a new pixel design that adds a bias electrode and a bias TFT to the ordinary pixel structure. Compared to PVA and MVA modes, the BVA has a distinct advantage of lower manufacture cost due to the elimination of a lithographic process step to form either ITO-cuts or protrusions on the color-filter substrates. The 15″ BVA-mode XGA prototype exhibits a high contrast ratio of 1200:1 and high cell transmittance of 4.3 %.