Vertical Alignment Liquid Crystal Research Articles

Wideband and wide-view circular polarizer for a transflective vertical alignment liquid crystal display

We propose an optical configuration for a transflective mode with a circular polarizer for the vertical alignment liquid crystal cell, which can show the wideband property of high contrast in oblique incidence. The proposed configuration consists of a biaxial film and two uniaxial films for satisfying both the transmissive and the reflective modes. Optimization of the optical configuration of the circular polarizer has been performed on the Poincaré sphere in the entire visible wavelength range using the Stokes vector and the Mueller matrix method. We also verify the excellent optical characteristics of the proposed configuration by comparing the viewing angle and contrast ratio in the oblique direction with a conventional optical configuration in calculation.

Improved Liquid Crystal Pretilt Angles by Patterned Dual Alignment Coating Structures

The pretilt angles for the optically compensated bend (OCB) mode liquid crystals have been improved using novel patterned dual alignment coating structures in this study. The transition from the splay configuration to the bend configuration can thus be effectively reduced. The dual alignment coating structures consisted of a horizontal alignment polyimide (PI) and a patterned vertical alignment liquid crystal polymer (LCP). Three patterning masks were designed for the photolithography process. The pretilt angles were demonstrated to be increased to 34 degrees for the triangle lattice array-patterned cells. It became 31 degrees for the square lattice array-patterned cells, and 24 degrees for the honeycomb lattice array-patterned cells. The improved pretilt angles were illustrated by the force balance model that can be predicted by the LCP area ratio. The effective control over the pretilt angle could improve the response time to 2 ms when the voltage was ramped up to 5.5 V for the OCB mode liquid crystal devices.

P‐128: Alignment Layer Free and Wide‐viewing Angle Vertical Alignment Liquid Crystal Display

AbstractNew alignment layer free vertical alignment (VA) liquid crystal device is proposed. In the device, the nanoparticles, reactive mesogen, and LC mixture are mixed together and filled into the cell with fishbone electrode structure. Through diffusion process, the particles moved to both surfaces of substrates, generating vertical alignment and then UV is exposed while applying proper voltage to the cell. Then, LC director is stabilized to form multi‐domain by polymerization of reactive mesogen. Consequently, the device shows, wide‐viewing‐angle and fast response time although conventional polymer layer is not used.

Liquid Crystal Alignment Capabilities on SiNx Thin Films Via Ion-Beam Irradiation

The aligned liquid crystals (LCs) display on SiNx thin films using ion-beam (IB) irradiation was studied with controllability of pretilt angle depending on incident energies of the IB. The LCs alignment property for the SiNx thin films were observed to verify the practical application potential. A good LCs alignment of vertical alignment LCs cells on SiNx thin film surfaces irradiated with incident IB energy of 1800eV was achieved. The atomic force microscopy (AFM) images of LCs on SiNx thin film surfaces irradiated with IB energy was used for the surface analysis.

Intermediate pre-tilt angle control by a composite alignment thin film structure for liquidcrystal displays

We designed a patterned composite alignment thin film structure using a horizontalalignment polyimide (PI) layer and vertical alignment liquid crystal polymer (LCP) pillars.The LCP polymer precursor concentration was varied at 0–10% and the pillars wereintroduced by a photolithography process. Both single-sided and double-sided liquid crystaldisplay cells were assembled for a series of electro-optical characterization techniques. Thehorizontal PI alignment layer alone had a designated control of the pre-tilt angle of7° after the prescribed mechanical rubbing process. The pre-tilt angle was improved to24° when the LCP precursor concentration was 5%. It was further increased to61° at the concentration of 10%. In addition, the study on the electrical response time and graylevel variation demonstrated promising results for potential applications. The field-onresponse time was only 2.79 ms and the field-off response time was 0.35 ms for thedouble-sided liquid crystal display cells using a ramping voltage of 5.5 V. The effectivecontrol of the cell pre-tilt angle suggested that the display power consumption and responsetime would be greatly improved.

New Approach to Enhance Contrast Ratio at Normal Incidence by Controlling the Retardation of Optical Compensation Film in Vertically Aligned Liquid Crystal Displays

We found that the light leakage at normal incidence of a vertically aligned liquid crystal display (VA-LCD) in black state is strongly affected by the polarization states of obliquely incident light. Experimental results were explained by a light scattering model of substrates on both sides of a liquid crystal layer. Applying this model, we found that the contrast ratio at normal incidence can be highly enhanced by controlling the retardation value of retardation films. This is a new, effective approach to enhancement of the contrast ratio in LCDs, different from the conventional approach of simply decreasing the depolarization of the scattering media.

Vertically Aligned Liquid Crystals on Tantalum Oxide Thin Films Using Ion Beam Irradiation Processing

In this study, we explored the effects of ion beam (IB) bombardment on the orientation of liquid crystal (LC) molecules on surfaces. Increasing the IB exposure time resulted in improved LC alignment characteristics. Through X-ray photoelectron spectroscopy analysis, it was also confirmed that IB irradiation caused Ta 4f and O 1s peak shifts in the positive direction. The breaking phenomenon of the O 1s bonds may cause anisotropic dipole–dipole forces to stably align the vertical LC molecules. Finally, measuring the contact angles on the IB irradiated surfaces, we concluded that increasing the IB irradiation time strengthened the anisotropic surface energy on the surfaces, and stable vertical LC alignment was achieved via the transformed surface energies.

Synthesis of soluble polyimides for vertical alignment of liquid crystal via one-step method

A series of polyimides (PIs) were copolymerized from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (DBPDA), 3,3′-dimethyl-4,4′-methylene-dianiline (DMMDA) and 4,4′-oxydi(phthalic anhydride) (ODPA) via one-step method. The PIs possessed excellent solubility in polar aprotic solvents and easily formed thin flexible films by solution casting. The glass-transition temperatures ( T g s) of the PIs were in the range of 219–242 °C and thermal decomposition temperatures in nitrogen occurred above 350 °C. The resultant PI films exhibited high transparency at wavelengths greater than 400 nm and induced excellent uniform vertical alignment of liquid crystal (LC). Even after the rubbing process, the pretilt angles of LC were still above 89°. The PI seems to be a prospective material for alignment layers in flexible displays.

Multi-Dimensional Liquid Crystal Alignment Effect of Polymer Wall on Vertically Aligned Liquid Crystal Cell

This paper investigates the multi-dimensional liquid crystal (LC) alignment effect of a polymer wall. The polymer wall is formed on the pixel boundary by an anisotropic phase separation method, which affects the nearby LC orientation. Vertically aligned (VA) LC cells are fabricated in order to show the effect of the polymer wall on LC displays. The electro-optic characteristics of the LC cell are affected by the shape and orientation of the molecules that form the polymer wall. The VA LC cell that has a polymer wall formed by vertically-aligned anisotropic-molecules on the pixel boundary shows much faster turn-on and turn-off times than the conventional LC cell without polymer wall, as well as no loss of transmittance within its pixel area.

Liquid Crystal Alignment Capabilities on SiNx Thin Films Via Ion-Beam Irradiation

The aligned liquid crystals (LCs) display on SiNx thin films using ion-beam (IB) irradiation was studied with controllability of pretilt angle depending on incident energies of the IB. The LCs alignment property for the SiNx thin films were observed to verify the practical application potential. A good LCs alignment of vertical alignment LCs cells on SiNx thin film surfaces irradiated with incident IB energy of 1800 eV was achieved. The atomic force microscopy (AFM) images of LCs on SiNx thin film surfaces irradiated with IB energy was used for the surface analysis.

Polyimide blend alignment layers for control of liquid crystal pretilt angle through baking

Polyimide films were used for liquid crystal (LC) alignment layers to control LC pretilt angles over the full range (8°–90°). The pretilt angles could be controlled using polyimide films prepared from polyamic acid for vertical LC alignment and using polyimide blend films prepared from two types of polyamic acids, one for vertical LC alignment and the other for planar LC alignment, by changing the baking times ranging from 40 to 180min at 230°C. The polyimide blend film could control the pretilt angle better than the polyimide prepared from just one polymer component. The LC alignment behavior was well correlated with the wettability of the polyimide films due to the fragmentation of the long alkyl side group on the polyimide surfaces by the baking process.

Vertical Alignment of Liquid Crystal on a-SiON Thin Film Exposed by Ion Beam

We study the properties of the vertical alignment of liquid crystal on amorphous SiON (a-SiON) thin film exposed by ion beam. a-SiON thin films are deposited on the indium-tin-oxide coated glass substrates by using radio-frequency magnetron sputtering system in the temperature range from 25 to 400°C. The transmittance and surface roughness of a-SiON thin films are enhanced with the increase in deposition temperature. After the substrates were deposited by sputtering system, we exposed them to ion beam and fabricated vertical alignment liquid crystal cells. As a result, the fabricated liquid crystal cells show uniform pretilt angle range from 88 to 89° and high thermal stability over 100°C for 20 hours.

Strong Vertical Alignment of Liquid Crystal on Porous Anodic Aluminum Oxide Film

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Properties of anodic aluminum oxide (AAO) film as a liquid crystal (LC) alignment material is studied. We deposit the transparent porous AAO film on glass with the diameter of the pores controlled between 17–65 nm. The liquid crystal can be aligned vertically against the substrate with the AAO film. The measured polar anchoring strength is about 1.5<formula formulatype="inline"> <tex Notation="TeX">$\,\times 10 ^{-5} {\rm J/m} ^{2}$</tex></formula>, which is comparable to that of N,N-dimethyl-N-octadecyl-3-aminopropyl -trimethoxysilyl chloride (DMOAP) layer. AAO films with smaller pore diameters exhibit higher anchoring strengths. On the other hand, the uniformity of the pore array in the AAO films does not affect the alignment quality significantly. </para>

A multi-domain vertical alignment liquid crystal display to improve the V–T property

The voltage–transmittance (V–T) property is important for the liquid crystal displays (LCDs). In this work, we propose a sub-pixel structure with two common electrodes of a multi-domain vertical alignment (MVA) mode. The sub-pixel is divided into two sub-areas and different common electrode voltages are applied to it. The optimal voltage difference of the common electrodes between sub-area 1 and sub-area 2 is proposed. The simulated results on the plotted displays and the voltage–transmittance property of the LCD, which has 1:1 sub-area ratio, have been carried out. The results show that the structure can form MVA liquid crystal display mode, such as 8-domain VA mode. It can improve the V–T property at large oblique viewing angle and make the transmittance difference between the normal direction and the oblique direction viewing angle less than that of conventional 4-domain MVA mode.

The correlation between electron density and anchoring strength in the inorganic vertical alignment layer

The relationship between the liquid crystal (LC) alignment and the density of the silicon oxide alignment layer was studied by theoretical and experimental approaches. The thin films were deposited by various methods and conditions, and then their densities were analyzed by x-ray reflectivity measurement. The alignment of LC was highly dependent on their densities, which we found to be closely related to the number of interacting dipoles. Ultimately, a-SiOx thin film with lower density gives rise to the uniform vertical alignment of liquid crystal.

Advanced wide viewing angle technology in liquid crystal display modes

This paper reviews the advanced optical configurations for in-plane switching (IPS) and vertical alignment (VA) liquid crystal (LC) cells with wide viewing angle in a visible wavelength range. Optical compensation and optimisation to eliminate off-axis light leakage in the dark state is performed on a Poincaré sphere using the trigonometric and the Muller matrix method. By optimising the wavelength dispersion of used optical retardation films, we could achieve wide-view characteristics for both the IPS and VA LC cells. In addition, we show the advanced wide-view technology for a reflective LC mode.

Optical bounce in a multi-domain vertical aligned liquid crystal display

Optical bounce phenomenon often occurs in a multi-domain vertical aligned (MVA) mode liquid crystal display (LCD). It results in the double image edge and motion image blur of the LCD. We find that the optical bounce phenomenon is due to nowadays overdriving method. Some suitable change of the method may eliminate the optical bounce phenomenon.

Vertical alignment of liquid crystals on a fully oxidized HfO2 surface by ion bombardment

High-performance liquid crystals (LCs) driven at a 0.9 V threshold were demonstrated on very thin HfO2 films with vertical (homeotropic) alignment by ion bombardment. Atomic layer deposition was used to obtain LC orientation on ultrathin high-quality films of double-layer HfO2/Al2O3. X-ray photoelectron spectroscopy indicated that full oxidization of HfO2 film surfaces was induced by ion bombardment, shifting the Hf 4f spectra to lower binding energies. The increased intensities of the Hf 4f peaks after ion bombardment confirmed that nonstoichiometric HfOx was converted to the fully oxidized HfO2 surfaces.

Vertical Alignment of Liquid Crystals with Negative Dielectric Anisotropy on an Inorganic Thin Film with a Hydrophilic Surface

The vertical alignment of liquid crystals having negative dielectric anisotropy on an amorphous silicon oxide (a-SiO(x)) thin film is the consequence of the anisotropic interaction between liquid crystals and a-SiO(x) thin films. To investigate the mechanism of the vertical alignment, we changed the physicochemical characteristics of alignment layers by controlling the composition, since the anisotropic interaction depends on the nature of both liquid crystals and an alignment layer. The variation of composition gives rise to a change in the polarizability, which is a simple measure of induced-dipole strength at the surface of the alignment layer. There is a critical transition point from planar to vertical alignment of liquid crystals, and it is the long-range van der Waals interaction that is responsible for the vertical alignment. The competition between long-range van der Waals interaction and short-range dipolar interaction were investigated and analyzed in terms of the interfacial energy between liquid crystals and an alignment layer.

Fast Switching of Vertical Alignment Liquid Crystal Cells with Liquid Crystalline Polymer Networks

This paper reports on the electro-optic characteristics of vertical alignment (VA) liquid crystal (LC) cells with liquid crystalline polymer networks. Optical bouncing, that occurs during the turn-on of VA cells, can be eliminated by introducing in-cell polymer networks. Furthermore, the turn-off also becomes much faster because of the anchoring effect caused by the anisotropy in the molecular shape of the liquid crystalline polymers. These response times have been found to vary for different LC/prepolymer mixtures. When the concentration of the liquid crystalline prepolymer in the initial LC/prepolymer mixture was 3, 5, or 10 wt %, the response times were measured to be 34, 56, and 87% faster than those of a VA cell with pure LC. These switching behaviors of VA cells with liquid crystalline polymer networks are demonstrated and compared with those using pure LC and with polymer networks made of isotropic prepolymers.