Masahito Ohe

Electro‐optical characteristics and switching behavior of the in‐plane switching mode

Electro‐optical characteristics related to the threshold behavior of liquid crystals when using the in‐plane switching (IPS) mode were investigated with interdigital electrodes. In order to analyze the switching behavior of liquid crystals, an equation, which expresses the threshold transition, was derived using the continuum elastic theory. It was made clear that it was the electric field and not the voltage that drives the liquid crystals in the IPS mode. Significantly, an inversely proportional relationship between the threshold voltage and the gap between the substrates was found to hold. Furthermore, the electro‐optical characteristics were recognized to change with the variation of the gap between the substrates. This behavior is due to the independence of electric field on liquid crystal layer normal.

Response mechanism of nematic liquid crystals using the in‐plane switching mode

A response mechanism of nematic liquid crystals following the switching‐on and off of an in‐plane electric field when using the in‐plane switching (IPS) mode was investigated. Simplified theoretical expressions, which were derived with an assumption that an in‐plane electric field was applied to the liquid crystals, were used to explain the dynamical switching process in the IPS mode. In particular, the relaxation time of the liquid crystals when removing the electric field was described as a proportional relationship to the square of the cell gap. A thinner cell gap also proved to be effective in obtaining a fast response time in the IPS mode. By contrast, the switching‐on time when applying the in‐plane electric field proved to be inversely proportional to the difference between the square of the electric field strength and the square of the critical electric field strength at which the liquid crystals begin to deform.

SWITCHING OF NEGATIVE AND POSITIVE DIELECTRO-ANISOTROPIC LIQUID CRYSTALS BY IN-PLANE ELECTRIC FIELDS

This article compares switching behaviors between negative (Nn) and positive (Np) dielectro-anisotropic nematic liquid crystals driven by an in-plane electric field which is generated with interdigital electrodes. Even for Np type liquid crystals, excellent viewing angle characteristics were obtained as expected. Theoretical descriptions of the switching principle, i.e., threshold behavior and response mechanism, could be applied to both the Nn and Np type liquid crystals. However, the orientational deformation of the Np type liquid crystals caused by a distorted electric field which occurred near the edges of electrodes was not the same as that of the Nn type liquid crystals. The switching of the Np type liquid crystals near the edges of the electrodes by this field was followed by the in-plane switching of the liquid crystals between the electrodes. This remarkably distinctive dynamical behavior implied a difference in the response of the longitudinal axes for Nn and Np type liquid crystal molecules. The longitudinal axes of the latter seemed to be sensitive to the electric field component perpendicular to the substrates when applying the in-plane electric field. Furthermore, these experimentally obtained results were supported by computer simulations which analyzed the liquid crystal director distribution and transmittance pattern from edge-to-edge of a pair of electrodes when applying the in-plane electric field.

The in-plane switching of homogeneously aligned nematic liquid crystals

We have investigated the electro-optical effects and physical switching principle of homogeneously aligned nematic liquid crystals when applying an in-plane electric field with interdigital electrodes. By using the in-plane switching (IPS) of the liquid crystals which is achieved by the in-plane electric field, the viewing angle characteristics of the electro-optical effects were confirmed to be far superior to those of the conventional twisted nematic mode in which the electric field is applied along the direction perpendicular to the substrates. The non-reversal region of grey scales was extremely wide in which a high contrast ratio was kept, even along quite an oblique direction in the IPS mode. In order to clarify the switching principle of the liquid crystals in the IPS mode, a simplified expression describing the threshold behaviour of the device was derived with the assumption that a uniform in-plane electric field was applied along a direction perpendicular to the director and parallel to the homo...

Complete Suppression of Color Shift in In-plane Switching Mode Liquid Crystal Displays with a Multidomain Structure Obtained by Unidirectional Rubbing

Complete suppression of color shift in in-plane switching mode liquid crystal displays (LCDs) has been realized. The LCDs have a multidomain structure obtained by unidirectional rubbing and use of zigzag electrodes. The design concept and performance of the devices are described in this paper.

Quantitative Analysis of Cell Gap Margin for Uniform Optical Properties Using In-Plane Switching of Liquid Crystals

The allowable range of marginal variation of the cell gap to ensure uniform optical properties when using in-plane switching (IPS) of liquid crystals was evaluated quantitatively. The analysis yielded relationships of transmittance into the cell gap in bright, selected bright and selected intermediate states. The selected state was defined as the state in which a particular constant voltage was applied to the liquid crystals to obtain the desired state in a series of evaluations for different cell gaps. The relationships were used to evaluate normalized variations of transmittance (|ΔT/T|) corresponding to the difference in the cell gap from the optimal value (Δd), and the variations were compared with those in the twisted and supertwisted nematic modes. As a result, the marginal variation for a uniform display using the IPS of liquid crystals was quantitatively confirmed to be much narrower than that in the twisted nematic mode. In the selected bright state, the variation was between those of the twisted and supertwisted nematic modes, while it was comparable to the cell gap margin of the supertwisted nematic mode in the intermediate state; i.e., the variation of ±0.1 µ m in the twisted nematic mode corresponded to that in a range of ±0.02 to ±0.04 µ m in the IPS mode.

X-ray studies on layer structure and bistability in ferroelectric liquid crystals

Abstract X-ray diffraction measurements of thin chiral smectic C (Sc∗) liquid crystals between solid plates coated with rubbed polymer film were performed under an electric field. A variety of local layer structures which depend upon the boundary conditions could be observed, including bookshelf and chevron, and some distorted structures. Moreover, the method of layer deformation could be seen in a stepwise induced field, and seven deformation types could be detected. The relation between a bistability effect and the type of layer deformation was clarified and four deformation types were found favourable to bistability. A vertical layer structure at the boundary, like the bookshelf structure, was necessary to produce bistability. A high pretilt film seemed to have a smectic layer reform a tilted structure even under an induced field and it was difficult to produce bistability with the high pretilt one.

Alkylene polyimides for aligning nematic liquid crystals : anomalous odd-even effect in tilt bias angles as a function of chain length of alkylenes

Abstract Measurements of tilt bias angles of a nematic liquid crystal were carried out, by using several alkylene polyimides synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydrides and diamines with different chain lengths as alignment layers. The tilt bias angle dependence on the alkylene chain length of polyimides shows an anomalous odd-even effect. Small tilt bias angles, less than one degree, were observed with odd-number alkylene chain lengths. Relatively high tilt bias angles, however, were observed with even-number lengths. We have made an attempt to explain this effect from X-ray diffraction and scanning electron microscope measurements and molecular mechanics calculations of the polyimides with alkylene chain. As a result, this odd-even effect of the tilt bias angle is due to the difference of surface structures between alkylene polyimides with odd alkylene lengths and those with even ones.

10.1: 18.0-in.-Diagonal Super-TFTs with a Fast Response Speed of 25 msec

Materials and components available for the mass production of Super-TFTs IPS-TFT-LCDS were optimized and redesigned on the basis of a fundamental analysis of the IPS mode. As a result, we developed 18.0-inch diagonal Super-TFTs with a fast response speed of 25 msec., which is the total on +off response time between white and black levels. Moreover, we found that the IPS mode has a relatively higher potential for displaying a moving gray level image compared with that of the conventional TN-mode.

Unusual Voltage-Holding Ratio Characteristics Using In-Plane Switching of Nematic Liquid Crystals

Unusual characteristics of the voltage-holding ratio were found when using in-plane switching of homogeneously oriented nematic liquid crystals. Even when employing liquid crystals with much lower resistivity than is applicable to the conventional active matrix driving technique, voltage-holding ratio characteristics were higher than those in conventional electric fields applied along the direction perpendicular to the substrate plane. The unusual holding ratio characteristics were attributed to the electric field direction being approximately parallel to the substrate plane.