Antiferroelectric Liquid Crystal Display Research Articles

An Optically Isotropic Antiferroelectric Liquid Crystal (OI-AFLC) Display Mode Operating over a Wide Temperature Range using Ternary Bent-Core Liquid Crystal Mixtures.

We report on the synthesis and characterization of bent‐core liquid crystal (LC) compounds and the preparation of mixtures that provide an optically isotropic antiferroelectric (OI‐AFLC) liquid crystal display mode over a very wide temperature interval and well below room temperature. From the collection of compounds synthesized during this study, we recognized that several ternary mixtures displayed a modulated SmCaPA phase down to below −40 °C and up to about 100 °C on both heating and cooling, as well as optical tilt angles in the transformed state of approximately 45° (optically isotropic state). The materials were fully characterized and their liquid crystal as well as electro‐optical properties analyzed by polarized optical microscopy, differential scanning calorimetry, synchrotron X‐ray diffraction, dielectric spectroscopy, and electro‐optical tests.

Renewed focus on the small temperature change of smectic layer spacing in ferroelectric and antiferroelectric LCs

We aim at becoming released from the invisible hand of de Vries scenario that a small temperature change in the smectic layer spacing must be caused by ‘the far-fetched orientational distribution called the de Vries diffused cone’ or ‘the in-layer directors statically tilted and randomly distributed around the smectic layer normal’. First, we show in a prototypal compound MC513 that all the unusual properties suggesting the revival of a de Vries-type SmA-Sm transition can be explained by ordinary SmA emergence just below the isotropic phase only in a narrow temperature range. Second, we take up the unconventional molecular structure of TSiKN65, thus far regarded as a typical de Vries material; the core part makes a large angle of with respect to the average molecular long axis. We insist that a slight change in can explain almost all the unusual properties without taking account of the de Vries scenario. Last but not least is a practical point of view. During a thermal shock cyclic reliability test between −20°C and 60°C in prototyping antiferroelectric liquid crystal displays (AFLCDs), DENSO noticed the importance of the transition from Sm to the hexatic or crystalline phase; the cell quality critically depends on the layer spacing change in the lower-temperature part of Sm. We point out that the relative displacements of molecules along their long axes and hence the layer undulational fluctuations play an important role in addition to the director tilting ones.

An Extended Bilayer Model for Domain Growth in Antiferroelectric Liquid Crystal Display

We have investigated the domain growth of antiferroelectric liquid crystal display under the switching process by means of CCD camera pictures. Its dynamic behavior was simulated by our extended bilayer model. In this model, we supplement free energy of antiferroelectric liquid crystal display with the coupling coefficient between adjacent molecules in the same layer, because domain growth propagates parallel to the smectic layer. The mechanism of the domain growth in antiferroelectric liquid crystal display can be described by the extended bilayer model.

Necessary conditions for display and fast switching of high contrast images by antiferroelectric liquid crystal displays

Antiferroelectric liquid crystals (AFLC) with 45° molecular tilt were stated to be the solution of the dark state problem in AFLC displays (AFLCDs). However, AFLCs that are available at present actually possess antiferroelectric (AF) to ferroelectric (F) transition of first order characterized by slow switching times which in turn deteriorates the dark state in AFLCDs. Here, we report on the switching behavior of a siloxane AFLC dimer with molecular tilt close to 45°. This material exhibits AF to F transition of second order at lower temperatures, something that enables display and fast switching of high contrast images in AFLCDs.

Relaxation of Surface Stabilised Antiferroelectric Liquid Crystals at Video Frequency

As part of our line of research into high resolution, video rate antiferroelectric displays we have studied the ferroelectric – antiferroelectric relaxation that follows the data pulse, in surface stabilised antiferroelectric liquid crystal (AFLC) displays, driven by biasless or low bias video frequency waveforms. This generally incomplete relaxation reduces the intensity of the light transmitted by a pixel in any partially ferroelectric (grey-level) state during a video frame, and hence limits the frame-averaged brightness of the AFLC display. The relaxation is a function of the surface stabilisation (i.e., the anchoring strength, pre-tilt and thickness of the device and the pitch of the AFLC), the rotational viscosity of the AFLC, the degree of relaxation of the AFLC between successive frames, the magnitude of the data pulse (grey level) and bias voltage. We have optimised the driving scheme for a number of symmetric and asymmetric (biasless) displays with different antiferroelectric materials and a variety of alignment surfaces, and present mathematical fits to the decaying transparency during single frames as functions of the data pulse. Finally we discuss the mechanisms governing the relaxation.

The Role of Alignment Layers on the Induced Relaxation of Passively Multiplexed Antiferroelectric Liquid Crystal Displays

ABSTRACT We have investigated the static and dynamic electrooptical effects of adding an alignment layer derived from a chiral liquid crystal having SmC* and N* phases to the standard polymer alignment layer in antiferroelectric liquid crystal cells. Our results show that the added layer reduces the relaxation time of the liquid crystal, the threshold voltage and the dynamic range required to generate a full greyscale. The electrooptical response of materials was tested using a 7 level waveform, and the relaxation was studied using a novel memory test.

New Design Technique for Driving Antiferroelectric Liquid Crystal Displays in Passive Matrix Using a Predetermined V-T curve

We propose a new voltage-transmittance (V-T) curve suitable to determine the driving pulse shapes. There are three alternating periods in the multiplex driving scheme; a selecting period, a holding period and a resetting period. Holding pulse shape and holding period affect the switching behavior of AFLCs. In this paper a new V-T characteristics of AFLCs applicable in the design of multiplex driving scheme is examined, where a square pulse is used for both selection and holding voltages.

Quasi-Achromatic and Wide Viewing Properties of a Reflective Liquid Crystal Display in In-Plane Optical Geometry

In the in-plane optical geometry, the achromatic and viewing properties of reflective liquid crystal displays (LCDs) are studied within the framework of the 2 ×2 Jones matrix formalism. For obtaining high brightness, high contrast and good achromaticity, LCD cell parameters such as the molecular rotation angle of LC and the effective phase retardation through the LC layer are optimized in the geometry such that the average optic axis of the LC layer lies on the plane parallel to the substrates. The resultant viewing properties are consistent with previous experimental results for a reflective antiferroelectric LCD.

Analytical approach to V-shaped characteristics in antiferroelectric liquid crystal displays

Not only in ferroelectric liquid crystal displays (LCDs), but also in antiferroelectric LCDs, grey levels are possible by actively addressing the ‘continuous director rotation mode’. For ferroelectric LCDs this was shown qualitatively and quantitatively in previous articles. In this article it is shown that an exact analytical approach is also possible for antiferroelectric LCDs. In two consecutive layers the director orientations are symmetric, and at zero voltage they are in a splayed state. The conditions on alignment layer thickness and interaction coefficients are related to those of ferroelectric liquid crystal but are easier to fulfill.

Antiferroelectric liquid crystal display with high image quality

The antiferroelectric liquid crystal display (AFLCD) is a unique display that can at demonstrate a moving image perfectly the passive matrix driving scheme. We optimised driving the waveform and introduced a dual‐driving method. Also, by improving this driving method and using line inversion method, we realize the AFLC display of high image quality with 160(RGB) × 240, 32768 colors, crosstalk free and flicker free contrast ratio is greater than 60:1, and the brightness is above 200 cd/m2.

A new reflective display mode for antiferroelectric liquid crystal

In this work, we proposed a reflective antiferroelectric liquid crystal display (AFLCD) using a half‐wave cell whose in‐plane tilt angle is 22.5°. To check the validity of our design, we fabricated a reflective half‐wave AFLC cell of which in‐plane tilt angle is 24.9°, and measured VIS reflection spectra, contrast ratio and response time. In the results, the half‐wave AFLC cell in the reflective configuration exhibits high brightness, high contrast ratio of 20:1. and fast response time of 700 μs.

Influence of the Alignment Process on the Switching of High Contrast Antiferroelectric Liquid Crystal Displays

In this paper we investigate the causes of poor alignment in antiferroelectric liquid crystal devices making use of spin coated and rubbed polyimide. We found that poor alignment is due to the deviation of the layer normal from the rubbing direction. By optimising the skew between the rubbing direction on opposite substrates we found that a high quality and stable alignment can be obtained. Quantitative experimental results showing a static extinction ratio higher than 3000:1 and dynamic contrast ratio higher than 2000:1 are presented.

L‐7: Late‐News Paper: Solution of the Dark State Problem for High Contrast AFLCDs

AbstractDespite more than ten years of R&D in antiferroelectric liquid crystal displays (AFLCDs), this very promising technology has not yet reached the market. The main reason for this is the bad dark state due to light leakage from imperfections of the LC alignment and from the pretransitional effect. We have found that both problems are eliminated by using AFLC materials with substantially 45° tilt angle in the surface stabilized AFLC geometry. The performance of conventional AFLCDs will be considerably enhanced with this kind of material, and furthermore, 45° AFLCs have a unique potential for both reflective mode and polarizer free displays.

Development of Alignment Layer for Antiferroelectric Liquid Crystal Display Devices.

A series of polymer alignment layers consisting of fluoro and non-fluoro polyimide, polyamide-imide and polyamide were developed for antiferroelectric liquid crystal display devices. These polymer alignment layers were evaluated for their electrooptical properties. The alignment characteristics such as double hysteresis behavior and driving margin strongly depend upon the polymer alignment structure. Alignment layers with less hindered or linear diamine unit in the polymer backbone showed superiority, in comparison with sterically hindered diamine, in aligning and electrooptical properties of antiferroelectric liquid crystal devices.

Analytical Treatment of Grey Levels in Antiferroelectric Liquid Crystal Displays

Abstract Under certain simplifying assumptions, which are believed to be non-essential, it is possible to treat grey levels in antiferroelectric liquid crystal displays analytically. This shows more clearly than simulations which parameters play an essential role.

Polyimide, Polyamide-Imide and Polyamide Alignment Layers for Antiferroelectric Liquid Crystal Display Cells and Their Structural Effect on Hysteresis Behavior

Abstract Polyimide, polyamide-imide and polyamide alignment layers for antiferroelectric liquid crystal cell devices were developed. Good quality alignment with improved electrooptical properties of the cell was observed by using structurally different fluorine and non-fluorine containing polymers as surface alignment layers. The double hysteresis loops characteristic of the antiferroelectricity arising from the chiral smectic phase were found to be different depending on the different molecular structure of polymer alignment layers. The switching between antiferroelectric (AF) and ferroelectric (F) states significantly depended on the molecular structure of the alignment layer and liquid crystals used. The liquid crystal blend (AFLCB-1) used in the experiment exhibited a wide temperature range of the smectic C*A phase. In the present paper, we report the broader hysteresis with higher driving margin for the non-fluoro polymer alignment layers as compared to those of the fluoro polymer aligning layers under identical conditions. The driving margin (M) varied from the 0.21 to 2.2 as the alignment layers were changed in the cells. All the alignment layers developed in our laboratory showed superior properties to that of the commercially available alignment layer material (LQ-1800).

Fluorine containing chiral synthons for ferro and antiferroelectric liquid crystal compounds and their application

Recently, the high performance and functional properties of the fluorine containing organic compounds are focussed in order to develop the organic functional materials in a wide research area of material science. Especially the chiral fluorine compounds are being utilized as the building block in many research fields such as agrochemicals, pharmaceuticals, LB films and liquid crystals. Recently, we have successfully synthesized novel fluorine containing antiferroelectric liquid crystal compounds which displayed new phenomenon ‘Tristable State’ switching from its new smectic phase (SmCA*). These liquid crystal compounds either alone or in a mixture attribute the wide antiferroelectric smectic phase, sharp DC threshold and characteristic hysteresis of the threshold voltage. The purpose of this study is to synthesize fluorine containing chiral synthons having CH 2F, CHF 2, CF 3 and C 2F 5 group directly linked to the chiral center of the novel antiferroelectric liquid crystal compounds. The effect of fluorine substituents at the chiral center was also evaluated with special interest on the appearance of tristability in optoelectronic application. They are explained briefly as follows; ▪ 1. Appearance of Tristability It is interesting to note that the CF 3 and C 2F 5 substituents, linked to the chiral center of molecular structures showed antiferroelectricity, whereas groups such as CH 2F and CHF 2 substituted at chiral center exhibited the ferroelectricity. 2. Opto-electronic Properties The ferro and antiferroelectricity are characterized by their single and double hysteresis loop which is really obtained by introducing ferro and antiferroelectric liquid crystal compounds in between the device cell and by applying the triangular wave voltage. The CF 3 and C 2F 5 substituents at chiral center of liquid crystal compounds displayed double hysteresis loop (Fig. 1), whereas chiral center of the compounds substituted by CHF 2 or CH 2F group exhibited the single hysteresis loop and correspond to the ferroelectricity. A prototype antiferroelectric liquid crystal display have been demonstrated at SAMPE 1989 Japan. ▪

Ferroelectric Liquid Crystal Display Using Tristable Switching

Smectic layer structure and switching behavior in antiferroelectric liquid crystals TFMHPOBC, TFMHPDBC and MHPOBC were studied. At the phase transition point between SmC* and SmCA*, discontinuity in the leaning angle and the layer spacing was observed. In the SmA phase, diffraction peaks attributable to the focal conic texture and the planar layer structure were observed, implying that the bookshelf structure and the focal conic texture coexist. Using the tristable switching, we designed the fundamental waveforms for multiplex driving and constructed a prototype antiferroelectric liquid crystal display which works at room temperature.