Electrically commanded surfaces: A new liquid‐crystal‐display concept

Abstract— Fast in‐plane switching of the optic axis was realized in liquid‐crystal displays (LCDs) based on the concept of Electrically Commanded Surfaces (ECS). According to this concept, the liquid‐crystal layer in such a display is aligned by means of thin ferroelectric liquid‐crystal‐polymer (FLCP) film deposited onto the inner side of the display substrates. An electric field, applied normal to the substrates, switches the molecules of the ferroelectric film, representing the commanded surface that, via elastic forces, further transfers to the liquid‐crystal layer. The concept of electrically commanded surfaces opens the door to a new generation of advanced LCDs exhibiting extraordinary performance such as fast in‐plane switching.     

Printing of polymer thin‐film transistors for active‐matrix‐display applications

Abstract— We present a process for active‐matrix flat‐panel‐display manufacture based on solution processing and printing of polymer thin‐film transistors. In this process, transistors are fabricated using soluble semiconducting, conducting, and dielectric polymer materials. Accurate definition of the transistor channel and other circuit components are achieved by direct ink‐jet printing combined with surface‐energy patterning. We have used this process to create 4800‐pixel 50‐dpi active‐matrix backplanes. These backplanes were combined with polymer‐dispersed liquid crystal to create the first ink‐jet‐printed active‐matrix displays. Our process is, in principle, environmentally friendly, low temperature, compatible with flexible substrates, cost effective, and advantageous for short‐run length and large display sizes. As well as polymer‐dispersed liquid crystal, this technology is applicable to conventional liquid‐crystal and electrophoretic display effects.     

A self‐aligned multi‐domain liquid‐crystal display on polymer gratings in a vertically aligned configuration

Abstract— We report on a new method of fabricating a vertically aligned multi‐domain liquid‐crystal display (LCD) using surface‐relief gratings. A linear array of surface‐relief gratings was produced by using a photosensitive polymer material coated on glass substrates by the illumination of the UV light through a photomask. The LCD cell was assembled with two substrates with polymer gratings in such way that the grating vectors were orthogonal to each other. In this LCD configuration, the nematic molecules were reoriented by distortions of an external electric field at the grating surfaces to make four different domains. The LC cell with self‐aligned four domains shows excellent extinction in the off‐state and wide‐viewing characteristics in the on‐state.     

Pixel‐isolated liquid‐crystal mode by using a patterned anisotropic phase separation for flexible LCDs

Abstract— A pixel‐isolated liquid‐crystal (PILC) mode for enhancing the mechanical stability of flexible‐display applications is proposed. Because liquid‐crystal (LC) molecules in this mode are isolated in each pixel by patterned or phase‐separated microstructures, and the two substrates are tightly attached to each other by a solidified polymer layer, the LC alignment is stable against external pressure, and the cell gap of our structure is uniformly preserved against bending deformation of the plastic substrates. The mechanical stability of the PILC structure having plastic substrates was tested for its electro‐optic properties.     

Full‐color photo‐addressable electronic paper using cholesteric liquid crystals and organic photoconductors

Abstract— Full‐color photo‐addressable electronic paper using cholesteric liquid crystals and organic photoconductors was developed. The electronic paper is comprised of two stacked photo‐addressable elements displaying blue/green and red images, respectively. Each photo‐addressable element was independently controlled by two different color‐addressing lights. Furthermore, blue and green images were selectively switched by one organic photoconductor using the threshold characteristics of cholesteric liquid crystals. A highly reflective polymer‐dispersed cholesteric liquid‐crystal (PDCLC) layer was obtained by a new formation process based on the sol‐gel transition behavior of a gelatin matrix and an agar overcoat layer. The PDCLC layer had a close‐packed honeycomb‐like monolayer structure with a flat surface. The A6‐sized prototype had paper‐like features and showed full‐color bistable images instantly written with a viewer‐type writing apparatus.     

Polarization‐independent and electrically tunable liquid‐crystal Fresnel lenses based on photoalignment in dye‐doped liquid crystals

Abstract— This work demonstrates polarization‐independent and electrically tunable liquid‐crystal (LC) Fresnel lenses based on photoalignment in dye‐doped liquid crystals (DDLCs). The LC alignments in the adjacent odd and even zones of the LC Fresnel lens are orthogonally hybrid alignments. Those are generated by surface treatment of homeotropic and homogeneous alignment layers, and the dye‐adsorption layer onto the UV‐cured surface‐relief Fresnel zone plate. The maximum focusing efficiency (～34.14%) of the fabricated LC Fresnel lens by applying a suitable AC voltage is close to the maximal theoretical focusing efficiency of a binary phase LC Fresnel lens (～40.5%). Additionally, the focusing efficiency is polarization‐independent and electrically tunable.     

A single‐substrate multicolor cholesteric liquid‐crystal display prepared through ink‐jet printing

Abstract— Ink‐jet printing was used to prepare a single‐substrate multicolor cholesteric liquid‐crystal (Ch‐LC) display incorporating three Ch‐LCs exhibiting different reflective wavelengths. A room‐temperature low‐vacuum chemical‐vapor‐deposition process was developed for coating a thin polymer film onto the Ch‐LC so that the top electrode could be coated onto the Ch‐LC layer. Herein, the successful operation of such a 10.4‐in. QVGA Ch‐LC display at 40 V will be described.     

Fast‐response study of polymer‐stabilized VA‐LCD

Abstract— Polymer stabilization is introduced in VA‐type LCDs, and fast response time can be achieved along with a high contrast ratio.¹ A small amount of reactive monomer is mixed with liquid crystal and forms a polymer layer above the alignment layer by using a UV process. The pre‐tilt angle of the liquid crystal is stabilized, and a faster response time can be realized when the bias pre‐tilt angle from 90° is increased. The properties of reactive monomers and liquid crystal and the conditions of the UV process were studied. Based on the application of the proper monomer and LC, and an optima UV process, a 65‐in. 240‐Hz full‐HD TFT‐LCD, with a faster response time and high contrast ratio, has been developed.     

Temperature dependence of residual DC voltage and its evaluation parameter related to liquid‐crystal and alignment‐layer materials

Abstract— The temperature dependence of residual DC voltage was studied based on the adsorption and desorption of ions in the liquid‐crystal (LC) layer to and from the interface between the LC and alignment layers during the application of an external DC offset voltage. The relaxation process of the adsorbed ions during the open‐circuit state was also studied after applying the DC offset voltage. Those processes were found to follow the Arrhenius rule, and a new evaluation parameter related to the temperature is proposed for the design of the LC and alignment‐layer materials.     

Technologies toward flexible liquid‐crystal displays

Abstract— Several leading technologies for flexible liquid‐crystal displays have been developed recently at ERSO. The roll‐to‐roll compatible techniques, polymer‐added liquid crystal, have been applied on a film‐like substrate. A flexible black‐and‐white cholesteric liquid‐crystal display was also implemented by photo‐induced phase separation. Color filters placed on a plastic substrate by a low‐temperature manufacturing process was successfully fabricated. A novel design of a wide‐viewing‐angle color plastic LCD was also proposed.     

A novel polarizer‐free dye‐doped polymer‐dispersed liquid crystal for reflective TFT displays

Abstract— A novel preparation method for dichroic dye‐doped polymer‐dispersed liquid crystals has been developed. This was achieved by creating a porous polymer matrix first by washing out the liquid crystal from a polymer‐dispersed liquid crystal (PDLC), which is then refilled with dye‐doped liquid crystal. Optimizing the liquid crystal used in the refilling results in decreased turn‐on voltage and faster response time. Poster‐standard reflectivity and newspaper‐standard contrast was demonstrated with a 3.8‐in. QVGA reflective TFT display with a drive voltage of 10 V.     

Polymer‐stabilized pretilt angle on the surface of nanoparticle‐induced vertical‐alignment surface for multi‐domain vertical‐alignment liquid‐crystal display

Abstract— By introducing polyhedral oligomeric silsesquioxane (POSS) nanoparticles along with a controlled amount of UV‐curable reactive mesogen (RM) into a liquid‐crystalline (LC) medium, a multi‐domain vertical‐alignment LC device was successfully demonstrated. The device, possessing a vertically aligned LC director in four different azimuthal directions, exhibited a fast response time and wide‐viewing‐angle characteristics, in the absence of conventional polymer‐type vertical‐alignment layers. Electro‐optic characteristics of the fabricated device, before and after UV curing of the cell, were studied. The surface morphology of the substrate surfaces were analyzed by using field‐emission scanning electron microscopy (FESEM). The experimental results show that the technology will possibly be applicable to cost‐effective vertical‐alignment liquid‐crystal devices and is suitable for green‐technology liquid‐crystal displays.     

Frictional properties of VA‐mode alignment‐film surfaces studied by frictional force microscopy

Abstract— The surface‐friction characteristics of the post‐bake temperature were compared to that of the rubbing condition on VA‐mode alignment films (AFs) using frictional force microscopy (FFM). The surface roughness and the surface frictional force increased when the temperature was elevated. However, the frictional asymmetry could not be detected by the frictional loop due to the instability of the side‐chain molecular arrangement. For the inter‐influences of the total friction and surface roughness, the density of the side chain was changed and it was intentionally rubbed for evaluation. The results showed that the total friction was higher when the rubbing was stronger and side‐chain density lower, and the surface roughness also increased in the same manner. This can be explained by the potential interaction due to the curved structure of the side‐chain surface that establishes the probe that is scanned and the top surface layer. In order to confirm the influence of the pre‐tilt angle on the tilt of the side chain, a test cell was produced under the same conditions and evaluated. The results showed that the pre‐tilt‐angle decreased according to the difference in density of the side chain and strength of rubbing after injecting liquid‐crystal (LC).     

Prospects of emerging polymer‐stabilized blue‐phase liquid‐crystal displays

Abstract— The prospects of emerging polymer‐stabilized blue‐phase liquid‐crystal displays, or more generally, Kerr‐effect‐induced isotropic‐to‐anisotropic transition, are analyzed with special emphases on the temperature effects. As the temperature increases, both the Kerr constant, induced birefringence, and response time decrease but at different rates. The proposed physical models fit well with experimental results. Some remaining technical challenges associated with this promising display technology are discussed.     

Nano‐domains formation on photoalignment thin film by self‐organized dewetting

A novel discontinuous photoalignment surface with nano‐domains for liquid crystal is developed. The formation of the discontinuous structure is created by self‐organized dewetting, which is regarded as one of the most promising bottom‐up approaches to fabricate nano‐structure. Different dewetting conditions, such as surface roughness, thickness and viscosity, have been investigated. Such discontinuous photoalignment layer can be fabricated on top of another continuous alignment layer to form a new kind of heterogeneous nano‐structured alignment surface – stacked alignment layers. This heterogeneous alignment surface can be used to produce arbitrary pretilt angles for the liquid crystal display. Simulation model has been built to understand the dewetting mechanism. Experiments using photo‐aligned and photo‐polymerisable polymer have been done to verify the dewetting theory. The produced stacked alignment layers are proved to be robust. Moreover, the dewetting processing is a fully controllable process and is compatible with existing manufacturing techniques.     

Conformable displays based on polymer‐dispersed liquid‐crystal materials on flexible substrates

Abstract— We have developed a process that enables one to conform polymer‐dispersed liquid‐crystal (PDLC) displays into a particular shape indefinitely. Planar PDLC displays are first fabricated between indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates. This fully functional display can then be conformed to a particular shape by heating above the glass‐transition temperature of PET and then allowing it to cool down to room temperature. The display retains its shape and is fully functional after processing. We have created spiral‐and wave‐like samples and have demonstrated their operation after the conformal process. The stress is relieved in the substrate by conforming. Temperature effects on polymer substrates were investigated for two types of polymer films (PET/ITO substrates and a conducting polymer PEDOT:PSS/PET substrate) to analyze the effects of temperature on the resistance and mechanics of the films under an applied uniaxial strain. We have found a decrease in contrast of the PDLC after conforming, but surprisingly, a reduced threshold voltage and reduced hysterisis occurs.     

Blue‐phase liquid‐crystal mixtures and their induced stabilization by photopolymerization

Abstract— Experiments on mixtures of different nematic‐liquid‐crystal hosts with a chiral dopant to induce the appearance of blue phases (BPs) is reported. The phase behavior and transition temperatures of the mixtures were studied by temperature‐dependent polarized optical microscopy, variable temperature X‐ray diffraction, and reflectance measurements. After completely characterized, a selected LC mixture presenting BPs was submitted to polymer‐stabilization using UV‐curable monomers. By using a specific combination of a pro‐mesogenic (B11) and a branched‐chain acrylate (EHA) monomer, a uniform porous microstructure is formed providing a stable BP.     

Low‐temperature‐applicable polymer‐stabilized blue‐phase liquid crystal and its Kerr effect

Abstract— A type of polymer‐stabilized blue‐phase liquid crystal, which can be used in a low‐temperature environment, is proposed. The blue‐phase range after polymerization was widened to more than 73°C, and the blue‐phase texture is very stable even at a temperature as low as ?35°C. The electro‐optical performances dependence on polymer concentration was investigated. The results indicate that the saturation voltage increases and the hysteresis enhances as the polymer concentration increases. The rise and decay times could reach as low as 391 and 789 μsec, respectively. Such material also shows good electro‐optical behavior at a temperature of ?35°C. In addition, the Kerr constant was tested under a uniformly distributed electric field to be 2.195 nm/V² at room temperature and 2.077 nm/V² at ?35°C. The Kerr constant tested under white‐light illumination was 1.975 nm/V², which shows a small dispersion.     

Driver‐circuits‐integrated LCDs based on novel amorphous In‐Ga‐Zn‐oxide TFT

Abstract— A liquid‐crystal panel integrated with a gate driver and a source driver by using amorphous In—Ga—Zn‐oxide TFTs was designed, prototyped, and evaluated. By using the process of bottom‐gate bottom‐contact (BGBC) TFTs, amorphous In—Ga—Zn‐oxide TFTs with superior characteristics were provided. Further, for the first time in the world, a 4‐in. QVGA liquid‐crystal panel integrated with a gate driver and a source driver was developed by using BGBC TFTs formed from an oxide semiconductor. By evaluating the liquid‐crystal panel, its functionality was successfully demonstrate. Based on the findings, it is believed that the novel BGBC amorphous In—Ga—Zn‐oxide TFT will be a promising candidate for future large‐screen backplanes having high definition.     

Tuning the alignment of liquid crystals by means of nano‐structured surfaces (Invited Paper)

Abstract— The solid‐surface/liquid‐crystal interactions, defining the field‐free alignment of the liquid crystal in conventional liquid‐crystal displays, are playing a vital role in their optical appearance and performance. Nano‐scale changes in the solid‐surface structure induced by light have been recently shown to affect the anchoring strength and the easy‐axis direction. Fine tuning of the anchoring strength is also demonstrated by nano‐structuring of the Langmuir‐Blodgett monolayer employed as liquid‐crystal alignment layers promoting homeotropic orientation. On the basis of nano‐engineering of the surface alignment properties, two novel alignment concepts have been introduced: electrically commanded surfaces (ECS) and high‐performance alignment layers (HiPAL). Nano‐structured polymers related to these concepts have been designed, synthesized, and used as materials for alignment layers in LCDs. ECS materials belong to the category of active alignment materials designed to mediate switching of the liquid crystal, whereas the HiPAL materials make possible the control of the molecular tilt angle in a broad range, from 0° to 90°, and they seem to enable the control of the anchoring strength as well. The nano‐structured alignment materials are strong candidates for implementation in a new generation of advanced liquid‐crystal displays and devices.