Advanced liquid‐crystal materials for TFT monitor and TV applications

Notebook applications have been one of the most important driving forces behind the remarkable growth of liquid‐crystal displays (LCDs). LCDs have recently been well accepted in the monitor market and large growth is forecasted because of the replacement of CRTs. The next challenge for LCDs is the TV market. These new application areas are supported by advanced LC technologies such as film‐compensated twisted nematic (TN), in‐plane switching (IPS), and vertically aligned (VA) modes. Each TFT technology requires a corresponding LC material improvement. We will review the recent liquid‐crystal material development for these advanced LC technologies.     

Photoalignment of LCoS LCDs

Because the pixel area of liquid‐crystal‐on‐silicon (LCoS) microdisplays is about 100 times smaller than that of direct‐view liquid‐crystal displays (LCDs), the limitations of the conventional alignment by brushing are obvious: Scratches and particle contamination caused by brushing become visible due to the strong optical magnification required for LCoS LCDs both in front or rear projection. As an alternative, photoalignment with the linear photo‐polymerization (LPP) technology avoids the generation of defects, thus increasing production yields considerably. For application in LCoS LCDs, alignment layer materials must match the high‐voltage holding ratio (VHR) specifications of TFT‐LCDs. The VHR performance of the newly developed second‐generation LPP materials is shown to be similar to standard TFT polyimides used for conventional brushed alignment. We report investigations of pretilt‐angle generation by photoalignment on reflective CMOS substrates. UV light reflected from the CMOS surface during LPP exposure affects the resulting pretilt angle. Compared to pretilt angles on transmissive substrates, the reflected UV light can induce lower, higher, or identical pretilt angles, depending on LPP material properties. In any case, the pretilt angles are well defined, which results in perfect LCD alignment.     

2‐D/3‐D displays based on switchable lenticulars

Abstract— An attractive concept for 3‐D displays is the one based on LCDs equipped with lenticular lenses. This enables autostereoscopic multiview 3‐D displays without a loss in brightness. A general issue in multiview 3‐D displays is their relatively low spatial resolution because the pixels are divided among the different views. To overcome this problem, we have developed switchable displays, using liquid‐crystal (LC) filled switchable lenticulars. In this way, it is possible to have a high‐brightness 3‐D display capable of fully exploiting the native 2‐D resolution of the underlying LCD. The feasibility of LC‐filled switchable lenticulars was shown in several applications. For applications in which it is advantageous to be able to display 3‐D and 2‐D content simultaneously, a 42‐in. locally switchable prototype having a matrix electrode structure was developed. These displays were realized using cylindrically shaped lenticular lenses in contact with LC. An alternative for these are lenticulars based on gradient‐index (GRIN) LC lenses. Preliminary results for such switchable GRIN lenses are presented as well.     

Energy‐efficient liquid‐crystal displays (e2‐LCDs) using a photonic‐crystal backlight system

Abstract— Cholesteric liquid crystals automatically form one‐dimensional photonic crystals. For a photonic crystal in which light‐emitting moieties are embedded, unique properties such as microcavity effects and simultaneous light emission and light reflection can be expected. Three primary‐color photonic‐crystal films were prepared based on cholesteric liquid crystal in which fluorescent dye is incorporated. Microcavity effects, i.e., emission enhancement and spectrum narrowing, were observed. Two types of demonstration liquid‐crystal displays (LCDs) were fabricated using the prepared photonic‐crystal films in a backlight system. One is an area‐color LCD in which a single photonic‐crystal layer is used for each color pixel and the other is a full‐color TFT‐LCD in which three stacked photonic‐crystal layers are used as light‐conversion layers. The area‐color LCD was excited by using 365‐nm UV light, and the full‐color TFT‐LCD was excited by using 470‐nm blue LED light. Because of the photonic crystal's unique features that allow it to work as light‐emitting and light‐reflecting layers simultaneously, both LCDs demonstrate clear readable images even under strong ambient light, such as direct‐sunlight conditions, under which conventional displays including LCDs and OLED displays cannot demonstrate clear images. In particular, an area‐color LCD, which eliminated color filters, gives clear images under bright ambient light conditions even without backlight illumination. This fact suggests that a backlight system using novel photonic‐crystal layers will be suitable for energy‐efficient LCDs (e²‐LCDs), especially for displays designed for outdoor usage.     

Deformed nanostructure of photo‐induced biaxial cholesteric films and their application in VA‐mode LCDs

Abstract— Novel biaxial retardation films made from photo‐induced deformed cholesteric liquid‐crystal (LC) nanostructures using reactive mesogen mixtures (RMMs) for a viewing‐angle compensation of vertically aligned liquid‐crystal displays (VA‐LCDs) was developed. The deformed cholesteric LC nanostructure has been observed by X‐ray‐diffraction (XRD) measurement. The birefringence of the film was described well by our optical model based on a form birefringence theory. The VA‐LCDs with photo‐induced biaxial cholesteric films have excellent viewing‐angle properties.     

A novel approach to LCD optimization

Abstract— A novel approach to optimization liquid‐crystal displays (LCDs) is presented. The optimization module allows for a prediction with a high accuracy for the best results, which can be obtained for one or the other configuration of the polarizers and phase retarders in various electro‐optical modes, if the LC parameters and the operating voltages are fixed. The module is a part of our program, MOUSE‐LCD, which is efficient software for LCD optimization and modeling.     

Analysis of the dependence of optical response time of liquid‐crystal displays on the viewing direction

Abstract— The optical response time of liquid‐crystal displays (LCDs) has recently been observed to be dependent on the viewing direction. For the vertically aligned LC mode, response time increased as the viewing angle increased when the final state is the zero gray level of the minimum luminance. This change in response time is analyzed to relate to the deformation of the normalized luminance curve of LCDs for different viewing directions. The dependency trends of the optical response time for the oblique direction can be estimated from the temporal luminance measurement data along the normal direction and the normalized luminance curve for oblique viewing directions.     

Robust flexible LCDs with paintable technology

Abstract— In this article, second‐generation liquid‐crystal displays (LCDs) made by Paintable LCD technology is presented. With this technology, LCDs are manufactured by a sequence of simple coating and UV curing processes. Since the process can be carried out on plastic substrates and the stack of optical layers is only tens of micrometers thick, the resulting LCDs are ultra‐thin and flexible.     

Optical design and roll‐to‐roll fabrication process of microstructure film for wide viewing LCDs

We have developed a new microstructure film for wide viewing liquid crystal displays (LCDs). By attaching it to the surface of a conventional LCD, the viewing angle characteristics of LCD has drastically improved without causing a blur of the frontal image and a decrease in the contrast ratio under bright ambient light conditions. This film can be applied to various LC modes including twisted nematic and multidomain vertical alignment by changing its internal micrometer‐size 3D structure. Further, this film can be mass‐produced efficiently by self alignment roll‐to‐roll process.     

Aggregate‐induced emission in light‐emitting liquid crystal display technology

Nowadays, liquid crystal displays (LCDs) with light‐emission are considered as energy‐efficient devices and are promising alternatives to conventional LCDs. To realize such possibility, strong fluorescent materials with a dichroic properties are required. Aggregate‐induced emission (AIE) is an unusual photophysical phenomenon shown by some luminogenic materials that will be highly emissive in their aggregate state. In this work, we studied the AIE effect of a luminescent liquid crystalline molecule TPE‐PPE in our LC system as a luminophore dopant. The result showed the excellent AIE effect that higher concentration of luminogen in the nematic LC host induced stronger luminescent intensity. Through exposure of a photoisomeriable alignment material sulfonic‐dye‐1, the photopatterning of a light‐emitting LC device was achieved with the use of the TPE‐PPE/nematic LC mixture.     

Polyimide‐free LCD by dissolving dendrimers

Vertical alignment (VA) and in‐plane switching modes have been widely used for liquid crystal displays (LCDs). They require a polyimide (PI) alignment layer in the pixel structure. PI‐free LCDs have been proposed to exploit the VA of liquid crystals (LCs) obtained by dissolving dendrimers without PI. In this paper, we report a new PI‐free VA mode with a pixel structure that has in‐plane electrodes. The PI‐free VA is achieved by dissolving an LC dendrimer in a positive LC mixture. We measured the test cell properties and obtained a lower voltage and a higher brightness in the voltage–brightness curve. In addition, we analyzed the alignment surface of LC dendrimer by time‐of‐flight secondary ion mass spectrometry and scanning electron microscopy observations. We found that dendrimer molecules are uniformly adsorbed on the glass surface and that the layer was generally one molecule thick. These properties are responsible for the lower voltage and higher brightness of the PI‐free VA mode. The use of dendrimers allows the PI process to be omitted and reduces the power consumption of the VA mode. It is thus possible to reduce the high manufacturing costs and improve the performance of the VA mode.     

Moving‐image‐sticking phenomenon induced by an outside force in liquid‐crystal displays

Abstract— Image deformation caused by an outside force is observed to remain for hours at high gray levels for liquid‐crystal displays (LCDs) in the multi‐domain (MD) vertical‐alignment (VA) mode. This so‐called moving‐image‐sticking phenomenon demonstrated a non‐symmetric luminance profile for the left and right viewing direction for MDVA‐mode LCDs which have original symmetric viewing‐angle characteristics. The generation of a stable reverse‐tilt domain by an outside force was assumed to be the cause of this phenomenon, and the stability of a reverse‐tilt domain under an electric fringe field was calculated by changing the electric‐fringe‐field distribution which determines the LC tilt direction. The domain of a given tilt direction is calculated to change to other tilt direction induced by a fringe field at a low gray condition, but to remain unchanged at a high gray condition. This agrees with the observed trends of duration time of the moving‐image‐sticking phenomenon.     

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.     

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.     

Active‐matrix and flexible liquid‐crystal displays with carbon‐nanotube pixel electrodes

Abstract— The necessary processes to use random carbon‐nanotube networks as transparent conductors in twisted‐nematic liquid‐crystal displays have been developed, replacing indium tin oxide. Because the nanotubes are deposited vacuum‐free from suspension, the potential advantages are lower costs for material, equipment, and production. Nanotube networks are also much better suited for flexible displays than the commonly used metal oxides. With the developed processes, the world's first full‐color active‐matrix LCDs as well as directly addressed flexible displays on plastic substrates with carbon‐nanotube pixel electrodes, have been realized.     

Scanning backlight for motion‐blur reduction on mobile displays

Abstract— Small‐form‐factor liquid‐crystal displays (LCDs) are mainly used in mobile applications (e.g., mobile phones, PDAs, and portable game consoles) but also in digital still cameras, video cameras, automotive applications, etc. Like all active‐matrix LCDs, mobile displays suffer from motion blur caused by the sample‐and‐hold effect. One option for improving the motion portrayal on active‐matrix LCDs is the use of a scanning backlight, which results in an imaging behavior similar to the one present in impulsive displays. In this paper, the realization of a scanning backlight for mobile displays is reported. This employs a backlight with seven individually lit segments for reducing the motion blur. Results of perception experiments performed with two identical displays confirm the benefit of using this technology. Optimal driving conditions result in a major improvement in motion portrayal on mobile LCDs.     

Electro‐optical characteristics of ion‐beam‐aligned FFS‐LCDs on diamond‐like‐carbon thin film

Abstract— A fringe‐field‐switching (FFS) mode cell having LC alignment has been developed by using a non‐rubbing method, a ion‐beam‐alignment method on a‐C:H thin film, to analyze the electro‐optical characteristics of this cell. The suitable inorganic thin film for FFS‐LCDs and the alignment capabilities of nematic liquid crystal (NLC) have been studied. An excellent voltage‐transmittance (V‐T) and response‐time curve for the ion‐beam‐aligned FFS‐LCDs were observed using oblique ion‐beam exposure on DLC thin films.     

Photoalignment and photo‐patterning of planar and homeotropic liquid‐crystal‐display configurations

Abstract— Optical alignment and micro‐patterning of the alignment of liquid‐crystal displays (LCDs) by linear photopolymerization (LPP) technology renders high‐quality multi‐domain twisted‐nematic (TN) and supertwisted‐nematic (STN) displays with broad fields of view over wide temperature ranges feasible. The prerequisites are the generation of photo‐induced high‐resolution azimuthal alignment patterns with defined bias‐tilt angles 0° ≤ θ ≤ 90°. For the first time, LPP‐aligned single‐ and dual‐domain vertically aligned nematic LCDs (VAN‐LCDs) are presented. Dual‐domain VAN‐LCDs are shown to exhibit broad fields of view which are further broadened by combining the displays with LPP‐aligned optical compensators made of liquid‐crystal polymers.     

Liquid‐crystal mixtures having tolane substances for field‐sequential‐color TN‐LCDs

Novel liquid‐crystal (LC) mixtures featuring high optical anisotropy Δn) and small rotational viscosity (γ¹) were developed for field‐sequential‐color TN‐LCD applications. The dynamic behavior of the TN cells in a narrow‐gap range was studied and new tolane LC substances were introduced. The newly developed LC mixtures, having a narrow‐gap cell, enable a TN‐LCD to switch fast enough to be applied to field‐sequential‐color displays not only at a room temperature but also at low temperatures. It was also confirmed that the voltage‐holding ratio (VHR) is sufficiently high in field‐sequential addressing conditions and, therefore, the LC mixtures can be used in active‐matrix LCDs. For practical use, a storage test of the TN cells under light irradiation was performed to evaluate their voltage‐holding property. It was also confirmed that their high VHR can be maintained for over 10,000 hours under practical conditions.     

Reduced cross‐talk in shutter‐glass‐based stereoscopic LCD

Abstract— It is expected that 3‐D will be the next step in the enhanced viewing experience. At present, there are two competing 3‐D technologies for glasses‐based consumer TVs: active shutter glasses and passive polarized glasses. With the ongoing reduction in response time of liquid‐crystal displays (LCDs), this article will focus on shutter‐glass‐based stereoscopic LCDs. In this paper, the properties of such a display system is described and it is demonstrated that by adding a line‐scanning backlight, the cross‐talk can be reduced to less than 1.4%, allowing for excellent 3‐D portrayal. For images of extreme contrast, this is perceivable, but not judged annoying by a panel of expert viewers. Which characteristics of the display and shutter glasses that should be optimized to create an inexpensive, cross‐talk‐free, 3‐D LCD are discussed.