Optical rewritable liquid‐crystal‐alignment technology

Abstract— A new optical rewritable (ORW) liquid‐crystal‐alignment technology has been developed to create a display and to demonstrate its maturity and potential. ORW displays have no electrodes and use polarizers as substrates. The display requires no photolithography on plastic. Its simple construction secures durability and low cost for mass production. The on‐screen information is optically changed in a writing unit that consists of an LCD mask and an exposure source that is based on LEDs, low power, and low cost in comparison with Hg lamps or lasers. A high contrast image can be easily written, viewed, and rewritten through a polarizer, while the multi‐stable gray‐level image requires zero power to maintain the image. Reconfigurable LC alignment using ORW technology best suits plastic‐card displays as well as for LC photonics and various one‐mask processes of patterned LC‐alignment applications.     

New properties and applications of rewritable azo‐dye photoalignment

Abstract— The rewritable azo‐dye photoalignment (ORW) of liquid crystals (LCs) for application in optical rewritable electronic paper has been investigated. It was observed that a periodic change in the azimuthal aligning direction with polarized UV light (365 nm) brings about homeotropic alignment, while utilization of visible light (450 nm) does not affect the LC tilt angle. The wavelength dependence of the ORW photoalignment result and the behavior of the photoinduced anisotropy was explored. The dark amplification of film anisotropy after exposure was observed, which is believed to be the relaxation process related to hydrogen bonding in azo‐dye film. New material, CD1, for azo‐dye rotation photoalignement that possesses a high azimuthal anchoring energy (about 2 × 10^?4 J/m²) was found.     

Plasma‐beam alignment technique for ferroelectric liquid crystals

Abstract— The plasma‐beam alignment procedure earlier developed for the alignment of nematic liquid crystals is successfully extended to ferroelectric liquid crystals (FLC). The highly uniform alignment of the “chevron” structure (before electrical treatment of FLC cells) and “quasi bookshelf” structure (after the electrical treatment) are realized. The contrast of bistable switching larger than 350:1 is achieved. This makes the non‐contact plasma‐beam alignment procedure especially attractive for high‐contrast bistable LCDs on an LCOS base, particularly used in PDA and e‐books. Fast switching and realization of gray scale in the plasma‐beam aligned FLC cells makes this technique also promising for full‐color displays including color LCD TV.     

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.     

Amorphous‐silicon thin‐film transistors made at 280°C on clear‐plastic substrates by interfacial stress engineering

Abstract— A process temperature of ～300°C produces amorphous‐silicon (a‐Si) thin‐film transistors (TFTs) with the best performance and long‐term stability. Clear organic polymers (plastics) are the most versatile substrate materials for flexible displays. However, clear plastics with a glass‐transition temperature (T_g) in excess of 300°C can have coefficients of thermal expansion (CTE) much larger than that of the silicon nitride (SiN^x) and a‐Si in TFTs deposited by plasma‐enhanced chemical vapor deposition (PECVD). The difference in the CTE that may lead to cracking of the device films can limit the process temperature to well below that of the T_g of the plastic. A model of the mechanical interaction of the TFT stack and the plastic substrate, which provides design guidelines for avoid cracking during TFT fabrication, is presented. The fracture point is determined by a critical interfacial stress. The model was used to successfully fabricate a‐Si TFTs on novel clear‐plastic substrates with a maximum process temperature of up to 280°C. The TFTs made at high temperatures have higher mobility, lower leakage current, and higher stability than TFTs made on conventional low‐T_g clear‐plastic substrates.     

Azo‐dye aligning layers for liquid‐crystal cells

Abstract— The photo‐induced alignment of liquid crystal onto a photochemical stable azo‐dye film was studied for liquid‐crystal display (LCD) applications. The photo‐aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV‐light polarization. The order parameters S = ?0.4 (80% of the maximum absolute value S_m = ?0.5) was measured at a wavelength of 372 nm from the polarized absorption spectra. The temperature‐stable pretilt angle of 5.3° was obtained by a two‐step exposure of azo‐dye film using normally incident polarized light followed by oblique non‐polarized light. The azimuthal anchoring energy of the photo‐aligned substrate was A_? > 10^?4 J/m², which is of the same magnitude as the anchoring of the rubbed PI layer. The VHR value of the photo‐aligned LC cell was also found to be very high (98–99%) at room temperature and more than 95% at T=80°C. The thermal stability of the photo‐aligned azo‐dye layers is sufficiently high, while UV stability has to be improved, _e.g., by polymerization. The new LCD aligning technology based on photochemical stable azo‐dye layers is envisaged.     

Liquid‐crystal displays using printed plastic TFT backplanes

Abstract— In this paper, we present results from a new liquid crystal over plastic printed thin‐film‐transistor (TFT) display. The display demonstrator shows that the processing incompatibilities between the plastic TFT backplane and the liquid‐crystal materials can be addressed to make a stable twisted‐nematic structure. New fabrication processes such as the photo‐alignment of liquid crystals have made it possible to create a new generation of displays, which pave the way towards fully integrated plastic liquid‐crystal‐display technologies.     

Synthesis and characterization of novel blue azo‐dye derivatives containing acrylate group for dye‐based color filters

Abstract— Even though dyes have a fine resolution and good chromaticities, they are not widely used as coloring materials for color filters (CFs) due to their low thermal stability and chemical resistance. A series of azo‐dye derivatives, which consist of two cross‐linkable acrylate or methacrylate groups to improve thermal and chemical properties, have been synthesized and used to fabricate color filters. The spectral properties and chemical/thermal stabilities of the fabricated CFs were investigated by comparing dye‐based CFs, without a complicated dispersion process, but with pigment‐based CFs using dispersed pigment. Also, more properties including the development test and surface morphologies lithographic properties were studied. The synthesized azo dyes were characterized by elemental analysis, UV‐visible spectra, IR, mass, and ¹H‐NMR spectra.     

Novel alignment technique for liquid crystal using hydrophilic/hydrophobic pattern layer

Abstract— We have developed a novel method of aligning liquid‐crystal (LC) molecules that employs a layer patterned with hydrophilic and hydrophobic regions fabricated by the photocatalysis of TiO₂. This method provides a resolution on the order of 10μm for the alignment of LC molecules without the need for protrusions or patterned electrodes. Using this method, we achieved the continuous multidomain vertical alignment of LC molecules.     

Organic thin‐film devices on paper substrates

Abstract— A high‐performance polymer thin‐film transistor fabricated on a paper substrate has been demonstrated. As a water barrier layer, parylene was coated on a paper substrate by using a vacuum deposition process. Using poly (3‐hexylthiophene) as an active layer, a polymer thin‐film transistor with a field‐effect mobility of up to 0.086 cm²/V‐sec and an on/off ratio of 10⁴ was achieved. The fabrication of a polymer thin‐film transistor built on an inexpensive paper substrate is expected to open a channel for future applications in flexible and disposable electronics with extremely low cost.     

Process study and optimization for fabrication of poly‐Si thin‐film transistors on plastic substrates

Abstract— A complete poly‐Si thin‐film transistor (TFT) on plastic process has been optimized to produce TFT arrays for active‐matrix displays. We present a detailed study of the poly‐Si crystallization process, a mechanism for protecting the plastic substrate from the pulsed laser used to crystallize the silicon, and a high‐performance low‐temperature gate dielectric film. Poly‐Si grain sizes and the corresponding TFT performance have been measured for a range of excimer‐laser crystallization fluences near the full‐melt threshold, allowing optimization of the laser‐crystallization process. A Bragg reflector stack has been embedded in the plastic coating layers; its effectiveness in protecting the plastic from the excimer‐laser pulse is described. Finally, we describe a plasma pre‐oxidation step, which has been added to a low‐temperature (<100°C) gate dielectric film deposition process to dramatically improve the electrical properties of the gate dielectric. These processes have been integrated into a complete poly‐Si TFT on plastic fabrication process, which produces PMOS TFTs with mobilities of 66 cm² /V‐sec, threshold voltages of ?3.5 V, and off currents of approximately 1 pA per micron of gate width.     

Synthesis and application of an alkylated pyrazole‐based azo dye for electrofluidic display

Electrofluidic display (EFD) is one of the most promising reflective displays for its full color and video speed. Colored EFD oil, normally formulated by soluble organic dyes in non‐polar solvent, dominates the color, electro‐optical behavior, and reliability performances for EFD devices. In this paper, a novel yellow electrofluidic dye with excellent solubility in non‐polar EFD solvent was achieved based on the introduction of long alkyl chain into pyrazole azo dye. The resulting EFD device fabricated by the dye showed excellent application properties, such as fast switching speed (17.8 ms), high aperture ratio (68.5%), low threshold voltage (24v), good light stability (240 h under accelerated conditions), and low backflow phenomenon.     

Plasma beam alignment for the large‐area substrates: Equipment and process

Abstract— We have developed an effective method for liquid‐crystal alignment of the large‐area substrates. This method is based on the oblique treatment of the alignment substrates with a “sheet” of accelerated plasma generated by the anode layer source of the “race track” geometry. During this treatment, the substrate or source is cyclically translated in the direction perpendicular to the plasma “sheet.” This method provides planar, tilted, and vertical liquid‐crystal alignment with excellent uniformity and reproducibility and easy axis control in the azimuthal and polar planes.     

Medical displays by using plasma‐beam‐alignment technology

Abstract— Novel anode layer plasma within minimum chamber space was developed for non‐contact alignment process. The plasma‐treated polyimide (PI) surface showed no particle contamination and no micro‐scratches. Surface morphology was investigated by using scanning electron microscope (SEM), an atomic force microscope (AFM), and X‐ray photoemission spectroscopy. The different oxygen‐to‐carbon ratio ([O]/[C] ratio) for XPS spectra indicated a composition change after plasma treatment. Surface pretilt angles were varied from 0 to 2.1° under different plasma exposure times. Finally, a prototype 20.8‐in. QXGA IPS‐mode gray‐scale medical liquid‐crystal display was successfully demonstrated with high contrast ratio, excellent uniformity, and wide viewing angle using this new plasma‐beam‐alignment technique.     

Field‐induced photo‐reactive alignment technology for the vertical‐alignment liquid‐crystal mode

Abstract— An advanced vertical‐alignment liquid‐crystal (VA‐LC) technology based on field‐induced photo‐reactive alignment (FPA) as an advanced alignment mode for VA is proposed. FPA realizes uniform alignment and a faster rising response time, especially at high voltage. This technology can generate a pre‐tilt angle only by using photo‐reactive alignment material so that the tact time is shorter and the long‐term reliability is higher than that of conventional photo‐reactive processes, which require additional photo‐reactive monomers. The advanced hybrid FPA was developed by adopting both the tilted alignment with a pre‐tilt angle and conventional vertical alignment. By using an advanced hybrid structure, the response time and contrast ratio can be further improved.     

Development of Binem® displays on flexible plastic substrates

Abstract— We have successfully fabricated Binem® displays on thin flexible plastic substrates. The fabrication is based on the standard Binem® process for glass, which has been adapted to plastic using new materials and technologies. The first application is targeted for an embedded display for smart‐card products.     

Alignment layers for cholesteric displays on glass and plastic substrates

Abstract— The optical performance of surface-stabilized cholesteric liquid-crystal displays (SCT-LCDs) has been optimized by using obliquely sputtered SiO₂ as the alignment layer. A comparison with polyimides used up to now showed an improvement in the contrast ratio by a factor of 2 for glass displays. A second advantage of this material is the absence of high temperatures during the manufacturing process. Therefore, it is possible to use obliquely sputtered SiO₂ for the preparation of plastic display prototypes. We have realized a 6-in. VGA display with approximately 130 dpi. This is the largest cholesteric display with the highest resolution fabricated on plastic substrates to date.     

Electro‐optical characteristics and optimization of vertically aligned STN‐LCDs

A new LCD for high‐duty passive‐matrix driving, referred to as a vertically aligned (VA) STN‐LCD, characterized by high transmittance and a high contrast ratio, has been developed. This LCD can display black‐and‐white images by using crossed polarizers without the use of a color compensator because the LCD has almost zero retardation at the initial alignment state. A fabricated black‐and‐white VA‐STN‐LCD without a black matrix shows high transmittance (>15%) and contrast (CR > 100) under 1/240‐duty direct‐multiplex driving.     

Characterization of flexible reflective liquid‐crystal cells

Abstract— We consider peculiarities in testing flexible reflective liquid‐crystal (LC) cells. Several new methods for measuring optical retardation of filled reflective LC cells on plastic substrates are proposed. Cases when the plastic is anisotropic and the LC cell consists of either one or two internal polarizers are studied. The majority of proposed methods can be applied for transmissive LC cells as well as for measuring twist angle.