Twisted Nematic Liquid Crystal Display Research Articles

Transparent conductive Al-doped ZnO films for liquid crystal displays

Al-doped zinc oxide (ZnO:Al) films were applied to liquid crystal displays (LCDs) as transparent electrodes substituting indium tin oxide (ITO). While the ZnO:Al-based twisted nematic LCD cell showed similar operational behavior to ITO-based counterpart, its electro-optical (EO) and residual dc (r-dc) characteristics were somewhat improved. Capacitance-voltage relations suggested that these improved EO and r-dc characteristics of the ZnO:Al-based LCD cell are due to the substantially lower density of charge carrier trapping centers in the polyimide layer∕electrode interface region, demonstrating high application potential of ZnO:Al films as transparent electrodes of LCDs.

Dielectric Spectroscopy of Metal Nanoparticle Doped Liquid Crystal Displays Exhibiting Frequency Modulation Response

Twisted nematic liquid crystal displays (TN-LCDs), doped with the nanoparticles of metal, such as Pd, Ag, or Ag-Pd, which are protected with ligand molecules, such as nematic liquid crystal, exhibit a frequency modulation (FM) electro-optical (EO) response with short response time of milliseconds (ms) or sub-ms order together with the ordinary rms voltage response. These devices are called FM/AM-TN-LCDs; they are distinct from the ordinary LCDs featured by the amplitude modulation (AM) response. The phenomena of the FM/AM LCDs may be attributed to the dielectric dispersion of a heterogeneous dielectric medium known as the Maxwell-Wagner effect. It is experimentally shown that the frequency range spreads from several tens hertz to several tens kilohertz and the spectrum is more or less centered about the dielectric relaxation frequency. We formulated a theory based on an equivalent circuit model to evaluate the dielectric relaxation frequency and the dielectric strengths; and we succeeded in explaining the dependence of the dielectric relaxation frequency on the concentration of nanoparticles and the their dielectric and electrical properties, whereas conventional theories based on electromagnetic theory are unable to explain this concentration dependence. This paper reports on the experimental results of the EO effects and the dielectric spectroscopy including the dielectric relaxation times and the dielectric strengths of nematic liquid crystal, 5CB (4-pentyl-4'-cyanobiphenyl), doped with the metal nanoparticles of I'd alone and Ag-Pd composite; and discusses how the observed dielectric relaxation frequency or dielectric relaxation time depend on the concentration of the doped nanoparticles and also their electrical and dielectric properties

High-resolution full-color LCD driven by OTFTs using novel passivation film

A full-color twisted-nematic type liquid crystal display (TN-LCD) of 1.4-in diagonal size driven by organic thin-film transistors (TFTs) has been fabricated. This TN-LCD has 80/spl times/80/spl times/3 (RGB) pixel arrays addressed by pentacene TFTs with a channel width of 50 /spl mu/m. The contact resistance between the pentacene film and the source/drain electrodes has been reduced by selecting the exposure condition of the photoresist in patterning the electrodes. In addition, a solution-processed passivation film with a novel structure, consisting of photosensitive polyvinylalcohol and organosiloxane glass resin, has been developed to protect the TFTs against degradation induced by integration with TN-LCD devices. Consequently, it has been confirmed that the organic-TFT-driven TN-LCD fabricated in this paper is capable of displaying full-color moving images at a resolution of 80 pixels per inch.

P-144: Low Voltage, High Contrast Ratio, and Wide Viewing Angle TN LCD's for Microdisplay, Mobile Phone and PDA Mobile Video Applications

The modeled contrast ratio of a twisted nematic liquid crystal display (TN/LCD) was improved by over a factor of 2 to greater than 500:1 at applied voltages of 2 to 4 volts via the addition of a thin negative birefringence optical film with a retardation value up to 50nm and the optical axis parallel to the surface. Combining wide-viewing-angle compensation films with this thin negative birefringence optical film can lead to significantly improve viewing angle as well. This performance improvement is important for high performance microdisplay HMD's, mobile phone, PDA mobile video TV's, and 3D applications.

P-156: Transflective Double Cell TN Liquid Crystal Display

Double cell Twisted Nematic (DTN) liquid crystal display is optimized for transflective liquid crystal displays, using MOUSE-LCD software. Two configurations of such DTN-LCD were simulated using a double and a single cell gap approach. DTN-LCD exhibits a high contrast ratio, wide viewing angles and achromatic (black/white) switching in both the transmissive mode and reflective mode. It also possesses a perfect dark state. DTN configuration is suitable for the high quality transflective TFT-LCD.

P-163: Design of Transflective Bistable Twisted Nematic LCD

We designed a transflective bistble twisted nematic (BTN) liquid crystal display with pattern compensation films. By using Meuller matrix method, we found the display parameters to optimize the performance of both the transmissive and the reflective modes. The display has a high contrast and high brightness.

Dielectric Properties of Twisted Nematic Liquid Crystal Displays Fabricated by Doping Ag-Pd Metal Nanoparticles Having A Long Term Stability

ABSTRACT TN-LCDs particularly doped with the metal nanoparticles of Ag-Pd composite, which are protected by chemical covering with nematic liquid crystal, 5CB (K-15, Merck) as a ligand, are shown to exhibit a long term stability in the electro-optical (EO) effect featured by a frequency modulation with short response times of ms or sub-ms order. This device is called FM-LCD. The unique electro–optical characteristics of the FM-LCD were clarified in terms of the Maxwell-Wagner effect of a heterogeneous dielectrics, where the analysis is done using an equivalent circuit model rather than the conventional potential theory; and we derived the formulae of the dielectric strength and the dielectric relaxation time. The former is used to explain the FM-LCD effect and also to evaluate the effective electrical conductivity of the metal nanoparticle that will be 4.4 × 104 S/m, which is about 1 × 10−3 times smaller than that of metal Ag; and the latter is expressed to demonstrate that the relaxation time decreases with increasing the concentration of metal nanoparticle for the first time.

Pentacene-based thin film transistors used to drive a twist-nematic liquid crystal display

The study addresses the factors of influence on the active matrix display that is driven by pentacene-based organic thin-film transistors (TFTs). The atmosphere and humidity conditions were found to seriously affect the performance of organic TFTs. An appropriate encapsulation layer was added to protect the organic TFTs from external damage. Organic TFTs reliability, the illumination effect and device uniformity, were also considered in the context of display application. Finally, a monochrome 3 inch 64 × 128 active-matrix twist-nematic liquid crystal display (LCD) was fabricated. The display is capable of showing video images with a refresh rate of 20 Hz. Obtained results reveal the potential of organic TFTs for active-matrix LCD technology.

Novel Photoaligned Twisted Nematic Liquid Crystal Cell

A novel photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle is sufficiently large (>75°), the p-polarization of the light is more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photo-alignment of the azo-dye layer on the bottom substrate is perpendicular to that of the upper one, and LC directors on the top and bottom substrates also become perpendicular. The method provides a new simple way of manufacturing photoaligned TN-LCDs.

A High-Performance Short-Channel Bottom-Contact OTFT and Its Application to AM-TN-LCD

We have demonstrated an organic thin-film-transistor (OTFT)-driven active-matrix twisted-nematic liquid crystal display (AM-TN-LCD) on a glass substrate, with a resolution of 160/spl times/120 pixels, 79 ppi. Substrate temperature was kept below the plastic-compatible temperature of 180/spl deg/C throughout the fabrication process. In order to realize an OTFT-driven display with fine resolution, we employed short-channel bottom-contact (BC) pentacene OTFTs. It has been known that their drivability is limited by contact resistance at source/drain (S/D). We found that the S/D contact resistance was markedly reduced when the thickness of the nonohmic Ti adhesion layer for ohmic Au S/D electrodes was reduced less than /spl sim/3 nm. We elucidate that this 3 nm corresponds to the thickness of the accumulating layer in a pentacene channel. When we use a self-assembled monolayer of mercapto-silane-coupling agent as the adhesion layer, the contact resistance becomes negligibly small and BC OTFTs scalable below 10 /spl mu/m were obtained. In addition to this OTFT-cell technology, we developed a low-damage pentacene patterning technique for integration of OTFTs and introduced low-temperature panel assembly process to suppress thermal-stress degradation of pentacene OTFTs, which are the key technologies to achieve OTFT-driven AM-TN-LCD.

Encoding fully-complex transmittance with coupled amplitude-phase liquid-crystal modulator

We propose a holographic code for synthesis of fully-complex transmittance, which can be implemented employing a twisted-nematic liquid-crystal display, two linear polarizers, and a He-Ne laser. This simple setup provides a reduced phase range and amplitude modulation with significant variance. Our holographic code efficiently exploits this constrained modulation for the accurate encoding of arbitrary complex transmittance. Two experimental examples illustrate the good performance of the holographic code.

Optical Investigation of Disclination Lines in Multidomain Twisted Nematic Liquid Crystal Display Created by Microrubbing

ABSTRACT A microrubbing (μ-rubbing) process was utilized to create a four-domain twisted nematic cell with improved viewing angle characteristics in comparison to a single domain twisted nematic display. A metallic sphere of 1 mm in diameter was used to μ-rub homeotropic polyimide, resulting in planar alignment with a 10° pretilt. With our set of initial conditions, a 10° pretilt cannot stabilize the four-domain structure in zero field; however, the four-domain structure is stable for voltages above 1 V. Here we optically study the formation of disclination lines and the stability of the four-domain configuration with respect to the applied voltage. The director orientation of the four-domain cell is simulated using the 2dimMOS simulation software and the optical transmission using extended Jones matrix method. Iso-contrast plots of the four-domain structure are compared with the single domain structure.

Synthesis and Physical Properties of Novel Fluorinated Liquid Crystalline Compounds

ABSTRACT Most twisted nematic liquid crystal displays (TN-LCD) uses liquid crystal mixtures with a positive dielectric anisotropy. In contrast, the recently introduced vertically aligned liquid crystal display (VA-LCD), which offers superior picture quality with a wide viewing angle, high contrast, and video-compatible switching times, is based on a liquid crystal mixture with negative dielectric anisotropy. We introduce here the synthetic route and physical and electro-optical properties of the liquid crystalline compounds with an ester group as linkage subsistent and with an alkyl chain as lateral subsistent. Also, we present physical and electro-optical properties of their mixture, and the LC-series for VA-LCD. Synthetic compounds exhibit broad range nematic phases up to 60 K degree and large negative dielectric anisotropies. And the viscosity of the LC-series is low in comparison with that of homologues with a lateral isothiocyanate group. The threshold voltage (Vth) was lower than that of a conventional mixture, enough so for it to be operated at a driving voltage. And the rising time of some LC-series improved to be faster than conventional LC mixtures. Our novel LC compounds are suitable materials for the improvement of the dielectric anisotropy of vertically aligned liquid crystal mixtures.

Adaptive correction to the speckle correlation fringes by using a twisted-nematic liquid-crystal display

An innovative technique for reducing speckle noise and improving the intensity profile of the speckle correlation fringes is presented. The method is based on reducing the range of the modulation intensity values of the speckle interference pattern. After the fringe pattern is corrected adaptively at each pixel, a simple morphological filtering of the fringes is sufficient to obtain smoothed fringes. The concept is presented both analytically and by simulation by using computer-generated speckle patterns. The experimental verification is performed by using an amplitude-only spatial light modulator (SLM) in a conventional electronic speckle pattern interferometry setup. The optical arrangement for tuning a commercially available LCD array for amplitude-only behavior is described. The method of feedback to the LCD SLM to modulate the intensity of the reference beam in order to reduce the modulation intensity values is explained, and the resulting fringe pattern and increase in the signal-to-noise ratio are discussed.

Cell-Gap Measurement of Reflective Twisted-Nematic Liquid Crystal Display with Internal Surface-Roughened Reflector

A new cell-gap measurement method is described for reflective twisted-nematic liquid crystal displays (TN-LCDs). The behavior of reflected-light intensity in the cell-gap measurement shows that the surface reflection of the display has a decisive influence on the obtained cell gap. The new measurement method, which takes into account the surface reflection, exhibits a good cell-gap agreement with a transmissive cell-gap measurement in an experiment using transmissive LC cells and an external reflector. Signal reflected-light intensity depends on the measurement point in the case of a display having a surface-roughened reflector. The signal becomes smaller on the slant face than both at the top and bottom of the reflector. Correction for signal variation is also included in the new method. The method is applied to microscopic cell-gap mapping using a microscope and a charge-coupled device (CCD) camera system. The cell-gap map obtained is consistent with the shape of the reflector.

Computer-generated holograms with optimum bandwidths obtained with twisted-nematic liquid-crystal displays

We discuss a computer-generated hologram for encoding arbitrary complex modulation based on a commercial twisted-nematic liquid-crystal display. This hologram is implemented with the constrained complex modulation provided by the display in a phase-mostly configuration. The hologram structure and transmittance are determined to obtain on-axis signal reconstruction, maximum bandwidth, optimum efficiency, and high signal-to-noise ratio. We employed the proposed holographic code for the experimental synthesis of first-order Bessel beams.

Unique measurement of the parameters of a twisted-nematic liquid-crystal display

The parameters of a twisted-nematic liquid-crystal display (TN-LCD) are measured with no ambiguity when we measure the intensity transmittance of a system that simply consists of a TN-LCD, two polarizers, a quarter-wave plate, and a monochromatic beam. First we show analytically that the director angle can have a 90 degree ambiguity and the twist angle of the liquid-crystal molecules a 180 degree ambiguity. Then we uniquely measure the parameters by fitting the theoretical predictions to the intensity transmittance measured with and without the quarter-wave plate and by using the quasi local-adiabatic condition. The birefringence of the TN-LCD is measured next as a function of the applied voltage. We design a phase spatial light modulator by using the measured parameters and measure the phase delay of the output beam to excellent agreement with the theoretical prediction.

High pretilt four-domain twisted nematic liquid crystal display by microrubbing: Process, characterization, and optical simulation

A microrubbing (μ-rubbing) technique was utilized to create four-domain alignment in a liquid crystal display sample. A small metallic sphere under sufficient load was used to directly rub a homeotropic polyimide alignment layer to create a large surface pretilt angle. We demonstrate a 47-μm linewidth and a variable pretilt angle with respect to the substrate plane for different loads. The homeotropic polyimide surface was μ rubbed in such a way that neighboring alignment regions are rubbed in opposite directions. liquid crystal cells were constructed with two μ-rubbed substrates where the rubbing directions are orthogonal to each other. With this cell configuration we obtained four domains consisting of two left-handed and two right-handed twisted nematic subpixels. We report on the electro-optic performance properties, viewing angle characteristics, the relationship between pretilt angle versus load, and thermal aging study of the four-domain liquid crystal cell. The optical simulations of the liquid crystal director orientation, viewing angle characteristics, and the contrast ratio of the four domains are also presented. The experimental results are in agreement with the simulation results.

Effects of carbon-nanotube doping on the performance of a TN-LCD

— The electro-optical characteristics of a 90° twisted-nematic liquid-crystal display (TN-LCD) were analyzed. The test cell was composed of a minute amount of dopant, multiwalled carbon nanotubes, and a standard nematic mixture, E7, used in TN-LCDs with direct addressing. Under the experimental conditions, no hystereses in optical transmittance were observed in either the doped cell or a neat counterpart under an applied ac voltage. Experimental results show that doping with nanotubes rectifies the electro-optical properties of the cells by reducing the driving voltage as well as the rise time. Similar results were found in a TN cell of a TFT-grade liquid crystal instead of the mixture consisting completely of polar compounds.

저온 공정을 이용한 플라스틱 STN 셀의 전기 광학 특성

We investigated the electro-optica](EO) performances of the super twisted nematic liquid crystal display(STN-LCD) on the polyimide(PI) surface using polymer film. The NLC(Nematic Liquid Crystal) pretilt angles generated are about 18<TEX>$^{\circ}$</TEX>by the rubbing alignment method on thin plastic substrates. However, the pretilt angle are at about 13<TEX>$^{\circ}$</TEX> lower on the glass substrate than on thin plastic substrate. Monodomain alignment of the plastic STN-LCD can be observed. A stable voltage-transmittance(V-T) curve of the plastic STN-LCD was observed on the polyimide(PI) surfaces using polymer film. Also, a faster response time for the plastic STN-LCD on the polyimide(PI) surfaces using polymer film can be achieved.