A liquid-crystal display device

Abstract: A liquid-crystal display device comprising a double-layered-type liquid-crystal cell that is composed of a first cell layer (C₁) and a second cell layer (C₂), said cell containing liquid-crystal molecules (4) with a twisted nematic orientation therein and a voltage-applying means (2, 22, 32) in one of the first and second cell layers (C₁), wherein the angle of twist of the liquid-crystal molecules (4) in the first cell layer (C₁) is opposite to that of twist of the liquid-crystal molecules (4) in the second cell layer (C₂), and the orientation of the liquid-crystal molecules (4) in the first cell layer (C₁) in the vicinity of the second cell layer (C₂) is at right angles to that of the liquid-crystal molecules (4) in the second cell layer (C₂) in the vicinity of the first cell layer (C₁).

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a multi-layered-type liquid-crystal display device with the use of a twisted nematic display process that can attain an excellent color display. 2. Description of the prior art: Liquid-crystal display devices are now being used in clocks and electronic calculating machines, and in computer terminals, word processor displays, televisions, and a variety of other uses in many fields. Recently, there has been an extremely large demand for liquid-crystal display devices because of the changes to multicolor and full-color displays, which are already being made use of in the fields of graphic display and image display. Color displays that have been widely put into practical use are attained by a liquid-crystal cell with color filter layers. The liquid-crystal cell functions as a light-switcher, and produces various colors. The main kind of display mode is a twisted nematic display mode attained by a liquid-crystal cell in which the liquid-crystal molecules are twisted 90°C so that high contrast, etc., can be obtained. However, with this TN display mode, the dependence of the display characteristics on the wavelength of light is great, and it is not possible to achieve switching of light uniformly over the entire spectrum of visible light. In particular, with the normally closed display process in which the axes of absorbance of the two polarizers are in parallel, there is the problem that light leakages at the time of the application of voltage gives rise to color. With a color display device that causes light-switching by the use of a TN display that has this kind of a color filter layer, there are two main kinds of driving methods. One of them involves the active-matrix drive method that uses a liquid-crystal cell that has picture elements provided with non-linear devices such as diodes or switching elements such as thin-film transistors. The other method involves the duty drive method in which the liquid crystals of a liquid-crystal cell without picture elements are successively driven. With the latter method, the steepness in the vicinity of the threshold value of the optical characteristics of the liquid crystals is important; this is a problem with the TN displays that are being used now. In order to improve the optical characteristics so that steepness in the vicinity of the threshold value can be attained, there has been suggested a supertwisted birefringence effect (SBE) process that gives liquid-crystal molecules that are twisted at angles of around 180°-270°C. With the SBE process, the curve in the vicinity of the threshold value increases sharply, and even when the duty ratio increases, it is possible to obtain a high contrast ratio. However, because birefringence effects of liquid crystals are used, the dependence of the display characteristics on the wavelength is theoretically higher than with the TN display, so it is very difficult to adapt it for use in a full-color display. GB-A-2 092 769, on which the preamble to claim 1 is based, discloses a liquid-crystal display device comprising: - a double-layered-type liquid-crystal cell that is composed of a first cell layer and a second cell layer, each said cell layer having first and second transparent substrates and liquid-crystal molecules with a twisted nematic orientation disposed between said first and second transparent substrates; and a voltage-applying means provided in one of the first and second cell layers; wherein the angle of twist of the liquid-crystal molecules in the first cell layer is opposite to the angle of twist of the liquid crystal molecules in the second cell layer; and wherein the projections onto the plane of said substrates of the longitudinal axes of liquid crystal molecules in said first cell layer adjacent to a region separating said first and second cell layers are perpendicular to the projections onto the plane of said substrates of the longitudinal axes of liquid crystal molecules in said second cell layer adjacent to said region separating said first and second cell layers. In the liquid crystal display device of GB-A-2 092 769, the angle of twist of the liquid-crystal molecules in the first and second cell layers is 90°. EP-A3-0 131 216 discloses a single layer liquid crystal display device. The angle of twist of the liquid-crystal molecules in the liquid crystal layer is between 180° and 360°. SUMMARY OF THE INVENTION According to the present invention there is provided a liquid-crystal display device as claimed in claim 1. In a preferred embodiment, the relationship between the pitch p of twist of the liquid-crystal molecules in the cell layer that has the voltage-applying means therein and the thickness d of the liquid-crystal layer in the said cell layer is as follows: ϑ/2π -1/4 < d/p ≲ ϑ/2π, wherein ϑ is the angle of twist of the liquid-crystal molecules. In a preferred embodiment, a color filter layer is disposed in at least one of the first and second cell layers. In a preferred embodiment, an active device is disposed on each picture element in at least one of the first and second cell layers. Thus, the invention described herein makes possible the objectives of (1) providing a liquid-crystal display device that produces a colored display image with superior color reproducibility and high contrast; and (2) providing a liquid-crystal display device that attains a full-color display or a multicolor display. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a sectional view showing the fundamental cell structure of a double-layered-type liquid-crystal display device of this invention. Figures 2a and 2b, respectively, are diagrams showing twists of liquid-crystal molecules in the right and left directions. Figure 3 is of characteristic curves showing the relationship between the Δn₂·d₂ of the second cell and the transmittance when a polarizer is placed in a crossed-Nicols manner with regard to the liquid-crystal display device of this invention. Figure 4 is of a characteristic curve showing the relationship between the angle of twist of liquid-crystal molecules and the contrast ratio of display images. Figure 5 is of a characteristic curve showing the relationship between the applied voltage and the transmittance with regard to a double-layered-type LCD. Figure 6 is a sectional view showing a liquid-crystal display device (i.e., a double-layered-type SBE-LCD) of this invention. Figure 7a is of characteristic curves showing the relationship between the applied voltage and the light transmittance with regard to a single-layered-type SBE-LCD. Figure 7b is of characteristic curves showing the relationship between the applied voltage and the light transmittance with regard to the display device of Figure 6 of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention provides a liquid-crystal display device, the fundamental double-layered cell structure of which is, as shown in Figure 1, composed of a first cell layer C₁ and a second cell layer C₂ that contains liquid-crystal molecules therein with a twisted nematic orientation. Each cell layer is composed of transparent substrates 1 of glass, acrylic resins, or the like, transparent conductive films 2 of ITO, nesa film, or the like positioned on the substrate 1, orientation films 3 of inorganic film made of SiO₂, SiO, or the like or an organic film made of polyimide, polyvinyl alcohol, nylon, acrylic resins, or the like for the orientation of liquid-crystal molecules positioned on the substrates 1 and the transparent conductive films 2, and a polarizer 5 positioned on the back face of the substrate 1. Both ends of each cell layer are sealed by sealing substances 6. A liquid-crystal layer 4 is disposed in each of the cell layers C₁ and C₂. The direction in which the liquid-crystal molecules of the liquid-crystal layer 4 in one cell layer are helically twisted is opposite to the direction in which the liquid-crystal molecules of the liquid-crystal layer 4 in the other cell layer. The directions of twist of the liquid-crystal molecules are set as shown in Figures 2a and 2b in which Figure 2a shows the twist of the liquid-crystal molecules in the right direction with regard to the direction in which light from a light source is incident upon the cell and Figure 2b shows twist of the liquid-crystal molecules in the left direction with regard to the light incidence direction. When an optically active substance is added to a nematic liquid crystal, the liquid-crystal molecules form a twisted structure. To cause the liquid-crystal molecules to give rise to a twist in the right direction, the substance that is represented by the following chemical structure is used as an optically active substance: To cause the liquid-crystal molecules to give rise to a twist in the left direction, cholesteryl nonanoate (Merck), S-811 (Merck), etc., are used as optically active substances. The angles ϑ₁ and ϑ₂ of twist of the liquid-crystal molecules of the liquid-crystal layers in the first and second cell layers are set to be in the optimum range. The values Δn₁·d₁ and Δn₂·d₂ (Δn₁ and Δn₂ being the birefringence of the liquid crystals in the first and second cell layers, respectively, and d₁ and d₂ being the thicknesses of the liquid-crystal layers in the first and second cell layers, respectively) of the liquid-crystal layers in the first and second cell layers are also set to be in the optimum range. The above-mentioned optimum range for the angles ϑ₁ and ϑ₂ of twist of the liquid-crystal molecules and the values of Δn₁·d₁ and Δn₂·d₂ are set considering the following three requirements: (1) Figure 3 shows the relationship between the values of Δn₁·d₁ and Δn₂·d₂ and the transmittance in the case where ϑ₁ and ϑ₂ are set to be 90°, and polarizers are disposed in a crossed-Nicols manner and moreover, voltage is not applied. Figure 3 indicates that when the value of Δn₁·d₁ of the first cell layer is equal to that of Δn₂·d₂ of the second cell layer, the transmittance becomes the lowest and a high contrast ratio can be achieved. This phenomenon arises because the light dispersion in the first cell layer is compensated by the second cell layer. The results mentioned above can be obtained not only when the angles ϑ₁ and ϑ₂ of twist are both set to be 90°, but also when they are set to be any degree. Moreover, even if the specific pitch of the twist of the liquid-crystal molecules in the first cell layer is different from that of the twist of the liquid-crystal molecules in the second cell layer, when the twisted structure has a desired angle of twist of the liquid-crystal molecules, the same results as mentioned above can be obtained. In addition, by considering the display contrast and the visibility, as shown in Figure 4, the angle of twist of the liquid-crystal molecules is preferably set to be in the range of about 180° to about 360° based on the relationship between the angle of twist and the contrast ratio. If the angle of twist of the liquid-crystal molecules exceeds 360°, a domain appears in which the liquid crystals have their orientation disarrayed at the time of the application of voltage, resulting in a dispersion of light, which readily gives rise to a decrease in contrast. (2) To obtain sharp threshold characteristics of contrast, the specific pitch p of the twist of liquid-crystal molecules in one cell layer with a voltage-applying means becomes very important. The ratio of the thickness d of the liquid-crystal layer to the pitch p of the twist of the liquid-crystal molecules, d/p, is preferably set by use of experimental data as follows: ϑ/2π - 1/4 < d/p ≲ ϑ/2π, wherein ϑ is the angle of twist of liquid-crystal molecules. This requirement applies in the case where the pretilt angle of the liquid crystals is about 10° or less. The said requirement is used in a normally closed display process in which the application of voltage makes the liquid crystals white. (3) The third requirement mentioned below is used in a normally white display process. For example, when the product of the birefringence Δn₁ and the thickness d₁ of the liquid-crystal layer in the first cell layer, Δn₁·d₁, is 0.7 (i.e., Δn₁·d₁ = 0.7) and the product of the birefringence Δn₂ and the thickness d₂ of the liquid-crystal layer in the second cell layer, Δn₂·d₂, is 0.5 (i.e., Δn₂·d₂ = 0.5), and when the angle of twist of the liquid-crystal molecules in each of the first and second cell layers is 270°, the transmittance of light is about 70% at the time of the application of zero voltage. The transmittance drastically decreases, as shown in Figure 5, with the application of voltage to the first cell layer. This is because the application of voltage causes the liquid-crystal molecules in the first cell layer to be raised up, which apparently makes the value of Δn₁·d₁ of the liquid-crystal layer in the first cell layer small, so that the value of Δn₁·d₁ becomes equal to that of Δn₂·d₂ of the liquid crystal layer in the second cell layer. To attain such a phenomenon, the value of Δn₂·d₂ must be smaller than that of Δn₁·d₁. If the value of Δn₂·d₂ is exceedingly close to the value of Δn₁·d₁, as seen from Figure 3, the transmittance at the time of the application of zero voltage becomes exceedingly low. Thus, the value of Δn₂·d₂ must meet the following inequality: Δn₂·d₂ < 0.85 Δn₁·d₁ Moreover, the requirements with regard to the desired angle of twist of the liquid-crystal molecules and the desired ratio of d/p are the same as those of the angle of twist and the ratio of d/p in the case of the normally closed display process. Moreover, to obtain the state in which the liquid-crystal layers become white, the thickness of the liquid-crystal layers is preferably set to be about 10 µm or less in the case where 180° ≦ ϑ ≦ 360°. Example 1 Figure 6 shows a liquid-crystal display device (i.e., a double-layered-type SBE) of this invention, in which the angle of twist of the liquid-crystal molecules in each of the first and second cell layers C₁ and C₂ is 270° (that is, the liquid-crystal-molecules therein are supertwisted). A electrode 32 is disposed on each of the glass substrates 23 of the first cell layer C₁ alone. On the glass substrates 23 (over the electrodes 32 in the case of layer C₁) liquid-crystal-molecule orientation films 26 of polyimide are formed with a thickness of about 1000 Å by a spin coating technique, the surfaces of which are treated by being rubbed with cloth, causing the liquid-crystal molecules to be oriented so that the direction of longitudinal axis of the liquid-crystal molecules becomes parallel to the substrates. The end portions of the cell layers are sealed by sealing substance 24. As a liquid-crystal substance, a nematic liquid crystal, ZLI-3281 (Merck), is used. To the liquid-crystal layer 25 of the first cell layer C₁, 1.1 wt% of cholesteryl nonanoate is added and 0.94 wt% of CB15 (trade name of British Drug House) is added to the liquid-crystal layer 25 of the second cell layer C₂. The pitch of the twist of the liquid-crystal molecules in the first cell layer C₁ is opposite to that of the liquid-crystal molecules in the second cell layer C₂. The thickness of the liquid-crystal layer in each of the first and second cell layers C₁ and C₂ (i.e., the thickness of each of the cell layers C₁ and C₂) is around 5 µm. Polarizers 21 are placed in a crossed-Nicols manner as shown in Figure 1. The values of Δn₁.d₁ of the first cell layer C₁ and Δn₂.d₂ of the second cell layer C₂ are adjusted to 0.7 and 0.5, respectively, by changing the thicknesses of the cell layers. Figure 7b shows the dependence of the light transmittance on the applied voltage with regard to the double-layered-type SBE of this invention, and Figure 7a shows the same characteristics as mentioned above with regard to a reference standard single-layered-type SBE in which the angle of twist of the liquid-crystal molecules in the single cell layer is 270°. It can be seen from Figures 7a and 7b that the double-layered cell has sharp threshold characteristics that are peculiar to SBE cells and also that the change in the characteristics of the double-layered cell with regard to the wavelengths for red, green, and blue is much smaller than that in the characteristics of the single-layered cell. Thus, the double-layered cell can attain a color display image with high contrast. Example 2 Inside the liquid-crystal cell layer with a voltage-applying means of the display device in the above-mentioned example, color (red, green, and blue) filter layers of a gelatin film are disposed. The display devices with the color filter layers are subjected to a duty drive, with the formation of a distinct, clear color image. These liquid-crystal display devices are useful for full-color display and multi-color display. Example 3 The liquid-crystal cell of this invention is used in place of a liquid-crystal panel with color filters that is provided with a TFT (Thin Film Transistor) as an active device, and are examined for color display tests by an active matrix drive. They produce a distinct and clear color image, so they are useful for full-color display and multi-color display. It is not intended that the scope of the claims appended hereto be limited to the description as set forth above.

Claims: A liquid-crystal display device comprising:- a double-layered-type liquid-crystal cell that is composed of a first cell layer (C₁) and a second cell layer (C₂), each said cell layer (C₁, C₂) having first and second transparent substrates (1, 13, 23) and liquid-crystal molecules (4) with a twisted nematic orientation disposed between said first and second transparent substrates (1, 13, 23); and a voltage-applying means (2, 22, 32) provided in one of the first and second cell layers (C₁, C₂); wherein the angle of twist of the liquid-crystal molecules (4) in the first cell layer (C₁) is opposite to the angle of twist of the liquid-crystal molecules in the second cell layer (C₂); and wherein the longitudinal direction of liquid-crystal molecules (4) in the first cell layer (C₁) directly adjacent to a region separating the first and second cell layers (C₁, C₂) are perpendicular to the longitudinal direction of liquid-crystal molecules in the second cell layer (C₂) directly adjacent to the region separating the first and second cell layers (C₁, C₂); said liquid-crystal display device being characterised in that the angle of twist of the liquid-crystal molecules in the first cell layer (C₁) and the angle of twist of the liquid-crystal molecules (4) in the second cell layer (C₂) are in the range 180° to 360°, and the angles of twist of the liquid-crystal molecules (4) in the first and second cell layers (C₁, C₂) are nearly equal to each other, and the product Δn₁.d₁ of the birefringence Δn₁ and the thickness d₁ of the liquid-crystal layer in the one of the first and second cell layers (C₁) in which the voltage means (2, 22, 32) is provided and the product Δn₂.d₂ of the birefringence Δn₂ and the thickness d₂ of the liquid-crystal layer in an other cell layer (C₂) in which the voltage means (2, 22, 32) is not provided are represented by the following inequality: Δn₂.d₂ < 0.85 Δn₁.d₁. A liquid-crystal display device according to claim 1 wherein the relationship between the pitch p of twist of the liquid-crystal molecules (4) in the cell layer (C₁) that has the voltage-applying means therein and the thickness d₁ of the liquid crystal layer in the cell layer (C₁) that has the voltage-applying means therein is as follows: ϑ/2π - 1/4 < d₁/p < ϑ/2π wherein ϑ is the angle of twist of the liquid-crystal molecules (4). A liquid-crystal display device according to claim 1 or 2 wherein a colour filter layer is disposed in at least one of the first and second cell layers (C₁, C₂). A liquid-crystal display device according to any preceding claim wherein an active device is disposed on each picture element in at least one of the first and second cell layers (C₁, C₂).

Inventors: ISHII YUTAKA, KIMURA NAOFUMI, KOZAKI SHUICHI, ISHII, YUTAKA, KIMURA, NAOFUMI, KOZAKI, SHUICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G09G3