Liquid crystal display device

Abstract: There is provided a liquid crystal display device comprising: a double-layered-type liquid crystal cell which is composed of a first cell layer and a second cell layer, said liquid crystal cell containing liquid crystal molecules with a twisted nematic orientation therein; and a voltage applying means in said first cell layer; wherein the dependency of the birefringence on the wavelength of light of said first cell layer is larger than that of said second cell layer. Accordingly, the liquid crystal display device produces a distinct and clear color image, so it is useful for full-color display and multicolor display.

Description: This invention relates to a multi-layered-type liquid crystal display device which uses a twisted nematic display process that can attain an excellent color display. Liquid crystal display devices are now being used in clocks and electronic calculating machines, in displays of computers terminal and word processor, in televisions, and in 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. A form of color display that has been widely put into practical use is 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 liquid crystal twist at 90° can be obtained. (The "twisted nematic" is referred to hereinafter as "TN".) However, when the TN display device is driven at a high duty ratio, the contrast of the image decreases. On the other hand, there has been suggested a supertwisted birefringence effect (which is referred to hereinafter as "SBE") process that uses TN oriented liquid crystal that is twisted at angles of approximately 180° to 270° larger than 90°. With the SBE process, the curve in the vicinity of the threshold value increases steeply, 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 of light is theoretically higher than with the TN display in which the liquid crystal is twisted at 90°, so it is very difficult to adapt it for use in a full-color display. The present invention aims to provide a liquid crystal display device that produces a colored display image with superior color reproducibility and high contrast ratio even when the liquid crystal display device is driven at a high duty ratio. According to the present invention, there is provided a liquid crystal display device comprising; a double-layered-type liquid crystal cell which is composed of a first cell layer and a second cell layer, said first and second cell layers each containing liquid crystal molecules with a twisted nematic orientation therein; and voltage applying means for applying a predetermined voltage to said first cell layer, wherein the direction of the twist angle of the liquid crystal in said first cell layer is opposite to that of the liquid crystal in said second cell layer; the absolute values of the twist angles of the liquid crystal molecules in the first and second cell layers are substantially equal to each other; and the orientation of the liquid crystal molecules in said first cell layer in the vicinity of said second cell layer is orthogonal to that of the liquid crystal molecules in said second cell layer in the vicinity of said first cell layer, said device being characterized in that the anisotropy of the indices of refraction of the liquid crystal material of said first cell layer is larger than that of the liquid crystal material of said second cell layer for all wavelengths of light, and the twist angle of the liquid crystal in each of said first and second cell layers is in the range from 180° to 360°. The liquid crystal display device according to the present invention, with the use of the TN liquid crystal layers in which the liquid crystal is twisted at an angle equal to or larger than 180°, produces a distinct and clear color image, so it is useful for full-color display and multicolor display. GB-A-2 092 769 discloses a liquid crystal display cell having two mutually superposed layers of twisted nematic liquid crystal. A polariser polarises light travelling through the cell, and the display state is controlled by applying a voltage across one of the layers. The other layer acts as an optical compensator to reduce the interference coloring phenomenon which tends to occur during de-energization. The content of earlier European application EP-A- 0 284 372, being relevant pursuant to Articles 54(3) and (4) EPC with regard to novelty, discloses a liquid crystal display device comprising a double-layered-type liquid crystal cell. The relation of the values of the anisotropy of the indices of refraction of the liquid crystal of the first and second cell layer with respect to each other is not discussed. An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a sectional view showing the fundamental cell structure of a double-layered-type liquid crystal display device of the preferred embodiment according to the present invention, Figs. 2a and 2b are diagrams showing the twists of liquid crystal molecules in the right and left directions, respectively, Fig. 3 is a graph of characteristic curves showing the relationship between the Δn₂·d₂ of the second cell and the transmittance when a polarizer is of a crossed-Nicols type with regard to the liquid crystal display device of the preferred embodiment according to the present invention, Fig. 4 is a graph of a characteristic curve showing the relationship between the twist angle of liquid crystal and the contrast ratio of display images, Fig. 5 is a graph of a characteristic curve showing the relationship between the applied voltage and the transmittance with regard to a double-layered-type SBE liquid crystal device, Fig. 6 is a sectional view showing the fundamental cell structure of a double-layered-type SBE liquid crystal display device of the preferred embodiment according to the present invention, Figs. 7a and 7b are graphs of characteristic curves showing the dependency of the birefringence Δn on the wavelength of light of the liquid crystals used in the liquid crystal device shown in Fig. 6, Fig. 8a is a graph of characteristic curves showing the relationship between the applied voltage and the light transmittance with regard to a double-layered-type liquid crystal cell comprising the liquid crystals EX-02 disposed in the first and second cell layers, and Fig. 8b is a graph of characteristic curves showing the relationship between the applied voltage and the light transmittance with regard to a double-layered-type liquid crystal cell comprising the liquid crystal EX-02 disposed in the first cell layer and the liquid crystal ZLI-3021-000 disposed in the second cell layer. Fig. 1 shows the fundamental structure of the double-layered-type liquid crystal display device of the preferred embodiment according to the present invention. As shown in Fig. 1, the liquid crystal display device has a double layered structure composed of a first and a second cell layers C₁ and C₂ each of which contains a liquid crystal layer 4 therein. In the liquid crystal layer 4, liquid crystal molecules are arranged in a twisted nematic phase. The first and second cell layers C₁ and C₂ are formed stacked on both surfaces of a common transparent substrate 1. Each cell layer has such a structure that the liquid crystal layer 4 is contained between two transparent substrates 1 and 1′ and is sealed by a sealing element 6 and orientation membranes 3 for orienting liquid crystal molecules in a twisted nematic phase are formed on respective inner surfaces of the substrates 1 and 1′. The first cell layer C₁ has transparent electrode films 2 for applying a voltage to the liquid crystal layer which are formed on respective inner surfaces of the substrates 1 and 1′. Further, each cell layer has a polarizer 5 formed on the outer surface of each of the outer substrate 1′. Each of the transparent substances 1 and 1′ may be made of glass, acrylic resin, or the like, the transparent conductive film 2 may be made of ITO (which is a film mainly made of indium oxide), a nesa film, or the like, and the orientation film 3 may be made of an inorganic material such as SiO₂, SiO, or the like or an organic material such as polyimide, polyvinyl alcohol, nylon, acrylic resin, or the like. In the cell structure shown in Fig. 1, axes of respective liquid crystal molecules in the liquid crystal layer 4 are twisted helically from the side of one substrate to the side of another substrate of each cell layer, as shown schematically in Figs. 2a or 2b. As is well known to those skilled in the art, the direction of twist is defined either that to the right (Fig. 2a) or that to the left (Fig. 2b) when seen in the direction of incident light. In order to give a twisting power to individual liquid crystal molecules, at least one optically active material is added in the nematic liquid crystal. In the case of the right twist, the material represented by the following general formula and offered by MERCK & Co., Inc. is added thereinto. In the case of the left twist, cholesteryl nonanoate S-811® offered by the same company is added thereinto. Values ϑ₁ and ϑ₂ of twist angles of the first and second liquid crystal layers 4 and 4 in the first and second cell layers and values Δn₁·d₁ and Δn₂·d₂ of those each of which is given by a product of the birefringence Δn of the liquid crystal layer and the thickness d thereof are adjusted so as to satisfy the following four requirements. (1) Fig. 3 shows the relationship between the values of Δn₁·d₁ and Δn₂·d₂ and the transmittance under conditions that both of ϑ₁ and ϑ₂ are set equal to 90°, respectively, and the polarizers 5 are of a crossed-Nicols type and moreover, a voltage is not applied to the first cell layer C₁. Fig. 3 indicates that when the value of Δn₂·d₂ of the second cell layer C₂ is equal to that of Δn₁·d₁ of the first cell layer C₁, the transmittance becomes the lowest and therefore, a high contrast ratio can be obtained. This phenomenon arises because the light dispersion in the first cell layer C₁ is compensated by the second cell layer C₂. The aforementioned results can be obtained not only when the twist angles ϑ₁ and ϑ₂ are set to be 90°, but also when they are set to be any degree, so long as both of them are set equal to each other. Moreover, the same results as mentioned above can be obtained, even if the specific pitch of the twist of the liquid crystal molecules in the first cell layer C₁ is different from that of the twist of the liquid crystal molecules in the second cell layer C₂, so long as the twisted structure of each liquid crystal layer has a desired twist angle with respect to the liquid crystal molecules contained therein. That is, the second cell layer C₂ functions as a compensation cell layer (a compensator) for the first cell layer C₁. This is the first requirement upon setting those values. In addition, taking the display contrast and visibility into considerations the relationship between which is shown in Fig. 4, the twist angle of the liquid crystal is set so as to fall in the range of about 180° to about 360° in which the contrast ratio becomes relatively high. If the twist angle of the liquid crystal exceeds 360°, a domain appears in which each liquid crystal has its orientation disarrayed at the time of the application of voltage, resulting in a dispersion of light, which readily causes a decrease in the contrast. (2) To obtain a steep threshold characteristic in the contrast, the specific helical pitch p of liquid crystal is considered to be very important. The ratio of the thickness d of the liquid crystal layer 4 to the pitch p of the helical twist of the liquid crystal, d/p, is preferably set so as to satisfy the following relation which is confirmed in the experiment performed by the inventors of the present invention: ϑ/2π - 1/4 < d/p ≾ ϑ/2π wherein ϑ is the twist angle of the liquid crystal. This is the second requirement. This requirement can be applied only to the case where the pretilt angle of the liquid crystal is about 10° or less. In the case that the pretilt angle of the liquid crystals is larger than 10°, the range to be satisfied by the ratio d/p becomes different from the aforementioned range. The aforementioned requirement is available to a normally closed displaying system in which the liquid crystal becomes white when a voltage is applied thereto. (3) The third requirement is also available to a normally white display system. For example, when the values Δn₁·d₁ and Δn₂·d₂ in the first and second liquid crystal layers C₁ and C₂ are set at 0.7 and 0.5, respectively, and when the twist angle of each of the first and second cell layers C₁ and C₂ 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 Fig. 5, with the application of voltage to the first cell layer C₁. This is because the application of voltage causes axes of liquid crystal molecules in the first cell layer C₁ to rise upward in the direction of thickness, which apparently contributes to make the value of Δn₁·d₁ of the liquid crystal layer 4 in the first cell layer C₁ smaller, so that the value Δn₁·d₁ becomes equal to Δn₂·d₂ of the liquid crystal layer 4 in the second cell layer C₂. To attain such a phenomenon, the value of Δn₂·d₂ must be smaller than that of Δn₁·d₁ . If the value Δn₂·d₂ becomes too close to the value Δn₁·d₁ , the transmittance at the time of the application of zero voltage becomes extremely low, as is shown clearly in Fig. 3. According to the experiential consideration, it is confirmed that the value Δn₂·d₂ should satisfy the following inequality: Δn₂·d₂ ≦ 0.85 Δn₁·d₁ Moreover, it is confirmed experientially that the requirements for the desired twist angle of the liquid crystal and the desired ratio of d/p thereof should be roughly same as those in the case of the normally closed display system. (4) The fourth requirement is used for making the dependency of the display characteristics on the wavelength of light small. To satisfy the fourth requirement, the liquid crystal in the first cell layer C₁ is set so that the dependency of the birefringence Δn₁ thereof on the wavelength of light is larger than that of the liquid crystal in the second cell layer C₂. Stated differently, the anisotropy of the indices of refraction of the liquid crystal material of the first cell layer C₁ is larger than that of the liquid crystal material of the second cell layer C₂ for all wavelengths of light. Moreover, by considering the speed of response, the thickness of the liquid crystal layer 4 is preferably set equal to or less than about 10 µm or less in the case where 180° ≦ Θ ≦ 360°. Example 1 Fig. 6 shows the double-layered-type cell structure of a liquid crystal display device of the preferred embodiment according to the present invention, in which the twist angle of the liquid crystal in each of the first and second cell layers C₁ and C₂ is 240° (i.e., the device is an SBE liquid crystal device). A transparent conductive film 8 for applying a display driving voltage to a liquid crystal layer 11 is formed on each of glass substrates 9 of the first cell layer C₁ alone by vapor deposition of ITO. On each of the transparent conductive films 8, a liquid crystal molecule orientation film 12 of polyimide is formed with a thickness of about 500 Å by a spin coating technique, the surface of which is treated by rubbing with cloth, causing the liquid crystal molecules to be in a parallel orientation. The periphery of each of the first and second cell layers C₁ and C₂ is sealed by a sealing element 10. Fig. 7a shows respective dependencies of the birefringence Δn on the wavelength of light of a nematic liquid crystal EX-02 made by Dainippon ink & chemicals, Inc. and a nematic liquid crystal ZLI-3021-000 (trade name) made by Merck & Co., Inc., which are used in the preferred embodiment. As shown in Fig. 7a, it is seen that the dependence of the birefringence Δn on the wavelength of light of the nematic liquid crystal EX-02 is larger than that of the nematic liquid crystal ZLI-3021-000. Therefore, the nematic liquid crystal EX-02 to which 0.8 wt% of an optically active material S-811 is added is used in the liquid crystal layer 11 of the first cell layer C₁, and the nematic liquid crystal ZLI-3021-000 to which 1.2 wt% of CB-15 is added is used in the liquid crystal layer 11′ of the second cell layer C₂. The pitch of the liquid crystal layer 11 in the first cell layer C₁ is about 10 µm and the pitch of the liquid crystal layer 11′ in the second cell layer C₂ is about 15 µm. The twist angle of the liquid crystal in the first cell layer C₁ is opposite to that of liquid crystal in the second cell layer C₂. The thickness of the liquid crystal layer 11 in the first cell layer C₁ is about 6 µm and that of the liquid crystal layer 11′ in the second cell layer C₂ is about 9 µm. A crossed-Nicols type polarizer is used for each of polarizers 7. Fig. 8b shows the dependence of the light transmittance on the applied voltage with regard to the double-layered-type TN (twisted nematic) liquid crystal device of the present example, comprising the liquid crystal layer 11 of the nematic liquid crystal EX-02 and the liquid crystal layer 11′ of the nematic liquid crystal ZLI-3021-000, and Fig. 8a shows the dependence of the light transmittance on the applied voltage with regard to the double-layered-type TN liquid crystal device of a comparative example, comprising the liquid crystal layers 11 and 11′ of the nematic liquid crystal EX-02, wherein the wavelengths λ used herein for red color, green color, and blue color are 610 nm, 550 nm, and 450 nm, respectively. As shown in Fig. 8b, since the characteristic curves for red color, green color and blue color of the TN liquid crystal device of the present example show substantially uniform characteristics in the range from the threshold voltage to a higher voltage, the TN liquid crystal device of the present example is useful for a color display with high contrast. Example 2 An experiment with regard to a double-layered-type TN liquid crystal device comprising the liquid crystal layer 11 of a nematic liquid crystal SP4024 made by Chisso and the liquid crystal layer 11′ of the nematic liquid crystal ZLI-3021-000 was made as well as the experiment of the Example 1, wherein the dependence of the birefringence Δn on the wavelength of light of the nematic liquid crystal SP4024 is larger than that of the nematic liquid crystal ZLI-3021-000, as shown in Fig. 7b. It is seen from the result of the experiment of the Example 2 that the TN liquid crystal device used in the Example 2 has the same effect as that of the TN liquid crystal device of the preferred embodiment used in the Example 1. In the aforementioned examples, color filter layers of red, green and blue are disposed inside of the liquid crystal cell layer with a voltage applying means of each of the TN liquid crystal display devices. The liquid crystal 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 multicolor display. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of the present invention.

Claims: A liquid crystal display device comprising; a double-layered-type liquid crystal cell which is composed of a first cell layer (C₁) and a second cell layer (C₂), said first and second cell layers each containing liquid crystal molecules with a twisted nematic orientation therein; and voltage applying means (2; 18) for applying a predetermined voltage to said first cell layer (C₁), wherein the direction of the twist angle of the liquid crystal in said first cell layer (C₁) is opposite to that of the liquid crystal in said second cell layer (C₂); the absolute values of the twist angles of the liquid crystal molecules in the first and second cell layers (C₁, C₂) are substantially equal to each other; and the orientation of the liquid crystal molecules in said first cell layer (C₁) in the vicinity of said second cell layer (C₂) is orthogonal to that of the liquid crystal molecules in said second cell layer (C₂) in the vicinity of said first cell layer (C₁), said device being characterized in that the anisotropy of the indices of refraction of the liquid crystal material of said first cell layer (C₁) is larger than that of the liquid crystal material of said second cell layer (C₂) for all wavelengths of light, and the twist angle of the liquid crystal in each of said first and second cell layers (C₁, C₂) is in the range from 180° to 360°. A liquid crystal display device as claimed in claim 1, wherein the product Δn₁·d₁ of the birefringence ▵n₁ and the thickness d₁ of the liquid crystal layer (4) in one of said first and second cell layers (C₁, C₂) that undergoes an optical change because of an external force and the product Δn₂·d₂ of the birefringence Δn₂ and the thickness d₂ of the liquid crystal layer (4) in the other cell layers (C₁, C₂) that does not undergo an optical change are represented by the following inequality: Δn₂·d₂ ≦ 0.85 Δn₁·d₁. A liquid crystal display device as claimed in claim 1 or claim 2, wherein the relationship between the pitch p of twist of the liquid crystal molecules in said first cell layer (C₁) and the thickness d of the liquid crystal layer (4) in the said first cell layer (C₁) is as follows: Θ/2π - 1/4 < d/p ≦ Θ/2π, where Θ is the twist angle of the liquid crystal. A liquid crystal display device as claimed in any preceding claim, wherein a color filter layer is disposed in at least one of said first and second cell layers.

Inventors: HAYASHI MARIKO, ICHIMURA YUKIKO KASUGA-RYO, KIMURA NAOFUMI, HAYASHI, MARIKO, ICHIMURA, YUKIKO, KASUGA-RYO,, KIMURA, NAOFUMI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G02F1
IPC codes: G02F1