LIQUID CRYSTAL DISPLAY DEVICE

Abstract: A liquid crystal display device comprises a liquid crystal cell in which a pair of substrates (1, 1′) having electrodes (2, 3) are opposed to each other with the surfaces having elec­ trodes located inside, a ferroelectric liquid crystal (7) which is located between said substrates (1, 1′) and which exhibits a chiral smectic C-phase, and a polarizer plate (8) on the side of incident light, one of said electrodes serving as a light-­ reflecting electrode (2). Since the number of parts is small, light absorption loss is small. Further, since the reflecting plate is kept off the atmospheric air, the reflecting plate is not oxidized and the coefficient of reflection is prevented from decreasing. The display device can be used for micro­ computers, word processors, and TV's.

Description: The invention relates to a liquid crystal display device as indicated in the precharacterizing part of claim 1. In solid state display panels, the system which controls each picture element independently is effective for displays with large area. As such a panel there has been known in the past a display in which the multiplexing drive system is applied to a simple matrix structure of the A4 size with 400 elements in the row and 200 elements in the column direction that is formed by the use of a two-frequency liquid crystal,for instance, a twistic nematic liquid crystal (referred to as TN liquid crystal hereinafter). However, to construct a reflection type liquid crystal display by the use of such a TN liquid crystal, two-polarizing plants have to be arranged on the outside of a pair of substrates, rotated mutually by 90°. Moreover, a reflecting plate has to be arranged on the outside (namely, on the farthest side with respect to the incident light) of the polarizing plate. Namely, the light path is as follows: The incident light goes through a first polarizing plate, an opposing glass, an opposing transparent conducting film, TN liquid crystals, a transparent conducting film, a glass substrate, a second polarizing plate, and an aluminium reflecting substrate. Further, light reflected there proceeds backward to the incidence side through the second polarizing plate. However, use of such TN crystals has many drawbacks such as those listed below. (1) The amount of reflected light is reduced due to the use of two polarizing plates. (2) There is observed a loss of light due to absorption of light by the glass substrates. (3) The reflecting plate becomes cloudy due to oxidation of aluminum by the atmosphere, which induces a reduction in the reflectivity. (4) When active elements are provided on the inside of the substrate, there is generated an absorption loss caused by the organic resin that exists in their surroundings. The precharacterizing part of new claim 1 considers the prior art as known from EP-A-142 326. This prior art liquid crystal display device comprises a transparent substrate, a transparent electrode forms thereon and a reflective metal electrode formed on a semiconductor substrate. A polarizing plate is mounted on the outer surface of the transparent substrate. In the space between the two electrodes, there is provided a quantity of ferroelectric liquid crystal material having smectic C phase. EP-A-092 181 discloses a liquid crystal element including a ferroelectric liquid crystal sandwiched between a pair of substrates having electrodes on the opposite surfaces. A polarizing plate is mounted on the outer surface of the upper substrate. On the outer surface of the lower substrate, a further polarizing plate or, instead of this, a reflector may be placed adjacent to this substrate. It is the object of the invention to improve a liquid crystal display device as indicated in the precharacterizing part of claim 1 such that loss of light can be minimized by improving the reflectivity of the reflective electrode. This object is solved by the features as claimed in the characterizing part of claim 1. The ferroelectric liquid crystals display the chiral smectic C phase (SmC*). Namely, a bistable state can be obtained by setting the separation between the cells to be 4 microns or less. By mixing liquid crystal material in its isotropic liquid crystal state ( high temperature state ) in such a thin cell, and by lowering the temperature, SmA and then bistable SmC* are obtained. By so doing, it is possible to loosen the spiral structure. When a voltage is applied to SmC*, molecules are aligned in one direction with an angle of about 45°. By the application of a voltage with opposite sign it is possible to obtain an angle of about-45° (within 45°±5°) in the opposite direction. These two states are nonvolatile in the sense that they hardly change even after the voltage is shut off, retaining an angle of about 90°(within ± 10°). With this observation, it became clear that transmission and nontransmission of light can be accomplished by the use of a single polarizing plate. Because of this, the present invention comprises a pair of substrates (one on the light incidence side is called the opposing substrate and the other one on the other side is called simply the substrate that form a cell), electrodes ( one on the light incidence side is called the opposing electrode and the other one on the inner side is called simply the electrode ) that are attached to the inside of the substrates, and FLC that is sealed between the electrodes. Further a polarizing plate is arranged on the incident side. In particular, in the present invention, a layer which refects light in the visible region was constructed by one of the electrodes, namely, the electrode on the substrate. In that case, the incident light passes through the polarizing plate, the opposing substrate, the opposing electrode, FLC, and the electrode where it is reflected, and proceeds backward on the reverse path. It should be mentioned that one of the unsolved problems namely, the range of the temperature for use, was found satisfactory currently, if if is in the range from 0 to 50°C for a combination ( blending ) of a plurality of different kinds of FLC. Therefore, the temprerature range was considered to be not much of a problem in the practical applications. In addition, as to the tone, it was found unnecessary provided that the number of colors employed is limited up to eight, so that it became clear that the present invention is usable satisfactorily as the display for microcomputers or the like. With the above arrangement, the following advantages can be realized. Namely: (1) Loss of light can be minimized by the use of only one polarizing plate. (2) The electrode for reflecting light is not exposed to the air so that it will not be oxidized and hence the reflectivity of light can be maintained at a high value. (3) The number of required parts can be reduced. (4) Because of the use of FLC, the field of vision can be increased, and in addition, energy can be saved. (5) There occur absolutely no absorption loss and reduction in aperture factor due to the thickness of the nonlinear elements (active elements ). Further, when forming each picture element by connecting a nonliniar element (NE) and ferroelectric liquid crystal (FLC) in series, it was accomplished for the first time to realize a matrix system for a large area equal to or greater than that of the A4 size, by removing the cross talks between the picture elements. In addition, the present invention has a feature that it became possible to realize a color display by providing red ( called R ), green ( called G ), and blue ( called B ) filters corresponding to the other X-axis arrangement of the ferroelectric liquid crystal. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a plan view and a longitudinal section view of passive liquid display panel of the present invention. Figure 2 shows the operation of the ferroelectric liquid crystal used in the present invention. In Figure 1(B) are shown a plan view and a longitudinal sectional view along A-A' of a passive liquid crystal display. As may be clear from the figure, the display has one electrode 2 which consists of linear patterns in the Y direction 2-1, 2-2, ...., and 2-n and a second electrode 3 which consists of linear patterns in the X direction 3-1, 3-2, ...., and 3-m, and their intersection constitutes a dot 9 (for example, the intersection of 2-2 and 3-1). Directly beneath the linear electrode pattern 2, there may be arranged nonlinear elements if need arises. Each of these linear patterns are provided on a first substrate 1 and a second substrate 1'. In addition, those linear patterns have external connection terminals 11 and 11', respectively, and they can be linked to the peripheral circuits by means of these regions. Each electrode has its periphery sealed with a sealing material 23, and its interior is filled with liquid crystals 7 with smectic phase. Further, on the inside of the first substrate 1 there is provided the first electrode 2 as a reflecting layer which is prepared in practice by vacuum deposition of aluminum. On the top surface of the other substrate 1', there is provided a polarizing plate 8. The figure illustrates the configuration for color display. In the present case, filters 4 are provided closely each other on the inner side of the substrate 1', arranged so as to repeat R(red), G(green), and B(blue) alternately. A protective covering film 5 (use was made of a polyimide-based film in this case) for the filters 4 was provided on top of them, with its surface 5' formed flat. Further, making a close contact with the top surface (bottom surface in the drawing) of the protective film, there are provided the linear transparent electrodes 3 in the X-axis direction. These electrodes are composed on conductive transparent film (CTF) and are formed specifically of indium-tin oxide (ITO) by sputtering. On the inner surfaces of the pair of electrodes 2 and 3, there are formed asymmetric orientation layers. A nonrubbed layer (layer to which the rubbing treatment was not given) which is one of the asymmetric orientation layers is formed by coating 1% methanol solution of 1.1.1. trimethylsilazane by monomolecular coating method and drying. A rubbed layer which is the other of the asymmetric orientation layers is formed by coating and drying 1.1.1. trimethylsilazane on the CTF layer as in the above, and then by coating on top of it, by spinning method, 1% b-nylon solution obtained by diluting nylon in a solution of 60% m-cresol and 40% methanol. Instead of b-nylon, PAN(polyacrylnitrile), and PVA(polyvinyl alcohol) are coated to a thickness of about 0.1 µm by spinning method, and rubbing treatment gas given according to the known method. As an example of rubbing treatment, nylon was rotated at 900 RPM in the rubbing apparatus, and the surface was formed by moving the substrate at a speed of 2m/min. Namely, on one of the electrodes 2 there was formed a unrubbed film to serve as an orientation film with no rubbing, and on the other electrodes 3 there was formed a film using the organic compounds mentioned above and a rubbing treatment was given to it. The unrubbed film may also be used as the inner surface of CTF3. Then, FLC E such as S8 (octyloxybenzylideneaminomethylbutyl benzoate ) was filled in between the orientation treated layers. Besides S8, FLC such as BOBAMBC or FLC's obtained by blending a pluralty of FLC's may also be used for filling. Some of these FLC's may be found in "Ferroelectrics Switing in the Tilted Smectic Phase of R-C-3-4-n-Hexyloxybenzylidence-4'-Amino(2-chloropropyl) Cinnamate (HOBACPC)", Ferroelectrics, 1984, Vol.59,pp.126-136 by J.W. Goodby et al., JP-A-59-98051 and JP-A-59-11874. Examples of the threshold characteristics of the picture element are shown in Fig.2(A) and Fig.2(B). Depending upon the way in which the alignment direction of the liquid crystals and the direction of the voltage are chosen, one or the other of these figures will be obtained. However, they are substantially equivalent graphs. From the figures it will be seen that curves 13 and 13' are obtained by the application of voltages of ±5V, transmission and nontransmission of light can be realized, and hysteresis with sufficient inversion and memory effect can be obtained. In Fig.2 the ordinate represents the transmissivity. In Fig.1, for light in the visible region to be reflected, incident light 10 has to pass through the polarizing plate 8, the second transparent substrate 1', the filter 4, the protective film 5, the transparent conductive film 6, the asymmetrically orientation treated layer (not shown) smectic liquid crystals 7, and the reflecting electrode 2 where it is reflected to follow the reversed light path as the reflected light 10'. In this case, if there exists some kind of color as a meduim in the filters 4 and the protective film 5 and it is desired to have an overall constant hue, the top surface of the reflecting electrode 2 may be varied with variable thickness in the range of 300 to 1500Å to form a CTF as an interference film of light. As may be clear from the figures, if use is made of a liquid crystal with smectic phase, especially SmC* layer, then the liquid crystal functions as memory, and "on" and "off" of light can be controlled by the use of just one polarizing plate, provided that the tilt angle of the smectic layer is chosen to be about 45°. As a result, there will be no need for providing a reflecting plate on the outside of the first substrate using TN liquid crystal in a manner well known in the past. It only needs to use one of the electrodes as the reflecting electrode. More important, the liquid crystal material has a thickness which is less than just 4microns (generally 1 to 2 µm). For this reason, the incident light 10 and the reflected light 10' pass through an identical filter once for each. On the contrary, in the case of arranging a reflecting plate on the outside of the rear substrate (bottom side of the first substrate in the figure), as is done in the prior case of using TN liquid crystal, an incident light that passed through a filter which may be red, for instance, could pass through an adjacent green filter as the reflected light. If it happens, the reflected light when it comes out will have a color which is neither red nor green, and as a result of their mixing it will have a whitish hue. In the case of the present invention, however, it is a significant feature that there will be no mixing with a color of the neighboring dot, because the distance between the filter 4 and the reflecting electrode 2 is less than 10 µm In Fig.1 for the present invention, there is shown a 6X6 segment of a matrix, but it represents only a portion of the 100X100 matrix that was actually used in the experiment. However, the dot numbers may be given other configurations such as 640X400 (in the case of color it will be 1,920X400), 720X400, and so on. It should be noted that varions other forms of the present invention can be considered. Thus, for example, a dichroic material may be added to apparently improve the anisotropy of light absorption by SmC*. As such substances, one may mention, for example, azo-based or anthraquinone-based dyes (with dichroic ratio greather than 8) which can be dissolved in liquid crystals.

Claims: A liquid crystal display device comprising: a pair of substrates (1,1') at least one (1') of which is transparent; a transparent electrode formed on an inside surface of the transparent substrate; a polarizing plate (8); a reflective electrode (2) formed on an inside surface of the other one (1) of said substrates (1,1') and comprising a reflective metal layer; and a ferroelectric liquid crystal material (7) having smectic C phase interposed between said substrates (1, 1'), characterized in that said reflective electrode (2) further comprises a conductive transparent film which is formed on the surface of said metal layer facing the liquid crystal material (7). The device of claim 1 wherein said reflective metal layer is an aluminum layer. The device of claim 1 or 2 wherein said conductive transparent film is 30 - 150 µm thick. The device of any one of claims 1 to 3, wherein said ferroelectric liquid crystal material (7) has a tilt angle 45 o . The device of any one of claims 1 to 4, wherein said polarizing plate (8) is placed adjacent to the transparent substrate (1'). The device of any one of claims 1 to 5, wherein a dichroic material is added to said ferroelectric liquid crystal layer (7). The device of claim 6, wherein said dichroic material is a dye with the dichroic ratio (CR) greater than 8. The device of claim 7, wherein said dye is an anthraquinone-based or azo-based dye.

Inventors: HAMATANI TOSHIJI, INUJIMA TAKASHI, KONUMA TOSHIMITSU, KOYANAGI KAORU, MASE AKIRA, MIYAZAKI MINORU, SAKAMA MITSUNORI, YAMAGUCHI TOSHIHARU, YAMAZAKI SHUNPEI, HAMATANI, TOSHIJI, INUJIMA, TAKASHI, KONUMA, TOSHIMITSU, KOYANAGI, KAORU, MASE, AKIRA, MIYAZAKI, MINORU, SAKAMA, MITSUNORI, YAMAGUCHI, TOSHIHARU, YAMAZAKI, SHUNPEI
Applicants: SEMICONDUCTOR ENERGY LAB, SEL SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1986
Main classification: G02F1
IPC codes: G02F1