Abstract:
A reflective liquid crystal display includes upper and lower substrates that are opposite to and are spaced apart from each other; a liquid crystal layer interposed between the upper and lower substrates; a transparent common electrode on the surface of the upper substrate opposite the lower substrate; a cholesteric liquid crystal (CLC) color filter that selectively reflects and transmits light, the CLC color filter formed over the lower substrate; a transparent pixel electrode on the CLC color filter; and a light absorption layer between the lower substrate and the lower substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Patent Application No. 2000-62803, filed on Oct. 25, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.  
         BACKGROUND OF THE RELATED ART  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a liquid crystal display (LCD) device, and more particularly to a reflective LCD device including a cholesteric liquid crystal (CLC) color filter.  
           [0004]    2. Description of Related Art  
           [0005]    In general, a liquid crystal display (LCD) device employing a thin film transistor (TFT) as a switching element is typically called a thin film transistor-liquid crystal display (a TFT-LCD) device. The TFT-LCD has a great advantage in displaying colored images.  
           [0006]    The TFT-LCD is generally comprised of upper and lower substrates and an interposed liquid crystal layer therebetween. The upper and lower substrates are respectively referred to as a color filter substrate and a TFT array substrate. Further, as a light source, the TFT-LCD also includes a backlight device under the lower substrate such that the light from this backlight device passes through the upper and lower substrates and is used for displaying images. However, only about 7% of the light generated from the backlight device pass through the pair of substrates. For this reason, the TFT-LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device.  
           [0007]    To solve these problems, a reflective LCD device has been researched and developed. Since the reflective LCD device is operated using ambient light other than an internal light source, such as a backlight device, battery life can be increased resulting in longer use times. Namely, only the drive circuitry that drives the liquid crystal uses the battery power in the reflective TFT-LCD device. Therefore, it is adopted for such application as the notebook computer and PDA (Personal Digital Assistant).  
           [0008]    For the reflective LCD device, a reflector or/and a reflective electrode is arranged in a pixel region where the transparent electrode would be formed in a transmissive LCD device. In other words, the reflective LCD device is driven using the light reflected from the reflective electrode or/and the reflector. However, the reflective LCD device is low in brightness due to the fact that the reflective LCD device uses the ambient light and the brightness depends on this ambient light from surroundings. One of the reasons for the low brightness is that the ambient light passes twice through the color filter. Due to the reflection on the reflector, the incident light from the outside passes the color filter and then is reflected from the reflector. Then, it is directed toward the color filter again and used for displaying the images. Therefore, most of the light is absorbed by the color filter, thereby decreasing the brightness.  
           [0009]    In order to overcome above-mentioned problem, it is essential to raise the transmittance of the color filter. Further, to get an excellent transmittance, the color filter ought to have low color purity. However, there is a limitation of lowering the color purity.  
           [0010]    Accordingly, for greater characteristics (such as brightness) of the reflective LCD device, a cholesteric liquid crystal (CLC) has been developed, which selectively transmits or reflects light while acting as a color filter. If the CLC color filter is used in the reflective LCD device, it is possible to omit the reflector from the reflective LCD device, thereby simplifying the manufacturing process. Furthermore, it has advantages of increasing color purity and contrast ratio.  
           [0011]    The CLC has a helical shape and the pitch of the CLC is controllable. Therefore, it is possible that the CLC color filter can selectively transmit and/or reflect light. In other words, as well known, all objects have their intrinsic wavelength, and the color that an observer recognizes is the wavelength of the light reflected from or transmitted through the object. The visible spectrum of light is about 400 nm to 700 nm. The visible light region can be broadly divided into red, green, and blue regions. The wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm. Due to the pitch of the liquid crystal, the CLC color filter can selectively transmit or reflect the light having the intrinsic wavelength of the color corresponding to each pixel thereby clearly displaying the colors of red (R), green (G) and blue (B) with high purity. In order to provide a precise color, a plurality of the CLC color filters can be arranged to display the color more clearly than the conventional color filter. Further, the CLC color filter selectively reflects or transmits the right- or left-handed circularly polarized light. Thus, it can transmit a large amount of light, compared to the conventional color filter.  
           [0012]    [0012]FIG. 1 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter. As shown, a reflective LCD device  50  includes upper and lower substrates  10  and  30  and an interposed liquid crystal layer  20  therebetween. The upper and lower substrates  10  and  30  are a transparent material such as glass. On the surface facing the lower substrate  30 , the upper substrate  10  includes a transparent common electrode  12  that induces voltage to the liquid crystal layer  20 .  
           [0013]    Still referring to FIG. 1, on the surface facing the upper substrate  10 , the lower substrate  30  includes an alignment layer  36 , a CLC color filter  38  formed on the alignment layer  36 , and a transparent pixel electrode  48  for applying voltage to the liquid crystal layer  20  on the CLC color filter  38 . On the other surface, the lower substrate  30  includes a light absorption layer  40 . The light absorption layer  40  is made of a material that greatly absorbs light to absorb the light passing through the CLC color filter  38 .  
           [0014]    In the above-mentioned structure of the reflective LCD device  50  shown in FIG. 1, the external ambient light is selectively reflected by or transmitted through the CLC color filter  38  as described before. Some portion of the ambient light passing through the CLC color filter  38  is absorbed by the light absorption layer  40 . And some portion of the light having a certain wavelength is reflected by the CLC color filter  38  to display a color. Therefore, a reflector is not required.  
           [0015]    [0015]FIGS. 2A to  2 D are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 1.  
           [0016]    Referring to FIG. 2A, the alignment layer  36  is formed on the transparent lower substrate  36 . The alignment layer  36  is necessary for allowing a cholesteric liquid crystal, which will be formed in a later step, to align in a particular direction relative to the light reflection or transmission. The alignment layer  36  is usually formed of polyimide-based polymer that aligns the liquid crystal in one direction. In general, the polyimide-based polymer has advantages of good adhesiveness to the liquid crystal and provides sufficient liquid crystal alignment.  
           [0017]    Now, referring to FIG. 2B, the alignment layer  36  formed on the lower substrate  30  is rubbed in a designated direction.  
           [0018]    The rubbing method is generally classified into a method in which the substrate itself is rubbed by a fabric or a rubber including: a method of rubbing an inorganic substance that is formed on the substrate; a method of rubbing a polyimide-based polymer that is formed on the substrate; and a method of rubbing a polymeric material that has a similar chemical structure as the liquid crystal. Here, the method of rubbing a polyimide-based polymer is employed.  
           [0019]    [0019]FIG. 2C shows a manufacturing step of forming the CLC color filter  38  on the alignment layer  36 .  
           [0020]    First, the cholesteric liquid crystal (CLC) is coated on the alignment layer  36 . Then, an exposure process and a baking process are performed in series. Thereafter, the CLC color filter  38  is finally formed.  
           [0021]    [0021]FIG. 2D shows a manufacturing step of forming the light absorption layer  40 . As shown, the light absorption layer  40  is coated or adhered to the transparent lower substrate  30 . A black paint or polymer is usually coated for forming the light absorption layer  40 . Further, the black film can be adhered to the lower substrate  30  in order to form the light absorption layer  40 .  
           [0022]    As described before, the reflective LCD device includes the light absorption layer and the CLC color filter on the alignment layer. The above-mentioned reflective LCD device needs to have the alignment layer for the CLC color filter and the light absorption layer. However, the CLC molecules can be arranged in a self-aligning manner rather than other liquid crystal molecules. Therefore, if the light absorption layer is made of the same material as the alignment layer and acts as the alignment layer, a separate alignment layer can be omitted in the reflective LCD device. Further, since the light absorption layer is formed under the lower substrate in the aforementioned structure of the reflective LCD device, the light passing through the CLC color filter can be reflected by the lower substrate before it is absorbed into the light absorption layer. Accordingly, the quality of the reflective LCD device is deteriorated.  
         SUMMARY OF THE INVENTION  
         [0023]    Accordingly, the present invention is directed to a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter, which substantially obviates one or more of problems due to limitations and disadvantages of the related art.  
           [0024]    An object of the present invention is to provide a method of fabricating a reflective liquid crystal display device (as well as the reflective liquid crystal display device itself), which decreases manufacturing steps and cost.  
           [0025]    Another object of the present invention is to provide a reflective liquid crystal display device having high color purity and an improved contrast ratio.  
           [0026]    Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
           [0027]    In order to achieve the objects, in one aspect, a reflective liquid crystal display includes upper and lower substrates that are opposite to and are spaced apart from each other; a liquid crystal layer interposed between the upper and lower substrates; a transparent common electrode on the surface of the upper substrate opposite the lower substrate; a cholesteric liquid crystal (CLC) color filter that selectively reflects and transmits light, the CLC color filter formed over the lower substrate; a transparent pixel electrode on the CLC color filter; and a light absorption layer between the CLC color filter and the lower substrate.  
           [0028]    The upper and lower substrate are made of a transparent glass substrate.  
           [0029]    The light absorption layer is formed of one of an organic material and an organic composite, such as polyamic acid, ployimide, acrylate, epoxy, siloxane, ester or styrene-based monomer. Further, the organic material comprises a black-colored additive such as dye, pigment or carbon.  
           [0030]    The present invention also provides, in another aspect, a reflective liquid crystal display device, including first and second substrates opposite to and spaced apart from each other; a liquid crystal layer interposed between the first and the second substrates; first transparent electrode for applying voltage to the liquid crystal layer, the first transparent electrode formed on the first substrate; a cholesteric liquid crystal (CLC) color filter that selectively reflects and transmits light, the CLC color filter formed on the the second substrate; second transparent electrode for applying voltage to the liquid crystal layer, the second transparent electrode formed on the CLC color filter; wherein the second substrate acting as a light absorption layer that includes an organic material and a black-colored additive.  
           [0031]    The above-mentioned first substrate is a transparent glass.  
           [0032]    The organic material is one of polyamic acid, ployimide, acrylate, epoxy, siloxane, ester and styrene-based monomer, and the black-colored additive is one of dye, pigment and carbon.  
           [0033]    The present invention also provides, in another aspect, a method of forming a lower substrate for use in a reflective liquid crystal display device, including forming a light absorption layer on a substrate; rubbing the light absorption layer in one direction; forming a cholesteric liquid crystal (CLC) color filter on the light absorption layer; and forming a transparent pixel electrode on the CLC color filter.  
           [0034]    The above-mentioned substrate is made of a transparent glass substrate.  
           [0035]    The light absorption layer is formed of one of an organic material and an organic composite, such as polyamic acid, ployimide, acrylate, epoxy, siloxane, ester and styrene-based monomer. Further, the organic material comprises a black-colored additive such as dye, pigment and carbon.  
           [0036]    The present invention also provides, in another aspect, a method of forming a lower substrate for use in a reflective liquid crystal display device, including preparing a light absorption layer as a substrate; forming a cholesteric liquid crystal (CLC) color filter on the light absorption layer; and forming a transparent pixel electrode on the CLC color filter.  
           [0037]    The light absorption layer acting as the substrate is formed of one of an organic material and an organic composite, such as polyamic acid, ployimide, acrylate, epoxy, siloxane, ester and styrene-based monomer. Further, the organic material comprises a black-colored additive such as dye, pigment and carbon.  
           [0038]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0040]    In the drawings:  
         [0041]    [0041]FIG. 1 is a schematic cross-sectional view illustrating a display area of a conventional reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter;  
         [0042]    [0042]FIGS. 2A to  2 D are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 1;  
         [0043]    [0043]FIG. 3 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter according to a first embodiment of the invention;  
         [0044]    [0044]FIGS. 4A to  4 C are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 3;  
         [0045]    [0045]FIG. 5 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter according to a second embodiment of the invention; and  
         [0046]    [0046]FIGS. 6A and 6B are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 5. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0047]    Reference will now be made in detail to the preferred embodiments of the invention, examples of which is illustrated in the accompanying drawings.  
         [0048]    [0048]FIG. 3 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter according to a first embodiment of the present invention. As shown, a reflective LCD device  150  includes upper and lower substrates  110  and  130  and an interposed liquid crystal layer  120  therebetween. The upper and lower substrates  110  and  130  are a transparent material such as glass or plastic. On the surface facing the lower substrate  130 , the upper substrate  110  includes a first transparent electrode  112  that induces voltage to the liquid crystal layer  120 .  
         [0049]    Still referring to FIG. 3, on the surface facing the upper substrate  110 , the lower substrate  130  includes a light absorption layer  136 , a CLC color filter  138  formed on the light absorption layer  136 , and a second transparent electrode  144  applying voltage to the liquid crystal layer  120  on the CLC color filter  138 .  
         [0050]    In the structure of the first embodiment of the present invention depicted in FIG. 3, the light absorption layer  136  is arranged between the CLC color filter  138  and the lower substrate  130 . Therefore, light passing through the CLC color filter  138  is not reflected by the lower substrate  130  because the light is absorbed by the light absorption layer  136  beforehand. Further, since the light absorption layer  136  acts as an alignment layer and the CLC molecules of the CLC color filter  138  are arranged in a self-aligning manner rather than other liquid crystal molecules, a separate alignment layer for aligning the CLC color filter is not needed, compared to the conventional art depicted in FIG. 1. In the present invention, when forming the CLC color filter  138  on the light absorption layer  136 , the CLC color filter  138  is aligned and orientated by applying a shear force to the CLC color filter  138  using coating processes such as knife coating, bar coating and gravure coating.  
         [0051]    [0051]FIGS. 4A to  4 C are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 3.  
         [0052]    [0052]FIG. 4A shows a step of forming a light absorption layer  136 . As shown, the light absorption layer  136  is formed on the entire surface of the lower substrate  130 . The light absorption layer  136  is preferably made of an organic material that includes black-colored additives such as dye, pigment and carbon. Carbon is mainly added to the light absorption layer  136 .  
         [0053]    Furthermore, the organic material for forming the light absorption layer  136  as described herein means a substance including a monofunctional group or a multifunctional group that has thermal reactivity and photo reactivity. The organic material for the light absorption layer  136  should be fluid-like when coating this material on the lower substrate  130 . This organic material, however, should be hardened and be stable. Therefore, in order to match these conditions, the organic material should have a high phase transition temperature or include a cross-linking agent.  
         [0054]    A black resin having a black-colored additive for forming the light absorption layer  136  generally comprises polyamic acid and polyimide; acrylate-, epoxy-, siloxane-, ester- or styrene-based monomer itself, or its oligomer or polymer. Also, the black resin having the black-colored additive can comprise the above-mentioned organic composition.  
         [0055]    Now, referring to FIG. 4B, the light absorption layer formed on the lower substrate is rubbed in order to act as an alignment layer for a CLC color filter that will be formed in a later step. As shown in FIG. 4B, the light absorption layer  136  is rubbed in a designated direction using the aforementioned rubbing method such as a method using fabric or rubber. As mentioned before, since the CLC is aligned in a self-aligning manner, the light absorption layer  136  is rubbed, to align the CLC color filter instead of an alignment in the conventional art.  
         [0056]    [0056]FIG. 4C shows a step of forming a CLC color filter on the light absorption layer. Again, the CLC color filter  138  is aligned and orientated by applying a shear force to the CLC color filter  138  using coating processes such as knife coating, bar coating and gravure coating when forming the CLC color filter  138 . This shear force applied to the CLC color filter  138  helps the CLC molecules to be properly self-aligned and self-oriented in a designated direction.  
         [0057]    As described before, since the CLC color filter  138  and the light absorption layer  136  are adhered to each other, the light reflection caused by the lower substrate is prevented, in contrast to the conventional art. Namely, the light passing through the CLC color filter  138  is prevented from being reflected on the lower substrate  130  because the light is absorbed by the light absorption layer  136 . Accordingly, since light not reflected by the CLC color filter  138  is absorbed by the light absorption layer  136 , back color is easily displayed in the reflective LCD device.  
         [0058]    [0058]FIG. 5 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a cholesteric liquid crystal (CLC) color filter according to a second embodiment of the invention. As shown, although a reflective LCD device  250  has a similar structure to the first embodiment depicted in FIG. 3, it does not include a lower substrate. Namely, a light absorption layer  234  acts as a lower substrate such that it is thicker than the light absorption layer  136  of the first embodiment. In other words, no separate lower substrate is required for the second embodiment. The light absorption layer  234  of the second embodiment is preferably made of the same material as that of the first embodiment, and it is thick enough to function as a lower substrate. Some explanation of the second embodiment will be omitted because this has a similar structure to the first embodiment.  
         [0059]    As shown in FIG. 5, a reflective LCD device  250  includes upper and lower substrates  210  and  234  and an interposed liquid crystal layer  220  therebetween. The upper and lower substrates  210  is a transparent material such as glass or plastic and the lower substrate  234  is a light absorption layer. On the surface facing the light absorption layer  234 , the upper substrate  210  includes a first transparent electrode  212  that induces voltage to the liquid crystal layer  220 .  
         [0060]    Still referring to FIG. 5, on the surface facing the upper substrate  210 , the lower substrate  234 , i.e., the light absorption layer, includes a CLC color filter  236  and a second transparent electrode  244  applying voltage to the liquid crystal layer  220  on the CLC color filter  236 .  
         [0061]    [0061]FIGS. 6A and 6B are perspective views illustrating manufacturing process steps of the reflective LCD device of FIG. 5.  
         [0062]    [0062]FIG. 6A shows a step of rubbing the surface of the light absorption layer  234 . As mentioned, the light absorption layer  234  is thick enough to act as a lower substrate and made of the same material as that of first embodiment. Due to the greater thickness, the light absorption layer  234  has a thermal, chemical, mechanical stability. Further, due to the rubbing process, the light absorption layer acts as an alignment layer for the CLC color filter that will be formed thereon in a later step.  
         [0063]    [0063]FIG. 6B shows a step of forming a CLC color filter  236  on the light absorption layer  234 . Again, in the present invention, when forming the CLC color filter  236 , the CLC color filter  236  is aligned and orientated by applying a shear force to the CLC color filter  236  using coating processes such as knife coating, bar coating and gravure coating. This shear force applied to the CLC color filter  236  helps the CLC molecules to be properly self-aligned and self-oriented in a designated direction.  
         [0064]    As described before, since the CLC color filter  236  acts not only as the alignment layer but also the lower substrate, the manufacturing process and cost are declined.  
         [0065]    Accordingly, according to the first embodiment of the present invention, the black resin acting as both the light absorption layer and the alignment layer is formed on the lower substrate. According to the second embodiment of the present invention, the black resin that is thick enough to be the substrate acts as the light absorption layer, the alignment layer and the lower substrate. Therefore, some steps of manufacturing the reflective LCD device can be omitted, thereby decreasing the manufacturing cost.  
         [0066]    As described herein, the reflective LCD device according to the first and second preferred embodiments of the present invention has the following advantages.  
         [0067]    First, the color purity is greatly improved and the brightness of the light is maximized due to the CLC color filter.  
         [0068]    Second, the manufacturing steps and cost are decreased, since the light absorption layer can act as the alignment layer and as both the alignment layer and the substrate.  
         [0069]    Third, the reflection from the lower substrate is prevented, since the light absorption layer is formed right under the CLC color filter.  
         [0070]    It will be apparent to those skilled in the art that various modification and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.