Patent Publication Number: US-6989875-B2

Title: Liquid crystal display device using cholesteric liquid crystal and a manufacturing method thereof

Description:
This application is a divisional of prior application Ser. No. 10/175,491, filed Jun. 20, 2002 U.S. Pat. No. 6,774,962. 
     This application claims the benefit of Korean Patent Application No. 2001-69443, filed on Nov. 8, 2001, which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) device, and more particularly to an LCD device using cholesteric liquid crystal and a manufacturing method thereof. 
     2. Discussion of the Related Art 
     Flat panel display (FPD) devices having small size, lightweight, and low power consumption have been a subject of recent research according to coming of the information age. Among many kinds of FPD devices, LCD devices are widely used for notebook personal computers (PCs) or desktop PCs because of their excellent characteristics of resolution, color display and display quality. Generally, in an LCD device, first and second substrates having respective electrodes are disposed to face each other with a liquid crystal layer is interposed therebetween. The liquid crystal layer has an optical anisotropy due to an electric field generated by applying a voltage to the respective electrodes. The LCD device displays images by using a transmittance difference according to the optical anisotropy of the liquid crystal layer. 
       FIG. 1  is a schematic cross-sectional view of a related LCD panel. 
     In  FIG. 1 , first and second substrates  10  and  20 , referred to as lower and upper substrates, are facing and spaced apart from each other. A thin film transistor “T” (TFT) having a gate electrode  11 , and source and drain electrodes  15   a  and  15   b  are formed on an inner surface of the first substrate  10 . The TFT “T” further has an active layer  13  and an ohmic contact layer  14 . A gate insulating layer  12  is formed on the gate electrode  11 . A passivation layer  16  is formed on the TFT “T”. The passivation layer  16  covers the TFT “T” and has a contact hole  16   c  exposing the drain electrode  15   b  of the TFT. A pixel electrode  17  is formed on the passivation layer  16  and connected to the drain electrode  15   b  through the contact hole  16   c.    
     A black matrix  21  is formed on an inner surface of the second substrate  20  at a position corresponding to the TFT “T”. A color filter layer  22   a  and  22   b , in which colors of red (R), green (G) and blue (B) are alternately repeated, is formed on the black matrix  21 . A common electrode  23  of transparent conductive material is formed on the color filter layer  22   a  and  22   b . The color filter layer  22   a  and  22   b  of a single color corresponds to the one pixel electrode  17 . 
     A liquid crystal layer  30  is interposed between the pixel and common electrodes  17  and  23 . When a voltage is applied to the pixel and common electrodes  17  and  23 , the arrangement of molecules of the liquid crystal layer  30  changes according to an electric field generated between the pixel and common electrodes  17  and  23 . Orientation films (not shown) respectively formed on the pixel and common electrodes determine an initial arrangement of liquid crystal molecules. 
     First and second polarizers  41  and  42  are formed on outer surfaces of the first and second substrates  10  and  20 , respectively. The first and second polarizers  41  and  42  convert natural light to linearly polarized light by transmitting only light whose polarizing direction is parallel to a transmission axis of the polarizer. The transmission axis of the first polarizer  41  is perpendicular to that of the second polarizer  42 . 
     In  FIG. 1 , the TFT and the pixel electrode are formed on the lower substrate and the color filter layer and the common electrode are formed on the upper substrate. Recently, however, structures in which the TFT and the color filter layer are formed on the lower substrate, or the color filter layer and the common electrode are formed on the lower substrate and the TFT and the pixel electrode are formed on the upper substrate have been suggested. 
     Since an LCD device does not emit light for itself, an additional light source is necessary. Therefore, a backlight is disposed over the first polarizer  41  of  FIG. 1  and light from the backlight is provided to a liquid crystal panel. Images are displayed by adjusting the light according to the arrangement of the liquid crystal layer. The LCD device of this structure is referred to as a transmissive LCD device. The pixel electrode  17  and the common electrode  23 , two electrodes generating an electric field, are made of transparent conductive material and the first and second substrates  10  and  20  are also transparent. 
     Since only one polarizing component of the incident light is transmitted through the polarizer used in the LCD device and the other components are absorbed and then converted into heat loss, brightness of the LCD device is reduced by more than 50% considering reflection at a surface of the polarizer. To improve the brightness of the LCD device by reducing the heat loss, an LCD device having a reflective circular polarizer under the device is suggested. The circular polarizer transmits one circular polarizing component of the incident light and reflects the other components. The reflected circular polarizing components are reflected again by several optical parts under the circular polarizer and converted into a light component capable of passing the circular polarizer. Theoretically, since all the incident light is converted into one component and then transmits through the circular polarizer, loss of light occurring in a conventional linear polarizer is remarkably reduced. 
       FIG. 2  is a schematic cross-sectional view of a related art LCD device. 
     In  FIG. 2 , a first polarizer  42  that is a linear polarizer is disposed under a liquid crystal cell  41 , in which a liquid crystal layer is interposed between two substrate having respective electrodes on inner surfaces. A retardation layer  43 , which converts linear polarization into circular polarization and vice versa, and a second polarizer  45  that is a linear polarizer are disposed under the first polarizer  42 . A compensation film  44  can be interposed between the retardation layer  43  and the second polarizer  45 . A sheet  46  for collecting and diffusing light and a backlight  47  are sequentially disposed under the second polarizer  45 . On the other hand, a third polarizer  48  whose transmission axis is perpendicular to that of the first polarizer  42  is disposed over the liquid crystal cell  41 . The liquid crystal cell  41  can have the same structure as or different structure from the liquid crystal cell of  FIG. 1 . 
     The second polarizer  45  can be made through forming a cholesteric liquid crystal layer  45   b  on a transparent substrate  45   a . The cholesteric liquid crystal has a selective reflection property that only light of a specific wavelength is selectively reflected according to a helical pitch of the molecules of the cholesteric liquid crystal. The polarization of the reflected light is determined according to a rotational direction of the liquid crystal. For example, if a liquid crystal layer has a left-handed structure where liquid crystal molecules rotate counter clockwise along a rotational axis, only left-handed circularly polarized light having a corresponding color, i.e., wavelength, is reflected. Since the pitch of the cholesteric liquid crystal that light experiences is varied according to an incident angle, a wavelength of reflected light is also varied. Accordingly, there is a color shift such that a color of transmitted light varies according to a viewing angle. To compensate for the color shift, a compensation film  44  may be disposed over the second polarizer  45 . 
     As shown in  FIG. 2 , a sheet  46  for collecting light from the backlight  47  and diffusing light to the liquid crystal cell  41  can be disposed between the second polarizer  45  and the backlight  47 . 
     In the LCD device of  FIG. 2 , brightness is improved through increasing transmitted light by using a circular polarizer to a conventional LCD device. However, a conventional linear polarizer is still necessary because polarizing efficiency of the circular polarizer is lower than that of the linear polarizer. Moreover, a retardation layer should be attached for light that passes the circular polarizer to transmit through the linear polarizer. Therefore, production cost is high due to a plurality of films required for an increase in brightness. However, the increase in brightness is not large and a viewing angle is narrow. 
     On the other hand, a LCD device using a cholesteric liquid crystal color filter (CLC) has been researched and developed recently. Since cholesteric liquid crystal has a selective reflection property, brightness can be improved in contrast with a LCD device using a color filter of absorption type. 
       FIG. 3  is a cross-sectional view of a related art LCD device using a CLC. 
     In  FIG. 3 , a circular polarizer  53  using cholesteric liquid crystal is disposed under a liquid crystal cell  51  having a CLC color filter  52 . A collection sheet  54  and a backlight  55  are sequentially disposed under the circular polarizer  53 . 
     A diffusing sheet  56  for diffusing light transmitted through the liquid crystal cell  51  is disposed over the liquid crystal cell  51 . A retardation layer  57  and a linear polarizer  58  are sequentially disposed over the diffusing sheet  56 . 
     The collection sheet  54  for collecting light entering the circular polarizer  53  and the CLC  52  is made by forming a film  54   b  having a high condensing pattern on a transparent substrate  54   a . The collection sheet  54  may be made of only the film  54   b  without the substrate  54   a . Moreover, the backlight  55  may include the high condensing pattern or means. 
     In a LCD device having the structure of  FIG. 3 , a wavelength variation of reflected light according to an incident angle to the cholesteric liquid crystal is solved by using a high condensing backlight and a collection sheet. Moreover, the light efficiency increases by using a circular polarizer and a reflective CLC, and the collected light is diffused through a diffusing layer over the liquid crystal cell. Therefore, the brightness is improved in contrast with a related art LCD device of  FIG. 2 , and the problem of a color shift according to a viewing angle is solved. However, the production cost and the thickness of the LCD device also increase due to the individual circular polarizer and collection sheet. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a liquid crystal display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art. 
     An advantage of the present invention is to provide a liquid crystal display device of high brightness, wide viewing angle and low thickness with low cost and short manufacturing process by forming a circular polarizer and a collection sheet on one substrate. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. 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. To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a liquid crystal display device includes a liquid crystal cell; a collimating polarizer under the liquid crystal cell, the collimating polarizer having a collimating layer and a circular polarizer fixed on the collimating layer; a backlight under the collimating polarizer; a diffusing layer over the liquid crystal cell; a retardation layer over the diffusing layer; and a linear polarizer over the retardation layer. 
     In another aspect, a liquid crystal display device includes a liquid crystal cell; a collimating polarizer under the liquid crystal cell, the collimating polarizer having collimating layer, a circular polarizer and a transparent substrate between the collimating layer and the circular polarizer; a backlight under the collimating polarizer; a diffusing layer over the liquid crystal cell; a retardation layer over the diffusing layer; and a linear polarizer over the retardation layer. In another aspect, a fabricating method of a collimating polarizer for a liquid crystal display device includes providing a first transparent substrate; coating a cholesteric liquid crystal on the first transparent substrate; exposing and hardening the cholesteric liquid crystal to form a pitch of the cholesteric liquid crystal; coating a resin on the cholesteric liquid crystal; and patterning and hardening the resin to form a collimating layer. 
     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 
       The accompanying drawings, which are included herewith to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. 
       In the drawings: 
         FIG. 1  is a schematic cross-sectional view of a related art LCD panel; 
         FIG. 2  is a schematic cross-sectional view of a related art LCD device; 
         FIG. 3  is a cross-sectional view of a related art LCD device using a CLC; 
         FIG. 4  is a schematic cross-sectional view of an LCD device according to a first embodiment of the present invention; 
         FIGS. 5A to 5G  are schematic cross-sectional views showing a fabricating process of a collimating polarizer according to a first embodiment of the present invention; 
         FIG. 6  is a schematic cross-sectional view of an LCD device according to a second embodiment of the present invention; and 
         FIGS. 7A to 7F  are schematic cross-sectional views showing a fabricating process of a collimating polarizer according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to the illustrated embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, similar reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 4  is a schematic cross-sectional view of an LCD device according to a first embodiment of the present invention. 
     In  FIG. 4 , a collimating polarizer  120  is disposed under a liquid crystal cell  110  and a backlight  130  is disposed under the collimating polarizer  120 . The liquid crystal cell  110  includes first and second substrates  111  and  112  facing each other and having respective electrodes  113  and  114  on inner sides, and a liquid crystal layer  115  between the first and second substrates  111  and  112 . A cholesteric liquid crystal (CLC)  117  is formed on an outer side of the first substrate  111 . A plurality of thin film transistors (TFTs) and pixel electrodes (not shown) may be formed on the inner side of the first substrate  111  and a common electrode (not shown) may be formed on the inner side of the second substrate  112 . Otherwise, a common electrode may be formed on the inner side of the first substrate  111  and a plurality of TFTs and pixel electrodes may be formed on the inner side of the second substrate  112 . The collimating polarizer  120  includes a circular polarizer  122  of a cholesteric liquid crystal on a transparent substrate  121 , and a collimating layer  123  under the transparent substrate  121  having a condensing pattern. The backlight  130  may include a lamp  131  as a passing the first and second transparent substrates  124  and  121  having the cholesteric liquid crystal layer  122   a  therebetween through two rollers spaced apart from each other. 
     In  FIG. 5C , after exposing the cholesteric liquid crystal layer  122   a  (of  FIG. 5B ) to light such as ultraviolet (UV) light to select a pitch and hardening the exposed cholesteric liquid crystal layer to form a circular polarizer  122 , material such as light curable resin is coated on the second transparent substrate  121  to form a resin  123   a . The organic film  123   a  is coated by a plane roll and then patterned by a stamp to provide a condensing pattern on the organic film  123   a . Otherwise the organic film  123   a  may be coated and simultaneously patterned by an embossed roll. A condensing film or collimating layer is completed by hardening the patterned organic film  123   a  with UV or heat. The organic film  123   a  may be hardened after coating and patterning or at the same time with patterning to form a condensing film or collimating layer  123 . The pattern of the condensing film or collimating layer  123  is one of prism shape, holography shape in which a incoherent light is added to transmitted light for reproduction and a micro lens shape having two side surfaces that circular, or circular and planar. 
     In  FIG. 5D , a first protection film  125  is attached on the condensing film  123  for protection. 
     In  FIGS. 5E and 5F , after the first transparent substrate  124  under the circular polarizer  122  is eliminated, a second protection film  126  is attached under the circular polarizer  122 . 
     Therefore, an integrated condensing or collimating polarizer is provided in a state of  FIG. 5F  to protect the circular polarizer  122  and the collimating layer  123 . When the integrated collimating polarizer is used for an LCD device, the first and second protection films  125  and  126  are eliminated as shown in  FIG. 5G  Moreover, the integrated collimating polarizer may be attached to the liquid crystal cell  110  (of  FIG. 4 ) or to the backlight  130 . diffusing layer  240 . Moreover, a linear polarizer  260  is disposed over the retardation layer  250 . 
       FIGS. 7A to 7F  are schematic cross-sectional views showing a fabricating process of an integrated condensing or collimating polarizer according to a second embodiment of the present invention. 
     In  FIG. 7A , a cholesteric liquid crystal layer  221   a  is formed on a transparent substrate  223 . The cholesteric liquid crystal layer  221   a  may be formed by a coating method, especially, a roll coating method using a roller so that a surface can be planarized and the cholesteric liquid crystal layer  221   a  can have a uniform thickness. 
     In  FIG. 7B , after exposing the cholesteric liquid crystal layer  221   a  (of  FIG. 7A ) to ultraviolet (UV) light to select a pitch and hardening the exposed cholesteric liquid crystal layer to form a circular polarizer  221 , material such as light curable resin is coated on the circular polarizer  221  to form a resin  222   a . The organic film  222   a  is coated by a plane roll and then patterned by a stamp to provide a condensing pattern on the organic film  222   a . Otherwise, the organic film  222   a  may be coated and simultaneously patterned by an embossed roll. A condensing film or collimating layer is completed by hardening the patterned organic film  222   a  with UV or heat. The organic film  222   a  may be hardened after coating and patterning or at the same time with patterning to form a condensing film or collimating layer  222 . The pattern of the collimating layer  222  is one of prism shape, holography shape and a micro lens shape. 
     In  FIG. 7C , a first protection film  224  is attached on the collimating layer  222  for protection. 
     In  FIGS. 7D and 7E , after the substrate  223  under the circular polarizer  221  is eliminated, a second protection film  225  is attached under the circular polarizer  221 . 
     Since the substrate between the circular polarizer and the collimating layer of the first embodiment is eliminated from the integrated collimating polarizer according to the second embodiment of the present invention, the thickness is reduced so that brightness can be further improved due to reduction of light loss. 
     In the first and second embodiments, a resin is hardened with UV light and then separated from a stamp to form a condensing film or collimating layer after being coated and patterned with the stamp. In the other embodiment, the organic film may be hardened with UV after the stamp is separated. 
     The step of forming a circular polarizer may include steps of annealing or aging for alignment of the cholesteric liquid crystal. Moreover, after completing an integrated collimating polarizer, an additional annealing for complete hardening may be included. 
     For attaching an integrated collimating polarizer to a liquid crystal cell, a glue is coated on a surface of the integrated collimating polarizer before the second protection film is formed or a film treated with a glue is used as the second protection film for the glue to be transcribed to the integrated collimating polarizer. 
     Consequently, the color shift problem according to the viewing angle of the cholesteric liquid crystal is solved due to the condensing backlight and the condensing film or collimating layer and transmittance increases by using the circular polarizer and the CLC. Accordingly, brightness and viewing angle of an LCD device is improved. The fabricating process is simplified and the production cost is reduced through forming a condensing film or collimating layer and a circular polarizer on one substrate. The thickness of the LCD device is also reduced. The thickness of the LCD device may be further reduced and brightness may be further improved by eliminating a substrate between the circular polarizer and the condensing film or collimating layer.