Patent Publication Number: US-2007121043-A1

Title: Liquid crystal display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to and the benefit of Korean Patent Application No. 10-2005-00115970, filed on Nov. 30, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
     BACKGROUND  
      1. Field of the Invention  
      The present invention relates to a liquid crystal display device, and, more particularly, to an electrically birefringence (ECB) mode transmissive type liquid crystal display device.  
      2. Discussion of Related Art  
      A liquid crystal display device can be classified into a twisted nematic (TN) type, an electrically birefringence (ECB) type, and an optically compensated birefringence (OCB) type, according to an operation mode. Further, the liquid crystal display device can be classified into a transmissive type liquid crystal display device and a reflective type liquid crystal display device according to the kind of a light source. The transmissive type liquid crystal display device uses internal light source such as backlight to display an image, and the reflective type liquid crystal display device uses external light source such as a natural sunlight.  
      Recently, a need has been developed that requires the advantages of a transmissive type liquid crystal display device and a reflective type liquid crystal display device. Under this circumstance, a transflective type liquid crystal display device has been proposed to have the advantages of both the transmissive type liquid crystal display device and the reflective type liquid crystal display device. The transflective type liquid crystal display device in general includes an ECB mode liquid crystal panel.  
       FIG. 1  is a schematic cross-sectional view showing a conventional transflective type liquid crystal display device.  
      As shown in  FIG. 1 , the conventional transflective type liquid crystal display device includes an electrically controlled birefringence (referred to as ‘ECB’ hereinafter) mode liquid crystal panel  100 , a first retardation film  110 , a first polarizing plate  120 , a second retardation film  130 , and a second polarizing plate  140 . The ECB mode liquid crystal panel  100  drives a liquid crystal cell, and includes first and second substrates  102  and  104 , first and second orientation films  103  and  105  respectively being at the first and second substrates  102  and  104  and oriented opposite to each other, and a liquid crystal  107  formed between the first and second substrates  102  and  104 . The first retardation film  110  is formed at an outer surface of the first substrate  102 . The first polarizing plate  120  is formed at an outer surface of the first retardation film  110 . The second retardation film  130  is formed at an outer surface of the second substrate  104 . The second polarizing plate  140  is formed at an outer surface of the second retardation film  130 .  
      Here, the first and second retardation films  110  and  130  function to change a polarized state of a light. For example, a λ/4 retardation plate for changing a light in a line polarization into a circle polarization may be used as the first and second retardation films  110  and  130 . Also, a λ/2 retardation plate for rotating a light in a line polarization or a circle polarization at a predetermined angle may be used as the first and second retardation films  110  and  130 . The λ/4 retardation plate and the λ/2 retardation plate can be simultaneously used as the first and second retardation films  110  and  130 .  
      Furthermore, the first and second polarizing plates  120  and  140  are arranged at outer surfaces of the first and second retardation films  110  and  130 , respectively. A light transmittance axis of the first polarizing plate  120  forms an angle of 90° with respect to a light transmittance axis of the second polarizing plate  140 .  
      Moreover, a backlight (not shown) is arranged at an outer surface of the second polarizing plate  140 , namely, at a lower portion of (or below) the second polarizing plate  140 .  
      However, in the conventional ECB mode transflective type liquid crystal display device, as shown in  FIG. 1 , retardation films and polarizing plates are arranged at upper and lower sides of an ECB mode liquid crystal panel, thereby greatly increasing a thickness thereof.  
      In addition, so as to embody a transmissive type liquid crystal display device using the ECB mode liquid crystal panel, when only polarizing plates included in the aforementioned transflective type liquid crystal display device are formed on the liquid crystal panel, a remaining retardation occurs in the ECB mode liquid crystal panel, with the result that a display in a dark state becomes impossible.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an aspect of the present invention to provide a liquid crystal display device that prevents an occurrence of a remaining retardation in an ECB mode liquid crystal panel by attaching first and second polarizing plates to upper and lower sides of the ECB mode liquid crystal panel, and by forming an optical viewing angle compensation film between the first and/or second polarizing plates.  
      An embodiment of the present invention provides a liquid crystal display device having: an electrically controlled birefringence mode liquid crystal panel for driving a liquid crystal cell and including first and second substrates, first and second orientation films respectively being at the first and second substrates and oriented opposite to each other, and a liquid crystal layer formed between the first and second substrates; a first polarizing plate formed at an outer surface of the first substrate; a second polarizing plate formed at an outer surface of the second substrate; and a compensation film formed between the electrically controlled birefringence mode liquid crystal panel and the first polarizing plate, and/or between the electrically controlled birefringence mode liquid crystal panel and the second polarizing plate.  
      An embodiment of the present invention provides a liquid crystal display device having: an electrically controlled birefringence mode liquid crystal panel including first and second substrates, first and second orientation films respectively being at the first and second substrates and oriented opposite to each other, and a liquid crystal layer formed between the first and second substrates; a first polarizing plate formed at an outer surface of the first substrate; a second polarizing plate formed at an outer surface of the second substrate; and a compensation film formed between the electrically controlled birefringence mode liquid crystal panel and the first polarizing plate.  
      In one embodiment, the compensation film is composed of an optical viewing angle compensation film or a retardation film.  
      Also, a light transmittance axis of the first polarizing plate and a light transmittance axis of the second polarizing plate are inclined by about 35° to 55°, or by about 45° with respect to an orientation direction of the liquid crystal layer included in the electrically controlled birefringence mode liquid crystal panel.  
      Further, the light transmittance axis of the first polarizing plate and the light transmittance axis of the second polarizing plate have an angle from about 80° to 100° with respect to each other, or are arranged orthogonal to each other.  
      Moreover, a negative retardation of the compensation film ranges from about −60 nm to −10 nm, or is about −35 nm.  
      Furthermore, an optical axis of the optical viewing angle compensation film is arranged horizontal to an orientation direction of the liquid crystal layer included in the electrically controlled birefringence mode liquid crystal panel.  
      In addition, an optical axis of the optical viewing angle compensation film is inclined by about −10° to 10° with respect to an orientation direction of the liquid crystal layer included in the electrically controlled birefringence mode liquid crystal panel, and an optical axis of the retardation film is inclined by about 80° to 100° with respect to an orientation direction of the liquid crystal layer included in the electrically controlled birefringence mode liquid crystal panel.  
      Also, an optical axis of the compensation film and a light transmittance axis of the first and/or second polarizing plate facing the compensation film are arranged to have an angle from about 35° to 55° with respect to each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.  
       FIG. 1  is a schematic cross-sectional view showing a conventional transflective type liquid crystal display device;  
       FIG. 2  is a cross-sectional view showing a liquid crystal display device according to an embodiment of the present invention;  
       FIG. 3  is cross-sectional view showing a liquid crystal display device according to another embodiment of the present invention; and  
       FIG. 4  is a view showing an internal construction of an optical viewing angle compensation film included in a liquid crystal display device according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      In the following detailed description, certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments may be modified in various ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, rather than restrictive. There may be parts shown in the drawings, or parts not shown in the drawings, that are not discussed in the specification as they are not essential to a complete understanding of the invention. Like reference numerals designate like elements. Here, when a first element is connected to/with a second element, the first element may be not only directly connected to/with the second element but also indirectly connected to/with the second element via a third element.  
       FIG. 2  is a cross-sectional view showing a liquid crystal display device according to an embodiment of the present invention.  
      With reference to  FIG. 2 , the liquid crystal display device according to an embodiment of the present invention includes an ECB mode liquid crystal panel, a first polarizing plate  70 , a second polarizing plate  80 , and a compensation film (e.g., an optical viewing angle compensation film)  90 . The ECB mode liquid crystal panel drives a liquid crystal cell. The ECB mode liquid crystal panel includes first and second substrates  50  and  10 , first and second orientation films  42  and  44  respectively being at the first and second substrates  50  and  10  and oriented opposite to each other, and a liquid crystal layer  40  formed between first and second substrates  50  and  10 . The first polarizing plate  70  is formed at an outer surface of the first substrate  50 . The second polarizing plate  80  is formed at an outer surface of the second substrate  10 . The optical viewing angle compensation film  90  is formed between the ECB mode liquid crystal panel and the first polarizing plate  70 .  
      Here, the first substrate  50  and the second substrate  10  are arranged at the ECB mode liquid crystal panel. A thin film transistor Tr is formed on the second substrate  10 , and is composed of a gate electrode  13  and source and drain electrodes  23   a  and  23   b.  The thin film transistor Tr further includes an active layer  19  and an ohmic contact layer  20 . A gate insulation film  16  is formed at an upper portion of the gate electrode  13 .  
      A passivation layer  25  is formed at an upper portion of the thin film transistor Tr and covers the thin film transistor Tr. The passivation layer  25  includes a contact hole  27  to expose the drain electrode  23   b.  Then, a pixel electrode  36  is formed at an upper portion of the passivation layer  25 , and is connected to the drain electrode  23   b  via the contact hole  27 .  
      In addition, black matrixes  53  are formed on an inner surface of the first substrate  50  at a location corresponding to the thin film transistor Tr (i.e., in  FIG. 2  at least one of the black matrixes  53  is shown to be formed at a location corresponding to the thin film transistor Tr). Color filter patterns  56   a,    56   b,  and  56   c  have red, green, and blue filters arranged repeatedly between the black matrixes  53 . At lower portions of the color filter patterns  56   a,    56   b,  and  56   c,  an overcoat layer  60  and a common electrode  63  are successively formed. The common electrode  63  is made of transparent conductive materials. Herein, one color of the color filter patterns  56   a,    56   b,  and  56   c  corresponds to one pixel electrode  36 .  
      Moreover, the first and second orientation films  42  and  44  are formed at inner surfaces of the first and second substrates  50  and  10 , respectively. The first and second orientation films  42  and  44  are oriented in directions opposite to each other.  
      The liquid crystal layer  40  is injected between the first and second orientation films  42  and  44 . When a voltage is applied to the pixel electrode  36  and the common electrode  63 , liquid crystal molecules of the liquid crystal layer  40  horizontally oriented by the first and second orientation films  42  and  44  are changed in an aligned state by an electric field generated between the pixel and common electrodes  36  and  63 , with the result that the liquid crystal display device is driven.  
      In addition, as a transmissive type liquid crystal display device, the liquid crystal display device needs an additional light source. Although it is not shown, a backlight is arranged at a lower portion of (or below) the second polarizing plate  80  as the additional light source, and light from the backlight is incident onto the liquid crystal panel to adjust an amount of the light according to an alignment of the liquid crystal molecules so that an image is displayed.  
      That is, the ECB mode liquid crystal panel according to an embodiment of the present invention is characterized in that orientation films  42  and  44  are respectively on the first and second substrates  50  and  10  and oriented opposite to each other, and the liquid crystal layer  40  is horizontally oriented by the orientation films  42  and  44  formed between the first and second substrates  50  and  10 , so that the liquid crystal cell is driven.  
      In a liquid crystal display device, e.g., as illustrated earlier in  FIG. 1 , an ECB mode liquid crystal panel is in general included in a transflective liquid crystal display device. However, the liquid crystal display device is characterized in that a transmissive type liquid crystal display device is also embodied through the ECB mode liquid crystal panel.  
      So as to do this (e.g., to embody the transmissive type liquid crystal display device through the ECB mode liquid crystal panel), first and second polarizing plates  70  and  80  are provided at upper and lower sides of the ECB mode liquid crystal panel in  FIG. 2 . The compensation film  90  is formed between the ECB mode liquid crystal panel and the first polarizing plate  70 . Through the aforementioned arrangement, a remaining retardation of the ECB mode liquid crystal panel is prevented so as to improve a contrast and a viewing angle.  
      Here, the compensation film  90  can be composed of an optical viewing angle compensation film or a retardation film.  
      Furthermore, a light transmittance axis of the first polarizing plate  70  and a light transmittance axis of the second polarizing plate  80  are inclined by 35° to 55°, or by 45°, with respect to an orientation direction of the liquid crystal layer  40  included in the electrically controlled birefringence mode liquid crystal panel.  
      Moreover, the light transmittance axis of the first polarizing plate  70  and the light transmittance axis of the second polarizing plate  80  have an angle from 80° to 100° with respect to each other.  
      In one embodiment, the light transmittance axis of the first polarizing plate  70  and the light transmittance axis of the second polarizing plate  80  are arranged orthogonal (90°) to each other.  
      Furthermore, in an embodiment of the present invention, the compensation film  90  is formed between the ECB mode liquid crystal panel and the first polarizing plate  70 . Here, the negative retardation of the compensation film  90  ranges from −60 nm to −10 nm. In one embodiment, the negative retardation of the compensation film  90  is −35 nm.  
      When the compensation film is the optical viewing angle compensation film, an optical axis thereof is inclined by −10° to 10° with respect to an orientation direction of the liquid crystal layer  40  included in the ECB mode liquid crystal panel, and, in one embodiment, the optical axis of the optical viewing angle compensation film is horizontal to the orientation direction of the liquid crystal layer  40 . When the compensation film is the retardation film, an optical axis thereof is inclined by 80° to 100° with respect to an orientation direction of the liquid crystal layer  40  included in the electrically controlled birefringence mode liquid crystal panel, and, in one embodiment, the optical axis of the retardation film is orthogonal to the orientation direction of the liquid crystal layer  40 .  
      Accordingly, an optical axis of the compensation film  90  and a light transmittance axis of the first polarizing plate  70  facing the compensation film  90  are arranged to have an angle from 35° to 55° with respect to each other.  
      Further, in one embodiment, the optical axis of the compensation film  90  and the light transmittance axis of the first polarizing plate  70  facing the compensation film  90  are arranged to have an angle of 45° with respect to each other.  
       FIG. 3  is a cross-sectional view showing a liquid crystal display device according to another embodiment of the present invention. There may be parts shown in  FIG. 3 , or parts not shown in  FIG. 3 , that are not discussed below as they are not essential to a complete understanding of the invention. Like reference numerals in  FIG. 3  designate like elements in  FIG. 2 .  
      Upon comparing the embodiment of the present invention shown in  FIG. 3  with the embodiment of  FIG. 2 , a first compensation film  90 ′ is disposed between the first polarizing plate  70  and a liquid crystal panel, and a second compensation film  90 ″ is disposed between the second polarizing plate  80  and a liquid crystal panel.  
      Accordingly, each of the first and second compensation films  90 ′ and  90 ″ can be composed of an optical viewing angle compensation film or a retardation film. The negative retardation of each of the first and second compensation films  90 ′ and  90 ″ ranges from −60 nm to −10 nm, or, in one embodiment, is −35 nm.  
      In addition, when the compensation film is the optical viewing angle compensation film, an optical axis thereof is inclined by −10° to 10° with respect to an orientation direction of the liquid crystal layer  40  included in the ECB mode liquid crystal panel, and, in one embodiment, the optical axis of the optical viewing angle compensation film is horizontal to the orientation direction of the liquid crystal layer  40 . When the compensation film is the retardation film, an optical axis thereof is inclined by 80° to 100° with respect to an orientation direction of the liquid crystal layer  40  included in the electrically controlled birefringence mode liquid crystal panel, and, in one embodiment, the optical axis of the retardation film is orthogonal to the orientation direction of the liquid crystal layer  40 .  
      Accordingly, an optical axis of the first compensation film  90 ′ and a light transmittance axis of the first polarizing plate  70  facing the first compensation film  90 ′ and/or an optical axis of the second compensation film  90 ″ and a light transmittance axis of the second polarizing plate  80  facing the second compensation film  90 ″ are arranged to have an angle from 35° to 55° with respect to each other.  
      Further, in one embodiment, the optical axis of the first compensation film  90 ′ and the light transmittance axis of the first polarizing plate  70  facing the first compensation film  90 ′ and/or the optical axis of the second compensation film  90 ″ and the light transmittance axis of the second polarizing plate  80  facing the second compensation film  90 ″ are arranged to form an angle of 45° with respect to each other.  
       FIG. 4  is a view showing an internal construction of an optical viewing angle compensation film included in a liquid crystal display device according to an embodiment of the present invention. The embodiment of  FIG. 4  is to be regarded as illustrative in nature, rather than restrictive, and the present invention is not thereby limited.  
      Referring to  FIG. 4 , in the interior structure of the optical viewing angle compensation film according to the embodiment of the present invention, an optical axis of a nematic liquid crystal molecule  405  and a discotic liquid crystal molecule  410  are identically arranged. As such, to some extent, this may compensate a variation of the retardation with respect to a moving direction of a light that has permeated the ECB mode liquid crystal panel.  
      Moreover, the nematic liquid crystal molecule  405  is a positive uniaxial material. An extraordinary refractive index n e  of the nematic liquid crystal molecule  405  is greater than an ordinary refractive index n 0  thereof. In contrast to this, the discotic liquid crystal molecule  410  is a negative uniaxial material. An extraordinary refractive index n e  of the nematic liquid crystal molecule  410  is less than an ordinary refractive index n 0  thereof.  
      According to the present invention, by attaching first and second polarizing plates to upper and lower sides of the ECB mode liquid crystal panel, and forming an optical viewing angle compensation film between the first and/or second polarizing plates, an occurrence of a remaining retardation in an ECB mode liquid crystal panel is prevented to improve a contrast and a viewing angle.  
      While the invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof.