Patent Publication Number: US-2005140901-A1

Title: Fringe field switching liquid crystal display

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a fringe field switching liquid crystal display (FFS-LCD), and especially to an FFS-LCD having good contrast ratio characteristics.  
      2. Description of Prior Art  
      The in-plane switching liquid crystal display (IPS-LCD) has been developed in order to improve on the narrow viewing angle of the standard twisted nematic liquid crystal display (TN-LCD). The IPS-LCD has a plurality of counter electrodes and a plurality of pixel electrodes all disposed on a same substrate of two opposite substrates, for driving liquid crystal molecules that are disposed between the substrates. The resulting electric field in each pixel is substantially planar, and is parallel to a surface of said same substrate. This structure provides an improved viewing angle.  
      However, the counter electrode and pixel electrodes are formed of opaque metals, giving the IPS-LCD a low aperture ratio and low transmittance. Thus the so-called fringe field switching liquid crystal display (FFS-LCD) has been developed in order to improve on the aperture ratio and transmittance of IPS-LCDs. The FFS-LCD is characterized by the liquid crystal molecules being driven by a fringe electric field at each pixel.  
      Referring to  FIG. 6 , this is a schematic, cross-sectional view of a conventional FFS-LCD  1 , which comprises an upper substrate  20  and a lower substrate  10  disposed opposite to each other and spaced apart a predetermined distance. A liquid crystal layer  50  having a multiplicity of liquid crystal molecules (not labeled) is disposed between the upper and lower substrates  20 ,  10 . A counter electrode  11  and a plurality of pixel electrodes  13  are disposed at the lower substrate  10 , with an insulating layer  12  interposed between the counter electrode  11  and the pixel electrodes  13 . A lower alignment film  14  is formed on the pixel electrodes  13  and the insulating layer  12 . A color filter  25  and an upper alignment film  24  are formed on an inner surface of the upper substrate  20 , in that order from top to bottom. The color filter  25  comprises a black matrix (not shown), and a color resin layer (not shown) having Red, Green and Blue segments. Two polarizers  40 ,  30  are attached on outer surfaces of the upper substrate  20  and the lower substrate  10 , respectively. The polarizers  40 ,  30  are ordinary type polarizers, which are made of polyvinyl alcohol (PVA). The polarizers  40 ,  30  function to allow passage of ordinary polarized light beams, while blocking extraordinary polarized light beams. Polarizing axes of the polarizers  40 ,  30  are perpendicular to each other; that is, the polarizers  30 ,  40  are crossed polarizers.  
      When the FFS-LCD  1  is driven, a fringe electric field is formed at upper portions of the counter electrode  11  and the pixel electrodes  13  at each pixel. The liquid crystal molecules disposed over the counter electrode  11  and pixel electrodes  13  are all driven, thus giving the FFS-LCD  1  improved transmittance compared to that of an IPS-LCD.  
      However, light leakage occurs in an oblique viewing azimuth because of the crossed ordinary type polarizers  30 ,  40 . As seen in  FIG. 7 , the leakage of light beams through the pair of crossed polarizers  40 ,  30  is proportional to the viewing angle. Referring to  FIG. 4 , the broken curve I is the contrast ratio for the polarizers  30 ,  40 . As seen in  FIG. 4 , the FFS-LCD  1  has a high contrast ratio at a 0 degree viewing angle; however, the contrast ratio diminishes rapidly once the viewing angle changes from 0 degrees. The overall display quality of the FFS-LCD  1  diminishes along with the diminution in the contrast ratio.  
      Furthermore, the color filter  25  has a de-polarizing effect on light beams passing therethrough due to pigment light scattering. Therefore light beams passing through the FFS-LCD  1  are at least partially de-polarized by the color filter  25  before reaching the polarizer  40 . This de-polarizing of the light beams prior to them reaching the polarizer  40  can reduce the contrast ratio of the FFS-LCD  1 . Even though such de-polarizing effects are generally small, they can have a significant effect on the contrast ratio of the FFS-LCD  1 .  
      It is desired to provide an FFS-LCD that can solve the above-mentioned contrast ratio and de-polarizing problems.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a fringe field switching liquid crystal display which achieves a good contrast ratio over large viewing angles.  
      Another object of the present invention is to provide a fringe field switching liquid crystal display which achieves a high contrast ratio by preventing any de-polarizing effects from occurring in a color filter thereof.  
      To achieve the above objects, a fringe field switching liquid crystal display of the present invention includes a first substrate and a second substrate disposed opposite each other and spaced apart a predetermined distance. A liquid crystal layer is interposed between the first substrate and the second substrate. A plurality of pixel electrodes is formed at the first substrate, and a counter electrode is formed between the first substrate and the pixel electrodes. The pixel electrodes are strip-shaped and parallel to each other. The counter electrode is overlapped partially by the pixel electrodes. Two polarizers are attached at the first substrate and the second substrate, respectively. A color filter is attached to an underside of the second substrate. At least one of the two polarizers is an extraordinary type polarizer. Therefore a good contrast ratio over large viewing angles is achieved by suppressing light leakage.  
      In certain embodiments of the present invention, the color filter is disposed above the polarizer. Therefore, a high contrast ratio is achieved by preventing any de-polarizing effects of the color filter.  
      Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic, side cross-sectional view of part of an FFS-LCD according to a first embodiment of the present invention;  
       FIG. 2  is a schematic, side cross-sectional view of part of an FFS-LCD according to a second embodiment of the present invention;  
       FIG. 3  is a schematic, side cross-sectional view of part of an FFS-LCD according to a third embodiment of the present invention;  
       FIG. 4  is a graph showing a relationship between contrast ratio and viewing angle, in respect of the FFS-LCD of the first embodiment of the present invention and in respect of the conventional FFS-LCD of  FIG. 6 ;  
       FIG. 5  is a diagram showing leakage of light beams through crossed extraordinary type polarizers of the first embodiment of the present invention;  
       FIG. 6  is a schematic, side cross-sectional view of part of a conventional FFS-LCD; and  
       FIG. 7  is a diagram showing leakage of light beams through crossed ordinary type polarizers of the conventional FFS-LCD of  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a schematic, cross-sectional view of a fringe field switching liquid crystal display (FFS-LCD)  100  according to the first embodiment of the present invention. The FFS-LCD  100  comprises a first substrate  110 , a second substrate  120 , and a liquid crystal layer  130  having a multiplicity of liquid crystal molecules (not labeled). The first substrate  110  and the second substrate  120  are spaced apart from each other, and the liquid crystal layer  130  is disposed therebetween. The first substrate  110  and the second substrate  120  are made of glass. Alternatively, the first substrate  110  and the second substrate  120  can be made of silicon dioxide (SiO 2 ).  
      A counter electrode  111  and a plurality of pixel electrodes  113  are disposed on the first substrate  110 , with a transparent insulating layer  112  interposed between the counter and pixel electrodes  111 ,  113 . A polarizer  141  and an alignment film  116  are formed on the pixel electrodes  113  and the insulating layer  112 , in that order from bottom to top. A color filer  127 , a polarizer  143  and an alignment film  126  are formed on an underside of the second substrate  120 , in that order from top to bottom.  
      The alignment films  116 ,  126  are horizontal alignment layers with a low pretilt angle below 3° for orientating the liquid crystal molecules. The counter electrode  111  is plate-shaped, and each pixel electrode  113  is strip-shaped.  
      The color filter  127  comprises a black matrix (not shown), and a color resin layer (not shown) having Red, Green and Blue segments. The black matrix is disposed between the segments of the color resin layer to prevent light beams from leaking. The insulating layer  112  is used to prevent electrostatic buildup and eliminate afterimage. The insulating layer  112  is made of SiO 2  or silicon nitride (SiNx).  
      The polarizers  141 ,  143  are extraordinary type polarizers composed of mixtures of narrow-band components. Each component consists of a modified organic dye material which exists in a liquid-crystalline phase. Polarizing axes of the polarizers  141 ,  143  are perpendicular to each other; that is, the polarizers  141 ,  143  are crossed polarizers. The polarizers  141 ,  143  pass extraordinary polarized light beams, while blocking ordinary polarized light beams. A thickness of each of the polarizers  141 ,  143  is less than  100  micrometers. This ensures that the operating voltage of the FFS-LCD  100  will not be affected when the polarizers  141 ,  143  are formed at inner surfaces of the first substrate  110  and the second substrate  120 , respectively.  
      When a voltage is applied to the counter electrode  111  and the pixel electrodes  113 , a fringe electric field having horizontal components is produced therebetween at each pixel. The long axes of the liquid crystal molecules are oriented parallel to the fringe electric field. A backlight (not shown) is positioned under the first substrate  110 , such that the FFS-LCD  100  operates as a transmissive mode LCD. The polarization state of light beams emitted from the backlight is changed when the light beams pass through the liquid crystal layer  130 . Therefore, the light beams cannot pass through the polarizer  143  formed on the second substrate  120 . As a result, the FFS-LCD  100  is in a dark state.  
      When no voltage is applied to the counter electrode  111  and the pixel electrodes  113 , the liquid crystal molecules are oriented substantially parallel to the first substrate  110  and the second substrate  120 . Long axes of the liquid crystal molecules are oriented parallel to the pixel electrodes  113 . The state of polarization of light beams is not changed when the light beams pass through the liquid crystal layer  130 . Therefore, the light beams can pass through the polarizer  143  formed on the second substrate  120 . The light beams emitted from the polarizer  143  subsequently pass through the color filter  127  and the second substrate  120 . As a result, the FFS-LCD  100  is in a bright state.  
       FIG. 5  shows leakage of the light beams passing through the crossed extraordinary type polarizers  141 ,  143 . Each sector of the circle corresponds to  5  degrees of viewing angle. The center of the circle corresponds to viewing at a normal angle of incidence. As seen in  FIG. 5 , the leakage of light beams through the pair of crossed polarizers  141 ,  143  is inversely proportional to the viewing angle. That is, light leakage at large viewing angles is reduced. Referring to  FIG. 4 , the continuous curve II is the contrast ratio for the polarizers  141 ,  143 . It illustrates that the polarizers  141 ,  143  have a good contrast ratio over large viewing angles. As a result, the display quality of the FFS-LCD  100  is improved.  
      Furthermore, the color filter  127  is disposed under the second substrate  120  above the polarizer  143 . This arrangement reduces or eliminates the adverse effects of color filter de-polarizing, and yields a higher contrast ratio.  
      Referring to  FIG. 2 , this is a schematic, cross-sectional view of an FFS-LCD  200  according to the second embodiment of the present invention. The FFS-LCD  200  is similar to the FFS-LCD  100  of the first embodiment, and comprises a first substrate  210  and a second substrate  220 . An extraordinary type polarizer  241  is formed at an inner surface of the first substrate  210 , and an ordinary type polarizer  243  is formed on an outer surface of the second substrate  220 . The polarizer  243  and the polarizer  241  are crossed polarizers. Referring to  FIG. 5  and  FIG. 7  again,  FIG. 5  shows leakage of light beams passing through extraordinary type polarizers, and  FIG. 7  shows leakage of light beams passing through ordinary type polarizers. A corresponding diagram for the FFS-LCD  200  would essentially be a combination of the leakage of light beams shown in  FIGS. 5 and 7 . As seen in  FIGS. 5 and 7 , the respective leakages of light beams are substantially complementary to each other. That is, dark regions of  FIG. 5  correspond to light regions of  FIG. 7 , and vice versa. The combination of the ordinary type polarizer  243  and the extraordinary type polarizer  241  can provide complete extinction of light beams at all viewing angles.  
      Referring to  FIG. 3 , this shows a schematic, cross-sectional view of an FFS-LCD  300  according to the third embodiment of the present invention. The FFS-LCD  300  is similar to the FFS-LCD  200 , and comprises a first substrate  310  and a second substrate  320 . An extraordinary type polarizer  341  is formed at an undersurface of the second substrate  320 , and an ordinary type polarizer  343  is formed on an undersurface of the first substrate  310 . Polarizing axes of the polarizers  343 ,  341  are perpendicular to each other.  
      A color filter (not labeled) is disposed under the second substrate  320  above the polarizer  341 . This arrangement reduces or eliminates the adverse effects of color filter de-polarizing, and yields a higher contrast ratio.  
      In the present invention, the FFS-LCDs  100 ,  200  and  300  are transmissive mode LCDs. In alternative embodiments, the FFS-LCDs  100 ,  200 , and  300  can each be configured as a transflective mode LCD or a reflective mode LCD.  
      It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.