Patent Publication Number: US-2005140902-A1

Title: In plane switching liquid crystal display with transflector

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to in plane switching liquid crystal displays (IPS-LCDs), and especially to a transflective mode IPS-LCD having low power consumption.  
      2. Description of the 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 more traditional 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 in a liquid crystal layer between the substrates. The resulting electric field is substantially planar and parallel to a surface of said same substrate. This configuration provides an improved viewing angle for the IPS-LCD.  
      Referring to  FIG. 3 , this is a cross-sectional representation of a conventional IPS-LCD  1 . The IPS-LCD  1  is a transmissive mode LCD, which utilizes a backlight for illuminating. The IPS-LCD  1  comprises an upper substrate  10  and a lower substrate  20  disposed opposite to each other and spaced apart a predetermined distance. A liquid crystal layer (not labeled) having a plurality of liquid crystal molecules  30  is disposed between the upper and lower substrates  10 ,  20 . A plurality of counter electrodes  11  and a plurality of pixel electrodes  13  are disposed on the lower substrate  20 , with an insulating layer  12  and an alignment film  14  disposed on the counter and pixel electrodes  11 ,  13 , in that order from bottom to top. A lower polarizer  41  is formed on an undersurface of the lower substrate  20 , the lower polarizer  41  being an ordinary type polarizer. A color filter  15  and an alignment film  16  are disposed on an undersurface of the upper substrate  10 , in that order from top to bottom.  
      An upper polarizer  40  is formed on a top surface of the upper substrate  10 , the upper polarizer  40  also being an ordinary type polarizer. Polarization axes of the upper polarizer  40  and the lower polarizer  41  are perpendicular to each other.  
      The color filter  15  comprises a black matrix (not shown), and a color resin layer (not shown) having Red, Green and Blue segments. The color filter  15  is disposed between (but not adjacent) the upper polarizer  40  and the liquid crystal layer. The color filter  15  has a de-polarizing effect on light beams passing therethrough due to pigment light scattering, therefore light beams passing through the IPS-LCD  1  are at least partially de-polarized by the color filter  15  before reaching the upper polarizer  40 . This de-polarizing of the light beams prior to them reaching the upper polarizer  40  can reduce the contrast ratio of the IPS-LCD  1 . Even though such de-polarizing effects are small, they can have a significant effect on the contrast ratio of the IPS-LCD  1 .  
      The counter electrodes  11  and the pixel electrodes  13  are strip-shaped, and are arranged parallel to each other in alternating fashion. The counter electrodes  11  and the pixel electrodes  13  are transparent conductors, being made of a material such as indium tin oxide (ITO). When the IPS-LCD  1  is driven, an electric field having a component parallel to main surfaces of the substrates  10 ,  20  is formed at upper portions of the counter electrodes  11  and the pixel electrodes  13 . The liquid crystal molecules  30  disposed over the counter and pixel electrodes  11 ,  13  are driven, thus giving the IPS-LCD  1  an improved wide viewing angle compared to that of a TN-LCD.  
      However, the IPS-LCD  1  is a transmissive mode LCD, which depends on a backlight for illumination. The backlight typically uses 50% or more of the total power consumed by the IPS-LCD  1 .  
      It is desired to provide an IPS-LCD that can solve the above-mentioned high power consumption problem.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide an in plane switching liquid crystal display which has low power consumption.  
      To achieve the above object, an in plane 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, with a plurality of liquid crystal molecules contained in the liquid crystal layer. A plurality of pixel electrodes and a plurality of counter electrodes are formed on the first substrate in alternating fashion. Two polarizers are disposed on the first substrate and the second substrate, respectively. A transflector capable of transmitting and reflecting light beams is disposed on the first substrate. The in plane switching liquid crystal display can thus utilize ambient light beams as well as light beams provided by a backlight, so that the in plane switching liquid crystal display has low power consumption.  
      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, cross-sectional view of part of an IPS-LCD according to a first embodiment of the present invention;  
       FIG. 2  is a schematic, cross-sectional view of part of an IPS-LCD according to a second embodiment of the present invention; and  
       FIG. 3  is a schematic, cross-sectional view of part of a conventional IPS-LCD. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a schematic, cross-sectional view of an in plane switching liquid crystal display (IPS-LCD)  2  according to the first embodiment of the present invention. The IPS-LCD  2  comprises a first substrate  220 , a second substrate  210 , and a liquid crystal layer (not labeled) having a plurality of liquid crystal molecules  230 . The first substrate  220  and the second substrate  210  are spaced apart from each other, and the liquid crystal layer is disposed therebetween. The first substrate  220  and the second substrate  210  are made of glass. Alternatively, the first substrate  220  and the second substrate  210  can be made of silicon dioxide (SiO 2 ).  
      A plurality of gate bus lines (not shown) and a plurality of data bus lines (not shown) are cross-arranged on an inner surface of the first substrate  220 . A plurality of thin film transistors (not shown) is disposed at intersections of the gate bus lines and the data bus lines. A plurality of counter electrodes  211  and a plurality of pixel electrodes  213  are disposed on the first substrate  220 , with a transparent insulating layer  212  and an alignment film  214  disposed on the counter and pixel electrodes  211 ,  213  in that order from bottom to top. A transflector  217  is interposed between the counter and pixel electrodes  211 ,  213  and the first substrate  220 . A color filer  215  and an alignment film  216  are formed on an underside of the second substrate  210 , in that order from top to bottom. Two polarizers  241 ,  240  are formed on two outer surfaces of the first substrate  220  and the second substrate  210 , respectively.  
      The alignment films  214 ,  216  are horizontal alignment layers. Alignment directions of the alignment films  214 ,  216  are parallel to each other, or alternatively an angle of  180  degrees may be formed between the alignment directions. Polarization axes of the polarizers  241 ,  240  are perpendicular to each other. The polarizers  241 ,  240  are ordinary type polarizers. The alignment direction of the alignment film  214  is parallel to the polarization axis of the polarizer  241 .  
      The counter electrodes  213  and the pixel electrodes  211  are strip-shaped, and are arranged parallel to each other in alternating fashion on the transflector  217 . The counter electrodes  213  and the pixel electrodes  211  are made of a transparent conductor, such as indium tin oxide (ITO) or indium zinc oxide (IZO). The transparent insulating layer  212  is made of SiO 2  or silicon nitride (SiNx), to prevent the counter electrodes  213  and the pixel electrodes  211  from shorting and to protect them.  
      The transflector  217  is a multiple-layered structure, with ITO layers (not shown) and titanium dioxide (TiO 2 ) layers (not shown) stacked one on the other in alternating fashion. Each ITO layer has a high refractive index, and each TiO 2  layer has a low refractive index. Therefore, the multiple-layer structure can reflect light beams as well as transmit light beams. A desired ratio of light beams reflected by the transflector  217  to light beams transmitted by the transflector  217  is obtained by selecting the thicknesses and the refractive indexes of the layers having different refractive indexes accordingly.  
      The color filter  215  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. The black matrix is used to prevent light beams from leaking from the IPS-LCD  2 , and to protect the thin film transistors from damage.  
      When no voltage is applied to the counter electrodes  213  and pixel electrodes  211 , the liquid crystal molecules  230  are arranged substantially parallel to the first substrate  220  and the second substrate  210 . The state of polarization of light beams is not changed when they pass through the liquid crystal layer. Therefore the light beams, whether they be light beams from a backlight or light beams from the ambient environment, cannot pass through the polarizer  240 . As a result, the IPS-LCD  2  is in a dark state. When a voltage is applied to the counter electrodes  213  and the pixel electrodes  211 , an electric field having horizontal components is produced therebetween. Long axes of the liquid crystal molecules  230  are parallel to the direction of the horizontal components of the electric field. The polarization state of light beams is changed when the light beams pass through the liquid crystal layer. Therefore the light beams can pass through the polarizer  240 . As a result, the IPS-LCD  2  is in a white state.  
      The transflector  217  can reflect ambient light beams and transmit light beams from a backlight for display. Therefore, the IPS-LCD  2  is a transflective mode liquid crystal display. Because the transflective mode LCD can make use of both internal and external light sources, it can be operated in a bright ambient light environment with low power consumption. Further, the IPS-LCD  2  has a wide viewing angle compared to other kinds of LCDs.  
      Referring to  FIG. 2 , this shows a schematic, cross-sectional view of an IPS-LCD  3  according to the second embodiment of the present invention. The IPS-LCD  3  is the same as the IPS-LCD  2  of the first embodiment, except that the IPS-LCD  3  has a transflector  317  disposed between a first substrate  320  and a polarizer  341 .  
      In alternative embodiments, the transflector  217 ,  317  of the present invention may be made of a material having high reflectivity, such as aluminum or silver. Further, the transflector  217 ,  317  may define a plurality of holes therein for free transmission of light beams therethrough. The holes may, for example, be circular, elliptical or polygonal. A desired ratio of light beams reflected by the transflector  217 ,  317  to light beams transmitted by the transflector  217 ,  317  is obtained by configuring areas of the holes accordingly.  
      In other alternative embodiments, at least one of the polarizers  240 ,  241 ,  341  of the present invention may be an extraordinary type polarizer. The extraordinary type polarizer can improve a transmission ratio and an aperture ratio of the IPS-LCD  2 ,  3 . Further, the polarizer  240  can be disposed between the color filter  215  and the alignment film  216 . This arrangement reduces or eliminates the adverse effects of color filter de-polarizing, and yields a higher contrast ratio.  
      In still another alternative embodiment, the counter electrodes  211  and the pixel electrodes  213  are disposed on the second substrate  210  and the first substrate  220  respectively, in order to generate an electric field having horizontal components parallel to the surfaces of the first substrate  220  and the second substrate  210 .  
      In summary, the IPS-LCD  2 ,  3  of the present invention is a transflective mode liquid crystal display, which can make use of both internal and external light sources. Therefore, the IPS-LCD  2 ,  3  can be operated in a bright ambient light environment with low power consumption. Furthermore, the IPS-LCD  2 ,  3  has a wide viewing angle compared to other kinds of LCDs.  
      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.