Abstract:
An exemplary transflective LCD ( 20 ) device includes first and second substrates ( 210, 220 ); a liquid crystal layer ( 23 ) interposed between the substrates; a common electrode ( 211 ) disposed at an inner surface of the first substrate; a transmission electrode ( 221 ) and a reflection electrode ( 222 ) disposed at an inner surface of the second substrate, with the reflection electrode defining an opening therein; a first retardation film ( 251 ) and a first polarizer ( 241 ) disposed at an outside surface of the first substrate; a second retardation film ( 252 ) and a second polarizer ( 242 ) disposed at an outside surface of the second substrate; and a discotic molecular film ( 261  and  862 ) disposed in a position selected from the group consisting of, between the first retardation film and the first substrate, and between the second retardation film and the second substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to an application by CHIU-LIEN YANG, WEI-YI LING and CHIA-LUNG LIN entitled LIQUID CRYSTAL DISPLAY DEVICE, filed after Dec. 1, 2005 but before the present application, and assigned to the same assignee as that of the present application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to liquid crystal display (LCD) devices, and more particularly to a reflection/transmission type LCD device capable of providing a display both in a reflection mode and a transmission mode.  
       BACKGROUND  
       [0003]     Conventionally, there have been three types of LCD devices commercially available: a reflection type LCD device utilizing ambient light, a transmission type LCD device utilizing backlight, and a semi-transmission type LCD device equipped with a half mirror and a backlight.  
         [0004]     With a reflection type LCD device, a display becomes less visible in a dim environment. In contrast, with a transmission type LCD device, a display becomes hazy in strong ambient light (e.g., outdoor sunlight). Thus researchers sought to provide an LCD device capable of functioning in both modes so as to yield a satisfactory display in any environment. In due course, a semi-transmission type LCD device was disclosed in Japanese Laid-Open Publication No. 7-333598.  
         [0005]     However, the above-mentioned semi-transmission type LCD device typically has the following problems.  
         [0006]     The semi-transmission type LCD device uses a half mirror in place of a reflective plate used in a reflection type LCD device, and has a minute transmission region (e.g., minute holes in a metal thin film) in a reflection region, thereby providing a display by utilizing transmitted light as well as reflected light. Since reflected light and transmitted light used for a display pass through the same liquid crystal layer, an optical path of reflected light is twice as long as that of transmitted light. This causes a large difference in retardation of the liquid crystal layer with respect to reflected light and transmitted light. Thus, a satisfactory display may not be obtained. Furthermore, a display in a reflection mode and a display in a transmission mode are superimposed on each other, so that the respective displays cannot be separately optimized. This results in difficulty in providing a color display, and tends to cause a blurred display.  
         [0007]     Accordingly, what is needed is an LCD device that can overcome the above-described deficiencies.  
       SUMMARY  
       [0008]     A transflective LCD device includes a first and a second substrate; a liquid crystal layer having homogeneous alignment liquid crystal molecules interposed between the first and second substrates; a common electrode disposed at an inner surface of the first substrate; a transmission electrode and a reflection electrode disposed at an inner surface of the second substrate, with the reflection electrode defining an opening therein, wherein a portion of the transmission electrode at the opening, a corresponding portion of the common electrode, and the liquid crystal layer therebetween form a transmission region, and the reflection electrode, a corresponding portion of the common electrode, and the liquid crystal layer therebetween form a reflection region; a first retardation film and a first polarizer disposed at an outside surface of the first substrate; a second retardation film and a second polarizer disposed at an outside surface of the second substrate; and a discotic molecular film disposed in a position selected from the group consisting of, between the first retardation film and the first substrate, and between the second retardation film and the second substrate.  
         [0009]     Other objects, advantages, and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic, side cross-sectional view of part of a transflective LCD device according to a first embodiment of the present invention.  
         [0011]      FIG. 2  shows a polarized state of light in each of certain layers of the transflective LCD device of  FIG. 1 , in respect of an on-state (no voltage applied) and an off-state (voltage applied) of the transflective LCD device, when the transflective LCD device operates in a reflection mode.  
         [0012]      FIG. 3  shows a polarized state of light in each of certain layers of the transflective LCD device of  FIG. 1 , in respect of an on-state (no voltage applied) and off-state (voltage applied) of the transflective LCD device, when the transflective LCD device operates in a transmission mode.  
         [0013]      FIG. 4  is a schematic, side cross-sectional view of selected layers of the transflective LCD device of  FIG. 1 , showing a discotic molecular film compensating a phase difference of a liquid crystal layer of the transflective LCD device while voltage is provided to the liquid crystal layer.  
         [0014]      FIG. 5  is a schematic, side cross-sectional view of part of a transflective LCD device according to a second embodiment of the present invention.  
         [0015]      FIG. 6  is a schematic, side cross-sectional view of part of a transflective LCD device according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]      FIG. 1  is a schematic, side cross-sectional view of part of a transflective LCD device  20  according to a first embodiment of the present invention. The LCD device  20  includes a first substrate  210 , a second substrate  220  disposed parallel to and spaced apart from the first substrate  210 , and a liquid crystal layer  23  having liquid crystal molecules (not labeled) sandwiched between the substrates  210  and  220 .  
         [0017]     A common electrode  211  and a first alignment film  271  are disposed on an inner surface of the first substrate  210 , in that order from top to bottom. A first discotic molecular film  261 , a first retardation film  251 , and a first polarizer  241  are disposed on an outer surface of the first substrate  210 , in that order from bottom to top. A transmission electrode  221 , an insulating layer  223 , a reflection electrode  222 , and a second alignment film  272  are disposed on an inner surface of the second substrate  220 , in that order from bottom to top. The insulating layer  223  and the reflection electrode  222  include an opening  225 . The second alignment film  272  covers the transmission electrode  221  at the opening  225 . A second retardation film  252  and a second polarizer  242  are disposed on an outer surface of the second substrate  220 , in that order from top to bottom.  
         [0018]     The first alignment film  271  has a rubbing direction parallel to that of the second alignment film  272 . An alignment direction of molecules of the first discotic molecular film  261  is parallel to the common rubbing direction of the alignment films  271  and  272 . A pre-tilt angle of the molecules of the first discotic molecular film  261  adjacent to the first substrate  210  is in the range from 0° to 45°, and a pre-tilt angle of the molecules of the first discotic molecular film  261  adjacent to the first retardation film  251  is in the range from 45° to 90°. The molecules of the first discotic molecular film  261  are negative liquid crystal molecules having a negative phase difference.  
         [0019]     A polarizing axis of the first polarizer  241  is perpendicular to that of the second polarizer  242 . A slow axis of the first retardation film  251  maintains an angle of 45° relative to the polarizing axis of the first polarizer  241 , and a slow axis of the second retardation film  252  maintains an angle of 45° relative to the polarizing axis of the second polarizer  242 . The slow axis of the first retardation film  251  is perpendicular to that of the second retardation film  252 . The first and second retardation films  251  and  252  are preferably quarter-wave plates.  
         [0020]     A portion of the transmission electrode  221  corresponding to the opening  225 , a corresponding portion of the common electrode  211 , and a corresponding portion of the liquid crystal layer  23  contained therebetween form a transmission region. The reflection electrode  222 , a corresponding portion of the common electrode  211 , and a corresponding portion of the liquid crystal layer  23  contained therebetween form a reflection region. The reflection electrode  222  is made of metal with a high reflective ratio, such as aluminum (Al) or an aluminum-neodymium (Al—Nd) alloy. The transmission electrode  221  and the common electrode  211  are made of a transparent conductive material, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO).  
         [0021]     The liquid crystal layer  23  in the transmission region has a thickness d 22 , and the liquid crystal layer  23  in the reflection region has a thickness d 21 . Typically, a retardation value of the liquid crystal layer  23  in the transmission region is in the range from 130 nm˜350 nm, and a retardation value of the liquid crystal layer  23  in the reflection region is in the range from 65˜175 nm. The liquid crystal molecules of the liquid crystal layer  23  are positive type liquid crystal molecules. The liquid crystal layer  23  is a homogeneous alignment liquid crystal layer.  
         [0022]      FIG. 2  shows a polarized state of light in each of certain layers of the LCD device  20  when the LCD device  20  operates in a reflection mode. When no voltage is applied to the LCD device  20 , the LCD device  20  is in an on-state (white state). Ambient incident light becomes linearly-polarized light having a polarizing direction parallel to that of the first polarizer  241  after passing through the first polarizer  241 . Thereafter, the linear-polarized light is incident upon the first retardation film  251  (a quarter-wave plate), and becomes circularly-polarized light. Then the circularly-polarized light is incident on the first discotic molecular film  261  and liquid crystal layer  23 . Since an effective phase difference of the first discotic molecular film  261  and the liquid crystal layer  23  in an on-state is configured to be a wavelength of λ/4 in order to obtain a white display, the incident circularly-polarized light becomes linearly-polarized light. The linearly-polarized light exiting the liquid crystal layer  23  is reflected by the reflection electrode  222 . The linearly-polarized light keeps its polarized state, and is incident on the liquid crystal layer  23  again. The linearly-polarized light passing through the liquid crystal layer  23  becomes circularly-polarized light having a polarizing direction opposite to that of the circularly-polarized light originally incident on the liquid crystal layer  23 . The circularly-polarized light exiting the liquid crystal layer  23  is converted to linearly-polarized light by the first retardation film  251 , and is output through the first polarizer  241  for displaying images.  
         [0023]     On the other hand, when a voltage is applied to the LCD device  20 , the LCD device  20  is in an off-state (black state). Up to the point where ambient incident light reaches the liquid crystal layer  23 , the ambient incident light undergoes transmission in substantially the same way as described above in relation to the LCD device  20  being in the on-state. Since an effective phase difference of the first discotic molecular film  261  and the liquid crystal layer  23  is configured to be 0 by applying a voltage in order to obtain a black display, the circularly-polarized light incident on the first discotic molecular film  261  and liquid crystal layer  23  passes therethrough as circularly-polarized light. The circularly-polarized light exiting the liquid crystal layer  23  is reflected by the reflection electrode  222 . The circularly-polarized light keeps its polarized state, and is incident on the liquid crystal layer  23  again. After passing through the liquid crystal layer  23  and first discotic molecular film  261  unchanged, the circularly-polarized light is converted into linearly-polarized light by the first retardation film  251  (a quarter-wave plate). At this time, the polarizing direction of the linearly-polarized light is rotated by about 90° compared with that of a white display state. Then the linearly-polarized light is absorbed by the first polarizer  241 . Thus the linearly-polarized light is not output from the LCD device  20  for displaying images.  
         [0024]      FIG. 3  shows a polarized state of light in each of certain layers of the LCD device  20  for an on-state (white state) and an off-state (black state) when the LCD device  20  operates in a transmission mode. Incident light undergoes transmission in a manner similar to that described above in relation to the LCD device  20  operating in the reflection mode. An effective phase difference of the first discotic molecular film  261  and liquid crystal layer  23  in an on-state is configured to be a wavelength of λ/2. An effective phase difference of the first discotic molecular film  261  and liquid crystal layer  23  in an off-state is configured to be 0.  
         [0025]      FIG. 4  shows a principle of the first discotic molecular film  261  compensating a phase difference of the liquid crystal layer  23  of the transflective LCD device  20  while a voltage is provided to the liquid crystal layer  23 . The liquid crystal molecules of the liquid crystal layer  23  may not be completely perpendicular to the substrates  210  and  220  while a voltage is provided thereto. Some of the liquid crystal molecules maintain an angle relative to the substrates  210  and  220 , with the angle generally decreasing along a direction from a middle of the liquid crystal layer  23  toward the substrate  210 , and similarly generally decreasing along a direction from the middle of the liquid crystal layer  23  toward the substrate  220 . The molecules of the first discotic molecular film  261  also maintain a pre-tilt angle relative to the substrates  210  and  220 . The liquid crystal molecules of the liquid crystal layer  23  have a positive phase difference, and the molecules of the first discotic molecular film  261  have a negative phase difference. The positive and negative phase differences counteract each other so as to compensate the effective phase difference of the liquid crystal layer  23 .  
         [0026]     With the above-described configuration, the first discotic molecular film  261  can compensate for any phase difference of the liquid crystal layer  23  due to the liquid crystal molecules of the liquid crystal layer  23  not being completely perpendicular to the substrates  210  and  220  when a voltage is provided to the liquid crystal layer  23 . This reduces leakage of light when the LCD device  20  in the off-state, and thereby increases a contrast of images displayed by the LCD device  20 . Moreover, the first discotic molecular film  261  can compensate contrast and color-shift of the LCD device  20  according to different viewing angles, so as to improve a wide viewing angle performance of the LCD device  20 .  
         [0027]      FIG. 5  is a schematic, side cross-sectional view of part of a transflective LCD device  80  according to a second embodiment of the present invention. The LCD device  80  has a structure similar to the LCD device  20 . However, the LCD device  80  includes a second discotic molecular film  862  disposed between a second retardation film  852  and a second substrate  820 . Further, only a first retardation film  851  and a first polarizer  841  are disposed on an outer surface of a first substrate  810 .  
         [0028]     An alignment direction of molecules of the second discotic molecular film  862  is parallel to that of alignment films  871  and  872 . A pre-tilt angle of molecules of the second discotic molecular film  862  adjacent to the second substrate  820  is in the range from 0° to 45°, and a pre-tilt angle of molecules of the second discotic molecular film  862  adjacent to the second retardation film  852  is in the range from 45° to 90°.  
         [0029]      FIG. 6  is a schematic, side cross-sectional view of part of a transflective LCD device  90  according to a third embodiment of the present invention. The LCD device  90  has a structure similar to the LCD device  20 . However, the LCD device  90  further includes a second discotic molecular film  962  disposed between a second retardation film  952  and a second substrate  920 . A first discotic molecular film  961 , a first retardation film  951 , and a first polarizer  941  are disposed on an outer surface of a first substrate  910 , in that order from bottom to top.  
         [0030]     An alignment direction of molecules of the second discotic molecular film  962  is parallel to that of alignment films  971  and  972 . A pre-tilt angle of molecules of the first discotic molecular film  961  adjacent to the first substrate  910  is the range from 0° to 45°. In this exemplary embodiment, the pre-tilt angle is 40°. A pre-tilt angle of molecules of the first discotic molecular film  961  adjacent to the first retardation film  951  is in the range from 45° to 90°. In this exemplary embodiment, the pre-tilt angle is 89°. A pre-tilt angle of molecules of the second discotic molecular film  962  adjacent to the second substrate  920  is the range from 0° to 45°. In this exemplary embodiment, the pre-tilt angle is 40°. A pre-tilt angle of molecules of the second discotic molecular film  962  adjacent to the second retardation film  952  is in the range from 45° to 90°. In this exemplary embodiment, the pre-tilt angle is 89°. The molecules of the first and second discotic molecular films  961  and  962  are negative liquid crystal molecules having a negative phase difference.  
         [0031]     It is to be understood, however, that even though numerous charcteristics and advantages of the present embodiments have been set out in the forgoing description, together with details of the structures and functions of the embodiments, 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.