Abstract:
A liquid crystal display device (LCD) including an OCB mode liquid crystal layer is provided, in which an even number of domains are formed in one unit pixel, the number of domains being at least two, and the arrangement of liquid crystal (LC) molecules in the domains is controlled so that the LCD has wide vertical and horizontal viewing angles. Also, a disclination line formed in the unit pixel allows the LC molecules to readily make a transition to an initial bend phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0098263, filed Nov. 26, 2004, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquid crystal display device (LCD) and, more particularly, to an LCD having wide vertical and horizontal viewing angles and including a liquid crystal layer that easily makes a transition to an initial bend phase. 
   2. Description of the Related Art 
   Nowadays, in order to overcome the shortcomings of conventional display devices such as cathode ray tubes (CRTS) that are -heavy and large-sized, much attention is being paid to flat panel display devices (FPDs), for example, LCDs, organic light emitting display devices (OLEDs), and plasma display panels (PDPs). 
   In a typical LCD, liquid crystals are injected between two substrates including electrodes for generating an electric field. The electric field is generated between the substrates by the application of different electric potentials to the electrodes, so that the arrangement of liquid crystal (LC) molecules is changed. Thus, optical transmittance is controlled so as to display an image on a screen. 
     FIGS. 1A and 1B  are perspective views illustrating an operating principle of a twisted nematic (TN) mode LCD, which is the most representative type of LCD. 
   Referring to  FIG. 1A , the TN mode LCD includes a first substrate  101 , a second substrate  103 , a light source (not shown), and a liquid crystal layer  106 . A first electrode (not shown) and a first alignment layer (not shown) are formed on one surface of the first substrate  101 , and a first polarizer  102  is formed on the other surface thereof. A second electrode (not shown) and a second alignment layer (not shown) are formed on one surface of the second substrate  103 , and a second polarizer  104  is formed on the other surface thereof. The light source supplies light  105  from above the other surface of the second substrate  103 . The liquid crystal layer  106  is filled between the first and second substrates  101  and  103 . 
   A polarization axis of the first polarizer  102  is located in the same direction as a direction  107  in which the first alignment layer formed on one surface of the first substrate  101  is rubbed. A direction  108  in which the second alignment layer formed on one surface of the second substrate  103  is rubbed is perpendicular to the direction  107  in which the first alignment layer is rubbed. A polarization axis of the second polarizer  104  is located in the same direction as the direction  108  in which the second alignment layer is rubbed. 
   Referring to  FIG. 1A , in an inactivated state where no voltage is applied between the first and second electrodes, it can be seen that the major axes (i.e., local optical axes) of LC molecules of the liquid crystal layer  106  filled between the first and second substrates  101  and  103  are gradually twisted due to the directions  107  and  108  in which the first and second alignment layers are rubbed. Thus, an LC molecule  106   a  close to one surface of the first substrate  101  is twisted at an angle of 90° to an LC molecule  106   b  close to one surface of the second substrate  103 . As a result, the light  105  emitted from the light source is linearly polarized by the second polarizer  104  (refer to  105   a ), rotated by the LC molecules of the liquid crystal layer  106  (refer to  105   b ), and then externally emitted through the first polarizer  102  having the polarization axis perpendicular to that of the second polarizer  104  (refer to  105   c ). 
   Referring to  FIG. 1B , in an activated state where a voltage is applied between the first and second electrodes, LC molecules of the liquid crystal layer  106  are not twisted any more and become parallel to each other due to an electric field generated by the voltage. Thus, light  105   a  obtained by linearly polarizing the incident light  105  is not rotated any more and is wholly absorbed in the first polarizer  102 . 
   However, the TN mode LCD has narrow horizontal and vertical viewing angles and a slow response speed. Thus, optically compensated bend (OCB) mode LCDs have been proposed to solve the problems of the TN mode LCD, but it is still necessary to improve horizontal and vertical viewing angles. 
   SUMMARY OF THE INVENTION 
   An exemplary embodiment of the present invention, therefore, solves aforementioned problems associated with conventional devices and methods by providing a liquid crystal display device (LCD), which has wide vertical and horizontal viewing angles and includes a liquid crystal layer that easily makes a transition to an initial bend phase. 
   In an exemplary embodiment of the present invention, an LCD includes a first substrate including a first alignment layer and a first electrode; a second substrate opposite to the first substrate and including a second alignment layer and a second electrode; and an OCB mode liquid crystal layer disposed between the first and second substrates and including an even number of domains in one unit pixel, the number of domains in the one unit pixel being at least two, wherein liquid crystal molecules in two adjacent domains are arranged in directions that are perpendicular to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the present invention will be described in reference to certain exemplary embodiments thereof with reference to the attached drawings in which: 
       FIGS. 1A and 1B  are perspective views illustrating an operating principle of a TN mode liquid crystal display device (LCD), which is the most representative type of LCD; 
       FIGS. 2A ,  2 B and  2 C are perspective views illustrating an operating principle of an OCB mode LCD; 
       FIGS. 3A and 3B  are diagrams illustrating viewing angles of a TN mode LCD and an OCB mode LCD; and 
       FIGS. 4 ,  5 A,  5 B, and  6  are plan views and a cross-sectional view, respectively, of an OCB mode LCD according to an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention will now be described in detail with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The thicknesses of layers or regions shown in the drawings are exaggerated for clarity. The same reference numerals are used to denote the same elements throughout the specification. 
     FIGS. 2A ,  2 B and  2 C are perspective views illustrating an operating principle of an OCB mode LCD. 
   Referring to  FIG. 2A , the OCB mode LCD includes a first substrate  201 , a second substrate  203  opposite to the first substrate  201 , and an OCB mode liquid crystal layer  205 . A first electrode (not shown) and a first alignment layer (not shown) are formed on one surface of the first substrate  201 , and a first polarizer  202  is formed on the other surface thereof. A second electrode (not shown) and a second alignment layer (not shown) are formed on one surface of the second substrate  203 , and a biaxial compensation film (not shown) and a second polarizer  204  are formed on the other surface thereof. The liquid crystal layer  205  is filled (i.e., disposed) between the first and second substrates  201  and  203 . 
   In this case, the polarization axes of the first and second polarizers  202  and  204  are perpendicular to each other. 
   The liquid crystal layer  205  has a thickness of 1.5 to 2.5 μm. Thus, a space between the first and second substrates  201  and  203  is also 1.5 to 2.5 μm. The liquid crystal layer  205  typically includes liquid crystals with positive dielectric anisotropy. 
   In this case, a direction  206   a  in which the first alignment layer is rubbed is the same as a direction  206   b  in which the second alignment layer is rubbed. In other words, the first and second alignment layers are formed in the same direction. The first and second alignment layers are formed such that LC molecules, which are filled between the first and second substrates  201  and  203  and close to the first and second alignment layers, have a pretilt angle of 5° to 20°. Also, each of the first and second alignment layers is formed to a thickness of 500 to 1000 Å (where Å is equal to 10 −8  cm). 
   When no voltage is applied between the first and second electrodes, the LC molecules of the liquid crystal layer  205  are naturally arranged in a splay phase due to the properties of the first and second alignment layers, the thickness of the liquid crystal layer  205 , and/or the intrinsic properties of the LC molecules. 
   Referring to  FIG. 2B , when a voltage for phase transition to the bend phase is applied between the first and second electrodes, because a line  208  that connects the centers of the LC molecules is affected by the directions  206   a  and  206   b  in which the first and second alignment layers are rubbed, the LC molecules of the liquid crystal layer  205  are arranged in a convex bend phase in the rubbed directions  206   a  and  206   b  with respect to a vertical line  207  perpendicular to the first and second substrates  201  and  203 . Here, the phase transition voltage is about 25 to 30 V. 
   In this case, the liquid crystal layer  205  that is phase-transitioned to the bend phase keeps almost the same shape in one domain. Typically, each unit pixel of an LCD includes one domain. 
   Light  209 , is incident on the other surface of the first substrate  201 , linearly polarized by the first polarizer  202 , transmitted through the phase-transitioned liquid crystal layer  205 , and externally emitted through the second substrate  203  and the second polarizer  204 . While the light  209  is being transmitted through the phase-transitioned liquid crystal layer  205 , birefringence is caused by the liquid crystal layer  205 . 
   Referring to  FIG. 2C , when a driving voltage is applied between the first and second electrodes, the LC molecules of the liquid crystal layer  205  are rearranged such that the line  208  that connects the centers of the LC molecules is similar to the vertical line  207 , and the light  209  cannot be transmitted through the second polarizer  204  because birefringence is not caused any more. 
   Thereafter, when no driving voltage is applied between the first and second electrodes, the liquid crystal layer  205  is arranged to the bend phase as described with reference to  FIG. 2B  so that the light  209  can be transmitted again. At this time, the phase transition voltage is maintained between the first and second electrodes. 
     FIGS. 3A and 3B  are diagrams illustrating viewing angles of the TN mode LCD as described with reference to  FIGS. 1A and 1B  and the OCB mode LCD as described with reference to  FIGS. 2A and 2B . Specifically,  FIG. 3A  shows the color shifts of the TN mode and OCB mode LCDs, while  FIG. 3B  shows the contrast ratios thereof. 
   In this case, each of the TN mode LCD and the OCB mode LCD has 854×480 pixels, a pixel pitch of 0.2865×0.2865 mm, and a contrast ratio of 400:1. 
   Referring to  FIGS. 3A and 3B , when a color shift  301   a  and a contrast ratio  301   b  of the TN mode LCD are compared with a color shift  302   a  and a contrast ratio  302   b  of the OCB mode LCD, it can be observed that the OCB mode LCD has better characteristics. 
   However, given only the color shift  302   a  and the contrast ratio  302   b  of the OCB mode LCD, it can be seen that horizontal characteristics are excellent, but vertical characteristics  303   a  and  303   b  are not very good. That is, a horizontal viewing angle is 160°, whereas a vertical viewing angle is 140°. 
     FIGS. 4 ,  5 A,  5 B, and  6  are plan views and a cross-sectional view, respectively, of an OCB mode LCD according to an exemplary embodiment of the present invention.  FIGS. 5A and 5B  are magnified plan views of 3 unit pixels of  FIG. 4 , and  FIG. 6  is a cross-sectional view taken along the line I-I′ of  FIG. 4 . 
   Referring to  FIG. 4 , the OCB mode LCD including an emission region and a peripheral region is disposed on a substrate  401 , which is a glass substrate or a plastic substrate. The emission region includes unit pixels  404  defined by gate lines  402  and data lines  403 . The peripheral region includes a gate driver  405  and a data driver  406 . In this case, each of the unit pixels  404  is one of red (R), green (G), and blue (B) pixels. 
   While not shown in the drawings, a first electrode and a first alignment layer are formed in the unit pixel  404  of the emission region. Also, a thin film transistor (TFT) and a capacitor may be formed in the unit pixel  404  of the emission region. 
   Referring to  FIGS. 5A and 5B , which are plan views illustrating directions in which first alignment layers are rubbed in a region A including three unit pixels (i.e., red, green, and blue pixels) of  FIG. 4 , the first alignment layer formed in each of the unit pixels  404 , namely, a red pixel  404   r , a green pixel  404   g , and a blue pixel  404   b , includes a first domain  450   a  and a second domain  450   b , which are rubbed in different directions. Specifically, the first domain  450   a  is formed in one direction of a first rubbing direction  451   a  for rubbing the first alignment layer from an upper-left end of the unit pixel toward a lower-right end thereof, a second rubbing direction  451   b  for rubbing the first alignment layer from a lower-left end of the unit pixel toward an upper-right end thereof, a third rubbing direction  451   c  for rubbing the first alignment layer from the upper-right end toward the lower-left end, and a fourth rubbing direction  451   d  for rubbing the first alignment layer from the lower-right end toward the upper-left end. A second domain  450   b  is formed in one direction of the first through fourth rubbing directions  451   a ,  451   b ,  451   c , and  451   d  other than the rubbing direction in which the first domain  450   a  is formed. Further, the first and second domains  450   a  and  450   b  should be rubbed in directions that are perpendicular to each other. 
   The first and second alignment layers may be formed using a spinning process, a dipping process, a roller coating process, or any other suitable process. By way of example, the first and second alignment layers may be formed using a roller coating process. That is, in order to form the first and second alignment layers, when each of the first and second alignment layers is rubbed in respectively different directions, a pattern mask for opening only domains that will be rubbed in one direction is formed, the alignment layer is rubbed, and the pattern mask is removed. Thereafter, domains that will be rubbed in another direction are rubbed using another pattern mask in the same manner as described above. 
   As the unit pixel  404  includes the first and second domains  450   a  and  450   b  rubbed in directions that are perpendicular to each other, the vertical viewing angle described with reference to  FIGS. 3A and 3B  can be improved. This is because when one unit pixel includes two domains rubbed in respectively different directions, a portion of the alignment layer rubbed in one direction can make up for another portion of the alignment layer rubbed in another direction. 
   A disclination line  452  is generated between the first and second domains  450   a  and  450   b  that are rubbed in different directions. Similar to disclination lines between unit pixels, the disclination line  452  in a unit pixel lowers a phase transition voltage. 
     FIGS. 5A and 5B  illustrate two selected from exemplary embodiments in which the red pixel  404   r , the green pixel  404   g , and the blue pixel  404   b  are formed using a combination of the first, second, third, and fourth rubbing directions  451   a ,  451   b ,  451   c , and  451   d . Although  FIGS. 5A and 5B  illustrate that one unit pixel includes two domains, other even-numbered domains, such as four, six, or eight domains, can be formed in one unit pixel. 
   Referring to  FIG. 6 , first electrodes  502   a  and  502   b  are formed on one surface of a first substrate  501 , which is a glass substrate or a plastic substrate, and a first alignment layer  503  is formed on the substrate  501 . 
   The first alignment layer  503  is formed on the first electrodes  502   a  and  502   b  such that it includes a first domain and a second domain rubbed in the direction perpendicular to each other as described with reference to  FIGS. 5A and 5B . 
   A second electrode  602  and a second alignment layer  603  are formed on a second substrate  601 , which is a glass substrate or a plastic substrate. The second alignment layer  603  corresponds to the first alignment layer  503  such that domains rubbed in the same direction face each other. 
   The first electrodes  502   a  and  502   b  and the second electrode  602  are formed of transparent conductive insulating materials, such as indium tin oxide (ITO) or indium zinc oxide (IZO). Each of the first electrodes  502   a  and  502   b  has a thickness of 1000 to 3000 Å and a width of 30 to 60 μm, and a space between the first electrodes  502   a  and  502   b  is about 5 to 15 μm. 
   In this case, a color filter  605  including a black matrix (BM)  604  may be formed between the second electrode  602  and the second substrate  601 . 
   Also, a first polarizer  504  may be adhered to the other surface of the first substrate  501 . Also, a light source device, i.e., a backlight including a light source having a red/green/blue LED or a white (W) LED, a reflector sheet, and a diffuser sheet may be adhered on the first polarizer  504 . 
   Further, a biaxial compensation film  606  and a second polarizer  607  may be formed on the other surface of the second substrate  601 . Here, the polarization axes of the first and second polarizers  504  and  607  are perpendicular to each other. 
   Thereafter, an OCB mode liquid crystal layer  701  is filled between the first and second substrates  501  and  601 , thereby completing the OCB mode LCD. 
   When a phase transition voltage is applied between the first electrodes  502   a  and  502   b  and the second electrode  602  of the completed OCB mode LCD, in each of liquid crystal layers  701   a  and  701   b , which is disposed in one unit pixel (i.e., on one of the first electrodes  502   a  and  502   b ), two domains  703   a  and  703   b  are arranged symmetrically with respect to a disclination line  702   a  formed in the unit pixel, due to the first and second alignment layers  503  and  603 . Also, LC molecules in the domain  703   a  are arranged in the direction perpendicular to the LC molecules in the domain  703   b . The LC molecules of one unit pixel in other embodiments may be arranged in other even-numbered domains, such as four, six, or eight domains. 
   In the exemplary embodiments of the present invention as described above, vertical and horizontal viewing angles can be widened, and the transition to a bend phase can be easily made by forming a disclination line in a unit pixel. 
   Although the present invention has been described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the present invention defined in the appended claims, and their equivalents.