Patent Publication Number: US-2013229605-A1

Title: Liquid crystal display and optical compensation film therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2012-0022388, filed on Mar. 5, 2012, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments of the present invention relate to an optical compensation film and a liquid crystal display including the same. 
     2. Discussion of the Background 
     Currently, a liquid crystal display (LCD) of a TN (twisted nematic) type is used as a monitor. In the TN LCD, the nematic liquid crystal material is horizontally aligned between two substrates while having a slight pre-tilt angle, and an azimuth angle of the liquid crystal molecules is twisted from one substrate to the other substrate by about 90 degrees. By applying an electric field in a vertical direction to the liquid crystal layer of the TN LCD, a director of the liquid crystal is controlled such that optical transmittance is controlled, thereby displaying images. 
     In the TN method, as opposed to a VA (vertical alignment) method or an IPS (in-plane switching) method, an average direction of the liquid crystal director may be toward a lower side, and in this case, deterioration of display quality may be generated when viewing the display device at vertical viewing angles. However, the display quality in the right and left directions is excellent. 
     To compensate for the deterioration of display quality at the vertical viewing angles of the TN LCD, a wide viewing angle (WV) film is used. 
     However, as a result of a limitation of a characteristic of the wide viewing angle film, the liquid crystal layer capable of compensating the viewing angle has a limited characteristic range. Accordingly, the display quality improvement of the transmittance of the liquid crystal display may be limited. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art. 
     SUMMARY 
     Exemplary embodiments of the present invention provide a compensation film having improved transmittance of a liquid crystal display. 
     Exemplary embodiments of the present invention also provide a liquid crystal display having excellent transmittance and an excellent viewing angle in all directions. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     An exemplary embodiment of the invention discloses a compensation film for a liquid crystal display including a phase difference induction layer and a supporting layer supporting the phase difference induction layer. The compensation film has an in-plane phase difference R 0  of about 40 nm-60 nm, a thickness phase difference Rth of about 140 nm-170 nm, and an average inclination angle β of about 15°-19°. 
     An exemplary embodiment of the present invention also discloses a liquid crystal display including: a first insulation substrate; a second insulation substrate facing the first insulation substrate; a first electrode disposed on at least one of the first insulation substrate and the second insulation substrate; a second electrode disposed on at least one of the first insulation substrate and the second insulation substrate; a liquid crystal layer disposed between the first insulation substrate and the second insulation substrate; and a first compensation film and a second compensation film respectively disposed outside the first insulation substrate and the second insulation substrate. The first compensation film and the second compensation film have an in-plane phase difference R 0  of about 40 nm-60 nm, a thickness phase difference Rth of about 140 nm-170 nm, and an average inclination angle β of about 15°-19°. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a schematic cross-sectional view of a liquid crystal display including a compensation film according to an exemplary embodiment of the present invention. 
         FIG. 2  is a perspective view of a compensation film according to the exemplary embodiment of the present invention. 
         FIG. 3  is a view showing experimental data for a viewing angle at up/down/left/right directions when applying compensation films of various characteristics to a high transmittance liquid crystal display. 
         FIG. 4  is a graph showing a wavelength dispersion characteristic of a supporting film of a compensation film according to the exemplary embodiment of the present invention and a conventional supporting film. 
         FIG. 5  is a view showing a change of a polarization state in a Poincare sphere color coordinate when red, green, and blue polarized light passes through a supporting film of a compensation film according to the exemplary embodiment of the present invention. 
         FIG. 6  is a view showing a change of a polarization state in a Poincare sphere color coordinate when red, green, and blue polarized light passes through a conventional supporting film. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ). 
     A compensation film and a liquid crystal display including the same according to an exemplary embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a schematic cross-sectional view of a liquid crystal display including a compensation film according to an exemplary embodiment of the present invention, and  FIG. 2  is a perspective view of a compensation film according to an exemplary embodiment of the present invention. 
     A liquid crystal display as a TN (twisted nematic) liquid crystal display including a nematic liquid crystal material shown in  FIG. 1  includes a lower panel  100 , an upper panel  200  facing the lower panel  100 , a liquid crystal layer  300  interposed between the lower panel  100  and the upper panel  200 , a lower compensation film  410  and an upper compensation film  420  respectively disposed outside the lower panel  100  and the upper panel  200 , and a lower polarization film  21  and an upper polarization film  22  positioned “outside” the lower compensation film  410  and the upper compensation film  420 , which means that the lower and upper polarization films  21 ,  22  are each respectively disposed on a side of the lower and upper compensation films  410 ,  420  which is opposite the side facing the liquid crystal layer  300 . Both lower and upper compensation films  410 ,  420  are not required in the present invention, and the relative positions of compensation film/polarization film may be swapped. 
     The lower panel  100  includes, for example, an insulation substrate  110 , a thin film transistor  120  disposed on the insulation substrate  110 , and a pixel electrode  130  connected to the thin film transistor  120 . The lower panel  100  also includes signal lines (not shown), such as gate lines and data lines, and an alignment layer (not shown) formed on the thin film transistor  120  and the pixel electrode  130 . 
     The upper panel  100  includes, for example, an insulation substrate  210 , a black matrix  220 , and a color filter  230  disposed on a lower surface of the insulation substrate  210 , and a common electrode  270  formed on the black matrix  220  and the color filter  230 . An alignment layer (not shown) is formed on the common electrode  270 . 
     The liquid crystal layer  300  is formed between the lower panel  100  and the upper panel  200 . In the liquid crystal layer  300 , the liquid crystal is aligned with a twisted nematic mode and has a retardation Δnd of 420 nm-470 nm. For example, if the liquid crystal has a refractive anisotropy Δn of 0.141 and the thickness of the liquid crystal layer  300  is 3.2 μm, the retardation of the liquid crystal layer  300  becomes about 452 nm. If the liquid crystal layer  300  has a retardation Δnd of 420 nm-470 nm, high light transmittance may be obtained as compared with a conventional liquid crystal display using a liquid crystal layer having retardation of about 410 nm. 
     Table 1 below shows changes in optical characteristics, such as transmittance and luminance, when only retardation of the liquid crystal layer is changed in the liquid crystal display of the same structure (in cases of 410 nm and 452 nm). As shown in Table 1, the case of a liquid crystal layer having a retardation of 452 nm has greater transmittance and luminance values than the case of a liquid crystal layer having a retardation of 410 nm. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Case 1 
                 Case 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Optical 
                 Actually measured Δnd 
                 410 nm 
                 452 nm 
               
            
           
           
               
               
               
               
               
            
               
                 characteristic 
                 Trans- 
                  9 positions 
                 4.82 
                 5.01 (3.84% 
               
               
                   
                 mittance 
                   
                   
                 improvement) 
               
               
                   
                   
                 Center 
                 4.77 
                 4.95 (3.67% 
               
               
                   
                   
                   
                   
                 improvement) 
               
               
                   
                 Luminance 
                 13 positions 
                 220.7 
                 227.4 (3.04% 
               
               
                   
                   
                   
                   
                 improvement) 
               
               
                   
                   
                 Center 
                 230.8 
                 246.8 (6.91% 
               
               
                   
                   
                   
                   
                 improvement) 
               
            
           
           
               
               
               
            
               
                 Response speed (Tr/Tf) 
                 5.4 ms 
                 5.0 ms (1.6/3.4) 
               
               
                   
                 (1.7/3.7) 
               
               
                 Cr 
                 971 
                 950 
               
               
                   
               
            
           
         
       
     
     The compensation films  410  and  420  include a supporting layer  41  and a phase difference induction layer  42 . The supporting layer  41  may be made of a TAC (triacetyl cellulose) film, and has a positive wavelength dispersion characteristic. The phase difference induction layer  42  may be formed by including a discotic liquid crystal that is hybrid-aligned. As shown in  FIG. 2 , the hybrid alignment means that the angles θ 1  and θ 2  at which the disc-type discotic liquid crystal is inclined with respect to the surface of the supporting layer  41  decreases as distance from the supporting layer  41  increases. That is, the discotic liquid crystal is inclined with a further spread shape the further away the discotic liquid crystal is from the supporting layer  41 . An average of the inclination angle β ((θ 1 +θ 2 )/2) of the discotic liquid crystal forming the phase difference induction layer  42  is 15°-19°. An in-plane phase difference R 0  of the compensation film is 40 nm-60 nm, and a thickness phase difference Rth is 140 nm-170 nm. 
     The compensation films  410  and  420  may be manufactured by coating a discotic liquid crystal material on a TAC film and curing the coated discotic liquid crystal layer with an appropriate condition. 
       FIG. 3  is a view showing experimental data for a viewing angle at up/down/left/right directions when applying compensation films of various characteristics to a high transmittance liquid crystal display. In the present experimental example, the liquid crystal display includes a liquid crystal layer having a retardation (Δnd) of 452 nm. 
     With reference to  FIG. 3 , among the several compensation films, when the compensation film (in the case B 2  of  FIG. 3 ) having an in-plane phase difference R 0  of 50 nm, a thickness phase difference Rth of 155 nm, and an average inclination angle β of 17° is applied to the liquid crystal display including the liquid crystal layer  300  having a retardation of 452 nm, a viewing angle of more than 80 degrees may be obtained in up/down/left/right directions. 
     The supporting layer  41  of the compensation films  410  and  420  has a positive wavelength dispersion characteristic. By adding an additive to the TAC (triacetyl cellulose), a supporting layer  41  having a positive wavelength dispersion characteristic may be formed. A is supporting layer  41  having a positive wavelength dispersion characteristic produces the effect described with reference to  FIG. 4  to  FIG. 6 . 
       FIG. 4  is a graph showing a wavelength dispersion characteristic of a supporting film of a compensation film according to an exemplary embodiment of the present invention and a conventional supporting film,  FIG. 5  is a view showing a change of a polarization state in a Poincare sphere color coordinate when red, green, and blue polarized light passes through a supporting film of a compensation film according to an exemplary embodiment of the present invention, and  FIG. 6  is a view showing a change of a polarization state in a Poincare sphere color coordinate when red, green, and blue polarized light passes through a conventional supporting film. 
     Having a positive wavelength dispersion characteristic for the supporting layer  41  means generating a larger phase difference as the wavelength of the passing light decreases, as shown in  FIG. 4 . As described above, if the supporting layer  41  has a positive wavelength dispersion characteristic, as shown in  FIG. 5 , the blue light has a larger phase difference than the green light or the red light, and the green light has a larger phase difference than the red light. Accordingly, the red light, the green light, and the blue light that are spread on a Poincare sphere color coordinate are gathered into positions close to each other after passing through the supporting layer  41 . As described above, if the red light, the green light, and the blue light are gathered on the Poincare sphere color coordinate into positions close to each other, a difference degree for the light amount passing through the polarization film  22  may be reduced. In contrast, if the supporting layer  41  has a negative wavelength dispersion characteristic, as shown in  FIG. 6 , the blue light has a smaller phase difference than the green light or the red light, and the green light has a smaller phase difference than the red light. Accordingly, the red light, the green light, and the blue light that are spread on the Poincare sphere color coordinate are scattered far away from each other after passing through the supporting layer  41 . As described above, if the red light, the green light, and the blue light are far away from each other on the Poincare sphere color coordinate, the difference degree for the light amount passing through the polarization film  22  may be increased, thereby generating a yellowish hue. 
     Each polarization film  21  and  22  may include a polarization layer and passivation layers positioned on opposite sides. The polarization films  21  and  22  may be made of a single film. Two polarization films  21  and  22  may be disposed such that the absorption axis thereof is crossed, and the absorption axes respectively form an angle of 0.5°-1.5° with the rubbing direction of the discotic liquid crystal of the compensation film  410  and  420  adjacent thereto. That is, the absorption axis of the lower polarization film  21  forms an angle of 0.5°-1.5° with the direction in which the discotic liquid crystals of the phase difference induction layer  42  of the lower compensation film  410  are rubbed and inclined, and the absorption axis of the upper polarization film  22  forms an angle of 0.5°-1.5° with the direction in which the discotic liquid crystals of the phase difference induction layer  42  of the upper compensation film  420  are rubbed and inclined. 
     As described above, when using a compensation film having an R 0  of 40 nm-60 nm, Rth of 140 nm-170 nm, and β of 15°-19°, viewing angle deterioration is not generated even though a high transmittance liquid crystal layer is applied to the liquid crystal display. Also, by providing a normal dispersion characteristic for the wavelength through the supporting film of the compensation film, the yellowish hue generated in the side of the liquid crystal display may be reduced. 
     It will be apparent to those skilled in the art that various modifications and is variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.