Abstract:
The present invention is directed to an alternative design of a multicolor display. The display device is capable of achieving high quality black and white states. The pixel electrodes and the microcups in the display device may be aligned only in one axis. In practice, a standard active matrix array may be used to drive the display device.

Description:
[0001]    This application claims priority to U.S. Provisional Application No. 61/141,566, filed Dec. 30, 2008; the content of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to multicolor display architecture comprising enhanced dark state. 
       BACKGROUND OF THE INVENTION 
       [0003]    In order to achieve a multicolor display, color filters are often used. The most common approach is to add color filters on top of black/white sub-pixels of a pixelated display to display the red, green and blue colors. When a red color is desired, the green and blue sub-pixels are turned to the black state so that the only color displayed is red. When the black state is desired, all three-sub-pixels are turned to the black state. When the white state is desired, the three sub-pixels are turned to red, green and blue, respectively, and as a result, a white state is seen by the viewer. 
         [0004]    The biggest disadvantage of such a technique is that since each of the sub-pixels has a reflectance of about one third (⅓) of the desired white state, the white state is fairly dim. To compensate this, a fourth sub-pixel may be added which can display only the black and white state, so that the white level is doubled at the expense of the red, green or blue color level (where each sub-pixel is only one fourth [¼] of the area of the pixel). Brighter colors can be achieved by adding light from the white pixel, but this is achieved at the expense of color gamut to cause the colors to be very light and unsaturated. A similar result can be achieved by reducing the color saturation of the three sub-pixels. Even with that, the white light level is normally substantially less than half of that of a black and white display, rendering it an unacceptable choice for display devices, such as e-readers or displays that need well readable black-white brightness and contrast. 
         [0005]    An alternative technique utilizing a dual switching mode is proposed in U.S. Pat. No. 7,046,228. In this patent, each pixel is formed of three sub-pixels, each one of which is a microcup filled with a display fluid of an appropriate color (i.e., red, green or blue) with charged white particles in the microcups and a dual switching mode is used. With the dual switching mode, an extra set of electrodes are added to each microcup so that the microcup can be switched into three states: white (with the particles on top), colored (with the particles on bottom), and black (with the particles moved to the sides). A black background layer is used so that when the particles are moved sideways, the black state is seen. The advantage of this is that high quality white and black states can be achieved while still offering red/green/blue colors. This dual mode design requires a special circuitry structure within each sub-pixel, which may reduce backplane yield and affect costs. In addition, a masking layer may be required to block light reflected by the white sub-pixels in the black state to maintain the desired contrast. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to an alternative design of a multicolor display. One of the key features of the invention is the intense dark color of the display fluid filled in the microcups, which allows the dark state to appear black. The display device may achieve high quality black and white states with only one pixel electrode per microcup and only one drive circuit per pixel, thus leading to simpler fabrication and lower manufacturing costs. In practice, a standard active matrix array may be used to drive the display device. 
         [0007]    One aspect of the invention is directed to a multi-color display device, which comprises a plurality of microcups wherein
       a) the microcups are separated by partition walls;   b) each of the microcups is filled with a display fluid comprising white charged pigment particles dispersed in a solvent of a dark color;   c) the microcups are sandwiched between a first layer and a second layer wherein the first layer comprises a common electrode and the second layer comprises a plurality of pixel electrodes; and   d) each of the microcups is capable of displaying the white color state, a dark color state and a medium color state.       
 
         [0012]    In this aspect of the invention—In one embodiment, the multi-color display device may further comprise an adhesive layer of a dark color or a color complementary to the color of the solvent. In another embodiment, the top surface of the partition walls is of a dark color or a color complementary to the color of the solvent. In a further embodiment, the partition walls may be of a black or dark color. In one embodiment, the reflectance of the medium color state achieved by driving the white charged pigment particles to an area between the common electrode and the pixel electrode in the microcup has at least five times, preferably at least ten times, the reflectance of the dark color state. In one embodiment, the dark color of the solvent is dark red, dark green or dark blue. In one embodiment, no more than 10%, preferably no more than 3%, of the light is reflected at the peak transmission of the dark color state. In one embodiment, the microcups and the pixel electrodes are aligned. In another embodiment, the microcups and the pixel electrodes are aligned only in one axis. In one embodiment, the particles are driven to the mid-level in the microcup. In another embodiment, the particles are distributed throughout the display fluid. In one embodiment, each microcup represents a sub-pixel and three sub-pixels form a pixel. In one embodiment, the device further comprises an active matrix driving system. 
         [0013]    Another aspect of the invention is directed to a multi-color display device, which comprises a plurality of microcups wherein
       a) each of the microcups is filled with a display fluid comprising black and white charged particles carrying charges of opposite polarities and dispersed in a clear and colored solvent;   b) the microcups are sandwiched between a first layer and a second layer wherein the first layer comprises a common electrode and the second layer comprises a plurality of pixel electrodes; and   c) each of the microcups is capable of achieving a white state, a black state and a color state.       
 
         [0017]    In this aspect of the invention—In one embodiment, the colored solvent is red, green or blue respectively. In one embodiment, the microcups and said pixel electrodes are aligned. In another embodiment, the microcups and said pixel electrodes are aligned only in one axis. In one embodiment, the color state achieved by driving both the white and black charged pigment particles to an area between the common electrode and the pixel electrode has a reflectance at least five times, preferably at least ten times, the reflectance of the black state. In one embodiment, the particles are driven to the mid-level in a microcup. In another embodiment, the particles are distributed in the display fluid. In one embodiment, each microcup represents a sub-pixel and three sub-pixels form a pixel. In one embodiment, the device further comprises an active matrix driving system. 
     
    
     
       BRIEF DISCUSSION OF THE DRAWINGS 
         [0018]      FIG. 1  depicts a cross-section view of a multicolor display device of the present invention. 
           [0019]      FIG. 2   a  depicts an aligned design. 
           [0020]      FIG. 2   b  depicts a design with alignment in only one axis. 
           [0021]      FIGS. 3   a - 3   c  illustrate how different color states may be displayed. 
           [0022]      FIG. 4  shows a scale of colors which may be displayed by the color display of the present invention. 
           [0023]      FIGS. 5   a - 5   e  shows how multiple color states may be displayed. 
           [0024]      FIGS. 6   a - 6   c  show alternative designs of the present invention. 
           [0025]      FIGS. 6   a - 6   c  illustrate how different color states may be displayed with a two particle system. 
           [0026]      FIGS. 7   a - 7   c  show another embodiment of this invention using a two particles system. 
           [0027]      FIGS. 8   a  and  8   b  show comparison of the display of the present invention against a prior art design utilizing color filters. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1  depicts a cross-section view of a color display device of the present invention. A microcup ( 100 ) is sandwiched between a first layer ( 101 ) and a second layer ( 102 ). The microcup ( 100 ) is surrounded by partition walls ( 107 ). The first layer comprises a common electrode ( 103 ). The second layer comprises multiple pixel electrodes ( 104 ). 
         [0029]    The microcup ( 100 ) is a micro-container filled with a display fluid ( 105 ). Details of the term “microcup” are given in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety. 
         [0030]    While microcups are specifically mentioned in the present application, it is understood that any micro-containers (e.g., microcapsules), regardless of their shapes or sizes, are within the scope of the present application, as long as the micro-containers are filled with a display fluid and have the same functions as the microcups. 
         [0031]    In one embodiment of the invention, the display fluid ( 105 ) is an electrophoretic fluid comprising only one type of charged pigment particles ( 106 ), usually white, dispersed in a colored dielectric solvent or solvent mixture, as shown in  FIG. 1 . The pigment particles may be positively charged or negatively charged. The color of the electrophoretic fluid (or the color of the dielectric solvent or solvent mixture) may be a dark red, dark green, dark blue or another dark color. The display fluids in the microcups are of different colors and each of the microcups represents a sub-pixel. 
         [0032]    The common electrode ( 103 ) is usually a transparent electrode layer (e.g., ITO), spreading over the entire top of the display device. 
         [0033]    The second layer ( 102 ) comprises multiple pixel electrodes ( 104 ). The pixel electrodes ( 104 ) are described in U.S. Pat. No. 7,046,228, the content of which is incorporated herein by reference in its entirety. It is noted that while active matrix driving electrodes are mentioned as pixel electrodes, the scope of the present invention encompasses other types of electrode addressing such as passive matrix driving, as long as the electrodes serve the desired functions. 
         [0034]    The pixel electrodes and the microcups may be aligned.  FIG. 2   a  shows an example. The microcups (solid lined,  100 ) and the pixel electrodes ( 104 , dotted lined) are aligned. In such an aligned design, each pixel electrode corresponds to one microcup. 
         [0035]      FIG. 2   b  is a top view of a design which requires only alignment in one axis. In this figure, the microcups (solid lined,  100 ) and the pixel electrodes (dotted lined,  104 ) are un-aligned in one direction (i.e., the Y-axis) and each pixel electrode is underneath parts of two neighboring microcups. In the context of the present invention, the term “un-aligned” or “non-aligned” is intended to encompass configurations in which at least one pixel electrode is underneath parts of more than one microcup and the unaligned configuration is only in one direction. The non-alignment is in one axis to simplify the manufacture of the display device, and in this case, the colored pixels are in vertical stripes, as shown. 
         [0036]      FIGS. 3   a - 3   c  illustrate how black, white and color states may be displayed. In this example, the display fluid comprises charged white pigment particles dispersed in a dark red fluid. 
         [0037]    In  FIG. 3   a , by applying appropriate voltages to the common ( 303 ) and pixel ( 304   a ) electrodes, the white particles move to be near or at the common electrode ( 303 ). As a result, the white color is seen at the viewing side. 
         [0038]    In  FIG. 3   b , by applying appropriate voltages to the common ( 303 ) and pixel ( 304   b ) electrodes, the white particles move to be near or at the pixel electrode ( 304   b ). Because the dark red dye absorption is strong, the color seen in this case is a very dark red color. At a high enough level of the color density, such a microcup will appear black to the viewers. 
         [0039]    The color of the fluid is dark enough that when the white particles are at or near the pixel electrode (i.e., at the bottom of the microcup), preferably not more than 10%, more preferably not more than 3%, of the light is reflected at the peak transmission of the dark color state. 
         [0040]    By properly adjusting the timing (i.e., duration) of a driving waveform, a substantial amount of the white particles may move to be in an area between the common electrode and the pixel electrode (i.e., about the mid-level in the microcup), as shown in  FIG. 3   c . In this scenario, the reflected color appears to be a medium red color. 
         [0041]    The degree of darkness of the display fluid preferably is such that the reflectance of the medium color state (as shown, for example, in  FIG. 3   c ) is at least 5 times, more preferably at least 10 times, the reflectance of the dark color state (as shown, for example in  FIG. 3   b ). 
         [0042]    In the context of the present invention, when the particles are driven to an area between the common electrode and the pixel electrode, the particles may be distributed throughout the display fluid or a substantial amount of the particles may gather at the mid-level in a microcup. A “substantial amount,” as used herein, refers to at least 60%, preferably 75%, more preferably 90%, of the particles. “The mid-level,” as used herein, refers to the area between 20 and 80%, preferably 30 and 70%, more preferably 40 and 60% of the height (h) of a microcup. In any case, the reflectance of the medium color achieved by driving the particles to an area between the common and pixel electrodes is at least 5 times, more preferably at least 10 times, the reflectance of the dark color state achieved by driving the particles to be at or near the pixel electrode. 
         [0043]      FIG. 4  shows a scale of colors which may be displayed by the color display of the present invention. The black state at the right end of the scale is achieved when the white particles are at or near the bottom of a microcup. The white state at the left of the scale is achieved when the white particles are at or near the top of a microcup. The blue color state in the middle of the scale is achieved when the white particles are distributed in the mid-level of a microcup. 
         [0044]      FIG. 5  illustrates how multiple colors are displayed with a display device of the present invention. Each microcup represents a sub-pixel and each pixel has three sub-pixels. The three microcups, each representing a sub-pixel, are filled with display fluids of red, green and blue color, respectively. 
         [0045]    In  FIG. 5   a , when a white pixel is desired, all three sub-pixels are turned to the white color state. In  FIG. 5   b , when a black pixel is desired, all three sub-pixels are turned to the dark color state.  FIG. 5   c , when a red color is desired, one of the sub-pixel is turned to red (pigment particles are driven to the middle level of the display fluid, as shown in  FIG. 3   c ) and the remaining two sub-pixels are turned to the black state for maximum color saturation. Similarly,  FIG. 5   d  and  FIG. 5   e  display the green and blue colors respectively. Alternatively, in  FIGS. 5   c ,  5   d  and  5   e , one of the sub-pixel is driven to the color state while the remaining two sub-pixels are driven to the white state for maximum brightness (at the expense of the color saturation). Further alternatively, in  FIGS. 5   c ,  5   d  and  5   e , one of the sub-pixel is driven to the color state while the remaining two sub-pixels are driven to the black and white states respectively. 
         [0046]      FIGS. 6   a - 6   c  show alternative designs of the present invention. 
         [0047]    In  FIG. 6   a , there is a dark color (e.g., black) adhesive layer ( 108 ) at the opposite side of the viewing side. The first layer ( 101 ), the second layer ( 102 ) and the partition walls ( 107 ) are all transparent. The purpose of the colored adhesive layer is to add intensity to the dark color state. The adhesive layer may be of the black color or a dark color complementary to the color of the display fluid. In the latter case, if the microcups are filled with a display fluid of a dark red color, the dark color state will have a tinge of red and in this case, the adhesive layer may be of a combined color of dark green and dark blue. The dark green and dark blue colors from the adhesive layer through the partition walls and the red color from the display fluid together will provide a neutral black color. 
         [0048]    Likewise, if the microcups are filled with a display fluid of a dark green color, the adhesive layer then may be of a combined color of dark red and dark blue (complementary color). If the microcups are filled with a display fluid of a dark blue color, the adhesive layer then may be of a combined color of dark red and dark green (complementary color). As shown, the adhesive layer has the function of enhancing the contrast and neutralizing the dark color state displayed by the display device. Suitable pigments or dyes are added to the adhesive layer to achieve the desired color of the adhesive layer. 
         [0049]    When a dark adhesive layer is present as shown in  FIG. 6   a , in the white state (as shown in  FIG. 3   a ) or the medium color state (as shown in  FIG. 3   c ), the much brighter color displayed would overwhelm the small amount of the dark color in the wall area, thus not being affected by the dark adhesive layer. 
         [0050]    The dark color seen through the partition walls may also be achieved by other alternative designs. In one embodiment, the top surface ( 107   b ) of the partition walls ( 107 ) may be colored black or a dark color state, as shown in  FIG. 6   b . The black or a dark color may be applied to the top surface of the partition walls by methods as described in U.S. Pat. No. 6,829,078, the content of which is incorporated herein by reference in its entirety. The top surface of the partition walls may be of a dark color complementary to the dark color of the display fluid within the microcups. In other words, the top surface may be of a combined color of dark red and dark green, or a combined color of dark green and dark blue or a combined color of dark blue and dark green, depending on the color of the display fluid. Suitable pigments or dyes may be used to achieve the desired color of the top surface layer ( 107   b ). 
         [0051]    In another embodiment, the entire partition walls ( 107 ) may be of a black or dark color. This may be achieved by forming the microcup structure ( 100 ) in a black or dark color, as shown in  FIG. 6   c . In this option, suitable pigments or dyes may be added to the composition for forming the microcup structure. 
         [0052]    In another embodiment of the invention, the display fluid may comprise two types of charged pigment particles of contrast color, usually white and black, dispersed in a clear and colored solvent. The two types of the charged pigment particles carry charges of opposite polarities. For example, while the white particles are positively charged, the black particles are negatively charged, or vice versa. In this case, the color of the solvent does not have to be as dark as the color of the solvent in the one particle system. 
         [0053]      FIGS. 7   a - 7   c  illustrate how black, white and color states may be displayed with this two particle system. In this example, the pigment particles are black and white and carry charges of opposite polarities. The solvent is, for example, of a red color. 
         [0054]    In  FIG. 7   a , the white particles are driven to be near or at the common electrode ( 703 ) and the black particles are driven to be near or at the pixel electrode ( 704   a ). As a result, the white color is seen at the viewing side. 
         [0055]    In  FIG. 7   b , the black particles are driven to be near or at the common electrode ( 703 ) and the white particles are driven to be near or at the pixel electrode ( 704   b ). As a result, the black color is seen at the viewing side. 
         [0056]    In  FIG. 7   c , a substantial amount of the black and white particles are driven to be in an area between the common electrode and the pixel electrode (e.g., near or at the middle level of a microcup). As a result, the red color of the solvent is seen at the viewing side. 
         [0057]    In the two particle system as described in  FIGS. 7   a - 7   c , the reflectance of the color state (as shown, for example, in  FIG. 7   c ) is at least 5 times, more preferably at least 10 times, the reflectance of the black state (as shown, for example in  FIG. 7   b ). 
         [0058]    In the two particle system, each pixel has three-sub-pixels, and the multiple color states are displayed as illustrated in  FIGS. 5   a - 5   e.    
         [0059]    The non-aligned design can be used in both the one particle and two particle systems. 
         [0060]    It is also noted that the display device of the present invention, may be viewed from either the side of the common electrode or the side of the pixel electrode. It is preferred that the display device is viewed from the side of the common electrode. This is applicable to both the one particle system and the two particle system. The dark colored adhesive layer, if present, is always on the non-viewing side of the display device. 
         [0061]    The display device of the present invention may be manufactured by methods known in the art. For example, the microcup layer may be formed on a layer of pixel electrodes followed by laminating a common electrode layer over the microcup layer, as described in U.S. Pat. No. 6,930,818. For the non-microcup type display devices, they may also be manufactured by methods known in the art. 
       Example 
       [0062]    This example compares the multiple color states displayed by the present invention against the multicolor states displayed utilizing color filters. 
         [0063]      FIG. 8   a  shows the five color states (white, red, green, blue and black) displayed by the present invention. Each pixel has three sub-pixels. The sub-pixels are microcups filled with display fluids comprising white pigment particles dispersed in a red, green and blue solvent, respectively. The five color states are achieved as described above. In the one particle system, the solvents are of dark colors. 
         [0064]      FIG. 8   b  shows the five color states displayed utilizing the color filters. Each pixel, in this case, has four sub-pixels. One of the sub-pixels is capable of displaying only black/white color states. The other three sub-pixels have color filters of red, green and blue placed on top to display the red, green and blue colors, respectively. 
         [0065]    The following table summarizes the results: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 The Present Invention 
                 The Color Filters 
               
               
                   
                 Test Data 
                 (FIG. 8a) 
                 (FIG. 8b) 
               
               
                   
                   
               
             
             
               
                   
                 Contrast Ratio 
                 10:1 
                 4:1 
               
               
                   
                 Whiteness 
                 30% 
                 15% 
               
               
                   
                 Color States 
                 Good (⅓ Fill Factor) 
                 Lower Brightness 
               
               
                   
                   
                   
                 (¼ Fill Factor) 
               
               
                   
                   
               
             
          
         
       
     
         [0066]    While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.