Abstract:
This invention relates to a display device comprising a plurality of display cells, wherein said display cells are separated by slanted partition walls. This invention also relates to a display device comprising a plurality of display cells, wherein said display cells are separated by indented partition walls having indented areas. The electrophoretic structures of the present invention may be manufactured by a continuous or semi-continuous roll-to-roll manufacturing process. The structures in which display cells are separated by slanted partition walls or partition walls having indented areas are capable of providing enhanced color states.

Description:
This application claims priority to U.S. Provisional Application No. 61/109,154, filed Oct. 28, 2008; the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a solvent. An EPD typically comprises a pair of electrodes, with at least one of the electrodes, typically on the viewing side, being transparent. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes. 
     An improved EPD technology and a roll-to-roll manufacturing process are disclosed in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety. 
     For full color displays with the normal up/down switching mode, color filters overlaid on the viewing side of the display may be used. However, poor whiteness and lack of a high quality “black” state are the major problems for reflective color displays using color filters. 
     Therefore, there is still a need for an improved EPD with high quality full color capability that can also be manufactured in an efficient manner, particularly by a roll-to-roll manufacturing process. 
     SUMMARY OF THE INVENTION 
     The present invention provides novel display structures. 
     The first aspect of the invention is directed to a display device comprising a plurality of display cells wherein the display cells are separated by slanted partition walls. In one embodiment, the display cells are filled with an electrophoretic display fluid comprising charged pigment particles dispersed in a dielectric solvent or solvent mixture. In one embodiment, the electrophoretic display fluid comprises one type of charged pigment particles. In one embodiment, the electrophoretic display fluid comprises two types of charged pigment particles. In one embodiment, the partition walls are of a dark opaque color. In one embodiment, the partition walls are formed from a composition comprising air pockets or a filler material. In one embodiment, the filler material is non-conductive carbon black, pigment black, silica, ZnO, TiO 2 , BaSO 4 , CaCO 3  or polymer particles. In one embodiment, the display is capable of displaying the color of the charged pigment particles, the color of the solvent or solvent mixture, or the color of a background layer. In one embodiment, the color of the background layer is black. In one embodiment, the display is capable of displaying a binary color system. In one embodiment, the slanted partition walls have an angle in the range of about 25° to about 80°. In one embodiment, the total active area is at least about 50% of the total area of the viewing surface. 
     The second aspect of the present invention is directed to a display device comprising a plurality of display cells wherein the display cells are separated by indented partition walls having indented areas. In one embodiment, the display cells are filled with an electrophoretic display fluid comprising charged pigment particles dispersed in a dielectric solvent or solvent mixture. In one embodiment, the electrophoretic display fluid comprises one type of charged pigment particles. In one embodiment, the electrophoretic display fluid comprises two types of charged pigment particles. 
     In one embodiment, the indented partition walls do not have open passageways. In one embodiment, each display cell comprises one or more indented partition walls. In one embodiment, the display is capable of displaying the color of the charged pigment particles, the color of the solvent or solvent mixture, or the color of a background layer. In one embodiment, the thickness of the indented area is about 5% to about 80% of the total thickness of the partition wall. In one embodiment, the height of the indented area is about 5% to about 80% of the height of the partition wall. In one embodiment, the indented areas are of a rectangular shape. In one embodiment, the indented areas are of a rectangular shape with an arched top. 
     In one embodiment, the indented partition walls are open partition walls having open areas. In one embodiment, the ceiling of the open areas is painted black. In one embodiment, the open areas are of a rectangular shape. In one embodiment, the open areas are of a rectangular shape with an arched top. In one embodiment, the height of the open areas is about 5% to about 80% of the height of the partition wall. In one embodiment, the display is a striped color display. In one embodiment, the open partition walls randomly appear in the display. 
     The electrophoretic display structures of the present invention may be manufactured by a continuous or semi-continuous roll-to-roll manufacturing process. The structures are capable of providing enhanced color states. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It is noted that all figures are shown as schematic and are not to scale. 
         FIG. 1  is a cross-section view of an electrophoretic display structure having slanted partition walls. 
         FIG. 2  is a cross-section view of an electrophoretic display capable of displaying multiple color states. 
         FIG. 3  shows the dimensions of an electrophoretic display having slanted partition walls. 
         FIGS. 4   a  and  4   b  are three-dimensional view of an electrophoretic display having indented partition walls. 
         FIG. 5  is a cross-section view of an electrophoretic display capable of displaying multiple color states. 
         FIGS. 6   a - 6   c  show the top view of display structures and their corresponding electrode configurations. 
         FIG. 7  shows the dimensions of an electrophoretic display having indented partition walls. 
         FIGS. 8   a  and  8   b  are three-dimensional view of an electrophoretic display having open partition walls. 
         FIG. 9  is a cross-section view of an electrophoretic display capable of displaying multiple color states. 
         FIG. 10  is a top view of a striped color display device. 
         FIG. 11  is the cross-section view of an electrophoretic display having open partition walls. 
         FIG. 12  is a top view which illustrates how the display of  FIG. 11  operates with increased contrast ratio. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first aspect of the present invention is directed to an electrophoretic display structure ( 100 ) having trapezoid-shaped partition walls, as shown in  FIG. 1 . The display cells (or microcups) ( 101 ) are separated by the trapezoid-shaped partition walls ( 102 ). The trapezoid-shaped partition walls may also be referred to as the slanted partition walls. The display cells are then filled with an electrophoretic fluid ( 104 ) and optionally sealed with a polymeric sealing layer ( 105 ). 
     It is preferable for the slanted partition walls to have an opaque color, especially a gray opaque color. This may be achieved by introducing air pockets or a filler material such as non-conductive carbon black, pigment black, silica, ZnO, TiO 2 , BaSO 4 , CaCO 3  or polymer particles, preferably non-conductive carbon black or pigment black, preferably in the amount of 0.01-20% by weight, more preferably in the amount of 0.01-10% by weight, into the composition for the formation of the display cells. Colored pigments may also be used to create special appearance. 
       FIG. 2  illustrates a full color display utilizing the structure of  FIG. 1 . In  FIG. 2 , the structure of  FIG. 1  has been turned 180°. In other words, the side opposite of the sealing layer ( 105 ) is now the viewing side. 
     For illustration purpose, there are only three display cells shown. Each of the display cells is sandwiched between a first layer ( 106 ) and a second layer ( 107 ). The first layer ( 106 ) comprises a common electrode ( 108 ) (i.e., the ITO layer). The second layer ( 107 ) comprises a center electrode and at least one side electrode for each display cell. In  FIG. 2 , there are two side electrodes ( 110   a  and  110   b ) on each side of the center electrode ( 109 ) for each display cell. The center electrode and the side electrodes are not in contact with each other. 
     The common electrode ( 108 ) may be an entire piece of ITO layer spreading across the display cells. 
     There is a black background layer ( 111 ) which is above the second layer ( 107 ). It is also possible to have the black background layer underneath the second layer or the second layer may itself serve as a black background layer. 
     Typically, the display cells are filled with a display fluid comprising a colored (e.g., red, green or blue) dielectric solvent or solvent mixture with white particles dispersed therein. The charged particles in each display cell may be of the same color or of different colors. Particles of mixed colors, when substantially evenly distributed, may be seen as one color, i.e., a composite color of different colors. 
     The particles may be positively or negatively charged. 
     Alternatively, the display fluid could also have a transparent or lightly colored solvent or solvent mixture and charged particles of two different colors carrying opposite charges, and/or having differing electro-kinetic properties. For example, there may be white pigment particles which are positively charged and black pigment particles which are negatively charged and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. 
     The display cells are separated by slanted partition walls ( 102 ). 
     For the purpose of illustration, it is assumed that the particles in  FIG. 2  and other figures are positively charged, throughout this application. 
     As shown in  FIG. 2 , the positively charged pigment particles are allowed to move in either the vertical (up/down) direction or the planar (left/right) direction. For example, for display cell  101   a , when the voltage of the common electrode ( 108 ) is set low, and the voltages of the center electrode ( 109 ) and the side electrodes ( 110   a  and  110   b ) are set high, the white particles would migrate to be near or at the common electrode ( 108 ). As a result, the white color (i.e., the color of the particles) is seen at the viewing side. 
     In display cell  101   b , when the voltage of the common electrode ( 108 ) is set high and the voltages of the center electrode ( 109 ) and side electrodes ( 110   a  and  110   b ) are set low, the white particles would migrate to be near or at the bottom of the display cell. As a result, the color of the fluid (e.g., red, green or blue) would be seen at the viewing side. 
     In display cell  101   c , when the voltages of the side electrodes ( 110   a  and  110   b ) are set low and the voltages of the common ( 108 ) and center ( 109 ) electrodes are set high, the white particles would migrate to be near or at the sides of the display cell. As a result, the color seen at the viewing side would be the color of the background layer ( 111 ) (i.e., black). In this current structure, the white charged pigment particles are hidden under the slanted partition walls ( 102 ), and therefore are not visible from the viewing side. 
     The full color displays of the present invention may be driven by an active matrix system or a passive matrix system as described in U.S. Pat. No. 7,046,228, the content of which is incorporated herein by reference in its entirety. 
       FIG. 3  illustrates the dimensions of the electrophoretic display structure of  FIG. 1 . In a full color display, the total active area (mark A in the figure) is preferably at least about 50% of the total area of the viewing surface. The term “active area” refers to the area which is not part of the partition walls. 
     The angle α of the slanted partition walls is preferably in the range of about 25° to about 80°, more preferably in the range of about 50° to about 80°. 
     The second aspect of the invention is directed to an electrophoretic display structure in which the partition walls are indented as shown in  FIGS. 4 and 8 . 
       FIG. 4   a  is a three-dimensional view of a display cell having two indented partition walls on the opposite sides of the display cell.  FIG. 4   b  is a three-dimensional view of an electrophoretic display structure having display cells with indented partition walls. The display cells of  FIG. 4   a  have been turned 180° in the structure of  FIG. 4   b.    
     The partition walls in  FIGS. 4   a  and  4   b  are indented but the indented areas still separate the neighboring display cells. En other words, the indented partition walls do not have open passageways. 
     While two indented partition walls are shown in  FIG. 4   a , a square-shaped display cell may have only one indented partition wall or may have two, three or four indented partition walls. In another embodiment, for a hexagon-shaped display cell, the display cell may have one, two, three, four, five or six indented partition walls. 
       FIG. 5  illustrates how a display structure of  FIG. 4   b  operates. In this example, it is shown that each display cell has two indented partition walls ( 515 ) on the two opposite sides of a square-shaped display cell. The side electrodes ( 510   a  and  510   b ) are placed underneath the indented areas of the partition walls. 
     As shown in  FIG. 5 , the charged pigment particles are allowed to move in either the vertical (up/down) direction or the planar (left/right) direction. For example, for display cell ( 501   a ), when the voltage of the common electrode ( 508 ) is set low, and the voltages of the center electrode ( 509 ) and the side electrodes ( 510   a  and  510   b ) are set high, the white particles would migrate to be near or at the common electrode ( 508 ). As a result, the white color (i.e., the color of the particles) is seen at the viewing side. 
     In display cell  501   b , when the voltage of the common electrode ( 508 ) is set high and the voltages of the center electrode ( 509 ) and side electrodes ( 510   a  and  510   b ) are set low, the white particles would migrate to be near or at the bottom of the display cell. As a result, the color of the fluid (e.g., red, green or blue) would be seen at the viewing side. 
     In display cell  501   c , when the voltages of the side electrodes ( 510   a  and  510   b ) are set low and the voltages of the common ( 508 ) and center ( 509 ) electrodes are set high, the white particles would migrate into the indented areas. As a result, the white charged pigment particles are hidden in the indented areas, and therefore are not visible from the viewing side. The color seen at the viewing side would be the color of the background layer ( 511 ) (i.e., black). 
       FIGS. 6   a - 6   c  are the top view of display cells and the electrode configuration on the second layer associated with the display cell. 
       FIG. 6   a  shows the configuration of the side electrode ( 510 ) and the center electrode ( 509 ) for a display cell having only one indented partition wall.  FIG. 6   b  shows the configuration of the side electrodes ( 510   a  and  510   b ) and the center electrode ( 509 ) of a display cell having two indented partition walls.  FIG. 6   c  shows the configuration of the side electrode ( 510 ) and the center electrode ( 509 ) of a display cell having four indented partition walls. 
     In one embodiment as shown in  FIG. 7 , the thickness (t) of one indented area is about 5% to about 80%, of the total thickness (T) of a partition wall and the height (h) of the indented area is about 5% to about 80%, of the height (H) of a partition wall. 
     In  FIGS. 4 and 5 , the indented areas are shown in a rectangular shape. The shape of the indented area is not limited to the rectangular shape and it may vary (such as a rectangular with an arched top), as long as it serves the same function and purpose of the indented area of a rectangular shape as shown. 
       FIG. 8   a  is a three-dimensional view of a display cell having two open partition walls on the opposite sides of the display cell.  FIG. 8   b  is a three-dimensional view of an electrophoretic display structure having display cells with open partition walls. The display cells of  FIG. 8   a  have been turned 180° in the structure of  FIG. 8   b.    
     In the structure of  FIG. 8   b , the open partition wall area becomes an open passage area. In other words, the display fluid in one display cell may interchange with the display fluid in a neighboring display cell when the two display cells are separated by the open partition walls. Because the display fluid can move freely underneath the open areas in the partition walls, in this structure, the partition wall areas essentially have become “active”. 
     The opaque wall will create a hiding place for the particles. Alternatively, the ceiling of the open areas in the partition walls may be painted a color (e.g., black). The colored ceiling may be achieved by a process described below. 
     In one embodiment, the open partition walls are only on two opposite sides of a square-shaped display cell. The shape of the open areas is not limited to a rectangular shape as shown; it may vary as long as it serves the intended purpose and function. 
       FIG. 9  illustrates how a display structure of  FIG. 8   b  operates. In this example, it is shown that each display cell has two open partition walls ( 915 ) on the two opposite sides of a square-shaped display cell. The side electrodes ( 910   a  and  910   b ) are placed underneath the open areas of the partition walls. 
     The height of the open areas is preferably about 5% to about 80%, of the height of the partition wall. 
     As shown in  FIG. 9 , the charged pigment particles are allowed to move in either the vertical (up/down) direction or the planar (left/right) direction. For example, for display cell ( 901   a ), when the voltage of the common electrode ( 908 ) is set low, and the voltages of the center electrode ( 909 ) and the side electrodes ( 910   a  and  910   b ) are set high, the white particles would migrate to be near or at the common electrode ( 908 ). As a result, the white color (i.e., the color of the particles) is seen at the viewing side. 
     In display cell  901   b , when the voltage of the common electrode ( 908 ) is set high and the voltages of the center electrode ( 909 ) and side electrodes ( 910   a  and  910   b ) are set low, the white particles would migrate to be near or at the bottom of the display cell. As a result, the color of the fluid (e.g., red, green or blue) would be seen at the viewing side. 
     In display cell  901   c , when the voltages of the side electrodes ( 910   a  and  910   b ) are set low and the voltages of the common ( 908 ) and center ( 909 ) electrodes are set high, the white particles would migrate into the open areas. As a result, the white charged pigment particles are hidden in the open areas, and therefore are not visible from the viewing side especially if the open partition walls have a colored or black ceiling. The color seen at the viewing side would be the color of the background layer ( 911 ) (i.e., black). 
     This type of display structure is particularly suitable for a striped color display device as shown in  FIG. 10 . In the display device as shown, all display cells of the first row are filled with a display fluid of the red color; all display cells of the second row are filled with a display fluid of the green color; and all display cells of the third row are filled with a display fluid of the blue color. 
     Each display cell in this case represents one color sub-pixel. There are no open areas in the partition walls in the horizontal direction. The partition walls in the vertical direction have open areas which allow the display fluid of the same color to move through, in each row. 
     The display cells with red, green and blue display fluids respectively aligned to the sub-pixels on a backplane to form one pixel unit of an electrophoretic display device. 
     The display structure of  FIG. 8   b  is also suitable for a binary color system. As shown in  FIGS. 11 and 12 , the open partition walls ( 1117 ) randomly appear in the structure. The non-open partition walls are marked  1118 . For illustration purpose, it is assumed that the display device shown in the figure is a black/white binary color display. In other words, the display fluid in this example comprises white particles dispersed in a black color solvent. To ensure high contrast ratio, an auxiliary layer ( 1107 ) is placed underneath the display cells. The auxiliary layer, for example, is of the black color and it may be an adhesive layer. 
       FIG. 12  illustrates how the display structure of  FIG. 11  operates. 
     When the white particles move to be at or near the common electrode ( 1108 ), the white color is seen in the display cells from the viewing side. In the non-open partition wall area ( 1118 ), the black color from the auxiliary layer ( 1107 ) is seen from the viewing side. In the open partition wall area ( 1117 ), the white color of the particles is seen, as the display cell structure is transparent. 
     When the white particles move to be at or near the center electrode ( 1109 ), the black color is seen in the display cells from the viewing side. In the non-open partition wall area ( 1118 ), the black color from the auxiliary layer is seen from the viewing side. In the open partition wall area ( 1117 ), the black color of the solvent is seen, although the black color is not as intense as that of the display cell area because in the open partition wall area the fluid level is not as deep as that in the display cell area. 
     The increase of the overall black color intensity results in an overall higher contrast ratio of the binary color display device. 
     All of the display structures described in the present application can be prepared by techniques such as microembossing. An embossing composition is usually first coated on an ITO layer. A male mold is then pressed onto the embossing composition from the top to form display cells of a desired configuration (e.g., slanted, indented or open partition walls). After the display cells are formed, the display fluid may be filled into the display cells and the filled display cells are optionally sealed. In order for the partition walls to achieve the function of hiding the particles, the structure formed after the embossing process is turned 180° in forming a display device. 
     The embossing male mold can be made using a diamond turn method. A photolithography method may also be used; but two masking steps are needed to create the open partition walls. All of the display structures of the present invention may be manufactured by a continuous or semi-continuous process in a roll to roll manner as described in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety. 
     In order to color the top surface of the partition walls without contaminating the display cells, a masking layer may be applied before spraying the black ink. An aqueous masking solution can be used in combination with a liquid black ink and the masking layer may be stripped with water to remove any black paint in the masking area. Details of the processes for coloring the top surface of the partition walls black may be found in U.S. Pat. No. 6,829,078, the content of which is incorporated herein by reference in its entirety. 
     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 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, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.