Abstract:
The present invention is directed to reflective display devices with a luminance enhancement structure on its viewing side and at least an auxiliary layer and/or edge sealing. The structure increases the overall reflectance by reducing the total internal reflection, and as a result, the brightness of a display device is increased.

Description:
[0001]    This application claims priority to U.S. Provisional Application No. 61/171,718, filed Apr. 22, 2009; the content of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to reflective display devices with a luminance enhancement structure on its viewing side. 
       BACKGROUND OF THE INVENTION 
       [0003]    The lack of satisfactory brightness is often a concern for electrophoretic display devices. Total internal reflection inevitably would occur with electrophoretic display devices because such a display device usually has components of a high refractive index. Due to the component having a higher refractive index (e.g., about 1.5) than the air (which has a refractive index of about 1) surrounding the display panel, some of the scattering light from the display panel may reflect back to the display device by total internal reflection. This total internal reflection phenomenon may result in a loss of about 30-50% of the scattering light, thus causing reduction in brightness of the display device. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed to a display assembly which comprises
       (a) a display panel which comprises an array of display cells filled with a display fluid;   (b) a luminance enhancement structure on the viewing side of the display device, wherein said luminance structure comprises columns and grooves in alternating order and each of said grooves has a cross-section comprising an apex angle and two edge lines; and at least one of (c) and (d):   (c) at least an auxiliary layer; and   (d) edging sealing.       
 
         [0009]    In one embodiment, the auxiliary layer is an anti-scratch layer, an anti-glare layer, a moisture protection barrier layer, a touch panel, a UV filter adhesive layer, an optical clear adhesive layer or a hard coat layer. 
         [0010]    In one embodiment, the luminance enhancement structure has a one dimensional configuration. In another embodiment, the luminance enhancement structure has a two dimensional configuration. 
         [0011]    In one embodiment, the two edge lines are straight lines and the apex angles of the grooves in the luminance enhancement structure are substantially equal throughout the structure. 
         [0012]    In one embodiment, the two edge lines are straight lines and the apex angles of the grooves in the luminance enhancement structure vary. 
         [0013]    In one embodiment, the two edge lines comprise two or more segments of straight line and the different segments of the straight line have different edge line angles in the luminance enhancement structure. 
         [0014]    In one embodiment, the edge lines in the luminance enhancement structure appear curved. 
         [0015]    In one embodiment, the curved edge lines in the luminance enhancement structure have more than one curvature. 
         [0016]    In one embodiment, the two edge lines of a single groove have different numbers of segments of the straight line in the luminance enhancement structure. 
         [0017]    In one embodiment, the apex angle is in the range of about 5° to about 50°. 
         [0018]    In one embodiment, the apex angle is in the range of about 20° to about 40°. 
         [0019]    In one embodiment, the surface of the grooves in the luminance enhancement structure is not coated. 
         [0020]    In one embodiment, the assembly further comprises a common electrode layer and a backplane. 
         [0021]    The luminance enhancement structure increases the overall reflectance by reducing the total internal reflection. As a result, the brightness of a display device is increased. Furthermore, the structure can be fabricated by a cost effective roll-to-roll manufacturing process. 
     
    
     
       BRIEF DISCUSSION OF THE DRAWINGS 
         [0022]      FIG. 1  depicts a cross-section view of a display device. 
           [0023]      FIG. 2   a  is a cross-section view of a luminance enhancement structure of the present invention. 
           [0024]      FIGS. 2   b  and  2   c  are a three-dimensional view of the luminance enhancement structures. 
           [0025]      FIG. 3   a  depicts a luminance enhancement structure having apex angles of substantially the same size. 
           [0026]      FIG. 3   b  depicts a luminance enhancement structure having apex angles of varying sizes. 
           [0027]      FIG. 3   c  depicts a luminance enhancement structure in which each of the two edge lines of the cross section of grooves comprises two or more segments of straight line. 
           [0028]      FIGS. 4   a - 4   c  depict a cross-section view of a display device with a luminance enhancement structure and at least one auxiliary layer. 
           [0029]      FIGS. 5   a  and  5   b  are a top view and a cross-section view respectively, of a display device with edge sealing which protects the display assembly. 
           [0030]      FIGS. 6   a - 6   g  show an example of how the luminance enhancement structure is fabricated. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Display Devices 
       [0031]      FIG. 1  illustrates a display device ( 100 ). The device comprises an array of display cells ( 101 ) which are filled with a display fluid ( 102 ) and sandwiched between two electrode layers ( 104  and  105 ). Each of the display cells is surrounded by partition walls ( 103 ). 
         [0032]    For an electrophoretic display, the display cells are filled with an electrophoretic fluid which comprises charged pigment particles dispersed in a solvent. The display fluid may be a system comprising one or two types of particles. 
         [0033]    In the system comprising only one type of particles, the charged pigment particles are dispersed in a solvent of a contrasting color. The charged particles will be drawn to one of the electrode layers ( 104  or  105 ), depending on the potential difference of the two electrode layers, thus causing the display panel to show either the color of the particles or the color of the solvent, on the viewing side. 
         [0034]    In a system comprising particles carrying opposite charges and of two contrasting colors, the particles would move to one electrode layer or the other, based on the charge that they carry and the potential difference of the two electrode layers, causing the display panel to show the two contrasting colors, on the viewing side. In this case, the particles may be dispersed in a clear solvent. 
         [0035]    The display cells may also be filled with a liquid crystal composition. In addition, it is understood that the present invention is applicable to all types of reflective display devices. 
         [0036]    For a segment display device, the two electrode layers ( 104  and  105 ) are one common electrode (e.g., ITO) and one patterned segment electrode layer, respectively. For an active matrix display device, the two electrode layers ( 104  and  105 ) are one common electrode and an array of thin film transistor pixel electrodes, respectively. For a passive matrix display device, the two electrode layers ( 104  and  105 ) are two line-patterned electrode layers. 
         [0037]    The patterned segment electrode layer (in a segment display device), the thin film transistor pixel electrodes (in an active matrix display device) or one of the line-patterned electrode layers (in a passive matrix display device) may be referred to as a “backplane”, which along with the common electrode drives the display device. 
         [0038]    The electrode layers are usually formed on a substrate layer ( 106 ) [(such as polyethylene terephthalate (PET)). The substrate layer may also be a glass layer. 
         [0039]    For a microcup-based display device disclosed in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety, the filled display cells are sealed with a polymeric sealing layer. Such a display device may be viewed from the sealing layer side or the side opposite the sealing layer side, depending on the transparency of the materials used and the application. 
         [0040]    While microcup is specifically mentioned, it is understood that the present invention is also applicable to microcapsules, microchannels, other conventional wall-typed display cells and equivalents thereof. 
       II. The Luminance Enhancement Structure or Film 
       [0041]      FIG. 2   a  is a cross-section view of the luminance enhancement structure ( 200 ) of the present invention in general. There are multiple columns ( 202 ) and grooves ( 203 ) across the structure. The cross-section ( 201 ) of the grooves ( 203 ) has a top point A and a base line (b). The dotted lines connecting the top point A to the two ends (E 1  and E 2 ) of the base line are referred to as “edge lines”. The dotted line means that the edge line may be a straight line or may comprise two or more segments of straight line. 
         [0042]    The two edge lines in a groove form an apex angle α. The surface ( 204 ) of the grooves ( 203 ) is optically flat or may be coated with a metal layer. 
         [0043]    In the context of this application, the terms “groove” or “grooves” refers to the groove or grooves the surface of which is either uncoated or coated. In one embodiment of the present invention, the surface of the groove or grooves is preferably uncoated. 
         [0044]    The columns ( 202 ) have a top surface ( 205 ). The thickness (“t”) of the luminance enhancement structure may be in the range of about 10 μm to about 200 μm. 
         [0045]    The luminance enhancement structure is formed from a material having a refractive index of about 1.4 to 1.7. The luminance enhancement structure is transparent. 
         [0046]    The fabrication of such a luminance enhancement structure is illustrated in a section below. 
         [0047]      FIG. 2   b  is a three-dimensional view of the luminance enhancement structure ( 200 ) in a one-dimensional configuration (i.e., the columns and grooves are in alternating order and are in continuous form in one direction).  FIG. 2   c  is a three-dimensional view of the luminance enhancement structure ( 200 ) in a two dimensional configuration. 
         [0048]      FIG. 3   a  shows a luminance enhancement structure wherein the grooves have two edge lines which are straight lines and the apex angles (a) are substantially equal for all grooves throughout the structure. 
         [0049]    In one embodiment, the two edge lines of the cross-section are substantially equal (i.e., isosceles triangular cross-section) for all grooves. In another embodiment, the two edge lines are substantially equal for some of the grooves and the two edge lines are not equal for the remaining grooves. In a further embodiment, the two edge lines are different for all grooves. 
         [0050]    In one embodiment, the heights “h” of the grooves are substantially equal throughout the structure. In another embodiment, the heights of the grooves vary. 
         [0051]    In one embodiment, the pitches (“p”) of the grooves are substantially equal for all grooves throughout the structure. In another embodiment, the pitches “p” of the grooves vary. The term “pitch” is defined as the distance between one end point (E 1 ) of the base line (b) of one groove and the corresponding point (E 1 ′) of the next groove. In other words, the term “pitch” is the sum of the width of the base line (b) and the width of the top surface of a column between the two grooves. 
         [0052]      FIG. 3   b  shows a luminance enhancement structure wherein the grooves have two straight edge lines and the apex angles α of the grooves are not all equal. For example, there may be 70% of the apex angles are substantially equal while the remaining apex angles vary. 
         [0053]    In one embodiment of this second design, the two edge lines of the cross-section are substantially equal (i.e., isosceles triangular cross-section) for all grooves. In another embodiment, the two edge lines are substantially equal for some of the grooves and the two edge lines are not equal for the remaining grooves. In a further embodiment, the two edge lines are different for all grooves. 
         [0054]    In one embodiment of the second design, the apex angles have no more than five different sizes throughout the structure. 
         [0055]    In one embodiment of the second design, the heights “h” of the grooves are substantially equal throughout the structure. In another embodiment, the heights of the grooves vary. 
         [0056]    In one embodiment of the second design, the pitches (“p”) of the grooves are substantially equal for all grooves throughout the structure. In another embodiment, the pitches (“p”) of the grooves vary. 
         [0057]    In any case, the grooves of different apex angles are randomly located in the luminance enhancement structure. 
         [0058]    The luminance enhancement resulted from different apex angles as described as the second design may be similarly achieved by maintaining the apex angles substantially equal while varying the angles of the edge lines of the grooves. In this alternative design, the edge lines of the cross section ( 301 ) may comprise two or more segments of straight line and the different segments of the straight line have different edge line angles (expressed as ELA in the drawings). The term “edge line angle” is referred to the angle of a segment of the straight line from the normal axis. 
         [0059]    In this type of design, the apex angles may be maintained substantially equal for all grooves throughout the structure. In one embodiment, the apex angles may vary; however, it is not needed. 
         [0060]    In  FIG. 3   c , each edge line is formed of two segments of straight line whereby ELA 1  is not equal to ELA 2  and ELA 3  is not equal to ELA 4 . 
         [0061]    It is noted that while the number of the segments increases, the edge lines would appear to be curved. It is also understood that the curved line may consist of more than one curvature, depending on how the segments of the straight line are connected. 
         [0062]    In one embodiment of this third design, the two edge lines of a single groove may have different numbers of segments of straight line. For example, one of the edge lines of a groove is formed of two segments of straight line while the other edge line is formed of three segments of straight line. 
         [0063]    In one embodiment of this third design, all grooves have the same set of two edge lines. 
         [0064]    In a further design, the columns of the luminance enhancement structure may have wavy edges. 
         [0065]    In yet a further design, the two edge lines of the cross section of the grooves are not equal (i.e., asymmetric configuration). 
         [0066]    The details of luminance enhancement structure may be found in US Publication No. 2009-0231245 and U.S. application Ser. Nos. 12/323,300, 12/323,315, 12/628,014, 12/686,197, 12/690,847, 12/719,702 and 61/162,561, the contents of all of which are incorporated herein by reference in their entirety. 
         [0067]    Regardless of the configurations, the size of the apex angles throughout this application, is preferably within a range of about 5 to about 50°, preferably about 20 to about 40°. 
         [0068]    In addition, in all of the structural designs illustrated above, the luminance enhancement structure may be one dimensional ( FIG. 2   b ) or two dimensional ( FIG. 2   c ). However, it is preferable that the structure is one dimensional. 
         [0069]    Unless otherwise stated, the term “substantially equal” or “substantially the same” is intended to refer to the fact that the variances for the angles or distances are within the range of manufacturing tolerances. 
       III. Display Device with the Luminance Enhancement Structure and Auxiliary Layer(s) 
       [0070]      FIG. 4   a  depicts a cross-section view of the luminance enhancement structure on the viewing side of the display device. As shown, the luminance enhancement structure of  FIG. 2   a  has been turned 180°, with the top surface ( 205 ) of the columns ( 202 ) now in optical contact with the top substrate layer ( 106 T) of the display device, which means that there is no air gap between the top surface  205  and the substrate layer  106 T. This may be achieved by an adhesive material, such as the Norland® optical adhesive. 
         [0071]    The space within the grooves ( 203 ) usually is filled with air. It is also possible for the space to be in a vacuum state. Alternatively, the space in the grooves ( 203 ) may be filled with a low refractive index material, lower than the refractive index of the material forming the luminance enhancement structure. 
         [0072]    The thickness of the top substrate layer ( 106 T) is usually between about 5 μm to about 175 μm, more preferably between about 1 μm to about 50 μm. In order to achieve the effect of the luminance enhancement structure, the top substrate layer is preferably as thin as possible (e.g., about 1 μm to about 25μ). During formation of a display cell layer on the substrate layer, preferably the substrate layer is adhered to a base layer for mechanical strength and the display cells are formed on the side of the substrate layer. After the display cells are formed, the base layer is removed and a luminance enhancement structure is laminated (optionally with an adhesive layer) to the substrate layer to complete the assembly. 
         [0073]    The present invention is directed to a display device with a luminance enhancement structure and 
         [0074]    (i) at least one auxiliary layer, or 
         [0075]    (ii) edge sealing, or 
         [0076]    (iii) both (i) and (ii). 
         [0077]    The auxiliary layer(s) ( 400 ) is/are on top of the luminance enhancement structure. The auxiliary layer may be an anti-scratch layer, an anti-glare layer, a moisture protection barrier layer, a touch panel, a UV filter adhesive layer, an optical clear adhesive layer, a hard coat layer or the like. 
         [0078]      FIGS. 4   b  and  4   c  are two examples of such an assembly. In  FIG. 4   b , the auxiliary layer ( 400 ) on top of the luminance enhancement structure ( 401 ) consists of an optical adhesive layer ( 402 ), a touch panel ( 403 ) and an anti-glare layer ( 404 ). In  FIG. 4   c , the auxiliary layer ( 400 ) on top of the luminance enhancement structure ( 401 ) consists of an optical clear adhesive layer ( 405 ), a moisture barrier layer ( 406 ) and an anti-glare and hard coat layer ( 407 ). 
         [0079]    In addition, in order to prevent moisture from seeping through the display device, especially through the grooves of the luminance enhancement structure, edge sealing is recommended.  FIG. 5   a  is the top view of a display assembly ( 500 ) which comprises a display device ( 503 ), a luminance enhancement structure ( 504 ), a backplane ( 501 ) and optionally an auxiliary layer ( 505 ). The edge sealing ( 502 ) covers the sides of the display device ( 503 ), the luminance enhancement structure ( 504 ) and the optional auxiliary layer(s) ( 505 ). Adhesive materials suitable for edge sealing may include, but are not limited to, commercially available sealing/adhesive materials such as Emerson &amp; Cuming brand of ECCOSEAL products, (e.g., 7200), DELO&#39;s KATIOBOND epoxy type of UV curable adhesives and the like.  FIG. 5   b  is a cross-section view. 
         [0080]    In  FIG. 5   b , it is also possible to have an auxiliary layer (e.g., a moisture barrier layer) the edges of which extend beyond the edges of the display device and the luminance enhancement structure. In this case, the edges of the auxiliary layer, the display device and the luminance enhancement structure are sealed. 
       IV. Fabrication of the Luminance Enhancement Structure 
       [0081]    The luminance enhancement structure may be fabricated in many different ways. 
         [0082]    In one embodiment, the luminance enhancement structure may be fabricated separately and then laminated over the viewing side of the display device. For example, the luminance enhancement structure may be fabricated by embossing as shown in  FIG. 6   a . The embossing process is carried out at a temperature higher than the glass transition temperature of the embossable composition ( 600 ) coated on a substrate layer ( 601 ). The embossing is usually accomplished by a mold which may be in the form of a roller, plate or belt. The embossable composition may comprise a thermoplastic, thermoset or a precursor thereof. More specifically, the embossable composition may comprise multifunctional acrylate or methacrylate, multifunctional vinylether, multifunctional epoxide or an oligomer or polymer thereof. The glass transition temperatures (or Tg) for this class of materials usually range from about −70° C. to about 150° C., preferably from about −20° C. to about 50° C. The embossing process is typically carried out at a temperature higher than the Tg. A heated mold or a heated housing substrate against which the mold presses may be used to control the embossing temperature and pressure. The mold is usually formed of a metal such as nickel. The hardening of the embossable composition may be accomplished by cooling, solvent evaporation, cross-linking by radiation, heat or moisture. 
         [0083]    The mold is preferably manufactured by the diamond turning technique. Typically the mold is made by diamond turning technique on a cylindrical blank known as a roll. The surface of the roll is typically of hard copper, although other materials may be used. The pattern on the mold (roll) is the opposite of the intended luminance enhancement structure. In other words, the roll will show sharp protruding patterns which are corresponding to the grooves of the luminance enhancement structure. The pattern on the roll is formed in a continuous manner around the circumference of the roll. In a preferred embodiment, the indentations on the surface of the roll are produced by a technique known as thread cutting. In thread cutting, a single, continuous indentation is cut on the roll while the diamond cutter is moved in a direction transverse to the turning roll. If the mold to be produced has a constant pitch, during manufacture of the mold, the roll will move at a constant velocity. A typical diamond turning machine will provide independent control of the depth that the cutter penetrates the roll, the horizontal and vertical angles that the cutter makes to the roll and the transverse velocity of the cutter. 
         [0084]    As shown in  FIG. 6   a , the mold creates the grooves ( 603 ) and is released during or after the embossable composition is hardened. 
         [0085]    The hardening of the embossable composition may be accomplished by cooling, solvent evaporation, cross-linking by radiation, heat or moisture. 
         [0086]    The refraction index of the material for forming the luminance enhancement structure is preferably greater than about 1.4, more preferably between about 1.5 and about 1.7. 
         [0087]    The luminance enhancement structure may be used as is or further coated with a metal layer. 
         [0088]    The metal layer ( 607 ) is then deposited over the surface ( 606 ) of the grooves ( 603 ) as shown in  FIG. 6   b . Suitable metals for this step may include, but are not limited to, aluminum, copper, zinc, tin, molybdenum, nickel, chromium, silver, gold, iron, indium, thallium, titanium, tantalum, tungsten, rhodium, palladium, platinum and cobalt. Aluminum is usually preferred. The metal material must be reflective, and it may be deposited on the surface ( 606 ) of the grooves, using a variety of techniques such as sputtering, evaporation, roll transfer coating, electroless plating or the like. 
         [0089]    In order to facilitate formation of the metal layer only on the intended surface (i.e., the surface  606  of the grooves), a strippable masking layer may be coated before metal deposition, over the surface on which the metal layer is not to be deposited. As shown in  FIG. 6   c , a strippable masking layer ( 604 ) is coated onto the surface ( 605 ) between the openings of the grooves. The strippable masking layer is not coated on the surface ( 606 ) of the grooves. 
         [0090]    The coating of the strippable masking layer may be accomplished by a printing technique, such as flexographic printing, driographic printing, electrophotographic printing, lithographic printing, gravure printing, thermal printing, inkjet printing or screen printing. The coating may also be accomplished by a transfer-coating technique involving the use of a release layer. The strippable masking layer preferably has a thickness in the range of about 0.01 to about 20 microns, more preferably about 1 to about 10 microns. 
         [0091]    For ease of stripping, the layer is preferably formed from a water-soluble or water-dispersible material. Organic materials may also be used. For example, the strippable masking layer may be formed from a re-dispersible particulate material. The advantage of the re-dispersible particulate material is that the coated layer may be easily removed without using a solubility enhancer. The term “re-dispersible particulate” is derived from the observation that the presence of particles in the material in a significant quantity will not decrease the stripping ability of a dried coating and, on the contrary, their presence actually enhances the stripping speed of the coated layer. 
         [0092]    The re-dispersible particulate consists of particles that are surface treated to be hydrophilic through anionic, cationic or non-ionic functionalities. Their sizes are in microns, preferably in the range of about 0.1 to about 15 um and more preferably in the range of about 0.3 to about 8 um. Particles in these size ranges have been found to create proper surface roughness on a coated layer having a thickness of &lt;15 um. The re-dispersible particulate may have a surface area in the range of about 50 to about 500 m 2 /g, preferably in the range of about 200 to about 400 m 2 /g. The interior of the re-dispersible particulate may also be modified to have a pore volume in the range of about 0.3 to about 3.0 ml/g, preferably in the range of about 0.7 to about 2.0 ml/g. 
         [0093]    Commercially available re-dispersible particulates may include, but are not limited to, micronized silica particles, such as those of the Sylojet series or Syloid series from Grace Davison, Columbia, Md., USA. 
         [0094]    Non-porous nano sized water re-dispersible colloid silica particles, such as LUDOX AM can also be used together with the micron sized particles to enhance both the surface hardness and stripping rate of the coated layer. 
         [0095]    Other organic and inorganic particles, with sufficient hydrophilicity through surface treatment, may also be suitable. The surface modification can be achieved by inorganic and organic surface modification. The surface treatment provides the dispensability of the particles in water and the re-wetability in the coated layer. 
         [0096]    In  FIG. 6   d , a metal layer ( 607 ) is shown to be deposited over the entire surface, including the surface ( 606 ) of the grooves and the surface ( 605 ) between the grooves. Suitable metal materials are those as described above. The metal material must be reflective and may be deposited by a variety of techniques previously described. 
         [0097]      FIG. 6   e  shows the structure after removal of the strippable masking layer ( 604 ) with the metal layer ( 607 ) coated thereon. This step may be carried out with an aqueous or non-aqueous solvent such as water, MEK, acetone, ethanol or isopropanol or the like, depending on the material used for the strippable masking layer. The strippable masking layer may also be removed by mechanical means, such as brushing, using a spray nozzle or peeling it off with an adhesive layer. While removing the strippable masking layer ( 604 ), the metal layer ( 607 ) deposited on the strippable masking layer is also removed, leaving the metal layer ( 607 ) only on the surface ( 606 ) of the grooves. 
         [0098]      FIGS. 6   f  and  6   g  depict an alternative process for depositing the metal layer. In  FIG. 6   f , a metal layer ( 607 ) is deposited over the entire surface first, including both the surface ( 606 ) of the grooves and the surface ( 605 ) between the grooves.  FIG. 6   g  shows that the film of grooves deposited with a metal layer ( 607 ) is laminated with a film ( 617 ) coated with an adhesive layer ( 616 ). The metal layer ( 607 ) on top of the surface ( 605 ) may be conveniently peeled off when the film of grooves is delaminated (separated) from the adhesive layer ( 616 ) coated film ( 617 ). The thickness of the adhesive layer ( 616 ) on the adhesive coated film is preferably in the range of about 1 to about 50 um and more preferably in the range of about 2 to about 10 um. 
         [0099]    The luminance enhancement structure comprising grooves (uncoated or coated with a metal layer) is then laminated over a layer of display cells as described above. 
         [0100]    For a two dimensional luminance enhancement structure, it may be manufactured by a self-aligned process as disclosed in U.S. Ser. No. 12/323,300, the content of which is incorporated herein by reference in its entirety. In the self-aligned process, the display cells are formed by a photolithography process, utilizing the luminance enhancement structure as a photomask. 
         [0101]    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, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.