Abstract:
An electrophoretic display includes a cell having a viewed region and a non-viewed region. The cell contains a suspending fluid and a first particle species and a second particle species dispersed within the suspending fluid. Application of a first electrical field causes the first particle species and the second particle species to vibrate and separate from: one another, the cell walls, the viewed region, and the non-viewed region. Application of a second electric field, in one direction, causes the first particles to migrate toward the viewed region and the second particles to migrate toward the non-viewed region, effecting a color state. The electrophoretic display may be fabricated from multiple display cells arranged on a substrate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to electrophoretic displays, particularly encapsulated electrophoretic displays, and to a method for enhancing the colored state(s) and contrast of such displays.  
           [0002]    Traditionally, electronic displays such as liquid crystal displays have been made by sandwiching an optoelectrically active material between two pieces of glass. In many cases, each piece of glass has an etched, clear electrode structure formed using indium tin oxide (ITO). A first electrode structure controls all the segments of the display that may be addressed, that is, changed from one visual state to another. A second electrode, sometimes called a counterelectrode, addresses all display segments as one large electrode, and is generally designed not to overlap any of the rear electrode wire connections that are not desired in the final image. Alternatively, the second electrode is also patterned to control specific segments of the display. In these displays, unaddressed areas of the display have a defined appearance.  
           [0003]    Electrophoretic displays offer many advantages compared to liquid crystal displays. Electrophoretic display media are generally characterized by the movement of particles through an applied electric field. Encapsulated electrophoretic displays also enable the display to be printed. These properties allow encapsulated electrophoretic display media to be used in many applications for which traditional electronic displays are not suitable, such as flexible displays. Additionally, electrophoretic displays typically have attributes of good brightness, wide viewing angles, high reflectivity, state bistability, and low power consumption when compared with liquid crystal displays. However, problems with the image quality, specifically the contrast, to date has been less than optimal. Contrast is defined as the ratio of the white state to the dark state reflectance of the display. Contrast enables the eye to easily distinguish between light and dark.  
           [0004]    One example of an electrophoretic display involves the use of an electrophoretic ink which uses cells or microcapsules filled with black and white particles. The particles can be electrically manipulated to position themselves on the top or the bottom of the microcapsule or cell and therefore generate black or white surface visibility to an observer. In electrophoretic displays, the particles are oriented or translated by placing an electric field across the cell. The electric field typically includes a direct current field. The electric field may be provided by at least one pair of electrodes disposed adjacent to a display comprising the cell. Actual display of black or white colors is accomplished by manipulating the position of the particles in correspondence with the observing angle. Once set for a black state or a white state, the display maintains its color until a different configuration is forced through the application of a subsequent electrical field.  
           [0005]    The purpose of this disclosure is to describe the switching of a two-particle electrophoretic display comprising two-particle electrophoretic ink consisting of a first particle species of a first color (e.g. white) and a second particle species of a second color (e.g. black) suspended in a clear medium. The different colored particles carry opposite charges. Current electrophoretic displays are switched by application of a DC voltage in order to move the charged pigment particles. The switching of the polarity of the DC voltage results in moving the white particles to a first electrode (i.e. viewed region) and the black particles to a second electrode (i.e. non-viewed region) and vice versa.  
           [0006]    Due to particle clustering, settling, adhesion, etc., particularly at high particle densities, the respective colored states and contrast ratio is often degraded because particles of one color are trapped near or at the viewing region by particles of the other color. This trapping of the undesired colored particles reduces the contrast ratio at the viewing region. In other words, a white state is not completely comprised of white particles and a black state is not completely comprised of black particles at the viewed region.  
         SUMMARY OF THE INVENTION  
         [0007]    This invention relates to an improved method for enhancing the colored states and improving the contrast image of an electrophoretic display. In particular, the present invention provides for a two-particle electrophoretic display, along with methods and materials for use in such displays. The electrophoretic display may be filled into a grid of cells made from, for example, a photopolymer material. In the electrophoretic display of the present invention, the particles are vibrated, rotated, and moved by application of electric fields. One electric field may be an alternating current (AC) field and another electric field may be a direct current (DC) field. The electric fields may be created by at least one pair of electrodes disposed adjacent a suspending fluid containing the particles. The particles may be made up of some combination of dye, pigment, and/or polymer. It will be appreciated that the present invention may also be applied to a one-particle electrophoretic display in which the particles are dispersed in a dyed suspending fluid or a display in which the particles have a positively charged hemisphere and a negatively charged hemisphere differentially colored, respectively.  
           [0008]    The electrophoretic display may take many forms. The display may comprise an array of cells each formed from a limitless variety of sizes and shapes. The perimeter of the cells may, for example, form a polygon, circle, or other geometric configuration and may have dimensions in the millimeter range or the micron range. The particles may be one or more different types of particles. The particles may be colored and may be positively or negatively charged. The display may further comprise a clear or dyed dielectric suspending fluid in which the particles are dispersed.  
           [0009]    This invention provides novel methods for controlling and electronically addressing particle-based displays. Additionally, the invention discloses applications of these methods and associated materials on substrates which are useful in large area, low cost, or high durability applications.  
           [0010]    In one aspect, the invention relates to an encapsulated electrophoretic display which includes a cell having a first or viewed region and a second or non-viewed region and containing a suspending fluid with a plurality of first particles of a first electrical charge and a plurality of second particles of a second electrical charge. The first particles and the second particles are dispersed within the suspending fluid. The first particles have a first color (e.g. white) and the second particles have a second color (e.g. black). The application of a first electrical field causes the first particles and the second particles to vibrate and separate from each other. Application of a second electrical field, having a first polarity, effects a first color state by causing the first particles to migrate towards the viewed region and the second particles to migrate towards the non-viewed region.  
           [0011]    In another aspect, the invention relates to a method of improving the colored states and contrast ratio of an encapsulated electrophoretic display comprising the steps of: providing a two-particle electrophoretic display consisting of at least one first particle of a first color and a first electrical charge and at least one second particle of a second color and a second electrical charge; suspending the first particles and the second particles in a clear medium contained in a matrix of photopolymer cells, each cell having a viewed region and a non-viewed region. Application of an alternating current electrical field causes the first particles and the second particles to vibrate and separate. This effect reduces the adhesion of the particles with: the other particles, the cell walls, the non-viewed region, and the viewed region. Application of a second direct current electrical field, having a first polarity, causes the migration of the first particles toward the viewed region and the second particles toward the non-viewed region.  
           [0012]    In yet another aspect, the invention relates to an encapsulated electrophoretic display which includes a cell having a first or viewed region and a second or non-viewed region and containing a dyed suspending fluid with a plurality of particles of an electrical charge. The particles are dispersed within the dyed suspending fluid. The particles have a first color (e.g. white) and the fluid has a second color (e.g. black). The application of a first electrical field causes the particles to vibrate and separate from each other. Application of a second electrical field, having a first polarity, effects a first color state by causing the particles to migrate toward the viewed region. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention may take physical form in certain parts and arrangements of parts, several preferred embodiments of which are described in the specification and illustrated in the accompanying drawings which form a part hereof and wherein:  
         [0014]    [0014]FIG. 1 shows a series of cells containing particles in a suspending fluid and having electrodes disposed adjacent thereto;  
         [0015]    [0015]FIG. 2 shows a top perspective view of a sample portion of several cells arranged in a grid or array;  
         [0016]    [0016]FIG. 3 is a chart showing the voltage sequences (voltage/time) for an alternating current electric field and a direct current electric field;  
         [0017]    [0017]FIG. 4A is a chart showing the linear application of the direct current electric field;  
         [0018]    [0018]FIG. 4B is a chart showing the non-linear application of the direct current electric field;  
         [0019]    [0019]FIG. 4C is a chart showing another non-linear application of the direct current electric field;  
         [0020]    [0020]FIG. 5A is a diagrammatic side view of a display cell of an initial colored (white) state in which the white particles are at the viewed region and the black particles are at the non-viewed region;  
         [0021]    [0021]FIG. 5B is a diagrammatic side view of the display cell in which the particles are stirred up as a result of the application of an alternating current electric field;  
         [0022]    [0022]FIG. 5C is a diagrammatic side view of the display cell in which the agitated black particles are in a state of migration toward the viewed region and the agitated white particles are in a state of migration toward the non-viewed region. The migration of both the black particles and the white particles is a result of the application of a direct current electric field;  
         [0023]    [0023]FIG. 5D is a diagrammatic side view of a final colored (black) state of the display cell in which the black particles are at the viewed region and the white particles are at the non-viewed region;  
         [0024]    [0024]FIG. 5E is a diagrammatic side view of another display cell representing the prior art in which some of the white particles are trapped by the black particles and some of the black particles are trapped by the white particles,  
         [0025]    [0025]FIG. 6 depicts concepts of the present application used in association with an active matrix display; and,  
         [0026]    [0026]FIG. 7 sets forth a pixel cell of the display shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The present application relates to improved encapsulated electrophoretic displays and, more particularly, to the colored states and resultant contrast of such displays. Generally, an encapsulated electrophoretic display includes one or more species of particles that either absorb or scatter light. One example, in which this invention relates, is a system in which the cells or capsules contain two separate species of particles suspended in a clear suspending fluid. One species of particles may be white, while the other species of particles may be black. The particles are commonly solid pigments, dyed particles, or pigment/polymer composites. The two species of particles may also have other distinct properties, such as, fluorescence, phosphorescence, retroreflectivity, etc.  
         [0028]    An encapsulated electrophoretic display can be constructed so that the optical state of the display is stable for some length of time. When the display has two states which are stable in this manner, the display is said to be bistable. The term bistable will be used to indicate a display in which any optical (colored) state remains fixed once the addressing voltage is removed. For the purpose of this invention, the bistable states represent a white state and a black state.  
         [0029]    Electrophoretic displays of the invention are described below. Preferably, these displays are microencapsulated two-particle species electrophoretic displays, but also may include one-particle species electrophoretic displays or particles with a positively charged hemisphere and a negatively charged hemisphere differentially colored, respectively. Concepts of the invention include providing a reflective display which provides improved colored states and a higher contrast ratio than heretofore realized.  
         [0030]    Referring to FIG. 1, a two-particle electrophoretic display  10  is shown, which consists of one particle species of a first color  12  (e.g. white) and another particle species of a second color  14  (e.g. black). The display  10  further comprises a clear suspending or carrier fluid  16  in which the two-particle species  12 ,  14  are dispersed. The particles  12 ,  14  and carrier fluid  16 , are together referred to as the particle dispersion and/or two particle electrophoretic ink  17 . An optically transmissive cell  24  surrounds the particle dispersion  17 . The first and second particles  12 ,  14  differ from each other optically and in terms of at least one other physical characteristic that provides the basis for their separation. For example, the particles  12 ,  14  are colored differently and have different surface charges. Such particles may be obtained by surrounding differently colored pigment core particles with transparent polymer coatings having different zeta potentials. As shown, the two-particle electrophoretic ink  17  consists of one particle species of a first white color  12  and another particle species of a second black color  14 . In one configuration, the black colored particles  14  carry a positive charge  15 , while the white colored particles  12  carry a negative charge  13 . The particle size can range from about 0.1 micron to about 10 microns. In the absence of an electric field, the particles  12 ,  14  are substantially immobile.  
         [0031]    There is much flexibility in the choice of particles for use in electrophoretic displays. For purposes of this invention, the particles  12 ,  14  are any components that are charged or capable of acquiring a charge (i.e. has or is capable of acquiring electrophoretic mobility). The particles  12 ,  14  may be neat pigments, dyed pigments, or pigment/polymer composites, or any other component that is charged or capable of acquiring a charge. The particles  12 ,  14  may be surface treated so as to improve charging or interaction with a charging agent, or to improve dispersability. A preferred white particle that may be used in electrophoretic displays according to the invention are particles of titania. The titania particles may be combined with a polymeric resin and may be coated with a metal oxide, such as aluminum oxide or silicon oxide, for example. The titania particles may have one, two, or more layers of metal oxide coating. For example, a titania particle for use in electrophoretic displays of the invention may have a coating of aluminum oxide and a coating of silicon oxide. The coatings may be added to the particle in any order. The coatings should be insoluble in the suspending fluid  16 . Additionally, the black particles  14  may be absorptive, such as carbon black or colored pigments used in paints and ink. The pigments should also be insoluble in the suspending fluid  16 .  
         [0032]    As discussed, the particles  12 ,  14  are dispersed in a suspending fluid  16 . The suspending fluid  16  should have a low dielectric constant. The fluid  16  should be clear, or substantially clear, so that the fluid  16  does not inhibit viewing the particles  12 ,  14 . The suspending fluid  16  containing the particles  12 ,  14  can be chosen based on properties such as density, refractive index, and solubility. The suspending fluid  16  may be made from a hydrocarbon including, but not limited to, dodecane, tetradecane, toluene, xylene, and the aliphatic hydrocarbons in the Isopar™ series. Isopar™ is a registered trademark of The Exxon Corporation, Houston, Tex.  
         [0033]    As shown in FIG. 1, three cells  24  are displayed. It will be appreciated that any number of grids or arrays  28  of cells  24  may be arranged (refer to FIG. 2). It is further appreciated that the actual display of a black color state  20  or a white color state  18  is accomplished by manipulating the position of the particles  12 ,  14  in each cell  24  in correspondence with the observing angle  30 . As shown, the cells  24  are cubical in geometry. It will be further appreciated that any number of geometric configurations may be utilized. The cells  24  represent a spacer layer and may be made from a photopolymer (i.e. SU-8). The cells may also be made by microencapsulation methods including, but not limited to, coacervation, or interfacial polymerization as described in U.S. Pat. No. 6,392,785 to Albert, et al., which is incorporated herein by reference. The cells may also be made by molding or embossing. The walls  26  of the cells  24  may be coated to prevent particle adhesion. For the invention described herein, the cell geometry is not essential. As an example, the visible square viewing region  32 , as shown in FIG. 2, is approximately 200 microns along each side. The use of separate cells  24  prevents agglomeration and settling of the particles  12 ,  14 .  
         [0034]    Referring again to FIG. 1, an addressing scheme for controlling the color state of the display  10  is shown in which an electrode  40  (or set of electrodes) is adjacent a non-viewed region  25  (i.e. bottom or rear surface) of the cells  24  and another continuous top electrode  42  is adjacent a viewed region  27  (i.e. top or front surface) of the cells  24 . The top electrode  42  may take the form of an indium tin oxide coating (ITO) of a transparent glass substrate  50  overlying the cell array  28 . The glass substrate  50  may be similar to those used in liquid crystal displays. The ITO top electrode  42  may be evaporated onto the top glass substrate  50 . The ITO top electrode  42  is transparent, and the colored states  18 ,  20  are viewed through the ITO top electrode  42 . Underlying the cell array  28  is a glass bottom substrate  52 . Alternately, the bottom substrate  52  may be a silicon wafer with patterned electrodes or an active matrix backplane, to be described hereinafter. It will be appreciated that the top and bottom electrodes  40 ,  42  may also be formed from flexible material, such as ITO coated Mylar™. Mylar™ is a registered trademark of E.I. DuPont Corporation, Wilmington, Del.  
         [0035]    It will also be appreciated that the viewed and the non-viewed regions can be arranged laterally (not shown) so that the non-viewed region (although observable) is significantly smaller in area with respect to the viewed region (such as in laterally driven electrophoretic displays).  
         [0036]    The electrodes  40 ,  42  are connected to a pair of voltage sources  60 ,  62 . One voltage source  60  provides an AC (alternating current) field while the other voltage source  62  provides a DC (direct current) field.  
         [0037]    As discussed, the different colored particles  12 ,  14  carry opposite charges  13 ,  15 , respectively. Current electrophoretic displays switch their color states using a DC voltage only in order to move the charged pigments to a viewing region. At high particle densities, the contrast ratio is often degraded because particles of one color are trapped near the viewed region by particles of the other color (FIG. 5E). In accordance with concepts of the present invention, a proposed method prevents such trapping, thereby improving the contrast of the display  10 . Specifically, the electric field generated by a DC voltage  62  is overlaid with an electric field generated by an AC voltage  60 . The voltages  60 ,  62  are applied between the top and bottom electrodes  42 ,  40 . The AC voltage  60  is used to set the particles  12 ,  14  into a vibrating motion. While the particles  12 ,  14  are vibrating and shaking back and forth, the DC voltage  62  is ramped up (increased) to its maximum value. This process enables particles  12 ,  14  to move past each other more easily, and prevents agglomeration of particles  12 ,  14  during the switching process and is helpful in shaking loose particles  12 ,  14  which are sticking to other particles  12 ,  14 , the viewed region  27 , the walls  26 , and/or the non-viewed region  25  of the cells  24 . The ramping of the DC voltage  62  involves moving from a lower to a higher voltage until the total voltage is either positive or negative. As long as the DC voltage  62  is less than the amplitude of the AC voltage  60 , the pair of voltages  60 ,  62  exhibit a reverse pulse which moves the particles  12 ,  14  slightly in a direction opposite to the direction of migration. Once the total voltage is either positive or negative, the AC voltage  60  may be switched off.  
         [0038]    As an example of addressing the display  10 , for particles  12 ,  14  of about 1-10 microns in diameter, an AC frequency in the range of 10-150 Hz may be applied. For smaller particles and/or particles with a higher charge and a higher mobility, a higher frequency (i.e. 500 Hz) may be applied. The amplitude of the AC voltage  60  is approximately equivalent to an electric field of about 1-2 volts/micron. While the AC voltage  60  is applied to the particles, a DC voltage  62  is added and may be slowly increased to a value that moves the particles  12 ,  14  to the opposite electrodes (described in detail below). During the time period that the DC voltage  62  is increasing, the black and white particles  14 ,  12 , respectively migrate to opposite electrodes. This driving method becomes particularly important when the particle density is high. High particle densities become necessary in thin displays in order to still provide good reflectivity, improved colored states, and high contrast.  
         [0039]    Referring to FIG. 3, the combined AC and DC voltages  60 ,  62  are diagramed. As applied to a black and white electrophoretic display  10 , initially (t 0  to t 1 ) the AC voltage  60  creates a grey state  19  (representing a mixture of the black and white particles) until the DC voltage  62  is applied which creates an electric field in one direction. As shown in FIG. 3, the DC voltage  62  is increased between time t 1 , and time t 2  to a value V 1  that moves the particles into an initial black state  20 . In order to further improve the arrangement of the electrophoretic particles in a single color state (i.e. black state  20 ), the DC voltage  62  may be changed or ramped  64  (V 1 →V 3 →V 1 ) one or more cycles between time t 2  and time t 3 . The duration of each ramping cycle  64  may be from approximately 10 milliseconds to 10 seconds. The actual duration of each ramping cycle  64  depends upon the cell  24  dimensions and the particle  12 ,  14  mobility. The ramping cycle  64  may be continuous (as shown in FIG. 3) or discontinuous (not shown). The higher the AC frequency the faster can be the ramping cycles  64  of the DC field. The repetitions of the ramping  64  are shown by the dashed lines on the DC voltage diagram. It will be appreciated that the AC voltage  60  may start at a higher voltage and gradually taper to a lower voltage (not shown). Once the black state  20  is complete (t 3 ), the AC voltage  60  may be switched off. The black color state  20  may be switched to a white color state  18  by first applying the AC voltage  60  from time t 3  to time t 6  and secondly applying a reversed polarity of the DC voltage  62  from time t 4  to time t 6 . As a result, the white particles  12  are attracted to the viewed region  27  and a white color state  18  results (t6).  
         [0040]    The DC voltage  62  may increase (V 0 →V 1 ) in a linear arrangement or in a non-linear arrangement (FIGS.  4 A- 4 C) from time t 1  to time t 2 . Changes in the DC field are slower than the frequency of the AC field. It will be appreciated that the AC component  60  may be a sine wave, a triangular wave, a sawtooth function, etc. (not shown). It will be further appreciated that the AC and DC voltage signals  60 ,  62  could be generated with discreet digital voltage levels.  
         [0041]    As shown in FIGS.  5 A- 5 D, the particle migration is displayed going from an observed initial white color state  18  to a black color state  20 , respectively. FIG. 5A represents the initial white color state  18 . FIG. 5B displays the application of an alternating current electric field  60 , whereby the particles begin to oscillate and separate from the other particles, the walls  26 , the rear or bottom surface  25 , and the top or front surface  27 . Once the direct current electric field  62 , FIG. 5C, is applied, the particles  12 ,  14  begin to migrate. As shown in FIG. 5C, the positively charged black particles  14  begin to migrate towards the negatively charged upper electrode  42 . At or near the same time, the negatively charged white particles  12  begin to migrate towards the positively charged bottom electrode  40 .  
         [0042]    [0042]FIG. 5D represents the observed final black color state  20 , in which all of the black particles  14  have migrated to the viewed region  27  and all of the white particles  12  have migrated to the non-viewed region  25 . It will be appreciated that the black particles  14  have not trapped any white particles  12 . Similarly, the white particles  12  have not trapped any of the black particles  14 . In contrast, FIG. 5E shows a final black color state  20 ′ of a display  10 ′ without the application of an alternating current electric field. As a result, some of the white particles  12  are trapped by the black particles  14 , and are visible to the observer  30 . This trapping results in a degradation of the observed colored states and the contrast of the resultant display.  
         [0043]    As an alternative embodiment, the addressing scheme applied to an electrophoretic display as described above may also apply to an active matrix electrophoretic display  100  (FIG. 6). In this embodiment, a typical backplane or back plate  102  architecture implemented using thin film transistors (TFT)  108  comprises an array of individual pixel cells  104  arranged on the substrate  106 . It will be appreciated that display  100  includes electrophoretic ink (not shown) and a counterelectrode (not shown) overlying the backplane  102 . Pixel cells  104  are selectively activated via the TFTs or pixel switches  108 . Gate lines  112  control the pixel switches  108  either block or passe voltage signals on a data line  110 . The writing of a frame (i.e. one computer image) involves applying a voltage to each individual pixel  104  so that an image appears. In the described active matrix addressing display  100 , the writing is done by addressing the gate line  112  with a voltage pulse. The transistors  108  on the same gate line  112  will go to an open state. The data (voltage levels) which is on the data lines  110  is then passed through the transistor  108  to the pixels  104  (pixel storage capacitors). After another gate line  112  is addressed, new data is written to the associated pixels  104  which are on this gate line  112 .  
         [0044]    [0044]FIG. 7 shows a circuit diagram of one pixel cell  104  in the TFT backplane  102  with example voltages A, B,  114 ,  116 . In this example, the AC voltage would be applied to the common counterelectrode or top transparent electrode (not shown) of the electrophoretic display  100 . The ramping of the DC voltage (similar to what is depicted in FIG. 4C) would be done in steps by writing frames (i.e. one computer image) with increasingly higher voltage amplitude on the data lines  110 . In the example of FIG. 7, the voltage levels may also be shifted (i.e. the common ground may be shifted to a positive value) so that only positive voltage levels are involved. Instead of addressing the active matrix display  100  “per frame” described above (where all the pixels  104  are addressed with one set of voltage levels, after which all transistors  108  are addressed again with a new set of voltage levels, etc.), one could also perform the addressing per line. In this “per line” addressing, one would switch “on” all the transistors  108  which are connected to a first gate line  112  and then repetitively write data signals (the DC component) to all the associated data lines  110  until the desired voltage state is reached. Then the transistors  108  on this first gate line  112  would be switched “off” and a second gate line  112  would be addressed (this means the transistors  108  on this second gate line  112  would be switched to the “on” state). Again, the data signals on the data lines  110  would be increased or decreased (in steps or continuously varying as shown in FIGS. 4A, 4B, and  4 C) until the desired voltage levels would be reached. Then the transistors  108  on this second gate line  112  would be switched “off” and yet another third gate line  112  would be addressed.  
         [0045]    Another embodiment for addressing an electrophoretic active matrix display employs a constant voltage potential on the common counterelectrode (point “B” in FIG. 7). A combined “AC/DC” signal similar to the ones described before (or as shown in FIG. 3) is approximated by only changing the voltage levels on the data lines  110 . This applies to “per line” addressing and to “per frame” addressing. In this case, “per frame” addressing requires a short frame time so that high enough frequencies (depending on the frequency requirement for the AC voltage requirement) on the pixel cells  104  can be achieved.  
         [0046]    The invention has been described with reference to several preferred embodiments. Obviously, alterations and modifications will occur to others upon a reading and understanding of the specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.