ELECTRODE CONFIGURATIONS FOR ELECTROPHORETIC DEVICE

Abstract: An electrophoretic display device is set forth having various electrode schemes in order to modulate transmis­ sion of light through the device. The electrophoretic parti­ cles (9, 16, 25) of the suspension form an imaging structure relating to an array of small unit cells or pixels which are turned on or off by appropriate electrical signals. Accord­ ingly, the pigment particles (9, 16, 25) totally cover transpa­ rent electrodes (3, 4, 13, 14, 23, 24, 33), which condition can be changed by applying the appropriate electric field to the opposite electrode (3, 4, 13, 14, 23, 24, 33). By this structure, various alphanumeric displays can be constructed.

Description: The present invention relates to a transmissive electrophoretic display device comprising, between a first light transparent substrate with a first electrode configuration and a second light transparent substrate with a second electrode configuration, an electrophoretic suspension comprising a fluid containing pigment particles the surface of a least one of the substrates being provided with opaque members. Electrophoretic display devices offer an improved technique for displaying images in various systems of optical displays. A device as mentioned in the opening paragraph is known from EP-A-0 023 741. In this device light leakage at non-selected areas is prevented by choosing the refractive indices of the fluid and the pigment particles different from each other and giving one of the substrates a refractive index greater than the refractive index of the fluid. The respective indices are chosen such that light leakage is prevented by internal reflection. To obtain this the substrate may comprise a prism or have a special profile. A device according to the invention is charachterized in that the first electrode configuration comprises at least one transparent first electrode which extends at least at the area of a picture element and in a light absorbing state is covered by the pigment particles and the second electrode configuration comprises at least one second electrode which has a small area relative to the area of said first electrode and in a light transmitting state is covered by the pigment particles. In such a device light transmission is controlled by the electrode structures. These structures make it possible to view directly an electrophoretic display device wherein the display is illuminated from the side opposite to the observer. Electrophoretic display devices provide visible display of images in an electrophoretic fluid by appropriate application of electric fields to the electrophoretic fluid. Such electric fields modulate light by the electrophoretic fluid. Electrophoretic fluids are constituted principally by a fluid or dye of one color containing a colloidal suspension of pigment particles of a different color or brightness contrast. Upon appropriate application of electric fields, the pigment particles are concentrated at the electrodes at one side or the other of the electrophoretic fluid to cause either viewing of the color of the colloidal particles, or viewing of the color of the electrophoretic fluid, as for example described in US-A-3 756 693. By appropriate electrode arrangements according to the present invention, the basic imaging structure in an electrophoretic display includes a rectangular array of small unit cells, or picture elements (pixels), which are turned "on" or "off" by appropriate electrical signals. That is, the colloidal pigment particles cover an appropriate electrode structure or region during the "off" period, but during an "on" period the pigment is transported to a relatively small region blocking only a small portion of light from passing through the pixel. When the entire device is illuminated from the rear, an image is viewed which is formed by dots of light against a dark background. This scheme can be used with almost any method of addressing each pixel where such addressing occurs by imposing a DC voltage level between two pixel electrodes. A first embodiment according to the invention is characterized in that said first transparent substrate includes a plurality of hemispherical cavities, each of said cavities having a hemispherical coating forming said first electrode configuration and wherein said second electrode configuraton includes a plurality of metal pin electrodes, each of said metal pin electrodes extending into a respective hemispherical cavity. Using cavities coated with a conductive layer as electrodes to change the optical properties of a fluid containing charged particles is known per se from In another scheme according to the present invention, the display device includes parallel plates being separated by a small distance. In such a structure, one electrode arrangement would have electrodes of a given width at one side of the display device and being at a reference potential, whereas electrodes at the other side of the display device are of a second width which is interspersed with the width of the electrodes at the first surface. These second electrodes are the driven electrodes, being asymmetrically driven to positive or negative values enabling the switching of the electrophoretic device. The ratio of the width of the electrodes to the distance between the parallel substrates should be small as possible to first, improve field uniformity, and second, to maximize the transmission of light when the pixel is turned "on" consistent with the ability of the driven electrodes to gather the pigment. Such an arrangement involves that the overall electrode size should be as small as possible so that many stripes of light per pixel will occur. A variation of this structure is provided by the provision of a single electrode covering the surface of one of the substrates, whereas the strip electrodes at the opposite surface have a given width and separation similar to the strip electrode sizes as noted above. This structure enables an electric field to be more uniform since the single electrode has no corners. Otherwise, a higher pigment concentration is needed to cover the electrodes. The features and advantages of the present invention will be described in more detail by way of example relative to the several embodiments of the present invention, with reference to the drawing figures, in which: Figure 1 illustrates in partial cross-section the operation according to the present invention; Figure 2 illustrates the arrangement of one set of electrodes according to a first embodiment of the present invention; Figure 3 illustrates a second embodiment of an electrode structure according to the present invention; and Figure 4 illustrates a variation of the embodiment of Figure 3. The arrangement of the present invention may be seen by reference to Figure 1 in which a first transparent substrate is provided at a small distance from a second transparent substrate 2. The first substrate 1 has a plurality of electrodes 3 on an inner surface thereof, while the substrate 2 has a plurality of electrodes 4 formed in facing relationship to electrodes 3. A light source 5 is provided at one side of the display structure to transmit light through the display device to an observer 6. From the electrophoretic solution 8, a plurality of pigment particles 9 are formed on the respective electrodes, according to the potentials applied thereto. For example, as seen in Figure 1, the pigment particles 9 are formed as a coating on electrodes 3 in two segments of the display device, whereas the pigment particles 9 are formed over the electrodes 4 at the opposite side of the electrophoretic fluid 8 in another segment of the display device. This latter corresponds to the "on" scheme of imaging in the electrophoretic device since the electrodes 4 being coated with pigment particles 9 only block a small portion of light from the source 5 from passing through the display device. Consequently, areas between the electrodes 4 are made visible by light passing through the display device. By appropriate arrangement of the electrode structures, alpha-numeric displays can be formed. The covering of the electrodes 3 by the pigment particles 9 covers the display device at those portions with the color of the particles so that no light passes through the display device. Appropriate opaque portions 7 are applied to each substrate surface 1 and 2 in order to separate the several electrode structures 3 and 4. By this means, light only passes through the area designated by the respective electrodes 3 and 4 in the instance when the pigment particles 9 are attracted to the electrodes 4. One of the advantages of the present invention is the provision of electrode patterns on the two substrates that enable illumination with transmitted light. A first scheme of accomplishing this may be seen by a first embodiment, as illustrated in Figure 2. In this embodiment a substrate 11 has a plurality of hemispherical cavities over which, at both the inner face of the substrate 11 and the cavity 15, an electrode 13 is coated. An indium-tin oxide or other transparent electrical conductor is provided for the electrode 13. Moreover, this electrode is maintained at ground potential. At a second substrate 12, metal pin electrodes 14 are inserted there through to extend into the hemispherical cavity. This arrangement enables a readially isotropic electric field to be arranged within the electrophoretic cell; and therefore, enables uniform pigment distribution over the surface of the hemispherical cavity. As such, light passing through the substrate 11 will be blocked by the pigment particles 16 in the cavity 15. During switching of the arrangement to an "on" time, the pigment 16 will travel to the metal pin electrode, thereby freeing the area in the hemispherical cavity around the pin 14 to pass light. Opaque layers 17 are provided for masking the areas between the hemispherical cavities. The electrode 14 is driven to either a plus or minus potential in order to switch the electrophoretic cell into the "on" or "off" condition. It has been found that a disadvantage of this structure is that fabrication of the hemispherical cells is difficult. Further, refraction and total internal reflection is possible within the cavities and index matching is required between the substrate surfaces and the electrophoretic suspension 18 between the substrates 11 and 12. A further arrangement of the present invention may be seen in Figure 3 which illustrates another embodiment of the present invention. In this embodiment, two parallel substrates 21 and 22 are separated by a distance t in which the electrophoretic suspension 26 with pigment particles 25 is provided. The pigment particles 25 are induced into electrodes 23 on the surface of the substrate 21. Such covering of the electrodes 23 with the pigment particles enables only the passage of light through the area between the electrodes 23. These areas correspond to the width w₂ being the size or width of the electrode strips 24 on the substrate 22. However, these electrodes 24 are opaque so that light does not pass through them. Upon being switched to the "on" condition, the electrophoretic particles on the electrodes 23 will migrate to the electrodes 24 thereby enabling light to pass through the structure. The electrodes 24 are the driven electrodes, being asymmetrically driven to either a plus or minus polarity in order to switch the electrophoretic cell. The electrodes 23, on the other hand, are at a reference potential, such as ground. These electrodes 23 may of an indium-tin oxide material or any other similar transparent material, while the electrodes 24 are of an opaque conductive material. The electrodes 23 extend perpendicular to the drawing figure along the length of the substrate 21. These electrodes can be crossed-linked in an H pattern. Similarly, the electrodes 24 extend perpendicular to the paper on the substrate 22. In this structure of Figure 3, the electrodes 23 have a width w₁, while the electrodes 24 have a width w₂. The width of the electrode 24 is equal to the separation of the electrodes 23, and the electrodes 24 extend along the areas of separation of the electrodes 23. The distance between the substrates 21 and 22 is the distance or thickness t. In this structure, the ratio of w₁ to t should be as small as possible to improve field uniformity. Further, the ratio of w₂ to w₁ should be as small as possible to maximize transmission of light when the pixel is in the "on" condition, consistent with the ability of the w₂ electrodes to gather with the pigment particles. These two considerations require that the overall electrode size should be as small as possible, so that there will be many electrode stripes per pixel. A variation of this embodiment may be seen in Figure 4 in which the electrode 33 is a single electrode covering the surface of the substrate 21. This electrode 33 is at a reference potential, such as ground, and is opposite to the plurality of electrodes 24 which are opaque. These electrodes 24 are shown in Figure 4 as having electrophoretic pigment particles 25 formed thereon so that the "on" condition is established. In this arrangement, the electric field is more uniform because the ground electrode 33 has no corners. Moreover, 100% of the front surface of the substrate 21 is transparent inasmuch as electrode 33 is transparent. In such an arrangement, a higher pigment concentration is necessary to cover this electrode during the "off" condition. Otherwise, the same relationships between w₁, w₂ and t exist as that shown in Figure 3.

Claims: A transmissive electrophoretic display device comprising, between a first light transparent substrate (1, 11, 21) provided with a first electrode configuration (3, 13, 23, 33) and a second light transparent substrate (2, 12, 22) provided with a second electrode configuration (4, 14, 24), an electrophoretic suspension comprising a fluid (8, 18, 26) containing pigment particles (9, 16, 25) the surface of at least one of the substrates being provided with opaque members (7, 17, 24) characterized in that the first electrode configuration comprises at least one transparent first electrode which extends at least at the area of a picture element and in a light absorbing state is covered by the pigment particles and the second electrode configuration comprises at least one second electrode which has a small area relative to the area of said first electrode and in a light transmitting state is covered by the pigment particles. An electrophoretic display device according to Claim 1, characterized in that said first transparent substrate (11) includes a plurality of hemispherical cavities (15), each of said cavities having a hemispherical conductive coating forming said first electrode configuration (13), and wherein said second electrode configuration includes a plurality of metal pin electrodes (14), each of said metal pin electrodes (14) extending into a respective hemispherical cavity (15). An electrophoretic display device according to Claim 1, characterized in that said first transparent substrate (1, 21) and said second transparent substrate (2, 22) are both planar plate-like structures being parallel to each other, said transparent substrates being separated by a distance t. An electrophoretic display device according to Claim 3, characterized in that said first electrode configuration includes a plurality of transparent parallel electrodes (23) extending in one dimension of said first transparent substrate (21) each of said first plurality of parallel electrodes (23) having a width w₁ and being separated from one another by a distance w₂, and wherein said second electrode configuration includes a second plurality of opaque parallel electrodes (24) extending in the same direction as said first plurality of electrodes, each of said second plurality of parallel electrodes having a width w₂ and being separated from one another by a distance w₁. An electrophoretic display device according to Claim 3, characterized in that said first electrode configuration includes a single transparent electrode (33) covering a surface of said first transparant substrate (21), and wherein said second electrode configuration includes a plurality of opaque parallel electrodes (24) extending in one dimension of said second transparent substrate (22), each of said plurality of electrodes (24) having a width w₂ and being separated from one another by a distance w₁.

Inventors: MARSHALL, THOMAS, STUPP, EDWARD HENRY
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: G02F1
IPC codes: G09F9, G02F1