An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between them. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates, respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate may be used for the active type EPDs.
An electrophoretic dispersion composed of a dielectric solvent and charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the charged pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, may be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD [M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. E D 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026 and U.S. applications, Ser. No. 60/443,893, filed Jan. 30, 2003 and Ser. No. 10/766,757, filed on Jan. 27, 2004) and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M. A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2000); and SID'02 Digest, pp. 522 (2002).
An improved EPD technology was disclosed in U.S. Pat. No. 6,930,818 filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Pat. No. 6,672,921 filed on Jun. 28, 2000 (corresponding to WO02/01281) and U.S. Pat. No. 6,933,098 filed on Feb. 15, 2001 (corresponding to WO02/65215). The improved EPD comprises isolated cells formed from microcups and filled with charged pigment particles dispersed in a dielectric solvent. To confine and isolate the electrophoretic dispersion in the cells, the filled cells are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
A liquid crystal display comprising display cells prepared by the microcup technology and filled with a liquid crystal composition optionally comprising a dichroic dye is disclosed in U.S. Pat. Nos. 6,795,138 and 6,784,953.
A display panel may also be prepared by the Gyricon technology (as disclosed in U.S. Pat. No. 6,588,131 assigned to Gyricon Media, Inc. and U.S. Pat. Nos. 6,497,942, and 5,754,332 assigned to Xerox). A Gyricon sheet is a thin layer of transparent plastic in which millions of small beads, somewhat like toner particles, are randomly dispersed. The beads, each contained in an oil-filled cavity, are free to rotate within those cavities. The beads are “bichromal” with hemispheres of two contrasting colors (e.g., black and white, red and white), and charged so they exhibit an electrical dipole. When a voltage is applied to the surface of the sheet, the beads rotate to present one colored side to the viewer. Voltages can be applied to the surface to create images such as text and pictures. The image will persist until new voltage patterns are applied.
For a direct drive display, a display cell layer (10) typically is sandwiched between a common electrode layer (11) and a backplane (12) as shown in FIG. 1.
The common electrode layer (11) is a single electrode layer which covers the entire display area. The backplane (12) comprises a substrate layer on which a desired graphic pattern (13) is printed or coated with a conductive material. In an electrophoretic display, the charged pigment particles in the display cell layer in the area of the desired graphic pattern may migrate to either the side of the common electrode layer or the side of the backplane, depending on the voltage difference between the common electrode layer and the conductive pattern.
The desired graphic pattern may be alphabet letters, numerical displays (such as those utilizing the well-known 7 or 14 segment electrodes), logos, signs or other graphic designs.
The backplane is usually formed of a patterned ITO substrate, a flexible printed circuit board or a rigid circuit board. However, these types of the backplane are not only expensive but also relatively complex in manufacture.
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