The 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 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. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026; U.S. Provisional Application No. 60/443,893, filed Jan. 30, 2003; and U.S. Pat. No. 7,184,197) 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 (2001); and SID'02 Digest, pp. 522 (2002).
An improved EPD technology was recently disclosed in U.S. Pat. No. 6,930,818 (WO01/67170), U.S. Pat. No. 6,672,921, (WO02/01281), and U.S. Pat. No. 6,933,098 (WO02/02/65215). The improved EPD comprises isolated cells formed from microcups and filled with charged 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 a group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
Other types of displays, namely magnetophoretic displays (MPDs) and electromagnetophoretic displays (EMPDs), are disclosed in U.S. Provisional No. 60/367,325, filed on Mar. 21, 2002; U.S. Pat. No. 6,927,892; U.S. Provisional No. 60/375,299, filed on Apr. 23, 2002; and U.S. Pat. No. 6,914,713. The magnetophoretic display generally comprises display cells sandwiched between two layers of substrate and filled with a magnetophoretic dispersion wherein the pigment particles are magnetic but not charged. The display is driven by a magnetic field. At least the substrate layer on the viewing side is transparent. In the electromagnetophoretic display, the display cells sandwiched between two substrate layers are filled with an electromagnetophoretic dispersion wherein the pigment particles are both charged and magnetic. One of the substrate layers, preferably on the non-viewing side, is coated with a conductive layer facing the filled display cells. The display is driven by a combination of electric and magnetic fields. The substrate layer on the viewing side is transparent.
For all types of displays, the dispersion contained within the display cells is undoubtedly one of the most crucial parts of the device. The dispersion, as stated earlier, usually is composed of pigment particles dispersed in a colored dielectric solvent or solvent mixture. The composition of the dispersion determines, to a large extent, the longevity, contrast ratio, switching rate, response waveform, threshold characteristics and bistability of the device. In an ideal dispersion, the dispersed pigment particles remain separate and do not aggregate or flocculate under all operating conditions. Furthermore, all components in the dispersion must be chemically and electrochemically stable and compatible not only with each other but also with the other materials present in the display, such as the electrodes and sealing and substrate materials.
The dispersing medium may be colored by dissolving or dispersing a dye or colorant in the dielectric solvent or solvent mixture.
Halogenated solvents of a high specific gravity have been widely used in EPD applications particularly in those involving an inorganic pigment, such as TiO2, as the charged whitening or coloring particles. The halogenated solvents of a high specific gravity are very useful in reducing the rate of sedimentation of the pigment particles in the solvent. Fluorinated solvents are among the most preferred because they are chemically stable and environmentally friendly.
However, most dyes or pigments are not soluble in fluorinated solvents particularly perfluorinated solvents having a high boiling-point. For example, phthalocyanines are highly desirable colorants due to their high extinction coefficients and chemical stability; but they are normally insoluble in most solvents, particularly insoluble in fluorinated solvents. Substitution on the ring with an organic group such as an alkyl or fluorinated alkyl may improve the solubility in organic solvents, particularly fluorinated solvents. However, the solubility is not sufficiently high and is also strongly temperature dependent. As a result, the EPDs colored by this type of dyes typically show poor shelf-life stability and narrow operation temperature latitude.
Certain soluble perfluorinated Cu phthalocyanine dyes have been disclosed in U.S. Pat. No. 3,281,426 (1966). The process for the preparation of these dyes involves heating a mixture of an aromatic starting compound and a perfluoroalkyliodide at a temperature in the range of from 200° C. to 350° C. The reaction is performed in an autoclave or a pressure ampoule due to the pressure developed. This synthesis involves complicated reaction conditions (e.g., high pressure and temperature) and long reaction time and has a low yield. Other phthalocyanine derivatives (U.S. Pat. Nos. 6,043,355 and 5,932,721) show improved solubility in various organic solvents or even in water, but not in highly fluorinated solvents.
A group of fluorinated silicon (IV) phthalocyanine and naphthalocyanine dyes are disclosed in U.S. Provisional Application No. 60/381,263, filed on May 17, 2002; and U.S. Publication No. 2004-0030125. However the solubility of the fluorinated Si phthalocyanine and naphthalocyanine dyes in perfluorinated solvents, such as HT-200, is still limited and the viscosity of the resultant electrophoretic dispersion containing this type of dyes is relatively high.
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