Abstract:
A dispersion medium, for dispersing electrophoretic particles in an electrophoretic display device, contains an organic compound having two or more rings in its structural formula, where the organic compound comprises at least about 30% by weight of the dispersion medium. The thus constituted dispersion medium improves the reliability and response of the electrophoretic device. The organic compound comprises one or more of the materials defined by the structural formulae (1) to (4), wherein each of A1 to A6, C1, and C2 are ring compounds.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophoretic display device, and more particularly to an electrophoretic display device having an improved dispersion medium. 
     2. Description of the Related Art 
     Electrophoretic display devices, which use electrophoresis, are one type of non-emitting display device. Electrophoresis is a phenomonon whereby the Coulomb force causes microparticles (e.g., electrophoretic particles) charged naturally by dispersion to migrate when an electrical field is applied to a fluid dispersion medium in which the microparticles are contained. 
     The basic structure of an electrophoretic display includes first and second electrodes disposed in opposition with a specific space therebetween. This space is filled with an electrophoretic suspension. When a potential difference is applied between the electrodes, the electrophoretic particles are pulled to one or the other of the electrodes depending on the direction of the electrical field. If the dispersion medium is dyed with dye and pigment particles are used for the electrophoretic particles, the color of the pigment particles or the color of the dye can be seen from the viewing side of the display, that is, the side where the transparent substrate and transparent electrodes are disposed. An electrophoretic display thus comprised can therefore be used to display images by forming the electrodes in a desired pixel pattern and controlling the voltage applied to the pixel electrodes. 
     Dodecylbenzene is described as the dispersion medium in this type of electrophoretic display in, for example, a paper (in Japanese) entitled “An electrophoretic display using microcapsules” (Japan Hardcopy 1999 collection of papers, pp. 237 to 240) written by an engineer at NOK corp. This paper describes placing the electrophoretic suspension inside the microcapsules instead of directly filling the dispersion medium between the electrodes. 
     Japanese Patent Publication (kokoku) S49-32038 is directed to an electrophoretic display having cells disposed between the electrodes, dividing the electrophoretic suspension into pixel units. Migration of the electrophoretic particles is restricted to within their designated cells. Using such cells makes it possible to increase the image resolution of the electrophoretic display and easily develop a color display. 
     Using dodecylbenzene as the dispersion medium of the electrophoretic display, however, causes certain problems relating to reliability and response. Dodecylbenzene attacks the sealants used to inject and seal the suspension inside the display, and there is therefore a danger of the suspension being incompletely sealed in the display. 
     OBJECT OF THE INVENTION 
     An object of the present invention is therefore to improve the reliability and response of the electrophoretic display device by employing an improved dispersion medium. 
     SUMMARY OF THE INVENTION 
     To achieve this object, the present invention provides a dispersion medium in which electrophoretic particles are dispersed, wherein the dispersion medium includes at least one organic compound having a structural formula containing at least two rings. An electrophoretic display device including such a dispersion medium is also provided. 
     The at least one organic compound preferably comprises at least one material defined by any of the following structural formulae (1) to (4) shown below: 
     
       
         X1-A1-B1-A2-Y1  (1) 
       
     
                               X3-C1  (3) 
     
       
         X4-C2-B4-A6-Y3  (4) 
       
     
     The content of such organic compound(s) in the dispersion medium is preferably 30% by weight or more; more preferably, the content of such organic compound(s) is 80% by weight or more; and even more preferably, such organic compound(s) constitute 100% by weight of the dispersion medium. 
     In the formulas (1) to (4), each of A1 to A6 is at least one of a benzene ring, a cyclohexane ring, a dioxane ring, a pyrimidine ring, a furan ring, a tetrahydrofuran ring, a dithiane ring, a piperidine ring, a pyrazine ring, a morpholine ring, a cyclopentane ring, a cyclobutane ring, a pyridine ring, a thiophene ring, a pyrrole ring, or a cyclohexene ring. 
     In the formulas (1) to (4), each of X1 to X4, Y1 to Y3, and Z is at least one substituent selected from the group consisting of hydrogen, fluorine, chlorine, bromine, iodine, cyano, nitro, amino, hydroxyl, alkanoyloxy, alkoxycarbonyl, carboxy, trifluoromethyl, trifluoromethoxy, alkoxymethylene, alkyl, alkoxy, alkenyl, sulfonate, thioalkyl, monoalkylamino, dialkylamino, and trialkylamino. 
     In the formulas (1) to (4), each of B1 to B4 is either a single bond or comprises one of the following: COO, OCO, CH═N, N═CH, CH═CH, (CH 2 ) n O, O(CH 2 ) n , (CH 2 ) n , O(CH 2 ) n O, CH═CH—COO, OCO—CH═CH, O, CO, NHCO, CONH, N═N, N 2 O, C≡C, SO, SO 2 , or NH, where n is an integer from 1 to 10. 
     In the formulas (1) to (4), each of C1 and C2 comprises naphthalene, tetrahydronaphthalene, decahydronaphthalene, indene, quinoline, coumarin, azulene, fluorene, or dihydrophenanthrene. 
     Formulas (5) to (32) shown in Appendix A are more specific examples of compounds having any of the above structural formulas (1) to (4), where R1 to R33, X, and Y are as follows. 
     Each of R1 to R33 is an alkyl, alkoxy, alkoxymethelene, halogen elements, hydrogen, alkenyl, cyano, nitro, amino, hydroxyl, alkanoyloxy, alkoxycarbonyl, trifluoromethyl, carboxy, monoalkylamino, dialkylamino, trialkylamino sulfonate, thioalkyl, trifluoromethoxy, acyl, acyloxy, or formyl. 
     Each of X and Y is either a single bond or comprises one of the following: COO, OCO, CH═N, N═CH, CH═CH, (CH 2 ) n O, O(CH 2 ) n , (CH 2 ) n , O(CH 2 ) n O, CH═CH—COO, OCO—CH═CH, O, CO, NHCO, CONH, N═N, N 2 O, C≡C, SO, SO 2 , or NH, where n is an integer from 1 to 10. 
     Of these organic compounds, ones with small optical anistropy are preferably used. Scattering, birefringence, and optical rotatory polarization depending on the orientation of the molecules occur easily when an organic compound with large optical anistropy is used as the dispersion medium, and such compounds thus contribute to a drop in light uniformity. By using materials with small optical anistropy, and more preferably optically isotropic materials, highly uniform light can be obtained from each pixel of the electrophoretic display device, and a wide viewing angle can be achieved. 
     Examples of the organic compounds with small optical anisotropy denoted by any of structural formulae (1) to (4) above include cyclohexyl cyclohexane, phenylcyclohexane, phenylethylcyclohexane, phenylcyclohexanecarboxylate, 4-phenyl-1,3-dioxane, and 2-phenylethyl-1,3-dioxane types. 
     The present invention is ideally suited to an electrophoretic display device wherein the space between opposing electrodes is divided into cells representing pixel units, and migration of each electrophoretic particle is restricted to within the particular cell in which it is contained. Each cell is filled with a suspension having electrophoretic particles dispersed in a dispersion medium. Units of three adjacent cells form a group, and the three cells of each group are respectively filled with a suspension corresponding to red (R), green (G), and blue (B). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an oblique view showing an electrophoretic display panel of an electrophoretic display device according to a preferred embodiment of the present invention; and 
     FIG. 2 is a partial section view of the electrophoretic display panel shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are described below with reference to the accompanying figures. 
     An electrophoretic display system according to embodiments of the invention comprises an electrophoretic display panel and peripheral circuitry. However, because the distinguishing feature of the present invention is the suspension used in the electrophoretic display panel, the peripheral circuitry is not described below. An electrophoretic display panel according to embodiments of the invention is described below with reference to FIG.  1  and FIG.  2 . 
     Display panel A has a component substrate  100  with an opposing substrate  200  disposed in opposition thereto, and a cell grid  11  disposed between the substrates. FIG. 1 is an oblique view showing the opposing substrate  200  removed and raised above the component substrate  100 , and FIG. 2 is a partial section view. 
     A common electrode  201  and then a sealant layer  202  are formed on the surface of the substrate  200  that is opposite the component substrate  100 . The opposing substrate  200 , common electrode  201 , and sealant layer  202  are all transparent, and the outside of the opposing substrate  200  is the display surface (the viewing side) of display panel A. 
     Pixel electrodes  104 , scan lines, data lines  102 , and thin-film transistors functioning as switching elements are formed in the center (display area  100 A) of the component substrate  100  on the surface opposing the opposing substrate  200 . The scan line drive circuit, data line drive circuit  140 , and external connection electrodes are formed in the area  100 B around the outside of the display area  100 A. 
     The cell grid  11  is formed over the display area  100 A. Walls  110  of a constant height divide the space between component substrate  100  and opposing substrate  200  into a grid of cells  11 C. The cell grid  11  can be formed by sequential steps of forming a film coating of a photosensitive polyimide precursor on the component substrate  100 , exposing the film coating to ultraviolet rays though a mask corresponding to the cell pattern, and then developing. 
     The cells  11 C divide the area between the electrodes (between common electrode  201  and pixel electrodes  104 ) into pixel units. Each cell  11 C is filled with a suspension  1  having electrophoretic particles  3  dispersed in a dispersion medium  2 . A surface active agent or other additive is added to this dispersion medium  2  as needed. 
     Because this display panel A is a full color display device, the cells  11 C are grouped into units of three. That is, three adjacent cells form one group of cells. One cell in each group is filled with a suspension corresponding to red (R), another with a suspension corresponding to green (G), and the third with a suspension corresponding to blue (B). 
     Each color of suspension contains electrophoretic particles of the same color as the display color and dyes the dispersion medium a color that absorbs the display color (such as the complement of the display color). More specifically, the suspension for red uses red particles (such as ferric oxide particles) for the electrophoretic particles and has the dispersion medium dyed cyan. The suspension for green uses green particles (such as cobalt green pigment particles) for the electrophoretic particles and has the dispersion medium dyed magenta. The suspension for blue uses blue particles (such as cobalt blue pigment particles) for the electrophoretic particles and dyes the dispersion medium yellow. 
     Therefore, when the electrophoretic particles  3  are attracted to common electrode  201 , and hence toward the display or viewing surface side of panel A, light passing through the opposing substrate  200  and common electrode  201  and incident on the suspension  1  is directly reflected by the electrophoretic particles  3  on the side of the common electrode  201 . Most of the reflected light (light of the wavelength corresponding to the display color) is therefore not absorbed and light of the wavelength corresponding to the display color is emitted. The display color can therefore be seen from the viewing surface side. 
     When the electrophoretic particles  3  are attracted to the pixel electrodes  104  on the side of component substrate  100 , light passing through opposing substrate  200  and common electrode  201  to the suspension  1  is reflected by the electrophoretic particles  3  on the side of the pixel electrodes  104  after passing through the suspension  1 . The reflected light is then absorbed by the dispersion medium  2  as it passes through the suspension  1  to the common electrode  201 , and red therefore cannot be seen from the viewing surface. 
     The position and distribution of the electrophoretic particles  3  between the electrodes can be controlled by the field strength applied to the suspension  1 . It is therefore possible to adjust the color pixels in each group to a desired color so that a full color image can be presented on the display panel A by controlling the field strength applied to the suspension  1  in the cells of each color and thereby controlling the position and distribution of electrophoretic particles  3  in the cells of each color. 
     Compositions of the dispersion medium  2 , according to embodiments 1 to 19, are shown in Appendix B. 
     Compared with a dispersion medium comprising organic compounds with a structural formula containing only one ring of dodecylbenzene, for example, dispersion media with compositions, according to embodiments of the present invention, do not easily evaporate while having low viscosity, and do not easily penetrate the sealant. It is therefore possible to improve electrophoretic display device reliability and response compared with using a dispersion medium comprising only dodecylbenzene. 
     While the present invention has been described in conjunction with preferred embodiments thereof, further alternatives, modifications, variations and applications will be apparent to those skilled in the art in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, variations and applications as may fall within the spirit and scope of the appended claims.                         
                         
                         
                         
                           
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Compound 
                 wt % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Embodiment 1 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 70 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                 Embodiment 2 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 60 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 40 
               
               
                   
               
               
                 Embodiment 3 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 100 
               
               
                   
               
               
                 Embodiment 4 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 100 
               
               
                   
               
               
                 Embodiment 5 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 60 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 40 
               
               
                   
               
               
                 Embodiment 6 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 40 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 40 
               
               
                   
               
               
                 Embodiment 7 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 35 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                 Embodiment 8 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 9 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 10 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 11 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                 Embodiment 12 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                 Embodiment 13 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                 Embodiment 14 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 15 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 16 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 40 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 17 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 30 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
                 Embodiment 18 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 35 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 15 
               
               
                   
               
               
                 Embodiment 19 
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 25 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 20 
               
               
                   
               
               
                   
                 
                   
                             
                     
                         
                         
                     
                   
                 
                 10