Patent Publication Number: US-2007096651-A1

Title: Plasma display panel

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a surface discharge type plasma display panel. More particularly, the present invention relates to improvement in a display electrode for generating surface discharge.  
      2. Description of the Related Art  
      The surface discharge type plasma display panel includes row electrodes that are arranged as display electrodes for generating surface discharge, a dielectric layer for covering the row electrodes, column electrodes intersecting the display electrodes, partitions that are discharge barriers between cells and fluorescent materials for reproducing colors. A screen is made up of a plurality of cells (display elements) arranged in a matrix form.  
      In general, the row electrodes and the dielectric layer are disposed on a front substrate, while the column electrodes, the partitions and the fluorescent materials are disposed on a rear substrate. In terms of the increase in luminance, a reflection type in which the fluorescent materials are disposed on the rear substrate has an advantage over a transmission type in which they are disposed on the front substrate.  
      A row electrode array comes in two types. Herein, one is referred to as an independent type and the other is referred to as a common type for descriptive purposes. The independent type is an array type in which a pair of the row electrodes is disposed for each row of a matrix display. The total number of row electrodes is twice as many as the number of rows (vertical resolution). The common type is an array type in which the row electrodes whose total number is the number of rows plus one are disposed at regular intervals. In the common type, each of the row electrodes and each of the neighboring row electrodes make a display electrode pair for surface discharge and surface discharge gaps are formed at all gaps between the row electrodes. The independent type has the advantage of easy drive control, while the common type has the advantage of high utilization rate of a screen.  
      With a general panel structure in which the row electrodes are disposed on the front substrate as described above, the row electrodes include a transparent conductive film and a metal film in both the independent type and the common type. The transparent conductive film serves to secure an electrode area necessary to spread surface discharge appropriately. The metal film serves to compensate for conductivity of the transparent conductive film.  
      The most basic shape of the row electrode is a strip shape that directly extends over the entire length of a row and has a constant width. This strip shape is simple and is patterned relatively easily. The shape, however, has the disadvantage that a large discharge current is apt to flow.  
      The flow of the large discharge current involves an expensive driving circuit with a proper current capacity and also increases power consumption. Further, the flow of the large concentrated discharge current for a short time drops a driving voltage substantially, which causes an uneven display including intensity disturbance called streaking.  
      There are various proposals regarding a row electrode in such a surface discharge type plasma display panel. Japanese patent No. 3,352,821 describes that a row electrode is patterned to have a shape including a strip portion extending over plural cells and a plurality of projections protruding from the strip portion in each of the cells and thereby a discharge current is suppressed. Japanese patent No. 2,734,405 describes that a row electrode is patterned to have a ladder shape including a plurality of long strips parallel to one another and short strips for coupling the log strips to one another at regular intervals and thereby current concentration is reduced.  
      The conventional improvement is directed to reduce the area of the row electrode in the cell. Accordingly, an attempt to sufficiently suppress a discharge current limits the spread of surface discharge excessively, which makes it impossible to obtain sufficient luminance.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to solve the problems pointed out above, and therefore, a first object of the present invention is to suppress a discharge current and to improve luminance. A second object of the present invention is to reduce the number of man-hours and the number of materials upon the formation of electrodes.  
      According to one aspect of the present invention, a plasma display panel for achieving the first object includes a pair of substrates disposed in face-to-face relation with each other, a discharge gas sealed between the substrates, and a plurality of first display electrodes and a plurality of second display electrodes disposed on one of the substrates to generate surface discharge, each of the first and second display electrodes having a shape including an elongated strip portion extending over plural cells and a plurality of projections protruding from the strip portion in each of the cells, the projection of the first display electrode and the projection of the second display electrode defining a surface discharge gap in each of the cells. Each of the projections of the first and second display electrodes is formed to have a serpentine strip shape having at least two bending portions.  
      The projection is made in the form of a serpentine strip, increasing electrical resistance of a current path between the strip portion and the surface discharge gap. When discharge is generated at the surface discharge gap, a current flowing from the strip portion to the surface discharge gap is suppressed by the electrical resistance. A sufficient current is supplied, through the strip portion, from a power source to ends of the respective projections on the strip portion side. Since a voltage drop is substantial across the projection, surface discharge spreads toward the strip portion. Thereby, a discharge area extends, resulting in the high luminance.  
      According to another aspect of the present invention, a plasma display panel for achieving the second object includes, in addition to the feature descried above, a feature that the first and second display electrodes are metal electrodes. In the present invention, the selection of pattern dimensions of the projection can increase electrical resistance only in the projection of the display electrode. Accordingly, even if the strip portion and the projection are made of metal that is a good conductor, a discharge current can be suppressed.  
      According to yet another aspect of the present invention, a plasma display panel includes a feature that the cells are made up of three kinds of the cells having different light emission colors and the projections in any of the three kinds of the cells have a length different from that of at least one of the other cells. In this plasma display panel, the selection of the projection length can compensate for the slight difference among the cells in discharge characteristics to synchronize discharge timing of the three kinds of the cells, leading to the enhancement of color reproducibility. Conversely, the discharge timing in the cells can be made to be different from one another actively.  
      These and other characteristics and objects of the present invention will become more apparent by the following descriptions of preferred embodiments with reference to drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an exploded perspective view showing a structure of a plasma display panel according to a first embodiment of the present invention.  
       FIG. 2  is a plan view showing a color array in a screen.  
       FIG. 3  is a plan view showing an outline of a row electrode shape.  
       FIG. 4  is a schematic diagram of a projection of a row electrode.  
       FIG. 5  is a plan view showing a first modification of a layered structure of the row electrode.  
       FIG. 6  is a plan view showing a row electrode shape in a plasma display panel according to a second embodiment of the present invention.  
       FIG. 7  is a plan view showing a second modification of the layered structure of the row electrode.  
       FIG. 8  is a plan view showing another example of a row electrode array. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  is an exploded perspective view showing a structure of a plasma display panel according to a first embodiment of the present invention, and  FIG. 2  is a plan view showing a color array in a screen.  FIG. 1  shows a portion corresponding to six cells in the plasma display panel  1 , i.e., cells  51 ,  52 ,  53 ,  54 ,  55  and  56  in the screen  50  shown in  FIG. 2 .  
      The plasma display panel  1  includes a front glass substrate  10 , a rear glass substrate  20  and a discharge gas (not shown) sealed in a space between the substrates  10  and  20 .  
      First row electrodes  11  and second row electrodes  12  both of which having a shape unique to the present invention are disposed on an inner surface of the glass substrate  10  as display electrodes for generating surface discharge. The row electrode  11  and the row electrode  12  make an electrode pair in each row. The electrodes  11  and  12  are covered by an insulation layer  13 . The insulation layer  13  is a layered film including a dielectric layer  14  and a thin protection film  15 .  
      Column electrodes  21  are disposed on an inner surface of the glass substrate  20  and are covered by a dielectric layer  22 . A plurality of partitions  23  having a strip shape when viewed from the top is disposed, in parallel with one another, on the dielectric layer  22 . The partitions  23  extend in the same direction as the column electrodes  21 . The illustrated partition pattern is a striped pattern. The partitions  23  abut on the protection film  15  practically although they are spaced away from each other in the illustrated example.  
      A red (R) fluorescent material layer  24 , a green (G) fluorescent material layer  25  or a blue (B) fluorescent material layer  26  is formed between the adjacent partitions  23 . The fluorescent material layers  24 ,  25  and  26  have a continuous strip shape extending over the plural cells arranged along the partitions  23 .  
      As shown in  FIG. 2 , the screen  50  is made up of many cells arranged in rows and columns.  FIG. 2  shows a part of a row including the three cells  51 ,  52  and  53  and a part of a row including the three cells  54 ,  55  and  56 . The color array in the screen  50  is a striped array in which the cells belonging to each column have the same light emission color and each column has a light emission color different from those of the neighboring columns. Three cells arranged along the horizontal direction correspond to one pixel.  
       FIG. 3  is a plan view showing an outline of the row electrode shape, and  FIG. 4  is a schematic diagram of a projection of the row electrode.  
      In this example, the entire row electrode  11  and the entire row electrode  12  are metal electrodes made of a metal thin film that is patterned using photolithography. More specifically, each of the electrodes  11  and  12  is a layered film including a base layer made of chromium (Cr) having a thickness of approximately 50 nm, a main conductor layer made of copper (Cu) having a thickness of approximately 3 μm and an upper layer made of chromium having a thickness of approximately 50 nm. Note that the materials for the electrodes and the film thickness are not limited to this example. Other metal such as aluminum, nickel, silver or gold can be used as the materials. Any other metal can be used as the materials for the electrodes, as long as they can provide sufficient conductivity even if they are arranged on a large screen with a diagonal of 50 inches or greater.  
      As shown in  FIG. 3 , the row electrode  11  is patterned to have a shape including a strip portion  111  and a plurality of projections  112 . The strip portion  111  extends over the cells arranged in the row direction and has a constant width. Each of the projections  112  protrudes from the strip portion  111  in the cell toward the row electrode  12  with which the row electrode  11  makes a pair. Likewise, the row electrode  12  is patterned to have a shape including a strip portion  121  and a plurality of projections  122 . The strip portion  121  extends over the cells arranged in the row direction and has a constant width. Each of the projections  122  protrudes from the strip portion  121  in the cell toward the row electrode  11  with which the row electrode  12  makes a pair. In each of the cells, the projection  112  of the row electrode  11  and the projection  122  of the row electrode  12  form a surface discharge gap  60  (a gap between the display electrodes).  
      As shown in  FIG. 4 , the projection  112  of the row electrode  11  is patterned in the form of one strip that meanders and has five bending portions. The end of the projection  112  extends in parallel with the strip portion  111 . Likewise, the projection  122  of the row electrode  12  also has one serpentine strip shape with five bending portions. The projection  112  of the row electrode  11  and the projection  122  of the row electrode  12  are disposed symmetrically with respect to the center position of the cell in the row direction and in the column direction. This is because surface discharge  62  is initiated at the center of the cell in the row direction. The connection point P 1  between the projection  112  and the strip portion  111  and the connection point P 3  between the projection  122  and the strip portion  121  lie at the center in the row direction of the cell.  
      In the projection  112  of the row electrode  11 , a current path between the connection point P 1  and a discharge initiation point P 2  has a length Ls greater than a linear distance L between the connection point P 1  and the discharge initiation point P 2 . For the purpose of increasing electrical resistance deliberately, the length Ls is preferably set to be twice or more the linear distance L. In order to increase the length Ls without narrowing the surface discharge gap  60 , it is preferable that a pattern width of the strip portion constituting the projection  112  be reduced and the number of bending portions be increased. The reduction of the pattern width increases an electrical resistance value without changing the length Ls. Similarly, in the projection  122  of the row electrode  12 , a current path between the connection point P 3  and a discharge initiation point P 4  has a length Ls greater than a linear distance L between the connection point P 3  and the discharge initiation point P 4 .  
      With the plasma display panel  1  including the projections  112  and  122 , the current path from each of the strip portions  111  and  121  to the surface discharge gap  60  has high electrical resistance. When the surface discharge  62  is generated, a discharge current flowing from each of the strip portions  111  and  121  to the surface discharge gap  60  is suppressed by the electrical resistance. Stated differently, the peak value of the discharge current is smaller than those of plasma display panels having conventional structures. Further, since voltage drops are substantial across the projections  112  and  122 , the surface discharge  62  easily spreads through the projections  112  and  122 . These operations can achieve the suppression of the discharge current and the improvement in luminance.  
       FIG. 5  is a plan view showing a first modification of the layered structure of the row electrode.  
      Referring to  FIG. 5 , the plasma display panel  2  includes a row electrode  11   b  that is a layered film having a transparent conductive film  101  and a metal film  102 . In the row electrode  11   b , a strip portion  113  extending over plural cells arranged in the row direction is made up of a part of the transparent conductive film  101  and the metal film  102  overlapping therewith. A projection  114  is the remaining part of the transparent conductive film  101  and is patterned to have a serpentine strip shape similar to that of the illustrated example of  FIG. 3  or  4 .  
      A row electrode  12   b  is a layered film including a transparent conductive film  201  and a metal film  202 . In the row electrode  12   b , a strip portion  123  extending over plural cells arranged in the row direction is made up of a part of the transparent conductive film  201  and the metal film  202  overlapping therewith. A projection  124  is the remaining part of the transparent conductive film  201  and is patterned to have a serpentine strip shape similar to that of the illustrated example of  FIG. 3  or  4 .  
      The transparent conductive films  101  and  201  have a resistance value per unit length that is higher than that of the metal films  102  and  202 . Accordingly, compared to the case where the row electrodes  11   b  and  12   b  are made of metal, the row electrodes  11   b  and  12   b  can provide desired electrical resistance even if the projections  114  and  124  are formed to have a shorter length or a larger pattern width.  
       FIG. 6  is a plan view showing a row electrode shape in a plasma display panel according to a second embodiment of the present invention.  
      Referring to  FIG. 6 , a plasma display panel  3  has a cell structure and a screen structure that are basically the same as those of the first embodiment described above. The plasma display panel  3  differs from the plasma display panel of the first embodiment in row electrode shape.  
      As shown in  FIG. 6 , a row electrode  11   c  is patterned to have a shape including a strip portion  115  and a plurality of projections  116  and  117 . The strip portion  115  extends over plural cells arranged in the row direction and has a constant width. The projections  116  and  117  protrude from the strip portion  115  toward a row electrode  12   c  with which the row electrode  11   c  makes a pair. Likewise, the row electrode  12   c  is patterned to have a shape including a strip portion  125  and a plurality of projections  126  and  127 . The materials for the row electrodes  11   c  and  12   c  may be metal as shown in the example of  FIG. 3  or  4 . Alternatively, the materials for them may be composite materials in the form of layered film including the transparent conductive film and the metal film as shown in the example of  FIG. 5 .  FIG. 6  shows a structure in which the strip portions  115  and  125  are made of a transparent conductive film and a metal film (shown by a broken line in the drawing) overlapping therewith.  
      The plasma display panel  3  is characterized in that the shapes of the projections  116 ,  117 ,  126  and  127  of the row electrodes  11   c  and  12   c  are selected for each cell. More specifically, among first, second and third cells arranged in the row direction, i.e., out of three kinds of the cells having different light emission colors, the projections  117  and  127  disposed in one kind of the cells has a length different from those of the projections  116  and  126  disposed in the other two kinds of the cells respectively. In the illustrated example, each of the projections  117  and  127  is a strip portion having three bending portions. The projections  117  and  127  are shorter than the projections  116  and  126  each of which has five bending portions. Stated differently, the projections  117  and  127  have electrical resistance lower than that of the projections  116  and  126 .  
      The peak current value upon discharge changes depending on the electrical resistance as described above. Besides, a discharge delay time also changes depending on the electrical resistance. The increase in the electrical resistance increases the discharge delay time. Accordingly, the electrical resistance of the projections is selected for each cell, which makes it possible to reduce or increase the variations in the discharge delay time among the cells. In some cases, for example, the discharge delay time is different depending on the light emission color due to the differences of the fluorescent material. In such cases, the discharge delay times in the three kinds of the cells having different light emission colors are synchronized with one another. Thereby, no color shift of additive color mixing is present, leading to the enhancement of color reproducibility. Further, the variations in the discharge delay time are actively increased, which allows current concentration to be reduced.  
      In the example shown in  FIG. 6 , the projection shape is different between one kind of the cells and the other two kinds of the cells. Instead, however, the projection shape can be different for each kind of the cells. In the control of electrical resistance through the selection of a shape, which is unique to the present invention, fine control is easier and more secure compared to the control through the selection of materials.  
      As an application of the different projection length among the cells, the projection length in each of the cells may be selected depending on the cell position in the row direction. In the case, for example, where an application voltage difference is generated, by a voltage drop across the strip portion, between the end portions and the central portion of the row electrode in the row direction, the projection in the cell where the application voltage is relatively high is elongated to increase the electrical resistance, leading to the equalization of discharge intensity of the cells in one row.  
       FIG. 7  is a plan view showing a second modification of the layered structure of the row electrode.  
      A plasma display panel  4  shown in  FIG. 7  includes a row electrode  11   d  and a row electrode  12   d . The row electrode  11   d  includes a strip portion  118  that is a metal film extending over plural cells arranged in the row direction. A projection  119  in each cell that protrudes from the strip portion  118  is a transparent conductive film. Likewise, the row electrode  12   d  includes a strip portion  128  that is a metal film and projections  129  each of which is a transparent conductive film.  
       FIG. 8  is a plan view showing another example of a row electrode array.  
      The present invention is applicable to a common type of row electrode array as shown in  FIG. 8  in addition to an independent type of row electrode array.  
      A plasma display panel  5  shown in  FIG. 8  includes a row electrode  11   e , a row electrode  12   e  and a mesh-patterned partition  29  that delimits a screen vertically and horizontally. The row electrode  11   e  is patterned to have a shape including a strip portion  111   e  and a plurality of projections  112   e . The strip portion  111   e  extends over plural cells arranged in the row direction and has a constant width. The projections  112   e  protrude from the both sides of the strip portion  111   e . Likewise, the row electrode  12   e  is patterned to have a shape including a strip portion  121   e  and a plurality of projections  122   e . The strip portion  121   e  extends over plural cells arranged in the row direction and has a constant width. The projections  122   e  protrude from the both sides of the strip portion  121   e . In each of the cells, the projection  112   e  of the row electrode  11   e  and the projection  122   e  of the row electrode  12   e  form a surface discharge gap. The materials for the row electrodes  11   e  and  12   e  may be metal. Alternatively, the materials for them may be composite materials in the form of layered film including a transparent conductive film and a metal film.  FIG. 8  shows an example of the row electrodes  11   e  and  12   e  as metal electrodes.  
      In the examples shown in  FIGS. 3 and 5 , all the strip portions (bus portions) and all the projections (discharge electrode portions) of the row electrodes are made up of a common metal conductor. According to the examples, an electrode forming process can be reduced compared to the case where the projections functioning as the discharge electrode portions are made up of a transparent conductive film. The present invention is beneficial to the case of using row electrodes that are entirely made of metal. The present invention makes it possible to sufficiently limit a discharge current in each of the cells due to the projections having multiple bending portions and to extract display light through clearances between the bending portions. Besides, in the case where the entire row electrode is patterned by a trilaminar metal film of Cr, Cu and Cr, as described earlier with reference to  FIG. 3 , some of the Cu film of the serpentine pattern of the projection can be cut to control resistance.  
      In the embodiments described above, the overall structure of the plasma display panel, the structures of various elements thereof, especially the structure of the row electrode as a display electrode may be changed as needed, in accordance with the subject matter of the present invention. The pattern dimensions of the row electrode may be selected depending on specifications of a cell size. The shape of the projection in the row electrode is not limited to the combination of lines bending at right angles and may include a curve. The bend includes a twist and a sinuosity. The strip making up the projection does not necessarily have a constant pattern width. The pattern width may be different between a part close to the strip portion and a part far therefrom.  
      The present invention enables the suppression of a discharge current and the improvement in luminance.  
      The present invention also enables the reduction in the number of man-hours and the number of materials upon the formation of electrodes.  
      Further, the present invention can reduce the variations in discharge delay among the cells having different light emission colors and can enhance color reproducibility.  
      While example embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended claims and their equivalents.