Abstract:
A plasma display panel (PDP) designed to reduce the amount of external light that is reflected. This is accomplished by having some phosphor material on portions of the tops of the barrier ribs outside the discharge cells. Since the reflectance of the barrier rib material is higher than that of phosphor material, such a design will reduce the amount of external light reflected off the screen of a plasma display panel. By reducing external light reflection, the contrast of the image is improved. This can be achieved while still preventing crosstalk between neighboring discharge cells.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application PLASMA DISPLAY PANEL earlier filed in the Korean Intellectual Property Office on 24 May 2004 and there duly assigned Serial No. 10-2004-0036835. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a plasma display panel, and more particularly, to a plasma display panel having a structure that can improve contrast by reducing reflective brightness from external light. 
     2. Description of the Related Art 
     A plasma display panel (PDP) displays images by exciting phosphor layers formed in a predetermined pattern with ultraviolet rays that are generated from a glow discharge. The glow discharge occurs when a predetermined voltage is applied to electrodes formed is a closed space where a discharge gas is filled. 
     PDPs can be classified into direct current PDP, alternating current PDP, and hybrid PDP according to driving methods. PDPs can be further classified into those having a minimum of two electrodes and those having a minimum of three electrodes. A direct current PDP includes address electrodes that increase addressing speed by performing an address discharge and a sustain discharge separately. Alternating current PDPs can be divided into those with an opposing discharge electrode structure and those with a surface discharge electrode structure according to the disposition of the electrodes. In the opposing discharge electrode structure, a discharge occurs in a direction perpendicular to the PDP by applying a potential difference to two sustain electrodes on different substrates. In the surface discharge electrode structure, a discharge occurs on one surface of a substrate by having both sustain electrodes on the same substrate. 
     In PDPs, reflective brightness of external light is an important factor affecting performance. Reflective brightness is from light external to the PDP that reflects off the display of the PDP during operation of the PDP. The more reflective brightness there is, the poorer the image quality becomes. The brightness and contrast of the PDP are reduced if the reflective brightness is increased. The reflective brightness can be further increased if a white pigment, such as TiO 2 , is added to a barrier rib to effectively reflect light emitted from a phosphor layer. Therefore, there is a need to reduce the reflective brightness. A conventional method of preventing the reflection involves placing a black stripe that can block external light on a non-discharge region of the PDP. A problem with incorporating the black stripe into the design of a PDP is that several extra manufacturing steps are required to make a PDP having the black stripe. For example, the black stripe material must be applied or deposited on the structure somehow. Then the black stripe material has to be patterned. All of these extra steps can be extremely expensive in a manufacturing production environment. Therefore, what is needed is a more effective solution to the reflection of external light problem that both results in an effective PDP structure that is also inexpensive, simple and easy to make. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved design for a PDP. 
     It is also an object of the present invention to provide a design for a PDP that results in less reflective brightness. 
     It is further an object of the present invention to provide a design for a PDP that provides improved image quality. 
     It is yet another object of the present invention to provide a design for a PDP that results in improved contrast of the displayed image. 
     It is still an object of the present invention to provide a design for a PDP that reduces the reflection of external light that is easy to make. 
     It is also an object of the present invention to provide a design for a PDP that is easy and inexpensive to make and manufacture. 
     It is yet an object of the present invention to provide a design for a PDP that is inexpensive and simple to make. 
     It is further an object of the present invention to provide a design for a PDP that is both effective at reducing or eliminating the reflection of external light off the screen and is easy to make and does not result in extra process steps. 
     These and other objects can be achieved by a PDP with phosphor layers located in some of the non-discharge regions. The PDP has an upper substrate, an upper dielectric layer formed on the upper substrate, a lower substrate facing the upper substrate, a lower dielectric layer formed on the lower substrate and facing the upper dielectric layer, a plurality of barrier ribs that define discharge cells between the upper substrate and the lower substrate, a plurality of sustain electrode pairs located within the upper dielectric layer corresponding to the discharge cells, a plurality of address electrodes located within the lower dielectric layer corresponding to the discharge cells, and disposed perpendicularly to the sustain electrode pairs, a plurality of main phosphor layers producing red, green, and blue light located in the discharge cells, and a plurality of dummy phosphor layers at least partially connected to the main phosphor layers by connector layers. The dummy phosphor layers covers some of the exposed portions of the barrier ribs. Since the phosphor material reflects less light than the barrier rib material, such a design reduces external light reflection thus improving contrast and overall image quality for the PDP. Unlike employing a black stripe, the present design is easy to make and does not require extra process steps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a perspective view of a plasma display panel (PDP) according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 ; and 
         FIG. 3  is a partial cross-sectional view illustrating a phosphor layer on the barrier rib of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the figures,  FIG. 1  is a perspective view of a plasma display panel (PDP)  100  according to an embodiment of the present invention and  FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 . Referring to  FIGS. 1 and 2 , the PDP  100  includes an upper substrate  111  on which an image is displayed and a lower substrate  121  facing the upper substrate  111 . A plurality of sustain electrode pairs  112  are arranged on a surface of the upper substrate  111  facing the lower substrate  121 . Each of the sustain electrode pairs  112  includes an X electrode  113  and a Y electrode  114 , and the X and the Y electrodes  113  and  114  can respectively corresponded to a common electrode and a scan electrode. 
     The X and Y electrodes  113  and  114  respectively include transparent electrodes  113   a  and  114   a  and bus electrodes  113   b  and  114   b  connected at sides of the transparent electrodes  113   a  and  114   a . Each of the transparent electrodes  113   a  and  114   a  has a stripe shape, and the bus electrodes  113   b  and  114   b  also have stripe shape but with a narrower width than the transparent electrodes  113   a  and  114   a . A gap between ends of the transparent electrodes  113   a  and  114   a  defines a discharge gap, and bus electrodes  113   b  and  114   b  are located on sides of transparent electrodes  113   a  and  114   a  opposite from the discharge gap. Therefore, as depicted in  FIG. 2 , the sustain electrode pairs  112  are located to correspond to the discharge cells  125 . 
     The transparent electrodes  113   a  and  114   a  are formed of a transparent conductive material, such as indium tin oxide (ITO), to allow visible light to pass through. The bus electrodes  113   b  and  114   b  apply a voltage to the transparent electrodes  113   a  and  114   a , and serve to reduce any voltage drop along the transparent electrodes  113   a  and  114   a . The bus electrodes  113   b  and  114   b  are formed of a material having high conductivity, such as Ag or Cu, to improve the electrical conductivity along the transparent electrodes  113   a  and  114   a  since the transparent electrodes  113   a  and  114   a  are formed of ITO which has a lower electrical conductivity than Ag or Cu. 
     In the drawing, the transparent electrodes  113   a  and  114   a  have a stripe shape, but the present invention is not limited thereto. Also, the X electrode and the Y electrode can instead be made of only the transparent electrodes or alternatively of only the bus electrodes. 
     The sustain electrode pairs  112  are covered by an upper dielectric layer  115  formed on the upper substrate  111 . The upper dielectric layer  115  is, in turn, covered by a protective layer  116  made of MgO. 
     Address electrodes  122  are formed on a surface of the lower substrate  121  facing the upper substrate  111  and extend perpendicular to the sustain electrode pairs  112 . The address electrodes  122  have a striped shape and are located to correspond to the discharge cells  125  and are formed below the discharge cells  125 . The address electrodes  122  are covered by a lower dielectric layer  123  formed on the lower substrate  121 . 
     Barrier ribs  124  are formed on the lower dielectric layer  123 . The barrier ribs  124  partition a space between the upper substrate  111  and the lower substrate  121  into the discharge cells  125 . The barrier ribs  124  are made up of first barrier ribs  124   a  spaced apart by predetermined intervals and second barrier ribs  124   b  that intersect the first barrier ribs  124   a . The first barrier ribs  124   a  extend parallel to each other and are located between adjoining or neighboring address electrodes  122 . In the figures, the first barrier ribs  124   a  run parallel to the address electrodes  122 , but the present invention is in no way limited to as such. Also, the figures show the second barrier ribs  124   b  as running orthogonal to the address electrodes  122  and parallel to the sustain electrode pairs  112 , but the present invention is in no way limited to as such. For example, the barrier ribs  124  could instead have a delta shape and still be within the scope of the present invention. 
     The discharge cells  125  have four side surfaces and are arranged in a matrix defined by the first and second barrier ribs  124   a  and  124   b , and cross talk between the discharge cells  125  is prevented. When the discharge cells  125  are defined in a matrix, fine pitch and high brightness can be achieved. The discharge cells  125  are filled with a discharge gas such as Ne mixed with Xe. The upper substrate  111  and the lower substrate  121  are sealed together by a sealing member such as frit glass applied along the edges of the upper and lower substrate  111  and  121  with the discharge cells  125  being filled with the discharge gas. 
     A main phosphor layer  131  is located within the discharge cells  125  on side surfaces of the barrier ribs  124  and on an upper surface of the lower dielectric layer  123 . The main phosphor layer  131  is made up of phosphor materials that produce red, green, and blue light to produce a visible color image. The main phosphor layer  131  can include a red main phosphor layer, a green main phosphor layer, and a blue main phosphor layer according to the emitted color from a fluorescent material. The discharge cells  125  in which of the main phosphor layers  131  producing red, green, and blue light are made respectively form red, green, and blue sub-pixels which make up a unit pixel by forming pairs. 
     According to an aspect of the present invention, dummy phosphor layers  132  are formed on least on a portion of an upper region of the barrier ribs  124 . More specifically, the main phosphor layer  131  is found within the discharge cells  125  and extend in a direction parallel to the address electrode  122  and produce one of red, green, and blue light. In a sense, the dummy phosphor layers  132  produce colors identical to that of adjacent main phosphor layers  131  and are located on the upper surfaces of the second barrier ribs  124   b.    
     The main phosphor layers  131  and the dummy phosphor layers  132  with identically emitted colors are placed along a direction parallel to that of the address electrode  122 . With such a configuration, the neighboring discharge cells  125  in this address electrode direction do not affect each other when the second barrier rib  124   b  is a boundary. This is because phosphor layers on either side of second barrier rib  124   b  produce the same colors. Thus, discharge cells  125  consecutively arranged in parallel to the address electrodes to not affect each other. On the other hand, the dummy phosphor layers  132  are not formed on the upper surfaces of the first barrier ribs  124   a . Thus, discharge cells on opposite sides of the first barrier ribs  124   a  do not affect each other. If the dummy phosphor layers  132  were to be formed on the upper surfaces of the first barrier ribs  124   a , cross talk would occur between the neighboring discharge cells  125  on opposite sides of first barrier ribs  124   a  because they are of different color. 
     The thickness of the dummy phosphor layer  132  is preferably less than the thickness of the main phosphor layer  131 . If the thickness of the dummy phosphor layer  132  is too thick, there is a high possibility of generating cross talk between the discharge cells  125  due to the dummy phosphor layer  132  interposed between the discharge cells  125 . Here, the thickness A of the main phosphor layer  131  is defined from the center of a side surface of the second barrier rib  124   b  to an outer surface of the main phosphor layer  131 , as illustrated in  FIG. 3 . The thickness B of the dummy phosphor layer  132  is defined from the center of an upper surface of the second barrier rib  124   b  to an outer surface of the dummy phosphor layer  132  as also illustrated in  FIG. 3 . One example would be to make the main phosphor layer  131  approximately 10-30 um thick and the dummy phosphor layer  132  to be 3-10 um thick. 
     Referring to  FIG. 3 , a connector phosphor layer  133  is positioned between the main phosphor layer  131  and the dummy phosphor layer  132  to connect the main phosphor layer  131  to the dummy phosphor layer  132 . The connector phosphor layer  133  may be curved to connect the main phosphor layer  131  to the dummy phosphor layer  132  smoothly and continually around an upper edge of the second barrier rib  124   b.    
     The connector phosphor layer  133  prevents a portion of the second barrier rib  124   b  between the main phosphor layer  131  and the dummy phosphor layer  132  from being exposed to external is light. Therefore, this portion of the second barrier rib  124   b  is covered by connector phosphor layer  133  so that no portion of second barrier rib  124   b  is exposed to external light. By covering the second barrier rib  124   b  as so, less of the barrier rib  124  is exposed to external light. This is important because the barrier ribs  124  are more apt to reflect external light than the phosphor layers, and if more of the barrier ribs  124  are covered by a phosphor layer, less external light is reflected, leading to improved image contrast and an improved overall image quality. 
     As with the dummy phosphor layer  132 , if the connector phosphor layer  133  is too thick, there is a possibility of generating cross talk between the discharge cells  125 . In the present embodiment, the maximum thickness of the connector phosphor layer  133  is less than the thickness A of the main phosphor layer  131 . Here, the maximum thickness C of the connector phosphor layer  133  is defined from an upper corner of the second barrier rib  124   b  to an outer surface of the connector phosphor layer  133 . 
     The main phosphor layer  131 , the dummy phosphor layer  132 , and the connector phosphor layer  133  can be formed by a method such as using a dispenser. External light incident the second barrier rib  124   b  will result in minimal reflection since the second barrier rib  124   b  is entirely covered by the main phosphor layer  131 , the dummy phosphor layer  132 , and the connector phosphor layer  133 , resulting in improved image contrast. The improved image contrast is brought about because the main phosphor layer  131 , the dummy phosphor layer  132 , and the connector phosphor layer  133  are made of a material that has a lower reflectance than that of the barrier rib  124 . The barrier rib  124  is formed of SiO 2 , PbO, B 2 O 3 , Al 2 O 3 , TiO 2 , CaO, or ZnO, and is white and thus reflects nearly all incident light due to uniformly arranged fine particles. The phosphor particles that make up the main phosphor layer  131 , the dummy phosphor layer  132 , and the connector phosphor layer  133  reflect red, green and blue light and has a predetermined surface roughness. Thus, this phosphor material reflects less incident light than the material used in the barrier ribs  124 . Dummy barrier ribs, which are non-discharge portions, may be further formed on the outermost surfaces of the barrier ribs that define the discharge cells. Dummy phosphor layers as described above may be formed on at least a surface portion of the dummy barrier ribs. 
     It is to be appreciated the advantages of the present invention, over that of a black stripe or a black matrix, in reducing external reflectance and improving image contrast. In the present invention, unlike the implementation of the black stripe or black matrix layer, a separate layer and thus separate process steps are not necessary. The dispenser can form the main phosphor layers  131  along with the dummy phosphor layers  132  and the connector phosphor layers  132  in one single swath. Extra application and patterning steps for an additional layer are thus avoided by the design of the present invention, while improving the image contrast. Thus, as compared with the black matrix and the black stripe designs, the design of the present invention is not only effective in reducing the reflection of external light, the design of the present invention is also easy to make and does not involve extra process steps, thus being simple and inexpensive to manufacture. It is this combination of reducing the reflection of external light and the absence of additional process steps that makes the design of the present invention superior over other designs used to reduce the reflection of external light. 
     The operation of the PDP  100  will now be briefly described. First, an address discharge occurs when an address discharge voltage is applied between the address electrodes  122  and the Y electrodes  114 . Then, a predetermined wall charge accumulates in the discharge cells  125 . In this state, a sustain discharge occurs when a sustain discharge voltage is applied between the X electrodes  113  and the Y electrodes  114 . The charged particles generated as a result of discharge collide with the discharge gas and produce ultra violet rays by forming a plasma. An image is displayed on the upper substrate  111  by the main phosphor layer  131  that is excited by the generated ultra violet rays. When the PDP  100  is operating, external light incident from the outside of the PDP  100  is prevented from reaching the second barrier ribs  124   b  and reflecting off the second barrier ribs  124   b  by the dummy phosphor layer  132  and the connector phosphor layer  133 . Compared to the second barrier ribs  124   b , a much smaller fraction of the light is reflected off the dummy phosphor layer  132  and the connector phosphor layer  133 , thus reducing the reflection of the external light for the entire PDP. 
     According to the present invention, the reflection of external light off the second barrier rib  124   b  can be prevented by forming a dummy phosphor layer  132  on at least a part of an upper region of the second barrier rib  124   b  and forming a connector phosphor layer  133  between the dummy phosphor layer  132  and a main phosphor layer  131 . The reflection of external light can be remarkably reduced by the inclusion of the dummy and the connector phosphor layers because phosphor reflects less light than the barrier rib. Accordingly, brightness of light reflected by the PDP can be reduced and contrast can be improved due to the reduction of reflected external light. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.