Bechtel, Hans-helmut (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
Glaeser, Harald (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
Mayr, Walter (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
PHILIPS CORPORATE INTELLECTUAL PROPERTY GMBH (Weisshausstrasse 2, Aachen, 52066, DE)
Juestel, Thomas (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
Bechtel, Hans-helmut (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
Glaeser, Harald (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
Mayr, Walter (Prof . Holstlaan 6, AA Eindhoven, NL-5656, NL)
| 1. | A plasma display screen equipped with a front plate (1) comprising a transparent plate (3) on which a dielectric layer (4) and a protective layer (5) are provided, which protective layer comprises a material that emits light upon excitation by UV photons, and equipped with a carrier plate (2) which is provided with a phosphor layer (9) having a ribbed structure (12), which divides the space between the front plate (1) and the carrier plate (2) into plasma cells that are filled with a gas, and equipped with one or more electrode arrays (6, 7,10) on the front plate (1) and the carrier plate (2) for generating corona discharges in the plasma cells. |
| 2. | A plasma display screen as claimed in claim 1, characterized in that the light emitting material comprises a dopant. |
| 3. | A plasma display screen as claimed in claim 1, characterized in that the light emitting material comprises doped MgO. |
| 4. | A plasma display screen as claimed in claim 3, characterized in that the MgO is doped with one or more ions selected among the group consisting of A13+, Ga3+, In3+, Nd3+, Pr3+, Sb3+, Bi3+, Pb2+, Sn2+, Zn2+, Cd2+, Br, Cl, Fand N3. |
| 5. | A plasma display screen as claimed in claim 1, characterized in that a scattering layer is provided between the dielectric layer (4) and the protective layer (5). |
| 6. | A plasma display screen as claimed in claim 5, characterized in that the scattering layer comprises particles having a particle size in the range between 50 and 300 nm. |
| 7. | A plasma display screen as claimed in claim 1, characterized in that the light emitting material emits light having a wavelength < 400 nm. |
| 8. | A plasma display screen as claimed in claim 7, characterized in that the light emitting material emits light in a wavelength range from 200 to 280 nm. |
The invention relates to a plasma display screen equipped with a front plate comprising a transparent plate on which a dielectric layer and a protective layer are provided, and equipped with a carrier plate which is provided with a phosphor layer having a ribbed structure, which divides the space between the front plate and the carrier plate into plasma cells that are filled with a gas, and equipped with one or more electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells.
Plasma display screens enable high-resolution color pictures having a large display screen diagonal to be obtained in a compact design. A plasma display screen comprises a gas-filled hermetically sealed cell with electrodes arranged in accordance with a grid pattern. The application of an electric voltage brings about a gas discharge that generates light in the ultraviolet range. By means of phosphors, this light can be converted to visible light and emitted towards the viewer through the front plate of the cell.
In principle, two types of plasma display screens can be distinguished: a matrix arrangement of the electrodes and a co-planar arrangement of the electrodes. In the case of the matrix arrangement, the gas discharge is ignited and maintained at the point of intersection of two electrodes on the front plate and the carrier plate. In the case of the co- planar arrangement, the gas discharge is maintained between the electrodes on the front plate and ignited at the point of intersection with an electrode, a so-termed address electrode, on the rear plate. The address electrode is situated, in this case, below the phosphor layer. By virtue of this arrangement, fifty percent of the UV light generated in the gas discharge reaches the front plate where it is absorbed in the layers present there. For a part of the UV light this effect is further enhanced, as the UV light is re-absorbed in the gas space as a result of the fact that gas atoms are excited so as to switch from the ground state to a higher energy state. The light is subsequently emitted again, however, it is deflected from its original direction so that also light originally spreading in the direction of the phosphor layer can reach the front plate. In this manner, in plasma display screens in accordance with the state of the art, only approximately 60% of the UV light generated is absorbed by the phosphors.
The luminance of a plasma display screen depends to a substantial extent from the efficiency with which the UV light excites the phosphors. To increase the luminance of a
plasma display screen, DE 199 44 202 describes a plasma display screen comprising an UV light-reflecting layer on the front plate. A drawback of said plasma display screen resides in that this layer must be provided on the protective layer of MgO since said MgO layer is not pervious to W light. As a result, the advantageous effect that MgO has a high ion-induced secondary electron emission coefficient and that it reduces the ignition voltage of the gas can hardly be utilized.
Therefore, it is an object of the invention to provide a plasma display screen having improved luminance.
This object is achieved by a plasma display screen equipped with a front plate comprising a transparent plate on which a dielectric layer and a protective layer are provided, which protective layer comprises a material that emits light upon excitation by UV photons, and equipped with a carrier plate which is provided with a phosphor layer having a ribbed structure, which divides the space between the front plate and the carrier plate into plasma cells that are filled with a gas, and equipped with one or more electrode arrays on the front plate and the carrier plate for generating corona discharges in the plasma cells.
The efficiency of the plasma display screen can be increased by using a material in the protective layer that emits light itself when it is excited by UV-photons.
In accordance with an advantageous embodiment, the light-emitting material of the protective layer contains dopants that bring about, for example, imperfections in the crystal lattice of the material and hence defect luminescence when the material is exposed to UV-photon radiation. On the other hand, upon excitation by the UV photons, the dopants themselves may switch to a higher energy state and, subsequently, return to the ground state while emitting radiation.
In accordance with a further advantageous embodiment, the light-emitting material of the protective layer comprises doped MgO. By virtue thereof, the advantageous effect that MgO has a high ion-induced secondary electron emission coefficient and reduces the ignition voltage of the gas can be utilized.
In accordance with a still further advantageous embodiment, a scattering layer of particles having a particle size ranging between 50 and 300 nm may be provided between the dielectric layer and the protective layer. The light emitted by the protective layer in the direction of the dielectric layer is reflected back into the plasma cell by this scattering layer.
In addition, the scattering layer precludes light conduction in the protective layer.
If, in accordance with an advantageous embodiment, the wavelength of the light emitted by the light-emitting material is below 400 nm, particularly in the UV-C
wavelength range between 200 and 280 nm, then the emitted light can be used to excite the phosphors in the phosphor layer. In this manner, the efficiency with which light is generated in the plasma cells is increased.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment (s) described hereinafter.
In the drawing: Fig. 1 shows the structure and the operating principle of a single plasma cell in an AC plasma display screen.
As shown in Fig. 1, a plasma cell of an AC plasma display screen with a co-planar arrangement of the electrodes comprises a front plate 1 and a carrier plate 2. The front plate 1 comprises a transparent plate 3, for example of glass, on which, in succession, a dielectric layer 4 and a protective layer 5 are provided. The glass plate 3 is provided with parallel, strip-shaped discharge electrodes 6,7 which are covered by the dielectric layer 4.
The discharge electrodes 6,7 are made, for example, from metal or ITO. The carrier plate 2 is made of glass, and said carrier plate 2 is provided with parallel, strip-shaped address electrodes 10 of, for example, Ag extending perpendicularly to the discharge electrodes 6,7.
These address electrodes are covered by a phosphor layer 9 emitting light in one of the three primary colors red, green or blue. For this purpose, the phosphor layer is divided into a plurality of color segments. Customarily, the red, green and blue-emitting color segments of the phosphor layer 9 are provided in the form of perpendicular triplets of strips. Individually drivable plasma cells wherein the corona discharges take place are formed by a ribbed structure 12 comprising separating ribs of, preferably, a dielectric material.
A gas is present in the plasma cell as well as between the discharge electrodes 6,7, of which one serves alternately as the cathode or the anode. The gas may comprise, for example, nitrogen or a mixture of nitrogen and at least one inert gas, such as He, Ne, Kr or Xe. After ignition of the surface discharge, enabling charges to flow on a discharge path situated between the discharge electrodes 6,7 in the plasma region 8, a plasma is formed in the plasma region 8, which plasma, dependent upon the composition of the gas, causes radiation 11 to be generated in the UV range, preferably in the range between 200 and 400 nm. The phosphor layer 9 is excited by said radiation 11 so as to emit visible light 13 in one
of the three primary colors, said light issuing to the exterior through the front plate 1 and hence forming a luminous pixel on the display screen.
The protective layer 5 comprises a material which, upon excitation by UV photons, emits light. This light-emitting material preferably comprises dopants. It is particularly preferred that the light-emitting material comprises doped MgO. For the doping ions use can be made, for example, of A13+, Ga3+, In3+, Nd3+, Pr3+, Sb3+, Bi3+, Pb2+, Sn2+, Zn2+, Cd2+, Br, Cl-, F-or N3-. The light-emitting material in the protective layer 5 may also comprise two or more of these doping ions.
By incorporating cations into the MgO lattice, on the one hand imperfections can be created which, upon excitation of the cation-doped MgO layer by UV photons, bring about defect luminescence having a longer wavelength. On the other hand, it is also possible to incorporate cations, for example Nd3+, Pr3+ or Pb2+, into the MgO lattice that exhibit an excited state in the UV range and, upon excitation by UV photons, return to the ground state while emitting light having a wavelength of, preferably, < 400 nm. It is alternatively possible for the protective layer 5 to comprise Tb3+-doped MgO as the light-emitting material. In this embodiment, the light-emitting material of the protective layer 5 emits colored, green light.
By incorporating monovalent anions into the MgO lattice, cation vacancies can be created which, upon excitation of the anion-doped MgO layer by UV photons, bring about defect luminescence having a longer wavelength.
The manufacture of a protective layer 5 comprising cation-doped MgO as the light-emitting material can take place by means of electron beam evaporation or wet- chemical processes. In the case of electron beam evaporation, the target comprises, apart from MgO, 0. 01 to 1 mol% of a suitable oxide. For the oxide use can be made, for example, of A1203, Ga203, In203, Nd203, Pr6OI l, Sb2Os, Bi203, PbO, SnO, ZnO or CdO.
The manufacture of a protective layer 5 comprising anion-doped MgO as the light-emitting material can take place by means of electron beam evaporation or wet- chemical processes. In the case of electron beam evaporation, the target comprises, apart from MgO, for example 0.01 to 1 mol% of a suitable magnesium halogenide, such as MgF, MgCl or MgBr.
The light emitted by the light-emitting material of the protective layer 5, which preferably has a wavelength in the range between 200 and 280 nm, may, in addition to the W photons from the plasma discharge, excite the phosphors in the phosphor layer 9 and hence increase the efficiency of the plasma display screen.
For the phosphors in the segmented phosphor layer 9 use is preferably made of phosphors which can be efficiently excited by light having a wavelength in the range between 200 and 400 nm. The blue color segments of the phosphor layer 9 preferably comprise a phosphor selected among the group consisting of (Ba1-xSrx)MgAl10O17 : Eu, where (0 # x # 1), (Bal-xSrx) MgAlloOl7 : Eu, Co, where (0 # x < 1), (Yl-xOdx) 2SiOs : Ce, where (0 < x # 1), (Y1-xGdx)BO3 : Ce, where (0 # x # 1) and a mixture of at least two of these phosphors. The green color segments of the phosphor layer 9 preferably comprise a phosphor selected among the group consisting of Zn2Si04 : Mn, (Bai-xSrx) MgAlloOl7 : Eu,Mn, where (0 < x < 1), (YI-XGdX) B03 : Tb, where (0 # x # 1) and a mixture of at least two of these phosphors. The red color segments of the phosphor layer 9 preferably comprise a phosphor selected among the group consisting of (Y1-xGdx)BO3 : Eu, where (0 < x < 1), Y203: Eu, (Yl-xGdx) 203: Eu, where (0 < x < 1), YV04 : Eu, Y (V1-xPx)O4 : Eu, where (0 < x # 1) and a mixture of at least two of these phosphors.
Advantageously, a scattering layer is situated between the dielectric layer 4 and the protective layer 5. This scattering layer preferably comprises particles having a particle size ranging between 50 and 300 nm. For the particles use can be made, for example, of metal oxides, metal phosphates or metal fluorides.
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