Display Panel With Corona Discharge Control

Abstract

An electroluminescent display panel having solid state storage layers, an excitation current source and an ion generating source. When excitation current is applied to the panel luminescence is induced. A corona discharge created by addressing a matrix of conductors with a coincident voltage injects ions into the semiconductor control layer of the panel and alters the impedance state thereof. The change in the impedance state of the control layer alters the current flow through the panel resulting in a corresponding change in the state of panel luminescence. By selectively writing into an addressed matrix element either sequentially or simultaneously a pattern or image is formed on the panel face. The panel may be selectively erased by the addressing voltage source or by a separate erasing voltage source.

Description

This invention relates to panel display devices. More particularly, this invention relates to an electroluminescent panel display device wherein an area of the panel may be written upon or erased by the application of a coincident control voltage to selected cross-points of a conductive wire grid.

The panel type display is a flat device in that its depth is usually a much smaller dimension than its square area dimension. The display device may be considered a transducer which converts an electrical input into an optical output adapted for human observation. There has been much interest in display panel devices of this type since they may afford the answer to a workable flat screen television which permits large information displays and which are observable by many individuals simultaneously such as for example air traffic controllers. Other uses or applications may be in radar plotting, reproduction of photographs and readout of computer data.

The electroluminescent panel display has certain distinct advantages over the conventional cathode ray tube. Among these are it obviates the need for deflection coils and associated circuitry. It is also capable of being constructed in large sizes such 3 .times.4 feet, 4.times.5 feet and up to 20 .times.40 feet and it may be made to give high light outputs with good contrast and high resolution. The device is relatively insensitive to vibration and shock and the space required with regard to depth is small.

In the conventional electroluminescent panel display device a layer of luminescent material is sandwiched between a pair of electrodes and the combination deposited on a substrate such as glass. See for example, U.S. Pat. No. 2,932,770 to Livingston. Generally, the electroluminescent material is made of phosphors which give off light when a changing electric field is applied to the electrodes. Where an X-Y addressable panel is desired the electrodes may be set up in a grid configuration. For example, positioned on one side of the phosphor layer there may be placed a first group of parallel conductors and on the other side of the layer a second group of parallel conductors perpendicular to the first group of conductors, forming a series of cross-points where the first and second groups of conductors intersect. Thus, a specific area of the phosphor layer may be caused to luminesce by applying a voltage simultaneously to selected conductors of the first and second group.

In these prior art devices the excitation source and the addressing source are the same. This feature has distinct disadvantages, among which is the problem of providing the panel with good storage capability. Where the same source is used for both exciting and addressing the panel, storage capability may be sacrificed in order to achieve rapid addressing. Another problem with prior art devices was in finding a way to provide total isolation between adjacent cross-points of the matrix system. A further problem encountered in prior art devices was in controlling the luminescent intensity or brightness. Brightness of the excited phosphor depends among other factors on the level and the frequency of the applied voltage. It is evident that where the addressing voltage and the excitation voltage emanate from the same source, control of the brightness of electroluminescent layer is limited and inflexible.

The disadvantages of the aforementioned devices have been overcome by the present invention wherein a separate means is provided for exciting the electroluminescent layer and for addressing the matrix cross-points. The electroluminescent display device of the present invention provides a flat panel having a depth of approximately one-half inches which has high storage capabilities, isolation between selected and unselected cross-points and the capacity to be made into large sizes. More particularly, the present invention provides a solid state storage electroluminescent display panel in which a first plurality of parallel conductive lines are mounted upon a substrate and are insulated from each other by a nonconductive material. Overlying the conductive lines there is a layer of electroluminescent material. Above the electroluminescent layer there is a control layer semiconductor material with electrically controllable impedance. Spaced from and in a plane parallel to the solid state layers is a wire grid network by means of which a point on the control layer may be addressed. A time varying excitation current is applied to alternate conductive lines of the panel to cause a current to flow in a path form one conductive line through the phosphor and control layers to an adjacent conductive line, thereby causing the panel to luminesce. When a coincident address voltage is applied to the grid conductors negative or positive ions are generated by corona discharge and are injected into the control layer below the selected cross-points, thereby either increasing or decreasing the control layer impedance in this area of the panel. The panel luminance in the selected area may, in this way be set to any desired level.

Accordingly, it is an object of this invention to provide an electroluminescent display device which has high storage capability.

Another object of this invention is to provide a separate source for exciting the phosphor layer and a separate source for addressing the matrix cross-points.

Another object of this invention is to provide means of addressing the display devices which eliminates interference with unselected cross-points.

Yet another object of this invention it to provide an electroluminescent display device which is not limited as to size.

These and further objects of the present invention will be more fully understood by reference to the description which follows and the accompanying drawings wherein:

FIG. 1 illustrates a plan view of the electroluminescent display device,

FIG. 2 is a side view of FIG. 1 along the lines 2-2 showing in detail the layers of the panel,

FIG. 2a is a view similar to FIG. 1 showing in addition the excitation current source for the panel,

FIG. 3 is a plan view similar to FIG. 1 showing a third or corona grid, and

FIG. 4 is a side view of FIG. 3 along the line 4-4 showing details of the panel layers.

Turning now to FIG. 1, there is shown generally at numeral 10 a section of the electroluminescent panel of the invention. Above and spaced from panel 10 in a plane parallel to the panel there are shown a plurality of metal horizontal wires X.sub.1 through X.sub.4. Above and spaced from wires X.sub.1 and X.sub.4 in a plane parallel to the X wires there are a plurality of vertical wires Y.sub.1 through Y.sub.4.

In FIG. 2 there is shown in enlarged detail the layers of a section of the display panel. Reference numeral 11 is a substrate or support means which may be glass, Mylar or any suitable nonconductor. Overlying substrate 11 are a plurality of conductive lines 13. The conductive lines are insulated from each other and bound to substrate 11 by an epoxy 12 or other adhesive. Each conductive line is coated with an insulating layer 14 and has been abraded to expose the conductive material. A portion of the conductive material has been etched away so that each wire line is contained in a trough made up of the insulating material 14. The area above the conductive material in the trough is filled with an electroluminescent material 18 which is capable of emitting radiation under the action of a strong electric field below its breakdown potential. Alternately, material 18 could be formed in a continuous layer overlying the conductive lines. The electroluminescent surface is flat and the control layer of semiconductive material 15 evenly overlies the electroluminescent material 18.

The conductive material of conductive line 13 may be any good electrically conductive material such as copper, silver, platinum, brass or steel alloys. Insulating material 14 should be selected so that it is capable of withstanding the etching agents used to form the trough. Although zinc sulfides may be used as a suitable electroluminescent material, a mixture of copper fluoride and magnesium activated zinc sulfide in an epoxy binder will yield similar results. Moreover, any of the well known electroluminescent phosphors may be utilized and tailored to furnish the desired response and spectral output.

For the semiconductor control layer material zinc sulfide, lead oxide, cadmium sulfide, cadmium oxide, germanium, zinc sulfide, zinc oxide and the like may be employed. The control layer should have the properties of a field effect semiconductor. A field effect semiconductor in this context refers to materials capable of conducting current through the body thereof. However, the conduction of such material is modified by applying an electric field perpendicular to the current flow of the material creating a region that effectively changes the cross-sectional conducting area of the material or the conductivity of the material itself. Some of the semiconductor materials listed may not perform efficiently as a field effect semiconductor. In some of these instances a dielectric used with the semiconductor may give satisfactory results.

At least one portion of the electroluminescent material forms part of the electrical circuit between the electrodes with the successive part of the electrical circuit being formed by a storing portion of the semiconductor material. The semiconductor material is capable of conducting current therethrough without substantially altering the charge pattern on the charge retaining surface. When an alternating current beyond a threshold level is applied to the spaced electrodes, electroluminescence will be induced, assuming that the semiconductor material is in a low impedance state. It can be demonstrated that the deposition and retention of an electrostatic charge on the retaining surface of the electroluminescent panel can be used to control the flow of current through the panel. When a negative electrostatic charge is deposited upon the panel, the impedance of the semiconductor is increased with a concomitant reduction or interruption of current flow in adjacent areas. The dimunition of current flow will result in a corresponding dimunition in light output from the electroluminescent layer resulting in contrasting areas of light and dark on the panel or half-toned response. Further reduction in current flow below a threshold value will cause that portion of the panel to cease luminescence altogether and that portion of the panel will appear dark. Conversely, the impedance of the semiconductor material is lowered and current flow increased as the charges are neutralized or removed from the panel surface. Accordingly, by selectively depositing and maintaining a charge pattern on the surface of the electroluminescent panel an image can be produced and stored by the device.

FIG. 2a depicts the layers of the solid state storage panel. Current flow through the solid state layers is from current source 9, lead 7 to conductive line 13, through the electroluminescent phosphor 18 through control layer 15 to adjacent conductive line 13 to lead 8 and thence back to source 9. The excitation voltage may range between 300 volts to 800 volts. The control layer 15 conducts a current without altering the charge pattern on the charge retaining surface. This current flow will cause the phosphor to luminesce and the control layer will determined the brightness emitted. If the control layer is in its high impedance state very little current will be permitted to flow. Conversely, when this control layer is in its low impedance state, current will flow freely thereby causing the panel to glow brightly. The degree of brightness will of course depend upon the level and the frequency of the applied voltage, as well as the impedance of the control layer.

In order to vary the impedance state of the control layer, ions generated by a corona discharge are injected into the control layer. The injection of positive ions into the control layer will lower the impedance state of the control layer while the injection of negative ions in the control layer will increase the impedance of the control layer. When ions are injected into the control layer at selected points as will be discussed in greater detail hereinafter, a pattern or image which may be stored is formed on the panel.

Turning again to FIG. 1, if a sufficiently high voltage is applied between the fine wires and a conducting surface parallel to it the air near the wires will become ionized and the ions created will be swept to the conducting surface. This movement of ions is called corona current. The voltage necessary to create this current is a function of the wire diameter. Corona emission begins to appear at a fairly well-defined threshold voltage. This threshold voltage is a function of the electric gradient at the surface. To inject negative ions at a single grid cross-point the selected X or control conductor wire passing over the selected point is set to a negative potential of approximately 200 volts from power supply 80 via switch S1d. The selected corona or Y conductor is driven to a negative value of approximately 7000 volts from power supply 81 via switch S2. If, in particular, cross-point X.sub.4, Y.sub.4 is to be addressed, control conductors X.sub.1 through X.sub.3 are set at a positive potential of approximately 100 volts by way of switch arms S1a, S1b, and S1c connected to the positive terminal of power supply 80. Y.sub.4 is driven sufficiently negative by operation of S2 to generate a corona discharge at the intersection of X.sub.4, Y.sub.4, causing negative ions to be injected into the control layer below the intersection. Any negative ions created by the corona discharge at the other cross-points are attracted to the positive X wires preventing negative ions from reaching the control layer at these points. However, negative ions are permitted to get through to the control layer at the selected cross-point. The injection of these negative ions into the control layer will increase the impedance of the control layer at the point of ion injection and will result in dimunition or cessation of luminescence at that point. A similar system may be used for injecting positive ions. It is noted that points on the panel may be addressed where the X-Y conductors are displaced from each other at angles other than at right angles and such displacement of the X-Y conductors are within the scope of the invention. Moreover, the panel may be erased point by point or overall by the use of the addressing voltage power supply or from a separate erase voltage source.

In FIGS. 3 and 4 there is illustrated another embodiment of the invention employing a different grid arrangement. Turning briefly to FIG. 4 there is shown generally at numeral 10 a side view of the panel along the line 4-4 of FIG. 3. The details of the panel layers are the same as discussed above with regard to FIG. 2. FIG. 4 also shows that the X, Y and C conductors are spaced from each other and from the panel 10 also in the manner discussed above. The X conductors are located closest to the panel and the corona or C conductors are located furthest away from the panel with the Y conductors between them.

Turning again to FIG. 3 there is schematically illustrated horizontal conductors X1 through X5 and vertical conductors Y1 through Y4. In addition, there is shown corona conductors C1 through C3 also oriented horizontally and interdigitated between the X conductors. The X conductors through the ganged switch arms of switch S3 are connected to the power supply 91. X conductors X1 through X4 are connected to the positive terminal of power supply 91. X5 conductor is connected to the negative terminal of the power supply 91. The Y conductors through the ganged switch arms of S4 are similarly connected to power supply 91. Y conductors Y1 through Y3 are connected to the positive terminal of power supply 91 and conductor Y4 is connected to the negative terminal of power supply 91. Connected to the corona wire at C through switch 55 is a negative potential from power supply 92.

It is understood that the grid networks and the panel layers of FIG. 1 through 4 represent only a portion of an a actual panel display device. In an actual panel display device having a dimension for example of 5 feet .times. 5 feet or larger the grid wires would be far more numerous and more closely spaced than shown schematically in FIGS. 1 through 4. In the contemplated large panel display device numerous cross-points at the intersection of the grid would be addressed or scanned sequentially or simultaneously so as to build visual data information upon the panel. This may be accomplished by varying the impedance of the semiconductor layer through ion injection at each cross-point. The contrast between array elements may also be varied by changes of excitation voltage or frequency.

It is also understood that the mechanical switches S1 through S6 are shown only for purposed of explanation and the invention is not intended to be limited thereto. It is within the scope of this invention that the mechanical switches be replaced by electronic switches with the necessary control circuits.

Assuming that the control layer 15 is in a low impedance state and that the excitation current has been permitted to flow, panel 10 will glow brightly since the phosphor has become excited by the current flowing through it. When a cross-point, for example, X.sub.5, Y.sub.4 is desired to be selected, conductors X.sub.1 through X.sub.4 are made slightly positive by connection to the positive terminal of power supply 91 through ganged switch arms S3a, S3b, S3c and S3d. X5 is made slightly negative by connection to the negative terminal of power supply 91 by S3e. Simultaneously, Y.sub.1 and Y.sub.3 conductors are made slightly positive by connection through the ganged switch arms S.sub.4a, S.sub.4b and S.sub.4c to the positive terminal of power supply 91 and Y.sub.4 conductor is made slightly negative by connection to the negative terminal of power supply 91 via S4d of switch S.sub.4. Corona wires C.sub.1, C.sub.2 and C.sub.3 are driven greatly negative by power supply 92 when switch S.sub.5 closes at C. A corona discharge is generated at this time and negative ions will be injected into the panel only at the intersection of X.sub.5 and Y.sub.4. All other cross-points will be isolated from the corona discharge because the ions, being negative will be attracted to the slightly positive X and Y conductors passing over the unselected array elements. The injection of negative ions into the control layers at X.sub.5 and Y.sub.4 will cause the control layer beneath the intersection of the selected cross-point to increase its impedance. The increase in impedance at this point will decrease current thereat and will cause the point to darken. A similar system may be used to selectively inject positive ions into the panel control layer. Where a pattern or image is to be formed successive cross-points will be modulated and a pattern or image will thereby be built up on the panel as the impedance control layer below these cross-points is altered. In order to produce contrast between adjacent areas on the panel the voltages on the X, Y and C conductors may be varied individually or simultaneously.

From the foregoing disclosure it has been demonstrated that the invention provides an electroluminescent display panel which is capable of isolating the selected areas from the unselected areas. Moreover, the invention provides a panel having good storage and resolution.

Claims

What we claim is:

1. An electroluminescent panel display device comprising:

a first layer of electroluminescent material,

a second layer of semiconductor material adapted to change its impedance state upon the injection of ions therein said second layer overlying said electroluminescent material,

a plurality of parallel conductors in overlying relation with said first layer,

means for supplying a current flow through said first and second layers and through alternate ones of said parallel conductors to cause said electroluminescent material to glow, and

means for generating ions to be injected into said second layer for altering the impedance thereof whereby said panel luminance can be controlled.

2. The apparatus of claim 1 comprising:

a support member for supporting said conductor and said first and second layers and said ion generating means.

3. The apparatus claim 2 comprising a layer of insulating material between said support and said first layer, said insulating layer having said plurality of spaced parallel conductors embedded therein.

4. The apparatus of claim 3 comprising:

strips of electroluminescent material overlying each of said plurality of parallel conductors.

5. In a panel display device having a layer of electroluminescent material overlying a layer of semiconductor material said semiconductor adapted to change its impedance state when subjected to ion injection, means for passing an excitation current through said layers causing said panel to luminesce and means for generating ions to be injected into said semiconductor material whereby said panel changes its state of luminescence.

6. The apparatus of claim 5 wherein said ion generating means is formed into a grid configuration of conductors and includes means to select points on said panel whereby said points change their state of luminescence.

7. An electroluminescent panel display device comprising:

a support member,

a layer of electroluminescent material overlying said support member,

a layer of semiconductor material overlying said first layer,

means for passing an excitation current through said first and second layers causing said panel to luminesce,

a plurality of parallel first conductors spaced from but in overlying relation with said second layer, and

a plurality of parallel second conductors spaced from and perpendicular to said first conductors, said second conductors in cooperation with said first conductors creating a corona discharge generating ions for injection into said second layer when a coincident voltage is applied to said first and second conductors whereby said panel luminance changes at the points of ion injection because of the increased or decreased impedance of said second layer.

8. The apparatus of claim 7 comprising:

a plurality of parallel third conductors interdigitated between said first conductors adapted to create a corona discharge generating ions for injection into said second layer when a voltage of a first polarity is applied thereon and a voltage of a second polarity is applied to said first and second conductors whereby the luminance of said panel is changed.

9. In a panel display having a layer of electroluminescent material overlying a layer of semiconductor material said semiconductor material adapted to change its impedance state when subjected to ion injection, means for passing an excitation current through said layers causing said panel to luminesce, a plurality of parallel first conductors spaced from but in overlying relation with said semiconductor layer, a plurality of parallel second conductors spaced from and positioned angularly in relation to said first conductors, a first array of switches connected to a voltage source for applying a voltage to said first conductors, a second array of switches connected to a voltage source for applying a voltage to said second conductors said second conductors in cooperation with said first conductors creating a corona discharge generating ions for injection into said semiconductor layer when a coincident voltage from said voltage sources is applied to said first and second conductors whereby said panel luminance changes at the points of ion injection because of the increased or decreased impedance of said semiconductor.