Electroluminescent Device

Abstract

An electroluminescent device wherein a memory switch, which requires no holding current or voltage for its maintenance, controls the operation of the electroluminescent layer. When the device is arrayed in a coordinate pattern, individual area of the device can be selectively addressed to form a pattern of visual data.

Description

This invention relates to an electroluminescent device. Specifically, the invention relates to an electroluminescent panel display device, controlled by a memory threshold switch, which is capable of displaying visual data in a matrix array.

BACKGROUND OF THE INVENTION

There has been considerable interest in display panel devices generally since they may afford the answer to a workable flat screen television which permit large information displays and which are observable by many individuals simultaneously. Other uses or applications may be in radar plotting and read out of computer data.

The panel display device has certain distinct advantages over the conventional cathode ray tubes which have become a standard visual display device. First of all the panel display obviates the need for deflection coils and associated power consuming circuitry. Secondly, panel displays as opposed to cathode ray tubes are capable of being constructed in large sizes such as 3 .times. 4', 4 .times. 5' and up to 20 .times. 40' and they may be made to give high light outputs with good contrast and high resolution. Thirdly, panel devices are relatively insensitive to vibration and shock and the space required with regard to depth is minimal.

In the conventional electroluminescent panel display device a layer of luminescent or phosphor material is sandwiched between electrodes and the combination deposited on a substrate such as glass. Generally, the electroluminescent material is made of phosphor which emits light when subjected to a time varying electric field. Where an X-Y or matrix addressable panel is desired the electrodes may be set up in a grid configuration by applying a coincident voltage to selected conductors of the X and Y group.

Although electroluminescent panel devices have had successes in many applications, there exist certain disadvantages in their usage which must be taken into consideration. One of the disadvantages of electroluminescent panels is that they generally require separate sources of voltages for exciting the electroluminescent layer and for addressing the crosspoints. This requirement represents a considerable current drain. Another problem ascribable to electroluminescent panels is that they tend to exhibit cross talk. That is, crosspoints adjacent to the selected crosspoint in the grid emit light to a disturbing degree interfering with visual data or even generating unreliable visual data. Thus, satisfactory isolation of crosspoints in electroluminescent displays is an objective which remains elusive.

The disadvantages of the aforementioned electroluminescent devices have been overcome by our invention. We provide isolation between selected and unselected crosspoints and utilization of the same voltage source for exciting the electroluminescent layer as well as for addressing the selected crosspoints. Moreover, we provide a semiconductor switching element which controls the operation of the electroluminescent layer and which has memory of its conductive state.

The memory element of the present invention is inexpensive to fabricate and comprises a bistable non-rectifying semiconductor of amorphous or polycrystalline material having variable resistance states. When a voltage of a threshold value is applied across the electrodes of the element it will rapidly change from a blocking or high resistance state to a conducting or low resistance state. The element retains its conductive state even under zero bias conditions and may be returned to its blocking condition by a further increase in current above a discrete level or by being subjected to radiation. The provision of a built-in memory mechanism in a display device has long been sought since it would reduce the memory storage requirements of computers. Our invention fulfills this need.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the invention to provide an electroluminescent display device which is inexpensive to construct and which is capable of being produced in large sizes.

It is another object of the invention to provide an electroluminescent device which yields positive or negative images.

It is a further object of the invention to provide an electroluminescent panel which furnishes isolation between selected and unselected crosspoints and which has memory of its conductive state.

It is yet another object of the invention to provide an electroluminescent panel which can be operated at high frequencies without affecting the "on-off" contrast.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a composite structure comprising a region of semiconductor switching material and a region of electroluminescent material disposed between a pair of electrodes. In one embodiment of the invention the switching material and the electroluminescent material is placed in parallel between the pair of electrodes at least one of which is transparent. Also in this embodiment a control impedance which may be a resistor or capacitor is placed in series with this composite structure. A low impedance source of AC voltage is applied across the electrodes and when the applied voltage reaches the threshold level of the bistable switching element it changes its conductive state permitting current to flow through the electroluminescent element. The change in current flow through the electroluminescent element will alter the amount of light emitted by it. In a second embodiment of the invention the bistable semiconductor switching material and the electroluminescent material are in series between the pair of electrodes. Alternately, in either embodiment the control impedance may be a resistive layer overlying the electrode. The composite electroluminescent structures are formed into a panel array and addressing circuits are provided to select either simultaneously or individually desired crosspoints on the panel. Thereby, an image or other visual data is displayed on the panel.

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. 1a shows the gross features of the I-V curve for the bistable switching element,

FIG. 1b is a schematic sectional view of one embodiment of the invention showing the control impedance in series with the composite structure of the bistable semiconductor switching material in parallel with the electroluminescent material placed between the electrodes,

FIG. 1c is a view similar to FIG. 1b showing a resistive layer overlying one of the electrodes;

FIG. 1d is cross sectional view of the electroluminescent device showing a capacitive impedance,

FIG. 2 is a schematic sectional view showing the bistable semiconductor switching material in series with the electroluminescent material situated between the electrodes,

FIG. 3 a schematic sectional view showing the bistable semiconductor switching material and the electroluminescent material dispersed heterogeneously between the electrodes, and

FIG. 4 is a simplified schematic plan view of a section of the electroluminescent panel showing the wiring used to address the panel.

Referring to the drawing wherein like reference numerals designate the same elements throughout the several views, there is shown in FIG. 1b a composite matrix element structure comprising a first transparent electrode 14 and a second electrode 17 which may be of transparent or opaque material. Disposed between electrodes 14 and 17 and in electrical relation therewith is a region of bistable semiconductor switching material 13 mixed in an epoxy binder material 15 and a region of electroluminescent material 12 in parallel with the switching material. Electrode 17 is connected to ground through a control impedance Z.sub.f 11. Electrode 14 is connected to a low impedance AC source 10 by a wire 16.

FIG. 1c is essentially the same as FIG. 1b except that in lieu of control resistor 11 a layer of resistive material 18 overlays electrode 17. Moreover, a capacitor may be substituted in lieu of a resistor as shown in FIG. 1d. In such event the capacitor would present a very low impedance to RF current and would allow more current to flow through the switching and electroluminescent material.

FIG. 1a depicts the gross features for the AC current-voltage I-V characteristics of a bistable memory element which for the purposes of illustration may consist of CuO powder in an epoxy binder. After fabrication the device is in its high resistance or blocking state R.sub.b, for voltages less than a threshold value V.sub.th. The sample current in trace (a ) increases monotonically with applied voltage. The details of I-V are dependent on the device material and the mode of operation AC or DC. When the voltage exceeds a threshold value which is typically between 10-30 volts for an approximately 40 microns thick switching material sample, the device makes a transition to a conducting state shown in trace (b ). The element remains in this conducting state even though the applied voltage is reduced or removed for periods of at least months unless specifically driven back to the high resistance state. The element makes a transition from a high resistance state 10.sup.9 .gtoreq. megohms) to a low resistance state (approximately 1 to 10.sup.3 ohms) in times on the order of microseconds when subjected to the threshold voltage. The high resistance state may be recalled by subjecting the element to a sufficiently high AC or DC current, radiation from a RF discharge, or to RF current through the element. Where variable resistance switching material is utilized, the I-V curve will show several resistance traces between traces (a ) and (b). The resistance states from high to low and the resistance states in between are described in greater detail in copending application, Ser. No. 879,061, filed Nov. 24, 1969 and assigned to the same assignee as the instant application.

The transparent electrode 14 may comprise thin layers of tin oxide, copper iodide or gold alone or on a transparent substrate. The opaque electrode may be made of any good electrically conductive material such as copper, silver, brass, platinum or steel alloys. For the electroluminescent material zinc sulfide or a mixture of copper chloride and magnesium activated zinc sulfide in a binder may be used. However, any of the well known electroluminescent phosphors may be utilized and tailored to furnish the desired response and spectral output. For the semiconductor switching material amorphous or polycrystalline ZnO:Zn, ZnO:Zn+ZnO or ZnO powders suspended in a binder such as an epoxy can be employed. Other suitable oxides include cupric oxide, cuprous oxide, ferric oxide, lead dioxide, manganese dioxide, mercuric oxide and aluminum oxide.

The reduced zinc oxide (ZnO:Zn) used in the switching element is a well known phosphor and is obtainable commercially. Moreover, the zinc oxides are variable resistance devices and provide a gray scale in the intensity of the electroluminescent material with which it is in parallel. Reducing the percentage of excess zinc in the switching element by mixing the ZnO:Zn with unreduced powders yields devices which perform satisfactorily as switching and memory elements. Zinc oxide with no deliberate reduction also performs satisfactorily. However, the unreduced material does not appear to work as well as a variable resistance device in some instances as the reduced material. The minimum amount of excess zinc for improved behavior may in fact be made available locally in a pure ZnO device by thermal or electronic processes during the initial forming of the device. Therefore, further devices made from pure ZnO may produce devices that work as well as ZnO:Zn systems.

The preparation of the zinc and the other modified oxides follow conventional procedures as generally given in U.S. Pat. No. 2,887,632 to Dalton. Specifically the variable resistance zinc oxides are fabricated by firing zinc oxide with small amounts of zinc and aluminum formates in a vacuum for five minutes at 700.degree. C. The ratios by weight are 60 gms. ZnO to 0.3 gm. aluminum formate and 60 gms. zinc formate. These procedures reduce the resistivity of the ZnO powders used about one order of magnitude. While the reason why the modified oxide has a reduced resistance is not fully understood, it is believed that in firing, the various mixtures of the metallo-organic compounds like zinc formate and aluminum formate decompose into a pure metal which becomes part of the oxide crystal lattice and a volatile organic compound. Modified CuO and A1.sub.2 O.sub.3 were made by adding 10 percent by weight of the above formates to the oxides and firing as stated above.

The active materials used and their characteristic resistivity are shown in Table I.

TABLE I

Material Vol. Resistivity __________________________________________________________________________ ZnO:ZN (p-15 phosphor) 7.5 .times. 10.sup.9 ohm-cm Zinc Oxide 7.5 .times. 10.sup.9 ohm-cm Zinc Oxide fired with Aluminum formate as described previously 4.4 .times. 10.sup.8 ohm-cm Zinc Oxide fired with Zinc formate 1.1 .times. 10.sup.9 ohm-cm High Conductivity Zinc Oxide 6 .times. 10.sup.5 ohm-cm Cupric Oxide 6.5 .times. .sup.5 ohm-cm Cupric Oxide fired with 10 % by weight Aluminum formate 8.8 .times.0 .sup.8 ohm-cm Cupric oxide fried with 10 % by weight copper 4.8 .times. 10.sup.7 ohm-cm Cuprous oxide 7.2 .times. 10.sup.11 ohm-cm Ferric Oxide 3.6 .times. 10.sup.9 ohm-cm Lead Dioxide 8 .times. 10.sup.3 ohm-cm Manganese Dioxide 1 .times. 10.sup.5 ohm-cm Al.sub.2 O.sub.3 2.5 .times. 10.sup.11 ohm-cm Al.sub.2 O.sub.3 with 10 % by weight Aluminum formate 3.8 .times. 10.sup.8 ohm-cm Mercuric Oxide 9 .times. 10.sup.7 ohm-cm __________________________________________________________________________

The following comments are to be made about Table I.

1. All samples are powder samples with varying powder size which may account for some of the unusual resistivities observed, e.g., .rho.CuO<.rho.Cu.sub.20. The powders used are U. S. P. grade J. T. Baker Chemicals unless otherwise noted. 2. The technique used for the resistivity measurement is essentially that outlined by the American Society for Testing and Materials (A.S.T.M.) for determining the electrical resistance of insulating materials. 3. Material modifications made by firing oxides with metal formates were particularly helpful in the zinc oxide-aluminum formate system. 4. A wide variety of metal oxides are observed to display variable resistance behavior. The differences observed were mainly found in the formation of the variable resistance state and the ease with which the highest variable resistance state could be recalled. If a best characteristic resistivity could be extracted from Table I, one would have to choose approximately 10.sub.9 -10.sup.10 ohm-cm.

The type of binders used and the percent (by weight) of the active powders in the binder successfully used thus far are given in Table II. These results are for the ZnO:Zn system and a fixed electrode material.

TABLE II

Binder Material (% wt.) Loading of ZnO:ZN (% wt.) __________________________________________________________________________ 60% Seezak* Epoxy SR 100+SC 301 40% ZnO:Zn 80% Plio Bond* cement 20% ZnO:Zn 70% Seezak SA 593 Adhesive 30% ZnO:Zn 98% Ciba* Araldite Epoxy 2% ZnO:Zn 95% Ciba Araldite Epoxy 5% ZnO:Zn 90% Ciba Araldite Epoxy 10% ZnO:Zn 60% Ciba Araldite Epoxy 40% ZnO:Zn 50% Ciba Araldite Epoxy 50% ZnO:Zn 60% Ciba Araldite Epoxy 40% ZnO 50% Ciba Araldite Epoxy 50% ZnO:+50% ZnO:Zn __________________________________________________________________________

The percentages given above are meant only to be indicative of the successful range of loading densities and are not meant to limit this disclosure.

Some of the electrical properties of the binders used are presented in Table III. ##SPC1##

It is to be expected from the above results of Table III that an even wider variety of binder materials (rubber based cements, epoxys, and plastics) might be used. The primary criteria being high breakdown strength and high resistivity. In choosing an appropriate binder, and percent mixture of active powder, such parameters as pot life of binder, consistency of mix (very thick or heavily loaded mixtures are more difficult to spread), mechanical stability, and the thermal and moisture resistance properties of the composite sample must also be given consideration. The powder-binder mix may be prepared in any way that will provide a reasonably uniform mixture.

Although probably desirable, extreme uniformity of mix may not be necessary since wide variation in loading densities are acceptable, noting Table II. If the mixture is thick the spreading of the film with a doctor's blade, spatula or similar spreading device provides adequate films. If the mixture is thin (or deliberately thinned with a binder solvent) the switching layer may be painted, sprayed or precipitated on to a base electrode. The counter electrode may then be placed atop the wet mixture or painted or sprayed on where the active portion of the device has been allowed to cure. The techniques described have obvious advantage for making large area devices or matrices of devices at room temperature without the need for special environmental chambers.

Though no completely verified theory of operation of the memory element has been found, empirical observations provide some possible explanation of the behavior of the memory element. The initial rapid switching to the low resistance state is thought to correspond to the formation of a permanent filamental conduction path by thermal or electronic processes resulting from high local diversities in the sample during an electrical breakdown process. This conduction path may be formed from a local reduction of the metal oxide to a metallic filament, the transport of electrode material through a gap in the bulk material (the hole or gap priginating during fabrication of the device or by catastrophic electronic breakdown of the device material) or by a combination of the two aforementioned processes.

While this operational description has not been definitely established it is consistent with the observation of a zero bias memory. In addition, such a filamental conduction mechanism is consistent with the means by which we can recall the high resistance state. A large current density perhaps explains the complete or partial rupture of fine conducting filaments. Finally, microscopic examination reveals the presence of local regions of structural change in the elements which have been switched to a low resistance state. It is believed that breakdown and filament formation is initiated by any means by which a large current increase can occur through an initially high resistance material such as by thermal, electronic or optical excitation of carriers from the intrinsic bulk material, traps therein or adjacent electrodes.

The operation of the devices of FIGS. 1b and 1c is identical. If the impedance of the composite switch Z.sub.ELS is such that the composite structure impedance, is greater than Z.sub.f, Z.sub.F, (i.e. Z.sub.ELS>> Z.sub.f), the voltage V from the low impedance source will be distributed primarily across the composite structure and as the voltage V is increased the electroluminescent element will luminesce. If, however, the voltage across the composite structure V.sub.ELS is greater than the threshold voltage V.sub.th, the bistable switching material will go to a low resistance state and Z.sub.ELS .ltoreq.Z.sub.f. In this condition the voltage across the composite structure is much lower than when V.ltoreq.V.sub.th and the electroluminescent material, (EL) will register an "off" or much more weakly luminescing state. The restrictions on Z.sub.EL and Z.sub.f are that Z.sub.ELS >>Z.sub.f in the blocking state and Z.sub.ELS .ltoreq.Z.sub.f in the conducting state. The high impedance state of the device may be recalled by pulsing the switching material with an AC or DC voltage or by subjecting the switching material to a source of radiation such as a tesla coil.

Experience has shown that for the voltages employed in activating the electroluminescent material EL a series impedance, Z.sub.f .apprxeq.4 K ohm is sufficient to limit the current through the switch to a level which will prevent the sample from reverting to its blocking state after switching. Thus, we need only restrict the composite structure impedance to Z.sub.ELS >>4 K ohm in the blocking state and Z.sub.ELS .ltoreq.4 K ohm in the conducting state. It is understood that the value of the control impedance Z.sub.f given above is only meant to be indicative and by no means restrictive since the optimum value will be dictated ultimately by the details of the device construction.

The above operation is to be contrasted with the composite switch and electroluminescent structure in a series configuration as shown in FIG. 2. Part 14 is a transparent electrode and part 12 is the electroluminescent material. A conducting film 19 is placed between the switching material 13 and the electroluminescent material. Electrode 17 contacts the other side of the switching material and is connected through control impedance 11 to ground. Transparent electrode 14 is connected to potential source 10 by wire 16 thus, completing the series circuit of the switching and electroluminescent elements. It should be noted that conductive film 19 is added to provide a greater area of conductive material between the switching and electroluminescent elements. However, satisfactory performance of the memory device is not limited to its utilization.

In operation of FIG. 2 the electroluminescent element is held in an "off" condition by the switching element which has a blocking impedance Z.sub.S greater than the electroluminescent impedance Z.sub.EL . In other words Z.sub.S >>Z.sub.EL. As a result, the voltage across the electroluminescent element is less than the voltage across the switching element or V.sub.EL <<V.sub.S as long as V.sub.S >V.sub.th. When V.sub.S >V.sub.th the switching element reverts to a low resistance state and the electroluminescent element luminesces.

Where the switching material is of the variable resistance type the brightness of the luminescence may be varied by altering the resistance of the switching material. The Z.sub.S >>Z.sub.EL requirement for the series configuration in the "off" condition calls for a blocking state Z.sub.S .apprxeq.10.sup.9 ohms, a much more stringent requirement than the Z.sub.ELS >>Z.sub.F required in the parallel configuration.

Among the advantages of the parallel configuration is the fact that the controlling elements are permitted to have a much lower value of impedance in the blocking state than would be permissible with the series configuration. This allows a much broader range of switching materials to be used. In fact even switches with Z.sub.S in the blocking state .ltoreq.Z.sub.EL will be acceptable.

Referring now to FIG. 3, a heterogeneous mixture of switching material with an electroluminescent powder in an epoxy binder is shown. The parts are arranged essentially the same as shown in FIG. 1b. This configuration is especially advantageous in that the entire active elements of switching material plus electroluminescent material may be fabricated in a single spraying procedure. Moreover, this configuration provides the desirable features of yielding positive or negative images depending on the sense of the coincident pulses. That is, the background areas may be made brighter than the image areas or vice-versa.

FIG. 4 is a plan view of an array of a plurality composite electroluminescent structures one of which is designated a. One electrode of each composite structure is connected to an X terminal and the other electrode is connected to a Y terminal. The whole array of composite structures may be mounted on a support 42 which may be made of a nonconductive material. For purposes of explanation we may assume that the electrodes connected to the Y terminals are the transparent electrodes so that the panel may be viewed from this side. It is evident however, that the position of the transparent electrodes could be placed in the reverse manner. Moreover, certain applications of the invention may utilize transparent electrodes on both sides of the panel providing positive and negative visual data. Depending upon circuit design requirements the composite switching and electroluminescent device may be of the series or parallel type described above. Since the composite structures may be fabricated in small sizes resolution can be easily controlled.

Switch S.sub.1 connects a source of AC potential 40 to the X terminals while switch S.sub.2 connects the Y terminals to ground through a control impedance 41. Although switches s.sub.1 and s.sub.2 are shown as mechanical devices, the invention is not intended to be limited thereto. It will occur to those skilled in the art that electronic devices such as vacuum tubes or transistors could be substituted in lieu thereof. Moreover, in computer or communications applications, logic circuits may be used to address the panel in order to process numerous types of input data. It is therefore within the scope of the invention to employ electronic switching and logic processing circuits where it is desired.

In operation of FIG. 4 it shall be assumed that the crosspoint x.sub.2, y.sub.2 is to be addressed and that the composite structures are a series combination of the bistable switching and electroluminescent elements. When switches S.sub.1 closes at terminal X.sub.2 and switches S.sub.2 closes at terminal Y.sub.2, the composite structure at this crosspoint will become actuated provided the applied voltage is above the threshold value of the switching material. Under these conditions the electroluminescent material will luminesce because the bistable switching material is now in its conducting state permitting current flow between the terminals. Now when switch S.sub.1 moves off terminal X.sub.2 the composite structure ceases luminescing. However, the bistable switching material presently in its conducting state remembers this condition. In order to return the bistable switching material to its former blocking state a large current may be sent through it or it may be subjected to radiation from an available radiation source. Sensing means may also be provided to ascertain the conductive state of any composite structure. Thus, a computer is relieved of the need for large storage equipment where the panel is used in conjunction therewith.

It is understood that FIG. 4 represents only a segment of panel array. In an actual array the composite structures and terminals would be far more numerous giving access to more panel coordinates. In an actual display panel numerous terminals could be addressed or scanned sequentially or simultaneously so as to build up visual data on the panel. The voltage to individual address terminals may also be modulated to control the brightness of the panel and to furnish degrees of contrast of visual data by varying the resistance state of the variable switching material.

From the foregoing, a panel display having memory capability has been disclosed.

Claims

What is claimed is:

1. A matrix addressable electroluminescent display device comprising:

a plurality of composite structures arranged in an array, each of said structures comprising a region of zero bias memory material adapted to block current below a threshold voltage and to pass current above said threshold voltage, said zero bias memory material retaining its last state under zero bias condition,

each of said composite structures also comprising a region of electroluminescent material and a pair of electrodes between which said regions of zero bias memory material and electroluminescent material are disposed at least one of said pair of electrodes being transparent, and

means for selectively applying an electrical field across individual pairs of electrodes whereby visual data is formed on said display device.

2. The apparatus of claim 1 wherein said zero bias memory material has variable resistance states for varying the state of luminescence of said electroluminescent material.

3. The apparatus of claim 1 including a capacitive control impedance in series with said composite structures.

4. The apparatus of claim 1 including pairs of transparent electrodes whereby said display device may be viewed from both sides.

5. Electroluminescent apparatus comprising:

a pair of electrodes, at least one of which is light transparent,

electroluminescent material interposed between said pair of electrodes, and

zero bias memory material having a low and a high conductive state and being interposed between said pair of electrodes, said memory material switching from one of said two conductive states to the other of said two conductive states when a potential applied thereacross exceeds a threshold potential level and remaining in said one of the two conductive states, even after the potential applied thereacross is reduced to zero until said the other of said two conductive states is driven back externally to said one of said two conductive states wherein said memory material activates and deactivates said electroluminescent material to luminesce selectively as its conductive state changes.

6. The apparatus of claim 5 wherein said electroluminescent material is positioned in series with said zero bias memory material between said electrodes.

7. The apparatus of claim 5 wherein said electroluminescent material is positioned in parallel with said zero bias memory material between said electrodes.

8. The apparatus of claim 5 wherein said zero bias memory material is adapted to revert to its low conducting state when subject to a source of radio frequency radiation or high amplitude pulses.

9. The apparatus of claim 5 comprising:

zero bias memory material adapted to have variable resistance states between said low and said high conducting states for varying the state of luminescence of said electroluminescent material.

10. The apparatus of claim 5 wherein said zero bias memory material and said electroluminescent material are combined in a heterogeneous mixture in an epoxy binder material whereby the background and image areas of said apparatus may be made to alternately brighten and darken with change in the voltage applied to said electrodes.

11. The apparatus of claim 5 wherein said zero bias memory material comprises an amorphous or polycrystalline film of semiconductor material selected from the group of metal oxides consisting of zinc, copper, lead, manganese, mercury and aluminum.

12. The apparatus of claim 5 wherein said zero bias memory material comprises a semiconductor material of reduced metal oxides.

13. The apparatus of claim 5 comprising a control impedance in series with said electrodes.

14. The apparatus of claim 13 wherein said control impedance comprises a layer of resistive material overlaying one of said electrodes and in electrical series with said electrode.

15. The apparatus of claim 13 wherein said control impedance comprises a capacitor in series with said electrodes.