US 3590252 A
Description (OCR text may contain errors)
United States Patent LIGHT-SENSITIVE SWITCHING DISPLAY DEVICE 5 Claims, 7 Drawing Figs.
Int. Cl 11011 17/00,
Field of Search 250/213,
 References Cited UNITED STATES PATENTS 2,920,232 l/1960 Evans 250/213 2,989,641 6/1961 Nicoll 250/213 2,988,647 6/1961 Duinker et al.. 313/108 3,059,118 10/1962 Koury 250/213 3,304,430 2/1967 Biard et a1. 250/217 Primary ExaminerWalter Stolwein Attorneys- F. H. Henson and C. F. Renz ABSTRACT: An electroluminescent storage and readout device in which 'a light-responsive switch is utilized to control an electroluminescent material and in which the light-responsive switch is controlled by a light-generating information system. A high voltage storage/readout device is optically coupled to a low-voltage electroluminescent drive panel in a two-stage device.
, 2e 2e 24 2e LIGHT-SENSITIVE SWITCHING DISPLAY DEVICE BACKGROUND OF THE INVENTION This invention relates to electroluminescent display systems and in particular to electroluminescent storage/readout devices in which each cell is combined with a switch activated by light.
A problem is encountered in connecting a high-voltage storage/readout device electrically to an information system which is generally composed of transistors and microcircuitry. The basic problem is to connect a high-voltage, high-current electrical device to low-voltage, low-current electrical information system. This problem has been overcome in the prior art by electrically isolating but optically coupling the storage/readout device to a controlling light-pulsing device which is part of the information system. The light-pulsing device can be a low-voltage, low-output, electroluminescent device, which is compatible with microcircuitry. Such an electroluminescent pulsing device usually has a short light-hour life but since it is only pulsed and does not operate continuously for long periods of time a short light-hour life is not deleterious.
The prior art devices that overcome this problem are all three-stage devices, a light-pulsing stage, a storage stage and a readout stage. The present invention simplifies this device into basically a two-stage device. Previously the storage portion was an electroluminescent layer that emitted light to latch on a photoconductive switch. The present invention accomplishes storage by utilizing a photoconductive switch which is designed to operate as a bistable switch.
Hysteretic material such as cadmium selenide has a current voltage characteristic which causes its impedance to suddenly decrease when the voltage gradient applied across the material exceeds a certain value. The voltage must then be decreased considerably below this value for a minimum period of time before the material will resume its former high-impedance state. This hysteretic characteristic of cadmium selenide has been used for electroluminescent switching but always with voltage pulse switching.
SUMMARY OF THE INVENTION This invention relates to the use of the characteristic that cadmium selenide, when used as a hysteretic material and electrically biased at a voltage just less than that required to switch to a low-impedance state can be switched from a state of high impedance to a state of low impedance by application of a suitable light pulse. This is accomplished when a hysteretic photoconductor with an impedance characteristic such as cadmium selenide is biased with a sinusoidal voltage such that it has two stable conditions, a low-impedance state or on" position and a high-impedance state or off position. The physical embodiment of the invention hereinafter described in a device comprising two panels, an electroluminescent panel combined with cadmium selenide as a hysteretic photoconductive switching element triggered by light pulses generated by a low-voltage electroluminescent drive panel. The latter is x-Y addressed and is electrically isolated but optically coupled to the electroluminescent hysteretic storage/readout panel.
BRIEF DESCRIPTION OF THE DRAWINGS The advantages and applications will become obvious from the following description and the drawings in which:
FIG. I is a graph of the voltage current characteristic of a hysteretic photoconductive material such as cadmium selenide when switched by light pulsing.
FIG. 2 represents a sectional view of one embodiment of the invention, a two-panel construction, hysteretic optically switched electroluminescent device.
FIG. 3 represents a cutaway plan view of the device shown in the upper part of FIG. 2.
FIG. 4 represents a plan view of the illuminating layer shown in FIGS. 2 and 3.
FIG. 5 represents a sectional view of a second embodiment of the invention, a single-panel-constructed, hysteretic photoconductively switched electroluminescent device.
FIG. 6 represents a cutaway plan view of the device shown in FIG. 5.
FIG. 7 represents a plan view of the illuminating layer also shown in FIGS. 5 and 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, where the horizontal ordinate is voltage and the vertical ordinate is current on a logarithmic base,t he line from the origin 0 to point a represents the 0, state, while the line from f to b represents the low-impedance state. It requires a voltage pulse greater than V or a light pulse of an appropriate duration and intensity combined with a suitable biasing voltage V,,, where V is greater than V, and less than V, to switch from the high-impedance state to the low-impedance state. The hysteretic material will revert to its former state of highimpedance if the voltage drops to zero for a specified time. If the hysteretic material is normally biased between V, and V a suitable light pulse will switch the material to the low-impedance state and a suitable negative voltage pulse will switch the hysteretic material back to its former high-impedance state.
FIG. 2 represents a cross-sectional view of a hysteretic optically switched electroluminescent device 30 comprising two separate panels, storage/readout panel 20 and drive panel 10, sealed between transparent front cover 32 and opaque back cover 31. These panels are electrically insulated but optically coupled by a light-transmitting medium comprising a transparent substrate 11, which does not diffuse light readily, thereby minimizing crosstalk.
The drive panel 10 is a known, prior art, x-Y-addressed electroluminescent panel. It comprises a transparent substrate 11; electroluminescent layer 12, linear X-electrodes 15 and transparent linear Y-electrodes l6 mutually at right angles. A voltage-pulsing source 35 supplies voltage pulses to drive panel 10 by selectively energizing appropriate X- and Y-electrodes to excite the chosen element at the crossing of the X- and Y-electrodes.
The storage/readout panel 20 comprises; a main substrate 21; an array of rear terminal pads 22, readout terminal pads 23 and through hole conductors 24; hole filler 25; general terminal 26; hysteretic material 27; electroluminescent readout layer 28; and transparent electrode 29. Electrically coupled to the storage/readout panel 20 is the voltage source 33 which applies a sinusoidal bias potential across the series combination of the hysteretic material and electroluminescent layer and can further provide negative voltage pulses when required to erase the information stored in the memory of the storage/readout panel 20.
FIG. 3 shows a layer-by-layer cutaway plan view of the device shown in FIG. 2. The outermost layer at the upper left is the transparent front cover 32 on the readout side of the storage/readout panel. Directly below that is the transparent electrode layer 29 then comes the electroluminescent readout layer 28. The next three sections represent the component parts of the electrically floating electrodes each consisting of a rear terminal pad 22, and a readout terminal pad 23 joined by a through hole conductor 24. Also shown in the three sections are, main substrate 21, common terminal 26, hysteretic material 27 and inert hole filler 25.
FIG. 4 is a plan view of the readout terminal pads 23 as they would appear when illuminated. Other parts designated in FIG. 4 are, through hole conductors 24, hole filler 25, and substrate 21.
FIG. 5, a preferred embodiment of the invention, is an electroluminescent device 60 constructed around a single main substrate 61. The drive panel and storage/readout panel are functionally but not physically seperable. Electroluminescent device 60 comprises: electroluminescent trigger layer 12; X-Y-electrode grid consisting of X-electrodes l and transparent Y-electrodes 16; floating electrodes each consisting of a solid through hole conductor 67 and a readout terminal pad 23; common terminal 65; hysteretic material 27 readout electroluminescent layer 28; transparent electrode 29; opaque rear cover 31; transparent front cover 32; main substrate 61 and opaque insulating layer 66. Also shown in FIG. 5 is voltage-pulsing source 35 and voltage source 33. Voltage-pulsing source 35 is functionally connected across the drive electroluminescent panel which is formed in strata on the first surface of the main substrate 61. Voltage source 33 is functionally connected across the storage/readout panel which is formed in strata on the second surface of the main substrate 61. The storage/readout panel and the drive panel are optically coupled through and electrically isolated by main substrate 61.
FIG. 6 is a cutaway sectional plan view of the storage/readout portion of the device 60. Starting from the upper left-hand corner are the following: transparent front cover 32; transparent front electrode 29; readout electroluminescent layer 28; readout terminal pads 23; opaque, low dielectric constant, epoxy lever 66, through hole conductors 67; hysteretic material 27; common terminal 65. and main substrate 61.
FIG. 7, similar to FIG. 4, is a plan view of the illuminated readout terminal pads 23, separated by opaque epoxy 66. Hidden, below the terminal pads 23, in dotted lines is the through hole conductors 67.
Voltage source 33 is electrically connected between the front transparent electrode 29 and the common terminal, numbered 26in device 30 and numbered 65 in device 60.
The hysteretic material 27, see FIGS. 2, 3, 5 and 6; in the electroluminescent device is isolated from all exterior ambient light by opaque back cover 31, the opaque floating electrodes 23 and common terminal 26, shown in FIGS. 3 and 4 for device 30. In device 60 the hysteretic material is similarly isolated from exterior light by the opaque rear cover 31, the opaque floating electrodes 23, and the opaque layer 66.
Information to be displayed is sequentially read into addressed positions in the X-Y grid with voltage pulses. These voltage pulses are transformed into light pulses by the drive electroluminescent layer 12. The light pulses from the drive electroluminescent layer impinge upon a particular section of the continuous layer of electrically biased hysteretic material which shall be called a segment of the hysteretic material. The segment switches from its normal state of high impedance to a state of low impedance. This change of impedance redistributes the voltage between the segment of the hysteretic material and a segment of the storage/readout electroluminescent layer such that the segment of the electroluminescent layer luminesces. If the biasing voltage on the hysteretic layer is maintained the display on the storage/readout device will remain luminescent even though the light pulse from the drive device has ended. To erase the information displayed, a suitable negative voltage pulse is provided by voltage source 33 through the storage/readout device and switches all segments of the hysteretic material back to their normal state of high impedance, redistributing the voltage between the segments of hysteretic material and the segments of the storage/readout electroluminescent layer such that no segments of the storage/readout electroluminescent layer will continue to luminesce. Information will normally be changed by erasing all the present information and reinserting the updated information by sequential voltage pulsing of the drive device.
In determining the sinusoidal biasing voltage for the hysteretic material from the hysteretic curve of FIG. I the voltage division between the hysteretic material and the storage/readout electroluminescent layer in both the low-impedance and high-impedance state must be considered. The impedances of the electroluminescent and hysteretic elements must be such that in high-impedance state the voltage across the hysteretic material is less than V,, (FIG. I). In low-impedance state the voltage division must be such that the voltage across a hysteretic material is maintained greater than V (FIG. 1). Thus the impedance of the hysteretic material is matched with that of the electroluminescent layer so that they hysteretic material will act as a bistable switch.
In addition to this criterion the impedance must be matched such that the device will not fail due to power overload after a limited period of use. This may occur when the hysteretic material dissipates an excessive amount of power thus causing the materials characteristic to deteriorate quickly. Prior to manufacturing, a single switching element of the hysteretic material should be tested in conjunction with a single element of the electroluminescent readout layer in order to establish a reasonable life. This is accomplished by varying the impedance of the electroluminescent readout segment and the impedance of the hysteretic switching material within the previously established limits and testing to obtain maximum life. Theimpedance of the hysteretic switching element can be varied by varying the bulk parameters in the hysteretic switch itself. Included in these bulk parameters are the physical and geometric parameters associated therewith, these parameters determine the critical power dissipation per switch and thereby the life of the device.
The device 60 (FIG. 5) is manufactured by the successive deposition of layers onto the two major surfaces of the transparent main substrate 61. On the second surface the following layers are deposited in their respective order: the common terminal layer 65; the continuous layer-of hysteretic material 27; the low dielectric constant, insulative, opaque layer 66 (a black epoxy) leaving predetermined separate circular portions of the hysteretic material exposed, to be covered with aluminum deposits which form the solid through hold connections 67; the aluminum readout electrode pattern comprising many readout terminal pads 23 (FIG. 7). Onto the combined aluminum and opaque layer, the continuous electroluminescent layer 28, followed by an optically transparent gold electrode layer 29 forming the transparent electrode and also an electrical connection to the high-voltage supply 33.
The following layers are deposited onto the first surface of the main substrate 61 in this order: the transparent Y-electrode grid 16, the continuous drive electroluminescent layer 12, and the X-electrode grid 15. Lastly the complete device 60 is sealed between a front and back cover with the necessary electrical connections brought through or between the sealing cover members.
For a more detailed description of the materials used and the methods of manufacturing similar devices, refer to copending US. Pat. application Ser. No. 671,273, filed Sept. 28, I961, entitled Electroluminescent Device by R. E. Lake et al. and assigned to the assignee of the present invention.
In the description of the drawings, the word panel has been used since it most aptly describes the geometry of the particular embodiment shown in the drawings, but the invention is broader and is not subject to such geometric limitations. Thus an example of a simple embodiment of this invention is a single cell, off/on, display or indicator utilizing hysteretic material as a bistable switch activated by light pulses, such as one segment or unit of the display as described or illustrated in the drawings.
, I claim as my invention:
1. An electroluminescent device comprising: a storage/readout section and a light-pulsing trigger section, said trigger section comprising an x-Y-addressed electroluminescent panel and electrically isolated from but optically coupled to said storage/readout section, said storage/readout section including an electroluminescent layer controlled by a light responsive switch, voltage means to bias said switch such that at least a portion of said readout/storage section has two stable conditions, an on" condition and off condition, said switch activating said portion of said readout/storage section to the on" condition upon the occurrence of a light pulse from said trigger section impinging on said switch and deactivating said readout/storage section to the off" condition by reducing the bias from said voltage means and an opaque layer to prevent light other than said light pulses from actuating said switch. v
2. An electroluminescent device comprising two sections and two covers; a storage/readout section and a light-pulsing trigger section; said storage/readout section including the following: a hysteretic material layer, opaque floating electrodes, an electroluminescent layer, a transparent front electrode, a main substrate, an opaque common terminal, said electroluminescent layer and said hysteretic material layer being electrically connected in series by said floating electrodes, said floating electrodes and said common terminal overlapping one another in parallel planes so that no light from said electroluminescent layers impinges upon the hysteretic material, said trigger section comprising an xY-addressed electroluminescent panel optically coupled to but electrically isolated from said hysteretic material, said first cover being a transparent light-pulsing and covering said storage/readout section, said second cover being an opaque insulator and covering the rear of said trigger panel.
3. An electroluminescent device as claimed in claim 2 including, voltage means to bias said hysteretic material such that at least an element of said electroluminescent layer has two stable conditions, an on condition and an off c0ndition, said hysteretic material layer being actuated by light pulses from said trigger section and in turn activating said element to said on condition.
4. An electroluminescent device comprising two panels stratified on a main substrate, said substrate having a first and second major surface, said substrate being an electrical insulator and a light-transmitting medium; a first panel being a trigger panel including an X-Y-addressed electroluminescent layer on the second surface of said main substrate, a second panel, being a storage/readout panel, provided on said first surface; said second panel comprising the following layers:
a conductive common terminal layer, a hysteretic material layer, an opaque layer, a floating electrode layer consisting of many separate floating electrodes, an electroluminescent layer electrically connected in series with said hysteretic material, a transparent electrode layer, said device sealed by a front transparent cover over said second panel and opaque rear cover over said first panel, said opaque layer in conjunction with said floating electrode layer optically isolating said hysteretic material from said electroluminescent layer and from ambient light entering through said front transparent cover, means to electrically bias the series combination of said electroluminescent layer and said hysteretic material such that said combination has two stable conditions and is activated from a first condition to a second condition by light pulses from said trigger panel. 5. An electroluminescent device as claimed in claim 4 wherein said opaque layer is an insulative layer having a low dielectric constant.