US2932770A - Electroluminescent device - Google Patents

Description

April 1960 D. c. LIVINGSTON 2,932,770

ELECTROLUMINESCENT DEVICE Filed April 29, 1958 INVENTOR DONALD C. LIVINGSTON ATTORNEY Patented Apr. 12, 1960 ELECTROLUMINESCENT DEVICE Donald C. Livingston, Bayside, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, DeL, a corporation of Delaware Application April 29, 1958, Serial No. 731,784

4 Claims. (Cl. 315-451) My invention is directed toward electroluminescent devices.

Certain types of phosphors, when under the influence of an externally applied electric field, will luminesce, the intensity of the emitted light being some function of the intensity of this field. Consequently, films or layers formed from such phosphors can be used as transducers for converting electrical energy into light energy. These layers, for example, can be formed by dispersing phosphor particles in dielectric media, or can be formed from one or more phosphor crystals without a dielectric.

One type of device utilizing such layers is known to the art as a bistable electroluminescent cell. This type of cell is a sandwich type structure employing separate photoconductive and electroluminescent layers electrically connected in series and so arranged that some of the light emitted from the electroluminescent layer, upon excitation of the latter impinges on the photoconductive layer. The electrical characteristics of the two layers are chosen such that the photoconductive impedance in the dark is high relative to the electroluminescent impedance. Further, when the photoconductive layer is illuminated, the photoconductive impedance is much lower than that of the electroluminescent layer.

As a consequence of these impedance characteristics, when a voltage is applied between the two layers and the photoconductive layer is in the dark, the electroluminescent layer remains quiescent and emits substantially no light. The cell is then in its first, or unexcited, electric state.

However, when a voltage is applied between the two layers and the photoconductive layer is stimulated by a light signal, the impedance of the photoconductive layer decreases sharply, and a larger portion of the applied voltage appears across the electroluminescent layer and causes it to luminesce. This luminescence produces additional illumination of the photoconductive layer, thus further decreasing the photoconductive impedance. As a result, the photoconductive impedance decreases to a minimum, and the electroluminescent layer luminesces brightly. The cell is then in its second, or fully excited, state. The cell will remain in its second state even after the light signal is extinguished since the photoconductive layer remains illuminated by light from the electroluminescent layer. conventionally, the cell is returned from its second state to its first state by removing the applied voltage therefrom.

In accordance with the principles of my invention, I provide a matrix of interconnected bistable cells of the type indicated, wherein each cell can be separately placed in either one of its two states and wherein any selected cell can be returned from its second state to its first state without the necessity of removing the applied voltage trom the selected cell.

More particularly, I employ a cross-grid structure provided with a first array of parallel, separated, electrical conductors extending in a first direction and a second array of parallel, separated, electrical conductors extending in a second and non-parallel direction. The conductors in at least one of the two arrays are light-transparent. Each conductor of one array passes over all conductors of the second array. The point at which any one conductor in the first array passes over any one conductor in the second array is termed a cross-over point.

A separate bistable electroluminescent cell is positioned at each cross-over point and is interposed and electrically connected between the appropriate first and second array conductors. Each cell comprises a photoconductive element and an electroluminescent element in series corinection. The photoconductive and electroluminescent elements of each cell are so oriented with respect to each other that light emitted from the electroluminescent element will impinge upon the photoconductive element.

When all first-array conductors are maintained at a first potential and all second-array conductors are maintained at a second and difierent potential, and the various cells are in the dark, each cell will be in its first state. When a light signal impinges upon a transparent conductor at any selected cross-over point, the cell positioned at this point will be placed in its second state, and will remain in this state when the light signal is extinguished.

However, a bistable cell will possess a stable second state only when the voltage drop across it equals or exceeds a predetermined minimum value. More particularly, for the cell to continue to produce light in the absence of incident light radiation, the impedance of its photoconductive element must be maintained at a low value. Hence, its electroluminescent element must emit a minimum amount of light sufilcient to maintain the impedance of the photoconductive element at the desired low value. The minimum value of voltage which, when applied across the cell, will cause the electroluminescent element to emit this minimum value is defined as the minimum holding voltage. This minimum value will have different values dependent upon the cell geometry, but for a given geometry, the minimum value is fixed and constant. Applied voltages falling below this minimum value will be insuflicient to maintain the cell in its Second state; the cell will then return to its first state.

When all first-array conductors are maintained at a first potential and all second-array conductors are maintained at a second and difierent potential, the potential difference between the two potentials being at least equal to the minimum holding voltage, one or more cells can be placed in the second state by directing a light beam successively upon the transparent conductors along the appropriate cross-over points. I further provide means coupled between the conductors of any selected cell to reduce the potential difference therebetween to a value less than the minimum holding voltage. When this selected cell is in the second state, and the potential difterence thereacross is so reduced, the selected cell will return to its first state, the state of the other cells remaining unchanged.

Illustrative embodiments of my invention will now be described with reference to the acompanying drawings wherein:

Fig. 1 illustrates one embodiment of my invention;

Fig. la is an enlarged fragmentary view in cross section of the matrix of bistable cells shown in Fig. 1; and

Fig. 2 illustrates a second embodiment of my invention.

Referring now to Figs. 1 and 1a, there is shown a matrix of bistable cells having a first array of separate, parallel, transparent, electrical conductors extending in a first direction, in this example, vertical conductors 20, 22, 24, 26, and 28. Further, the matrix includes a second array of parallel, separate, transparent, electrical con-. ductors extending in a second and different direction, in

this example, horizontal conductors 30, 32, 34, 36, and 38. A selected horizontal conductor 32 is coupled to the arm of potentiometer 42, one end of the potentiometer being grounded, the other end being coupled to terminal 46. All other horizontal conductors, namely 30, 34, 36, and 33, are connected directly to terminal 46. Similarly, a selected vertical conductor 24 is coupled to the arm of potentiometer 44, one end of potentiometer 44 being grounded, the other end being connected to terminal 48. All unselected vertical conductors 2t 22, 26, and 28 are connected directly to terminal 48.

As will be apparent, each vertical conductor crosses over all horizontal conductors to form a plurality of cross-over points thereat. interposed at each cross-over point and electrically connected between the vertical and horizontal conductor defining this point is a circuit component 190. 7 Component 1&0 includes in series 'connection a separate photoconductive element 102 and an elec troluminescent element 104, these two elements having relative positions at which light emitted from the electroluminescent element 164 impinges upon the photoconductive element 102. The electroluminescent element can be a separate element or alternatively, can be part of a continuous electroluminescent layer.

A first alternating voltage having an amplitude V is applied between terminal 46 and grounded terminal 50. A second alternating voltage opposed in phase with respect .to the first voltage but otherwise identical thereto is applied between terminal 48 and terminal 50. (The phase opposition of these voltages results in opposite instantaneous polarities indicated in Figs. 1 and 2.) Consequently, the voltage difference between any horizontal conductor other than conductor 32and any vertical conductor other than conductor 24 is equal to 2V.

By virtue of the settings of potentiometers 42 and 44,

an alternating voltage having the same phase as the first applied voltage, but having a different and lower amplitude'V is applied between horizontal conductor 32 and ground. Similarly, an alternating voltage having the same phase as the second applied voltage but having a second and lower amplitudeV' is applied between vertical .couductor 24 and ground.

' Hence, a voltage drop' of 2V appears across the component 1% located at the cross-over point 40 between horizontal conductor 42 and vertical conductor 24; a voltage drop of V-l- V appears across all components positioned at the points at which vertical conductor 24 crosses over horizontal conductors 30, 34, 36, 38 as well as the components positioned at the points at which vertical conductors 20, 22, 26, and 28 crossover horizontal conductor 32; and a voltage drop of 2V appears across all components positioned at the points at which horizontal conductors 30,134, 36, and 3% cross over vertical conductors 2t), '22, 26, and 28. The voltage values are so selected that the'voltage drop of 2V has a value below that of the minimum holding voltage, while the voltage drops V+ V or 2V have values at least equal to the minimum holding voltage.

Consequently, when the cross-over points are successively scanned by a light beam directed along the vertical transparent conductors, the component located at each cross-over point will be triggered from its first or unexcited to its second or excited state in the manner previously described. Thereafter, the component at any cross-over point other than cross-over point 40 will remain in its second state in the manner previously indicated, since the voltage applied across this component will be at least equal to the minimum holding voltage. However, when the cross-over point'40 is no longer irradiated by a light beam, its circuit component will return to its first state since the voltage applied thereacross is less than the minimum holding voltage. 7

It is often necessary to produce a pattern of excited cells on the matrix as, for example, by scanning with a light beam and subsequently extinguish some of these excited cells after the scanning operation is completed.

This action can beaccomplished in the manner shown in Fig.2.

In Fig. 2, all horizontal conductors are connected 2 through corresponding isolation resistors 60, both di- U rectly to terminal 76 and, through potentiometer 72, to ground. Similarly, all vertical conductors are connected throughthe corresponding isolation resistors 58 both directly to terminal 78 and, through potentiometer 74, to

ground. The'junction between any horizontal conductor and its resistor 60 is connected through a separate fixed" contact of switch 82. Similarly, the junction between each vertical conductor and its resistor 58 is coupled to a separate fixed contact of switch 84. The arm 66 of switch 82 is coupled through one side of switch 80 to the arm of potentiometer'72. The arm 62 of switch 84 is coupled through a second arm of switch 80 to potentiometer 74. With switch 70 closed, a first alternating voltage having an amplitude of V isapplied between terminal 76 and ground, while a second alternating voltage opposed in phase but otherwise identical to the first alternating voltage is applied between terminal 78 and ground.

If the, settings of potentiometer- s 72 and 74 are adjusted in the same manner as in Fig. l, the component at any selected cross-over point can be maintained at a voltage of 2V less than the minimum holding voltage, while all other components are maintained at a voltage at least equal to the minimum holding voltage. In this example,

the component located at cross-over point 57 defined by.

the intersection of vertical conductor 57 and horizontal conductor 54 is maintained at this voltage of 2V. However, by appropriate settings of switches 82and184, any selected cell can be extinguished. More particularly, with switch 70 open, switches 82 and 84 are set to extinguish the selected cell; this cell is then returned to its first stat by momentarily closing switch 7 0.

While I have shown and pointed out my invention as applied above, it will beapparent to those skilled in the art that many modifications can be made within the scope and sphere of my invention.

What is claimed is:

1. An electroluminescent device comprising, a first array of parallel, separate, electrical conductors extending in a first direction; a second array of parallel, separate, electrical conductors extending in a second and different direction, the conductors in at leastone of said arrays being transparent, each first'array conductor passing over each second array conductor to form a cross-% over point thereat; and a plurality of separate circuit components, each component being positioned at a corresponding cross-over point and being interposed and electrically connected between the two conductors defining said corresponding point, each component including a photoconductive element and an electroluminescent ele.

ment inseries connection, these elements having respective positions at which light emitted from the electroluminescent element will impinge upon the photoconducrate, electrical conductors extending in a second and different direction, the conductorsin at least one of said arrays being transparent, each first array conductor passing over each second array conductor to form a crossover point thereat; and a plurality of separate circuit components, each component being positioned at a corresponding cross-over point and being interposed and elec-' trically connected between the two, conductors defining said corresponding point, each component including a photoconductive element and an electroluminescent element in series connection, these elements having respec tive positions at which light emitted from the electroluminescent element Will impinge upon the photoconductive element, one selected first array conductor being maintained at a first potential, all unselected first array conductors being maintained at a second potential, one selected second array conductor being maintained at a third potential, all unselected second array conductors being maintained at a fourth potential, the difi'erence between said first and third potentials being less than the minimum holding voltage, the difference between said second and fourth potentials being at least equal to said minimum holding voltage.

3. An electroluminescent device comprising, a first array of parallel, separate, electrical conductors extending in a first direction; a second array of parallel, separate, electrical conductors extending in a second and difierent direction, the conductors in at least one of said arrays being transparent, each first array conductor passing over each second array conductor to form a crossover point thereat; and a plurality of separate circuit components, each component being positioned at a corresponding cross-over point and being interposed and electrically connected between the two conductors defining said corresponding point, each component including a photoconductive element and an electroluminescent element in series connection, these elements having respective positions at which light emitted from the electroluminescent element will impinge upon the photoconductive element, and first and second otentiometers, each potentiometer having first and second fixed contacts and a movable contact, the first fixed contacts of both potentiometers being interconnected, the movable contact of said first potentiometer being coupled to a selected conductor in said first array, the movable contact of said second potentiometer being coupled to a selected conductor in said second array, all unselected first array conductors being coupled to the second fixed contact of said first potentiometer, all unselected second array conductors being coupled to the second fixed contact of said second potentiometer.

4. An electroluminescent device comprising, a first array of parallel, separate, electrical conductors extending in a first direction; a second array of parallel, separate, electrical conductors extending in a second and diiferent direction, the conductors in at least one of said arrays being transparent, each first array conductor passing over each second array conductor to form a cross-over point thereat; and a plurality of separate circuit components, each component being positioned at a corresponding cross-over point and being interposed and electrically connected between the two conductors defining said corresponding point, each component including a photoconductive element and an electroluminescent element in series connection, these elements having respective positions at which light emitted from the electroluminescent element will impinge upon the photoconductive element; first, second and third terminals; a first set of resistors, each of which is coupled between said first terminal and a corresponding first array conductor; a second set of resistors, each of which is coupled between said second terminal and a corresponding second array conductor; first and second potentiometers; each potentiometer having first and second fixed contacts and a movable contact, the first contacts of both potentiometers being coupled to said third terminal, the second contacts of said first and second otentiometers being coupled to said first and second terminals respectively, the movable contact of said first potentiometer being coupled to the junction of a selected first set resistor and its corresponding first array conductor, the movable contact of said second potentiometer being coupled to the junction of a selected second set resistor and its corresponding second array conductor.

References Cited in the file of this: patent UNITED STATES PATENTS 2,650,310 White Aug. 25, 1953 2,698,915 Piper Jan. 4, 1955 2,774,813 Livingston Dec. 18, 1956 OTHER REFERENCES An Improved High-Gain Panel Light Amplifier, E. Kazan, Proceedings of the IRE, October 1957, pages 1358-1364.

Transient Voltage Indicator and Informational Display Panel, A. Bramley and J. E. Rosenthal, Review of Scientific Instruments, vol. 24, No. 6, June 1953, pages 471 and 472.

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