US2930897A

US2930897A - Electroluminescent device

Info

Publication number: US2930897A
Application number: US708661A
Authority: US
Inventors: Donald C Livingston
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1958-01-13
Filing date: 1958-01-13
Publication date: 1960-03-29
Anticipated expiration: 1977-03-29

Description

Filed Jan. 13, 1958 IIIII ww w INVENTOR DONALD C. LIVINGSTON United States Patent ELECTROLUMINESCENT DEVICE Donald C. -Livingston, Bayside, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Application January 13, 1958, Serial No. 708,661

6 Claims. (Cl. 250213) My invention is directed toward electroluminescent image display 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 strength of this applied field. Consequently, films or layers formed from such phosphors can be used as transducers for transforming electrical energy to 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.

It is known that first and second mutually orthogonal, for example, horizontal and vertical, arrays of parallel, separated electrical conductors can be positioned on each side of such a film or layer to form a crossed-grid structure wherein a portion of the film defined as a cell is connected between one horizontal conductor and one vertical conductor. When a suitable electrical potential difference'is applied between any pair of horizontal and vertical conductors, the cell connected between this pair will luminesce.

The applied potentials can be switched or commutated in such manner as to successively energize each cell in turn, thus producing an effect analogous to cathode ray tube scanning. Under certain circumstances, however,

"electroluminescent panels of this type will exhibit a spurious efiect, which I call the cross effect. This eflect is produced in the following manner. When a positive potential, as for example +V, is applied to a selected horizontal conductor and a more negative potential, as for example '-V, is applied to a selected vertical con- ..ductor, all other conductors being held at an intermeand vertical'conductors are maintained at the same potential, the cells connected between these unselected conductors do not luminesce.

It is an object of the present invention to improve electroluminescent devices in such manner that the cross elfect is eliminated.

Y Another object is to provide a new and improved electroluminescent panel characterized by the absence of the cross elfect.

Still another object is to eliminate the cross effect in electroluminescent devices of the crossed-grid type by applying a signal to the particular cell to be energized and at the same time applying a small portion of said signal to all unselected cells.

These and other objects of my invention will either be explained or will become apparent hereinafter.

crossed-grid structure.

?,930,897v Patented Mar. 29, 1960 In accordance with the principles of my invention, I

array of parallel, separated electrical conductors extending along a first direction; a second array of parallel, separated electrical conductors extending along a second and non-parallel direction; and an electroluminescent layer interposed between said arrays and electrically coupled therebetween. A first impedance network is coupled between a first terminal and each of the conductors of the first array to establish a first current path between each first array conductor and the first terminal; all of the first current paths having substantially the same impedance. A second impedance network is coupled between a second terminal and each of the conductors of the second array to establish a second current path between each second array conductor and the second terminal, all of the second current paths having substantially the same impedance.

A signal, the amplitude of which can vary with time,

is impressed across the first and second terminals and,

after being attenuated by an amount determined by the impedance of the first and second current paths, is also impressed across each of the cells in the crossed-grid structure. In addition, switching or commutation means, coupled between the conductors in both arrays and the first and second terminals, applies the same signal, substantially unattenuated but reversed in phase, to any selected cell and causes this cell to luminesce to a degree dependent upon the amplitude of the incoming signal.

With the above arrangement, the cross efiect can be substantially eliminated. More particularly, when the applied voltages are appropriately adjusted, a spurious luminescent pattern in the shape of a cross will not appear, the selected cell luminescing "brightly against a dim uniform background.

An illustrative embodiment of my invention will now be described in detail with reference to the accompanying drawing.

' Referring to the drawing, a plurality of parallel, separated, vertical transparent conductors 20, 22, 24, 26 and .28, form a first array and are each coupled through a separate resistor 30 to one end of a variable resistor 32. The other end of this resistor 32 is connected to terminal 56. Similarly, a plurality of parallel, separated,

horizontal conductors

34, 36, 38, 40 and 42, form a second array and are each coupled through a separate resistor 44 to one end of a variable resistor 46. The other end of the resistor 46 is connected to terminal 58.

An electroluminescent film or layer, not shown, is interposed between the horizontal and vertical conductors and is electrically coupled thereto.

A first amplitude-variable signal is impressed between terminal 56 and grounded terminal 60. A second signal, opposed in instantaneous polarity or phase to the first signal but otherwise identical thereto, is supplied between

terminals

58 and 60. At a selected instant in time, therefore, terminal 56 will be at some positive potential, as

for example +V volts with respect to the potential at terminal 60, while terminal 58 be at a negative potential of V volts with respect to the potential at terminal 60.

These voltages, attenuated by a first impedance network composed of

resistors

32 and 30 and coupled between terminal 56 and each of the

vertical conductors

22, 24, 26, 28, and by a second impedance network composed of

resistors

46 and 44 and coupled between terminal 58 and each of

thehorizontal conductors

34, 36, 38, 40 and 42,v

are impressed across all the cells in the crossed-grid structure; as a result the same potential, the value of which is determined by the attenuation voltage established by the two impedance networks, is appliedacross all cells in the In addition, a commutator 52 is'connected between terany one of the horizontal conductors, in this example conductor 36, to terminal 56. A second commutator 48 is connected between terminal 58 and all of the vertical conductors to couple any one of the five vertical conductors, in this example 26, to terminal 58. Conse quently, a voltage of 2V is applied across the cell 62 defined by the intersection of horizontal conductor 36 and vertical conductor 26 and this selected cell will luminesce brightly against a dim uniform background.

As is well known, all electroluminescent films or layers, whether formed of a crystalline film or as a dielectric suspension of phosphor particles, exhibit nonlinear brightness-voltage characteristics. More particularly, at low levels of applied voltages the light output from these films increases relatively slowly with an incremental increase in applied voltage, but at higher voltage levels, the light output increases much more rapidly with an incremental increase in applied voltage. Hence, the operation point can be so selected that the background level is substantially dark while the selected cell is relatively bright.

I have found that when the

variable resistors

32 and 46 are appropriately adjusted, each of the vertical conductors, other than 26, can be maintained at a potential of V/3, while all horizontal conductors other than 36 can be maintained at a voltage of -|V/3. As a consequenee, the voltage across the cell 62 being 2V, the voltage across all other cells in the panel is 2V/3. Thus, the voltage ratio between the energized cell and the background can be 3:1 as distinguished from known structures, wherein a voltage ratio of 2:1 is established between the spot and the spurious cross.

Since the uniform background does not contain a conspicuous geometrical form as does the spurious cross, it tends to dissolve into the ambient illumination. If, as indicated above, the voltage is appropriately chosen with respect to the non-linear characteristic of the electroluminescent film, the background can be regarded as esentially dark.

For proper operation of the circuit, the values of resistors i and 44, which can be but need not be equal, must be large enough to isolate the conductors in each array from each other. Further, the values of variable resistors 32 and 45 should be respectively small compared to those of the

respective resistors

30 and 44; otherwise all of the unselected strips will tend to be pulled to the potential of the selected strip.

Further, it will be obvious to those skilled in the art that a single amplitude-variable signal can be applied to the above arrangement instead of the two phaseopposed signals as heretofore used, provided that terminal 60 is eliminated and terminal 58 is grounded. For a given signal level, however, the voltages across all the cells, established when both signals are used, will be halved when one signal is employed.

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

What is claimed is:

1. In combination, a first array of parallel separated electrical conductors extending along a first direction; a second array of parallel separated electrical conductors extending along a second and non-parallel direction; an electroluminescent layer interposed between said arrays and electrically coupledthereto; a first set of resistors, one end of one selected resistor in said first set being connected in common to one end of all unselected resistors in said first set, the other end of said selected first set resistor being coupled to a first terminal, the other end of any one of said unselected first set resistors being connected to a corresponding one of the conductors in said first array; and a second set of resistors, one end of one selected resistor in said second set being connected in com- ,rnon -to one end of all unselected resistors in said second 4 set, the other end of said selected second set resistor being coupled to a second terminal, the other end of any one of said unselected second set of resistors being connected to a corresponding one of the conductors in said second array.

2. The combination as set forth in claim 1 wherein both of said selected resistors are variable resistors.

3. A device responsive to an incoming signal and comprising, in combination, a first array of parallel separated electrical conductors extending along a first direction; a second array of parallel separated electrical conductors extending along a second and non-parallel direction; an electroluminescent layer interposed between said arrays and electrically coupled thereto; first and second terminals, said signal being applied across said terminals; a first impedance network coupled between said first terminal and each of the conductors of said first array to establish a first current path between each first array conductor and said first terminal, all said current paths having substantially the same impedance; a second impedance network coupled between said second terminal and each of the conductors of said second array to establish a second current pathbetween each second array conductor and said second terminal, all said second current paths having substantially the same impedance, whereby said signal, attenuated by an amount determined by the resistance of said first and second current paths, is impressed across each first array conductor-second array conductor pair.

4. in combination, a first array of parallel separated electrical conductors extending along a first direction; a second array of parallel separated electrical conductors extending along a second and non-parallel direction; an electroluminescent layer interposed between said arrays and electrically coupled thereto; a first set of resistors, one end of each resistor in said first set being connected in common to a first terminal, the other end of any one resistor in said first set being connected to a corresponding one of the conductors in said first array; a second set of resistors, one end of each resistor in said second set being connected in common to a second terminal, the other end of any one resistor in said second set being connected to a corresponding one of the conductors in said second array; and a commutator coupled between said first and second terminals and the conductors in both arrays to couple a selected second array conductor to said first terminal and a selected first array conductor to said second terminal.

5. A device responsive to first and second push-pull signals and comprising, in combination, a first array of parallel separated electrical conductors extending along a first direction; a second array of parallel separated electrical conductors extending along a second and nonparallel direction; an electroluminescent'layer interposed between said arrays and electrically coupled thereto; first, second and third terminals, said first signal being applied between said first and second terminals, said .second signal being applied between said third and second terminals; a first impedance network coupled between said first terminal and each of the conductors of said first array to establish a first current path between each first conductor and said first terminal, all said first current paths having substantially the same impedance; at second impedance network coupled between said third terminal and each of the conductors of said second array to establish a second current path between each second conductor and said second terminal, all said second current paths having substantially the same impedance, whereby a third signal proportional to the difference between said first and second signals, attenuated by an amount determined by the resistance of said first and second current paths, is impressed across each first array conductorsecond array conductor pair.

6. A device responsive to first and second push-pull signals and comprising, in combination, a first array of parallel-separated electrical conductors extending along aaa eo'r a first direction; a second array of parallel separated electrical conductors extending along a second and nonparallel direction; an electroluminescent layer interposed between said arrays and electrically coupled thereto; first, second and third terminals, said first signal being applied between said first and second terminals, said second signal I being applied between said third and second terminals; a

first impedance network coupled between said first terminal and each of the conductors of said first array to establish a first current path between each first conductor and said first terminal, all said first current paths'having substantially the same impedance; a second impedance network coupled between said third terminal and each of the conductors of said second array to establish a second current path between each second conductor and said second terminal, all said second current paths having substantially the same impedance, whereby a third signal proportional to the difierence between said first and second signals, attenuated by an amount determined by the resistance of said first and second current paths is being impressed across each first array conductor-second array" conductor pair; and a commutatorv coupled between said first and third terminals and the conductors of both arrays to couple a selected first array conductor to said third array conductor pair.

References Cited in the file of this patent UNITED STATES PATENTS 2,698,915

Piper Jan. 4, 1955