US2873380A - Electroluminescent device - Google Patents

Description

Feb. 10, 1959 B. KAZAN ELECTROLUMINESCENT DEVICE Filed Oct. 20, 1952 2 Sheets-Sheet l PEZFLTIC .NMQQQWRQ BL. V6275 WTOR.

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United States Patent ELECTROLUMINESCENT DEVICE Benjamin Kazan, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 20, 1952, Serial No. 315,696

13 Claims. (Cl. 250-213) This invention relates to devices for producing light or light images or other radiation by subjecting a luminescent body to a varying electric field. In particular, the invention relates to such devices for producing light or other radiant energy representative of an energizing influence which may be light or other radiantenergy, electrical signals, an electrical charge pattern, particle radiation, pressure or the like. The invention further relates to light amplifiers and light storage devices. This invention contemplates the provision of devices for use in promoting the productionand storage of light by an electroluminescent body under the energizing influence of a variable reactance device, such reactance being varied by an applied electric force.

- It is known in the electronic arts that an electroluminescent body can be made to produce light by the application of an electric field across said body. This phenomenon is known as electroluminescence. The theory of electroluminescence is not well understood, and several theories are in force at present. One of these is the Field emission theory. According to this theory, an electric field exists between particles of phosphor in an electroluminescent body. If this field is sufficiently strong, electrons may be drawn out of oneparticleand across the separating gap toward the opposing particle which is thus bombarded in a manner similar to the usual low velocity electron bombardment of a.

phosphor? screen. Under 1 this electron-bombardment, phosphor particles are induced to give oft" light.

= Another theory is that electrons are retained in shallow traps within a phosphor particle. These electrons may bedrawn out of the traps by an electric field, accelerated and caused to energize other particles,thereby producing light. i l

There are devices presently known which may be used to producelightimages in response to an energizing force of one kind or another. Electrical storage devices are vices are complicated, have critical structuralrelationships; and are expensive and diificult to manufacture. Accordingly,theprincipal object of this invention is to provide a device capable of producing and storing light images by subjecting an electroluminescent body to an electric field. l i 1 A further object is to provide an improved and sim plifiedelectronic device for use in producing, storing, and amplifying light images. i l A further objectof this invention is to provide an improved electro-optical device for use in converting an electrical charge pattern to a visual light image.

Another object is to provide an improved light storage and light amplifying device which is comparatively expensive andis easy to manufacture.

In general, the purposes and objects of this invention are accomplished by theprovision of an electrolumines cent body and a variable reactance device associated with the body and adapted to varythe field across the body "ice , '2 under the control of an applied electrical force, either a voltage or a field or the like. The variable reactance device may be a variable dielectric condenser, a saturable reactor or the like whose A. C. impedance is varied by a D. C. voltage or a field appliedto the device.

The invention is described with reference to the drawings wherein:

Fig. 1 is a sectional elevational view of one embodiment of an electroluminescent device controlled by a 0 variable dielectric condenser;

Fig. 2 is an alternative arrangement of the device shown in Fig. 1;

Fig. 3 is a curve showinggenerally how dielectric constant and impedance vary with D. C. voltage across a variable dielectric condenser;

Fig.4 is a sectional elevation of another embodiment of the invention wherein a saturable reactor is employed;

Fig. 5 is a curve showing the variation of impedance with D. C. voltage applied to a saturable reactor;

Fig. 6 is a sectional elevation of a further embodiment of the invention showing one step in its operation; Fig. 7 is the device of Fig. 6 .at another step in its operation;

Fig. 8 shows one apparatus for charging the surface of a body;

Fig. 9 is a sectional elevation of, another embodiment of the invention; and

Fig. 10 is the device of position. l 1

In Fig. l is shown an electroluminescent device having an electroluminescent bodywhich receives its energizing influence from a variable reactance device whose reactance is varied by a'modnlating D. C. voltage. In this instance the variable reactance device iis .a variable dielectric condenser. I p

In the. form. of the invention shown t in Fig. .lan electroluminescent cell 10. is connectedin seriesyvith. a

Fig. 9 in light-producing variable dielectric condenser 12 The cell comprises a material such as glass having autransparent conductive coating 20. The transparent conductive coatingmay be of the type formedfrom the vapors of stannic chloride, water, and methanol, l H H The variable dielectric condenser 12 comprises a sheet or layer 22 of ferro-electricmaterial mounted bee tweenthe conductive plate 16and another conductive plate 24bothof which may be made of metal or the like. Ferro-electric materials such as Rochelle salt, barium I titanate, bariumstrontium titanate, or the like may be also well known inthe art. However such known de- A Fig.2. An alternating voltage source 28 is connected between the plate 24 and the coating 20. A source 30 of D. modulating voltage is connected across the condenser by connection to the plates 16 and 24 and isarraznged to bevaried in accordance with input signals.

In order to prevent shunting of the alternating voltage from the source 28 through the D. C. voltage modulator 30, thelatter should have an A. C. impedance considerably higher than that of the ferrof-electric condenser 12 at the alternating voltage supply frequency which is used. Since the term-electric material 22 ha s the property of changing its dielectric constant, and

hence, its capacitiveimpedance, as a function of the D. C. field across it, the amount of alternating voltage applied across the electroluminescent cell 10 can be varied by the varying D. C. voltage across th e f errqa electric condenser 12. In particular, if the alternating voltage source'28 is adjusted so that with zero D. C. voltage across the ferro-clectric condenser, sufficient A. C. voltage appears across the electroluminescent cell to cause it to emit a reference amount of light, the application of D. C. voltageto the ferro-electric condenser 12 will cause its A. C. impedance and the voltage across it to increase (Fig. 3), whereby the voltage across the electroluminescent cell is lowered, and the light output therefrom reduced. It is understood that in such an arrangement as indicated in Fig. 1, the A. C. impedance of the ferro-electric condenser 12 with D. C. voltage on it is made relatively high compared to that of the electroluminescent cell so that the voltage on the electroluminescent c'ell can be efiectively modulated by the changes in capacity ofthe ferro-electric condenser. Since ferro-electric materialshave been made with dielectric constants which vary over a range of 10. to 1 as a function of D. C. voltage. applied across them, the electroluminescent cell can be modulated over a considerable range of light output by the input D. C. voltage.

Because of the fact'that the electroluminescent light can be modulated by the voltage across a condenser the arrangement of Fig.1 can be used as a self-indicating storage device. If instead of providing a D. C. modulator, the ferro-electric condenser '12 is given a charge, the D. C. voltage across it will remain relatively fixed-and the amount of light from the electroluminescent c-ell'will remain at a corresponding fixed level. As soon as the charge is removedythe electroluminescent light will assume a new level. In other words, the electroluminescent cell can in'dicate'the amount of static charge stored on the condenser.

'An'otherembodiment of the device of Fig. 1 can also be constructed with a saturable reactor placed in series with the electroluminescent cell instead of a ferro-electric condenser. ISuchIan arrangement is shown in. Fig.4 and comprisesan iron core 32 having an alternating voltage source 34 connected in series with an electroluminescent cell 36 by means of a winding'38 around one leg '40 ofthe core 32. A modulated D. .C. source 42;is linked to'the reactance core 32 through a winding 44wound arounda leg 46. In this case, variation of the D. C. voltage across the winding 44 varies the A. C. impedance of the winding 38 and the light emitted from the electroluminescent cell isvaried proportionally. The winding 38, core 32, and winding constitute a variable reactan'ce'member connected in series with the electroluminescent cell 36, and having its A. '0. impedance varied by' the'voltag'e applied by the D. C. source 42. .The magnetic core 32 can be made of such material that it will retain an amount of permanent magnetization. Since the amount of magnetization of the core determines the A. C. impedance of the winding 38, the electroluminescent cell will indicate the state of "magnetization of the core. Fig. shows the variation of'impedance with D. 'C. voltage applied to a saturable reactor.

In Figures 6 and 7, an apparatus is shown for making a stored charge "pattern directly visible. The apparatus comprises two portions movable with respect to each other. One portion 48 inc'ludes an insulating layer 50, for example of photoconductivemateriaL-having a conductive backing plate 52. The other portion 54-includes a layer '56 of electroluminescent material having on one side, a sheet of glass 58 having a transparent conductive coating '59 and, on the other side, a sheet or layer 60 of ferro-electric material. Asheet 61 .of protective insulating material may be provided .on the free surface of the"ferro-electric sheet 60.

' In using this device with an alternating voltage source 62 connectedbetween the conductive members 52 and 59, the free surface of the photoconductive layer 50 is first'charged uniformly. Thischarging step -may be efiect'ed,for"exampleby'a corona discharge process. In

.4 I one embodiment of such a process, as shown in Fig. 8, an assembly of several fine wires 63, of the order of three mils in diameter, are positioned approximately onehalf inch away from the surface of the photoconductive layer 50 to be charged. A metal backing plate 64 is positioned behind and in contact with the conductive backing plate 52 and a potential of six or seven thousand volts is applied between the wires and the plate whereby a corona discharge takes place. The sign of the applied voltage determines the sign of the charge deposited on the surface. The assembly of wires is slowly moved across and charges the entire surface of the photoconductive layer. After this charging step, the photoconduc'tive layer must be kept in the dark or otherwise protected from light or other radiant energy which might affect its conductivity and its established charge. The charged photoconductive layer is then exposed to a light image and a surface electrical charge pattern is produced which is representative ofthe image. The charge pattern is produced by the increased conductivity of the layer whichallows' conduction of charge across elemental volumes of the photoconductive layer exposed to the light image. After exposure to the light image, the conductivity of the photoconductive layer returns to its normal high value in the dark.

Next, as indicated in Fig. 7, the portion 48 is brought into contact with the portion 54 with the photoconductive layer 5d in contact with the ferro-electric layer 60 or insulating layer 61 where used. This step must also he carried out in the dark or some other precautionary measures must be taken to prevent distortion of the charge pattern onthe photoconductive layer. At this time, the charge patternon the surface of the photoconductivelayer comes in contact with "the surface of the ferro-electric layer 6t or-layer 61 where used. Where the insulating layer 61 is used, the photoconductive layer 56 is capacitively connected through the layer 61 to the ferro-elec tric laycr 6d. Thus, an electric field is established across each elemental area of the ferro-electric layer, and the capacity and-impedance of the layer vary accordingly. As a result the alternating voltage applied across each elemental area of the electroluminescent layer 56 produces thereon alight output representative of the original light image.

Another form of two-part device for making visible a charge pattern is shown in Fig. 9. In this case, one portion 65 includes a-ferro-electric layer 66 provided with a conducting backing sheet-67- on one surface and a protective layer 68 of insulating material on the other surface. The other portion 69 includes an electroluminescent layer 70 having on one side a sheet of glass '72 provided with a layer of conductive material 73 on one surface and a layer 74 of photoconductive material onthe other surface. A uniform charge distribution is initially provided on the exposed surface of the photoconductive layer 74 by any'suitable-m-ethod such as that described above. The photoconductor is then exposed to a radiation image which causes charges to leak from the photoconductor surface to the electroluminescent layer 70 wherebya charge pattern representative of the radiant energy remains on the free surface of thephotoconductor. The t'wopo'rtion-s of .-the device are then pressed together as in Fig. 10 and an alternating voltage is applied across the sandwich bythe usual connections between the layers '65 and 73. As described above, the charge pattern on the surface of the photoconductor causes a pattern of potential variations to be applied across the ferro-electric layer 66. The resultant fields across elemental portions of the ferro-electric layer affect the capacitance and impedance of these portions and. as a result, the amount of alternating voltage appearing across elemental portions of the electroluminescent layer 70.

As a result, the electroluminescent layer produces a light.

image representative oftheoriginallight image. 7

' In the embodiments shown in Figures 6, 7, 9, 10, the

asvaaeh photoconductive and electroluminescent layers may also be made to operate regeneratively so that an electroluminescent picture initially produced by the charge pat-, tern will cause a continuous pattern of conductivity over the photoconductive layer, thereby sustaining the light picture for an indefinite period. This phenomenon of regeneration is described in a copending Kazan application Serial No. 315,694 filed concurrently herewith.

What is claimed is: t I v 1. An electroluminescent device comprising. ari-ielectroluminescent body adapted to produce light wherijeliec cally energized, a variable reactance member adip d to have its impedance varied by an applied electric field and connected in series with said electroluminescent body, electrode means electrically connected to said body and said member for establishing an alternating electric field across said electroluminescent body and said member, and means electrically connected to said variable reactance member for applying an electric field thereto to vary the reactance thereof and thereby varying the alternating voltage applied to said electroluminescent body.

2. An electroluminescent device comprising an electroluminescent body, a variable reactance member electrically connected in series with said body, said variable reactance member being adapted to be subjected to a direct current voltage for varying the impedance thereof, and means electrically connected to said body and said variable reactance member for applying an alternating voltage across the series combination, whereby upon application of a direct current voltage to said variable reactance member the impedance of said variable reactance member may be varied to produce corresponding variations in the electric field applied across said electroluminescent body.

3. The invention according to claim 2, said means including a source of alternating voltage of constant amplitude applied across said series combination.

4. The invention according to claim 3, said device further including a source of variable voltage connected to said variable reactance member.

5. An electroluminescent device comprising a laminated structure including a layer of electroluminescent material, a layer of a variable reactance material adapted to have its impedance varied by an applied electric field, and a pair of conducting plates on the outer surfaces of said layers, said plates and layers being electrically connected in series.

6. An electroluminescent device comprising a laminated structure including a layer of electroluminescent material and a conducting plate on each side of said layer for establishing an electric field across said luminescent material, a saturable reactor connected to one of said conducting plates and in series with said structure, said saturable reactor being connected to a source of variable direct current voltage for varying the impedance thereof, the other of said conducting plates and the free end of said saturable reactor being adapted to be connected to a source of alternating voltage whereby upon varying the direct current voltage applied to said saturable reactor, the impedance thereof may be varied to produce corresponding variations in the electric field applied to said electroluminescent material.

7. An electroluminescent device comprising a sheet of photoconductive material having a conductive backing sheet, a layer of electroluminescent material having a transparent conductive sheet on One surface and a layer of variable reactance material on the other face thereof, said variable reactance material adapted to have its impedance varied by an applied electric field, a source of alternating voltage connected between said conductive sheets, said photoconductive sheet being electrically connected to said variable reactance layer whereby said alternating voltage is applied across each elemental area of said electroluminescent layer in proportion to the charge pattern on said photoconductive layer.

8. An electro-optical apparatus comprising a sheet of variable reactance material having a conductive backing sheet, said variable reactance material adapted to have its impedance varied by an applied electricfield, a sheet of electroluminescent material having a layer of photoconductive material on one surface thereof and a layer of transparent conductive material on the other surface, said conductive sheet and said conductive layer having a source of alternating voltage connected therebetween, said variable reactance sheet being electrically in series with said photoconductive layer whereby said alternating voltage is applied across each elemental area of said electroluminescent layer in proportion to the charge pattern on said photoconductive layer.

9. The method of visually reproducing a stored charge pattern, said method comprising establishing a uniform charge pattern on a surface of a photoconductive sheet, exposing said charged surface to radiant energy representative of an image to be viewed to discharge elemental areas of said uniformly charged surface in varying amounts and thus establish a new charge pattern corresponding to said image, connecting said photoconductive sheet in series with a layer of electroluminescent phosphor and a layer of ferroelectric material with said ferroelectric layer adjacent to the charged surface of said photoconductive sheet, and applying a source of alternating voltage across the series combination, whereby the impedance of said ferroelectric material is varied through elemental volumes thereof accordng to the variations in charge on said photoconductive sheet to vary the electric field developed across elemental areas of the electroluminescent phosphor and thus vary the light output therefrom.

10. The method of visually reproducing a charge pattern, said method comprising establishing a uniform charge pattern on one surface of a photoconductive sheet, exposing said charged surface to radiant energy representative of an image to be viewed to discharge elemental areas of said uniformly charged surface in varying amounts and thus establish a new charge pattern corresponding to said image, disposing said photoconductive sheet adjacent to a laminated structure including a layer of electroluminescent phosphor and a sheet of ferroelectric material, applying a source of alternating voltage between said phosphor and the unexposed surface of said photoconductive sheet, and electrically connecting the I charged surface of said photoconductive sheet and said ferroelectric material.

11. The method of visually reproducing a charge pattern, said method comprising arranging a laminated structure including a transparent conductive support mem her, a layer thereon of an electroluminescent phosphor and a layer of photoconductive material, establishing a uniform charge pattern on the free surface of said photoconductive material, exposing said charged surface to a light image to be viewed in order to discharge elemental areas of said uniformly charged surface in varying amounts and thus establish a new charge pattern corresponding to said light image, disposing said laminated structure adjacent to a second laminated structure including a conductive backing sheet and a ferroelectric electrically "connected to saidinmbr for applying -'a References Cited in the file of this patent direct current vol'tagethereto, forvaryin'g the'reactance E STATES PATENTS ofsaid member and thereby varying the'alternatiiig volt- 2 566 349 Mager Sept 1'951 P S .e V '-2,'603:757 Sheldon July 15, ;19s2

13. An electroluminescent device comprising an elec- 5 624 857 Mager 7 Jan 6 1,953 troluminesce t body, 'a variable reactance member 10 White 1953 adaptedto haveits'impedance varied by an applicdclec- '7 7 trio field and connected in .series with said body, and OTHER REFERENCES electrode means electrically connected to said body and Mellon Inst. of Industrial Res.: Cfimputer Ccmpomember fer establishing an alternating electric field 10 neiits F 1l0 WSi1ip"NO. 347, Quar terly Rpt. No. 3 (1951); thereacross. (same) No.6 (June 1952

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