US2948823A - Electroluminescent device - Google Patents

Description

Aug. 9, 1960 M. WASSERMAN 2,943,823

ELECTROLUMINESCENT DEVICE Filed July 14, 1959 szsc'moos/o PHOTO CONDUCT/VF ZAYER I2 1 EL ECTROL UM/NES CEN T LAYER l6 INVENTOR MOE WASSERMAN ATI'ORNEY 2,948,823 Ice 7 --P atented Aug. 9,1960

Myinvention relates to electroluminescent devices.

One type of electroluminescent device known to the art comprises separate photoconductiveand electroluminescent layers electrically connected in series. Thetphoto conductive impedance in the dark is high relative to the electroluminescent impedance. Further, when the photoconductive layer is illuminated, the photoconductive impedance is significantly reduced. These layers are so arranged that a portion of the light emitted from the electroluminescent layer (when this layer is excited by applying a voltage across the series connected layers),

impinges on the photoconductive layer.

As a result, 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 device 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; consequently, a larger portion of the applied voltage appears across the electroluminescent layer and light is emitted. The device is then in its second, or energized, state. As long as the photoconductive layer remains illuminated by light from the electroluminescent layer, the device will remain energized despite removal of incident light. Conventionally, the device is deenergized by removing the applied voltage.

Such known devices, however, have certain inherent disadvantages. More particularly, the sensitivity of the device depends upon the ratio of the dark photoconductive impedance to the illuminated photoconductive impedance. Since only the light receiving surfaces of the photoconductive layer are excited by incident light, this ratio decreases as the thickness of the photoconductive layer increases. Stated differently, high device sensitivi ties require thin photoconductive layers.

On the other hand, for bistable operation, the dark impedance of the photoconductive layer should be high relative to the impedance of the electroluminescent layer. Since the dark photoconductive impedance increases with increasing thickness of the photoconductive layer, the photoconductive layer must be relatively thick for this type of operation.

As a result, electroluminescent devices of the type described above utilize relatively thick photoconductive layers and, consequently, have low sensitivity.

In accordance with the principles of my invention, my electroluminescent device comprises first, second and third transparent electrodes, an electroluminescent layer interposed between the second and third electrodes, and a photoconductive layer interposed between the first and second electrodes. The first electrode is smaller in area than the second electrode. The surface area of the photoconductive layer is substantially equal to the surface area of the electroluminescent layer. 7

By varying the ratio of the first electrode area to the second electrode area, the dark impedance of the photo conductivewlayer can be varied accordingly; i.e. asthe ratio of these electrode areas decreases or increases, the dark impedance is increased or decreased. Furthen'the photoconductive layer can be extremely thin, thus permitting "high device sensitivity. invention the device sensitivity and the dark photoconductive impedance are separate and independent parameters which can becontrolled independently.

An illustrative embodiment of my invention will now be described-with reference to the accompanying figure.

Referring now toth'e figure, there is shown a bistable electroluminescent device having a first transparent electrode I10, asecond transparent electrode 14 and a third transparent electrode '18. -A photoconductive layer-12. is interposed between electrodes 10 and 14. An'electroluminescent layer 1*6is interposed between electrodes '14 and "18. The entire structure including'electrod'e 18 is supportedona glass-substrate 20.

The electrodes 10, 14 and 18 can be transparent tin oxide films. The photoconductive layer can be composed of cadmium sulfide particles, activated with copper, coactivated with a chloride, and embedded in a glass enamel, as disclosed, for example, in my copending patent application Serial No. 796,155, filed February 27. 1959 now Patent No. 2,937,353. The electroluminescent layer can comprise a dispersion of electroluminescent (copper activated zinc sulfate) particles also embedded in a glass enamel. Typically, the electroluminescent layer can be 1.5-2.5 mils thick. The photoconductive layer typically can be 2-5 times as thick as the electro luminescent layer.

-As shown in the figure, the area of the first electrode is smaller than the area of the second electrode. adjusting the ratio of the area of the first electrode to that of the second electrode, the dark photoconductive impedance can be suitably adjusted for a photoconductive layer of given thickness. More particularly, as this ratio decreases, the dark impedance is increased, Typically, this ratio can fall within the range 0.25-0.50.

In the device thus far described, bistable operation is obtained in the manner previously indicated. Alternatively, the first electrode 10 can be opaque. The electroluminescent layer 16 is sufiiciently thin to permit incident light to be directed through layer 16 and electrode 13 upon the photoconductive layer 12, thus again obtaining bistable operation.

If desired, the second transparent conductive layer 12 in the above structure can be replaced by'an opaque conductive film. Under these circumstances, however, the electroluminescent layer will be energized only when the photoconductive layer is illuminated by incident light. As soon as the incident light is removed, the electroe luminescent layer will be deenergized and emit no light.

What is claimed is:

1. An electroluminescent device'comprising first, second and third electrodes, the third electrode and at least one of the first and second electrodes being transparent, the area of the first electrode being less than that of said second electrode, the ratio of the area of the first electrode to that of the second electrode falling within the approximate range of 0.25-0.50; an electroluminescent layer interposed between said second and third electrodes; and a photoconductive layer interposed between said first and second electrodes, the surface areas of both of said layers being substantially equal.

2. An electroluminescent device comprising first, secand third electrodes, the first and third electrodes being transparent, the area of the first electrode being less than that of said second electrode, the ratio of the area of the first electrode to that of the second electrode falling within the approximate range of 0.25-0.50; an electroluminescent layer interposed between said second In other words, in my and third electrodes; and a photoconductive layer interposed between said first and second electrodes, the surface areas of both of said layers being substantially equal.

3. An electroluminescent device comprising first, second and third electrodes, the second and third electrodes being transparent; the area of the first electrode being less than that of said second electrode, the ratio of the area of the first electrode to that of the second electrode falling within the approximate range of 0.25-0.50; an electroluminescent layer interposed between said' second and third electrodes; and a photoconductive layer interposed between said first and second electrodes, the surface areas of both of said layers being substantially equal.

4. An electroluminescent device comprising first, second and third electrodes, the first, second and third electrodes being transparent, the area of the first electrode being less than that of said second electrode, the ratio of the area of the first electrode to that of the second electrode falling Within the approximate range of 0.25-0.50; an electroluminescent layer interposed between said second and third electrodes; and a photoconductive layer interposed between said first and second 4 electrodes, the surface areas of both of said layers being substantially equal.

5. An electroluminescent device comprising first, second and third electrodes, the first and third electrodes being transparent, said second electrode being opaque, the area of the first electrode being less than that of said second electrode, the ratio of the area of the first electrode to that of the second electrode falling within the approximate range of 0.25-0.50; an electroluminescent layer interposed between said second and third electrodes; and a photoconductive layer interposed between said first and second electrodes, the surface areas of both of said layers being substantially equal.

References Cited in the file of this patent UNITED STATES PATENTS 2,836,766 Halstead May 27, 1958 2,873,380 Kazan Feb. 10, 1959 2,874,308 Livingston Feb. 17, 1959 2,882,419 Diemer et a]. Apr. 14, 1959

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