US3604938A

US3604938A - Method for operating electroluminescence display device

Info

Publication number: US3604938A
Application number: US820136A
Authority: US
Inventors: Tadao Kohashi; Kazunobu Tanaka
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1969-04-29
Filing date: 1969-04-29
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14

Abstract

A method of operating an electroluminescent display device in which a unidirectional electric field of a predetermined direction is established across an electroluminescent layer comprising electroluminescent phosphor and glass enamel thereby to control the luminescence of said electroluminescent layer caused by excitation with an AC electric field.

Description

I] ranted-:1 States Patent [72] Inventors Tadao Kohashi Yokohama; Kazunobu Tanaka, Kawasaki-shi, both of, Japan [21] Appl. No. 820,136 [22] Filed Apr. 29,1969 [45] Patented Sept. 14, 1971 [73] Assignee Matsushita Electric Industrial Co., Ltd.

Osaka, Japan Continuation of application Ser. No. 618,380, Feb. 24, 1967, now abandoned.

[54] METHOD FOR OPERATING ELECTROLUMINESCENCE DISPLAY DEVICE 13 Claims, 1 1 Drawing Figs.

[52] U.S.Cl 250/213 R, 313/108 A [51] InLCl H011 17/00 [50] Field of Search 250/213,

83.3 IR; 313/108; 340/173 MS, 174.1 MO

INC/DENT ENERGY I-Iausmann & Slack: Physics;" 4th Ed; June, 1957; pp. 429, 430; D. Van Nostrand Co. Inc., QC 21-H37-l957.

Thornton: AC-DC Electroluminescenee; Physical Review; March 1, 1959; pp. 1l871 190. 2502l3 Primary Examiner--Walter Stolwein Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: A method of operating an electroluminescent display device in which a unidirectional electric field of a predetermined direction is established across an electroluminescent layer comprising electroluminescent phosphor and glass enamel thereby to control the luminescence of said electroluminescent layer caused by excitation with an AC electric field.

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MMMQW ATTORNEYS PATENTED 8EP14|97| 3,604; 938

sum 2 or 2 FIG 4 SUPPORT PLATE L2 LUM/NOUS OUTPUT SUPPORT PLATE 1 LW/IVOUS OUTPUT INVENTORS TADAO KOHASHI KAZUNOBU TANAKA ATTORNEYS METHOD FOR OPERATING ELECTROLUMINESCENCE DISPLAY DEVICE CROSS-REFERENCES TO RELATED APPLICATIONS This is a continuation application of the copending US Pat. application Ser. No. 618,380 filed on Feb. 24, l967, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating an electroluminescent display device comprising an electroluminescent layer made of electroluminescent phosphor and resistive binding material, wherein luminescence caused by an AC excitation of the layer is controlled by a unidirectional electric field.

2. Description of the Prior Art A conventional electroluminescent display device is supplied with an alternating electric power and thereby excited to electroluminescence. In general, an electroluminescent layer has a large capacitance representing a low capacitive impedance resulting from its construction in which an electroluminescent phosphor is suspended in a dielectric medium. Accordingly, it is required to control a large alternating electric power for controlling the intensity of luminescence of an electroluminescent display device.

Therefore an effective luminescent display device which can be controlled without resort to a direct control by an alternating electric power is desired.

Further, a number of proposals have been made concerning a so-called energy-sensitive luminescence displaying device, comprising in combination a photoconductive layer having an impedance varying with an incident energy, such as a radiant ray, and an electroluminescent layer, wherein the luminous output of the electroluminescent layer is reversibly controlled in accordance with the variation in the impedance of the photoconductive layer. Such a conventional device is based on a principle wherein the luminescence caused by an AC excitation of the electroluminescent layer is controlled by variation in the AC impedance of a photoconductive layer resulting from the variation in the intensity of an incident ray such as a light ray or a radiant ray. However, as is well known, the photoconductive layer possesses a photoconductive sensitivity far lower to an AC voltage than to a DC voltage.

Further, a photoconductive layer is generally expressed equivalently by a parallel circuit of a capacitance, which is determined by its material and geometrical configuration, and a variable resistance which varies according to the energy excitation thereof. Accordingly, such a layer represents a low AC impedance due to the parallel capacitance effect even if the variable resistance is of high resistance value. Accordingly, the effective variation in the AC impedance of a photoconductive layer due to incident energy excitation is effectual only when the impedance presented by the variable resistance is reduced to a value, such as, the same as or below the impedance of the parallel capacitance. Therefore, it is impossible to control the luminescent AC power of an electroluminescent layer at an intensity band of incident energy excitation whereby the impedance presented by the variable resistance is higher than that of the parallel capacitance.

Owing to the above described facts, a highly sensitive operation is difficult to be performed with a conventional electroluminescent display device.

SUMMARY OF THE INVENTION In view of the above description, the main object of the present invention is to provide an electroluminescent displaying device wherein the luminescence caused by an AC excitation is reversibly controlled by a DC (unidirectional) electric field (voltage) of which polarity is defined.

Another object of the invention is to provide an electroluminescent display device wherein the AC luminescent output of the electroluminescent layer is reversibly controlled with a high sensitivity by a unidirectional electric field having a defined polarity through utilization of the variation of the resistance of an energy sensitive element such as a photoconductive layer, a piezoresistance element or a magnetoresistance element of which at least the resistance varies according to excitation by an incident energy.

In summary, the object of the present invention is to provide an electroluminescent display device characterized in that an AC electric field and a unidirectional electric field are superposing applied to an electroluminescent layer formed of a mixture of an electroluminescent phosphor and a resistive material, said unidirectional electric field being applied in such a direction that the potential of the luminous output takeout surface side of the electroluminescent layer is lower compared with the opposing opposite surface side thereof, whereby the luminescence of the electroluminescent layer is controlled by means of the unidirectional electric field.

An embodiment of the invention will be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a vertical sectional view of an embodiment according to the present invention illustrating the construction thereof and the electric power feeding system;

FIGS. 2A-D illustrate oscilloscopic waveforms of AC voltage and luminescence in the embodiment shown in FIG. 1;

FIGS. 3a-d illustrate oscilloscopic waveforms of AC voltage and luminescence in another device for the purpose of comparison;

FIGS. 4 and 5 are schematic drawings showing other embodiments of the present invention in vertical section.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, which shows the construction in section and the electric power feeding system of an embodiment of the invention, wherein a photoconductive layer, for example, at least the resistance of which varies according to the excitation by an incident energy such as a light ray, radiant ray or electron beam, or the like is correlated with an electroluminescent layer in such a manner that the photoconductive layer controls the AC excited luminescence of the electroluminescent layer by a DC voltage (field) relating to the intensity of energy excitation.

In FIG. I, the numeral 11 denotes a support plate made of transparent glass or ceramic having a high melting point, the surface of which is coated with a transparent, conductive film 12 formed of a metal oxide such as stannic oxide or the like. The numeral 13 denotes an electroluminescent layer, (which will be described as EL-layer hereunder,) which is formed as a layer having a thickness in the order of 50p, for example, by mixing an EL powder phosphor such as ZnSzCu with glass enamel having a specific resistance of the order 10 to 10 0 cm., which serves as a binder. The numeral 14 denotes a resistive light interception layer, of a thickness in the order of 10p, having a specific resistance of the same order as or below that of the EL-layer 13. The light interception layer 14 is formed by mixing pulverized graphite with glass enamel. The numeral 15 denotes an energy sensitive layer of which, at the least, the resistance varies in accordance with an incident energy. The energy sensitive layer 15 is a photoconductive layer having a thickness in the order of to 200p., formed by sintering CdS, CdSe, CdS-CdSe or the like, or by binding powdered CdSe, CdS-CdSe or the like by glass enamel of a high specific resistance as a binder. The maximum resistance of the layer 15 is selected to be the same or higher than that of the EL-layer 13.

The numeral 16 denotes an electrode having such a property that admits an incident energy L The electrode 16 is formed as an evaporated metal film such as aluminum, when the incident energy L, is a radiation ray such as X-ray, and is formed as a transparent electrode when the incident energy is a light ray. An AC voltage supply source 17 and DC voltage supply source 18 are connected between the

electrodes

12 and 16 to apply an AC voltage V, and DC voltage V therebetween.

The AC supply source is so adjusted that the magnitude and frequency of the voltage supplied are large enough to cause the EL-layer 13 to have a sufficiently large luminous output L during a dark period, that is, in the absence of an incident radiation energy L Under this condition, the' resistive impedance of the energy sensitive layer which is, for example, the photoconductive layer is sufficiently high, and an alternating current flows into the EL-layer via a low parallel capacitive impedance.

A DC voltage V,,, which is equal to or higher than the peak value of the AC voltage shared by the EL-layer 13, is applied across the EL-layer in superposed relationship to the AC voltage V As shown in FIG. 1, when the DC voltage or DC electric field is applied in such a manner that the potential at the luminous output take-out surface side of the EL-layer 13 (the electrode 12 side) is lower than that at the opposite surface side (the electrode 16 side). Under this condition, when the incident energy L increases or decreases, the waveform of the luminous output L varies reversibly, as shown in FIG. 2.

In FIG. 2, there are shown a set of waveforms (A, B, C and D) OF the AC voltage shared by the EL-layer l3 and luminous output appearing at the luminous output takeout surface side of the EL-layer. The AC voltage designated at (A) is set at 85 volts with a frequency of l kc. by adjusting the AC voltage V FIG. 2(B) shows an example of the waveform of the luminous output L when there is not incident energy L When L =0, them the DC voltage shared by the EL-layer 13 is 40 volts. When the incident energy 1.. is increased to a small extent with the suitable DC voltage V applied as described above, the AC voltage shared by EL-layer I3 is not varied appreciably with respect to the decrease of resistive impedance of the energy sensitive layer 15 resulting from the increase of the incident energy L because the parallel capacitive impedance of the energy sensitive layer 15 is low and the parallel resistive impedance of the layer 15 is extremely high.

On the other hand, the DC voltage shared by the EL-layer l3 remarkably increases due to the relation between the resistive impedances of the EL-layer l3 and the energy sensitive layer 15. The experiments conducted by the inventors have revealed that the intensity of the AC field luminescence pulse decreases continuously when the DC voltage across the EL- layer 13 is increased. The waveform of the luminous output L occurring when the DC voltage across the EL-layer 13 is at 260 volts is shown in FIG. 2(C).

Under this condition, when the incident energy L is shut off, the waveform of the luminescence of the EL-layer 13 changes to that of FIG. 2(D), which is the same as that of FIG. 2(8). The luminescence control by the unidirectional electric field as described above is completely reversible with respect to the incident energy L Thus, an intensified EL-image of negative polarity is displayed on the EL-layer 13.

In FIGS. 3(a), (b), (c) and (d), there are shown the oscilloscopic waveforms corresponding to FIGS. 2(A), (B), (C) and (D), respectively. These waveforms appear when the unidirectional electric field is applied in the opposite direction to that as shown in FIG. 1, namely, when the DC supply source is connected between the

electrodes

12 and 16 in such a manner that the luminous output takeout surface side of the EL-layer 13 is at a positive potential or a higher potential. In this case, as shown in FIG. 3(0), the intensity of luminescence pulse decreases according to the increase of the DC voltage shared by the EL-layer 13 in a similar manner as in the case shown in FIG. 2, while the transition form (c) to (d) is absolutely irreversible, as is apparent by comparing (b) with (d), and a luminescent storage effect is presented. This phenomenon is not preferable for a luminescence display.

The irreversibility to avoid only when the DC voltage V 0r unidirectional field (voltage) is applied in such a manner that the luminous output takeout surface side of the EL-layer 13 is at a lower potential (negative potential) than the opposite surface side of the EL-layer 13.

In the device of this embodiment, the control of luminescence is performed by changing the unidirectional field through utilization of the resistance variation in the energy sensitive layer 15. The device has the superior advantage that it can be made to have an extremely high sensitivity in comparison with the device wherein the control is performed through utilization of an AC impedance variation in the ener gy sensitive layer.

According to the present invention, various luminescent display devices can be realized. For example, as intensified image of positive polarity can be displayed as the output light image for an incident infrared image on a device using a layer of an infrared-quenching photoconductive material such as CdS:Cu or Ga and the like as the energy sensitive layer 15, by radiating a biasing light to decrease the resistance of the layer 15 and subsequently radiating infrared rays as L, is superposed relationship to the biasing light. Generally, the greater the dark conductivity and the photoconductivity of an infrared ray photoconductive material, the smaller the infraredquenching sensitivity and the range of variation in conductivity of an infrared quenching photoconductive material. The device according to the present invention operates with the higher sensitivity the smaller the minimum conductivity of the layer 15 is. The high sensitivity can be attained by suitably selecting the relation between the resistances of the layer 15 and the layer 13, even when the variation in resistance of the layer 15 is small. Accordingly, an infrared positive image intensifying device having a high sensitivity can be obtained by using an infrared quenching photoconductive material having a low dark conductivity and photoconductivity and a high quenching sensitivity. Also, as shown in FIGS. 4 and 5, a magnetoresistance element 21, piezoresistance element 22 and the like may be used as an energy sensitive layer.

According to the principle of the present invention, various luminescent display devices can be realized wherein a DC (unidirectional field) control is performed by providing the elec trode 16 having various configurations, with the elimination of the layers 14 and 15. Also various electroluminescent display devices can be realized by providing means to directly or indirectly apply a unidirectional field to the EL-layer without using the electrode 16. Thus a number of embodiments can be carried out. This invention is intended to include such various embodiments.

As described above, according to this invention, a luminescence control by a DC electric field can thoroughly be performed reversibly without causing luminescent storage phenomenon.

We claim:

1. A method of operating an electroluminescent display device comprising an electroluminescent layer containing powder of electroluminescent phosphor, said method comprising establishing an AC electric field across said electroluminescent layer to excite the electroluminescent layer and establishing a variable unidirectional electric field across said electroluminescent layer is superposed relationship to said AC electric field to control the luminescence caused by the AC excitation of said electroluminescent layer, said unidirectional electric field being established in such a manner that the luminous output takeout surface side of the electroluminescent layer is of negative polarity against the opposite side, and the maximum value of said variable unidirectional electric field is not less than the peak value of said AC electric field, so that when the incident energy to said energy sensitive layer increases or decreases, the luminescent output of the electroluminescent layer decreases or increases, respectively.

2. A method of operating an electroluminescent display device comprising an electroluminescent layer containing powder of electroluminescent phosphor, a first electrode capable of transmitting light and disposed on the luminous output takeout surface of said electroluminescent layer, an energy sensitive layer disposed on the other surface of said electroluminescent layer and a second electrode capable of transmitting an incident energy and disposed on the energy sensitive layer, said method comprising applying an AC voltage between said first and second electrodes to excite said electroluminescent layer and applying a unidirectional voltage between said first and second electrodes in superposed relationship to said AC voltage to control the luminescence caused by the AC excitation of said electroluminescent layer, said unidirectional voltage being applied in such a manner that the first electrode is of negative polarity, the second electrode is of positive polarity and said unidirectional voltage is not less than the peak value of the AC voltage across the electroluminescent layer so that when the incident energy to said energy sensitive layer increases or decrease, the luminescent output of the electroluminescent layer decreases or increases, respectively.

3 A method of operating an electroluminescent display device as set forth in claim 2, wherein said electroluminescent layer contains glass enamel for binding together said powder of electroluminescent phosphor which is ZnSzCu.

4. A method of operating an electroluminescent display device as set forth in claim 3, wherein said glass enamel has a specific resistance of to 10 ohm-cm.

5. A method of operating an electroluminescent display device as set forth in claim 2, wherein said energy sensitive layer is composed of photoconductive material.

6. A method of operating an electroluminescent display device as set forth in claim 2, wherein said energy sensitive layer is composed of magnetoresistive material.

7. A method of operating an electroluminescent display device as set forth in claim 2, wherein said energy sensitive layer is composed of piezoelectric material.

8. A method of operating an electroluminescent display device as set forthin claim 2, wherein said energy sensitive layer is composed of infrared quenching photoconductive material.

9. A method of operating an electroluminescent display device comprising an electroluminescent layer containing powder of electroluminescent phosphor, a first electrode capable of transmitting light and disposed on the luminous output takeout surface of said electroluminescent layer, a resistive light-interception layer disposed on the other surface of said electroluminescent layer, an energy sensitive layer disposed on the light-interception layer and a second electrode capable of transmitting an incident energy and disposed on the energy sensitive layer, said method comprising applying an AC voltage between said first and second electrodes to excite said electroluminescent layer and applying a unidirectional voltage between said first and second electrodes in superposed relationship to said AC voltage to control the luminescence caused by the AC excitation of said electroluminescent layer, said unidirectional voltage being applied in such a manner that the first electrode is of negative polarity, the second electrode is of positive polarity, and said unidirectional voltage is not less than the peak value of the AC voltage across the electroluminescent layer so that when the incident energy to said energy sensitive layer increases or decreases, the luminescent output of said electroluminescent layer decreases or increases, respectively.

10. A method of operating an electroluminescent display device as set forth in claim 9, wherein said electroluminescent layer contains glass enamel for binding together said powder of electroluminescent phosphor which is ZnSzCu.

11. A method of operating an electroluminescent display device as set forth in claim 9, wherein said glass enamel has a specific resistance of 10 to 10 ohm-cm.

12. A method of operating an electroluminescent display device as set forth in claim 9, wherein said energy sensitive layer is composed of photoconductive material.

13. A method of operating an electroluminescent display device as set forth in claim 9, wherein said energy sensitive layer is composed of infrared quenching photoconductive material.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,604, 938 Dated September 14, 1971 Inventor(s) Tadao KOHASHI et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The Claim for Convention Priority is missing and should be listed as follows:

Sinned and sealed this 1L; th day of March 1972.

(SEAL) Attest:

EDWARD NJLL IICTHQR, JR. Attesting: Officer ROBERT GOTTSCHALK Commissioner of Patents -Japan, Patent Appln. N 12 989/66 filed March 1, l966 USCOMM-UC (wave-em

Claims

2. A method of operating an electroluminescent display device comprising an electroluminescent layer containing powder of electroluminescent phosphor, a first electrode capable of transmitting light and disposed on the luminous output takeout surface of said electroluminescent layer, an energy sensitive layer disposed on the other surface of said electroluminescent layer and a second electrode capable of transmitting an incident energy and disposed on the energy sensitive layer, said method comprising applying an AC voltage between said first and second electrodes to excite said electroluminescent layer and applying a unidirectional voltage between said first and second electrodes in superposed relationship to said AC voltage to control the luminescence caused by the AC excitation of said electroluminescent layer, said unidirectional voltage being applied in such a manner that the first electrode is of negative polarity, the second electrode is of positive polarity and said unidirectional voltage is not less than the peak value of the AC voltage across the electroluminescent layer so that when the incident energy to said energy sensitive layer increases or decrease, the luminescent output of the electroluminescent layer decreases or increases, respectively. 3 A method of operating an electroluminescent display device as set forth in claim 2, wherein said electroluminescent layer contains glass enamel for binding together said powder of electroluminescent phosphor which is ZnS:Cu.

4. A method of operating an electroluminescent display device as set forth in claim 3, wherein said glass enamel has a specific resistance of 106 to 109 ohm-cm.

8. A method of operating an electroluminescent dIsplay device as set forth in claim 2, wherein said energy sensitive layer is composed of infrared quenching photoconductive material.

10. A method of operating an electroluminescent display device as set forth in claim 9, wherein said electroluminescent layer contains glass enamel for binding together said powder of electroluminescent phosphor which is ZnS:Cu.

11. A method of operating an electroluminescent display device as set forth in claim 9, wherein said glass enamel has a specific resistance of 106 to 109 ohm-cm.