Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Patent
1986-08-13
1989-02-28
Moore, David K.
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
313509, 34082581, H05B 3322
Patent
active
048088803
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a memory effect display means comprising thin electroluminescent and photoconductive films.
The interest of combining the properties of electroluminescent bodies and photoconductive bodies has long been recognised.
It is based on the possibility of obtaining a memory effect, which can be diagrammatically explained in the following way. A display means essentially comprises an electroluminescent layer (or a stack of layers comprising an electroluminescent layer which can be called a "electroluminescent structure") placed between two systems of electrodes, which are connected to driving circuits. A photoconductive layer can be arranged in series with the electroluminescent structure, so as to establish, under the effect of an optical excitation, an electrical conduction between certain of these electrodes. This conduction leads to the establishment of appropriate electrical potentials and the appearance of an excitation of the electroluminescent layer, which then emits radiation. The latter is mainly used for the display of information, but it also makes it possible to maintain the conduction of the photoconductive layer, even after the end of optical addressing. Thus, there is auto-maintenance or, in other words, a memory effect.
In an article entitled "The use of photoconductive CdS:Cu-cl films in a laser-addressed electroluminescent display screen" published in the journal Thin Solid Films, 41, 1977, pp 151-160, by G. OLIVE et al was described a device of this type and its structure is diagrammatically shown in the attached FIG. 1. On an insulating, transparent support 10 are deposited opaque electrodes (Au) 12 and 14, a photoconductive material layer 16 and a transparent electrode 18 connected to electrode 12. An electroluminescent material 20 covers electrode 18. A transparent electrode 22 is connected to electrodes 12 and 18 and an opaque electrode (Al) 24 is placed in the electroluminescent material, in such a way that the latter is inserted between on the one hand electrode 24 and on the other the two electrodes 18, 22. Laser 26 is able to emit a light beam 28, which strikes the photoconductive material 16 in the area located between electrodes 12 and 14.
This device functions in the following way. In the inoperative state, an a.c. voltage is applied to electrode 24 and electrode 14, but laser 26 is stopped. The photoconductive material 16 is not optically excited and its behaves like an insulator. Thus, electrodes 14 and 12 are electrically insulated from one another and the potential of electrode 12 floats, as does that of electrodes 18 and 22. The electroluminescent material is not excited and consequently emits no light.
Excitation is optically controlled by laser 26, which emits a beam 28, which strikes photoconductor 16 between electrodes 12 and 14 making said zone electrically conductive. The two electrodes 12 and 14 are then connected by a conductor channel (symbolically indicated by arrow 36) and the potential of electrodes 12, 18 and 22 is established at the value fixed by the potential applied to electrode 14. A potential difference then appears between electrode 24 on the one hand and electrodes 18 and 22 on the other. This leads to the appearance of an electric field and the excitation of the electroluminescent material. The radiation 30 emitted by the electroluminescent material towards the front of the device makes it possible for information to be displayed by an observer at 32. With respect to the rear part 34 of the emitted radiation, it excites the photoconductor and maintains the photoconduction thereof. The laser 26 can then be placed in the inoperative state without the electroluminescence stopping, giving a memory effect. The display stops on eliminating the electrical excitation.
A somewhat identical device was described by A. H. KITAI et al in an article entitled "Hysteretic Thin-Film EL Devices Utilizing Optical Coupling of EL Output to a Series Photoconductor", published in the journal SID 84 DIGEST, pp 255-256.
The complexity of the prior a
REFERENCES:
patent: 2931915 (1960-04-01), Jay, Jr.
patent: 3358185 (1967-12-01), Lally
patent: 3711719 (1973-01-01), Szepesi
patent: 4035774 (1977-07-01), Chang
"The Use of Photoconductive CdS:Cu,Cl Thin Films in a Laser Addressed Electroluminescent Display Screen" Olive et al., Thin Solid Films 41(1977), 151-160.
"Hysteretic Thin-Film EL Devices Utilizing Optical Coupling of EL Output to a Series Photoconductor" Kitai et al., SID 84 Digest pp. 255-256.
"Device Characterization of an Electron-Beam-Switched Thin Film ZnS:Mn Electroluminescent Faceplate" Sahni et al., IEEE Transactions on Electron Devices, vol. ED 28 No. 10, Jun. 81, p. 708.
Centre National d'Etudes des Telecommunications
Moore David K.
O'Shea Sandra L.
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