Electro-luminescent devices

Dec. 29, 1959

2,919,366

D. H. MASH

ELECTRO-LUMINESCENT DEVICES

Filed Oct. 22, 1958

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&gt; 10 f //WENTOR

Derek Hubert Mash

BY

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ATTORNEY

United States Patent Office

2,919,366

Patented Dec. 29, 1959

2,919,366

ELECTRO-LUMINESCENT DEVICES

Derek Hubert Mash, London, England, assignor to Thorn Electrical Industries Limited, London, England

Application October 22, 1958, Serial No. 768,976

Claims priority, application Great Britain October 23, 1957

Claims. (Cl. 313—108)

The present invention relates to electro-luminescent devices of the kind in which light is emitted only over predetermined parts of the front surface of the device. Examples are luminous signs and other devices for displaying legends or diagrams, the parts from which light emission is required being hereinafter referred to as a pattern.

In one known lamp of this kind, the electro-luminescent phosphor is applied only to that part of the front of the lamp from which emission of light is required. This suffers from the disadvantage that unless the remainder of the front of the lamp is covered with a non-luminescent substance of exactly the same self-colour as the phosphor, the pattern can be seen even when the lamp is switched off. A further disadvantage is that electric current is passed by the lamp over the whole area and not only over the area of the pattern.

In another known lamp of the kind set forth, one of the electrodes, usually the back, opaque electrode, is formed in the shape of the pattern. However, when a number of separated patterns, such a individual letters, have to be illuminated, individual contacts have to be made to each separate pattern.

If conducting leads are taken to these individual patterns across the back of the lamp, light is emitted in the area traversed by the leads and to overcome this an insulating layer has to be applied between the lead and the back of the lamp. These operations are time-consuming and difficult to apply in quantity production.

The present invention has for its principal object to provide an electro-luminescent lamp of the kind set forth in which the disadvantages referred to are substantially reduced or eliminated.

Another object of the invention is to provide a method of making an electro-luminescent lamp of the kind set forth in which little or no individual attention is required to be paid to separated parts of the pattern, and which, therefore, is relatively inexpensive.

According to the present invention, an electro-luminescent device of the kind set forth comprises, in the order named proceeding from the front of the device, a transparent electrically conducting layer, a layer of electroluminescent phosphor, insulating material extending over the areas of the phosphor other than the pattern from which light emission is required, and a second electrically conducting layer extending over the said insulating material and the pattern. The last-named conducting layer may be opaque and constitutes the back electrode. A reflecting layer may be provided between the phosphor and the insulating material in the said areas and also between the phosphor and the last-named conducting layer over the pattern.

The invention also provides a method of making an electro-luminescent lamp of the kind set forth comprising the steps of applying to a transparent support a first, transparent, electrically conducting layer, applying a second electro-luminescent layer or series of layers over the whole of the first layer, applying over parts of the surface of the second layer, constituting a pattern from which light emission is desired, a substance which is water-repellent or which becomes water-repellent after drying, applying to the surface thus formed a coating of plastic in aqueous suspension or solution or as an emulsion in water, this coating being repelled from the regions of the pattern, drying the said coating, removing the said substance, and applying a third electrically conducting layer over the whole surface so formed. Before applying the last-named layer, the water-repellent material when in the form of a wax or grease may be removed by wiping or brushing after the plastic suspension or emulsion has dried. Alternatively the water-repellent material may be removed by means of a solvent which does not affect the dried plastic.

The invention will be described by way of example, with reference to the accompanying drawing in which:

Fig. 1 is a much enlarged view in cross-section of part of one embodiment of the invention, and

Figs. 2 to 5 show in much enlarged cross-section four stages in the manufacture according to .another form of the invention.

Referring to Fig. 1, a transparent base plate 10 of glass or plastic has its back surface coated with a transparent electrically conducting layer 11 over which is applied a layer 12 of electro-luminescent material. A stencil 13 of insulating material, such as paper or sheet plastic, is cut to the shape of the area from which no light emission is required, and this stencil is stuck to the layer 12. For instance the stencil may be coated with rubber which can be heat-sealed under pressure to the electro-luminescent layer 12. A back electrode 14, for example in the form of an electrically conducting paint, is then applied over the stencil, this electrode extending over the material of the stencil and also over the spaces therein. If desired, a light reflecting layer may be provided over the whole surface of the electro-luminescent layer 12 before the stencil 13 is applied.

This method is not very suitable when symbols such as letters of the alphabet, or other complicated shapes, are required and when only small quantities of the lamps are needed, because the cost of tools for punching the stencils is usually unacceptably high.

Another method makes use of a silk-screening printing process, the silk-screen being masked over those areas from which light emission is required, that is the areas of the pattern formed by the spaces between the stencil 13 of Fig. 1. A suitable viscous material, adapted when dry to form an insulating layer, is forced through the screen on to the electro-luminescent layer, thus forming the layer 13 in Fig. 1. Usually it is not possible to produce a thick enough layer in one screening operation and the process has, therefore, to be repeated, with careful registration, until a thick enough layer of insulating material has been built up.

A preferred process according to the invention will now be described with reference to Figs. 2 to 5. This also uses a silk-screen printing process, but in this case the screen is masked over the areas from which no light emission is required. The layers 11 and 12 (and if desired a reflecting layer 15) are applied to the glass plate 10 as described with reference to Fig. 1. The pattern 16 is then printed as shown in Fig. 2 upon the electroluminescent layer 12, or upon the reflecting layer 15 when one is provided, using a material which has, or develops upon drying, a water-repelling property. Such a material may be a grease or wax or a solution or dispersion of grease or wax. The required insulating coating over the areas fram which no light emission is required is then produced by brushing or applying by other

3,919,366 means an aqueous solution or emulsion of a suitable plastic over the whole surface, the solution or emulsion being repelled from the areas which have been rendered water-repellent.

In the remaining areas the surface is wetted by the aqueous emulsion or solution and on drying an insulating layer 17 of the required thickness remains as shown in Fig. 3. A single application of the material is thus sufficient.

A suitable plastic emulsion for the layer 17 is an aqueous emulsion of a plasticised styrene-methacrylate co-polymer known as “Stymultex Beta 4D.” However, many other materials may be used such as poly-vinyl acetate emulsion, vinyl acetate-methyl methacrylate copolymer emulsion, poly-butyl methacrylate emulsion, or an aqueous solution of poly-vinyl alcohol.

After the insulating material 17 has dried, the grease or wax 16 may be wiped off, with or without warming to produce the result shown in Fig. 4, and a back electrode 18 is then applied as before.

A preferred variation of the method just described is to use as the silk-screening to form the layer 16 a solution of a plastic which is either water-repellent on drying or can be made so by the addition of a silicone to the solution. The plastic solution or emulsion which is used subsequently to produce the insulating areas 17 is then so chosen that the water-repellent material can be removed with the aid of a solvent which does not affect the material on the insulating areas.

In one example the silk-screening ink 16 is an ethyl cellulose solution with added silicone and the material 17 to form the insulating areas is an aqueous emulsion of the material “Stymultex Beta 4D” hereinbefore referred to. The ethyl cellulose solution is silk-screened on to the electro-luminescent layer 12 in the normal manner, and when dry it is carefully flooded or brushed with the “stymultex” solution. This is allowed to dry and a further coat applied if desired. After drying, the ethyl cellulose 16 is removed by dipping or washing with alcohol, which does not affect the “Stymultex” film 17. The back electrode material 18 is then applied in the form of a conducting paint over the whole area of the plate. In this way clean-edged letters or numbers can be obtained in the finished device.