GB2054935A - Electro-Optic Display Cell - Google Patents

Abstract

A liquid crystal display cell to produce a display of coloured segments on a light background comprises substrates 1,2 embracing 4 a nematic liquid crystal medium with added dichroic dye, and having on their inner faces diffusing reflective coatings 5,9,10 such as Al on frosted glass. Apertures 6,7 in the front coating define the display segments. The front electrode 8, transparent eg In2O3, either covers the substrate over the coating and apertures, or local electrodes lie on an insulating transparent layer. The rear electrode may be the coating 9,10, possibly present only opposite the apertures, or In2O3 over an insulating transparent layer over the coating. The apertures contrast in colour against the background but disappear when energised, the sear coating then having the same appearance as the front. <IMAGE>

Description

SPECIFICATION Electro-Optic Display Cell The invention relates to a passive electro-optic display cell.

The previously known electro-optic display cells all possess disadvantages: U.S. Patent No. 3,833,287 discloses a cell of the above-mentioned type which provides only a clear display on a dark background, a so-called "negative display", but which cannot provide a dark display on a clear background, or "positive" display. In the publication 4'Molecular Crystals and Liquid Crystals" 1969, volume 8, pages 233 to 304, a cell of this type is disclosed which, again, permits a clear display on a dark background but not the reverse. Swiss Patent Application No.

1,272/78 discloses a cell which provides a dark display on a clear background, with the disadvantage of requiring a liquid crystal having a negative dielectric anisotropy. Such liquid crystals have generally an anisotropy AE which is lower, in absolute value, than that of the liquid crystals having a positive anisotropy. As a result, the voltages necessary for the control of such a cell are relatively high. Swiss Patent Application No.

3,135/78 discloses a cell which provides a dark display on a clear background while using a liquid crystal having a positive dielectric anisotropy.

However, this cell- is difficult to manufacture, because the layer of liquid crystal must have an accurately determined and uniform thickness.

The object of the present invention is to provide a display cell with a positive display, i.e. a dark display on a clear background, by means of a mixture of a liquid crystal having a positive dielectric anisotropy and of dichroic molecules, which does not require an especially high control voltage, and which is very simple to manufacture.

To this end, a display cell according to the invention has a front plate which carries an opaque reflecting and diffusing screen provided with apertures the shape of which determines the shape of the display elements, and a transparent counter electrode. The rear plate also carries, opposite to the apertures, reflecting and diffusing electrodes with the same appearance as the front plate screen. In the absence of an electric field between the plates, the display elements appear in colour on the clear background formed by the screen. With an electric field between the plates, a mixture of liquid crystal and dichroic molecules between the plates becomes transparent and the electrodes become visible. Since the appearance of the electrodes is similar to that of the screen, the corresponding display elements become invisible.

The invention will now be further described with reference to the accompanying drawings which illustrate, by way of example, several illustrative embodiments of the invention and modifications thereof. In the drawings: Fig. 1 is a sectional view of a passive electrooptic display cell representing a first illustrative embodiment of the invention, Fig. 2 is a sectional view of a passive electrooptic display cell representing a second embodiment; Fig. 3 is a partial sectional view of a modification of the embodiment of Fig. 1; Fig. 4 is a plan view of a further embodiment in the form of a quasi-analogue display cell; Fig. 5 is a partial transverse sectional view, on a larger scale, along the line V-V in Fig. 4; Fig. 6 is a partial longitudinal sectional view, again on a larger scale, along the line VI--VI in Fig. 4;; Fig. 7 is a partial sectional view of a modification of the cell of Figs. 4 to 6; and Fig. 8 is a partial sectional view of a modification of the cell of Fig. 2.

The display cell illustrated in Fig. 1 comprises two glass plates, namely a front plate 1 and a rear plate 2, separated from each other by a sealing frame 3 providing, with said plates, an enclosed space 4 imprisoning a mixture of liquid crystal having a positive dielectric anisotropy and dichroic molecules.

The front plate 1 carries an opaque screen 5, constituted by a layer of a reflecting material, such as aluminium, deposited on the inner face of the plate 1 ,which is frosted so that the screen 5 is also diffusing. The screen 5, which consequently has a very clear appearance, is provided with apertures such as 6 and 7 having the shape and dimensions of the individual elements of the pattern to be displayed, or display elements.

The term "display elements" as used herein is to be understood as referring to each of the portions of the display pattern that have to be rendered selectively visible or invisible, that is to say which must present a certain contrast with respect to the remainder of the display or, alternatively, be relatively undistinguishable therefrom. The several possible combinations of individual visible elements each permit the display of particular information. These elements can have the shape of segments in a classic digital display having seven segments, or have a different shape, in another type of display.

The front plate moreover carries, over its whole surface, a conductive and transparent layer 8 formed of indium oxide (In203), for example, deposited on the screen 5. This layer 8 serves as a counter-electrode.

The rear plate 2 carries control electrodes, two of which are shown at 9 and 10. These electrodes are constituted, similarly to the screen 5, by a reflecting layer, for example of aluminium, deposited on the inner surface of the plate 2 which is frosted. These electrodes are consequently reflecting and diffusing. They are disposed on the plate 2 opposite to the apertures 6 and 7, respectively, of the screen 5 so as to entirely cover these apertures when the cell is seen in plan view, and even to extend slightly beyond the boundaries thereof all around the apertures.

It is to be noted that, to simplify the drawing, the irregularities of the inner frosted surfaces of the plates 1 and 2 have not been shown in Figure 2, and they are also not shown in the later drawing figures, in spite of the fact that, on the scale of illustration used in these figures, they would normally be visible. The operation of the cell of Fig. 1 is as follows: In the absence of an applied electric field, as it is the case in zone 11 of Fig. 1, the molecules of the mixture have random orientations which are almost entirely aleatory. Consequently, the light passing through the aperture 6 is mostly absorbed by the dichroic molecules and this zone 11, as is the case with the other non-activated zones, that is to say zones not subjected to an electric field, appear as being colored on the clear background constituted by the screen 5.The color of these zones depends upon the nature of the dichroic molecules and from the concentration thereof in the mixture. When an electric fields is created, as it is the case in the display zone 12 in Fig. 1, by the application of a voltage between an electrode such as 10 and the counter-electrode 8, the molecules of the mixture become aligned perpendicularly to the plane of the cell, so that the zone 12 becomes almost transparent.

Consequently, the electrode 10 which is reflecting and diffusing becomes visible through the aperture 7 and, as it has the same appearance as the screen 5, the corresponding display element is erased and becomes as clear as the screen 5.

Thus the display is positive and has a control characteristic referred to as "reversed" with the visible display elements (appearing in color on a clear background) being those which are not subjected to an electric field.

It will be seen that, in this embodiment, the electrodes such as 9 and 10 have an electrical function, for applying the fields in conjunction with the counter-electrode 8, and an optical function since they are visible when the field is applied. This is why they are, like the screen 5, in the form of an aluminium layer deposited on the surface, previously frosted, of the plate 2. In practice, a layer of aluminium is deposited over the whole surface of the plate 2. This layer is then engraved, employing known photo-lithographic techniques, so as to keep only the electrodes and the associated tracks which connect them with the outside of the cell.

It is to be noted that the only visible portions of the mixture of liquid crystal and dichroic molecules are those which are situated behind the apertures such as 6 and 7. Consequently, it is only the shape of these apertures which determines the shape of the elements of the pattern to be displayed when, in the absence of an electric field, they are visible. The electrodes such as 9 and 10 need not necessarily have the same shape as the apertures. It is sufficient that, when the cell is seen in plan view, the electrodes should at least cover the corresponding apertures such as 6 and 7.

The fact that the electrodes are of larger dimensions than the apertures has no influence on the appearance of the display. The tolerances which have to be maintained, so far as the reciprocal alignment of the two plates is concerned, can consequently be widened, which is a considerable advantage in the manufacture of these cells. Moreover, the connecting tracks of the electrodes are always hidden by the screen 5.

This results in a greater freedom in choosing the shape of these connecting tracks which can, contrary to that which occurs in known cells, pass without inconvenience opposite the counterelectrode. This is another advantage in this type of cell. The present cells moreover have the advantages of simplicity of production, the provision of a positive display, in color on a clear background, and the need for only a relatively very low control voltage as the liquid crystal they contain has a positive dielectric anisotropy.

The mixture of the liquid crystal and dichroic molecules is moreover protected from the light by the screen 5 except, obviously, opposite to the apertures such as 6 and 7, and this constitutes an important supplementary advantage. Dichroic molecules which have a tendency to be destroyed by exposure to the light can even be used in these cells, since the Brownian movement of the molecules ensures a constant mixing of those molecules which have been exposed to the light with those which have been protected from the light by the screen 5. This results in a considerable increase in the lifetime of the display.

It is also to be noted that the cell which has been described above does not have to be provided with a polarizer for its operation, which still further increases the simplicity of manufacture.

This cell is moreover particularly suitable for the production of displays of large dimensions. As a matter of fact it is necessary, in such displays, to place spacers between the front and rear plates to ensure that the distance which separates them is constant. The presence of the screen 5, which is opaque, permits these spacers to be placed in any position in the cell except, obviously, opposite to the apertures provided in this screen, without the appearance of the display being modified as a result.

The display cell illustrated in Fig. 2 comprises, similarly to the cell of Fig. 1, a front plate 1 carrying a reflecting and diffusing screen 5 provided with apertures such as 6 and 7, the shape of which determines that of the elements of the pattern to be displayed. Electrodes 13 and 14 are arranged opposite apertures 6 and 7, respectively, on an insulating and transparent layer 15 made of sintered amorphous glass, for example. This layer is intended to prevent short circuiting of these electrodes between themselves and with the screen 5.

The rear plate 2 carries a counter-electrode 16, which is constituted by a continuous layer of a conducting and reflecting material such as aluminium deposited on the inner face of the plate 2 which is frosted. This counter-electrode 16, which is reflecting and diffusing, has the same shape as the screen 5.

The space between the plates 1 and 2 is filled, as in the case of Fig. 1, with a mixture of liquid crystal and dichroic molecules. The operation of this cell is exactly the same as that of the cell shown in Fig. 1.

In zones such as 11, not subjected to an electric field, the molecules of the mixture have almost aleatory orientations and the dichroic molecules absorb the light traversing the apertures such as 6, which thus appear in color on the clear background constituted by the screen 5.

In zones such as 12, where an electric fields is applied, the molecules of the mixture are aligned perpendicularly to the 'plates 1 and 2. The screen constituted by the counter-electrode 16 becomes visible through apertures such as 7 and the corresponding display element becomes invisible.

The manufacture of a cell according to this second embodiment is still further facilitated with respect to that of a cell according to the first embodiment by the fact that the control electrodes such as 13 and 14 are directly deposited on the apertures 6 and 7 provided in the screen 5; as the counter-electrode 16 covers the whole rear plate 2, there is no alignment problem in the relative positioning of the two plates. In the case of the cell of Fig. 1, if the plates 1 and 2 were to be shifted one with respect to the other, the result could be that one or the other edge of one of the display elements would remain visible when this element is erased. With the cell according to Fig. 2, this possibility is excluded.

A modification of the embodiment of Fig. 1 is illustrated in Fig. 3 which shows only the rear plate 2 of the cell. In this modification, a screen 17 which is reflecting and diffusing, similar to the screen 5 of Fig. 1, is arranged over the whole surface of the plate 2. It is covered by an insulating transparent layer 18. The control electrodes such as 19 and 20, deposited on this insulating layer, are constituted by a conducting transparent material such as In203.

Consequently the optical functions, fulfilled by the reflecting and diffusing screen 17, and the electrical functions, fulfilled by the electrodes 1 9 and 20, are now distinct.

The cells as described above are provided with no alignment layer, contrary to most of the liquid crystal cells previously known. This is a supplementary advantage of these cells which are thus simpler and cheaper to manufacture.

The absence of an alignment layer, however, results in a slight reduction of the contrast of these cells. To improve this situation, it is possible to provide on each of the plates a known homogenous planar alignment layer. The molecules of the mixture in contact with these layers will then take orientations which, while being independent with respect to each other, are all parallel to these plates. The contrast of such cells is increased and their light transmission characteristic as a function of the applied electrical voltage exhibits a threshold which facilitates their control by multiplexing.

It is also possible to add to the mixture of liquid crystal and dichroic molecules an optically active component as a chiralic compound. In the absence of alignment layers a contrast as marked as in the previous case can be obtained. A cell having this optically active component can also be provided with homeotropic alignment layers of the molecules of the liquid crystal. In this case, the necessary voltage for erasing the display elements is lower than in the previous cases.

The cell represented in Figs. 4 to 6 is intended to provide a quasi-analogue display, that is to say a display where, for instance, a reference mark which plays the role of a hand of a conventional measuring apparatus, is moved by discrete steps opposite a stationary scale. This reference mark can be constituted by a sole display element the color of which is contrasted with respect to that of the bottom or by an assembly of display elements also of contrasted color, juxtaposed so as to constitute a continuous strip extending from the origin of the scale to the position corresponding to the value to be displayed.

This cell comprises two plates, a front plate 21 and a rear plate 22, separated from each other by a frame 23 providing an enclosed space 24 containing a mixture of liquid crystal and dichroic molecules. The rear plate 22 carries a set of electrodes such as 26 and 26 which are elongated, closely juxtaposed and realized similarly to the electrodes 9 and 10 of Fig. 1, that is to say made of a reflecting material, such as aluminium, deposited on the inner and previously frosted face of the plate 22. The front plate 21 carries a diffusing and reflecting screen 27 of the same nature as the screen 5 of Figs. 1 and 2, provided with an aperture 28. A counterelectrode 29, made of a conductive transparent material such as In203, covers the screen 27 and the aperture 28.

The screen 27 is also provided with rectangular apertures such as 30 and with apertures having the shape of data or of any symbols such as 31.

In the display zones which are not subjected to an electric field, the mixture situated between the plates strongly absorbs the light which reaches it through the apertures 28. Consequently, these zones appear as colored on the clear background of the screen 27. In the zones subjected to a field, on the contrary, the molecules of the mixture are aligned perpendicularly to the planes of the plates 21 and 22. The light passing through the aperture 28 reaches electrodes such as 25 and 26, which reflect it. But, as the dichroic molecules of the mixture nevertheless absorb a portion of the light, these zones appear slightly more colored than the screen 27. The totality of the zone intended to change in appearance is consequently always visible, which improves the general appearance of the display.

The zones situated under the apertures 30 and 31, being never subjected to an electric field, are always absorbent. The graduated scale constituted by the graduations 30 and the data 31 consequently always appears in color.

It is obvious that such a display can be realized in a way similar to that of Figs. 2 and 3, that is to say with transparent electrodes deposited on the front plate, an insulating film being interposed between them and the screen 27, or with a reflecting and diffusing screen covering the whole rear plate 22, this screen being covered with an insulating film on which the electrodes, which in this case are also transparent, are deposited.

Likewise, the aperture 28 can, if necessary, have a different shape such as the shape of a circular arc to look more like the dial of a conventional measuring apparatus.

The electrodes such as 25 and 26 of the cell shown in Figs. 4 to 6 must be closely juxtaposed to prevent undesired colored lines from appearing between the zones corresponding to these electrodes when they are subjected to an electric field. To overcome difficulties of manufacture due to this close juxtaposition, the rear plate of the cell can be produced according to the modification of Fig. 7: In this modification, alternate electrodes such as 25 and 26 are respectively deposited directly on the rear plate 22 and on an insulating layer 32.

One can thus extend the electrodes to slightly overlap one another, as seen in plan view, which overcomes the risk of having, in the display zone which is weakly contrasted, strongly colored lines marking the divisions between the electrodes.

Fig. 8 is a partial sectional view of a display cell of the same type as that of Fig. 2 but in which, however, there is provided at 33 a supplementary aperture, of rectangular shape-for instance, in the screen 5. This aperture is covered,-as is the rest of the front plate 1, by the insulating layer 1 5.

Transparent electrodes such as 34 and 35, made of In203 for example, are deposited on this layer 1 5. The previously frosted inner face of the rear plate 2 carries, opposite to these electrodes 34 and 35, a transparent counter-electrode 36 also made of In203. These electrodes and this counterelectrode are absolutely similar to those of a conventional display; especially, the shape of these electrodes, as seen in plan view, determines the shape of the display elements. The molecules of the mixture situated in the zone surrounding the display elements delimited by the electrodes 34 and 35 are never subjected to an electric field.

Consequently, they have an aleatory orientation and the light which reaches them through the aperture 33 is absorbed. As a result this zone, which constitutes the background of this portion of the display, has a strongly colored appearance.

The same applies to the display elements corresponding to electrodes such as 34, which have the same voltage as the counter-electrode 36. Consequently, these display elements are not distinguished from the background.

The molecules of the mixture situated behind an electrode such as 35, to which a voltage is applied with respect to the counter-electrode 36, are, on the contrary, aligned perpendicularly to the plates 1 and 2 so that they do not substantially absorb the light which reaches them.

This light is diffused by the rear plate 2, which is frosted, so that these zones appear clear on the colored background. The zone delimited by the aperture 33 consequently permits a "negative" display of information, that is to say a clear display on a colored background. It will thus be seen that one can easily produce a cell comprising both a positive display zone, as has been described for instance with reference to Fig. 2, and a negative display zone. Such a cell permits the contrasting display of information of different types.

The residual contrast shown by the zones subject to an electric field, which has been used in the pseudo-analogue display of Figs. 4 to 6, becomes troublesome when it is desired to produce, with any one of the cells described above, a positive display in which the elements of the display pattern must be absolutely invisible in their erased state, such as a digital or alphanumeric display.

Several solutions can be employed to compensate for this residual contrast: For example, the appearance of the screen carried by the front plate can be modified by providing it with a more or less dense pattern of very fine holes, so as to render visible a small portion of the mixture situated between the display zones. If the density of this pattern and the ratio between the total area of these holes and the total surface area of the screen are conveniently chosen, the screen can have a color similar to that of the erased display elements, which then merge perfectly with the screen.

One can also deposit on the inner face of the front plate, between this plate and the screen, one or more interferential layers which are interrupted in the region of the openings in the screen and are chosen so as to give to the screen a color such that, in a different manner, the display elements which are erased are no longer visible. One can also replace the interferential layers by a layer of polymer containing molecules of the same type as the dichroic molecules which are present in the mixture situated between the plates, with a concentration such that the color of this layer of polymer is the same as that of the erased display elements. One can also color the body of the front plate, outside the display zones, by a method such as disclosed in U.S. Patent No. 4,057,408.

It is obvious that all the cells which have been described herein can be adapted to multiplex control. To this end, it is sufficient to separate the counter-electrode into several partial counterelectrodes and to connect electrically each of the electrodes situated opposite one partial counterelectrode to the corresponding electrodes situated opposite the other partial counterelectrodes. To prevent the partial counterelectrodes from being short-circuited by the screen situated on the same plate as they are, as in the embodiments of Figs. 1 and 4 to 6, a transparent insulating layer must be inserted between this screen and these counterelectrodes.

Finally, in all the cells which have been described above, one could cover all the surfaces in contact with the liquid crystal with a thin transparent and insulating layer of silicium oxide, for example, to prevent any possible contamination of the liquid crystal by the material with which, but for this layer, it would be in contact.

Claims (16)

Claims

1. An electro-optic display cell for displaying information elements, comprising: a front plate and a rear plate each having an inner face and an outer face, with the inner faces arranged substantially parallel and opposite to each other; a mixture of nematic liquid crystal having a positive dielectric anisotropy and dichroic molecules, said mixture being imprisoned between the inner faces of said plates; an array of display electrodes for individually displaying each of said elements and at least one counter-electrode, said display electrodes and said counter-electrode being respectively located on the inner faces of said plates and each element being displayed or not in response to the application or to the absence of an electrical field between the corresponding display electrodes and said counter-electrode;; a first reflecting and diffusing screen carried by said front plate and provided with at least one aperture facing said display electrodes; and a second reflecting and diffusing screen carried by said rear plate and facing at least said aperture.

2. The cell of claim 1, wherein: said display electrodes and said second screen are formed by a conductive reflecting and diffusing layer deposited on the inner face of said rear plate; and said counter-electrode is formed by a transparent conductive layer deposited on said first screen, at least opposite to said aperture.

3. The cell of claim 1, wherein: said counter-electrode and said second screen are formed by a conductive reflecting and diffusing layer deposited on the inner face of said rear plate at least opposite to said aperture; and said display electrodes are formed by a transparent conductive layer deposited at least opposite to said aperture, on an insulating layer covering said first screen.

4. The cell of claim 1, wherein said first screen is provided with a plurality of apertures respectively facing said display electrodes, the shape of said apertures determining the shape of said elements.

5. The cell of claim 1, further comprising means for providing said first screen with an appearance similar to the appearance of said elements when they are not displayed.

6. The cell of claim 5, wherein said means is constituted by a plurality of smail holes provided in said first screen.

7. The cell of claim 5, wherein said means comprises at least one interferential layer located between said front plate and said first screen, said layer being provided with at least one aperture facing the aperture of said first screen.

8. The cell of claim 5, wherein said means is a coloration of the body of said front plate, outside at least a zone thereof facing the aperture of said first screen.

9. The cell of claim 5, wherein said means comprises a colored layer.

10. The cell of claim 9, wherein said colored layer is constituted by a film of polymer including molecules similar to said dichroic molecules.

11. The cell of claim 1, wherein each of said plates further carries a planar alignment layer for the molecules of said liquid crystal.

12. The cell of claim 1, wherein said mixture further comprises an optically active component.

13. The cell of claim 12, wherein each of said plates further carries a planar alignment layer for the molecules of said liquid crystal.

14. The cell of claim 12, wherein each of said plates further carries an homeotropic alignment layer for the molecules of said liquid crystal.

1 5. The cell of claim 1, wherein said first screen further has at least one aperture disposed out of line with said electrodes.

16. The cell of claim 1, wherein said first screen is provided with a second aperture and further comprising a second array of electrodes arranged opposite to said second aperture so as to provide a free space, when seen in plan view, around said second array of electrodes.

1 7. An electro-optic display cell constructed and arranged substantially as herein particularly described with reference to Fig.1, Fig.2, Fig. 3, Figs. 4 to 6, Fig. 7 or Fig. 8, of the accompanying drawings.