GB2023866A

GB2023866A - Guest-host liquid crystal display

Info

Publication number: GB2023866A
Application number: GB7921268A
Authority: GB
Original assignee: Ebauches SA
Current assignee: Ebauches SA
Priority date: 1978-06-22
Filing date: 1979-06-19
Publication date: 1980-01-03
Also published as: GB2023866B; CH624492A5; DE2902126A1; JPS5529897A

Abstract

In display zones 9 of an electro- optic cell, a mixture 4 of dichroic material in a positive dielectric anisotropy nematic liquid crystal is in contact with a non-homogeneous planar alignment layer 7, 10 on one or both cell plates 1, 2 whereby it appears optically absorbing in the unsuited state. There is thus a contrast with other regions, where both plates carry homotropic alignment layers 8, 11, which is removed by a field E between associated electrodes 5, 6. The latter are absent from any zones 9 intended for permanent display, and other areas of the cell may contain zones which require activation to produce contrast. The layers 7, 10 may be of silicon or tin oxide deposited at normal incidence; if layer 7 is absent, layer 8 is continuous. <IMAGE>

Description

SPECIFICATION Passive electro-optic display cell The present invention relates to a passive electrooptic display cell comprising two plates at least one of which is transparent, imprisoning between them a mixture of nematic liquid crystal having a positive dielectric anisotropy and of dichioic molecules, the innerfaces of the said plates carrying control electrodes and alignment layers.

Such devices have been disclosed especially by Heilmeier (Applied Physics Letters, vol. 13, 1968, pages 91 and 92), Taylor (US Patent No 3.883.287), Nagasaki (US Patent No 3.900.248). In such devices, using a liquid crystal having a positive dielectric anisotropy, the colour of the area situated outside the display zones is defined by the absorption bands of the dichroic molecules of the mixture and of all the other optical compounds which can be associated to the cell such as polarizers, filters, reflectors.

etc..., while the color of the display zones (segments) is defined only by the absorption bands of these other optical compounds. Therefore, the area situated outside the display zones can be only darker than the segments, which is generally not very appreciated.

For some of these cells (Heilmeier, Nagasaki), a display in dark on a clear ground could be realized while using a liquid crystal having a negative dielectric anisotropy, associated with homeotropous alignment layers on the two plates of the cell.

However, this type of liquid crystal shows some drawbacks, especially that of necessitating higher control tensions.

The purpose of the present invention is to remove these drawbacks, the display appearing in dark on a clear ground, by means of a liquid crystal having a positive dielectric anisotropy, with the advantages thereby produced while simplifying the determination of the topology of the control electrodes of the cell. Moreover, the invention permits to realize display zones of permanent information, in the same plane as the segments, without supplementary operation and without increasing the consumption of the cell.

The drawing shows, by way of example, two embodiments of the invention.

Figures 1 and 2 are sectional views of two embodiments of passive electro-optic display cells.

It is to be noted that, in these two figures, the thickness of the elements of the cells has been exaggerated for increasing the clearness of the drawing.

The display cell represented in Figure 1 is constituted by two plates made of transparent glass 1 and 2 separated by a frame 3, imprisoning a mixture 4 consisting of a nematic liquid crystal having a positive dielectric anisotropy and of dichroic molecules.

The front plate 1 carries, on its inner face, transpa- rent control electrodes 5 made of SnO2 for instance, while the rear plate 2 carries, on its inner face, a control electrode 6 also made of SnO2. The plate 1 is moreover innerly coated with a non homogeneous planar alignment layer 7 of the molecules of the mixture 4. This layer 7 is itself covered with a homeotropous alignment layer 8 interrupted opposite the display zones (segments) designated by 9.

The rear plate 2 of the cell is inner coated with a non homogeneous planar alignment layer 10 itself covered with a homeotropous alignment layer 11 interrupted opposite the display zones 9.

In the area of the cell situated outside the display zones 9, the molecules of the mixture have a homeotropous structure, that is to say are perpendi cular to the plane of the cell, in the whole thickness of the layer of the mixture 4. The incident light is thus not absorbed and, consequently, the said area of the cell is transparent, having the appearance of the ground, especially its color.

In the display zones (segments) not subjected to the action of an electric field, such as the zone 9 situated at the left side of Figure 1, the molecules of the mixture 4 have, under the effect of the alignment layers 7 and 10, a non loonoogeneois planar structore which is absorbent, while the surrounding area is transparent.

The application of an electric field E to those display zoloes that leave to reiooain invisible, such as zone 9 situated at the right side of Figtiro 1, has the effect or orielotating the molecules of tho mixture 4 according to a lioiooeotropotls structure, tloat is to say perpendicular to the plane of the cell.The zones which are thus activate dO no more absorb the light and are "cancelled" or "effaced'. becoming transparent and confotioding themselves with the surroundinn area which is also transparent. TIle display is thtis effected by reversed control. This cell works in a transmission motie.

The display cell represented in Figure 2 differs from that of Figure 1 by tho fact that the non holologelleous planar alignment layer 7 is eliminated and that the homeotropous alignment 8 entirely covers the plate 1, 111(0 by the fact that a diffuser 12 is interposed between the rear plate 2, respectively the electrode 6 and the layer 10. The cell tlotis works in a reflection node.In the display zones 9 which are not subjected to the action of toll electric field, the planar stricture is replaced toy a mixed structure, the molecules situated in the vicinity of tlon plate 1 having a homeotropous alignment and those situated in the vicinity of the plate 2 a planar alignment.

This structure is also absorbent.

The aliglllmellt layers used can be realized in several ioianners: Tloe homeotropous alignment can be obtained by means of layers of alumina, of magnesium fluoride (sec German Patent Application No. 23 30 909 of the firm SIEMENS) or still of magnesium oxide, which can be deposited in any known way, for instance by cathodic sputtering, toy depositiolo in vapor phase or by evaporation tind'.r vacuum, according to an incidence substantially perpendicular to the subs trade. One can still use a surfactant such as, for instance, the lecithin.

As concerns the non homogeneous planar alignmint, it can be oblained by means of layers of silicium oxide or of SnO2, for instance, deposited according to the same techniques as those hereabove disclosed.

If, instead of using non homogeneous planar alignment layers, one deposits homogeneous planar alignment layers, the operation of the cell remains the same, without improvement of the contrast or of the appearance. This is due to the fact that the several components of the light are unequally absorbed.

It must be understood that "planar" does not mean that the axes of the molecules are exactly parallel to the plane of the cell; likewise, "homeotropous" does not mean that these axes are exactly perpendicular to this plane. The parallelism and the perpendicularlity are only limit cases.

It is to be noted that the present arrangement eliminates the usual constraints to which is subjected the determination of the topology of the electrodes. As a matter of fact, where the projection of the pattern of an electrode of the front plate on the rear plate crosses the pattern of an electrode of this rear plate, there is created an electric field which tends to orientate the molecules of the mixture according to a homeotropous structure, which is precisely the structure of the mixture in the area of the cell situated outside the display zones. Consequently, the crossing points do not risk to be untimely visible. The realisation of the pattern of the nets of the electrodes carried by the two plates is thereby greatly facilitated. Thus, one of the plates could be entirely covered with a conductive layer.

This improvement can be considered as an important simplification.

At last, it is to be noted that one can provide, in the present device, permanent display zones, that is to say zones of the same construction as the display zones but without control electrodes which, consequently, will steadily remain contrasted. Such zones can, for instance, constitute a frame surrounding the whole display zones.

Likewise, the present arrangement can be easily combined with the conventional arrangement in which the segments to be displayed must be activated. As a matter of fact, one could imagine a display cell comprising, in the same space, two display areas, one realized according to the present arrangement and the other one such as disclosed by Nagasaki, for instance.

Claims

1. Passive electro-optic display cell comprising two plates at least one of which is transparent, imprisoning between them a mixture of nematic liquid crystal having a positive dielectric anisotropy and of dichroic molecules, the inner faces of the said plates carrying control electrodes and alignment layers, characterized by the fact that, in the area situated outside the display zones, the two plates are coated with homeotropous alignment layers inducing in the layer of the mixture a homeotropous structure, consequently transparent, and by the fact that, in the display zones, at least one of the said plates is covered with a planar alignment layer inducing, in the vicinity of this plate, in the absence of an electric field, a substantially planar alignment of the molecules of the mixture, these zones thus absorbing the incident light, while, in the presence of an electric field, the molecules of the mixture have a homeotropous structure which is consequently transparent, thus confounding itself with the area situated outside the display zones.

2. Display cell as claimed in claim 1, characterized by the fact that, in the display zones, the second plate is also covered with a substantially planar alignment layer, the layer of the mixture having therefore, in these zones, a planar structure, which is consequently absorbent, in the absence of an electric field.

3. Display cell as claimed in claim 1, characterized by the fact that the second plate is entirely covered with a homeotropous alignment layer, the layer of the mixture having therefore, in the display zones and in the absence of an electric field, a mixed structure, the molecules adjacent to the first plate having an alignment substantially planar and the molecules adjacent to the second plate having a homeotropous alignment, so that these zones are absorbent.

4. A passive electro-optic display cell constructed and arranged substantially as herein particularly described with reference to and as illustrated in Figure 1, or Figure 2, of the accompanying drawings.