GB2030720A

GB2030720A - Gust Host Liquid Crystal Display

Info

Publication number: GB2030720A
Application number: GB7921267A
Authority: GB
Original assignee: Ebauches SA
Current assignee: Ebauches SA
Priority date: 1978-06-22
Filing date: 1979-06-19
Publication date: 1980-04-10
Also published as: DE2902127A1; CH623938A5; JPS5529896A

Abstract

The molecular alignment in display regions 33 of a dichroic gust- host liquid crystal display cell 25-34 differs from that in other regions such that in the unsuited state regions 33 absorb light linearly polarised in a direction a as transmitted by an adjacent polariser 24 of constrast with other areas of the cell. The liquid crystal (nematic, but may contain optically active material) has positive dielectric anisotropy so that a field E between associated electrodes 29, 30 destroys the contrast; permanent display areas may have no electrodes, and further display regions may be such as to require activation to produce contrast. In both embodiments, areas of the cell plate 25 adjacent the polariser corresponding to regions 33 produce parallel homogeneous alignment extending parallel to direction a. In all other areas plates 25 and 26 both produce parallel homogeneous alignment extending at 90 DEG to direction a (regions 33 have a twist structure, as shown), or both produce homotropic alignment. <IMAGE>

Description

SPECIFICATION Passive Electro-optic Display Device The present invention relates to a passive electro-optic display device comprising a polarizer situated opposite a display cell comprising two plates at least one of which, the front one, is transparent, imprisoning between them a mixture consisting of a liquid crystal with a positive dielectric anisotropy and at least localiy a nematic order, and of dichroic molecules, the inner faces of the said plates carrying control electrodes and alignment layers.

The display device according to the invention is derived from that disclosed by Heilmeier s al. in Applied Physics Letters 13 (1 968), pages 91 and 92, an example of which is given by Fig. 1 of the drawing and by the specification relating thereto.

The drawback of the Heilmeier's device lies in the fact that, for a liquid crystal having a positive dielectric anisotropy, the display is effected in clear on a dark ground, which is generally not appreciated. A display in dark on a clear ground could be obtained while using a liquid crystal having a negative dielectric anisotropy associated with homeotropous alignment layers applied on the two plates of the cell. This solution, however, has the disadvantage, in the present state of the technique, of necessitating higher control tensions.

The purpose of the present invention is to remove these drawbacks while bringing to the cells disclosed by Heilmeier a modification such that the display is effected in dark on a clear ground with a liquid crystal having a positive dielectric anisotropy permitting the use of a low control tension while simplifying the determination of the topology of the control electrodes of the cell.

The drawing illustrates the prior art and represents two embodiments of the invention and a modification.

Fig. 1 is a sectional view of a part of a known display device such as disclosed by Heilmeier.

Figs. 2 to 4 are partial sectional views of two embodiments of display devices according to the invention and of a modification.

It is to be noted that, in all these figures, the thicknesses of the several elements represented have been exaggerated so as to increase the clearness of the drawing.

The display device as represented in Fig. 1 comprises a polarizer 1 behind which is located a display cell including two transparent glass plates 2 and 3 carrying, on their inner face, the first one transparent control electrodes 4, made of SnO2 for instance, and the second one a control electrode 5, also made of SnO2. These two plates are moreover innerly coated with two homogeneous planar alignment layers 6, respectively 7, with interposition, in the case of the plate 3, of a reflecting and diffusing layer 8.

These two plates are maintained at a distance from each other by a frame 9, made of sintered glass, to which they are tightly assembled, imprisoning between them a mixture 10 consisting of a nematic liquid crystal having a positive dielectric anisotropy, and of dichroic molecules.

In the zones of the cell which are not subjected to the action of an electric field, the molecules of the mixture have a homogeneous planar structure parallel to the axis of the passage a of the polarizer 1. The layer of the mixture thus plays the role of a polarizer absorbing this one of the two components of the light that is not absorbed by the polarizer 1. The zones of the cell which are not subjected to the action of an electric field are thus absorbent.

If a fields is applied to the cell, as represented in the zone 11 of the display device of Fig. 1, the molecules of the mixture orientate themselves according to a homeotropous structure, that is to say perpendicularly to the plane of the cell. The zones which are thus activated make a contrast with the rest of the cell since they do not absorb the component of the light that the polarizer 1 has allowed to pass.

In this case, the display is thus effected in clear on a dark ground.

The display device represented in Fig. 2 comprises a polarizer 12 behind which is located a display cell comprising two plates 1 3 and 14 at least the front one of which is transparent, made for instance of glass. A frame 1 5 maintains these plates at a distance from each other, a mixture 1 6 consisting of a nematic liquid crystal having a positive dielectric anisotropy, and of dichroic molecules, being imprisoned between these plates. The control electrodes of the plate 13 are designated by 1 7 and the control electrode of the plate 14 by 18. They are made of Sino2, for instance.The plate 1 3 is innerly coated with a homogeneous planar alignment layer 1 9 which is itself coated with a homeotropous alignment layer 20 interrupted opposite the display zones, designated by 21. The plate 14 is coated with a reflecting and diffusing layer 22 itself coated with a non-interrupted homeotropous alignment layer 23.

This display device operates in a reflection mode, as follows: In the area of the cell situated outside the display zones 21, the molecules of the mixture are orientated by the homeotropous alignment layers 20 and 21 perpendicularly to the plane of the cell in the whole thickness of the latter. Hence, in this area, the component of the incident light which the polarizer 12 allows to pass is not absorbed.

The appearance of this area is thus determined by the optical characteristics of the polarizer 12 and of the layer 22.

In the display-zones (segments) which are not subjected to the action of an electric field, as it is the case of the zone 21 situated at the left side of Fig. 2, the molecules of the mixture which are in contact with the homeotropous alignment layer 23 are perpendicular to the plate 14 while the molecules in contact with the homeogeneous planar alignment layer 19 are orientated in a parallel direction to the plate 13. The absorption axis of the dichroic molecules of the mixture which are in the neighbourhood of the layer 19 is parallel to the axis of passage a of the polarizer 12. Hence, the display zones 21 which are not subjected to the action of an electric field thus appear as absorbent.

The display is effected by reversed control while "cancelling" or "effacing" the zones 21 which must not be visible by the application of an electric field such as the fields existing in the zone 21 situated at the right side of Fig. 2. The molecules of the mixture are orientated perpendicularly to the plane of the cell; since the dichroic molecules orientate themselves in the same way, they cease to absorb the incident light.

Consequently, the zones 21 subjected to an electric field become transparent and are confounded with the surrounding area.

The display device according to Fig. 3 comprises a polarizer 24 behind which is located a display cell comprising two transparent plates 25 and 26, for instance made of glass, and a frame 27 which maintains them at a distance from each other, between which is imprisoned a mixture 28 of a nematic liquid crystal having a positive dielectric anisotropy and of dichroic molecules. The electrodes of plate 25, designated by 29, and the electrode of plate 26, designated by 30, are made of SnO2. The plate 25 is innerly covered with a homogeneous planar alignment layer 31. This layer 31 is covered with another homogeneous planar alignment layer 32 interrupted opposite the display zones designated by 33.The layer 32 is such that its alignment direction is substantially perpendicular to the alignment direction defined by the layer 31 appearing opposite the display zones 33, which is itself parallel to the axis of passage a of the polarizer 24.

The plate 26 is covered with an uninterrupted homogeneous planar alignment layer 34 which defines an alignment direction which is parallel to that of the layer 32. This arrangement results in a homogeneous planar structure of the layer of the mixture in the area situated outside the display zones while, in the said zones and in the absence of any electric field, the layer of the mixture adopts a twisted nematic structure.

This display device operates, in a transmission mode, as follows: In the area of the cell situated outside the display zones 33, the structure of the mixture is such that the dichroic molecules do not absorb the component of the incident light that is allowed to pass by the polarizer 24.

Consequently, the appearance of this area is determined by the optical characteristics of the polarizer 24.

In the display zones (segments) which are not subjected to the action of an electric field, such as the zone 33 situated at the left side of Fig. 3, the twisted nematic structure of the mixture causes the plane of polarization of the light which traverses it to rotate, the direction being thus orientated in any point according to the axis of absorption of the dichroic molecules. Hence, the non-activated display zones are thus absorbent.

If a tension is applied to the electrodes, producing an electric field such as the field existing in the display zones 33 at the right side of Fig. 3, the molecules of the mixture orientate themselves according to a homeotropous structure and thus cease to absorb the component of the light which is allowed to pass by the polarizer 24. Hence, the thus activated zones 33 have the same appearance as the area situated outside the display zones and confound themselves with these display zones.

The control is thus reversed, as in the case of Fig. 2, the display zones (segments) which must not be visible being those which are subjected to the action of the electric field under the effect of which they are "cancelled" or "effaced".

It is to be noted that, for instance for impelling a sense of rotation to the helix such as represented in the display zone 33 at the left side of Fig. 3, one can add to the nematic liquid crystal an optically active compound having the effect of inducing, in the absence of outer constraints, a helicoidal structure. Such a mixture has a nematic local order (See E. B. Priestey, R. C. A. Review, Vol. 35, March 1974, pages 84 and foilowing).

The alignment layers used can be realized diversely: The homeotropous alignment can be obtained by means of alumina, of magnesium fiuoride (cf.

German Patent Appiication No.23 30 909 of the firm "Siemens"), or still of magnesium oxide, which can be deposited by any known way, for instance by cathodic sputtering, by deposition in vapor phase or by evaporation under vacuum, according to an incidence which is substantially perpendicular to the substrate. One can still use a surfactant such as, for instance, the lecithin.

Regarding the homogeneous planar alignment, it can be obtained by means of silicium oxide or other, especially according to the techniques exposed by J. L. Janning in Applied Physics Letters, Vol. 21, 1972, pages 1 73 and following.

The homogeneous planar alignment layers can also be realized by a mere rubbing of the substrate, the direction of this rubbing determining the alignment direction.

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 perpendicularity 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, an electric field is created which tends to orientate the molecules of the mixture according to a homeotropous structure, which is thus transparent.

Consequently, the crossing points do not risk to be untimely visible. The realization 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 constituting an important simpiification.

At last, it must 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 remain steadily displayed. Such zones will permit to realize a mark, a frame, etc., within the cell without complicating thereby the manufacture, these zones lying in the same plane as the segments.

Likewise, the present arrangement can easily be combined with the conventional arrangement in which the segments to be displayed have to 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 thve other one as disclosed by Heiimeier. Such a solution has been represented in Fig. 4, the left portion of which is constituted by a known arrangement, of the type disclosed by Heilmeier, corresponding to the arrangement of Fig. 1, but without display zone and without diffusing and reflecting layer, and the right side of which is realized according to the present invention, corresponding to the arrangement of the embodiment of Fig. 3. In this modification, the elements which are common to the cells of Figs. 1 and 3 have been designated by compound reference numerals repeating precisely those of Figs. 1 and 3, while the elements which are proper to one or the other of the two executions have been designated by the same reference numerals as in the said Figs. 1 and 3, respectively.

Claims

1. Passive electro-optic display device comprising a polarizer situated opposite a display cell comprising two plates at least the front one of which is transparent, imprisoning between them a mixture consisting of a liquid crystal having a positive dielectric anisotropy and at least locally a nematic order, and of dichroic molecules, the inner faces of the said plates carrying control electrodes and alignment layers, characterized by the fact that at least one of the said plates is covered, opposite the display zones, with a homogeneous planar alignment layer orientated according to a direction such that, in the absence of an electric field, the dichroic molecules of the mixture absorb the component of the light which is not absorbed by the polarizer, while, in the presence of an electric field, the structure of the mixture is homeotropous, consequently transparent, while, in the area situated outside the display zones, the two plates are coated with alignment layers inducing a structure of the molecules of the mixture such that the component of the light which is allowed to pass by the polarizer is not absorbed by the dichroic molecules, this area being therefore transparent, so that only the display zones which are not activated remain visible.

2. Display device as claimed in claim 1, characterized by the fact that the alignment layers with which the plates are coated, in the area situated outside the display zones, are layers producing a homeotropous structure of the molecules of the mixture.

3. Display device as claimed in claim 2, characterized by the fact that the alignment layer with which the second plate of the cell is coated is a layer which covers it entirely and produces a homeotropous alignment of the molecules of the mixture.

4. Display device as claimed in claim 1, characterized by the fact that this one of the two plates which is situated in the vicinity of the polarizer is coated, in the area situated outside the display zones, with a homogeneous planar alignment layer defining a direction substantially perpendicular both to the axis of the passage of the polarizer and to the direction of homogeneous planar alignment produced by the alignment layer with which the same plate is coated in the display zones.

5. Display device as claimed in claim 4, characterized by the fact the the alignment layer with which the second plate of the cell is coated is a layer which covers it entirely and which produces a homogeneous planar alignment of the molecules of the mixture.

6. A passive electro-optic display device constructed and arranged substantially as herein particularly described with reference to and as illustrated in Figure 2, Figure 3, or Figure 4, of the accompanying drawings.