GB2023865A - Guest-host liquid crystal display - Google Patents

Abstract

In a display system comprising a quarter wave plate 21 between a reflector 22 and an electro-optic dichroic guest-host liquid crystal cell 10-13, the (non-absorbing) background areas of the liquid crystal layer 13 have a homotropic alignment at both cell plates whereas display regions 18 are homogeneously aligned at one or both plates, and the liquid crystal has positive dielectric arisotropy. There is thus a contrast between regions 18 and background areas where the cell is not suited, which is destroyed by a field E between associated electrodes 14, 15, although the latter are absent from any regions 18 arranged for a permanent display. Further portions of the display may be of the type requiring the pressure of a field to produce contrast. The liquid crystal material is nematic; if it contains an optically active additive the layer thickness must be below a critical value. Homotropic aligning layers 20, 23 are of surfactant or produced by cathodic sputtering or vapour deposition; layer 20 partially overlies a continuous homogeneous aligning layer 19 produced by rubbing or vapour deposition. <IMAGE>

Description

SPECIFICATION Passive electro-optic display device The present invention relates to a passive electrooptic display device comprising a reflector before which is arranged a quarter wave blade in front of which is located a dispay cell comprising two transparent glass plates imprisoning between them a mixture consisting of a liquid crystal having at least locally a nematic order and having a positive dielectric anisotropy, to which are added dichroic molecules, the inner faces of the said plates carrying control electrodes and alignment layers.

These cells are known per se, such as, for instance, those which are disclosed by Nagasaki in the US Patent No. 3.900.248.

The drawback of these ceils 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 can be obtained while using a liquid crystal having a negative dielectric anisotropy. However, this solution has the main drawback of necessitating higher control tensions.

The purpose of the present invention is to remove these drawbacks while bringing to this type of cells a modification such that the display is effected in dark on a clear ground, with a liquid crystal having a positive dielectric anisotropy, and that the determination of the topology of the control electrodes of the cell is thereby simplified.

The drawing shows the prior art and represents, by way of example, two embodiments of the invention.

Figure 1 is a sectional view of a part of an electro-optic display device such as disclosed in the US Patent No 3.900.248 and Figures2 and 3 are partial sectional views of two embodiments of display devices according to the invention.

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

The electro-optic display device represented in Figure 1 comprises a display cell constituted by two glass plates 1 and 2 carrying transparent electrodes 3 and 4, respectively, innerly coated with a planar homogeneous alignment layer 5, respectively 6.

Between these plates 1 and 2 is located a nematic liquid crystal 7 having a positive dielectric anisotropy, loaded with dichroic molecules, the axis of absorption of the light of which is parallel to the great axis of the said molecules, for instance orangemethyl. A reflector 8 is located behind the rear plate 2, with interposition of a quarter wave blade 9. The latter is placed in such a way that its neutral axes make an angle of 45" with the orientation direction, at rest, of the molecules of the liquid crystal which are adjacent to the rear plate 2 of the cell. Moreover, the optical characteristic of the quarter wave blade 9 is centered on the absorption peak of the dichroic molecules. The quarter wave blade could also be achromatic.

This cell operates as follows: The dichroic molecules being orientated in a parallel direction to the molecules of the liquid crystal 7, it results therefrom that, in the absence of any electric field, they are parallel to the plane of the cell. Consequently, for an intensity of the non polarised incident light, normalized to 1, the component of the light traversing the cell, parallel to the orientation of the dichroic molecules, will have, at its output at the rear of the cell, an intensity: ISarg = 1/2 e -aL, while the intensity of the perpendicular component is equal to Tsar| = 1/2, a being the coefficient of absorption of the mixture according to the direction of the great axis of the dichroic molecules and L being the thickness of the layer of the mixture.

The light then passes through the quarter wave blade 9, is reflected by the reflector 8, then passes again through the quarter wave blade, which has for effect to rotate its plane or polarization through 90".

Therefore, the intensity of the component of the light which is parallel to the orientation of the dichroic molecules and which enters by the rear of the cell is equal to: Earl/ /2 = 1/2 ls01r,whiletheintensiWofthe perpendicular component is equal to: IEarl= 1/2 e = Isaril- During the return way, the perpendicular component does not undergo any modification, so that at the output of the cell one has: Sisal e On the other hand, the parallel component will be absorbed so that the output intensity of this component be also equal to : 1SavII /2 e-'lL.

Taken as a whole, the output intensity is equal to 1Sav = eaL In the case where the dichroic molecules are perpendicular to the plane of the cell, there is no absorption. Consequently, the luminosity is not affected as this would occur with the use of a polarizer.

The display device represented in Figure 2 comprises a cell constituted by two transparent glass plates 10 and 11, separated by a frame 12, and which imprison between them a mixture 13 consisting of a nematic liquid crystal, at least with nematic local order, having a positive dielectric an isotropy, and of dichroic molecules.

The front plate 10 carries, on its inner face, transparent control electrodes 14, made of SnO2 for instance, while the rear plate 11 carries, on its inner face, a control electrode 15, also made of SnO2. The plate 10 is moreover inneriy coated with a homogeneous planar alignment layer 16 of the molecules of the mixture 13. This layer 16 is itself coated with a homeotropous alignment layer 17, interrupted opposite the display zones (segments), designated by 18. The rear plate 11 of the cell is innerly coated with a homogeneous planar alignment layer 19, which is itself coated with a homeotropous alignment layer 20, interrupted opposite the display zones 18.

The display device comprises, behind the cell, a quarter wave blade 21 and, behind this blade 21, a reflector 22.

In the area of the cell situated outside the display zones 18, the molecules of the mixture have a homeotropous structure, that is to say perpendicular to the plane of the cell, in the whole thickness of the layer of the mixture 13. Thus the incident light is not absorbed and the said area of the cell is consequent lytransparent, 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 18 situated at the left side of Figure 2, the molecules of the mixture 13 have, under the effect of the homogeneous planar alignment layers 16 and 19, a planar structure, the layer of the mixture then operating as a polarizer. Hence, a component of the incident light is absorbed in the outward way, and then, owing to the rotation through 90 of the plane of polarization of the light produced by the quarter wave blade which is traversed twice, the second component of the light is absorbed at the return way.

In these zones, the operation is the same as that disclosed for the Figure 1 (prior art). Thus, in the absence of a field, the display zones 18 are all absorbent, the surrounding area being transparent.

The application of an electric fieldtto the display zones which have not to become appearent, such as the zone 18 situated at the right side of Figure 2, has the effect of orientating the molecules of the mixture 13 according to a homeotropous structure, that is to say perpendicular to the plane of the cell. The zones which are thus activated do no more absorb the light and become transparent, being then confounded with the surrounding area which is also transparent.

Thus the display is effected by a reversed control.

The embodiment of Figure 3 differs from the preceding by the fact that the homogeneous planar alignment layer 16 innerly covering the front plate 10 is suppressed, and by the fact that the homeotropous alignment layer 17, interrupted opposite the display zones 18, is replaced by an uninterrupted homeotropous alignment layer 23, which is easier to realize. For the rest, the display cell of the device of Figure 3 is identical with that of Figure 2.

In the area of the cell situated outside the display zones, the behaviour is the same as that disclosed for the device of Figure 2. In the display zones (segments) not subjected to the action of an electric field, such as the zone 18 situated at the left side of Figure 3, the molecules of the mixture 13 which are adjacent to the plate 10 have a homeotropous alignment under the action of the alignment layer 23 while those which are adjacent to the plate 11 have a homogeneous planar alignment, under the effect of the alignment layer 19. In this way, one has a mixed structure, the molecules of the mixture progressively passing from a homeotropous alignment on the plate 10 to a homogeneous planar alignment on the plate 11.Hence, in these non activated display zones 18, the first component of the light is absorbed at the outward way and the second at the return way, so that these zones are absorbent zones.

If a tension is applied to the electrodes 14 and 15, there appears, in the concerned display zone 18, an electric field E having the effect of giving to the molecules of the mixture 13 a homeotropous structure which is transparent. These activated zones are then "cancelled" or "effaced", since they cease to be visible with respect to the surrounding area.

The alignment layers used could be realized on several ways: The homeotropous alignment can be obtained by means of alumina, of magnesium fluoride (see German Patent Application No 2330 909 of the firm SIEMENS) or still by means of magnesium oxide, which can be deposited by any known way, for instance by cathodic sputtering, by deposition in vapor phase or by vacuum evaporation, according to an incidence substantially perpendicular to the substrate. One can also use a surfactant such as, for instance, the lecithin.

Concerning 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 173 and following. The planar alignment layers can also be realized by a mere rubbing of the substrate, the direction of this rubbing defining 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 submitted 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 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 realization of the pattern of theelctrode nets carried by the two plates is thereby greatly facili- tated. Thus, one of the plates could be entirely covered with a conductive layer. This improvement can be considered as an important simplification.

One can also note that, if one adds to the mixture an opticaily active compound such as defined in the US Patent No 3.833.287, for instance CB15 of the British firm BDH, the present arrangement is then near from that disclosed in the Swiss Patent Application No 3.135/78-9 relating to a display cell of the White Taylor type similarly modified as the Nagasaki cell is here modified. Such an optically active compound induces a helicoidal structure in the mixture, at least in the absence of constraints. This mixture, if not nematic, has in any case a local nematic order. The pitch of the helix is a function of the concentration of the optically active compound.

If the pitch of the helix is large with respect to the wave length of the incident light, a wave guide system is established in the mixture layer (cf. The Physics of Liquid Crystals, P.G. De GENNES, Oxford University Press, 1975, pages 224 to 226). Even if the pitch of the helix is such that the wave guide system is only partial, the two components of the incident light remain unequally absorbed in the absence of the quarter wave blade. It is obvious that, if one adds an optically active compound to the nematic liquid crystal, the thickness of the layer of the mixture will be lower than the critical value such as defined in the Swiss Patent Application No 3.135/78-9, at least in the area situated outside the display zones.

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 permanently remain contrasted. Such zones could, for instance, constitute a frame surrounding the whole display zones.

Likewise, the present arrangement could easily be combined with the conventional arrangement in which the segments which are to be displayed must be activated. One could, as a matter of fact, 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.

Claims (6)

1. Electro-optic passive display device comprising a reflector before which is arranged a quarter wave blade in front of which is located a display cell comprising two transparent glass plates imprisoning between them a mixture consisting of a liquid crystal having at least locally a nematic order, and having a positive dielectric an isotropy, and of dichroic molecules, the inner faces of the sa;;d plates carrying control electrodes and alignment layers, characterized by the fact that the mixture is in contact with a homogeneous planar alignment layer, opposite the display zones of at least one of the said plates, while, in the area situated outside the display zones, the two plates are covered with a homeotropous alignment layer of the molecules of the mixture, these two alignment layers inducing a homeotropous structure of the said molecules, such that the incident light be not absorbed by the dichroic molecules, in such a way that this area is thus transparent while, opposite the display zones not subjected to the action of an electric field, the homogeneous planar alignment of the molecules of the mixture on at least one of the plates has for effect that the two components of the incident light are absorbed by the dichroic molecules, one in the outward way and the other one at the return way, the quarter wave blade, traversed twice, rotating the plane of polarization of the light through 90 , which has the effect of rendering these display zones absorbent, while, in the presence of an electric field, the structure of the mixture is homeotropous, consequently transparent, only the non activated display zones remaining visible.

2. Display device as claimed in claim 1, characterized by the fact that the second plate (1) of the cell is also covered, opposite the display zones, with a layer (7) of homogeneous planar alignment of the molecules of the mixture, these molecules having thus a planar structure.

3. Display device as claimed in claim 1, characterized by the fact that the second plate (1) of the cell is innerly covered with an uninterrupted layer (14) of homeotropous alignment of the molecules of the mixture, in such a way that, opposite the display zones, the molecules of the mixture in contact with the first plate (2) have a planar alignment and that the molecules in contact with the second plate (1) have a homeotropous alignment.

4. Display device as claimed in claim 1, characterized by the fact that the said liquid crystal having a positive dielectric anisotropy if of nematic type.

5. Display device as claimed in claim 1, characterized by the fact that the mixture of liquid crystal and of dichroic molecules contains in addition an optically active compound.

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