GB2354337A

GB2354337A - Surface mode liquid crystal device

Info

Publication number: GB2354337A
Application number: GB9921812A
Authority: GB
Inventors: Harry Garth Walton; Michael John Towler
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1999-09-16
Filing date: 1999-09-16
Publication date: 2001-03-21
Also published as: JP2001125145A; GB9921812D0

Abstract

A surface mode LCD, such as a pi-cell, containing a layer of nematic liquid crystal (1) having positive optical anisotropy, disposed between alignment layers (2 and 3). Further, layers (8 and 10) of negative uniaxial material have uniform optic axis which are tilted relative to the adjacent surfaces of the liquid crystal layer. Layers (9 and 11) have uniform optic axes which are perpendicular to layer 1. Layers 8-11 provide compensation for layer 1.

Description

2354337 SURFACE MODE LIQUID CRYSTAL DEVICE The present invention relates

to a surface mode liquid crystal device. Such a device may be used as a liquid crystal display or an optical processing device, for example in flat panel computer monitors, televisions, three dimensional displays and televisions, time sequential colour displays, head mounted displays and projection displays. Such surface mode devices are commonly used with a so-called "active matrix", for example in the form of thin film transistors (TFTs) or an array of gas-filled channels (so-called 1cplasma addressed liquid crystal" or "PALC") for selectively supplying voltages to an array of liquid crystal pixels.

Liquid crystal electro-optical displays have advantages over conventional cathode ray tube displays, for example in that much thinner displays may be provided. However, many liquid crystal modes have the disadvantage that the optical properties, and hence the visual appearance, of the display depend greatly upon the angle from which the display is viewed. For example, a uniform grey level displayed on a screen may appear to change intensity as the viewing angle is altered. Also, display colour may vary with viewing angle.

Liquid crystal molecules have anisotropic optical, or "birefringent", properties. The variation of optical properties with viewing angle may be understood by considering the optical indicatrix of the liquid crystal.

Figure I of the accompanying drawings illustrates the optical indicatrix of a uniformly oriented layer of nematic liquid crystal such that the long axis of the liquid crystal molecules, or the "director", is oriented along the z axis of Cartesian coordinate axes x,y,z. The optical indicatrix represents the speed at which polarised light travels through the liquid crystal layer. Thus, light polarised along the z axis and travelling along the x axis experiences a refractive index n, and travels at a speed of c/nz, where c is the speed of light in a vacuum. Similarly, light travelling along the z axis and polarised along the y axis travels at a speed of c/ny. In general, a beam of light travelling in an arbitrary direction and polarised along an arbitrary axis experiences a 2 combination of the three refractive indices ri, ny, n, and suffers some change in its polarisation state. The values of the refractive indices depend on the wavelength of light and, for example, may be given for a wavelength of 550 nanometres corresponding approximately to the middle of the visible spectrum. The properties and theory of index ellipsoids of birefringent materials are well known and are described, for example, in Max Born and Emil Wolf "Principles of Optics", sixth edition, Pergamon Press, 1989, ISBN 0-09-026481.

Materials in which the refractive indices n,,, ny, n, all have different values are known as biaxial materials. Materials having two refractive indices, for example n,, and ny. equal but different from the third refractive index n. are known as uniaxial materials. The z axis of symmetry is then referred to as the optic axis and the indices n,, (=ny) and nz are referred to as the ordinary (n.) and the extraordinary (n,,) refractive indices, respectively. The difference A n = N - n. is known as the optical anisotropy. Uniaxial materials of positive optical anisotropy are known as positive uniaxial materials whereas uniaxial materials of negative optical anisotropy are known as negative uniaxial materials.

Materials other than liquid crystal materials can also exhibit bireffingence. For example, sheets of plastics can be made optically anisotropic by stretching so as to align the polymer chains preferentially along one or more axis.

A liquid crystal layer whose molecular director varies through the layer has an optical indicatrix which similarly varies its orientation through the layer.

A material having refractive indices n, = ny = n, at all points is optically isotropic and has a spherical optical indicatrix. Such a material shows no variation in birefringence with viewing angle.

Surface mode liquid crystal devices are devices in which, during operation, the molecules in the centre of the liquid crystal layer remain substantially aligned in a given direction whereas molecules at the surfaces of the liquid crystal layer undergo reorientation in response to varying electric fields. One example of such a surace modje

3 device is known as a pi-cell, for example as disclosed in: US 4 566 758; P.J.Bos and K.R.Koehier-Beran, Mol. Cryst. Liq. Cryst., 1984, Vol. 113, pp. 329-339; and Miyashita et at, Euro display' 93 Digest (1993) pp. 149152. The pi-cell is an example of a surface liquid crystal mode with a rapid switching speed which makes it suitable for use in optical devices such as video displays requiring fast optical switching. The picell operates with the molecular director in a bend state with the optical indicatrix of the liquid crystal varying its orientation across the liquid crystal layer.

Another type of surface mode device is disclosed in WO 97/12275. This device uses a liquid crystal of negative dielectric anisotropy and also has a fast switching speed.

Miyashita et al disclose the use of a biaxial compensation film for improving the viewing angle characteristics of a pi-cell.

US 5 410 442 discloses the use of a negative bireffingence compensator for improving the viewing angle characteristics of a pi-cell.

US 5 825 445 discloses a normally white pi-cell in which the liquid crystal adopts a twisted structure. The use of a biaxial film for improving the viewing angle characteristics is also disclosed.

US 5 883 685 discloses an arrangement for a pi-cell as illustrated in Figure 2 of the accompanying drawings. The arrangement comprises a layer I of nematic liquid crystal material disposed between alignment layers 2 and 3 and, for example, comprising rubbed polyimide. External polarisers 4 and 5 are arranged with their polarising axes parallel or perpendicular and oriented at 45' to the rubbing direction of the alignment layers 2 and 3.

In order to provide viewing angle compensation, layers 6 and 7 are disposed on opposite sides of the liquid crystal layer 1. Each of the layers comprises a material of negative bireffingence whose optic axis orientation varies across the layer. Disk-shaped or discotic molecules are used to form the negative bireffingence layers as illustrated 4 diagrammatically in Figure 2. The variation in orientation of the optic axis in each layer 6 and 7 attempts to match the tilt of the optic axis of the positive uniaxial pi-cell liquid crystal in the layer I so as to improve the viewing angle characteristics. However, it is difficult to make birefringent layers whose optic axis varies in direction in a desired and predictable way across the thickness of the material.

US 5 184 237, US 5 375 006, US 5 245 456, US 5 189 538, US 5 568 290, EP 0 887 691 and JP 9-54315 disclose other liquid crystal modes such as twisted nernatic and super twisted nematic in which various techniques are disclosed for trying to improve the viewing angle performance of liquid crystal displays. Some of these techniques involve the use of birefringent layers with tilted optic axes. However, these liquid crystal modes are substantially different from surface liquid crystal modes so that the disclosures of these documents are not relevant.

According to the present invention, there is provided a surface mode liquid crystal device comprising a liquid crystal layer having first and second surfaces disposed between first and second uniaxial birefringent layers, respectively, the liquid crystal layer having a first polarity of optical anisotropy and the first and second birefi-ingent layers having a second polarity of optical anisotropy opposite the first polarity, the first and second birefringent layers having uniformly tilted optic axes with respect to the first and second surfaces, respectively.

The term "tilted" in this context means that the optic axis is oriented at an acute angle (i.e. between 0' and 90') with respect to the adjacent liquid crystal surface. The expression "uniformly tilted" means that the optic axis points in the same direction throughout the layer.

The liquid crystal layer may have first and second pretilts at the first and second surfaces, respectively, and the optic axes of the first and second birefi-ingent layers may be tilted in the same senses with respect to the first and second surfaces as the first and second pretilts, respectively.

The first and second pretilts may be of opposite senses in a plane substantially perpendicular to the first and second surfaces.

The liquid crystal layer may be of positive dielectric anisotropy and each of the first and second pretilts may be less than 45', for example greater than 10'.

The liquid crystal layer may be of negative dielectric anisotropy and each of the first and second pretilts may be greater than 45', for example greater than 80'.

The optic axes of the first and second birefiringent layers may be tilted by substantially the same angles in opposite senses with respect to the first and second surfaces, respectively. The first and second bireffingent layers may be tilted with respect to the first and second surfaces, respectively.

The device may comprise at least one further uniaxial bireffingent layer of the second polarity of optical anisotropy having a uniform optic axis substantially parallel to the optic axis of the liquid crystal at the middle of the liquid crystal layer. The at least one further birefringent layer may comprise first and second further bireffingent layers with the liquid crystal layer disposed therebetween. The first and second bireffingent layers may be disposed between the first and second further birefi-ingent layers.

The first and second polarities may be positive and negative, respectively. Each of the birefi-ingent layers may contain discotic molecules.

It is thus possible to provide a surface mode liquid crystal device of substantially improved angular viewing characteristics while using bireffingent layers of uniform optic axisorientation. Such layers are therefore easier and cheaper to manufacture. The uniformly tilted bireffingent layers approximately compensate for the nonuniformly tilted surface regions of the liquid crystal layer. The one or more 44perpendicular" layers, when present, compensate for the central region of the liquid crystal layer where the liquid crystal molecules are not substantially reoriented in response to a varying applied electric field across the liquid crystal layer.

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

Figure I is a diagram illustrating the optical indicatrix of an anisotropic material; Figure 2 Is a diagrammatic cross-sectional view of a known type of surface mode liquid crystal display; Figure 3 is a diagrammatic cross-sectional view of a pi-cell constituting an embodiment of the invention; Figure 4 shows graphs of transmission in percent against vertical and horizontal viewing angles in degrees illustrating the viewing characteristics of the display of Figure 3; Figure 5 shows graphs corresponding to those of Figure 4 for a known type of display; Figure 6 is a diagrammatic cross-sectional view of a pi-cell constituting another embodiment of the invention; Figure 7 is a diagrammatic cross-sectional view of a pi-cell constituting a further embodiment of the invention; Figure 8 shows graphs corresponding to those of Figure 4 for the display of Figure 7; and Figure 9 is a diagrammatic cross-sectional view of a display of the type disclosed in WO 97/12275 and constituting yet another embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

7 The pi-cell shown in Figure 3 comprises a layer I of nernatic liquid crystal, parallelaligned polyimide alignment layers 2 and 3 and polarisers 4 and 5 of the same type as shown in Figure 2. Substrates, electrodes and addressing arrangements are well known and are not shown in Figure 3 for the sake of clarity.

The alignment layers 2 and 3 are, for example, formed by spin-coating a polyimide prepolymer material SE7792 (available from Nissan) from a solution of N-methyl-2pyrrolidone onto a suitable glass substrate, baking at 180'C for one hour to produce cross-linking, and rubbing the resulting polyimide layer to provide a pretilt alignment surface for the liquid crystal layer 1.

The liquid crystal layer I may comprise a 10 micrometre thick nernatic liquid crystal material such as ZLI-2248 (available from Merck Gn-d3K Darmstadt, Germany) which has refractive indices n. = 1.5115 and n. = 1. 6639 and a dielectric anisotropy of +7A A surface pretilt of 5' is provided by the alignment layers 2 and 3. The layer 6 of Figure 2 is replaced by layers 8 and 9 as shown in Figure 3. Similarly, the layer 7 of Figure 2 is replaced by the layers 10 and I I in Figure 3. The layer 8 comprises a uniaxial bireffingent material of negative optical anisotropy. The optic axis is tilted uniformly throughout the layer 8 at an angle of 0 to the upper surface of the liquid crystal layer 1. The layer 9 likewise comprises a uniform negative uniaxial material whose optic axis is substantially perpendicular throughout the layer 9 to the upper surface of the liquid crystal layer 1.

The layers 10 and I I are similar to the layers 8 and 9 but with the optic axis of the layer 10 oriented at -0 to the lower surface of the liquid crystal layer I and with the optic axis of the layer I I shown in the opposite direction to that of the layer 9.

The negative bireffingent layers 8 to I I may be prepared by suitably stretching a sheet of plastic polymeric material. Alternatively, these layers may be formed by synthesising, aligning and cross-linking suitable discotic, materials, such as acrylate based materials. Such techniques are disclosed in US 5 883 685, the contents of which are incorporated herein by reference.

8 The viewing angle characteristics of the display shown in Figure 3 are illustrated by the upper and lower graphs shown in Figure 4. These graphs relate to the specific examples of the various parameters mentioned hereinbefore. In addition, the negative uniaxial layers 8 to I I have ordinary and extraordinary refractive indices ri,, = 1.6 and ne = 1.5. The layers 8 and 10 are 1.6 micrometres thick and 0 = 45'. The layers 9 and I I are 5.3 micrometres thick. A voltage of 2.1 volts across the liquid crystal layer I with the polarisation axes of the polarisers 4 and 5 orthogonal gives a bright or maximally transmissive display whereas a voltage of 5.3 volts gives a dark or minimally transmissive display. The upper graph in Figure 4 illustrates transmission against vertical viewing angle for the two voltages when viewing within a plane defined by the substrate normal and the rubbing direction of the alignment layers 2 and 3 whereas the lower graph illustrates the variation of transmission against horizontal viewing angle when viewing perpendicularly to the rubbing direction of the alignment layers 2 and 3.

By way of comparison, Figure 5 illustrates the viewing angle characteristics for a known type of display which is equivalent to that shown in Figure 3. In particular, for the known display, the layers 8 to I I are replaced by a single compensating layer comprising a uniform positive retarder with a retardafion of 170 nanometres. Bright and dark states of the device occur at the same voltages as for the display shown in Figure 3. A display of this type is disclosed in EP 0 616 240.

The display shown in Figure 3 thus has a substantially improved viewing angle performance compared with the known type of display. The contrast ratio is generally more uniform and, in particular, the contrast inversion which occurs at large horizontal viewing angles as illustrated in the upper graph of Figure 5 is eliminated in the display shown in Figure 3.

The display shown in Figure 6 differs from that shown in Figure 3 in that the layers 8 and 10 comprise layers in which the optic axis is substantially perpendicular to the layer surface. However, the layers 8 and 10 are tilted relative to the adjacent surfaces of the liquid crystal layer I so as to provide compensation for the surface regions of the liquid crystal layer 1. Such an arrangement may, for example, be used in a projection system.

9 Figure 7 illustrates a display which differs from that shown in Figure 3 in that the layer I I is omitted. The display of Figure 7 thus comprises layers 8 and 10 of uniform tilted bireffingent material and a single layer 9 with the optic axis perpendicular to the plane of the liquid crystal 1.

Figure 8 illustrates the performance of a display of the type shown in Figure 7 in which the examples of the various materials and parameters are the same as described hereinbefore except that the angle of tilt 0 of the films 8 and 10 is 55', the films 8 and 10 have a thickness of 2.4 micrometres and the layer 9 has a thickness of 10. 17 micrometres. The viewing angle performance is thus substantially improved compared with that of the known arrangement illustrated in Figure 5 but the improvement is not as large as that illustrated in Figure 4.

Figure 9 illustrates a display which differs from that shown in Figure 3 in that it is of the type disclosed in WO 97/12275. The liquid crystal of the layer I is of negative dielectric anisotropy and the alignment layers 2 and 3 provide a high pretilt greater than 450 and preferably greater than 80'. Also, the optic axis of the layers 9 and 10 are parallel to the surfaces of the liquid cryst-al layer 1. In practice, because the device is described with reference to Figures 3 to 9 are sur-face mode devices, the optic axis of the liquid crystal at the middle of the liquid crystal layer I is substantially unaffected by the varying applied field across the layer 1. For all of the these embodiments, the optic axes of the layers 9 and I I are substantially parallel to the optic axis of the liquid crystal at the middle of the layer 1.

Claims

CLAIMS:

1. A surface mode liquid crystal device comprising a liquid crystal layer having first and second surfaces disposed between first and second uniaxial bireffingent layers, respectively, the liquid crystal layer having a first polarity of optical anisotropy and the first and second birefringent layers having a second polarity of optical anisotropy opposite the first polarity, and the first and second bireffingent layers having uniformly tilted optic axes with respect to the first and second surfaces, respectively.

2. A device as claimed in claim 1, in which the liquid crystal layer has first and second pretilts at the first and second surfaces, respectively, and the optic axes of the first and second bireffingent layers are tilted in the same senses with respect to the first and second surfaces as the first and second pretilts, respectively.

3. A device as claimed in claim 2, in which the first and second pretilts are of opposite senses in a plane substantially perpendicular to the first and second surfaces.

4. A device as claimed in claim 3, in which the liquid crystal layer isof positive dielectric anisotropy and each of the first and second pretilts is less than 45'.

5. A device as claimed in claim 4, in which each of the first and second pretilts is I ess than 10'.

6. A device as claimed in claim 3, in which the liquid crystal layer is of negative dielectric anisotropy and each of the first and second pretilts is greater than 45'.

7. A device as claimed in claim 6, in which each of the first and second pretilts is greater than 80'.

8. A device as claimed in any one of claims 3 to 7, in which the optic axes of the first and second bireffingent layers are tilted by substantially the same angles in opposite senses with respect to the first and second surfaces, respectively.

9. A device as claimed in any one of the preceding claims, in which the first and second birefringent layers are tilted with respect to the first and second surfaces, respectively.

10. A device as claimed in any one of the preceding claims, comprising at least one further uniaxial birefringent layer of the second polarity of optical anisotropy having a uniform optic axis substantially parallel to the optic axis of the liquid crystal at the middle of the liquid crystal layer.

11. A device as claimed in claim 10, in which the at least one further birefringent layer comprises first and second further birefringent layers with the liquid crystal layer disposed therebetween.

12. A device as claimed in claim 11, in which the first and second birefringent layers are disposed between the first and second further bireffingent layers.

13. A device as claimed in any one of the preceding claim, in which the first and second polarities are positive and negative, respectively.

14. A device as claimed in claim 13, in which each of the birefringent layers contains discotic molecules.