GB2108698A

GB2108698A - Liquid crystal displays

Info

Publication number: GB2108698A
Application number: GB08230326A
Authority: GB
Inventors: Frances Carolyn Saunders
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1981-10-26
Filing date: 1982-10-22
Publication date: 1983-05-18
Also published as: GB2108698B

Abstract

A liquid crystal display device comprises a layer 5 of liquid crystal material contained between slides 2, 3, all arranged between two polarisers 11, 12. The slides 2, 3, are surface treated to give a progressive molecular twist which rotates the plane of plane polarised light passing through the liquid crystal layer 5. A small amount, typically 1-5%, of a neutral pleochroic dye is added to the liquid crystal material. This dye absorbs unguided light in the layer and improves a display appearance. The dye order parameter is greater than 0.6. <IMAGE>

Description

SPECIFICATION Liquid crystal displays This invention relates to liquid crystal displays.

Such displays commonly comprise a thin layer of a liquid crystal material between two glass slides. Electrode structures on the slides inner surface allow an electric field to be applied across the liquid crystal layer causing an observable molecular change. This forms a basis for a variety of different types of liquid crystal displays.

One type is often termed a twisted nematic display and typically comprises a layer of nematic liquid crystal material between two slides that have been surface treated to align liquid crystal molecules. The alignment directions on each slide may be at 900 to one another. This causes a progressive molecular twist across the nematic layer which guides and twists linear polarised light. A typical display then includes two crossed polarisers one attached to each slide parallel or perpendicular to the adjacent alignment direction.

In an OFF state with zero applied voltage the display is clear since linear polarised light is guided through the device. However in the ON state, with a suitably high applied voltage, the liquid crystal molecules align parallel to the applied electric field, thereby losing their twisted structure and the ability to rotate the plane of incident polarised light. As a result no light is transmitted through the crossed polarisers. The opposite effect is obtained with polarisers aligned with their optical axis parallel.

Small amounts of a cholesteric material may be added to the nematic material to give a more uniform appearance i.e. a single twist direction in a 900 twisted display. Also carefully controlled amounts of cholesteric may be added to give a 2700 twisted structure and an enhanced turn OFF time. Such devices are also termed twisted nematic devices, although the liquid crystal is strictly speaking a cholesteric of long pitch.

Coloured pleochroic dyes have been added to liquid crystal materials to enhance contrast between ON and OFF states e.g. blue and clear.

When used in twisted nematic displays only one polariser is used because of the pleochroic nature of the dyes.

A desirable property of liquid crystal displays is operation over a large ambient temperature range. Also a quick response time at low temperatures is desirable. This has led to the use of very thin layers of liquid crystal materials e.g. 6 ,*4m instead of the more typical 12 4m. When used in twisted nematic displays the effect of thin layers and low birefringence materials to obtain low temperature operation, is a loss in guiding the plane polarised light across the liquid crystal layer. This is observed as coloured fringes when the display is viewed in the OFF state.

It is an object of this invention to overcome this problem of loss of guidance in twisted layers.

According to this invention a liquid crystal display device comprises a liquid crystal cell capable of rotating the plane of incident plane polarised light having a thin layer of liquid crystal material contained between slides carrying electrode structures and surface treated to align adjacent liquid crystal molecules with a progressive twist across the layer and two polarisers arranged either side of the cell with their optical axis perpendicular or parallel to one another, characterised by a neutral pleochroic dye having a high order parameter in the liquid crystal material and at least one polariser arranged with its optical axis perpendicular to the alignment direction on the adjacent slide.

The order parameter S is greater than 0.6 preferably greater than 0.7 and ideally approaches unity.

The liquid crysal material may be a nematic material alone or with cholesteric additives.

The neutral dye may be a mixture of red, blue, and yellow dyes in proportions that give a neutral appearance.

Liquid crystal cells of 6 ,um layer thickness and low birefringence e.g. An~0.15 or less, have been found to show a poor guidance of plane polarised light; light emerges with an amount of eliptical polarisation instead of plane polarisation.

Incorporating small amounts, e.g. 15%, of a neutral pleochroic dye absorbs unguided light and removes fringes previously observed when the cell is OFF.

The invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 is an exploded diagrammatic view of a liquid crystal display; Figure 2 is a front view of Figure 1 showing a seven bar numeric display; Figure 3 is a cross section of Figure 2.

As shown by the drawings a liquid crystal display cell 1 comprises two glass slides 2, 3 spaced about 6 ,um apart by a spacer ring 4 to contain a layer 5 of liquid crystal material incorporating a neutral pleochoic dye. On the inside face of both slides 2, 3 are electrodes 6, 7 of e.g. tin oxide shaped into a conventional seven bar arrangement 8 as indicated in Figure 2, electrical connections to each bar are omitted for clarity but are conventional. Prior to assembly the slides 2, 3 inner face are unidirectionally rubbed with a soft tissue to provide surface alignment of liquid crystal molecules. The direction of rubbing is indicated by arrows 9 and 10; on assembly the two alignments are orthogonal resulting in a progressive 900 twist of liquid crystal molecules across the layer 5.

Techniques of alignment e.g. rubbing and oblique evaporation of MgF2 are described in U.K.

Patents Nos. 1,472,247 and 1,478,592.

Two linear polarisers 11, 12 are arranged either side the cell 1. The rear polariser 11 has its optical axis 13 orthogonal to the alignment direction 9 on the adjacent slide 2. The front polariser 12, acting as an analyser, is shown with its optical axis 14 crossed with respect to the rear polariser 11 although it may also be parallel.

As shown the device is illuminated at the rear by a light 1 5 and used in a transmission made by an observer 16. Alternatively the device may be used in a reflective mode in which case a reflector (not shown) is arranged behind the rear polariser 11.

The liquid crystal material may be the commercially available materials: ZLI 1695 mixture of CCH's ZLl 1565 ZLl 1964

mixture of PCH's and BCH's available from Merck 8 Co.

These materials have a low birefringence e.g.

An of approximately 0.15 or less; An being ne- nO where ne is extraordinary refractive index and nO is ordinary refractive index.

The dye is one having a high order parameter (S) greater than 0.5 preferably greater than 0.7.

Se is defined as A A 11 1 A,,+2A, where A"=absorbence parallel to alignment direction A,=absorbence perpendicular to alignment direction.

Dye molecules having high order parameters, when introduced in a liquid crystal host material, align themselves with the liquid crystal molecules; the guest host effect. An ideal dye molecule would align parallel to adjacent liquid crystal molecules. In practice thermal fluctuations cause random movement of molecules, both liquid crystal and dye, and the alignment is a statistical concept.

A neutral coloured dye is one that appears to have a neutral colour e.g. has an absorption curve s matching the human eye's response or matches sufficiently to appear neutral.

The dye may be a mixture of dyes e.g. red, blue, and yellow dyes. The amount of neutral dye added depends upon the lack of guidance in the liquid crystal layer; lower birefringence and thinner layers require more dye. Preferably the amount of dye is kept low because it also absorbs slightly in the cell-ON state and reduces ON-OFF contrast.

One example of dye is dye D8 1 available from BDH, Poole. The dye has blue:red:yellow in the proportions 1:1.8:2.5 and is 4% of dye in a liquid crystal material.

The blue dye has the structure:

U.K. Patent Application 2,081,736 A describes this material.

The red dye is a mixture of anthraquinones of structures

where X1,X2, X3, X4 may be tertiary or hydrogen independently.

The yellow dye has a structure

These red and yellow structures are described in U.K. Patent Application 2,093,475 A.

In operation light from the bulb 1 5 is horizontally polarised by the rear polariser 11. For the cell-OFF state, zero applied voltage, horizontally polarised light enters the liquid crystal layer 5 and is rotated to emerge as vertically polarised light which passes through the front polariser 12 to the observer 1 6. If the guiding of the polarised light in the layer 5 were perfect then all the light would be rotated (ignoring any small absorptions) on its passage through the layer 5. Unfortunately with thin layers 5 and low birefringent material the guiding is imperfect and light having a component parallel to the liquid crystal long molecular axis is emitted. This stray light causes observable fringes in cells not incorporating dyes of this invention.

The dye molecules absorb light whose polarisation is parallel to their long molecular axis.

Thus the parallel component of stray light from the liquid crystal molecules is absorbed by the dye molecules. As a result no fringes are observed when light is transmitted through the display.

On application of a voltage above the display threshold value to the electrodes, the liquid crystal molecules align parallel to the applied field. Due to the guest host effect the dye molecules rotate with the liquid crystal molecules.

In this condition the cell 1 loses its ability to rotate the plane of plane polarised light. Since the polariser axes 13, 14 are crossed no light is transmitted to the observer 1 6 at positions corresponding to the appropriate electrode bars 8.

Other portions of the display continue to transmit light. Removing the voltage allows the whole cell to relax back to an overall light transmitting state.

In an alternative device the liquid crystal layer is about 12 ym thick and includes an amount of cholesteric material such that the natural twist is about 1 6 ym. This means that plane polarised light passing through the layer is rotated by 2700 in the cell-OFF state. Again a neutral dye is incorporated into the liquid crystal material.

Different layer 5 thicknesses can be chosen and the cholesteric pitch arranged to give a 3 T/2 twist to plane polarised light.

Examples of materials are:-- ZLI:?l 132 a mixture of PCH's (available from Merck)+N1S/o CB 1 5 (available from BDH Ltd.).

CB 15 is

Claims

1. A liquid crystal display device comprising a liquid crystal cell capable of rotating the plane of incident plane polarised light having a thin layer of a liquid crystal material contained between slides carrying electrode structures and surface treated to align adjacent liquid crystal molecules with a progressive twist across the layer and two polarisers arranged either side of the cell with their optical axis perpendicular or parallel to one another, characterised by a neutral pleochroic dye having a high order parameter in the liquid crystal material and at least one polariser arranged with its optical axis perpendicular to the alignment direction on the adjacent slide.

2. The display of claim 1 wherein the order parameter is greater than 0.6.

3. The display of claim 1 wherein the order parameter is greater than 0.7.

4. The display of claim 1 wherein the liquid crystal material is a nematic liquid crystal material having a.positive dielectric anisotrophy.

5. The display of claim 4 wherein the liquid crystal material includes an amount of a cholesteric liquid crystal material.

6. The display of claim 5 wherein the amount of cholesteric liquid crystal material is arranged so that the cholesteric twist matches the surface alignment induced twist of the cell.

7. The display of claim 6 wherein the cholesteric twist and the surface alignment induced twist is about 7t/2.

8. The display of claim 6 wherein the cholesteric twist and the surface alignment induced twist is about 37t/2.

9. The display as claimed in any one of claims 1 to 8 wherein the liquid crystal material has a birefringence of 0.1 5 or less.

10. The display as claimed in any one of claims 1 to 9 wherein the amount of pleochroic dye is between 1 and 5% by weight.

11. The display of claim 1 wherein the thickness of the liquid crystal layer is about 6 ,um.

12. The display of claim 1 wherein the thickness of the liquid crystal layer is about 12 ym.

1 3. The display of claim 1 constructed, arranged, and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.