GB1579480A

GB1579480A - Liquid crystal displays

Info

Publication number: GB1579480A
Application number: GB53601/77A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-12-22
Filing date: 1977-12-22
Publication date: 1980-11-19
Also published as: CH639811GA3; DE2854733A1; FR2412899A1; FR2412899B3; CH639811B; JPS54116952A

Abstract

A nematic liquid-crystal material (13) having a positive or negative dielectric anisotropy is switched in a display cell from a homeotropic into a randomly homogeneous alignment by applying an electric field to electrodes (12). The nematic material contains a bichromatic dye, with the result that the display alternates between a clear and a coloured state. <IMAGE>

Description

(54) LIQUID CRYSTAL DISPLAYS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 790 Strand London, W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to liquid crystal displays and in particular to displays of the type in which a liquid crystal material is switchable between clear and coloured coneditions.

Liquid crystal display devices are finding increasing use in such applications as digital watches and battery powered calculators. Such displays normally comprise a twisted nematic material disposed between crossed polarisers, the display being operated by the application of an electric field to vary the degree of Itwist and hence the optical transmission of the display device.

Whilst such display devices are simple and effective, the necessity to provide polarising plates for each device increases the cost of the display.

According to the present invention there is provided a liquid crystal display device, including first and second parallel transparent plates arranged parallel to and in register with one another and each of which has a transparent electrode disposed on its inwardly facing surface and a nematic liquid crystal material of negative or positive dielectric anisotropy, in which at least one said electrode is so constructed as to promote random parallel alignment of the nematic material, and in which the nematic material contains a dichroic dye whereby the display device is switchable between a clear and a coloured condition by the application of an electric potential condition to the electrodes.

In random parallel alignment, at any plane parallel to the plates the molecules lie in random directions substantially parallel to the plates.

The cell is so constructed that, in the case of a negative dielectric anisotropic material, random parallel alignment occurs on the application of an electric field across the cell, and in the case of a positive dielecteric anisotropic material, the liquid crystal material experiences random parallel alignment in the unactivated state.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figs. 1 and 2 are schematic diagrams of part of a liquid crystal cell containing a nematic material of negative dielectric anisotropy; and Figs. 3 and 4 show a similar cell containing a nematic material of positive dielectric anisotropy.

Referring to Figs. 1 and 2, the display device, which may for example form part of a calculator or a timepiece, includes a pair of parallel transparent glass plates 11 only one of which is shown. One plate 11 has an electrode pattern 12 corresponding to the configuration of the display. Thus, for example the electrode pattern may correspond to the conventional seven segment numeric display. The other (not shown) plate is provided with a uniform coating of the electrode material. Both electrodes are transparent and may be formed from an indium tin oxide.

As shown in Fig. 1 the negative dielectric anisotropic liquid crystal material 13, which may for example be methoxy benzilidene butyl aniline (MBBA) is homeotropically aligned in the absence of an electric field across the cell between the electrodes. Initial homeotropic alignment may be provided by treating the inwardly facing glass surface of each plate with dilute aqueous solutions of either hexadecyl trimethyl ammonium bromide or lecithin.

Alternative methods of providing homeotropic alignment include using very clean glass surfaces, which can align homeotropically without treatment, or using rubbed or evaporated alignment layers which would normally provide parallel homogeneous alignment but which provide homeotropic alignment if applied to the same surface in two orthogonal directions.

The nematic material also contains one or more dyes, two usually being required.

One dye is the dichroic dye which gives the cell its colour and a small quantity of a complementary isotropic dye may also be added. The function of the latter dye is to "tint out" any residual colour from the dichroic dye in the nominally clear condition. A suitable dye combination for this purpose consists of 1.3 weight % of the blue anthraquinone dye N-1-(4-hydroxyan- thraquinone)-4-butoxy aniline, together with 0.5 weight % of the isotropic dye Waxoline yellow A, the weight percentage being based on the weight of the liquid crystal solution.

As shown in Fig. 2, application of an electric field across the cell via the electrodes causes random homogeneous alignment of the nematic material within the region of the electrode pattern 12 to which the potential is applied thus causing the appearance of the cell to change from clear to coloured in the electrode region.

In order to obtain a good random parallel alignment of the nematic material when the electric field is applied, some surface treatment of the glass plate may be required prior to deposition of the electrode materials. One method by which this may be achieved is to deposit a silica layer on the glass surface followed by an etching process to form a rough surface. Alternatively the glass surface may be abraded.

In further applications a transparent roughened electrode layer may be used.

Figs. 3 and 4 show a similar liquid crystal cell in which a nematic liquid crystal 14 of positive dielectric anisotropy is employed.

A suitable material for this purpose is cyano-biphenyl mixture E7 supplied by BDH Chemicals Ltd. The nematic material, as before contains a small quantity of a dichroic dye and may also include a complementary isotropic dye. As shown in Figs. 3 and 4 the positive anisotropic material is aligned in a random homogeneous manner (Fig. 3) in the absence of an electric field and in this condition appears coloured. Application of a field produces homeotropic alignment (Fig. 4) of the nematic material causing it to appear clear.

There are various ways of producing the random parallel alignment of the positive anisotropic nematic material. Thus a chemical vapour deposited layer of silica 3,000-5,000 A thick may be provided over the electrodes. Alternatively the surfaces of the plates 11 may be treated with a surfactant material such as those described by Matsume et al, in Applied Physics Letters 29, No. 2, 15th July, 1976. In either case it may be necessary to roughen the underlaying glass surface to ensure that the alignment is sufficiently random.

Only one of the plates needs to be transparent the other plate reflecting light back through the liquid crystal material and the transparent plate.

WHAT WE CLAIM IS: - 1. A liquid crystal display device, including first and second electrode plates, at least one of which is transparent, arranged parallel to and facing one another, and a nematic liquid crystal material of negative or positive dielectric anisotropy, in which at least one of said plates is so constructed as to promote random parallel alignment of the nematic material, and in which the nematic material contains a dichroic dye whereby the display device is switchable between a clear and a coloured condition by the application of an electric potential condition to the electrodes.

2. A display device as claimed in claim 1, in which the nematic material has negative dielectric anisotropy, and in which the inwardly facing surfaces of the plates are treated with a dilute aqueous solution of hexadecyl trimethyl ammonium bromide or lecithin so as to provide initial homeotropic alignment of the nematic material.

3. A display device as claimed in claim 1 or 2, and in which the nematic material is methoxybenzilidenebutyl aniline (MBBA).

4. A display device as claimed in claim 1, in which the nematic material has positive dielectric anisotropy, and in which the inwardly facing surfaces of the plates are each provided with a chemical vapour deposited layer of silica so as to provide initial random parallel alignment of the nematic material.

5. A display device as claimed in any one of claims 1 to 4, and which includes an isotropic dye adapted to tint out the residual colour of the dichroic dye in the clear condition of the display.

6. A display device as claimed in claim 5, in which the dichroic dye is N-1-(4- hydroxyanthraquinone)-4-butoxy aniline and the isotropic dye is Waxoline Yellow A.

7. A liquid crystal display device, including a mixture of a nematic material of negative or positive dielectric anisotropy and a dichroic dye, the mixture being in contact with a surface so constructed as to promote random parallel alignment of the nematic material.

8. A liquid crystal display device substantially as described herein with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. or evaporated alignment layers which would normally provide parallel homogeneous alignment but which provide homeotropic alignment if applied to the same surface in two orthogonal directions. The nematic material also contains one or more dyes, two usually being required. One dye is the dichroic dye which gives the cell its colour and a small quantity of a complementary isotropic dye may also be added. The function of the latter dye is to "tint out" any residual colour from the dichroic dye in the nominally clear condition. A suitable dye combination for this purpose consists of 1.3 weight % of the blue anthraquinone dye N-1-(4-hydroxyan- thraquinone)-4-butoxy aniline, together with 0.5 weight % of the isotropic dye Waxoline yellow A, the weight percentage being based on the weight of the liquid crystal solution. As shown in Fig. 2, application of an electric field across the cell via the electrodes causes random homogeneous alignment of the nematic material within the region of the electrode pattern 12 to which the potential is applied thus causing the appearance of the cell to change from clear to coloured in the electrode region. In order to obtain a good random parallel alignment of the nematic material when the electric field is applied, some surface treatment of the glass plate may be required prior to deposition of the electrode materials. One method by which this may be achieved is to deposit a silica layer on the glass surface followed by an etching process to form a rough surface. Alternatively the glass surface may be abraded. In further applications a transparent roughened electrode layer may be used. Figs. 3 and 4 show a similar liquid crystal cell in which a nematic liquid crystal 14 of positive dielectric anisotropy is employed. A suitable material for this purpose is cyano-biphenyl mixture E7 supplied by BDH Chemicals Ltd. The nematic material, as before contains a small quantity of a dichroic dye and may also include a complementary isotropic dye. As shown in Figs. 3 and 4 the positive anisotropic material is aligned in a random homogeneous manner (Fig. 3) in the absence of an electric field and in this condition appears coloured. Application of a field produces homeotropic alignment (Fig. 4) of the nematic material causing it to appear clear. There are various ways of producing the random parallel alignment of the positive anisotropic nematic material. Thus a chemical vapour deposited layer of silica 3,000-5,000 A thick may be provided over the electrodes. Alternatively the surfaces of the plates 11 may be treated with a surfactant material such as those described by Matsume et al, in Applied Physics Letters 29, No. 2, 15th July, 1976. In either case it may be necessary to roughen the underlaying glass surface to ensure that the alignment is sufficiently random. Only one of the plates needs to be transparent the other plate reflecting light back through the liquid crystal material and the transparent plate. WHAT WE CLAIM IS: -

1. A liquid crystal display device, including first and second electrode plates, at least one of which is transparent, arranged parallel to and facing one another, and a nematic liquid crystal material of negative or positive dielectric anisotropy, in which at least one of said plates is so constructed as to promote random parallel alignment of the nematic material, and in which the nematic material contains a dichroic dye whereby the display device is switchable between a clear and a coloured condition by the application of an electric potential condition to the electrodes.

9. A timepiece or a calculator provided

with one or more display devices as claimed in any one of claims 1 to 8.