DE2459533A1 - ELECTRO-OPTICAL DISPLAY UNIT - Google Patents

Description

The invention relates to an electro-optical display device using liquid crystal material. It affects in particular alphanumeric display devices using homeotropically aligned positive nematic liquid crystal materials.

The nematic materials consist of organic compounds, whose molecules have an elongated, table-shaped structure. The compounds generally contain polar groups and the resulting dipole moment forms a certain angle with the long axis of the molecule. Depending on the position of these polar groups in the molecule and the strength of their influence the angle between the molecular dipole moment and that of the longitudinal axis of the molecule can be much smaller than 45 °. links of this type are called positive nematic because the component of the dipole moment is in the direction of the longitudinal axis of the molecule is larger than the component perpendicular to this molecular axis. It is also a class of nematic compounds known to have a large angle between the dipole moment and the molecular

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own killing axis. These compounds are called negative-nematic because the component of the dipole moment in the direction of the molecular axis is smaller than the component perpendicular to it. All nematic materials, positive and negative, are birefringent and of uniaxial symmetry. The optical axis is always parallel to the longitudinal axis of the molecule.

Liquid crystal materials are in certain temperature ranges, including those around room temperature, in an intermediate phase between a crystalline state with high Principle of order and the liquid state with arbitrary arrangement of the building blocks. In nematic liquid crystal materials the molecules in a given area tend to align with each other parallel to their long axis. If such materials are placed between electrodes and an electric field is applied, the molecules tend to rotate and align themselves again parallel to each other in a direction which is determined by the dipole moment of the molecule. So will when an electric field is applied and the molecules rotate, the direction of the optical axis changes. To now include commercially available field effect displays Liquid crystal material arranged in parallel. In such cells, a preferred direction is established by rubbing or the like and the cell is arranged so that the preferred direction of the lower surface is perpendicular to the preferred direction of the upper surface is. The orientation is rotated by 90 ° across the thickness of the cell. When the cell is at rest between When viewed from crossed polarizers, it is bright. When an electric field is applied, the molecules rotate and the optical axis is substantially parallel to the field and the viewing direction, and therefore is the Cell dark. It has been found that it is difficult to obtain a uniform parallel orientation with prolonged use to manufacture over the entire range of the display device and

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maintain. As a result, cells, even when new, do not have uniform brightness over the entire area and will be canceled soon,

Most nematic materials tend to have their molecules oriented randomly on the substrate surface. However, additives are known which are added to negative nematic materials and which have the effect that the molecules are oriented perpendicular to the substrate surface in the resting state. This type of alignment is called homeotropic. It has been found that such additives, which were previously only added to negative nematic materials, are equally useful for homeotropic alignment for positive nematic materials. Assuming a homeotropically oriented material, if a negative nematic material is used, the direction of the electric field is generally parallel to the viewing direction and, conversely, the direction of the electric field must be perpendicular to the viewing direction if a positive nematic material is used. However, a negative nematic material shows turbulent motion when subjected to the influence of an electric field. This phenomenon is well known and is called dynamic scattering. The turbulence partially depolarizes the material, and cells of this type show little contrast between on and off. As described in US Pat. No. 3,687,515, a display device can be made using negative nematic material in which the field direction is perpendicular to the viewing direction and the dynamic scattering is based on partial depolarization and a "satisfied ^{: 1"} condition 'Here, too, the contrast is very low.

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Positive nematic material shows no dynamic scattering, and this invention therefore relates to positive nematic material which is homeotropically oriented and having means for applying an electric field is provided perpendicular to the viewing direction. In this way, improved display devices are made obtain.

The object of the invention is achieved by an electro-optical display device composed of two transparent ones arranged parallel to one another Carrier elements, at least one pair of electrodes on the surface of one of the carrier elements, a layer of positive nematic, liquid crystalline material with homeotropic orientation with respect to the surface of the support elements, control and Addressing circuits for generating an electric field in the liquid crystal material with a direction parallel to the surface of the support elements and perpendicular to the optical axis of the assembly, and polarizers on both sides of the assembly, where one polarizer is oriented parallel and the other perpendicular to the optical axis of the arrangement.

The invention relates to positive nematic cells with improved Display properties.

In this invention, at rest (turned off) the material is turned off viewed in the direction of the optical axis of the material, and therefore the cell appears uniformly dark between crossed Polarizers. The applied electric field causes the molecular orientation to change by nearly 90. The film is now birefringent and therefore appears bright between crossed polarizers.

The display device according to the invention comprises a liquid crystal cell, in which a positive-nematic, liquid-crystalline material is used which contains an additive that

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direction of the molecules between the carrier elements. "The Electrodes are formed on the surface of one of the support elements, and means are provided to apply an electric field to the liquid crystal material between adjacent ones Apply electrodes and generate an electric field with a direction parallel to the surface of the carrier elements and a To cause rotation of the molecules. The display device is provided with crossed polarizers with the optical axis perpendicular to the direction of the applied field and perpendicular to the plane of the support elements.

Details of the invention are given below with reference to the particular Description and the figures indicated.

Show it;

Figs. IA and B schematic drawings, partly in section,

from which the structure and the mode of operation of the arrangement can be seen;

Fig. 2 shows in an exploded manner a numerical one

Display device;

Figs. 3A and B are a diagrammatic representation of an electro-optical Display device according to the invention.

As can be seen from Fig. 1A, a liquid crystal cell 11 is formed from transparent support elements 15 and 17, for example made of glass, plastic or another inert material that has good translucent properties. The support elements have flat surfaces which are arranged parallel to each other, and on the inner surface of the Carrier element 17 are electrically conductive electrodes 19 and with a layer thickness of the order of 1000 to 3000 Å

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arranged. Examples of materials from which such electrodes can be made are In ₃ O ₃ , SnO and chromium. A layer 13 of a liquid crystal material is arranged between the carrier elements 15 and 17, the molecules 27 of which, as indicated schematically, have longitudinal axes in a direction perpendicular to the planes of the carrier elements. The distance between the carrier elements 17 and 15 is determined by suitable elements, for example by raised parts on the edges of the carrier elements themselves, or by clamping pieces or sealing rings 12 made of any suitable inert material such as glass or plastic. A distance between 0.0127 to 0.0254 mm (0.5 to 1 mil) is preferred and the gaps between electrodes 19 and 21 are about 0.0254 mm to 0.51 mm (1 to 20 mils). The electrodes 19 and 21 are connected to the electrical lines 24 and 25, the electrode 19 being connected to a voltage source 26 via a switch 28.

When viewed in the resting state through the crossed polarizers (not shown) in the direction of the optical axis, no Light from a diffuse light source (not shown), which is arranged behind the support element 17, through the liquid crystal layer 13, and the field appears uniformly black. When switch 28 is closed, there is an electric field between electrodes 19 and 21 parallel to the planar surface of the carrier element 17 applied. The molecules 27A (Fig. 1B) of the liquid crystal layer 13 rotate or twist in the Way that the longitudinal axes of the molecules are parallel to each other between the electrodes and parallel to the surface of the support element 17 are aligned. This causes a change in the optical properties, so that the liquid crystal material in Viewing direction is now optically anisotropic (birefringent) and appears bright between crossed polarizers. Outside of of the area defined by the electrodes, the orientation of the crystals and the field remain unchanged stays black. Alignment is a collaborative effect

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and there is a voltage threshold and no field threshold.

Examples of suitable positive nematic liquid crystal materials for the layer 13 are compounds whose structural formulas are given below:

RO (f \ CH = N fy C = N

in which R = C H ",. and η = 4 to 6, for example!

η 2n + l>

CH _n - 0 - (/ \ CH = N- / \ -C s N -;

B R-C

0 ~ f \ - CH = N - / Λ— C = N

where R = CH _{2 + 1} and η = 4 to 6, for example C ₆ H ₁₃ ;

in which R = C H _{2n + 1} and η = 4 to 6, ^R "- = ^C n ^H 2n + 1 and ⁿ = 4 to 7, for example 0 O
/ f ~ \ Il ο- (I \ _ö _
\ = / FI 973 046 509 g 27 / Q8 38

D NC (/ ^ C - O ff V- S - O ff \ C ₄ H ₉

All of these molecules have dipole moments at which the component in the direction of the longitudinal axis of the molecule is greater than the component perpendicular to the longitudinal axis of the molecule.

It has been found that the homeotropic alignment of positive nematic materials can be obtained by the incorporation of suitable additives such as long chain alkyl pyridinium or quaternary ammonium salts. Materials of this type have already been described as suitable additives for generating the homeotropic texture for negative nematic materials. These are compounds that are described, for example, in German patent application P 21 60 788.5 and the general formula R (R ¹ J ₃ N ⁺ X ^- Have, in which R is an alkyl radical having 10 to 24 carbon atoms, R ^{1 is} a methyl or ethyl radical and X is an acid anion. Typical examples of specific compounds are hexadecylpyridinium bromide, dodecylpyridinium bromide, hexadecyltrimethylammonium bromide and the like. The materials are used in amounts in one Range from about 0.25 to

-4

2.5 10 mole percent of the liquid crystal material, depending the solubility characteristics of the particular nematic liquid crystal material being used.

It was found that in a test arrangement in which the electrode spacing was about 0.127 mm (5 mils) and the spacing of the support elements was about 0.0127 mm (0.5 mils) and when C-type positive nematic liquid crystal material with an additive on

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7 χ 10 mole percent hexadecylpyridinium bromide was used as the alignment agent, giving the cell a threshold voltage of was about 8 volts and the display was uniformly black except for the areas where voltage was applied.

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FI 973 Ο46

Lower threshold voltages were obtained with mixtures of compounds of type A and B.

Fig. 2 is an exploded view. Way of a numerical display device with an electrode pattern of seven segments of opposing pairs of electrodes 31-32, 33-34, 35-36, 37-38, 39-40, 41-42 and 43-44, which consists of In ₃ O ₃ on a Glass substrate 45 was formed. One electrode of each pair is connected to ground _v 47 and the second electrode of each pair is connected to a suitable control and addressing circuit, which is shown schematically in box 49. Teflon (trademark of Du Pont for polytetrafluoroethylene) spacer 51 about 0.0127 mm (0.5 mil) thick and glass substrate 53 complete the assembly and provide space for the containment of the liquid crystal material which is homeotropically aligned by one of the suitable additives described above.

The FIGS. 3A and B are schematic representations of the optical system in which a cell 30 according to the invention, for example one shown in FIG. 2 and crossed polarizers 61 and 62, a lamp 63 and a diffuser 64 for generating diffuse back exposure can be used. In the idle state, the light beam 65 is polarized by the polarizer 62 and passes cell 30 without change, so that it is stopped by polarizer 61 and the cell appears dark. As shown in Fig. 3B, when a field is applied in certain areas of the cell, there is between selected pairs of electrodes a reorientation of the molecules in the liquid crystal material takes place, as a result of which the polarization state of the light is changed is that it is elliptically polarized in the areas in which the field was applied and the light now the polarizer 61 and observed in the form of bright areas in the space between the electrode pairs to which the voltage was applied can be displayed and any desired digit from zero to nine can be.

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The cells according to the invention advantageously have uniform brightness and long life because homeotropic alignment of the liquid crystal material, as opposed to parallel alignment, are easily obtained and maintained
can. Because the electrodes are on a single substrate material
can be produced, accurate electrode patterns and spacings can be obtained by conventional photographic methods and this allows a positive nematic homeotropically aligned material to be used in a liquid crystal display device.

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Claims (1)

PATENT CLAIMS Electro-optical display device characterized by two transparent carrier elements (15, 17; 45, 53) arranged parallel to one another, at least one pair of electrodes (19, 21; e.g. 31, 32) on the surface of one of the carrier elements (17, 45), a layer (13) made of positive-nematic, liquid-crystalline material (27) with homeotropic orientation with respect to the surface of the carrier elements (15, 17), Control and addressing circuits (24, 49) for generating an electric field in the liquid crystal material (13, 27) with a direction parallel to the surface of the carrier elements and perpendicular to the optical axis of the arrangement (11) and polarizers (61, -62) on either side of the array (30, Fig. 3), one polarizer (62) parallel and the other (61) perpendicular to the optical axis of the Arrangement (30) is aligned. 2. Display device according to claim 1, characterized in that that the liquid crystal layer (13) by adding a Alkylpyridinium salt or a quaternary ammonium base is homeotropically aligned. 3. Display device according to claims 1 and 2, characterized in that that the liquid crystal layer (13) the alkylpyridinium salts or quaternary ammonium salts in a concentration of 0.25 to 2.5 of the liquid crystal material. -4 a concentration of 0.25 to 2.5 χ 10 mol percent 4. Display device according to claims 1 to 3, characterized in that that the liquid crystal layer (13) compounds of the general formeini ^{FI 973 O46} 50 9 827 / Q 83 8 A R- O ff V-CH = N ff VC = N in which R = CH ", _Λ and η = 4 to 6 η 2n + 1 CO - ff \ - CH = N- / \ - C β Il in which R = CH _{9 1} and η = 4 to 6, Xl £ * lit J for example V - / in which R ¹ = CH ₃ , λ and η = 4 to 6 and R "= ^c _n ^H 2n + 1 ^{and n = 4 to 7} O D NC- / VC - O ff J C - O- (/ J C ₄ H ₉ contains, 5, display device according to claim 1, characterized in that the distance between the carrier elements (15, 17; 45, 53) between about 0.0127 to 0.254 mm (0.5 to 10 mils), the distance between the electrode pairs (19, 21; z. B, 31, 32) between about 0.0254 to 0.51 mm (1 to 20 mils) and the layer thickness of the electrodes between about 1000 and 3000 8. 6. Use of the display device according to claims 1 to for displaying alphanumeric characters. 509827/0838 FI 973 046 41 . L e r s e i t e