DE3911620A1 - Electro=optical LCD system using ECB principle - uses conventional liquid crystal layer and non-controllable liquid crystal addition - Google Patents

Abstract

By adding another self-contained layer (3) of liquid crystal material between two parallel and transparent plates (2, 4) and placing it on top of the nematic controllable liquid crystal mixture layer (6) between its two electrodes (10, 11). A phenomenon has been noticed where not only has the contrastability of the electrically controlled display not suffered by the overlay but the rather narrow viewing angle has widened. The upper layer (3) has no electrodes since its structure are not electrically sensitive; its molecules are symbolised as broad paltelets (13). Below that layer are the electrically controllable lay of nematic crystal molecules, here indicated as little rods (12). ADVANTAGE - Has improved contrast and greater independence from viewing angle.

Description

The invention relates to an electro-optical liquid crystal display according to the ECB principle with at least one electrically controllable nematic liquid crystal layer with negative dielectric anisotropy, the Molecules by applying a voltage from a homeotropic Orientation in a tilted planate orientation can be transferred.

The ECB principle (electrically controlled birefringence) or also DAP principle (deformation of aligned phases) was first described in 1971 (M. F. Schieckel and K. Fahrenschon, "Deformation of nemative liquid crystals with vertical orientation in electrical fields ", Appl. Phys. Lett. 19 (1971), 3912). Work by J. F. Kahn (Appl. Phys. Lett. 20 (1972), 1193), G. Labrunie and J. Robert (J. Appl. Phys. 44 (1973), 4869) and S. Matsumoto, M. Kawamoto and K. Mizunoya (J. Appl. Phys. 47: 3842 (1976).

The application of the ECB principle is both for liquid crystal displays with high multiplex rate (J. Robert and F. Clerc, SID 80 Digest Techn. Papers (1980), 30; H. P. Schad, SID 82 Digest Techn. Papers (1982), 244; H. P. Schad, M. Kaufmann and P. Eglin, Proc. 13th Freiburg working conference on liquid crystals, Freiburg (1983), 26 and J. Duchene, display 7 (1986), 3) as well as for TFT-addressed liquid crystal displays (J.F. Clerc and J.C. Deutsch, Proc. Eurodisplay 87, London (1987), 111).  

However, liquid crystal displays based on the ECB principle exhibit a very strong viewing angle dependence of the contrast on, so far, despite her very steep electro-optical characteristics and the resulting high multiplexability could.

In French patent application FR 84 07 767 Described ECB liquid crystal displays with a large layer thickness, through a significantly better viewing angle dependency of the contrast are marked. These However, cells assign due to the large layer thickness high switching times and also serious imaging errors that they are not a viable solution.

In European patent application EP 02 40 379 hence an arrangement of two parallel ECB displays proposed in each case with a significantly smaller layer thickness, to improve the switching times. It turned out that at the same time an improvement of the viewing angle dependency of contrast can be achieved when the liquid crystal molecules have the same pretilt in both cells and in the switched state the same angle to the homeotropic Have direction, but the direction of rotation of this angle is opposite. The defined setting of the same Pretilt angle in different cells is practical difficult to implement, so that such an arrangement particularly difficult for series production is.

In the same application, the use of a compensation film based on a thermoplastic polymer is proposed as a further concept for improving the viewing angle dependence of the contrast. This compensation foil can be combined both with the arrangement from EP 02 40 379 consisting of two ECB cells and with simple ECB cells, as described in a report in Nikkei Mikrodevices 1 (1988), 69. However, the production of these plastic films is complex and expensive; in addition, the compensation deteriorates considerably in the case of temperature fluctuations, since the compensation film and the liquid-crystalline material have a significantly different temperature dependence of the optical anisotropy Δ n .

There is therefore still a considerable need ECB liquid crystal displays, which have a high viewing angle independence the contrast with good values of the others Have cell parameters and their production in one industrial production is possible.

The invention was based, liquid crystal displays the task to provide, which are based on the ECB effect and show a high viewing angle independence of the contrast, without at the same time the above-mentioned disadvantages Show ECB ads.

Surprisingly, it has now been found that this task can be solved if you have an electro-optical Liquid crystal display according to the ECB principle with at least an additional, uncontrolled liquid crystal layer provided with a uniaxial, optically negative liquid crystal material.

The invention thus relates to electro-optical liquid crystal displays according to the ECB principle with at least an electrically controllable nematic liquid crystal layer with negative dielectric anisotropy, whose molecules by applying a voltage from a homeotropic orientation in a tilted planar orientation can be transferred, characterized in that that the display to improve the viewing angle dependency  of contrast at least one additional not controlled liquid crystal layer in a uniaxial, has optically negative liquid crystal material.

Ads where the additional Liquid crystal layers on planar oriented discotic and / or cholesteric liquid crystals.

The additional liquid crystal layers preferably consist of a nematic discotic (N _D ) liquid crystal material which is located between plane-parallel carrier plates. Since these materials generally orient themselves spontaneously, orientation layers can be omitted in many cases.

In a preferred embodiment of the invention yourself on at least one of the two sides of the different Arrangement consisting of liquid crystal layers a polarizer.

The sum of the optical path differences d × Δ n of the driven liquid crystal layers is preferably between 0.1 μm and 2.5 μm.

The sum of the optical path differences d × Δ n of the driven liquid crystal layers and the sum of the optical path differences d × Δ n of the additional liquid crystal layers are preferably matched to one another in such a way that a minimal viewing angle dependence of the contrast results.

The sum of the layer thicknesses of the additional liquid crystal layers is in particular smaller than 50 µm.  

The display is preferably operated in transmissive mode, the cell being two complementary devices for polarization on both sides of the arrangement includes.

The invention further relates to the use of discotic and / or cholesteric liquid crystals as materials for at least one additional, not controlled liquid crystal layer of electro-optical Liquid crystal displays according to the ECB principle with improved Viewpoint dependence of the contrast.

The liquid crystal displays according to the invention contain at least one electrically controllable nematic liquid crystal layer with negative dielectric anisotropy, the term being nematic here broadly and also chiral nematic (cholesteric) liquid crystal layers are included. Parallel to this Layer (s) is at least one additional one Liquid crystal layer with a uniaxial, optical negative liquid crystal material arranged.

The structure of the controlled liquid crystal layers corresponds to the design customary for ECB or DAP arrangements, the term conventional design also including all modifications and modifications. The liquid crystal layers are each between two plane-parallel carrier plates, which form a cell with a border. Electrode layers with overlying orientation layers are arranged on the inside of the carrier plates. Through these orientation layers, the liquid crystal molecules on the surfaces of the carrier plates are oriented homeotropically up to a pretilt angle of a maximum of a few degrees. The surface tilt angle on the carrier plates can be the same or different. However, the same tilt angle is preferred. The tilt angle is preferably 0.1-5.0 ° and in particular 0.3-2.5 °. The nematic liquid crystal used in these layers has a negative dielectric anisotropy Δε , so that when a voltage is applied, the liquid crystal molecules are converted from the homeotropic orientation into a tilted planar one. If the liquid crystal display is to be operated at high multiplex rates, the liquid crystal should not only have a negative dielectric anisotropy, but also high values for the ratio of the elastic constants K ₃ / K ₁, high values for the optical anisotropy Δ n and small values for the quotient Δε / ε _{| |} exhibit.

The controllable liquid crystal layer (s) in the Liquid crystal mixture used consists of 2-40 in particular however, 3-30 components are known either are or are prepared analogously to known methods. Mixtures containing at least one are preferred Tolan derivative included. Such mixtures are further preferred which contain at least one component, whose molecules are at least one of the structural elements A-F have:

where X = CN, or halogen.

Parallel to the controllable liquid crystal layer (s) is at least one additional liquid crystal layer between two parallel carrier plates arranged, which are completed by a border. The use of one or two, in particular but from an additional liquid crystal layer prefers. This additional liquid crystal layer (s) is (are) not controlled, so that the carrier plates no need to have electrode layers. Surface orientation layers can be present to the desired one Ensure orientation of the liquid crystal molecules, but they can also be left out. The additional liquid crystal layer (s) is (are) of a uniaxial, optically negative liquid crystal material educated.  

Through this additional liquid crystal layer (s) the viewing angle dependence of the contrast is decisive improved.

The additional liquid crystal layer (s) is based (based) preferably on planar oriented discotic ones and / or cholesteric molecules. These molecules have a more or less flat, two-dimensional, e.g. B. disk-like shape. So they tend to align parallel to each other d. that is, the director  points towards the normal of the molecular plane. In particular discotic liquid crystals are suitable. These generally have a flat and more or less disc-shaped central core, that of side chains is surrounded. The type of discotic liquid crystal materials used is not very critical. Exemplary be a series of discotic liquid crystalline compounds called, this enumeration the invention should only explain without limiting it:

(1) hexasubstituted benzene

(2) 2-, 3-, 6-, 7-, 10-, 11-hexasubstituted triphenyls

(3) 2-, 3-, 7-, 8-, 12-, 13-hexasubstituted truxenes or their oxidized homologs

(4) 1-, 2-, 3-, 5-, 6-, 7-hexasubstituted antrachinones

(5) substituted Cu complexes

(6) Tetraarylbipyranylidum

(7) Porphyrin derivatives

wherein R is independently an alkyl group with up to 30 carbon atoms, in which one or more CH₂- Groups through

can be replaced, whereby 2 O atoms are not directly linked.

Compounds (1), (2), (3) and (4) are preferred; in particular however, compounds (1), (2) and (3). Especially discotic liquid crystal compounds are also preferred, in which in all radicals R at least one -CH₂ group is replaced by a 1,4-phenylene group.

Discotic liquid crystals which have a nematic discotic phase N _D are preferred. In contrast to the columnar discotic phase, in which the molecules are packed together in columns, the arrangement of the molecules in the nematic discotic phase is less rigid. The molecules can rotate freely and can orient themselves more or less freely, but their planes are arranged parallel to one another on average. Cholesteric nematic discotic phases N _D * can also be used.

Because the discotic and especially the nematic discotic molecules show a high tendency to become spontaneous Aligning parallel to each other can be done on orientation layers to be dispensed with on the carrier plates, through the z. B. a preferred for nematic liquid crystals Orientation of the liquid crystal between the Plates is enforced. This is the manufacture of the additional liquid crystal layer (s) considerably simplified.  

The liquid crystal display can be in transmissive or reflective mode. In transmissive fashion two polarizers are required, on either side the display are arranged. Here is the use two complementary polarizers preferred. Under complementary Polarizers are understood e.g. B. two crossed Linear polarizers or two circular or elliptical Polarizers with respect to one in a homeotropic direction incident light wave are complementary. Does that work Display in reflective mode, is the use of a Sufficient polarizer, that of the light rays will go through twice. But there are also those in reflective fashion includes working ads where two polarizers are used on either side the display are arranged.

The display contains at least one controllable Liquid crystal layer. The use of one or two, but especially one that can be controlled Liquid crystal layer, preferred.

Displays with only one controllable liquid crystal layer are characterized by a comparatively small Manufacturing effort and are therefore for a Series production particularly suitable. Through the Use of the additional liquid crystal layer with these displays a high viewing angle independence of contrast guaranteed.

Displays with several, but in particular two controllable, liquid crystal layers are characterized by a considerably higher production outlay, but their use can be indicated in the case of high requirements. The high viewing angle independence of the cell given by the additional liquid crystal layer can be further improved, for example, by a cell geometry based on two controllable liquid crystal layers. The molecules in the two controllable liquid crystal layers are oriented in such a way that after application of the voltage with the homeotropic direction they enclose angles which are the same in amount but have an opposite sign in the two layers. Furthermore, the switching times of the display can be considerably shortened by replacing a liquid crystal layer of thickness d with two liquid crystal layers for which the sum of the layer thicknesses is less than or equal to d .

The sum of the optical path differences d × Δ n of the controllable liquid crystal layers is between 0.1 µm and 2.5 µm. The range between 0.1 μm and 1.5 μm is preferred for d × Δ n and between 0.1 μm and 1.0 μm is particularly preferred. When using a liquid crystal layer, d × Δ n is preferably between 0.2 μm and 1.5 μm and the layer thickness d is preferably <10 μm, but in particular <8 μm. Layer thicknesses that are smaller than 6 μm are particularly preferred.

The optical path difference d × Δ n of the additional liquid crystal layers is adapted to the optical path difference of the controllable liquid crystal layers in such a way that the contrast is dependent on the viewing angle to a minimum. The layer thickness d of the additional liquid crystal layer (the sum of the layer thicknesses of the additional liquid crystal layers) is preferably <50 μm, in particular, however, less than 25 μm and very particularly less than 10 μm. The additional liquid crystal layer (s) can also be formed as a λ / 4 plate.

In Fig. 1, a preferred implementation of the liquid crystal display according to the invention is schematically outlined, which is intended to explain the invention in more detail, but without limiting it. The proportions of the individual components are assumed purely arbitrarily and do not allow any conclusions to be drawn about the geometry of a cell to be realized. A corresponding statement applies to the shape and size of the schematically indicated liquid crystal molecules.

In this special liquid crystal display, an additional liquid crystal layer and a controllable, nematic liquid crystal layer ( 6 ) are used, which is shown in the non-driven state. The nematic liquid crystal molecules of the controllable liquid crystal layer, which are indicated as cylindrical rods ( 12 ), are essentially homeotropically oriented and have a small pretilt angle α with the homeotropic direction. This alignment is forced by the surface orientation layers ( 5, 7 ) which are arranged on the transparent electrodes ( 10, 11 ). The electrodes are attached to the glass plates ( 4, 8 ) used as carrier plates. Parallel to the nematic liquid crystal layer is the additional liquid crystal layer ( 3 ) based on a discotic liquid crystal material between the glass plates ( 2 ) and ( 4 ). The liquid crystal molecules of this additional liquid crystal layer are shown as small disks ( 13 ) and are arranged essentially perpendicular to the normal of the glass plates, ie to the homeotropic direction. The display is provided with two polarizers ( 1 ) and ( 9 ) on both sides of the display.

Example 1

It becomes an electro-optic liquid crystal display the ECB principle, which is produced from a non-controllable Liquid crystal layer I and a controllable Liquid crystal layer II consists, the in the two Liquid crystal layers used liquid crystal mixtures are composed as follows:

Liquid crystal mixture I

34% 2,3,7,8,12,13-hexa-nonoyloxy-truxen
33% 2,3,7,8,12,13-hexa-decoyloxy-truxen
33% 2,3,7,8,12,13-hexa-undecoyloxy-truxen

Liquid crystal mixture II

9% trans-4- (trans-4-propyl-cis-4-cyanocyclohexyl) propylcyclohexane
5% 4- (trans-4-propyl-cis-4-cyanocyclohexyl) -4'-propylbiphenyl
28% 1- (trans-4-propylcyclohexyl) -2- (4- (4-ethyl-2-fluorophenyl) phenyl) -et-han
28% 1- (trans-4-propylcyclohexyl) -2- (4- (4-pentyl-2-fluorophenyl) phenyl) -e-than
26% 1- (trans-4-pentylcyclohexyl) -2- (4- (4-ethyl-2-fluorophenyl) phenyl) -et-han
4% 4- (trans-4-propylcyclohexyl) -2-fluoro-4 ′ - (trans-4-propylcyclohexyl) -b-iphenyl

The liquid crystal mixture in layer II has a dielectric anisotropy of Δε (20 ° C, µ kHz) = - 1.0 and an optical anisotropy of Δ n (20 ° C, 589 nm) = 0.1387. The thickness of the liquid crystal layer II is about d ∼7 microns. The liquid crystal mixture in layer I has a clearing point of 85 ° C; their thickness is chosen so that a minimal viewing angle dependence of the contrast of the electro-optical display results.

Claims (9)

1. Electro-optical liquid crystal display according to the ECB principle with at least one electrically controllable nematic liquid crystal layer with negative dielectric anisotropy, the molecules of which can be converted from a homeotropic orientation into a tilted planar orientation by applying a voltage, characterized in that the display improves the viewing angle dependence the contrast has at least one additional, non-controllable liquid crystal layer with a uniaxial, optically negative liquid crystal material. 2. Display according to claim 1, characterized in that the additional liquid crystal layers on planar oriented discotic and / or cholesteric Liquid crystals are based. 3. Display according to claim 2, characterized in that the additional liquid crystal layers from a nematic discotic liquid crystal material exist, which is between plane-parallel Carrier plates are located without orientation layers.   4. Display according to at least one of claims 1-3, characterized in that at least one of the two sides of the different Arrangement existing liquid crystal layers Polarizer located. 5. Display according to at least one of claims 1-4, characterized in that the sum of the optical path differences d × Δ n of the controllable liquid crystal layers is between 0.1 µm and 2.5 µm. 6. Display according to at least one of claims 1-5, characterized in that the sum of the optical path differences d × Δ n of the controllable liquid crystal layers and the sum of the optical path differences d × Δ n of the additional liquid crystal layers are adapted to one another so that a minimal viewing angle dependence of contrast results. 7. Display according to at least one of claims 1-6, characterized in that the sum of the layer thicknesses d of the additional liquid crystal layers is less than 50 microns. 8. Display according to at least one of claims 1-7, characterized in that the display in transmissive Mode is operated and that the cell two complementary devices for polarization included on both sides of the arrangement.   9. Use of discotic and / or cholesteric Liquid crystals as materials for additional, non-driven liquid crystal layers of electro-optical Liquid crystal displays according to the ECB Principle with improved viewing angle dependency of the Contrast.