EP1090327A1

EP1090327A1 - Monostable ferroelectric active matrix display

Info

Publication number: EP1090327A1
Application number: EP99929152A
Authority: EP
Inventors: Hans-Rolf DÜBAL; Rainer Wingen; Toshiaki Nonaka
Original assignee: Aventis Research and Technologies GmbH and Co KG
Current assignee: AZ Electronic Materials GmbH
Priority date: 1998-06-08
Filing date: 1999-06-08
Publication date: 2001-04-11
Also published as: US6661494B1; JP4728479B2; JP2002517798A; DE19825487A1; EP2348356A1; WO1999064925A1

Abstract

The invention relates to a monostable ferroelectric active matrix display containing a liquid crystal layer in the form of a monodomain having a clearly defined direction of the normals z to the layer of the smC* phase, whereby the liquid crystal layer preferably comprises a chevron C-1, Chevron C-2 or a bookshelf geometry.

Description

Monostable ferroelectric active matrix display

The replacement of the cathode ray tube (picture tube) with a flat screen requires a display technology that simultaneously has high image resolution, ie more than 1000 lines, high image brightness (> 200 Cd / m ² ), high contrast (> 100: 1), a high image frequency (> 60 Hz), sufficient color display (> 16 million colors), a large image format (> 40 cm screen diagonal), low power consumption and a wide viewing angle enable and also inexpensive to produce. So far, there is no technology that fully fulfills all of these features at the same time.

Many manufacturers have developed screens based on nematic liquid crystals, which have been used in the area of notebook PCs, personal digital assistants and desktop monitors for several years. The technologies STN (Supertwisted Nematics), AM-TN (Active Matrix - Twisted Nematics), AM-IPS (Active Matrix - In Plane Switching), AM-MVA (Active Matrix - Multidomain Vertically Aligned) are used in the literature are described in detail, see, for example, T. Tsukuda, TFT / LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and the literature cited therein; SID Symposium 1997, ISSN-0097-966X, pages 7 to 10, 15 to 18, 47 to 51, 213 to 216, 383 to 386, 397 to 404 and the literature cited therein. In addition, the technologies PDP (Plasma Display Panel), PALC (Plasma Addressed Liquid Crystal), ELD (Electro Luminescent Display) and FED (Field Emission Display) are applied, which are also explained in the quoted SID report. Clark and Lagerwall (US 4,367,924) were able to show that the use of ferroelectric liquid crystals (FLC) in very thin cells leads to optoelectric switching or display elements which, compared to the conventional TN ("twisted nematic") cells, by up to a factor 1000 faster switching times, see also EP-A 0 032 362. Because of these and other favorable properties, for. B. the bistable switching option and the almost independent viewing angle contrast, FLCs are basically suitable for applications such as computer displays and televisions, as a monitor marketed by Canon in Japan since May 1995 shows.

For the use of FLCs in electro-optical or completely optical components, one requires either connections that form smectic phases and are themselves optically active, or one can optically by doping compounds that form such smectic phases but are not themselves optically active active compounds induce ferroelectric smectic phases. The desired phase should be stable over the largest possible temperature range.

The individual picture elements (pixels) of an LC display are usually arranged in an x, y matrix, which is arranged by a series of electrodes

(Conductor tracks) along the rows and the columns on the bottom and top of the display. The crossing points of the horizontal (row) and vertical (column) electrodes form addressable pixels.

This arrangement of the pixels is usually referred to as a passive matrix. Various multiplexing schemes have been developed for addressing, as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93 and contacts 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler manufacture of the display and the associated low manufacturing costs, but the disadvantage that passive addressing can always be done line by line, which means that the addressing time of the entire screen is N lines is N times the row addressing time. With the usual line addressing times of approx. 50 microseconds, this means a screen addressing time of approx. 60 milliseconds with, for example, HDTV standard (High Definition TV, 1152 lines), ie a maximum frame rate of approx. 16 Hz. This frequency is too low for the display of moving pictures . The display of grayscale is also difficult. Mizutani et.al. who on the occasion of the FLC Conference in Brest, France (20 to 24 May 1997, see Abstract Book 6 ^th International Conference on Ferroelectric Liquid Crystals, Brest / France) a passive FLC display with digital gray scale presented in which each of the RGB Pixels (RGB = red, green, blue) were subdivided into sub-points, which enables the display of gray values in digital form by means of partial switching. With N gray values

N using three basic colors (red, green, blue) results in 3 colors. The

The disadvantage of this method is a large increase in the number of display drivers required and thus in the costs. In the case of the screen shown in Brest, three times as many drivers were required as with a normal FLC display without digital grayscale.

In so-called active matrix technology (AMLCD), a non-structured substrate is usually combined with an active matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated on each pixel of the active matrix substrate. The nonlinear element can also be diodes, metal insulator metal, etc. Act elements which are advantageously produced using thin-film processes and are described in the relevant literature, see e.g. T. Tsukuda, TFT / LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach 1996, ISBN 2-919875-01-9 and the literature cited therein.

Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in plane switching) mode. In any case, the active matrix generates an electric field of individual strength at each pixel, which produces a change in orientation and thus a change in birefringence, which in turn is visible in polarized light. A serious disadvantage of this method is the lack of video capability due to the long switching times of nematic liquid crystals.

For this reason, among others, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, see e.g. WO 97/12355 or Ferroelectrics 1996, 179, 141-152, W.J.A.M. Hartmann, IEEE Trans. Electron. Devices 1989, 36, (9; Pt. 1), 1895-9.

Hartmann used a combination of the so-called 'quasi-bookshelf geometry' (QBG) from FLC and a TFT (thin-film transistor) active matrix and at the same time received a high switching speed, grayscale and high transmission. However, the QBG is not stable over a wide temperature range because the field-induced layer structure breaks up or rotates due to the temperature dependence of the smectic layer thickness. In addition, Hartmann uses an FLC material with a spontaneous polarization of more than 20 nC / cm ² , which leads to large electrical charges with pixels with realistic dimensions of, for example, 0.01 mm ² area (Q = 2 AP, A = applies when saturated) Pixel area, P = spontaneous polarization), which cannot reach the pixel during the opening time of the TFT, for example with inexpensive amorphous silicon TFT. For these reasons, this technology has not been pursued until now.

While Hartmann uses the charge-controlled bistability to display an almost continuous gray scale, Nito et. al. proposed a monostable FLC geometry, see Journal of the SID, 1 12, 1993, pages 163-169, in which the FLC material is oriented with the aid of relatively high voltages in such a way that only a stable position arises, from which then by application of an electric field via a thin film transistor Intermediate states are generated which correspond to a number of different degrees of brightness (gray values) when the cell geometry between crossed polarizers is adapted.

However, a disadvantage of this approach is the appearance of a stripe texture on the display that limits the contrast and brightness of this cell (see Fig. 8 in the above quotation). The disadvantageous stripe texture can be corrected by treatment with a high electrical voltage (20-50 V) in the nematic or cholesteric phase (see p. 168 of the above quotation); however, such field treatment is not suitable for the mass production of screens and generally does not lead to temperature-stable textures. In addition, this method only results in switching in an angular range of up to a maximum of the simple tilt angle, which in the case of Nito et. al. The material used is approx. 22 ° (see p. 165 Fig. 6) and thus only results in a transmission of a maximum of 50% of the transmission of two parallel polarizers.

It is an object of the present invention to provide a ferroelectric active matrix liquid crystal display which contains a ferroelectric liquid crystal mixture, the liquid crystal mixture assuming a monostable position but not forming a stripe texture, being temperature-stable and allowing a very high maximum transmission and a very high contrast.

The object is achieved according to the invention by a monostable ferroelectric active matrix display comprising a liquid crystal layer, preferably in chevron C2 geometry, in the form of a monodomain with a clearly defined direction of the layer normals z of the chiral-smectic phase, the ratio of the sum of pretilt - and layer tilt angle to tilt angle (WR = (LkW + Prw) / Tiw) is greater than 0.1 and preferably the amount of the dielectric anisotropy DA is less than 3.

The spontaneous polarization P in the liquid crystal layer is preferably between 0.1 and 15 nC / cm ² . The tilt angle Tiw is preferably between 9 and 40 ° in the liquid crystal layer.

The ratio of the product of anchorage strength and the sine of the tilt angle to the spontaneous polarization (AS sin Tiw / P) is preferably less than 20 V / μm.

The object is further achieved by a monostable ferroelectric active matrix display containing a liquid crystal layer in chevron C1 geometry in the form of a monodomain with a clearly defined direction of the layer normals z of the SmC * phase, the pretilt angle Prw being at least 5 ° and that The ratio of the pretilt angle to the tipping angle (Prw / truck) is greater than 0.7.

The object is further achieved by a monostable ferroelectric active matrix display containing a liquid crystal layer in bookshelf geometry in the form of a mono-domain with a clearly defined direction of the layer normal z of the smC * phase, the pretilt angle Prw being at least 1 °.

The object is further achieved by a monostable ferroelectric avtiv matrix display containing a liquid crystal layer in the form of a monodomain with a clearly defined direction of the layer normal z of the smC ^* phase, which has the following properties:

Spontaneous polarization P between 0.1 and 15 nC / cm,

Tiltwinkel Tiw between 9 and 45 °,

Absolute amount of the ratio of layer tilt angle to tilt angle of at least 0.2, Helical pitch in the chiral nematic (cholesteric) phase within the temperature range of 5 ° C above the smectic-nematic phase transition or, if the existential range of the nematic (cholesteric) phase is less than 5 ° C, then in a temperature range of at least 80% of the nematic phase range of at least 50 μm and

Dielectric anisotropy absolute value less than 3.

The liquid crystal layer preferably has one or more, in particular all, of the following features:

The angle between the layer normal z of the smC ^* phase and the preferred direction n of the nematic or cholesteric phase (N * phase) is in a range of 0.5 to 1.0 times the smC ^* tilt angle, at least however 5 °,

The ferroelectric liquid crystal layer has a phase sequence

l ^* -N ^* -smC *

whereby between the N ^* and the smC ^* phase there can be an smA * phase with a range of existence of at most 2 °, preferably at most 1 °.

The active matrix FLCD according to the invention preferably contains, as an optically active layer, a ferroelectrically liquid-crystalline medium (liquid-crystal phase) with a phase sequence of

Isotropic - Nematic or Cholesteric (N *) - Smectic C * or a phase sequence

Isotropic - nematic or cholesteric (N *) - smectic A ^* - smectic C *,

wherein the smectic A * phase has a range of existence (phase range) of at most 2 ° C., preferably at most 1 ° C., particularly preferably at most 0.5 ° C. The asterisk (*) on the phase label indicates that it is a chiral phase.

The displays are preferably produced by a process in which the liquid crystal layer is introduced into the space between a rubbed upper substrate plate and a rubbed lower substrate plate of the active matrix display, the rubbing directions on the upper and lower substrate plates being essentially parallel, and the Liquid crystal phase cools from the isotropic phase, with an electrical voltage being present on the display at least during the phase transition N ^* - »smC ^* or N * → smA * → smC ^* .

The FLC mixture is filled into an active matrix display. The manufacture and components of such an AM display is described in detail in the Tsukuda literature listed above. However, unlike nematic displays, the thickness of the FLC layer is only 0.7 to 2.5, preferably 1-2 μm. In addition, the rubbing directions on the upper and lower substrate plates are essentially parallel. The term "substantially parallel" includes anti-parallel or weak, i.e. friction directions crossed by up to 10 °.

It is important for the functioning of this display that an electrical direct voltage, preferably below 5 V, is applied during the production of the display during controlled cooling and during the phase transition N ^* -> smC ^* or N -> smA * -> smC * is maintained, which leads to the fact that the entire display occupies a monostable mono-domain that appears completely dark between crossed polarizers. After receiving this domain, the DC voltage is switched off. The texture thus obtained is monostable in contrast to Hartmann's approach mentioned above or in contrast to conventional bistable FLCD. This means that the preferred n-director (which indicates the preferred direction of the molecular longitudinal axes) is located in the rubbing direction of the cell, whereas the z-director (which indicates the preferred direction of the smectic layer normal) is inclined approximately by the amount of the tilt angle Rubbing direction is located. This constellation is exactly the opposite of the usual bistable cell according to Clark and Lagerwall, where the z-director lies in the rubbing direction.

In contrast to Nito's approach, with this orientation there are no two positional normals and thus no two orientation domains that ultimately lead to the disturbing stripe texture mentioned above, but only a clear direction of the z director and therefore a monodomain. In addition, the double tilt angle, which leads to 100% transmission in relation to parallel polarizers, is now accessible, i.e. double brightness is achieved.

The display thus obtained appears completely dark with a suitable angle of rotation between crossed polarizers. When a control voltage of only a few volts is applied, it appears bright, whereby the brightness can be varied continuously over the voltage and, when saturated, has almost the brightness of two parallel polarizing foils. The angle between the preferred direction of the nematic (or cholesteric) phase and the layer normal (z director) is ideally and therefore preferably the tilt angle of the smectic C phase, or at least substantially the tilt angle. "In essence" in the sense of this invention preferably means a value range from half to the full, particularly preferably 0.5 to 1 times the tilt angle, but at least 5 °. The ferroelectric active matrix liquid crystal display according to the invention is highly practical, in particular for TV and HDTV or multimedia, since it agrees high transmission, short switching times, gray scale and therefore full color capability, cost-effective production and a wide temperature range. In addition, the display can preferably be operated at voltages of <10 volts, preferably <8 V, particularly preferably <5 V.

For the display of gray values or as many natural colors as possible, the characteristic curve (transmission plotted against voltage) of the liquid crystal mixture should be flat enough on the one hand to address the gray values with the available voltages, on the other hand the saturation voltage should not be too high.

The saturation voltage V90, at which 90% of the maximum transmission is reached, should not be too high so that the display can be operated with voltages below 30V, preferably below 15 V, preferably below 10 V, particularly preferably below 8 V, especially below 5 V. . The threshold voltage V10 should preferably be adapted to V90 to ensure that the characteristic curve width KLB is large enough to be able to address a sufficiently high number of gray values. This is generally the case if the characteristic curve width KLB = V90-V10 is at least 100 mV, preferably at least 200 mV, preferably at least 500 mV, particularly preferably at least 1 V, especially at least 1.5 V.

In addition, the maximum transmission of the cell should generally be at least 40% (based on an empty cell between two parallel polarization foils), preferably at least 50%, preferably at least 70%, particularly preferably at least 80%, especially at least 90%. Furthermore, the T, V- Characteristic curve should be strictly monotonically increasing (with increasing voltage). A decrease in transmission after reaching a transmission maximum is not desirable.

The subject of this invention is therefore the selection of liquid crystals and mixtures thereof with suitable material parameters for the advantageous setting of the characteristic curve.

The invention further relates to a monostable active matrix FLC display, in which an optimal characteristic is obtained by the selection of liquid crystals or their mixtures through a targeted combination of several material properties of the ferroelectric liquid crystal, and the use of liquid crystal mixtures of these properties for active Matrix FLC displays.

In particular, active matrix display in the sense of the present invention is also understood to mean an LCD in which one of the two substrates is replaced by the back of an IC chip (IC = integrated circuit), as is the case, for example, with D.M. Walba, Science 270, 250-251 (1995).

Abbreviations are used in the examples and in the description of the invention, which are explained in the table below.

Definitions and abbreviations (abbr.):

DC Ulrich and SJ Eiston describe in Ferroelectrics, volume 178, pp. 177 to 186 (1996) the C1, C2 geometries etc. The characteristic curve of the ferroelectric liquid crystal cell according to the invention is influenced by a number of parameters, which should preferably take on their own or in combination preferred value ranges so that optimal switching conditions are achieved. In particular, this involves spontaneous polarization (P), the tilt angle (Tiw), the layer tilt angle (truck), the pretilt angle (Ptw), the anchorage strength (AS), the dielectric anisotropy (DA), and also the layer rotation angle (Sdw), the cell thickness (d), the helical pitch of the cholesteric phase (pitch) and the smectic C ^* phase, and the optical anisotropy (OA).

It is found that all of these parameters influence the characteristic, albeit to different degrees. The characteristic (T, V) should preferably have the following properties.

In contrast to all the usual experiences with ferroelectric LCDs, it was found that the characteristic curve is not influenced or only influenced to a very small extent by the rotational viscosity, instead, e.g. the saturation voltage V90 is strongly determined by the spontaneous polarization (P) and the anchoring strength (AS).

The spontaneous polarization (P) should preferably be between 0.1 nC / cm ² and 15 nC / cm ² (here the absolute amount of P is always meant), preferably between 0.2 and 10 nC / cm ² , preferably 0.4 to 8 nC / cm ² 0.5 to 6 nC / cm ² is particularly preferred, especially 0.8 to 3.5 nC / cm ² . The tilt angle should preferably be in the range from 9 ° to 45 °, preferably between 12 ° and 35 °, preferably 14 ° to 31 °, particularly preferably 17 ° to 27 °, especially 19 ° to 25 °.

The layer rotation angle (Sdw = angle between the preferred direction of the nematic phase and the smC ^* layer normal) should preferably be at least 5 °.

The absolute amount of dielectric anisotropy should preferably be less than 3 (three), preferably less than 2.5, particularly preferably less than 1.8, especially less than 1.2.

The product of the anchoring energy (AS) and the sine of the tilt angle divided by the spontaneous polarization (P) should preferably be less than 20 V / μm, preferably less than 15 V / μm, preferably less than 12 V / μm, particularly preferably less than 9 V / μm, especially less than 6 V / μm.

The parameter specifications relate to at least one temperature in the working range of the ferroelectric liquid crystal display.

The display according to the invention can be operated not only in the area of the smectic C ^* phase, but also, at least in part, in the area of another tilted smectic phase, with analogous transmission of the properties mentioned above.

The monostable active matrix FLC display according to the invention can be in the Chevron C1, the Chevron C2 or the bookshelf or quasi-bookshelf Arrangement (or 'geometry') are operated. The preferred combination of value ranges for spontaneous polarization and tilt angle applies to all three geometries:

Even more particularly preferred are the above-mentioned combinations of P and Tiw together with an absolute amount of dielectric anisotropy which is less than three (3), preferably less than two (2), particularly preferably less than 1.5, especially less than 1.2.

The pitch of the cholesteric helix (pitch) should be longer than 50 μm in a temperature range of 5 ° above the transition to the smectic phase or, in the case of a smaller range of existence of the cholesteric phase, preferably in the range of 80% of this range of existence. A pitch of at least 70 μm, very particularly preferably of at least 100 μm, is preferred in order to achieve high contrasts.

In the C2 geometry, which is normally preferred for small pretile angles and completely disappears for high pretile angles in the limit case (Ptw> Tiw), the Layer tilt angle (truck) together with the pretilt angle (Ptw) preferably have the following relation to the tilt angle (Tiw): the ratio WR from the sum of the pretilt angle and layer tilt angle to the tilt angle (i.e. (truck + Ptw) / Tiw) should generally be at least 0.1 , preferably at least 0.15, preferably at least 0.25, particularly preferably at least 0.5, especially at least 0.7. The amount of dielectric anisotropy DA should be less than 3.

In particular, there are advantageous characteristic curves with the combinations listed in the table below:

Even more particularly preferred are the above-mentioned combinations of P and Tiw together with an absolute amount of dielectric anisotropy which is less than 3, preferably less than 2, particularly preferably less than 1.5, especially less than 1.2.

In general, the C2 geometry is preferred over C1 and 'bookshelf. In the C1 geometry, which occurs preferentially at high pretile angles, the pretile angle should be at least 5 ° with a suitable position tilt angle, since otherwise no switching occurs at low voltage.

With a suitable tipping angle, the tipping angle (truck) and the pretilt angle (Prw) should advantageously have the following relationship: the ratio of the pretilt angle and the tipping angle should be greater than 0.7.

In particular, there are advantageous characteristic curves with the combinations listed in the table below (for C1 geometry):

Even more particularly preferred are the above-mentioned combinations of P and Tiw together with a dielectric anisotropy of greater than -1, preferably a positive dielectric anisotropy, ie> 0. In the bookshelf geometry, which in the sense of the present invention should be defined for the range -5 ° <truck> + 5 ° (ideal truck = 0), the pretilt angle should generally be at least 1 °, preferably at least 2 °. Particularly preferably, the pretilt angle should be at least 1 °, the spontaneous polarization should be at least 0.1 nC / cm ² , at most 15 nC / cm ² and the tilt angle should be at least 12 °.

The spontaneous polarization in the tables above can also preferably be 0.1 to 15 nC / cm ² .

A mixture particularly suitable for use in the display according to the invention has at least six (6), preferably at least eight (8), particularly preferably at least nine (9), especially at least eleven (11) components, which are selected such that the spontaneous polarization is between 0.1 and 15 nC / cm ² , the tilt angle is between 17 ° and 27 °, the ratio of the tilt angle to the tilt angle is at least 0.3, the pitch of the chiral nematic phase is at least 50 μm (range of 5 ° C above the phase transition), which Absolute amount of dielectric anisotropy is less than 3, and the phase sequence

isotropic-nematic-smectic C *

or

isotropic-nematic-smectic A-smectic C ^*

with a smectic A phase existence area of at most 2 ° C. The following examples are intended to explain the invention in more detail.

Examples:

1 . Production of an oriented, monostable FLC cell:

A glass substrate coated with transparent - conductive indium tin oxide is patterned in a photolithographic process, so that an electrode pattern is obtained. The transparent conductor tracks of this

Electrode structures are used for the electrical control of the display by means of a function generator, and so the switching behavior of a thin film transistor is simulated. Two such structured glass panes, which form the top and bottom of the display - that is, the carrier plates - are provided with orientation layers, these are rubbed and with the help of an adhesive frame with the addition of a concentration of 0.5% by weight spacer balls with a diameter of 1, 3 μm assembled. The adhesive is cured by careful heating, the liquid crystal mixture is filled at 100 ° C. by capillary forces and the cell is brought to a temperature above the IN ^* phase transition by slow cooling. At this

Temperature is applied to a direct voltage of 4 V and then the cooling process is continued to 22 ° C. The DC voltage is then switched off. A monostable monodomain is obtained which appears completely dark between crossed polarizers.

2. Determination of the characteristic curve (T, V characteristic):

The cell is now powered by monopolar pulses with a length of 10 ms and a spacing of 30 ms with voltages of variable amplitude by means of a function generator (model Wavetech) and amplifier (manufacturer: Krohn-Hite) wired, and the transmission is measured using a photodiode and oscilloscope. This gives the transmission (as a photodiode signal) as a function of the voltage. This characteristic curve generally shows a saturation of the transmission at high voltages, this value is referred to as 100%. V10 is now the threshold voltage at which the brightness is just 10% of the saturation transmission, V90 is the voltage at which 90% of the saturation transmission is achieved.

3. Determination of anchorage strength (AS)

The empirical anchoring strength AS is obtained from the value for V90 obtained in Example 2, the layer thickness and the spontaneous polarization according to the following formula.

AS (in J / m ³ ) = 10 * V90 (in volts) ^* P (in nC / cm ² ) / sin (Tiw) ^* d (in μm)

Once the value for AS has been determined for a mixture and a cell type, variations of P, d but also of other parameters can be made, provided the chemical nature of the mixture composition does not change too much and therefore the value for AS refers to the design of a AM-FLC displays can be transmitted.

4. An FLC display with a cell thickness of 1.25 μm is provided with an FLC mixture with the phase sequence lN * -smC ^* , which has the following physical data:

Geometry: C2

Tilt angle: 21 ° Layer tilt angle: 18 °

Truck / Tiw: 0.86

Pretilt angle: 0 °

Optical anisotropy: 0.17

filled and oriented.

The following TN characteristics (characteristic curves) are obtained when the P and AS are varied:

Advantageous switching is possible in the range of P from 0.1 to 15 nC / cm ² , but at low P values it is preferably associated with weak anchoring.

An FLC display with a cell thickness of 1.25 μm is equipped with an FLC mixture with the phase sequence lN * -smC *, which has the following physical data: Geometry: C2

Pretilt angle: 0 °

Optical anisotropy: 0.17

filled and oriented.

The following characteristic curves are obtained when the tilt angle Tiw is varied:

here pretilt 5 °, ** with intermediate maximum at 5 V / 73%

Advantageous switching is possible in the range from Tiw = 10 ° to 35 °. In these examples, the product of optical anisotropy and cell thickness was not optimized, so that there is a maximum transmission of only about 80%. By adjusting the cell thickness, for example, all of the maximum transmissions obtained here can be increased by a factor of 1.25. This gives you almost 100% in preferred configurations.

An FLC display with a cell thickness of 1.25 μm is filled with an FLC mixture with the phase sequence l-N * - smA * (with a range of existence <2 °) -smC *, which has the following physical data:

Geometry: C2

Dielectric anisotropy: -1 Optical anisotropy: 0.17

filled and oriented.

When the truck and Prw are varied, the following T, V characteristics (characteristic curves) are obtained WR = (truck + Prw) / Tiw:

Advantageous switching is possible in the range of WR> 0.1. With smaller values, the characteristic curve is too steep to realize gray values or no switching is even observed.

7. An FLC display with a cell thickness of 1.25 μm is provided with an FLC mixture with the phase sequence lN ^* -smC ^* , which has the following physical data:

Geometry: C2

Tilt angle: 25 °

Layer tilt angle: 15 °

TruckTiw: 0.60

Pretilt angle: 0 °

Dielectric anisotropy: -1 Optical anisotropy: 0.17 Spontaneous polarization: 1.5 nC / cm ²

filled and oriented.

The following T, V characteristics (characteristic curves) are obtained when the AS is varied:

^* here Ps = 0.75 nC / cm ²

Advantageous characteristic curves are obtained at low values of AS ^* sin (Tiw) / P, in particular at AS ^* sin (Tiw) / P <20 V / μm, preferably <16 V / μm.

8. An FLC display with a cell thickness of 1.25 μm is provided with an FLC mixture with the phase sequence lN ^* -smC ^* , which has the following physical data:

Geometry: C2

Tilt angle: 22 °

Layer tilt angle: 15 °

Truck / Tiw: 0.682

Pretilt angle: 0 °

Optical anisotropy: 0.17

Spontaneous polarization: 0.5 nC / cm ²

filled and oriented.

Advantageous characteristics are obtained with low values of the absolute value of the dielectric anisotropy, in particular with | DA | <3, preferably | DA | <2, particularly preferred | DA | <1.5, especially | DA | <1.2.

An FLC display with a cell thickness of 1.25 μm will have the following physical data with an FLC mixture with the phase sequence lN ^* -smC \:

Geometry: C1

Spontaneous polarization: 1.5 nC / cm ²

Tilt angle: 20 °

Layer tilt angle: 15 ° (C1, tilt direction other than C2)

Truck / tiw: 0.75

Dielectric anisotropy: -1 Optical anisotropy: 0.17

filled and oriented.

The following T, V characteristics (characteristic curves) are obtained when the pretilt angle is varied:

In the C1 geometry, no switching is observed at small pretilt angles. With the usable minimum value of the tilt angle of more than 9 °, at least a pretilt of 5 ° is required in order to obtain a usable characteristic.

If the position tilt angle is set below 5 ° (according to the amount), switching is only observed with a pretilt angle of at least one degree.

Prw is at least 5 °, Prw / truck greater than 0.7.

Claims

claims

1. Monostable ferroelectric active matrix display, containing a liquid crystal layer in the form of a monodomain with a clearly defined direction of the layer normal z of the smC * phase, the ratio of the sum of the pretilt and layer tilt angle to tilt angle (WR = (truck + Prw) / Tiw) greater than 0.1 and the amount of the dielectric anisotropy DA is less than 3.

2. Active matrix display according to claim 1, characterized in that in the liquid crystal layer the spontaneous polarization P is less than 15 nC / cm2.

3. Active matrix display according to claim 1 or 2, characterized in that in the liquid crystal layer the tilt angle Tiw is between 9 and 40 °.

4. Active matrix display according to one of claims 1 to 3, characterized in that the ratio of the product of anchorage strength and the sine of the tilt angle to the spontaneous polarization (AS sin Tiw / P) is less than 20 V / μm.

5. Monostable ferroelectric active matrix display, containing a liquid crystal layer in chevron C1 geometry in the form of a monodomain with a clearly defined direction of the layer normal z of the smC ^* phase, the pretilt angle Prw being at least 5 ° and the ratio of pretilt angle to layer tilt angle (Prw / Lkw) is greater than 0.7.

6. Monostable ferroelectric active matrix display, containing a liquid crystal layer in bookshelf geometry in the form of a mono-domain with a clearly defined direction of the layer normal z of the smC ^* phase, the pretilt angle Prw being at least 1 °.

7. Monostable ferroelectric active matrix display, comprising a liquid crystal layer in the form of a monodomain with a clearly defined direction of the layer normals z of the smC * phase, which has the following properties:

Spontaneous polarization P between 0.1 and 15 nC / cm2,

Tiltwinkel Tiw between 9 and 45 °,

Absolute amount of the ratio of layer tilt angle to tilt angle of at least 0.2,

Helical pitch in the chiral nematic (cholesteric) phase within the temperature range of 5 ° C above the smectic nematic phase transition of at least 50 μm and

Dielectric anisotropy absolute value less than 3.

8. Active matrix display according to one of claims 1 to 7, in which the liquid crystal layer has one or more of the following properties: The angle between the layer normal z of the smC * phase and the preferred direction n of the nematic or cholesteric phase (N ^* phase) is in a range of 0.5 to 1.0 times the smC ^* tilt angle, at least however 5 ° C

The ferroelectric liquid crystal layer has a phase sequence

l * -N ^* -smC ^*

whereby between the N ^* and the smC * phase there can be an smA * phase with a maximum existence of 2 °.

9. A method for producing active matrix displays according to one of claims 1 to 8, in which the liquid crystal layer is introduced into the space between a rubbed upper substrate plate and a rubbed lower substrate plate of the active matrix display, the rubbing directions on the upper and lower substrate plates essentially in parallel, and the liquid crystal phase cools from the isotropic phase, an electrical voltage being present on the display at least during the phase transition N ^* smC * or N ^* smA * smC *.

10. active matrix displays, producible by the method according to claim 9.

11. Use of active matrix displays according to one of claims 1 to 8 and 10 in the TV, HDTV or multimedia area or in the field of information processing, in particular in notebook PCs, personal digital assistants and desktop monitors.