WO2018172232A1

WO2018172232A1 - Light modulation element

Info

Publication number: WO2018172232A1
Application number: PCT/EP2018/056785
Authority: WO
Inventors: Phil Baker; Alex HOLT; Joseph SARGENT
Original assignee: Merck Patent GmbH
Current assignee: Merck Patent GmbH
Priority date: 2017-03-21
Filing date: 2018-03-19
Publication date: 2018-09-27
Anticipated expiration: 2019-09-21
Also published as: GB2575579B; GB2575579A; GB201915019D0

Abstract

The invention provides a defect-free light modulation element utilizing the vertically aligned deformed helix ferroelectric liquid crystal (VADHFLC) mode. Further, the present invention relates to a method of production of such light modulation element, to the use of such light modulation element in an electro-optical device, i.e. in a LC display device, and to an electro- optical device comprising the light modulation element according to the present invention.

Description

Light modulation element

TECHNICAL FIELD The invention provides a light modulation element utilizing the vertically aligned deformed helix ferroelectric liquid crystal (VADH FLC) mode.

Further, the present invention relates to a method of production of such light modulation element, to the use of such light modulation element in an electro-optical device, i.e. in a LC display device, and to electro-optical devices comprising the light modulation element according to the present invention.

STATE OF THE ART Liquid Crystal Displays (LCDs) are widely used to display information. LCDs are used for direct view displays, as well as for projection type displays. The electro-optical mode, which is employed for most displays, is still the twisted nematic (TN)-mode with its various modifications. Besides this mode, the super twisted nematic (STN)-mode, more recently the optically compensated bend (OCB)-mode, the electrically controlled birefringence (ECB)-mode with their various modifications, as e. g. the vertically aligned nematic (VAN), the patterned ITO vertically aligned nematic (PVA)-, the polymer stabilized vertically aligned nematic (PSVA)- mode, the multi domain vertically aligned nematic (MVA)-mode, as well as others, have been increasingly used.

Further to the above-mentioned modes, the so-called ferroelectric liquid crystal (FLC) mode is known for example from US 2016/0017226 A1 . For practical use of ferroelectric liquid crystals in electro-optical displays, chiral, tilted smectic phases, such as smectic C phases, are required [R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Physique 36, L-69 (1975)], which are stable over a broad temperature range. This aim can be achieved by means of compounds which themselves form such phases, for example smectic C phases, or by doping compounds, which do not form chiral, tilted smectic phases, with optically active compounds [M. Brunei, C. Williams, Ann. Phys. , 3, 237 (1978) ].

It is well reported in literature that vertically aligned smectic modes allow for easier alignment than planar smectic modes, which often exhibit so called zig-zag defects, as for example disclosed by Lee et al. IDW 99 LCT4-1 1999 .

The vertically aligned deformed helix ferroelectric helix liquid crystal (VADH FLC) mode comprises a chiral smectic C mixture with a pseudo

homeotropic alignment of the liquid crystal director where the helix axis is vertically aligned with respect to the cell substrate's main plain, such as, for example disclosed in US 8,158, 020 B2. When 2-P-n_aVe, wherein n_ave is the average refractive index of the LC and P is the pitch as the length over which the tilted molecules in the vertical aligned helix processes 360 degrees, is shorter than the wavelength of visible light (< 380 nm) wave guiding and Bragg reflections in the visible part of the spectrum are eliminated and a good dark state is observed between cross polarisers.

The cell can be switched by applying an electric field perpendicular to the helix axis. The electric field couples with the spontaneous polarization (Ps) of the helix mode causing the helix to distort, producing a birefringence and hence transmission of light between crossed polarisers. The transmission varies with the applied field strength meaning that grey scales can be realized.

One of the advantage of this VADH FLC mode in comparison with nematic modes is that it is very fast, where the switching time is of the order of tens of microseconds, which allows e.g. field sequential colouring or switching and eliminates the need for colour filters in displays thereby improving transmission efficiency. In this regard, Kim et al. Journal of SID 16/9 (2008) disclose microsecond switching utilizing the VADH FLC mode with driving fields of 3 V/μιτι. Furthermore, Lee et al. suggest in Optical Express 13 7732 (2005) a working cell in the VADH FLC mode with in-plane electrodes. However, a non-uniform transmission and edge defects are observed. Typically, VADH FLC modes utilizing IPS electrodes show alignment defects along the electrodes after switching above a certain threshold voltage, like e.g. described by Wu et al. in Opt. Express 13, 7732 (1005) or by Lee et al. in Mol. Cryst. Liq. Cryst. 453, 343 (2006). These alignment defects spoil the dark state and hence the contrast ratio for display applications. The electrode edge defects are a consequence of the nonuniform electric field profile of in-plane electrodes, which contain a large vertical electric field component near the electrode edge. This couples to the spontaneous polarisation of the vertical helix mode creating a torque, which forces the helix to turn over into the horizontal plane.

US 2008/0204608 A1 discloses the use of electrodes that extend through a liquid crystal cell in the standing vertical helix chiral mode. However, this application uses very thick cells (> 50 μιτι) with a long chiral pitch to deflect light by switching the chiral pitch. Consequently, through cell electrodes are required because of the large thickness of the cell, as IPS electrodes would suffer from electric field deterioration. However, US 2008/0204608 A1 does not refer to field induced defects forming when switching and the use of the extended electrodes as means to prevent such defects.

Moreover, the disclosed devices require an extremely large cell gap of 50 μιτι or more in order to realize a device that has sufficient beam deflection because they are operated in a refractive mode rather than by altering the polarisation state of the light.

Cho et al. suggest in IMID Digest 2012, 776 a polymer stabilization in order to overcome the alignment defects in the VADH FLC mode. However, polymer stabilization requires another processing step and the effect of the polymer stabilization on the switching voltage and times is unknown.

A general object of the present invention is to alleviate the above problems and to provide an alternative to the known light modulation elements of the prior art, or preferably, to provide an improved light modulation element. In particular, an object of the invention is to provide a light modulation element operated in the VADH FLC mode having the capability of generating high contrast and wide viewing angle images, favourable dark states, and exhibiting fast switching, more particularly to reduce the total switching time enabling a satisfactory display of moving images. Further objects of the present invention are to increase the optical aperture ratio and to increase the transmittance of the light modulation element. In view of the numerous requirements and parameters summarized above, surprisingly, the inventors of the present invention have found that electrodes that pass through the vertical thickness of the cell can realize defect free light modulation elements utilizing the VADH FLC mode. Thus, the invention relates to a light modulation element comprising a pair of transparent substrates, a pair of polarizers, a liquid crystal medium capable of forming a chiral smectic C phase in homeotropic orientation that is located in between the pair of substrates, and an electrode structure which generates an electric field (parallel electric field) in directions parallel to the substrates main plane, characterized in that the electrode structure is formed by at least two electrodes protruding toward the interior of the formed cell.

TERMS AND DEFINITIONS

The term "light modulation element" relates to devices capable of altering the phase or polarisation state of the light. Devices that are operated in refractive modes are excluded. The term "liquid crystal (LC)" relates to materials having liquid-crystalline mesophases in some temperature ranges (thermotropic LCs) or in some concentration ranges in solutions (lyotropic LCs). They obligatorily contain mesogenic compounds.

The terms "mesogenic compound" or "liquid crystal compound" are taken to mean a compound comprising one or more uniaxial calamitic (rod-, brick-, or board/lath-shaped) or uniaxial discotic (disk-shaped) mesogenic group. The term "mesogenic group" means a group with the ability to induce liquid-crystalline phase (or mesophase) behaviour. The compounds comprising mesogenic groups do not necessarily have to exhibit a liquid- crystalline mesophase themselves. It is also possible that they show liquid- crystalline mesophases only in mixtures with other compounds.

A calamitic mesogenic group usually comprises a mesogenic core. The mesogenic core consists of one or more aromatic or non-aromatic cyclic groups, which are connected to each other directly or via linkage groups and optionally comprising terminal groups attached to the ends of the mesogenic core. Optionally, the mesogenic group comprises one or more groups that are laterally attached to the long side of the mesogenic core, wherein these terminal and lateral groups are usually selected e.g. from carbyl or hydrocarbyl groups, polar groups like halogen, nitro, hydroxy, etc..

For the purposes of the present invention, the term "liquid-crystalline medium" or "liquid crystal material" is taken to mean a material, which exhibits liquid-crystalline properties under certain conditions. In particular, the term is taken to mean a material, which forms a liquid-crystalline phase under certain conditions. A liquid-crystalline medium may comprise one or more liquid-crystalline compounds and in addition further substances. The term "alignment" or "orientation" relates to the alignment (orientational ordering) of anisotropic units of material such as small molecules or fragments of big molecules in a common direction named "alignment direction". In an aligned layer of liquid-crystalline material or medium, the liquid-crystalline director coincides with the alignment direction so that the alignment direction corresponds to the direction of the anisotropy axis of the material.

The term homeotropic orientation/alignment for VADHF mode refers to the chiral smectic C helix axis being normal to the substrate. The term "director" is known in prior art and means the preferred

orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axes (in case of discotic compounds) of the liquid-crystalline molecules. In case of uniaxial ordering of such anisotropic molecules, the director is the axis of anisotropy.

For the purposes of the present application, the term in-plane electric field is taken to mean employing an AC or DC electrical field substantially parallel to the substrates, respectively the liquid crystal layer.

The term through cell electrodes refers to electrodes protruding toward the interior of the formed cell. In detail, electrodes which preferably extend over the over the entire thickness and entire length of the control layer or the formed cell. Suitable electrode structures are disclosed, for example, in WO 2004/029697 A1 . The electrodes are preferably arranged substantially parallel to each other. Preferably, the electrodes can have a circular cross- section, in the form of a solid wire or a cylinder, or the electrodes can have a rectangular or almost rectangular cross section. Especially preferred is a rectangular or almost rectangular cross section of the electrodes.

The term "chiral" in general is used to describe an object that is non- superimposable on its mirror image.

"Achiral" (non- chiral) objects are objects that are identical to their mirror image.

The birefringence Δη herein is defined as,

Δη = n_e - n₀ wherein n_e is the extraordinary refractive index and n₀ is the ordinary refractive index, and the effective average refractive index n_ave. is given by, nave. = [(2n₀ ² + n_e ²)/3]^1/2 The extraordinary refractive index n_e and the ordinary refractive index n₀ can be measured using an Abbe refractometer. The birefringence (Δη) can then be calculated.

All temperatures are quoted in degrees Celsius. All temperature differences are quoted in differential degrees.

The term "clearing point" means the temperature at which the transition between the mesophase with the highest temperature range and the isotropic phase occurs.

Throughout this application and unless explicitly stated otherwise, all concentrations are given in weight percent and relate to the respective complete medium. All physical properties have been and are determined according to "Merck Liquid Crystals, Physical Properties of Liquid

Crystals", Status Nov. 1997, Merck KGaA, Germany and are given for a temperature of 20 °C, unless explicitly stated otherwise.

In case of doubt the definitions as given in C. Tschierske, G. Pelzl and S. Diele, Angew. Chem. 2004, 1 16, 6340-6368 shall apply.

For the present invention,

denote 1 ,4-c clohex lene, and in particular

denote trans-1 ,4-cyclohexylene.

and denote 1 ,4-phenylene. Preferred alkyl groups are, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl, 2-methyl butyl, n-pentyl, s-pentyl, cyclo- pentyl, n-hexyl, cyclohexyl, 2 ethylhexyl, n-heptyl, cycloheptyl, n-octyl, cyclooctyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, dodecanyl, trifluoro- methyl, perfluoro-n-butyl, 2,2,2-trifluoroethyl, perfluorooctyl, perfluoro- hexyl, etc.

Preferred alkoxy groups are, for example, methoxy, ethoxy, 2-methoxy- ethoxy, n-propoxy, i-propoxy, n-butoxy, i-butoxy, s-butoxy, t-butoxy, 2- methylbutoxy, n-pentoxy, n-hexoxy, n-heptoxy, n-octoxy, n-nonoxy, n- decoxy, n-undecoxy, and n-dodecoxy.

Preferred alkenyl groups are, for example, ethenyl, propenyl, butenyl, pentenyl, cyclopentenyl, hexenyl, cyclohexenyl, heptenyl, cycloheptenyl, octenyl , cyclooctenyl .

Preferred alkynyl groups are, for example, ethynyl, propynyl, butynyl, pen- tynyl, hexynyl, octynyl. The ranges of the parameters that are indicated in this application all include the limit values, unless expressly stated otherwise.

The different upper and lower limit values indicated for various ranges of properties in combination with one another give rise to additional preferred ranges.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example

"comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components. On the other hand, the word "comprise" also encompasses the term "consisting of but is not limited to it. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is cross-section diagram showing the structure of a light modulation element according to the present invention.

FIGS. 2A, 2B, 2C and 2D are schematic diagrams showing the direction of an electric field and the direction of tilt of liquid crystal molecules in a liquid crystal element according to FIG. 1 . FIGS. 3A, 3B and 3C are diagrams modelling the arrangement of liquid crystal molecules in the chiral smectic C phase thereof.

FIG.4A, FIG. 4B and FIG.4C show the microscope images through crossed polarisers of a through electrode cell containing the VADH FLC mixture in the original off state before switching, switched state with 0.7 V/μιτι and the final off state after switching, respectively.

FIG.5A and FIG.5B show exemplarily the optical transmission

measurements and switching times as a function of electric field across a through cell electrode containing the VADH FLC mixture taken between crossed polarisers.

EXPLANATION OF LETTERS OR NUMERALS 1 ; Light modulation element

° 2: Substrate

° 3: homeotropic alignment layer

° 4: Electrode

° 5: Liquid crystal layer

° 5 a: Liquid crystal molecule

° C: Ideal cone

° E: Electric field

° Ps: Spontaneous polarization DETAILED DESCRIPTION

The light modulation element according to the present invention is described based on FIG. 1 . In the figure, numeral references 1 , 2, 3, 4, and 5 refer to a light modulation element, substrate, homeotropic alignment layer, electrodes, and a layer of liquid crystal in a smectic C phase, respectively. For sake of simplicity, the polarizers on the outer side of the opposing substrates are omitted. In accordance with the invention, the substrate material is preferably a transparent material with no birefringence. The thickness of the substrate material may be several dozen μιτι to several hundred μιτι.

In a preferred embodiment, the substrate material is selected each and independently from another, from polymeric materials, glass or quartz plates.

Suitable and preferred polymeric substrate materials are, for example, films of cyclo olefin polymer (COP), cyclic olefin copolymer (COC), polyester such as polyethyleneterephthalate (PET) or polyethylene-naphthalate (PEN), polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), very preferably PET or TAC films. PET films are commercially available for example from DuPont Teijin Films under the trade name Melinex ®. COP films are commercially available for example from ZEON Chemicals L.P. under the trade name Zeonor ® or Zeonex ®. COC films are commercially available for example from TOPAS Advanced Polymers Inc. under the trade name Topas ®.

In a preferred embodiment, both substrates are glass plates. The space between both substrates is substantially regulated by the electrodes, preferably only by the height of the electrodes. The layer of the liquid-crystalline medium is thereby located in the interspace.

It is preferable to use a metal or metal oxide electrode having a thickness or height similar to the thickness of the liquid crystal layer in order to apply a uniform, as possible, horizontal electric field to the liquid crystal layer. More preferably, a metal or metal oxide electrode is commonly used as a spacer and the thickness of the liquid crystal layer is regulated by the thickness or height of the metal or metal oxide electrode. In a preferred embodiment, the substrates are arranged with a separation from 1 μιτι or more to approximately 20 μιτι or less from another, preferably in the range from approximately 1 μιτι or more to approximately 15 m or less from another, and more preferably in the range from approximately 2 μιτι or more to approximately 8 μιτι or less from another. The layer of the liquid-crystalline medium is thereby located in the interspace.

Accordingly, the height of each electrode can be up to approximately 20 μιτι or less, preferably in the range from approximately 1 μιτι or more to approximately 15 μιτι or less, and more preferably in the range from approximately 2 μιτι or more to approximately 8 μιτι or less.

In a preferred embodiment, the gap between to electrodes is in a range of approximately 500 nm or more to approximately 150 μιτι or less, preferably in a range of approximately 1 μιτι or more to approximately 100 μιτι or less, more preferably in a range of approximately 1 μιτι or more to approximately 50 μιτι or less.

In a preferred embodiment, the width of each electrode is in a range of approximately 500 nm or more to approximately 3 mm or less, preferably in a range of approximately 1 μιτι or more to approximately 2 mm or less, more preferably in a range of approximately 1 μιτι or more to approximately 100 μιτι or less.

Suitable electrode materials are commonly known to the expert, as for example electrodes made of metal, such as for example, tin, copper , aluminium or metal oxides, such as, for example transparent indium tin oxide (ITO).

In a preferred embodiment, the electrode structure generates a

homogenous electric field (parallel electric field) in directions parallel to a principal face of the substrate. In a preferred embodiment, the electrodes of the light modulation element are associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD).

The light modulation element in accordance with the present invention comprises two or more polarisers, at least one of which is arranged on one side of the layer of the liquid-crystalline medium and at least one of which is arranged on the opposite side of the layer of the liquid-crystalline medium. The layer of the liquid-crystalline medium and the polarisers here are preferably arranged parallel to one another. In a preferred embodiment, the polarisers are located on the outer side of the substrates.

The polarisers can be linear polarisers. In a preferred embodiment, precisely two polarisers are present in the light modulation element. In this case, it is furthermore preferred for the polarisers either both to be linear polarisers. If two linear polarisers are present in the light modulation element, it is preferred in accordance with the invention for the polarisation directions of the two polarisers to be crossed.

It is furthermore preferred in the case where two circular polarisers are present in the light modulation element for these to have the same polarisation direction, i.e. either both are right-hand circular-polarised or both are left- hand circular-polarised.

The polarisers can be reflective or absorptive polarisers. A reflective polariser in the sense of the present application reflects light having one polarisation direction or one type of circular-polarised light, while being transparent to light having the other polarisation direction or the other type of circular-polarised light. Correspondingly, an absorptive polariser absorbs light having one polarisation direction or one type of circular-polarised light, while being transparent to light having the other polarisation direction or the other type of circular-polarised light. The reflection or absorption is usually not quantitative; meaning that complete polarisation of the light passing through the polariser does not take place. For the purposes of the present invention, both absorptive and reflective polarisers can be employed. Preference is given to the use of polarisers, which are in the form of thin optical films. Examples of reflective polarisers which can be used in the light modulation element according to the invention are DRPF (diffusive reflective polariser film, 3M), DBEF (dual brightness enhanced film, 3M), DBR (layered-polymer distributed Bragg reflectors, as described in US 7,038,745 and US 6,099,758) and APF (advanced polariser film, 3M). Examples of absorptive polarisers, which can be employed in the light modulation elements according to the invention, are the Itos XP38 polariser film and the Nitto Denko GU-1220DUN polariser film. An example of a circular polariser, which can be used in accordance with the invention, is the APNCP37-035-STD polariser (American Polarizers). A further example is the CP42 polariser (ITOS).

In a preferred embodiment, the light modulation element according to the present invention comprises one or more alignment layers, which are provided on the inner side (adjacent to the liquid crystalline medium) of at least one substrate. In another preferred at least two alignment layers are provided, each of them provided on the inner side of each of the opposing substrates.

In a preferred embodiment, the alignment layer are capable of inducing a homeotropic alignment, tilted homeotropic alignment, or pseudo

homeotropic alignment to the adjacent liquid crystal molecules.

Typical alignment layer materials capable of inducing a homeotropic alignment, tilted homeotropic alignment, or pseudo homeotropic alignment are commonly known to the expert, such as, for example, layers made of alkoxysilanes, alkyltrichlorosilanes, CTAB, lecithin or polyimides, preferably polyimides, such as, for example JALS-2096-R1 .

The alignment layer materials can be applied onto the substrate by conventional coating techniques like spin coating, roll-coating, dip coating or blade coating. It can also be applied by vapour deposition or conventional printing techniques, which are known to the expert, like for example screen printing, offset printing, reel-to-reel printing, letter press printing, gravure printing, rotogravure printing, flexographic printing, intaglio printing, pad printing, heat-seal printing, ink-jet printing or printing by means of a stamp or printing plate.

The light modulation element may furthermore comprise filters, which block light of certain wavelengths, for example, UV filters. In accordance with the invention, further functional layers commonly known to the expert may also be present, such as, for example, protective films and/or compensation films.

Typically, a chiral smectic liquid crystal material can be utilized as the liquid crystal medium capable of forming a chiral smectic C phase in homeotropic orientation in the light modulation element in accordance to the present invention, which can be provided by adding a chiral compound into a base liquid crystal material with a phase sequence of an isotropic liquid phase, a nematic phase, a smectic A phase and a smectic C phase from the side of higher temperature. The expert commonly knows such materials.

Herein, a "smectic liquid crystal material" is a liquid crystal material, which can be provided by aligning the directions of the major axis of a liquid crystal molecule such that a layer (as a smectic layer) is formed. Among such smectic liquid crystal material, a liquid crystal in which the directions of normal of the layer (layer normal direction) correspond to the directions of the major axis of a liquid crystal molecule is referred to as a "smectic A phase", and liquid crystal in which the directions of the major axis of a liquid crystal molecule do not correspond to the directions of normal of the layer is referred to as a "smectic C phase".

Generally, a ferroelectric liquid crystal material in a smectic C phase thereof has a so-called spiral structure in which the directions of liquid crystal directors are spirally twisted for each smectic liquid crystal material when no external electric field is applied and is referred to as a "chiral smectic C phase". On the other hand, with respect to an anti-ferroelectric liquid crystal in a chiral smectic C phase, the liquid crystal directors oppose to each other between adjacent layers. These liquid crystals materials in a chiral smectic C phase contain a chiral compound having an asymmetric carbon in its molecular structure, which causes spontaneous polarization. Then, the optical characteristics thereof may be controlled by means of reorientation of the liquid crystal molecules in the directions determined by the spontaneous polarization Ps and the external electric field E. Preferred liquid crystalline media exhibit a spontaneous polarization (Ps) of 1 nC/cm² or more, preferably of 10 nC/cm² or more, more preferably of 100 nC/cm² or more.

Preferred liquid crystalline media are selected from liquid crystalline media wherein the following equation is fulfilled:

where n_ave is the effective average refractive index of the liquid crystalline media and P is the chiral smectic pitch of the liquid crystalline media.

The liquid-crystal media in accordance with the present invention preferably have a clearing point of approximately 65°C or more, more preferably approximately 70°C or more, still more preferably 80°C or more, particularly preferably approximately 85°C or more and very particularly preferably approximately 90°C or more.

Typically, the Δη of the liquid-crystal media in accordance with the present invention, at 589 nm (NaD) and 20°C, is preferably in the range from approximately 0.020 or more to approximately 0.35 or more, more preferably in the range from approximately 0.10 or more to approximately 0.35 or more, even more preferably in the range from approximately 0.15 or more to approximately 0.35 or more and very particularly preferably in the range from approximately 0.17 or more to approximately 0.35 or more. A suitable liquid-crystalline medium in accordance with the present invention comprises 3 or more, preferably 5 or more, particularly preferably 8 or more and very particularly preferably 10 or more, different liquid- crystalline compounds exhibiting a smectic C phase.

In a preferred embodiment, the liquid crystalline media for the light modulation element according to the present invention comprise one or more compounds selected from formulae I, II or III,

R¹ -(A ¹-Z¹¹ )a-A'-(Z¹²-A¹²)b-(-Z¹³-A¹³)c-R¹² I

R² -(A²¹-Z² )d-A"-(Z²²-A²²)e-R²²

R31 _ _A31 __Z31 _)f__Alll__(Z32__A32_)g__R32 wherein

A¹ denotes

N-N

7 V

A¹¹ denotes X

A¹¹ denotes

X denotes S or O, preferably S,

31 denotes F or H, preferably F

R¹¹ to R³² are each independently a straight-chain or branched alkyl group with 1 to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non- adjacent Ch groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH₃)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another, are each independently selected from -COO-, -OCO- -O-CO-O-, -OCH2-, -CH2O-, -CH2CH2-, -(CH₂)₄-, -CF2CF2-, -CH=CH-,-CF=CF-, -CH=CH-COO-, -OCO-CH=CH-, -C≡C- or a single bond, are each independently in each occurrence 1 ,4- phenylene, wherein in addition one or more CH groups may be replaced by N, trans-1 ,4-cyclo- hexylene in which, in addition, one or two non- adjacent CH2 groups may be replaced by O and/or S 1 ,4-cyclohexenylene, 1 ,4-bicyclo-(2,2,2)-octylene, piperidine-1 ,4-diyl, naphthalene-2,6-diyl, decahydro- naphthalene-2,6-diyl, 1 ,2,3,4-tetrahydro-naphthalene 2,6-diyl, cyclobutane-1 ,3-diyl, spiro[3.3]heptane-2,6- diyl or dispiro[3.1 .3.1 ] decane-2,8-diyl, it being possible for all these groups to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, CI, CN or alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms may be substituted by F or CI, are each and independently 0 or 1 , whereby (a + b + c) > 1 , are each and independently 0 or 1 , whereby (d + e) 1 , preferably both d and e denote 1 , and are each and independently 0 or 1 , whereby (f + g) > 1 , preferably both f and g denote 1 . In a preferred embodiment, R¹¹ and R³² are different and each and independently selected from alkyi groups, alkyi ester groups, alkoxy groups or alkenyl groups.

In a preferred embodiment, Z¹¹ to Z³² are each independently selected from - COO-, -OCO-, or a single bond.

In a preferred embodiment A¹¹ to A³² are each independently in each occurrence 1 ,4-phenylene, or 1 ,4-cyclo-hexylene, whereby for both groups it being possible to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, CI, CN or alkyi, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms may be substituted by F or CI, more preferably unsubstituted trans-1 ,4-cyclo-hexylene or 1 ,4-phenylene, which might be unsubstituted, mono-, disubstituted with F, CI, CN or alkyi, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms may be substituted by F or CI,

further preferred unsubstituted trans-1 ,4-cyclo-hexylene or 1 ,4-phenylene, which might be unsubstituted, mono-, disubstituted in 2 and/or 3 position with F, CI, CN.

In a preferred embodiment, a is 0 and b or c is 0 or 1 , whereby (b + c) > 1 .

Preferred compounds of formula I are selected from compounds of the following formulae:

wherein R¹¹ and R¹² have each and independently from another one of the meanings as given above under formula I.

Preferred compounds of formula II are selected from compound of the following formula:

wherein R²¹ and R²² have each and independently from another, one of the meanings as given above under formula II.

Preferred compounds of formula III are selected from compound of the following formula:

wherein R³¹ and R³² have each and independently from another one of the meanings as given above under formula III.

In a preferred embodiment, the liquid crystalline media for the light modulation element according to the present invention comprises one or more compounds of formula I and one or more compounds of formula II.

In a preferred embodiment, the liquid crystalline media for the light modulation element according to the present invention comprises one or more compounds of formula I and one or more compounds of formula III. In a preferred embodiment, the liquid crystalline media for the light modulation element according to the present invention comprises one or more compounds of formula II and one or more compounds of formula III. In a preferred embodiment, the liquid crystalline media for the light modulation element according to the present invention comprises one or more compounds of formula I, one or more compounds of formula II, and one or more compounds of formula III. The amount of compounds of formulae I and/or II and /or III in the liquid- crystalline medium is preferably from 10 % or more to 80 % or less, more preferably from 12 % or more to 75 % or less, even more preferably 15 % or more to 70 % or less, by weight of the total mixture. Suitable liquid-crystalline media comprise one or more chiral compounds with a suitable helical twisting power (HTP), which are commonly known to the expert.

In a preferred embodiment, the chiral compounds are selected from the group of compounds of formulae IV:

denote each and independently a chiral alkyl or alkoxy chain having 3 to 12 C-atoms and wherein one or more -CH2- groups are replaced by -CHF-.

A suitable the chiral compound is selected from the group of compounds of formulae IVa:

wherein R⁴¹ and R⁴² denote each and independently a straight alkyl chain having 1 to 10 C-atoms.

The amount of chiral compounds in the liquid-crystalline medium is preferably from 10 % or more to 45 % or less, more preferably from 12 % or more to 30 % or less, even more preferably 15 % or more to 35 % or less, by weight of the total mixture.

In a preferred embodiment, the liquid crystalline component, formed by all utilized liquid crystalline compounds of the liquid crystalline medium consists of compounds of formula I and/or formula II and/or formula III and formula IV.

The liquid-crystalline medium in accordance with the present invention optionally comprises further compounds, for example stabilisers and/or antioxidants, such as for example compounds selected from HALS- stabilizers or compounds selected from the Irganox® series, which are commercially available from CIBA, Switzerland. They are preferably employed in a concentration of 0.01 % or more to approximately 5 % or less, particularly preferably 0.01 or more % to approximately 3 % or less, and very particularly preferably 0.01 % or more to approximately 1 % or less. The liquid-crystal media utilized in the light modulation element according to the present invention are prepared in a manner conventional per se. In general, the desired amount of the components used in lesser amount is dissolved in the components making up the principal constituent, preferably at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing. It is furthermore possible to prepare the mixtures in other conventional manners, for example using pre-mixes, for example

homologue mixtures, or using so-called "multibottle" systems. The functional principle of the light modulation element according to the present invention will be explained with reference to FIGS. 2A-2D in detail below. It is noted that no restriction of the scope of the claimed invention, which is not present in the claims, is to be derived from the comments on the assumed way of functioning.

FIGS. 2A-2D are schematic diagrams illustrating the direction of an electric field and the direction of tilt of a liquid crystal molecule in the configuration shown in FIG. 1 . In FIGS. 2A and 2C, the illustrated liquid crystal molecule 5a tilts such that a broad end thereof is above the paper plane and a narrow end thereof is the below the paper plane. The spontaneous polarization (Ps) of the liquid crystal is shown by an arrow. In FIGS. 2A and 2C, the relationship between the direction of an applied electric field and the tilt direction of the liquid crystal molecule is illustrated in the case where the spontaneous polarization is positive. When the direction of the tilt is inverted, the liquid crystal molecule undergoes a rotational motion within an ideal conical plane as illustrated in the perspective views of FIGS. 2B and 2D would be made. In FIGS. 3A, 3B and 3C, models of various orientations of liquid crystal molecules in a chiral smectic C phase are shown. Generally, a layer having a spiral structure is formed on an oriented surface in which the liquid crystal molecules 5a having a tilt angle Θ are stacked and twisted around an axes. When no electric field is applied (E=0), the liquid crystal directors are spatially averaged by a bilaterally symmetric spiral structure, as shown in FIG. 3A. Thus, the average optical axis of the liquid crystal layer 5 in a device according to FIG.1 is oriented in the normal direction of the layer and the configuration is optically isotropic with respect to incident light, which is parallel to the average optic axis. If a conoscopic image of such a liquid crystal material in a chiral smectic C phase under no electric field is observed from the normal direction of the layer by using a polarization microscope, an image of a cross may be positioned at the center portion and thus it may be confirmed that it has a uniaxial optic axis. Next, when a relatively low electrical field below the critical field (0<E<Es) is applied in the horizontal direction of the liquid crystal layer, a torque is provided to the liquid crystal molecules, which is an action of the electric field E on the spontaneous polarization Ps. Thus, the spiral structure is distorted to become asymmetric, as shown in FIG. 3B, and the average optic axis is tilted in one direction. The distortion can be increased with the increase of the strength of the electric field and in turn, the tilt angle of the average optical axis can be also increased. This effect can be confirmed by the movement of the image of cross in the conoscopic image. When the strength of the electric field is further increased to be equal to or greater than a certain threshold electric field Es, the spiral structure disappears, as shown in FIG. 3C, and a uniform orientation is provided. In this case, the tilt angle of the optical axis is equal to the tilt angle Θ of the liquid crystal director. Even when the electric field is further increased, the tilt angle Θ cannot be changed and the tilt angle of the optical axis stays constant.

The light modulation element according to the present invention can preferably be operated with a bipolar or biphasic square wave with a driving frequency of 1 Hz or more to 1000 Hz or less, more preferably 50 Hz or more to 100 Hz or less, most preferably 100 Hz, and an amplitude (Vamp) from 0.05 V or more to 750 V or less, more preferably 0.5 V or more to 100 V or less and most preferably 0.6 V or more to 40 V or less.

Typically the applied bipolar or biphasic square wave has a period comprising a positive pulse with the application of positive voltage, followed by the application of 0 voltage, followed by a negative pulse with the application of negative voltage and followed by the application of 0 voltage. The amplitudes (V_amp) of the applied pulses, negative and positive, are preferably identical.

Typically, the applied electric field strength is in the range of 0.1 V/μιτι or more to 5 V/μιτι or less, preferably 0.5 V/μιτι or more to 1 V/μιτι or less, and most preferably 0.6 V/μιτι or more to 0.8 V/μιτι or less.

A typical process for the production of a light modulation element according to the invention comprises the following steps:

• cutting and cleaning the substrates,

· coating the substrates with an alignment layer agent,

• providing the trough cell electrodes on the substrates • assembling the cell using a UV curable adhesive, and

• filling the cell with the liquid-crystalline medium.

•

The light modulation element of the present invention can be used in various types of optical and electro-optical devices.

Said optical and electro optical devices include, without limitation electro optical displays, liquid crystal displays (LCDs), non-linear optic (NLO) devices, and optical information storage devices.

It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention.

Independent protection may be sought for these features in addition to, or alternative to any invention presently claimed.

Throughout the present application it is to be understood that the angles of the bonds at a C atom being bound to three adjacent atoms, e.g. in a C=C or C=O double bond or e.g. in a benzene ring, are 120° and that the angles of the bonds at a C atom being bound to two adjacent atoms, e.g. in a C≡C or in a C≡N triple bond or in an allylic position C=C=C are 180°, unless these angles are otherwise restricted, e.g. like being part of small rings, like 3-, 4- or 5-atomic rings, notwithstanding that in some instances in some structural formulae these angles are not represented exactly.

It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Alternative features serving the same, equivalent or similar purpose may replace each feature disclosed in this specification, unless stated otherwise. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples are, therefore, to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. The following abbreviations are used to illustrate the liquid crystalline phase behaviour of the compounds: X = crystalline; N = nematic; X1 = unidentified crystal phase; X2 = unidentified crystal phase; SmX = unidentified smectic phase; SmA = smectic A; SmC = smectic C; I = isotropic. The numbers between the symbols indicate the phase transition temperatures in °C.

Table A

Table A indicates possible stabilizers, which can be added to the LC media according to the invention.

-27-



The LC media preferably comprise one or more dopants selected from the group consisting of compounds from Table A.

Examples

LC Mixture

The following mixture (H) is prepared.

To mixture H, the chiral compound IVa

is added in the various amounts and the resulting mixture (M) is dissolved by heating to 130°C.

The measured phases of a resulting mixture comprising 30 % of compound IVa is: SmC 101.5°C SmA 111.4°C I.

EXAMPLE 1

Test Cell

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), and two through cell electrodes made from tin foil and having a height of 15 μιτι, an electrode width of approximately 2mm and an electrode spacing of 150 μιτι, Ι ΟΟμιτι - please refer to table 1 ), is assembled. The cell is filled with a mixture M comprising 30 % of the chiral compound IVa in its isotropic phase via capillary filling. The voltage used to switch the LC mode is an AC bipolar square wave source with driving frequency of 100 Hz and amplitude from 0 to 1 10 V.

Optical performance

FIG. 4A to 4C show microscope images through crossed polarisers of a through electrode cell containing the VADH-FLC mixture (M) comprising 30 % w/w of chiral compound IVa in the original off state before switching, switched state with 0.7 V/μιτι and the final off state after switching, respectively.

FIG. 5 A and FIG. 5B show the optical transmission measurements and switching times as a function of electric field across a through cell electrode containing the VADH-FLC mixture comprising 30 % w/w of chiral compound IVa taken between crossed polarisers, respectively.

All the data was taken at 30°C, with a biphasic driving voltage wave signal (see inset in FIG. 5A) at 100 Hz under 100 times magnification with the sample between crossed polarisers. A good initial dark state is observed with transmission below 0.3 %. The transmission increases monotonically with increasing electric field strength. At 0.7 V/μιτι (100 V) a transmission of 72 % is observed. FIG.4B shows uniform illumination between the electrodes. There is a slight hysteresis which can be observed as a higher transmission (-10%) when decreasing the voltage. At 0 V there is a small residual blue texture, which is too small to resolve in the transmission data.

EXAMPLE 2

Test Cell

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), and two through cell electrodes made from tin foil and having a height of 12 μιτι, an electrode width of approximately 2mm and an electrode spacing of Ι ΟΟμιτι is assembled. The cell is filled with a mixture M comprising 30 % of the chiral compound IVa in its isotropic phase via capillary filling. The voltage used to switch the LC mode is an AC bipolar square wave source with driving frequency of 100 Hz and amplitude from 0 to 1 10 V and the optical performance is measured.

EXAMPLE 3 Test Cell

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), and two through cell electrodes made from tin foil and having a height of 7.5 μιτι, an electrode width of approximately 2mm and an electrode spacing of Ι ΟΟμιτι is assembled.

The cell is filled with a mixture M comprising 30 % of the chiral compound IVa in its isotropic phase via capillary filling. The voltage used to switch the LC mode is an AC bipolar square wave source with driving frequency of 100 Hz and amplitude from 0 to 1 10 V and the optical performance is measured.

EXAMPLE 4 Test Cell

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), and two through cell electrodes made from tin foil and having a height of 12 μιτι, an electrode width of approximately 2mm and an electrode spacing of Ι ΟΟμιτι is assembled.

The cell is filled with a mixture M comprising 19.9 % of the chiral compound IVa in its isotropic phase via capillary filling. The voltage used to switch the LC mode is an AC bipolar square wave source with driving frequency of 100 Hz and amplitude from 0 to 1 10 V and the optical performance is measured. COMPARATIVE EXAMPLE 1 :

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 5.0 μιτι and an IPS-electrode structure (electrode spacing 8 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 30 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 . COMPARATIVE EXAMPLE 2:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 9.0 μιτι and an IPS-electrode structure (electrode spacing 20 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 30 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

COMPARATIVE EXAMPLE 3:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 1 1 .9 μιτι and an IPS-electrode structure (electrode spacing 50 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 30 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

COMPARATIVE EXAMPLE 4:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 13.9 μιτι and an IPS-electrode structure (electrode spacing 100 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 30 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

COMPARATIVE EXAMPLE 5:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 5.0 μηη and an IPS-electrode structure (electrode spacing 8 μηη) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 19.6 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

COMPARATIVE EXAMPLE 6:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 7.2 μιτι and an IPS-electrode structure (electrode spacing 20 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 19.6 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

COMPARATIVE EXAMPLE 7:

A test cell consisting of two glass substrates with a thickness of 2.5 mm, each provided with a homeotropic alignment layer (JALS 2096-R1 from JSR, Japan), having a cell gap of 8.6 μιτι and an IPS-electrode structure (electrode spacing 50 μιτι) instead of through cell electrodes is prepared. The cell is filled with mixture M comprising 19.6 % of the chiral compound IVa. The optical performance is measured in analogy to example 1 .

SUMMARY

Table 1 gives a summary of the cell types tested with the different electrode spacing, cell gaps and chiral dopant concentrations and whether defects were observed and if so above what threshold electric field (Es).

All data in table 1 is taken using a 100 Hz bi-phasic signal at 30°C.

++ no defects

significant amount of defects

n.o. not observed

As can be seen from table 1 no defects were observed on any of the four through-electrode cells tested up to the highest transmission. Furthermore, the values for the maximum transmission are significantly higher for any of the four through-electrode cells tested in comparison with IPS-test cells

Claims

Patent Claims

Light modulation element comprising a pair of transparent substrates, a pair of polarizers, a liquid crystal medium capable of forming a chiral smectic C phase in homeotropic orientation that is located in between the pair of substrates, and an electrode structure which generates an electric field in directions parallel to the substrates main plane, characterized in that the electrode structure is formed by at least two electrodes protruding toward the interior of the formed cell.

The light modulation element according to claim 1 , wherein

substrates are arranged with a separation from 1 μιτι up to 20 μιτι from another.

The light modulation element according to claim 1 or 2, wherein the gap between to electrodes is in a range of 500 nm to 150 μιτι.

The light modulation element according to one or more of claims 1 to

3, wherein at least one homeotropic alignment layer is provided on the inner side of at least one substrate.

The light modulation element according to one or more of claims 1 to

4, wherein the liquid crystalline media exhibit a spontaneous polarization (Ps) of 100 nC/cm² or more.

The light modulation element according to one or more of claims 1 to

5, wherein the liquid-crystalline medium liquid crystalline medium is selected from liquid crystalline media wherein

where n_ave is the effective average refractive index of the liquid crystalline media and P is the chiral pitch of the liquid crystalline media.

7. The light modulation element according to one or more of claims 1 to 6, wherein the liquid crystalline media for the light modulation element according to the present invention comprise one or more compounds selected from formulae I, II or III,

R¹ -(A ¹-Z^{1 1} )a-A'-(Z¹²-A¹²)b-(-Z¹³-A¹³)c-R¹² I

R² -(A²¹-Z² )d-A"-(Z²²-A²²)e-R²²

R31 _ _A31 __Z31 _)f__Alll__(Z32__A32_)g__R32 wherein

A¹ denotes

N-N

7 V

A¹¹ denotes X

A¹¹ denotes

X denotes S or O,

31 denotes F or H,

R¹¹ to R³² are each independently a straight-chain or branched alkyl group with 1 to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent Ch groups to be replaced, in each occurrence independently from one another, by -O-, -S-, -NH-, -N(CH₃)-, -CO-, -COO-, -OCO-, -O-CO-O-, -S-CO-, -CO-S-, -CH=CH-, -CH=CF-, -CF=CF- or -C≡C- in such a manner that oxygen atoms are not linked directly to one another,

Z¹¹ to Z³² are each independently selected from -COO-, -OCO-,

-O-CO-O-, -OCH2-, -CH2O-, -CH2CH2-, -(CH₂)₄-,

-CF2CF2-, -CH=CH-,-CF=CF-, -CH=CH-COO-, -OCO-CH=CH-, -C≡C- or a single bond,

A¹¹ to A³² are each independently in each occurrence 1 ,4- phenylene, wherein in addition one or more CH groups may be replaced by N, 1 ,4-cyclo-hexylene in which, in addition, one or two non-adjacent CH2 groups may be replaced by O and/or S, 1 ,4-cyclohexenylene, 1 ,4-bicyclo- (2,2,2)-octylene, piperidine-1 ,4-diyl, naphthalene-2,6-diyl, decahydro-naphthalene-2,6-diyl, 1 ,2,3,4-tetrahydro- naphthalene-2,6-diyl, cyclobutane-1 ,3-diyl,

spiro[3.3]heptane-2,6-diyl or dispiro[3.1 .3.1 ] decane-2,8- diyl, it being possible for all these groups to be unsubstituted, mono-, di-, tri- or tetrasubstituted with F, CI, CN or alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H atoms may be substituted by F or CI, a, b and c are each and independently 0 or 1 , whereby a + b + c >

1 , d and e are each and independently 0 or 1 , whereby d + e > 1 , and f and g are each and independently 0 or 1 , whereby f + g > 1 .

Process for the production of a light modulation element according to one or more of claims 1 to 7 comprising the following steps:

cutting and cleaning the substrates,

coating the substrates with an alignment layer agent, providing at least two trough cell electrodes on the substrates or the alignment layer

assembling the cell using a UV curable adhesive, and filling the cell with the liquid-crystalline medium.

Use of a light modulation element according to one or more of claims 1 to 7 in an electro-optical device.

Electro-optical device comprising the light modulation element according to one or more of claims 1 to 7.