Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-05
2003-04-15
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000, C349S141000
Reexamination Certificate
active
06549255
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to the field of liquid crystals. More specifically, the present invention relates to a liquid crystal device including a liquid crystal bulk layer, and a dynamic surface alignment layer interacting with the bulk layer for obtaining a preferred orientation of the surface director of the bulk layer. The invention also relates to a method for producing a liquid crystal device including such a dynamic surface alignment layer. Furthermore, the present invention relates to a method for accomplishing an in-plane switching in a liquid crystal bulk layer of a liquid crystal device.
Technical background
The operation of almost all liquid crystal devices is based on a direct coupling between, on the one hand, an electric field applied in a perpendicular direction across a liquid crystal bulk layer within the device and, on the other hand, an induced polarization of the liquid crystal layer (in dielectric or paraelectric liquid crystals) or a spontaneous polarisation of the liquid crystal layer (in ferroelectric crystals). As a direct result of said direct coupling with the applied electric field, the orientation of the liquid crystal molecules within the bulk layer is changed, which in its turn results in an optical response so the device due to birefringent properties of the liquid crystal. The applied electric field will normally interact not only with the crystal molecules within the volume, but also with those molecules of the bulk layer that are located at the surface of the bulk layer. Typically, such interaction between the electric field and he surface molecules of the bulk layer may be less strong due to surface constraints.
There exists a number of different types of liquid crystal displays using liquid crystal devices, especially (1) Dynamic scattering displays; (2) Displays using deformation of homeotropically aligned nematics; (3) Schadt-Helfrich displays; (4) Supertwist-displays; (5) In-plane switching displays controlled by electric fields oriented in parallel with the substrates; and (6) Surface Stabilized Ferroelectric Liquid Crystal Displays (SSFLC Displays) and Displays with antiferroelectric liquid crystals.
For modern applications, a liquid crystal display should present several important properties, such as a low power consumption, a low threshold voltage, a steep electro-optical characteristic or bistability, a low viewing angle dependence of the contrast, short switching times, a high contrast, brightness, etc.
Today, some liquid crystal displays are advantageous concerning some of the above-mentioned desired properties, but there exists no ideal display which is optimized concerning all of the important properties.
Conventional nematic displays with a dielectric coupling to the electric field are usually slow, and nearly all suffer from non-satisfactory angular dependence of the contrast due to the out-of-plane switching of the liquid crystal molecules. The term “out-of-plane switching” refers to the fact that the nematic liquid crystal molecules, when subjected to an external electric field, typically will tilt in relation to the plane in which the molecules normally are located in.
The dynamic scattering displays relies on strong movements of the molecules and inherently need relatively high electric field for switching and therefore, this display type is seldom used anymore. Displays with deformation of homeotropically aligned nematics as well as Schadt-Helfrich displays have a strong viewing angle dependence of the optical contrast, and the latter also possesses a low steepness of the electro-optical characteristic. Supertwist displays (twist angles of e.g. 270°) have an improved steepness of the electro-optical characteristic, but they present longer switching times and not yet satisfying viewing angle dependence of the optical contrast.
In contrast to the use of fields which are oriented perpendicular to the confining substrates, displays of the in-plane-switching type are controlled by electric fields oriented in parallel—not perpendicular—with the substrates. These displays possess very small dependence of the optical contrast from the viewing angle, but the brightness and switching times are not satisfying. A specific disadvantage of this display type is the requirement for an in-plane applied electric field which causes manufacturing problems.
Next, the surface stabilized ferroelectric liquid crystal devices (SSFLCs) will by considered, but first a short description of the nature of smectic liquid crystals will be given for a better understanding of the SSFLCs.
In a smectic liquid crystal, the molecules are arranged in adjacent smectic layers. Smectic A phase and smectic C phase are the two most important representatives of these “layered” or smectic liquid crystals. In the C phase, the molecules are inclinded with an angle &bgr; (typically in the order of 22, 5°) with respect to the smectic layer normal, whereas in the A phase the molecules are perpendicular (&bgr;=0°) to the smectic layers, i.e. directed along the smectic layer normal. Furthermore, a smectic liquid crystal can be non-chiral (e.g. A or C) or chiral (e.g. A* and C*), where the term chiral, means lack of mirror symmetry. It should be noted that the term “chiral” does not refer to the occurrence of a twisted or helical molecular arrangement that may or may not appear as a secondary effect as a result of the medium's chirality.
A chiral smectic liquid crystal, such as C*, possesses a director that rotates in a cone in going from one smectic layer to the next. The apex angle &thgr;=2&bgr; of the cone may typically be in the order of 45°. Thereby, a helix texture is formed across the layers with the helix axis being perpendicular to the smectic layers and parallel to the axis of said cone. However, the local polarisation which is coupled to the director will then also turn around in a helical fashion, with the same period or pitch. Such a helical structure of the local polarisation means that the local polarisation is self-cancelling, i.e. the bulk liquid crystal will present no macroscopic polarisation.
Now, if an electric field is applied parallel to the smectic layers in the helical smectic C* bulk state, the electric field will couple to the permanent dipoles and align them with the field direction. In other words, the applied field will unwind the helix and create an induced macroscopic polarisation of the bulk liquid crystal.
In a so-called SSFLC device, a chiral smectic liquid crystal is used (e.g. C*), but the above-mentioned helix is suppressed by the confining substrate surfaces and thereby not present. This is accomplished (i) by arranging the smectic layers non-parallel with the confining planes or substrates of the device (bookshelf or quasi-bookshelf structure), and (ii) by making the thickness of the smectic liquid crystal layer perpendicular to the substrates so small (in the order of microns) that the interaction of the liquid crystal molecules with the substrate surfaces produces a liquid crystal texture in which there is no longer any helical arrangement of the director within the cell. Instead, the liquid crystal molecules align in a predetermined direction, e.g. parallel, to the substrate despite the fact that a chirai material is used. Specifically, the director lying parallel with the substrates forms an angle (e.g. 22, 5°) to the smectic layer normal. Since the uniform surface conditions at the boundaries are in conflict with the helical bulk condition and, therefore, will quench the helix, the helix will be elastically untwisted by the boundaries when the cell thickness is chosen below a certain value. The result is what is called the surface-stabilized smectic C* state, presenting a non-zero macroscopic polarisation.
The material such as C* used in this device is in the ferroelectric phase, which means that, in the absence of an electric field, it presents a permanent polarisation along the smectic layers, i.e. perpendicular to the long molecule axis. Thereby, the director can be “digitally”
Demus Dietrich
Komitov Lachezar
Lagerwall Torbjörn
Stebler Bengt
Ecsibeo AB
Qi Mike
Ton Toan
LandOfFree
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