Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With viewing layer of specified composition
Reexamination Certificate
2001-01-04
2003-05-06
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With viewing layer of specified composition
C428S001310, C428S001400, C428S001600, C252S299100, C252S299620, C349S067000, C349S178000, C349S186000
Reexamination Certificate
active
06558759
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved Liquid Crystal Display (LCD) device which is used in products ranging from watch displays to flat panel colour TV screens
The Liquid Crystal Device industry is currently a multi-billion dollar industry. In this industry, the products range from simple watch displays to flat panel colour TV screens. The device described in the present invention has advantages over the conventional LCD devices in that it has a wide and symmetrical viewing angle, no reversal of the contrast ratio in any direction, and also results in a simplification of the fabrication process. Accordingly, the device of the present invention will be very useful for various applications in liquid crystal industry.
When molecular crystals are heated to their melting point they usually change into the liquid phase. The periodic structure of the lattice as well as the orientational ordering of the molecules are destroyed simultaneously. However if the constituent molecules have a pronounced anisotropy of shape, such as a rod or a disc, the melting of the lattice may precede the disappearance of the orientational ordering. One, then, has an intermediate phase composed of molecules which are more or less parallel to each other, but at the same time exhibiting a certain degree of fluidity. The molecules can slide over on one another while still preserving their parallelism. The fluid is therefore anisotropic, turbid and, like a crystal, shows optical birefringence and dielectric anisotropy. At a higher temperature there is orientational melting and the anisotropic fluid transforms into the ordinary isotropic clear liquid. Such intermediate phases, which occur as a result of heating or cooling, are referred to as thermotropic liquid crystals.
Thermotropic liquid crystals can be classified broadly into two types, (i) those composed of rod-shaped molecules (called “calamitic” liquid crystals) which are known from the end of 19th century and form the majority of the currently known liquid crystals and (ii) those composed of disc-shaped molecules (called “discotic” liquid crystals) which have been developed recently.
2. Description of the Related Art
Discotic liquid crystals which were discovered by us as recently as 1977 represent a new class of thermotropic liquid crystal. In this context reference may be made to the publication of S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh,
Pramana,
9, 471-480 (1977). In this case the discs are stacked one on top of the other to form columns, the different columns constituting a two-dimensional lattice. The basic columnar structure is shown in
FIG. 1
of the drawing accompanying this specification. A number of modifications of the above said basic structure have been identified. Some discotic compounds exhibit a nematic (N
D
) phase as well. It is a fluid phase consisting of an orientationally ordered arrangement of discs, but with no long range translational order, somewhat like a pile of coins as shown in FIG.
2
. However, unlike the usual nematic calamitic, the nematic discotic is optically negative. The preferred orientation of the axis of the disc is termed as the director as shown in FIG.
2
.
The advent of discotic liquid crystals triggered off a spate of activity in this field and well over a thousand discotic compounds have been reported to date. A few discotic compounds are exemplified below: Hexaalkonoyloxy benzenes, hexaalkoxy triphenylenes, bis-(4-n-decylbenzoyl)methanato copper (II), hexa-n-alkanoates of truxene and octasubstituted phthalocyanines.
The potential uses of such materials are as quasi-one-dimensional conductors, photoconducting systems, ferroelectrics, light emitting diodes, photovoltaic solar cells, optical data storage devices and hybrid computer chips for molecular electronics.
Display devices based on calamitic liquid crystals are well known. A widely used device is the twisted nematic (TN) display device. In a twisted nematic display device two transparent glass plates are coated on their inner surfaces with a thin layer of transparent electrically conducting material, such as indium tin oxide, and further with a thin layer of a polyimide. The method of unidirectionally rubbing the said substrates with cotton or rayon or nylon fabric is widely used to achieve a macroscopic orientation of the liquid crystal director. The two glass plates are held apart at a distance of approximately 6-10 &mgr;m by means of spacers to form a cell, with the rubbing directions of the polyimide layers orthogonal to each other. The gap between the substrates of the cell is filled with a calamitic nematic liquid crystal. Owing to the boundary conditions the nematic liquid crystal will become oriented parallel to the rubbing direction of each glass plate and consequently the director will undergo a twist of 90° over the nematic layer distance. Polarizer sheets are attached to the outer surfaces of the glass plates with the axis of vibration (polarizing axis) of each sheet parallel to the rubbing direction of the plate to which it is attached. Unpolarized light is transformed into linearly polarized light by the polarizer fixed on the entrance side of the cell and emerges on the exit side with the polarization axis rotated through 90°. The emergent light will be transmitted by the second polarizer. Thus in this configuration, the so called normally white mode, the display appears bright in the unactivated state. A white mode with enhanced viewing angle can be achieved by setting the polarizers with their polarizing axes perpendicular to the rubbing directions. The application of an electric field normal to the layer orients the liquid crystal molecules (of positive dielectric anisotropy, &Dgr;&egr;>0) with their long axes along the layer normal. In this activated state the polarization axis of light is not rotated by the liquid crystalline medium and the display appears black. Orientation of one polarizer parallel and the second polarizer perpendicular to the rubbing direction results in a black appearance in the unactivated state and a bright appearance in the activated state. This so called black mode is useful for automobile dash board applications.
The major disadvantage of the above type of device is that when it is viewed obliquely, the viewing angle characteristic is poor, resulting in a loss of contrast, and even contrast inversion at certain azimuthal angles. See
FIG. 3
, which is taken from the publication of Y. Toko, T. Sugiyama, K. Katoh, Y. Imura and S. Kobayashi,
J. Appl. Phys.,
74, 2071-75 (1993) showing a typical polar plot of the contrast ratio (CR) for a conventional TN device.
Another widely used device is the supertwisted nematic (STN) device. The construction of such a device is similar to that of a TN device explained above except that the twist angle of the director is between 180° and 270°, instead of 90°. The higher twist angle is achieved by incorporating a suitable quantity of a chiral compound as a dopant in the nematic material before it is filled into the cell. However this device does not lead to any improvement in the viewing angle characteristic.
Both the TN and STN devices suffer from the additional disadvantage that for multiplexed displays there is a large difference in the pixel capacitance between the ON and OFF states, which gives rise to the problem of cross talk between pixels.
The viewing angle profiles, the symmetry and the angle dependence of the intensity contrast ratio between the ON and OFF states of any display device are important criteria for determining the quality of performance of the device. Several attempts have been made to enhance the performance of such devices. These attempts are directed mainly to improve the viewing angle characteristics using different techniques such as dividing each pixel into sub-pixels, adding retardation films, or by applying an electric field parallel to the substrate plane. The noteworthy point in all these attempts is the fact that the liquid crystalline material used is of the nematic c
Chandrasekhar Sivaramakrishna
Kumar Sandeep
Manickam Mayandithevar
Nair Geetha Gopinathan
Prasad Subbarao Krishna
Centre for Liquid Crystal Research
Norris McLaughlin & Marcus P.A.
Wu Shean C.
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