Ferroelectric liquid crystal panel

Details Ferroelectric liquid crystal panel Ferroelectric liquid crystal panel

2000-12-20

2004-01-13

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

With specified nonchemical characteristic of liquid crystal...

Within smectic phase

C349S088000, C349S094000

Reexamination Certificate

active

06678034

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 1999-0059463, filed on Dec. 20, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal panels for liquid crystal display (LCD) devices. More particularly, the present invention relates to a liquid crystal panel that uses a ferroelectric liquid crystal.
2. Discussion of the Related Art
A conventional liquid crystal display (LCD) includes a display panel. A display panel typically has upper and lower substrates that are attached with each other, and an interposed liquid crystal, usually a nematic, a smetic, or a cholesteric liquid crystal. A liquid crystal display device utilizes the electro-optic effects of the liquid crystal. A display panel is operationally divided into a plurality of liquid crystal cells. On the exterior surfaces of the upper and lower substrates, polarizers or retardation films are selectively attached.
A major design consideration of a liquid crystal cell is the characteristics of the particular liquid crystal that is used. A good liquid crystal should have a fast response time, a good gray scale, and a wide viewing angle, all while operating at a low driving voltage. However, it is very difficult to find a liquid crystal that has all of those characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of liquid crystals, a low twisted nematic (LTN) liquid crystal has advantages of a short response time and a good gray scale. However, it typically has low contrast ratios and relatively poor color-dispersion properties. Other twisted nematic (TN) liquid crystals have higher twist angles (such as 90 degrees) or employing an in-plane switching (IPS) mode. While those liquid crystals can provide a wide viewing angle, afterimages are produced when displaying moving images, and their brightness is relatively low. The anti-ferroelectric liquid crystal (AFLC), or the optically compensated birefringence (OCB), have advantages of a wide viewing angle and a fast response time, although there are problems with contrast ratios and cell gap control.
FIG. 1
is a cross-sectional view illustrating a conventional TN-LCD panel
20
. As shown in
FIG. 1
, the TN-LCD panel has lower and upper substrates
2
and
4
and an interposed liquid crystal layer
10
. The lower substrate
2
includes a substrate
1
having a TFT “S” that is used as a switching element to change the orientation of the liquid crystal molecules. The TFT “S” includes a pixel electrode
14
that applies a voltage to the liquid crystal layer
10
in accordance with signals that are applied to the TFT “S”. The upper substrate
4
has a color filter
8
for implementing color, and a common electrode
12
on the color filter
8
. The common electrode
12
serves as an electrode for applying a voltage to the liquid crystal layer
10
. The pixel electrode
14
is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the liquid crystal layer
10
between the substrates
2
and
4
, those substrates are sealed by a sealant
6
.
FIGS. 2A
to
2
C illustrate various alignments of possible liquid crystal molecules in the liquid crystal layer. As shown in
FIG. 2A
, in the nematic liquid crystal, each rod-like molecule fluctuates quite rapidly, but the molecules have a definite orientational order expressed by a unit vector “
” called a director. As shown in
FIG. 2B
, in the smetic liquid crystal the molecules have a layered structure in which the molecular orientation is perpendicular or nearly perpendicular to the layers. As shown in
FIG. 2C
, in the cholesteric liquid crystal, the director
changes its orientation gradually along a helical axis. The helical axis coincides with the optical axis of this material. Among the three different types of liquid crystals, the nematic liquid crystal is most widely used in liquid crystal display devices.
Liquid crystals for liquid crystal display devices should:
a) have a liquid crystal phase that extends from low to high temperatures, and thus are operable over a range of temperatures;
b) be chemically and optically stable over time;
c) have a low viscosity and a fast response time;
d) have highly ordered molecular alignments and thus provide a good contrast; and
e) have a large dielectric anisotropy and a low operating voltage.
The electro-optic effect enables electrical modulation of light by changing the alignment of the liquid crystal molecules using an applied electric field.
Among the various types of nematic liquid crystals, a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal are often used. For a TN liquid crystal panel, a nematic liquid crystal is interposed between transparent lower and upper electrodes (reference the common electrode
12
and the pixel electrode
14
of FIG.
1
). Those electrodes induce a definite molecular arrangement such that a gradual rotation of the molecules occurs between the lower transparent electrode and the upper transparent electrode until a twist angle of 90 degrees is achieved. In an STN liquid crystal panel the angle of twist rotation is increased to 180 to 360 degrees.
The basic configuration and operation of a twisted nematic liquid crystal display device will now be explained. As shown in
FIG. 3A
, opposed and spaced apart first and second polarizers
10
and
16
, respectively, have perpendicular first and second transmittance axis directions
40
and
42
. Between the two polarizers
10
and
16
are first and second transparent substrates
12
and
14
, which are also opposed to and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to
FIG. 3A
, the first and the second transparent substrates
12
and
14
include first and second orientation films
20
and
22
, respectively, on their opposing surfaces. Between the first and second orientation films
20
and
22
is a positive TN liquid crystal
18
.
The positive TN liquid crystal
18
has a characteristic that it arranges according to an applied electric field. The first and second polarizer
10
and
16
, respectively, transmit light that is parallel with their transmittance-axis directions
40
and
42
, but reflect or absorb light that is perpendicular to their transmittance-axis directions
40
and
42
.
The first and second orientation films
20
and
22
were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and second transparent substrates
12
and
14
to become tilted several degrees. First and second rubbing directions
50
and
52
of the first and second orientation films
20
and
22
are, respectively, parallel with the transmittance-axis directions of the first and second polarizers
10
and
16
. With no electric field applied across the positive TN liquid crystal
18
, the orientation of the liquid crystal molecules twists between one substrate to the other at a definite angle, that angle being the twisted angle of the positive TN liquid crystal
18
.
During operation, a back light device
24
irradiates white light onto the first polarizer
10
. The first polarizer
10
transmits only the portion of the light that is parallel with the first transmittance-axis direction
40
. The result is a first linearly polarized light
26
that passes through the polarizer
10
. The first linearly polarized light
26
then passes through the positive TN liquid crystal
18
via the first transparent substrate
12
.
As the first polarized light
26
passes through the positive TN liquid crystal
18
, the first polarized light
26
changes its phase according to the twisted alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light

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