IPS LCD having an organic conductive layer outside the subtrate

Details IPS LCD having an organic conductive layer outside the subtrate IPS LCD having an organic conductive layer outside the subtrate

1997-09-19

2001-02-20

Dudek, James A. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S040000

Reexamination Certificate

active

06191837

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display based on an active matrix and a method for producing the same. More particularly, the present invention relates to a liquid crystal display based on an active matrix wherein orientation of the liquid crystal is regulated by a transversely-activated electric field, and a method for producing the same. Furthermore, the present invention relates to a liquid crystal display wherein the display quality is protected against deterioration due to external static electricity, and a method for producing the same.
2. Description of the Related Art
Generally, a conventional active matrix liquid crystal display comprises a nematic liquid crystal sandwiched between a pair of transparent substrates. A voltage is applied between the transparent substrates to align the crystal in an upright position and thereby control the transmission of light (vertical drive mode).
A liquid crystal display based on the above mechanism includes switching elements, an active matrix substrate having various electrodes disposed thereon, a second substrate facing the foregoing substrate, a liquid crystal inserted between the substrates, and polarizing plates arranged on the outside surfaces of the two substrates. Gate and drain electrodes are formed in the horizontal and vertical directions of the active matrix substrate, and switching elements are formed at the intersections thereof.
A transparent pixel electrode is formed in the area surrounded by the gate and drain electrodes, and that area forms a pixel. Numerous pixels having the above constitution are formed on the transparent substrate. A second transparent substrate carries a common electrode thereon. A voltage applied between the electrodes formed on the two substrates changes the orientation of the liquid crystal, to thereby modulate the transmission of light.
FIG.
3
(
a
) is a plan view illustrating the construction of a conventional liquid crystal display based on an active matrix. FIG.
3
(
b
) is a sectional view along the line A-A′ of FIG.
3
(
a
) when the display is not activated, and FIG.
3
(
c
) shows the same cross-section when the display is activated.
As shown in these figures, the structure of the liquid crystal display is such that drain electrodes
14
and gate electrodes
10
are disposed vertically and horizontally on transparent substrate
1
, respectively, and a transparent pixel electrode
17
is formed in each area surrounded by the two electrodes. The pixel electrode
17
is connected through a switching element
18
and a source electrode
15
to a drain electrode
14
.
The switching element
18
often consists of a thin-film transistor. The thin-film transistor includes a drain electrode
14
which contacts a source electrode
15
through a semiconductor layer
30
, and has a gate insulating membrane
20
beneath the semiconductor layer
30
and a gate electrode
10
beneath the semiconductor layer
30
.
Another transparent substrate or paired substrate (also referred to as a “transparent electrode”)
508
facing the foregoing substrate
1
has a common electrode
16
formed thereupon. A liquid crystal composition (as shown by liquid crystal molecules
513
) is sandwiched between the two transparent substrates
1
and
508
. The crystal orientation of the liquid crystal (the liquid crystal molecules
513
) is nearly parallel to the transparent substrates
1
and
508
, and is twisted under the influence of the alignment layers OR
11
and OR
12
, from substrate
1
towards the other substrate
508
, until the shift in orientation angle reaches 90 degrees.
The operation of this system is described below.
When a specific voltage is applied to the gate electrode
10
, the thin-film transistor (or the switching element
18
) is turned on, and electric charge moves from the drain electrode
14
through the semiconductor layer
30
and source electrode
15
to the pixel electrode
17
. As shown in FIG.
3
(
c
), the liquid crystal (the liquid crystal molecules
513
) is aligned in an upright position with a specified orientation under the influence of an electric field developed between the pixel electrode
17
and the common electrode
16
, which alters the polarizing activity of the subject liquid crystal. Through this operation, the transmission of light through individual pixels is altered, the light tone of these pixels is modulated, and images are reproduced.
It is known, however, that in such a conventional liquid crystal display, the tone intensity in the display varies depending on the angle from which the viewer watches the display. For example, when viewed from the front with the panel placed upright, or along an axis normal to the panel, one can see images having good contrast. However, when viewing the same image along an axis that is slightly tilted downward, the image appears darker. If the tilted angle is further increased, there will be a boundary beyond which a reverse in tone will occur. In contrast, when viewing the same image along an axis slightly tilted upward, the image appears lighter.
The above phenomenon occurs because a vertically oriented electric field (an electric field having a direction vertical to the transparent substrates) is applied to align the liquid crystal molecules in an upright position. This modulates the polarizing activity of the liquid crystal molecules, such that the direction towards which the liquid crystal molecules are aligned is determined in advance.
To solve the problem of tone variation which depends on the angle from which the display is viewed, a number of solutions have been proposed. Such solutions include, for example, the use of optically compensated films, liquid crystals having a multi-domain orientation, and driving with a transversely oriented electric field.
The method using optically compensated films includes two varieties: one incorporates diffraction and the other is based on the dispersion of converged parallel beams. These methods, however, present problems such as reduced brightness when viewed from the front and tone reversal.
Recently a method has been proposed in which a discotic liquid crystal having a disc-like molecular structure is orderly arranged in accordance with the inclination of nematic liquid crystal molecules and converted into a film. The film is then applied to a polarizing plate. This method solves the above problems.
Multi-domain orientation is a method which divides pixels into a plurality of groups having different orientations, to thereby improve the asymmetric optical performance inherent in a twisted nematic liquid crystal.
The two-domain method or one of the varieties of the multi-domain orientation includes dividing each pixel into an upper part and a lower part, and providing orientation regulating films (alignment layers) each rubbed in a direction opposite the other. As a result, the liquid crystal molecules associated with the two respective parts are oriented opposite each other. This arrangement allows each pixel to provide symmetrical optical performance between the upper and lower halves, and light distribution on the pixel is averaged when viewed from the outside. Accordingly, the problem of tone variation as a function of viewing angle is improved.
Driving by a transversely-oriented electric field has a special significance in this invention. As described above, this method includes controlling the polarizing activity of the liquid crystal with an electric field having a direction transverse to the transparent substrates, as opposed to a conventional liquid crystal display system where the orientation of the liquid crystal is controlled by a vertically-oriented electric field. This method provides a wide-view angle and the display undergoes less change in color tone. Thus, this method is considered to be the most promising means of improving viewability, and many developmental efforts have been directed thereto.
As described in Japanese Unexamined Patent Publication No. 6-160878, driving by a transversely-ori

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