Array substrate for liquid crystal display device

Details Array substrate for liquid crystal display device Array substrate for liquid crystal display device

2001-07-24

2004-05-11

Parker, Kenneth (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S040000, C349S139000

Reexamination Certificate

active

06734939

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2000-42534, filed on Jul. 24, 2000, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin, and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
FIG. 1
is a schematic cross-sectional view illustrating a conventional LCD cell in an active matrix LCD. As shown, the LCD cell
20
has lower and upper substrates
2
and
4
and a liquid crystal (LC) layer
10
interposed therebetween. The lower substrate
2
has a thin film transistor (TFT) “S” as a switching element that switches a voltage that changes the orientation of the LC molecules. The lower substrate
2
also includes a pixel electrode
14
that is used to apply an electric field across the LC layer
10
in response to signals applied to the TFT “S”. The upper substrate
4
has a color filter
8
for producing a color, and a common electrode
12
on the color filter
8
. The common electrode
12
serves as an electrode that produces the electric field across the LC layer (with the assistance of the pixel electrode
14
). The pixel electrode
14
is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the LC layer
10
, a pair of substrates
2
and
4
are sealed by a sealant
6
.
As described above, since the common and pixel electrodes
12
and
14
of the conventional LCD panel are positioned on the upper and lower substrates
4
and
2
, respectively, the electric field induced between them is perpendicular to the lower and upper substrates
2
and
4
. The described liquid crystal display device has advantages of high transmittance and a high aperture ratio. Furthermore, because the common electrode
12
on the upper substrate
4
acts as a ground, the liquid crystal is shielded from static electricity.
However, the conventional LCD panels having the longitudinal electric field has a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates.
A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to
FIGS. 2 and 3A
to
3
D.
As shown in
FIG. 2
, lower and upper substrates
30
and
32
are spaced apart from each other, and a liquid crystal
10
is interposed therebetween. The lower and upper substrates
30
and
32
are often referred to as array substrate and color filter substrate, respectively. On the lower substrate
30
are a pixel electrode
34
and a common electrode
36
. The pixel and common electrodes
34
and
36
are aligned parallel to each other. On a surface of the upper substrate
32
is a color filter layer
42
that is commonly positioned between the pixel electrode
34
and the common electrode
36
of the lower substrate
30
. An overcoat layer
44
, which protects the color filter layer
42
, is formed on the color filter layer
42
. A voltage applied across the pixel and common electrodes
34
and
36
produces an electric field
35
through the liquid crystal “LC.” The liquid crystal “LC” has a negative dielectric anisotropy, and thus it aligns parallel to the electric field
35
. An edge sealant
40
is formed around the edges of the lower and upper substrates
30
and
32
, and bonds the upper substrate
32
to the lower substrate
30
to prevent leakage of the liquid crystal “LC”.
FIGS. 3A
to
3
D conceptually help illustrate the operation of a conventional IPS-LCD device. When no electric field is produced by the pixel and common electrodes
34
and
36
, i.e., off state, as shown in
FIGS. 3A and 3B
, the longitudinal axes of the LC molecules “LC” are parallel and form a definite angle with the pixel and common electrodes
34
and
36
. For example,
FIG. 3B
shows a common angle of 45 degrees between a line that is perpendicular to the pixel and common electrodes
34
and
36
and the longitudinal axes of the LC molecules.
On the contrary, when an electric field is produced by the pixel and common electrodes
34
and
36
, i.e., on state, as shown in
FIGS. 3C and 3D
, because the pixel and common electrodes
34
and
36
are on the lower substrate
30
, an in-plane electric field
35
that is parallel to the surface of the lower substrate
30
is produced. Accordingly, the LC molecules “LC” twist to bring their longitudinal axes into coincidence with the electric field. Thus, as shown in
FIG. 3D
, the LC molecules align with their longitudinal axes parallel with a line perpendicular to the pixel and common electrodes
34
and
36
.
In the above-mentioned IPS-LCD panel, there is no common electrode on the color filter. Furthermore, since the above-mentioned IPS-LCD panel has the pixel electrode and the common electrode on the array substrate, it uses the parallel electric field to the array substrate.
Now, referring back to
FIG. 2
, the overcoat layer
44
is formed on the color filter layer
42
so as to cover and protect the color filter layer
42
. Further, the edge sealant
40
is formed around the periphery of the IPS-LCD panel. However, there are some problems in the edge sealant
40
and the overcoat layer
44
.
In general, a number of ions are contained in the edge sealant
40
. As time passes, these ions migrate into the liquid crystal layer
10
after the LCD panel is complet

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