Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-20
2004-08-10
Ngo, Huyen (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S096000
Reexamination Certificate
active
06774962
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2001-69443, filed on Nov. 8, 2001, which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to an LCD device using cholesteric liquid crystal and a manufacturing method thereof.
2. Discussion of the Related Art
Flat panel display (FPD) devices having small size, lightweight, and low power consumption have been a subject of recent research according to coming of the information age. Among many kinds of FPD devices, LCD devices are widely used for notebook personal computers (PCs) or desktop PCs because of their excellent characteristics of resolution, color display and display quality. Generally, in an LCD device, first and second substrates having respective electrodes are disposed to face each other with a liquid crystal layer is interposed therebetween. The liquid crystal layer has an optical anisotropy due to an electric field generated by applying a voltage to the respective electrodes. The LCD device displays images by using a transmittance difference according to the optical anisotropy of the liquid crystal layer.
FIG. 1
is a schematic cross-sectional view of a related LCD panel.
In
FIG. 1
, first and second substrates
10
and
20
, referred to as lower and upper substrates, are facing and spaced apart from each other. A thin film transistor “T” (TFT) having a gate electrode
11
, and source and drain electrodes
15
a
and
15
b
are formed on an inner surface of the first substrate
10
. The TFT “T” further has an active layer
13
and an ohmic contact layer
14
. A gate insulating layer
12
is formed on the gate electrode
11
. A passivation layer
16
is formed on the TFT “T”. The passivation layer
16
covers the TFT “T” and has a contact hole
16
c
exposing the drain electrode
15
b
of the TFT. A pixel electrode
17
is formed on the passivation layer
16
and connected to the drain electrode
15
b
through the contact hole
16
c.
A black matrix
21
is formed on an inner surface of the second substrate
20
at a position corresponding to the TFT “T”. A color filter layer
22
a
and
22
b
, in which colors of red (R), green (G) and blue (B) are alternately repeated, is formed on the black matrix
21
. A common electrode
23
of transparent conductive material is formed on the color filter layer
22
a
and
22
b
. The color filter layer
22
a
and
22
b
of a single color corresponds to the one pixel electrode
17
.
A liquid crystal layer
30
is interposed between the pixel and common electrodes
17
and
23
. When a voltage is applied to the pixel and common electrodes
17
and
23
, the arrangement of molecules of the liquid crystal layer
30
changes according to an electric field generated between the pixel and common electrodes
17
and
23
. Orientation films (not shown) respectively formed on the pixel and common electrodes determine an initial arrangement of liquid crystal molecules.
First and second polarizers
41
and
42
are formed on outer surfaces of the first and second substrates
10
and
20
, respectively. The first and second polarizers
41
and
42
convert natural light to linearly polarized light by transmitting only light whose polarizing direction is parallel to a transmission axis of the polarizer. The transmission axis of the first polarizer
41
is perpendicular to that of the second polarizer
42
.
In
FIG. 1
, the TFT and the pixel electrode are formed on the lower substrate and the color filter layer and the common electrode are formed on the upper substrate. Recently, however, structures in which the TFT and the color filter layer are formed on the lower substrate, or the color filter layer and the common electrode are formed on the lower substrate and the TFT and the pixel electrode are formed on the upper substrate have been suggested.
Since an LCD device does not emit light for itself, an additional light source is necessary. Therefore, a backlight is disposed over the first polarizer
41
of FIG.
1
and light from the backlight is provided to a liquid crystal panel. Images are displayed by adjusting the light according to the arrangement of the liquid crystal layer. The LCD device of this structure is referred to as a transmissive LCD device. The pixel electrode
17
and the common electrode
23
, two electrodes generating an electric field, are made of transparent conductive material and the first and second substrates
10
and
20
are also transparent.
Since only one polarizing component of the incident light is transmitted through the polarizer used in the LCD device and the other components are absorbed and then converted into heat loss, brightness of the LCD device is reduced by more than 50% considering reflection at a surface of the polarizer. To improve the brightness of the LCD device by reducing the heat loss, an LCD device having a reflective circular polarizer under the device is suggested. The circular polarizer transmits one circular polarizing component of the incident light and reflects the other components. The reflected circular polarizing components are reflected again by several optical parts under the circular polarizer and converted into a light component capable of passing the circular polarizer. Theoretically, since all the incident light is converted into one component and then transmits through the circular polarizer, loss of light occurring in a conventional linear polarizer is remarkably reduced.
FIG. 2
is a schematic cross-sectional view of a related art LCD device.
In
FIG. 2
, a first polarizer
42
that is a linear polarizer is disposed under a liquid crystal cell
41
, in which a liquid crystal layer is interposed between two substrate having respective electrodes on inner surfaces. A retardation layer
43
, which converts linear polarization into circular polarization and vice versa, and a second polarizer
45
that is a linear polarizer are disposed under the first polarizer
42
. A compensation film
44
can be interposed between the retardation layer
43
and the second polarizer
45
. A sheet
46
for collecting and diffusing light and a backlight
47
are sequentially disposed under the second polarizer
45
. On the other hand, a third polarizer
48
whose transmission axis is perpendicular to that of the first polarizer
42
is disposed over the liquid crystal cell
41
. The liquid crystal cell
41
can have the same structure as or different structure from the liquid crystal cell of FIG.
1
.
The second polarizer
45
can be made through forming a cholesteric liquid crystal layer
45
b
on a transparent substrate
45
a
. The cholesteric liquid crystal has a selective reflection property that only light of a specific wavelength is selectively reflected according to a helical pitch of the molecules of the cholesteric liquid crystal. The polarization of the reflected light is determined according to a rotational direction of the liquid crystal. For example, if a liquid crystal layer has a left-handed structure where liquid crystal molecules rotate counter clockwise along a rotational axis, only left-handed circularly polarized light having a corresponding color, i.e., wavelength, is reflected. Since the pitch of the cholesteric liquid crystal that light experiences is varied according to an incident angle, a wavelength of reflected light is also varied. Accordingly, there is a color shift such that a color of transmitted light varies according to a viewing angle. To compensate for the color shift, a compensation film
44
may be disposed over the second polarizer
45
.
As shown in
FIG. 2
, a sheet
46
for collecting light from the backlight
47
and diffusing light to the liquid crystal cell
41
can be disposed between the second polarizer
45
and the backlight
47
.
In the LCD device of
FIG. 2
, brightness is improved through increasing transmitted light by using a circular polarizer to a conventional LCD device. Howe
LG.Philips LCD Co. , Ltd.
McKenna Long & Aldridge LLP
Ngo Huyen
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