Cholesteric liquid crystal display device with reflectors...

Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only

Reexamination Certificate

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Details

C349S096000, C349S113000, C349S115000, C349S117000, C349S187000

Reexamination Certificate

active

06833889

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 2001-08248, filed on Feb. 19, 2001 in Korea, 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 a liquid crystal display (LCD) device and more particularly, to a reflective cholesteric liquid crystal (CLC) display device and a manufacturing method for the same.
2. Discussion of the Related Art
Flat panel display devices, which have properties of thin, low weight and low power consumption, have been required as the information age rapidly evolves. Flat panel display devices may be classified into two types depending on whether the device emits light or not. One type is a light-emitting type display device that emits light to display images, and the other is a light-receiving display device that uses an external light source to display images. Plasma display panels (PDPs), filed emission display (FED) devices and electro luminescence (EL) display devices are examples of the light-emitting type display devices, and liquid crystal displays are an example of the light-receiving type display device. The liquid crystal display device is widely used for notebook computers and desktop monitors, etc. because of its superior resolution, color image display and quality of displayed images.
Generally, the liquid crystal display device has upper and lower substrates, which are spaced apart and facing each other. Each of the substrates includes an electrode, and the electrodes of each substrate are facing each other. Liquid crystal is interposed between the upper substrate and the lower substrate. Voltage is applied to the liquid crystal through the electrodes of each substrate, and thus an alignment of the liquid crystal molecules is changed according the applied voltage to display images. Because the liquid crystal display device cannot emit light as described before, it needs an additional light source to display images. Accordingly, the liquid crystal display device has a back light behind a liquid crystal panel as a light source. An amount of light incident from the back light is controlled according the alignment of the liquid crystal molecules to display images. The electrodes of each substrate are formed of transparent conductive material and the substrates must be transparent. The liquid crystal display devices like this are called transmissive liquid crystal display devices. Because the transmissive liquid crystal display device uses an artificial light source such as the back light, it can display a bright image in dark surroundings. However, the transmissive liquid crystal display device has high power consumption.
The reflective liquid crystal display device has been suggested to overcome the power consumption problem of the transmissive liquid crystal display device. Because the reflective liquid crystal display device controls transmittance according the alignment of liquid crystal molecules by irradiating light using an external light source such as ambient light or artificial light, it has a low power consumption compared with the transmissive liquid crystal display device. In the reflective liquid crystal display device, an electrode on the lower substrate is formed of conductive material that has a high reflectance, and an electrode on the upper substrate is formed of transparent conductive material to transmit the incident light.
The conventional reflective liquid crystal display device will be described hereinafter more in detail with reference to FIG.
1
.
FIG. 1
is a cross-sectional view of a conventional reflective liquid crystal display device. In the conventional liquid crystal display device of
FIG. 1
, a plurality of switching elements (not shown) are formed in an array matrix on a first substrate
10
and a plurality of reflective electrodes
12
, which are respectively connected to each of the switching elements, is formed on the first substrate
10
. The reflective electrode
12
, which is formed of conductive material such as metal, reflects the incident light and serves as a pixel electrode. A color filter
22
, which includes sub-color-filters red (R), green (G), and blue (B) in a repeated order, is formed beneath a second substrate
20
and corresponds to the reflective electrode
12
. A common electrode
24
is formed of transparent conductive material beneath the color filter
22
. Liquid crystal is interposed between the reflective electrode
12
and the common electrode
24
. An alignment of liquid crystal molecules is changed if a voltage is applied between the reflective electrode
12
and the common electrode
24
.
A retardation layer
40
is formed on the second substrate
20
. The retardation layer
40
of the conventional reflective liquid crystal display device of
FIG. 1
has a phase difference of &lgr;/4 and thus is called a quarter wave plate. The quarter wave plate
40
changes a linear polarization of light into a circular polarization of light and the circular polarization into the linear polarization. A polarizer
50
, which changes ambient light into linearly polarized light by transmitting only the light that is parallel to a light transmission axis, is formed on the quarter wave plate
40
. If the ambient light is irradiated to the reflective liquid crystal display device when the voltage is not applied, the incident light is changed into linearly polarized light as it passes through the polarizer
50
, and the linearly polarized light is changed into circularly polarized light as it passes through the quarter wave plate
40
. The circularly polarized light then passes through the second substrate
20
, the color filter
22
and the common electrode
24
in sequence, and there is no phase change during this process. The circularly polarized light then passes through the liquid crystal layer
30
. If the liquid crystal layer
30
is formed to have a phase difference of &lgr;/4 the circularly polarized light is changed into linearly polarized light as it passes through the liquid crystal layer
30
. The linearly polarized light is reflected at the reflective electrode
12
and then is changed into circularly polarized light as it passes again through the liquid crystal layer
30
. The circularly polarized light is changed into the linearly polarized light as it passes again through the quarter wave plate
40
, and then the linearly polarized light passes through the polarizer
50
. If a polarized direction of the linearly polarized light is parallel to the light transmission axis of the polarizer
50
, all of the linearly polarized light transmits through the polarizer
50
and if the polarized direction of the linearly polarized light is perpendicular to the light transmission axis of the polarizer
50
, the linearly polarized light cannot transmit through the polarizer
50
.
Because the reflective liquid crystal display device uses external light for its light source as described before, it has a low power consumption. However, because the reflective liquid crystal display device uses the external light for its light source, its luminance is changeable depending on external circumstances. In addition, the conventional reflective liquid crystal display device has a lower luminance than a transmissive liquid crystal display device under normal office environment. In addition, because the liquid crystal display device uses an absorption type color filter and thus incident light is lost at a high rate as it passes through the color filter, the conventional reflective liquid crystal display device has a rather low brightness. The absorption type color filter and the polarizer usually absorb over eighty percent of the incident light. Though the luminance can be increased by reducing a purity of the color filter in this case, there exists a limitation in increasing the luminance only by reducing the purity of the color filter.
Therefore, cholesteric liquid crystal (CLC) display devices, which use a CLC color filter to display color images, has been

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