Transflective color LCD device

Details Transflective color LCD device Transflective color LCD device

2001-04-06

2003-05-27

Tsai, Jey (Department: 2812)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S114000

Reexamination Certificate

active

06570634

ABSTRACT:

This application claims the benefit of Korean patent application No. 2000-18321, filed Apr. 7, 2000, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display (LCD) device implementing a liquid crystal layer having different cell gaps.
2. Discussion of the Related Art
As an information-oriented society rapidly develops, display devices are accordingly developed. The display device processes and displays a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display device field. However, to meet the needs of the times, a flat panel display device having small size, light weight, and low power consumption is a subject of research.
A thin film transistor liquid crystal display (TFT LCD) device is an example of the flat panel display device. The TFT LCD device is very thin and provides superior color display properties. For operation, a thin film transistor serves as a switching element of the TFT LCD device. The thin film transistor of the TFT LCD device switches a pixel such that the pixel controls a transmittance of light, which is incident from a back light of the TFT LCD device. An amorphous silicon layer is widely used for a silicon (active) layer of the thin film transistor, because it can be formed on a large, but relatively cheap, glass substrate at a relatively low temperature. The above-mentioned amorphous silicon TFT (a-Si:TFT) is frequently used for thin film transistors.
In general, the LCD devices are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an interior or exterior light source.
A typical transmissive TFT LCD device includes a liquid crystal panel and a back light. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed in between. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTS) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the back light. However, since the transmissive TFT LCD transmits just 3 to 8% of the incident rays of light from the back light, it is very inefficient in terms of its power consumption.
Specifically, the two polarizers have a transmittance of 45%, and the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive TFT LCD device has a transmittance of about 7.4% as shown in
FIG. 1
, which shows an accumulated transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive TFT LCD device requires a high back light brightness, and thus electric power consumed by the back light increases. A relatively heavy battery is needed to supply sufficient power to the back light of such a device. However, the battery rapidly discharges.
Unlike transmissive TFT LCD device, the reflective LCD device uses an ambient light incident from a natural light source or an exterior artificial light source. Because of its low power consumption, the reflective TFT LCD device is often preferred. However, the reflective TFT LCD device is useless when the weather or exterior light source is dark.
Accordingly, a transflective TFT LCD device has been developed to compensate for the reflective TFT LCD device. The transflective TFT LCD device is useful regardless of the weather or exterior light source.
FIG. 2
is a partial cross-sectional view illustrating a typical transflective TFT LCD device. For the sake of convenience,
FIG. 2
shows just one pixel portion of the transflective TFT LCD device.
As shown in
FIG. 2
, the transflective LCD device includes upper and lower substrates
60
and
50
opposing to each other, a liquid crystal layer
80
interposed between the upper and lower substrates, and a back light
70
under the lower substrate
50
. First and second electrodes
62
and
64
apply electric field to the liquid crystal layer
80
. The first electrode
62
is called a common electrode, since it covers the entire substrate
60
, whereas the second electrode
64
is called a transparent pixel electrode, since it is independently arranged for every pixel unit. The first and second electrodes are arranged, respectively, at the upper and lower substrates
60
and
50
that sandwich the liquid crystal material
80
. The first and second electrodes
62
and
64
are formed of transparent conducting material, such as indium tin oxide (ITO) or indium zinc oxide (IZO). In addition, a color filter layer
68
is interposed between the upper substrate
60
and first electrode
62
.
Surrounding the second electrode
64
there is a reflective pixel electrode
52
having a through hole
54
. The through hole
54
corresponds to the second electrode
64
such that it has a width of “&Dgr;L” in its cross-section, and serves to transmits incident light that is incident from the back light
70
. For a reflective mode, first incident rays
72
are incident from an exterior light source (not shown) to the reflective pixel electrode
52
, and the reflective pixel electrode
52
reflects them to the upper substrate
60
having the color filter layer
68
. For a transmissive mode, second incident rays
74
are incident from the back light
70
, and the transparent pixel electrode
64
, the second electrode, transmits the rays to the upper substrate
60
without a phase difference.
For operation, a switching element (not shown) formed on the lower substrate
50
applies electric signals to the reflective and transparent pixel electrodes
52
and
64
such that an electric field is induced across the liquid crystal layer
80
. Then, the liquid crystal layer
80
changes its alignment in accordance with the electric field such that the first or second incident rays
72
and
74
gets a selective phase difference after passing through the liquid crystal layer
80
. Then, the first and second rays
72
and
74
pass through the color filter layer
68
such that a color image is produced. At this point, retardation layers and polarizers (see
FIG. 3
) are further formed on exterior surface of the upper and lower substrates
60
and
50
, to modulate the first and second incident rays
72
and
74
.
Various elements for a transflective LCD device according to a related art will be explained with reference to FIG.
3
.
FIG. 3
is a conceptual cross-sectional view illustrating the transflective LCD device according to the related art. A color filter layer (reference
68
of
FIG. 2
) is omitted in
FIG. 3
for the sake of convenience, because it does not affect the modulation of the incident rays.
As shown in
FIG. 3
, a lower substrate
100
includes a first transparent substrate
106
, a reflective electrode
108
, a transparent electrode
110
, a first retardation film
104
, and a lower polarizer
102
. The reflective and transparent electrodes
108
and
110
are formed on an inner surface of the first transparent substrate
106
, whereas the first retardation film
104
and the lower polarizer
102
are sequentially formed on an exterior surface thereof. A back light
70
is positioned below the lower substrate
100
. The first retardation film
104
is beneficially a quarter wave plate involving a phase difference of &lgr;/4 (&lgr; being the wavelength of incident light).
In the meanwhile, an upper substrate
200
includes a second transparent substrate
206
, a common electrode
208
, a second retardation film
204
, and an upper polarizer
202
. The common electrode
208
is formed on an inner surface of the second transparent substrate
206
, whereas the second retardation film
204
and the upper polarizer
202
are sequentially formed on an exterior surface thereof. Between the upper and lower substrate
200
and
100
, a liqu

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