Liquid crystal display device

Details Liquid crystal display device Liquid crystal display device

2000-12-14

2004-03-09

Sikes, William I. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S096000, C349S117000

Reexamination Certificate

active

06704078

ABSTRACT:

CROSS REFERENCE
This application claims the benefit of Korean Patent Application No. 1999-67847, filed on Dec. 31, 2000, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having a retardation film.
2. Description of Related Art
LCD devices are divided into three types: a transmissive LCD device; a reflective LCD device; and a transflective LCD device. The transmissive LCD device has a back light device as a light source, and the reflective LCD device uses ambient light instead of the back light device. The transflective LCD device has a transmissive mode and a reflective mode.
FIG. 1
is an exploded view illustrating a typical transflective LCD device. As shown in
FIG. 1
, the transflective LCD device
1
includes upper and lower substrates
7
and
22
with a liquid crystal layer
15
interposed therebetween. The upper substrate
7
includes a black matrix
3
, a color filter
5
, and a common electrode
9
. The lower substrate
22
is called an array substrate and includes gate lines
21
arranged in a transverse direction and data lines
26
arranged in a longitudinal direction perpendicular to the gate line
21
. A pixel region P is defined by the gate and data lines
21
and
26
. A pixel electrode
18
is formed on the pixel region P. As a switching element, thin film transistors (TFTs) T are formed at a crossing point of the gate and data lines
21
and
26
. The pixel electrode
18
is made of indium tin oxide (ITO) or indium zinc oxide (IZO).
FIG. 2
is a cross-sectional view illustrating a conventional reflective LCD device. As shown in
FIG. 2
, a lower substrate
11
includes a reflective electrode
16
, and an upper substrate
13
includes a transparent electrode
17
. A liquid crystal layer
19
is interposed between the lower and upper substrates
11
and
13
. As the liquid crystal layer
19
, twisted nematic (TN) liquid crystal is mainly used. The twisted nematic liquid crystal molecules have a molecule axis that is twisted in 90° and a phase difference of &lgr;/4. Therefore, a linearly polarized light incident to the twisted nematic liquid crystal layer is changed into a circularly polarized light. On the upper substrate
13
, a retardation film
23
(i.e., a quarter wave plate) and a polarizer
25
are arranged. Ambient light is changed into a linearly polarized light through the polarizer
25
. The retardation film
23
changes the linearly polarized light into a circularly polarized light.
FIG. 3A
shows light state after passing through each layer when voltage is not applied to the liquid crystal layer and assuming that the observer traces light. Incident light passes through the polarizer
25
and become a linearly polarized light parallel to the light transmission axis of the polarizer
25
. The linearly polarized light is changed into a left-handed circularly polarized light after passing through the retardation film
23
. The left-handed circularly polarized light is changed into the linearly polarized light after passing the TN liquid crystal layer
19
. The linearly polarized light is reflected on the reflective electrode
16
and is changed into the left-handed circularly polarized light having a phase difference of &lgr;/4after passing through the TN liquid crystal layer
19
. The left-handed circularly polarized light is changed into the linearly polarized light after passing through the retardation film
23
. The linearly polarized light is parallel to a transmission axis of the polarizer
25
and thus passes through the polarizer
25
, whereupon the LCD device becomes a white state.
Alternately,
FIG. 3B
shows light state after passing through each layer when voltage is applied to the liquid crystal layer. When the liquid crystal display device goes to an on state, the TN liquid crystal molecules that are twisted in 90° are polarized in a direction of applied electric field to have a certain direction. As shown in
FIG. 3B
, incident light passes through the polarizer
25
and become a linearly polarized light parallel to the light transmission axis of the polarizer
25
. The linearly polarized light is changed into a left-handed circularly polarized light after passing through the retardation film
23
. The left-handed circularly polarized light passes through the TN liquid crystal layer
19
“as is”. The left-handed circularly polarized light is reflected on the reflective electrode
16
and is changed into the right-handed circularly polarized light. The right-handed circularly polarized light passes through the TN liquid crystal layer
19
“as is”. The right-handed circularly polarized light is changed into the linearly polarized light after passing through the retardation film
23
. The linearly polarized light is parallel to a transmission axis of the polarizer
25
and thus passes through the polarizer
25
, whereupon the LCD device becomes a dark state.
The retardation film
23
serves to improve a viewing angle and a contrast ratio. Even though the thickness of the retardation film
23
is changed a little, error of the phase difference become greater. Further, when it is assembled to the substrate, assembly or attachment error may occur, leading to an assembling error of the polarizer.
FIG. 4
is a cross-sectional view illustrating a transflective LCD device according to the conventional art. As shown in
FIG. 4
, on an upper substrate
43
, an upper retardation film
42
and an upper polarizer
45
are sequentially stacked. On a bottom surface of a lower substrate
53
, a lower polarizer
47
and a lower retardation film
50
are arranged. A liquid crystal layer
55
are interposed between the upper and lower substrates
43
and
47
. A back light device is arranged under the lower substrate
53
. A reflective electrode
49
is arranged on the lower substrate
53
and includes at least one light transmitting hole
51
. The light transmitting hole
51
serves to transmit light from the back light device
41
.
The transflective LCD device described above requires the lower polarizer
47
as well as the upper polarizer
45
. This is because light should not leak out in both a transmissive mode and a reflective mode when the transflective LCD device goes to a dark state in order to achieve a high contrast ratio.
FIG. 5A
shows light state after passing through each layer in a transmissive mode when voltage is applied to the liquid crystal layer
55
, and assuming that the lower retardation film
50
does not exist. As shown in
FIG. 5A
, light generated from the back light device
41
passes through the lower polarizer
47
and is changed into a linearly polarized light parallel to a transmission axis of the lower polarizer
47
. The linearly polarized light passes through the light transmitting hole
51
and the liquid crystal layer
55
“as is”. In other words, the linearly polarized light passing through the liquid crystal layer
55
has no phase difference. Then, the linearly polarized light passes through the upper retardation film
42
and is changed into left-handed circularly polarized light. The left-handed circularly polarized light passes through the upper polarizer
45
. At this point, of the left-handed circularly polarized light, only elements of light parallel to a transmission axis of the upper polarizer
45
are viewed by an observer. In other words, about a half of the circularly polarized light passes through the upper polarizer
45
, leading to a gray state other than a complete dark state, leading to a deterioration of contrast ratio.
FIG. 5B
shows light state after passing through each layer in a transmissive mode when voltage is applied to the liquid crystal layer
55
and the lower retardation film
50
is arranged. As shown in
FIG. 5B
, light generated from the back light device
41
passes through the lower polarizer
47
and is changed into a linearly polarized light parallel to a transmission axis of the lower polarizer
47
. The linearly polarized lig

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