Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-05-01
2004-05-25
Nguyen, Dung (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000
Reexamination Certificate
active
06741308
ABSTRACT:
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 device.
2. Discussion of the Related Art
A liquid crystal display device has characteristics of light weight, thin thickness, and low power consumption. Thus, it has been highlighted as a next generational display device. Generally, an LCD device is a non-emissive display device displaying images by using a refractive index difference according to the optical anisotropy of the liquid crystal interposed between array and color filter substrates.
In the conventional LCD device, a displaying method using a backlight behind the array substrate as a light source is commonly used. However, the incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The backlight of the conventional LCD device requires high brightness, and thus power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device, and the battery cannot be used for a long period of time.
In order to overcome the problems described above, a reflective LCD has been developed. Since the reflective LCD device uses the ambient light instead of the backlight, it becomes light and easy to carry. In addition, power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used for a portable display device such as an electric diary or a personal digital assistant (PDA).
However, brightness of the reflective LCD device may vary with the surroundings. For example, brightness of the indoor ambient light differs largely from that of the outdoors. Therefore, the reflective LCD device cannot be used where the ambient light is weak or does not exist. In order to overcome these problems, a transflective LCD device has been researched and developed. The transflective LCD device is switchable according to the user's selection from a transmissive mode using transmission of light to a reflective mode using reflection of light.
To increase a light efficiency between the transmissive and reflective modes, retardations (&dgr;) of liquid crystal layer of the transmissive and reflective modes should be equal. The retardation of the liquid crystal layer is defined by the following equation:
&dgr;=&Dgr;
n·d
wherein &dgr; is a retardation of the liquid crystal layer, &Dgr;n is a refractive index anisotropy of the liquid crystal layer, and d is a cell gap of the liquid crystal layer.
Therefore, the retardation of the liquid crystal layer in the transflective LCD device may be constant by forming a cell gap of the transmissive portion larger than that of the reflective portion.
FIG. 1
is a schematic cross-sectional view of a conventional transflective LCD device.
In
FIG. 1
, upper and lower substrates
10
and
30
are spaced apart from each other and a liquid crystal layer
20
is interposed therebetween. A backlight
38
is disposed at the outside of the lower substrate
30
. On the inner surface of the upper substrate
10
, a color filter layer
12
for passing only the light having a specific wavelength and a common electrode
14
functioning as one electrode for applying a voltage to the liquid crystal layer
20
are subsequently formed. On the inner surface of the lower substrate
30
, a transparent pixel electrode
32
functioning as another electrode for applying a voltage to the liquid crystal layer
20
, a passivation layer
34
having a transmissive hole
31
exposing a portion of the pixel electrode
32
, and a reflective layer
36
are subsequently formed. The area corresponding to the reflective layer
36
is a reflective portion “r” and the area corresponding to the portion of the pixel electrode
32
exposed by the transmissive hole
31
is a transmissive portion “t”.
A cell gap “d
1
” at the transmissive portion “t” is about twice of a cell gap “d
2
” at the reflective portion “r” to reduce a light path difference. However, even though the light efficiency of the liquid crystal layer between reflective and transmissive modes becomes equal by making the cell gap different, the number of light passing through the color filter layer at different portions is different. Thus, the brightness becomes different at the front of the display device.
Transmittance of the color filter resin having a high absorption coefficient only for a specific wavelength satisfies the following equation when Fresnel reflection is not considered and the transmittance is inversely proportional to the absorption coefficient and the distance that light passes:
T=exp
(−&agr;·
d
)
wherein T is transmittance, &agr;is an absorption coefficient of the color filter layer and d is a distance that light passes in the color filter layer.
At the reflective portion “r”, light passes the color filter layer
12
twice. Since the transmittance and the color purity are determined by an absorption coefficient and a thickness of the color filter layer according to the above equation, the values of exp(−&agr;·d) at the transmissive and reflective portions should be controlled to be equal to avoid differences of the transmittance between the transmissive and reflective portions. Therefore, the transmittance becomes constant at the transmissive and reflective portions by forming the color filter layer of the reflective portion thicker than that of the transmissive portion with the same absorption coefficient. For example, the color filter layer at the reflective portion is formed to be twice as thick as that at the transmissive portion. Alternatively, the absorption coefficient of the color filter layer at the reflective portion is formed to be lower than that at the transmissive portion.
Generally, color purity increases and transmittance decreases when a color filter layer becomes thicker. Therefore, the transmittance and the color purity of the transmissive and reflective portions are maintained by increasing the transmittance and decreasing the color purity of the reflective portion, or by increasing the color purity and decreasing the transmittance of the transmissive portion.
A color filter layer is classified into a dye type and a pigment type depending on the material of the organic filter. Depending upon the method of fabricating the color filter layer, it may also be divided into a dyeing method, a printing method, a pigment dispersion method, and an electro-deposition method. The pigment dispersion method is most widely employed.
FIG. 2A
is a cross-sectional view of a conventional transflective LCD device having a color filter layer. Pigment concentrations at the transmissive and reflective portions of the color filter layer are different from each other. Since the structure of
FIG. 2A
is similar to that of
FIG. 1
, the explanation about the same structure will be omitted for convenience.
In
FIG. 2A
, reflective and transmissive color filters
40
a
and
40
b
are disposed at the reflective and transmissive portions “r” and “t”, respectively. The transmittance of the reflective and transmissive color filters
40
a
and
40
b
becomes different from each other when the absorption coefficients of the reflective and transmissive color filters
40
a
and
40
b
are adjusted. For example, the pigment concentrations of the reflective and transmissive color filters
40
a
and
40
b
may be formed differently from each other. Since the pigment concentration of the color filter layer is proportional to the absorption coefficient of the color filter layer, the transmittance of the reflective color filter layer
40
a
can be higher than that of the transmissive color filter layer
40
b
by making the pigment concentration of the reflective color filter layer
40
a
lower than that of the transmissive color filter layer
40
b.
FIG. 2B
is a cross-sectional view of a conventional transflective LCD device having a color filter layer whose thicknesses at the transmissive and reflec
LG. Philips LCD Co. Ltd.
Morgan & Lewis & Bockius, LLP
Nguyen Dung
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