Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-02-14
2004-08-17
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S115000, C349S110000, C349S117000, C349S176000
Reexamination Certificate
active
06778238
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2001-7321, filed on Feb. 14, 2001, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) device, and more particularly, to a reflective LCD device using a cholesteric liquid crystal color filter.
2. Discussion of the Related Art
Generally, thin film transistor-liquid crystal displays (TFT-LCDs) with high color quality and small depth dimension are mainly being used as LCDs.
Conventional LCD devices include a first substrate and a second substrate with a liquid crystal layer interposed therebetween. The first substrate and the second substrate are generally referred to as a color filter substrate and an array substrate, respectively.
The LCD devices use backlight sources disposed over the outer surface of the second substrate to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD device. Since the backlight should emit light of a relatively high brightness, corresponding power consumption increases. Accordingly, a large capacity heavy battery is commonly used to supply sufficient power for the backlight. Moreover, use of the large capacity battery limits operating time.
Recently, to solve the above-mentioned problems, reflective LCD devices without the backlight are being researched and developed. Because power consumption of reflective LCD devices greatly decreases due to use of ambient light as a light source, operating time increases. Such reflective LCD devices are used for portable information apparatuses such as electric diaries and personal digital assistants (PDAs). In reflective LCD devices, a pixel area, which is covered with a transparent electrode in conventional transmissive LCD devices, is covered with a reflective plate or reflective electrode having opaque reflection characteristics. However, brightness of reflective LCD devices is very poor because the devices use only ambient light as a light source. The poor brightness results from operational characteristics of the reflective LCD devices in which ambient light passes through a color filter substrate, is reflected on a reflective electrode on a second substrate, passes through the color filter substrate again and then displays an image. Accordingly, brightness is decreased as a result of reduction of the transmittance when the ambient light passes through a color filter layer twice. Since overall thickness of the color filter layer is inversely proportional to transmittance and is directly proportional to color purity of the light, the problem of inadequate brightness of the reflective LCD devices can be remedied by forming a thin color filter layer with high transmittance and low color purity. However, there is a limit in fabricating the color filter layer below a threshold thickness due to characteristics of the resin used to form the color filter layer.
Accordingly, one possible solution to this problem is forming LCD devices using cholesteric liquid crystal (CLC) having selective reflection and transparency characteristics.
Generally, liquid crystal molecules have liquid crystal phases according to their structure and composition. The liquid crystal phases depend on the temperature and the concentration. Nematic liquid crystal material in which the liquid crystal molecules are aligned along one direction is mainly being researched and applied. Especially, the nematic liquid crystal material is nowadays being applied to the commercialized LCD. The CLC has an alignment state that the director of the nematic liquid crystal is distorted by mixing the nematic liquid crystal and the molecule whose molecular axis is distorted or whose reflected molecular phase has a different chiral characteristic from its original molecular phase.
The nematic liquid crystal phase has regularity that the liquid crystal molecules are aligned along one direction. In contrast with the nematic liquid crystal, the CLC has a structure of multi-layers, in each layer of which the liquid crystal has the regularity of the nematic liquid crystal. However, the liquid crystal alignment between the layers rotates, thereby the reflectance between layers being different. Therefore, the difference of the reflectance can display colors by reflection and interference of the light.
The rotation of the CLC molecule can be understood as a kind of helical structure. Two characteristics of the helical structure are a direction of rotation and a pitch that is repetition period of the helical structure.
The pitch can be understood as a distance through which the liquid crystal layer has the same alignment and is a variable to determine the color of the CLC. The central wavelength of the reflected light can be expressed the multiple of the pitch and the average refractive index (&lgr;
c
=n
avg
·p). For example, if the pitch of the CLC whose average refractive index is 1.5 is 430 nm, the central wavelength of the reflected light is about 650 nm, thereby the reflected light being red. Similarly, green and blue can be displayed by making the CLC have an adequate pitch.
In the CLC structure, another important characteristic is the direction of the rotation, which is an important element to make polarization in the reflection characteristic of the CLC. The direction of circular polarization of the reflected light is determined according to direction of the helical structure of the CLC, i.e., left-handed or right-handed helical structure. For example, the CLC of the right-handed helical structure reflects the right-handed circular polarized light having the corresponding pitch.
The ambient light can be considered as a sum of the right-handed and left-handed circular polarized lights, and a specific circular polarized light can be divided by using the CLC. In the conventional LCD devices, the polarization (linear polarization) property is used. In the LCD devices using the CLC, since the practical use of the light is improved in contrast with the color filter using conventional pigments or dyes, the power consumption can be effectively decreased.
FIG. 1
is a schematic cross-sectional view of a conventional reflective LCD device using a CLC color filter.
A first substrate
10
and a second substrate
30
are facing and spaced apart from each other, and a liquid crystal layer
50
is interposed between the first and second substrates
10
and
30
. On an inner surface of the first substrate
10
, a first transparent electrode
12
, which is a first electrode for applying a voltage to the liquid crystal layer
50
, is formed. On the outer surface of the first substrate
10
, a quarter wave plate (QWP)
14
, that is, a &lgr;/
4
retardation plate converting linearly polarized light into right- or left-handed circularly polarized light and conversely converting right- or left-handed circularly polarized light into linearly polarized light, is formed. Then, a polarizing plate
16
through which only light coinciding with the polarizing axis of the polarizing plate
16
can be transmitted is disposed on the QWP
14
. Over an inner surface of the second substrate
30
, a CLC color filter layer
32
, which selectively reflects light of corresponding wavelength and transmits light of the other wavelength, is formed. The CLC color filter layer
32
composes a pixel “P” including three sub-pixels “S” of red (R), green (G) and blue (B). A light absorption layer
34
absorbing the transmitted light through the CLC color filter layer
32
is interposed between the CLC color filter layer
32
and the second substrate
30
. A second transparent electrode
36
, which is a second electrode for applying a voltage to the liquid crystal layer
50
, is formed on the CLC color filter layer
32
.
Accordingly, since the CLC color filter layer
32
not only displays colors but also reflects light, an additional reflecting plate is not necessary. As the reflecting plate much affects the reflective brig
Moon Jong-Weon
Yoon Sunghoe
Duong Tai
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge
Ton Toan
LandOfFree
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