Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-12-13
2003-12-09
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000, C349S110000
Reexamination Certificate
active
06661485
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 10-2001-0005973, filed on Feb. 7, 2001, which is hereby incorporated by reference.
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. Description of the Related Art
Until recently, the cathode-ray tube (CRT) has been mainly used for display systems. However, flat panel displays are beginning to be implemented for display systems because of their small depth dimensions, desirably low weight, and low voltage power supply requirements. Currently, thin film transistor-liquid crystal displays (TFT-LCDs) with high resolution and small depth dimension are being developed.
Generally, conventional thin film transistor liquid crystal display (TFT-LCD) devices include an upper substrate and a lower substrate with a liquid crystal molecule layer interposed therebetween. The upper substrate and the lower substrate are generally referred to as a color filter substrate and an array substrate, respectively.
LCD devices use backlight sources disposed under the lower substrate to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD devices. 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.
Since power consumption of reflective LCD devices greatly decreases due to use of ambient light as a light source which increases operating time, 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 lower 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 the color filter substrate twice. Since overall thickness of the color filter substrate 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 with high transmittance and low color purity. However, there is a limit in fabricating the color filter below a threshold thickness due to characteristics of the resin used to form the color filter. Accordingly, one possible solution to this problem is forming LCD devices using cholesteric liquid crystals (CLCs) having selective reflection and transparency characteristics.
In reflective LCD devices using a CLC color filter layer, the fabrication processes are simplified due to omission of the reflective layer, and a high color purity and high contrast ratio is achieved. Moreover, since CLCs have a spiral structure and spiral pitch determines a selective reflection bandwidth of the CLCs, the reflection bandwidth can be controlled by a distribution of the spiral pitch at one pixel. To illustrate this in more detail, a wavelength range of visible light is from about 400 nm to 700 nm. The wavelength of the red light region is centered at about 650 nm, the wavelength of the green light region is centered at about 550 nm, and the wavelength of the blue light region is centered at about 450 nm. The CLC color filter is formed having characteristics that can selectively reflect or transmit right-handed or left-handed circularly polarized light at a bandwidth that corresponds to a pitch deviation by selecting bandwidths corresponding to the red, green, and blue light regions. In addition, the CLC color filter is formed having characteristics that control conditions for right or left pitch deviations with respect to the center wavelength. Accordingly, the pitch of the liquid crystal can be artificially adjusted so that a CLC color filter can selectively reflect light of an intrinsic wavelength of the color corresponding to each pixel.
FIG. 1
is a schematic cross-sectional view of a conventional reflective LCD device using a CLC color filter. The LCD device is an active matrix liquid crystal display (AMLCD) in which one TFT, which is an on/off switching device of a pixel voltage, controls a voltage of the liquid crystal of one pixel and adjusts transmittance of the pixel. In
FIG. 1
, upper and lower substrates
10
and
30
face each other and are spaced apart with a liquid crystal layer
50
interposed therebetween. At the bottom of the transparent substrate
1
of the upper substrate, a TFT “T” is formed. Beneath the TFT “T,” a pixel electrode
16
connected to the TFT “T” is formed in each pixel area. Beneath the pixel electrode
16
, a black matrix
14
that screens light of non-operating areas of the liquid crystal is formed. At the top of the transparent substrate
1
, a quarter wave plate (QWP)
18
and a polarization plate
20
are sequentially layered. On the transparent substrate
1
of the lower substrate
30
, the CLC color filter
32
, which reflects light of the bandwidth corresponding to the red, green, and blue regions and transmits light of other bandwidths, is formed. Beneath the CLC color filter
32
, a light absorption layer
34
that absorbs the transmitted light through the CLC color filter
32
is formed. Upon the CLC color filter
32
, a common electrode
36
is formed for applying an electric field to the liquid crystal layer
50
along with the pixel electrode
16
. Reflective LCD devices using the conventional CLC color filter include the TFT, pixel electrode, and black matrix formed on the upper substrate with the CLC color filter and light absorption layer being formed on the lower substrate.
The following descriptions demonstrate relationships of aperture ratio of a pixel electrode of a reflective LCD device. The aperture ratio is related to brightness and a ratio of effective pixel area to total display area. The higher the aperture ratio, the higher the brightness. Furthermore, since the aperture ratio of reflective LCD devices is related to the brightness of the reflected light that determines characteristics of the devices, it is necessary to increase the aperture ratio of the reflective LCD devices.
FIG. 2
is a schematic plane view of the upper substrate of FIG.
1
. In
FIG. 1
, a matrix structure is formed by orthogonal gate and data lines
11
and
13
. A pixel electrode
16
is formed in a pixel area which is defined by the intersections of the gate line
11
and data line
13
. A TFT “T” is formed to be connected to the pixel electrode
16
and a black matrix
14
is formed at an oblique-lined region. Since the pixel electrode
16
is formed separately from the gate line
11
and data line
16
in order to avoid any electrical interference therebetween, the aperture ratio is less than about 80%. Moreover, the black matrix
14
is formed overlapping an edge portion of the pixel electrode
16
to prevent light leakage at the edge of the pixel electrode caused by any cross-talk generated between the pixel electrode
16
and the data line
14
, thereby deteriorating display quality. Consequently, since a total area of the pixel electrode
16
is not used as an operating area of the display in the reflective LCD device, this causes the deterioration in the brightness of the reflected light.
BRIEF SUMMARY OF T
LG. Philips LCD Co. Ltd.
Morgan & Lewis & Bockius, LLP
Qi Mike
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