Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
2001-12-20
2003-07-29
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S171000, C349S184000
Reexamination Certificate
active
06600544
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display device that uses a liquid crystal material having spontaneous polarization and displays an image by driving a switching element to be on/off.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. Further, with the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices have been widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
By the way, the liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) positioned on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount that varies depending on environmental conditions, transmission type liquid crystal display devices are generally used as display devices of, particularly, personal computers displaying a multi-color or full-color image.
In addition, the current color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type, based on the liquid crystal materials to be used. The STN type liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a moving image because they are susceptible to crosstalk and comparatively slow in the response speed. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a portable battery power source. Moreover, since a color image is displayed using a color filter, a single pixel must be constructed by three sub-pixels, and thus there are problems that it is difficult to achieve high definition and the purity of display color is not sufficient.
In order to solve the above problems, the present inventors et al. have developed a liquid crystal display device employing a field-sequential method. The liquid crystal display device employing a field-sequential method can easily realize a more definite display compared with a liquid crystal display device employing a color-filter method because it does not require sub-pixels, and it can also realize display color of excellent purity because the color of emitted light from a light source can be used directly for display without using a color filter. Moreover, since the utilization efficiency of light is high, the liquid crystal display device employing a field-sequential method has the advantage of low power consumption. However, in order to realize a liquid crystal display device employing a field-sequential method, a high-speed responsiveness of liquid crystal is essential. Therefore, in order to achieve a high-speed response of a liquid crystal display device employing a field-sequential method having the above-described advantage or of a liquid crystal display device employing a color-filter method, the present inventors et al. are carrying out the research and development on the driving of a liquid crystal, such as a ferroelectric liquid crystal having spontaneous polarization, which is expected to achieve a response at a speed 100 to 1000 times higher than a conventional speed, by a switching element such as a TFT (Thin Film Transistor).
In the ferroelectric liquid crystal, as shown in
FIG. 1
, the long axis direction of the liquid crystal molecules changes by only 20 with the application of a voltage. The intensity of transmitted light is changed by sandwiching a liquid crystal panel holding a ferroelectric liquid crystal therein by two polarizers whose polarization axes cross each other at a right angle and by using birefringence caused by a change of the liquid crystal molecules in the long axis direction. When the ferroelectric liquid crystal is driven by a switching element such as a TFT, the spontaneous polarization is switched according to the charge amount injected (stored) in a pixel through the switching element and the intensity of transmitted light changes.
By the way, in a conventional liquid crystal display device in which a liquid crystal such as a ferroelectric liquid crystal having spontaneous polarization is driven by a switching element such as a TFT, 2P
S
·A (the total charge amount of a switching current resulting from complete reversal of spontaneous polarization), where P
S
is the magnitude of spontaneous polarization per unit area and A is the electrode area of each pixel, is not larger than a charge amount Q to be charged in each pixel through a switching element. In other words, in order to satisfy the condition 2P
S
·A≦Q, the liquid crystal material, pixel electrode, TFT, etc. are designed.
In the conventional example, as described above, the cone angle 2&thgr; (&thgr;: tilt angle) of the liquid crystal is set at 45° or less, the spontaneous polarization is completely reversed under the condition of 2P
S
·A≦Q, and the maximum intensity of transmitted light is obtained. Therefore, with the application of a voltage of not higher than 7 V, the magnitude P
S
of spontaneous polarization that satisfies the above-described condition is reduced to 8 nC/cm
2
or less, and a slow responsiveness will result because P
S
is not increased much, and there is demand for an increase in the magnitude of spontaneous polarization in view of the responsiveness, particularly the responsiveness at low temperature. Moreover, there is a problem that the degree of freedom in selecting a liquid crystal material is low. Due to the relation between the responsiveness and the selectable liquid crystal material, when a liquid crystal material having large spontaneous polarization is used, Q must be increased, resulting in a problem of an increase of the applied voltage. Furthermore, as shown in
FIG. 2
, at near end of the switching of spontaneous polarization, since the change of the optical axis due to the reversal of liquid crystal is small, the proportion of the change of the intensity of transmitted light due to the increase of the applied voltage becomes smaller, and therefore a high applied voltage is required in order to obtain the maximum intensity of transmitted light.
BRIEF SUMMARY OF THE INVENTION
A principal object of the present invention is to provide a liquid crystal display device capable of using a liquid crystal material having large spontaneous polarization and decreasing the applied voltage to the liquid crystal material to a low voltage.
A liquid crystal display device of the first aspect is a liquid crystal display device comprising a liquid crystal material having spontaneous polarization between two substrates facing each other; and a plurality of electrodes and switching elements provided on an inner face of one of the substrates so that the electrodes and switching elements correspond to pixels, respectively, wherein a maximum charge amount injected to each pixel by the sw
Betsui Keiichi
Shiroto Hironori
Yoshihara Toshiaki
Fujitsu Limited
Greer Burns & Crain Ltd.
Ngo Julie
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