1. Field of the Invention
The present invention relates to a pixel structure. More particularly, the present invention relates to a pixel structure suitable for an optically compensated birefringence liquid crystal display (OCB LCD).
2. Description of Related Art
The development of displays has become the focus of the industry due to the increasing demands. Cathode ray tube (CRT) display has been leading the market because of its excellent display quality and mature technique. However, in recent years, along with the rising consciousness of environmental conservation, CRT display has become unsatisfactory for it cannot meet the requirements of lightness, slimness, shortness, smallness, and low power consumption due to its high power consumption, high radiation, and its limitation in size reduction. Thus, thin film transistor liquid crystal display (TFT-LCD) having such characteristic as high image quality, high space efficiency, low power consumption, and no radiation is becoming the main stream in the market.
LCDs can be categorized into many different types according to the type of liquid crystal, the driving method, and the disposition of light source thereof. Wherein optically compensated birefringence liquid crystal display (OCB LCD) has very quick response time and can provide smooth imaging performance when it is used in a computer for playing quickly changing images such as animation or movie, thus, OCB LCD is very suitable for high-level LCD. However, an OCB LCD can only display normally with quick response time after liquid crystal molecules are transformed from splay state to bend state.
FIG. 1A is a diagram illustrating liquid crystal molecules in splay state in a liquid crystal display (LCD) panel. FIG. 1B is a diagram illustrating liquid crystal molecules in bend state in a liquid crystal display (LCD) panel. Referring to both FIG. 1A and FIG. 1B, in the OCB LCD panel 10, the liquid crystal layer 11 is disposed between the top substrate 12 and the bottom substrate 13. The top substrate 12 and the bottom substrate 13 respectively have an alignment layer (not shown) and the two alignment layers have the same rubbing direction. When no external electric field is supplied to the liquid crystal molecules in the liquid crystal layer 11, the liquid crystal molecules are arranged in splay state. When the OCB LCD is entering stand-by status, an electric field vertical to the top substrate 12 has to be supplied to the liquid crystal molecules so that the liquid crystal molecules are transformed into bend state. In a conventional OCB LCD, a few minutes time is required for the transition to drive all the pixels of an entire panel normally. In other words, a long transition time is required before the OCB LCD enters stand-by status, which is very disadvantageous to the instant-on characteristic of LCD. Thus, fast transition is prerequisite to OCB LCD to be acceptable to customers.
Conventionally, to quickly transform liquid crystal molecules in an OCB LCD from splay state to bend state, the voltage supplied is increased to produce stronger electric field so that the liquid crystal molecules can be transformed from splay state to bend state quickly. However, it is difficult to obtain suitable drive IC which can sustain high voltage, accordingly it is difficult to carry out related research and mass production, thus, a technique for producing transition threshold has to be used for reducing transition voltage and transition time and ensuring complete transition of the entire LCD panel. According to another conventional technique, polymer is added into the liquid crystal layer and irradiated with ultraviolet light when liquid crystal molecules are in bend state to form a polymer wall, so that the liquid crystal molecules can be maintained in bend state. Such a method may cause light leakage to an OCB LCD panel even though a simple process is adopted.
In the LCDs disclosed in U.S. Pat. Nos. 6,661,491, 6,600,540, and 6,603,525, the pixel electrode has slit and the transition of liquid crystal molecules is accelerated by the voltage differences between the pixel electrode and the top/bottom storage capacitance, the pixel electrode and the counter electrode, and the transverse electric field. In any of the LCDs disclosed in foregoing three patents, even though transition threshold can be produced by transverse electric field, slit in pixel electrode of the LCD may reduce the aperture ratio and the brightness of the LCD panel.
Besides, in the OCB LCD disclosed in U.S. Pat. No. 6,597,424, a plurality of protrusions or indentations are designed around the pixel electrode and voltage differences between the pixel electrode and the gate, the pixel electrode and the data line are used to produce transition threshold which can help transition of liquid crystal molecules, and further to shorten the time required for transforming liquid crystal molecules from splay state to bend state.