A research group led by Tohoku University and Sony Corp developed a new liquid crystal alignment technology. The technology may contribute to improving the contrast ratio and response time of VA LCD panels.
It was presented at the International Display Workshops (IDW) taking place from Dec 5 to 7, 2007.
With the use of the new technology, the liquid crystal molecules aligned substantially vertically can be tilted in the predetermined direction when a voltage is applied without providing a protruding structure or slit in the pixel electrode, which may lower the contrast ratio.
In a general VA LCD panel, the liquid crystal molecules are upright when no voltage is applied to the liquid crystal cell whereas they are tilted horizontally when a voltage is applied. To tilt the molecules in the predetermined direction, a protruding structure is formed on the substrate or a slit is provided in the pixel electrode.
The provision of the structure or slit, however, causes the disturbance in the alignment of surrounding liquid crystals, thus resulting in the degradation in the contrast ratio due to light leakage from the backlight unit. The latest alignment technology eliminates the need for the protruding structure or slit in the pixel electrode, thereby significantly reducing light leakage due to the disturbance in the liquid crystal alignment.
The new technology is also effective for improving the response time of VA LCD panels. In a regular VA LCD panel, liquid crystal molecules in the vicinity of a protruding structure or slit are first tilted in the predetermined direction when a voltage is applied. Then, other vertically aligned molecules are successively tilted horizontally like dominos, which takes a substantial amount of time for all the molecules to be tilted.
According to the latest alignment technology, almost all molecules are aligned in such a manner that they are tilted in the predetermined direction by 1 degree from the vertical plane (pretilt angle: 89 degrees). Therefore, the molecules are tilted horizontally all at once when a voltage is applied.
The following outlines the latest liquid crystal alignment technology. First, liquid crystal monomer molecules are applied on a glass substrate on which an ITO electrode is formed. Next, the monomer molecules are aligned vertically by controlling the thermal state. Then, the substrate is placed in the magnetic field and exposed to the UV irradiation.
As a result, the monomer film is polymerized and the liquid crystal molecules are slightly tilted in the predetermined direction from the vertical plane. The alignment direction and the pretilt angle of liquid crystal molecules (negative type) injected in the cell are thus determined.
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