A liquid crystal display has been widely supplied to the public for various uses according to development and progress of technology of widening a viewing angle and technology of a motion picture.
Recently, a method of driving liquid crystals has been improved in order to provide a liquid crystal display (LCD) having higher precision and a wide viewing angle. For example, Patent Document No. 1 discloses an IPS (In-Plane Switching) mode liquid crystal display using an electric field parallel to the surface of a glass substrate. In addition, Patent Document No. 2 discloses a liquid crystal display (LCD) having an improved aperture ratio by using FFS (Fringe-Field Switching) technologies obtained by further improving the IPS technology.
Further, a liquid crystal composition, an alignment film material, and the like of the liquid crystal display (LCD) as well as a liquid crystal display device have been improved. For example, the IPS mode or FFS mode device drives liquid crystal molecules on a liquid crystal layer by an electric field parallel to a substrate, but generates a “vertical electric field” in which electric field lines become perpendicularly close to the substrate around the electrode. When a liquid crystal composition having positive (+) dielectric anisotropy is used for these devices, liquid crystal molecules are aligned in an electric field line direction in a state that a voltage is applied thereto and thus in a vertical direction around an electrode, and this may cause display defects. Since these display defects need to be masked in order to use these devices as a liquid crystal display (LCD), there is a problem of not sufficiently increasing an aperture ratio. On the contrary, when a liquid crystal composition having negative (−) dielectric anisotropy is used for the FFS device, a long axis of liquid crystal molecules rotates in a direction perpendicular to an electric field applied thereto and thus is driven to orient parallel to the substrate in a region where a vertical electric field is applied to the liquid crystal molecules. In other words, since the FFS device is prevented from having the display defects in a region where the vertical electric field is generated around the electrode, the aperture ratio may be improved (Patent Document Nos. 3 and 4).
On the other hand, for the alignment film material, a photo-alignment method of radiating polarized ultraviolet (UV) light to an alignment film to cause a photodegradation reaction, a photodimerization reaction, or a photoisomerization reaction of the alignment film to apply a pretilt angle and alignment property to liquid crystal molecules, instead of a conventional rubbing method has been suggested (Patent Document Nos. 5 and 6). The photo-alignment method has an advantage of causing highly uniform alignment compared with the rubbing method, doing no damage to the alignment film since the photo-alignment method is a non-contact alignment method, and decreasing display defects of the liquid crystal display device such as those caused by dust, or static electricity.
However, this method of using a photodegradation reaction of an alignment film (hereinafter referred to as a photodegradation method) has a problem of deteriorating display performance such as leaving degradation products (monomolecule components) of the alignment film and deteriorating image sticking characteristics, thermal stability, electrical stability, and the like (Patent Document No. 7).
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