1. Field of the Invention
The present invention relates to a liquid crystal display element.
2. Description of Related Art
Smectic liquid crystal materials having spontaneous polarization, such as ferroelectric liquid crystals and anti-ferroelectric liquid crystals, are expected as materials of next-generation liquid crystal display elements, since these materials have characteristics, such as rapid response and wide viewing angles, in a surface stabilized display mode. Particularly in recent years, it has been attempted to provide various moving picture displays which are combined with an active matrix driving system. As materials which are suitable for this use and which have no hysteresis, thresholdless anti-ferroelectric liquid crystals (which will be also hereinafter referred to as “TLAF liquid crystals”) and polymer stabilized ferroelectric liquid crystals (which will be also hereinafter referred to as “PS-FLC liquid crystals”) are widely noticed.
However, as one of the features of liquid crystal display elements which use liquid crystal materials having spontaneous polarization, there is the difficulty of controlling alignment or orientation. The TLAF liquid crystals have a phase series of Iso phase→SA phase→SC* phase (TLAF phase) in that order from a high temperature side. When a phase transition from Iso phase to SA phase occurs, a layer structure is formed, and when a phase transition from SA phase to SC* phase occurs, a chevron structure wherein the variation in spacing between smectic layers cause the layers to be bent is produced (see FIGS. 8(a) and 8(b)). The chevron structure is divided into two kinds of C1 and C2 alignments in accordance with the relationship between a bent direction and a pretilt angle (see FIG. 8(b)). The liquid crystal display element using the TLAF liquid crystal preferably has C2 alignment in view of display characteristics. If a rib structure and a rubbing system are selected (see, e.g., Japanese Patent Application No. 10-184903 filed by the applicant) and if an alternating electric field is applied during a slow cooling from Iso phase to SC* phase, it is possible to selectively obtain uniform C2 alignment. However, it was revealed that the same C2 alignment has different alignment characteristics in accordance with the kind of alignment layers.
In an ideal TLAF liquid crystal, an average optical axis during the application of a voltage of 0 V is coincident with the normal direction of a smectic layer. When the TLAF liquid crystal is used as a display element, homogeneously aligned TLAF panel is placed between crossed polarizers the axis of which are parallel and perpendicular to the smectic layer normal (crossed polarizers configuration). In this case, there is obtained a voltage-transmittance characteristic shown in FIG. 9, wherein black is displayed when a voltage of 0 V is applied and gray scale to white is displayed when a positive or negative voltage is applied.
However, some kinds of used alignment layers produce a phenomenon that a domain, which has a stripe shape parallel to the direction of the smectic layer and which has a deviated optical axis from layer normal, is produced to grow in the course of time and/or to increase the shift of the optical axis. It was observed that some kinds of liquid crystal materials promoted the same phenomenon when a high voltage approximating a saturation voltage was applied. Examples of observed alignment deterioration are shown in the schematic diagrams of FIGS. 10A, 10B and 10C. FIG. 10A is a diagram viewed from the surface of a substrate. The alignment layers provided on both substrates for sandwiching a liquid crystal, which are rubbed at predetermined angles, and a TLAF liquid crystal material is introduced between the substrates, so that a smectic layer structure shown in the figure is formed. The rubbing angle is determined by the combination of the used liquid crystals and alignment layers. The initial alignment state of the liquid crystal display element thus formed, and the alignment, in which the domain having the partially deviated optical axis was produced, were observed by a microscope having the crossed polarizers configuration. The observed results are shown in FIGS. 10B and 10C, respectively.
In order to facilitate understanding, the polarizing direction of a polarizer or analyzer is shifted from the normal direction of a smectic layer by x° (<22.5°) as shown in FIG. 10D.
In an alignment wherein no alignment deterioration occurs, the optical axis is one direction as shown in FIG. 10B, so that light uniformly transmits to be visible. However, in an alignment after deterioration wherein a domain having an optical axis deviated by ±x° is produced, it is observed that a domain having an optical axis coincident with the polarizing direction is dark, and a domain having an optical axis deviated in the opposite direction is bright, in a uniform alignment as shown in FIG. 10C. If the optical axis is thus deviated, it is not possible to obtain a satisfied black level, so that it is required to completely inhibit the optical axis from being deviated from the layer normal direction as a display element.
As way of inhibiting the optical axis from being deviated, Japanese Patent Laid-Open No. 10-319377 has proposed a method for introducing a polymer precursor into a TLAF liquid crystal material, injecting them between substrates, and photopolymerizing them in SA phase to stabilize the structure when a voltage of 0 V is applied.
However, the inventors studied and verified that according to the method for introducing the polymer precursor as disclosed in Japanese Patent Laid-Open No. 10-319377, the alignment itself of the TLAF liquid crystal is disturbed by foreign molecules other than the liquid crystal material, to increase the leakage of light during a black level independent of polymerization methods, so that contrast lowers.