Negative A-plates for broadband wide-view liquid crystal displays

High contrast ratio and wide viewing angle are critical requirements for direct view liquid crystal displays LCDs . Various optical compensation schemes with phase-matched films have been developed to widen the viewing angle of LCDs. A-plate, whose optic axis is located in the plane parallel to film surface, has been widely used in wide-view LCDs. Although positive and negative A-plates are usually equivalent to each other, in some applications negative A-plate has its unique functions. For instance, a negative A-plate together with a positive A-plate reduce the phase mismatch at oblique viewing angle and thereby improve the off-axis image quality Besides, the adoption of negative A-plate makes it easier to achieve wide view and broad bandwidth. Negative A-plates have found emerging applications in transmissive and transflective LCDs. For large LCD panels, the compensation films are commonly made by stretching or compressing polymers at their glassy states. The stretching method offers good uniformity, low cost, and high yield. While the fabrication technique for C-plates and positive A-plates is relatively mature, negative A-plates have not been well explored. So far, several approaches have been proposed, such as photoinduced polymerization and self-assembled lyotropic liquid crystal films using coating or printing method. Both approaches are effective for obtaining a large negative in-plane birefringence but neither is ready for widespread applications. In this letter, we fabricated negative A-plates by stretching polystyrene PSt around its glass-transition temperature and measured its physical properties, such as refractive indices and birefringence dispersion. We also developed a molecular model to explain why the stretched polystyrene exhibits a negative birefringence. Finally, a broadband wideview LCD using the negative A-plate is proposed and the suppression of light leakage over visible spectrum is demonstrated. Negative A-plates can be obtained by stretching negative birefringence materials, such as PSt. The molecular structure of PSt is shown as follows: We have modeled a core structure of PSt using HYPERCHEM molecular modeling software. A semiempirical method of modified neglect of diatomic overlap was used with Polak–Ribiere geometry optimization algorithm. As a result, a single molecular core with side phenyl units was calculated in vacuo to its possibly lowest energy gradient, which suggests most possible configuration of the molecule. Figure 1 shows molecular configuration of PSt optimized to total energy gradient of 10−5 kcal / mol Å calculated as a rootmean-square value. The energy gradient is the rate of change first derivative of total energy with respect to displacement