The Study Of Vertical Alignment Technique For Cholesteric Liquid Crystal Displays

Cholesteric liquid crystal displays(Ch-LCDs) have bistable states,ie.the planar state or the focal conic state has pixel memory with the electric field being removed.This mode has many advantages,such as low power consumption,high contrast ratio and low cost and so on,and it is suitable for flexible displays.Thus there is a good future for Ch-LCDs in the static displays like e-book,e-paper and e-label etc.The free energy of the focal conic texture is larger than that of the planar texture,so the focal conic state is a semistable state in thermodynamics.Therefore,the performance of Ch-LCDs is strongly dependent on the stability of the focal conic state.If the PI layer is vertically aligned in order that the LC molecules are vertically aligned at the boundary as well,it is helpful to keep the focal conic state.This is the widely used "Surface Stablized Cholesteric Liquid Crystal Displays"(SSCh-LCDs).The anchoring effect of vertical alignment at the surface plays a direct and important role in the stability of the focal conic state.That an in-depth study of the anchoring effect to the stability of the focal conic state,is more meaningful to the SSCh-LCDs research. Because the polar anchoring energy is an important physical parameter for the evaluation of the anchoring effect,from the viewpoint of polar anchoring energy,we adjusted the polar anchoring energy by changing the temperature,and investigated the mechanism of how the polar anchoring energy affects the stability of the focal conic state.This study brings out a new way for the further study of surface boundary conditions effect to the performance of SSCh-LCDs.In our research,we established the automatic system for the measurement of polar anchoring energy,which is controlled by the LabVIEW software.With the high electric field method,we measured the polar anchoring energy of both rubbed and non-rubbed VA cells with the same physical parameters at different temperatures.The experimental results show that the polar anchoring energy decreases with the increment of temperatures.Within the experimental errors,the anchoring energy of rubbed VA cells is larger than that of non-rubbed VA cells.Treating the change of transmittance of the focal conic texture as the performance index,we measured the transmittance of the focal conic texture at different temperatures.And it is found that the change of transmittance of the focal conic texture becomes larger with the increment of temperatures,and the stability of rubbed VA cells is better than that of non-rubbed VA cells.These consequences imply that the polar anchoring energy of vertical alignment plays an important part in the stability of the focal conic texture,and the more larger polar anchoring energy is,the more better stability the focal conic texture has.All the above results are significant to both the study and application of cholesteric bistable LCDs.