Black-white Reflective Liquid Crystal Display Prepared with Two Color Reflective Layers

Reflective cholesteric liquid crystal displays (Ch-LCD) with bistability are very attractive because of their excellent readability, low power consumption with image memory, high reflectivity, high contrast ratio at a wide viewing angle, and a low manufacturing cost[1]. Cholesteric liquid crystal displays have several unique features suitable for mobile displays, public information displays, signboard applications, and future e-books. However, such technical problems as high operation voltage and full-color capability should be enhanced to yield a robust product. A cholesteric liquid crystal is similar to a nematic liquid crystal since it has a long-range orientational order but no a long-range positional order. The Ch-LCD, however, has a helical structure due to the dopants in the cell. Cholesteric liquid crystal displays use two stable states at zero field; in the planar state the helical axes of LC align perpendicular to the substrate surface and a selected wavelength of incident light is reflected by Bragg reflection. In the focal conic state the helical axes of LC align more or less parallel to the substrate surface, and the cell is weakly scattering the incident light without Bragg reflection[1]. Especially, low operation voltage is very important in cost and power consumption. In general, the ideal liquid crystal mixture for low driving voltage should have a high birefringence, high dielectric anisotropy, and low viscosity[2]. In addition, the panel structure and alignment layer of the display would be crucial factors for obtaining a higher quality display. We reported on the initial work of the driving techniques related to Ch-LCD[3] and blue-white cholesteric displays by single layer[4]. There are different types of color combination with ChLCD such as green/black and red/green/blue color. Technically, black-white chloesteric liquid crystal display is not simply obtained due to the reflectivity characteristic of cholesteric texture. In general, white color can be made by