Optical properties and polymer wall formation in cholesteric displays.

摘要

Stacks of thin layers of cholesteric liquid crystals (CLCs) reflecting light selectively with no polarizers or color filters have proved to be very promising for bright reflective displays. Two- and three-layer stacked displays are capable of showing multiple colors without parallax when addressed with a binary drive and full color when gray scale addressing schemes are used with three CLC layers of primary colors.;Although several types of reflective color displays with stacked cholesteric layers have been made, their performance has not been fully optimized and little effort has been made to determine factors influencing it in order. In addition, the ruggedness of CLC-based displays in many cases (especially with plastic substrates) is not sufficient.;In this study, reflective properties of double- and triple-layer stacks of various color, chirality, and stack order were analyzed. It has been found that stack order breaks the equity of light propagation direction in stacks comprised of cells with imperfect planar structures. To explain this inequity, a light propagation model incorporating forward scattering in CLC layers with angular distribution of the helical axes has been proposed. By using results of the optical characterization of single and multiple CLC layers, an optimized configuration of color and chirality of stacked CLC layers for full color reflective displays was determined.;Electric-field-induced polymer walls are proved to provide ruggedness and pixel separation in CLC displays. Formation of such walls in multiplexed cells was investigated through microscopic observations and numerical calculations and its mechanism was determined. Numerical calculations of the charge distributions on electrodes, as well as potential and electric fields in patterned cells, showed that discontinuity of the electrode surfaces caused a nonuniform distribution of charges in the electrodes leading to a highly nonuniform electric field. Together with a mismatch of dielectric properties of mixture components, this fringing field was found to affect the phase separation in LC-monomer mixtures through creating concentration gradient and lowering the energy barrier for forming LC droplets. In addition, the fringing field was found to determine the dynamics of spatial segregation of mixture components after the phase separation, leading to polymer wall formation.