Structures and Properties of Ferroelectric Liquid Crystal Displays

Doktorsavhandling, 1996

Liquid crystals have unique physical properties as they combine the features of solids and liquids. In thermotropic liquid crystals the degree of order varies with temperature resulting in an appearance of intermediate phases. The least ordered phase is called the nematic phase and has been commercially utilized in a range of practical applications for the last two decades. This work deals with higher ordered, chiral liquid crystals and their potential for Information Technology applications. The principles of the Surface Stabilized Ferroelectric Liquid Crystal Display (SSFLCD) are explained and compared with alternative technologies on the flat panel market. The liquid crystal materials which are to be used in practical applications have to fulfil several requirements from chemical, physical and environmental points of view. These parameters are discussed with a special focus on the dielectric properties of ferroelectric liquid crystals. Major steps in liquid crystal cell preparation are presented. Different aligning techniques suitable for SSFLC devices are examined, including both evaporated SiOx films and rubbed polymer aligning layers. The influence of varying rubbing strengths on the surface topography of polyimide aligning layers was investigated by means of atomic force microscopy. The electro-optic characteristics under addressing condition for cells prepared with selected aligning layers in combination with a variety of ferroelectric liquid crystal mixtures are reported. Bistability and the influence of dielectric biaxiality on the contrast enhancement of electro-optic response characteristics has been studied and suitable combinations of liquid crystal mixtures and aligning layers are presented. As ferroelectric smectic liquid crystals are intrinsically bistable grey scale is difficult to achieve. Several solutions to overcome this problem are presented. Utilizing of the electroclinic effect for grey scale applications and in light intensity modulators is considered. Finally the field induced defects caused by the addressing pulses and their utilization in grey scale generation are discussed.