Improved liquid crystals for active-matrix displays using high-tilt-orientation layers

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Abstract
Liquid crystal mixtures containing new Super Fluorinated Materials as highly dielectrically positive compounds have been developed. The new experimental LC mixtures have threshold values as low as 1.42 V in Viewing-angle Independent Panels, utilizing TN cells with an optical retardation of 0.5 micrometers . Broad nematic phase ranges from -40 to +100 and even +110 degree(s)C have been realized with threshold voltages of 1.8 V and below. Only a small increase of the flow viscosity had to be accepted. Also experimental LC mixtures with higher birefringence values suitable for displays with an optical retardation of 1.0 to 1.1 micrometers , having threshold values of about 2.3 V, have been developed. The surface tilt angles of the new liquid crystal mixtures have been investigated on several commercially available, as well as on new experimental polyimide orientation layers. All polyimides investigated were pre-imidized materials and showed suitable voltage holding ratios for AMDs. On all different alignment layers investigated here, the new liquid crystals lead to increased surface tilt angles, compared to the SFM mixtures available so far. The temperature dependent decrease of the surface tilt angle of the LCs on these new polyimides has also been investigated. Especially for the new LCs optimized for high surface tilt angles the temperature dependence of the surface tilt angle is stronger than for the same LC on high tilt orientation layers for STN application. Nevertheless, even at a temperature of 60 degree(s)C tilt angles of about 2 degree(s) have been obtained. This helps to reduce the occurrence of reverse tilt domains in AM displays. The strong temperature dependence of the tilt angle, found for some combinations of liquid crystals and orientation layers, can compensate the temperature dependence of the threshold voltage in the range from -20 to +40 degree(s)C.

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