Harald Hirschmann

Improved Liquid Crystal Mixtures for STN Displays

Supertwisted nematic (STN) liquid crystal displays remain a leading technology for mobile communications applications, personal digital assistants (PDA) and personal convergence devices. Higher demands on display information content place ever increasing demands on the physical parameters of the liquid crystal mixture. To satisfy current and future display requirements, it is necessary to develop mixtures showing: ? Fast response times achieved by rotational viscosity reduction and birefringence increase. ? Small temperature dependence of the operating voltage (dV/dT), especially at low temperatures. ? Small frequency dependence of the operating voltage (dV/df), necessary for multi-level greyscale operation. ? Improved steepness (V90/V10) of the electro-optical curve to enable higher duty multiplexing of the display. In order to fulfil these requirements, liquid crystal mixtures with improved physical properties were developed. Examples of these mixtures are presented.

Highly Birefringent Nematic Liquid Crystals for Reflective STN Applications

Abstract An important parameter of any LCD containing portable electronic device is a low power requirement. The reflective STN display mode offers a moderate to high information content display with low power consumption. One method to realise colour in this mode is to use a large retardation (dΔn) value in the cell. For a cell gap of 6 μm, and a retardation of −1.5, a birefringence (Δn) of greater than +0.2 is necessary. To achieve such a value in an STN LC mixture, materials with broad nematic range, good solubility and a Δn value of <+0.25 are required. A study of the trans-cyclohexyldifluorophenyltolane core structure was made and a large number of dialkyl, alkenyl-alkyl and alkoxy materials were synthesised and characterised. These materials possess low to moderate melting points, broad purely nematic mesophases, a high Δn and very good solubility in nematic LC hosts. They are suitable materials for inclusion in mixtures for colour reflective STN displays.