Gerard Cnossen

A wide-viewing-angle video display based on the deformed-helix ferroelectric liquid-crystal effect and a diode active matrix

— A wide-viewing-angle video display has been realized using a deformed-helix ferroelectric liquid crystal (DHF-LC) in combination with a diode active matrix. The DHF-LC is operated in the symmetric mode (ac driving), preventing the occurrence of image sticking. The application of a reset pulse before the select pulse results in a much better step response of the display and in lower drive voltages. These effects have also been calculated by making use of the electronic equivalent circuit of DHF-LC. The display has a contrast ratio of 25 and a transmission 55% that of a twisted-nematic cell.

Active-matrix deformed-helix ferroelectric liquid crystal displays

Abstract Active-matrix DHFLC displays have potential for a better performance than can be obtained with TN AM-LCDs. Alignment of DHFLC is still difficult and the defect structure lowers the contrast. A forming voltage improves the contrast, but the defects reappear during temperature cycling in the S*C-phase. At least four different types of defects can be distinguished. Active-matrix driving of DHFLC is thoroughly investigated. The symmetric DHFLC mode is the preferred mode, for it prevents the occurence of image retention. Voltage driving and charge driving are both possible. Special measures have to be taken to improve the step response of the display. A diode-AM-DHFLC display is described which operates at video speed and shows a good step response and a wide angle of view.

18.2: Galvanic Activity in Reflective LCDs

Reflective LC displays using an internal reflector are inherently asymmetric. This asymmetry affects the electro-optic behavior of the LC and has been identified as the major cause of flicker. In this paper we report experiments which associate this flicker with electrochemical activity within the LCD itself.

Grey level control by variation of the surface interaction in AFLCD's.

Abstract We report the influence of the rubbing treatment of the alignment layers on the electro-optic characteristics of AFLCDs. We applied a different rubbing treatment on different regions within a pixel. This variation of rubbing strengths causes domains with different surface interaction, which results in a different threshold voltage for each domain. In-pixel variation of the rubbing treatment and of the surface interaction influences the double hysteresis of the transmission-voltage characteristic, which is made less steep. The difference between the voltages at which the transmission starts to increase and the voltage at which the transmission reaches its maximum increases and so the selection of analogue grey levels is made easier. The results also show that the finite slope of the double hysteresis which is observed in normal AFLCDs, without extra rubbing treatment is probably caused by variations of the surface interaction.

Pretilt angle inducing mechanisms and surface-stabilized ferroelectric liquid crystals

Abstract We have investigated the role of fluorine containing groups and of alkyl side chains in the generation of pretilt angles. From X-ray photoelectron spectroscopy it was concluded that the fluorine-containing groups force the polyimide to adapt a certain conformation and that this conformation governs the pretilt angle. The effect of alkyl side chains was studied by modification of the surface of poly(vinylalcohol) with alkanoyl chlorides. The pretilt angle was found to depend on the length and surface density of the grafted chains. A uniform and tilted orientation could only be obtained by grafting long alkyl chains. The influence of the pretilt angle on the effective cone angle in ferroelectric liquid crystals has been calculated and was found to agree reasonably well with the experimental values. Angles between the memory states of 40° have been obtained leading to brightness levels comparable to those of the twisted nematic effect.