Rob van Asselt

Long-term Ion Transport in Nematic Liquid Crystal Displays

Leakage current measurements are performed on prototype cells revealing an S-shaped current versus voltage curve. This result indicates the presence of ion generation and recombination in the liquid crystal bulk. The compensation of the electrical field and the saturation current are related to the alignment layer thickness and to the temperature. The slope of the I–V curve corresponds with Onsagers dependency of the ion generation. The time dependency of the current is modelled by an empirical fitting of the generation constant. A complete theoretical model is thus presented. The simulation fits with the measurements and gives a profound insight in the long term ion transport in nematic liquid crystal displays.

Measurements of lateral ion transport in LCD cells

The ions present in liquid crystal devices modulate the applied electric field and lead to deterioration of the expected good optical response. In addition to the flicker and ghost images, a boundary image-retention effect is also possible. It occurs near the edges of a stressed pixel. We have attributed this effect to ions moving in the plane perpendicular to the applied electric field. This hypothesis has been proven using a combination of electrical and optical measurements. The observed optical non-homogeneity and its evolution with stress time were explained using the new model of lateral ion transport. The physical cause of this phenomenon is subject to further study.

23.3: Light-Efficient Liquid Crystal Displays Using Photoluminescent Color Filters

Liquid crystal displays using a UV backlight and photoluminescent color filter have the advantages of improved light utilization efficiency and wide viewing angle. In order to fully exploit these benefits, dedicated components are developed. Furthermore, technology needed to integrate these components and to make such a display are described.

The Influence of the Driving Voltage and Ion Concentration on the Lateral Ion Transport in Nematic Liquid Crystal Displays

Nematic liquid crystal displays (LCDs) contain ions that influence the electrooptical characteristics of the display. A typical super-twisted nematic (STN) display for mobile phone applications becomes darker at a standard driving frequency if it contains many impurity ions. We have discovered that ions can travel in the plane of the glass plates in the absence of a lateral electric field, leading to lateral nonhomogeneity in transmission (dark and bright stripes). In this paper, we present our research on the lateral ion transport dependence on the driving square wave (SQW) amplitude and dc component at a wide range of ion concentrations. The existence of a dc component, a high ion concentration and high SQW amplitudes increase the lateral ion speed.

10.4: Ion Transport Simulations and DC Leakage Current Measurements in Nematic LCDs

Prototype liquid crystal cells with different alignment layers were studied at different temperatures. Long-term leakage current measurements were then performed revealing an S-shaped IV-curve typical for the presence of ion generation and recombination as well as the Onsagers dependency. Highly and weakly resistive alignment layers can be clearly distinguished.

P‐52: Birefringent Colour Reflective Liquid Crystal Displays Using Broadband Cholesteric Reflectors

The application of broadband cholesteric reflectors in birefringent colour reflective LCDs is described. As compared to metallic reflectors, contrast and colour saturation are better, while the brightness is comparable. Moreover, when a cholesteric reflector is used the displays can be operated in the transmissive mode, in which case complementary colours are obtained. In this way transflective displays are obtained without sacrificing brightness of the reflective or the transmissive state.

Asymmetrical Properties of Reflective LCD'S

Abstract In this paper we present results concerning reflective LCDs with different alignment layer types. We made thought-out interpretations of the asymmetrical reflectance and current responses. The asymmetry is the result of an internal voltage. In our measurements, we distinguish alignment layers, which introduce mobile ions in the LC bulk and other alignment layers, which have a layer of trapped ions at the interface with LC.

Multidomain Structures for Improved Viewing Angle Twisted Nematic Liquid Crystal Displays

Abstract The orientation of iiquid crystals on spherulitic polymeric alignment layers can give both a radial and a concentric director pattern. These types of orientation lead to LCDs having a wide and symmetric viewing angle. Moreover, greyscale inversion is not observed within 60° polar angles.