Stefaan Vermael

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.

Lateral ion transport in nematic liquid-crystal devices

Liquid crystals used in electronic displays usually contain small amounts of ions that move under the influence of the varying applied electric field. It is well known that the motion of ions perpendicular to the substrates may lead to modified electric fields resulting in image sticking effects. During operation, the modulation in the director tilt angle can also lead to a net residual lateral component of the ion motion, parallel with the glass plates. A sustained ac driving voltage will accumulate the lateral ion motion and may result in image sticking effects near the pixel edge. Such effects have indeed been observed in supertwisted nematic cells.

Dispersive Ion Generation in Nematic Liquid Crystal Displays

Ion generation and recombination have been characterized in nematic liquid crystal displays. A model for dispersive generation may explain the time dependency of the leakage current through the cell. This current leads to the build-up of a charge layer near the electrodes depending on the resistivity of the alignment layer (a.l.) used. The amount of transported charge can then be calculated and related to a compensating voltage, which gives rise to image retention problems. Agreement between the results of theoretical analysis and experiments was reached when the capacitances of the a.l. and the diffusion layer were taken into account. Furthermore, the gradual removal of the charge layer during a short circuit was analyzed. This effect, depending on the type of a.l. used, is related to the diffusion and recombination of the ions involved.

3-dimensional ion transport in liquid crystals

One-dimensional ion transport has been studied in the past decade because of the importance of image retention and voltage holding ratio. Ions inside a liquid crystal (LC) disturb the expected optical behavior of LC molecules when the applied voltage is higher than the LC threshold. In LC devices with complex electrode patterns, interesting ionic effects occur. Advanced and reliable simulation programs are a necessary to investigate this. This paper describes the theory and results of a Monte Carlo 3D ion transport simulation program for LC devices.

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.

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.

Dependence of the lateral ion transport on the driving frequency in nematic liquid crystal displays

The presence of ions in a liquid crystal (LC) influences the transmission characteristics of LC displays. These ions follow the electric field perpendicular to the electrodes and move back and forth under the influence of the ac field. Because of their charge, they can distort the electric field, which leads to transmission changes. Recently it was discovered that due to the LC anisotropy, ion motion parallel with the plane of the electrodes (perpendicular to the electric field) is also possible, even without lateral fields. After driving a pixel for a long time, the ions will accumulate at one pixel edge, which leads to unwanted image artifacts. In this paper, we investigate the frequency dependence of the lateral ion transport in twisted nematic liquid crystal displays at high and low ion concentrations, different ion mobilities, and LC rotational viscosities, for a fixed voltage just above the LC threshold.

Simulation of V-shaped stability in AFLC.

The V-shaped transmission-voltage characteristics in FLC have been explained by the existence of a splayed state, caused by strong polar interaction with the alignment layers. Both simulation results and analytical calculations have been used to confirm this statement. The conditions that guarantee V-shaped characteristics have been described. There is no consensus on whether V-shaped characteristics can exist in AFLC. In tri-state switching the AFLC will be in the so-called ferroelectric up- (or down-) state FU (FD) for sufficiently high applied positive (negative) voltage. By means of the uniform-(phi) theory it has been shown that if V decreases to zero, one first follows the symmetrical up- (down-) state SU (SD), and then switches back to the normal alternating state so called antiferroelectric state AF. In this article we investigate switching from a strong positive voltage to zero and check under which conditions one ends up in a special alternating state SA, with both polarizations parallel to the glass surfaces, instead of in the normal alternating state, with both polarizations perpendicular to the glass surfaces. The first case guarantees V-shaped switching in AFLC, the second case leads to tri-stable switching. The simulation program is based on implicit iteration and on the Newton-Raphson linearization method. Several simulation results will be shown and discussed. They confirm that V-shaped switching occurs in AFLC under approximately the same conditions as for FLC, i.e. with strong interactions with the alignment layers.

Detailed comparison of several ion-transport algorithms in a 1-dimensional liquid crystal model

In this paper we investigate three one-dimensional ion transport simulation algorithms and compare the results. The ion transport algorithms are incorporated in a one- dimensional Liquid Crystal Display (LCD) model that calculates the director orientation and the influence of ions on the electrical field. The aim is to improve calculation speed and accuracy. The first algorithm is the traditional explicit forward method using finite differences. The second algorithm is based on the first, but it assumes an exponential variation, instead of a constant ion concentration in each interval. The third is Monte Carlo based. It does not use any intervals but calculates drift for individual ions and treats diffusion as a random walk. We investigated the frontiers of stability and speed with respect to the accuracy, by varying the time steps and the number of intervals. The main conclusion of our work is that the calculation speed can be improved by using the new algorithms without loss of accuracy. The exponential algorithm proves to be very helpful in the simulation in the case of ions piling up near the alignment layer. The Monte Carlo algorithm is the most appropriate and at the same time a promising candidate for extension to two-dimensional simulations.

Influence of the Ion Mobility on the Exact Location of the Regime Bump in NLCDs

Abstract In this article, we propose an improvement of the ion transport model in NLCDs, taking into consideration the possible dependence of the ion mobility on the ion density. The presence of ion channels in the LC bulk, initiated by a Carr-Helfrich-like mechanism, would lead to a higher mobility at the start of the regime pulse. A better explanation of the exact location of the current bump during the regime pulse can then be reached.