Herman Pauwels

Solution of the dark state problem in antiferroelectric liquid crystal displays

So far, it has proven impossible to achieve an acceptable dark state between crossed polarizers for antiferroelectric liquid crystals (AFLCs), which otherwise would have an enormous potential for electro-optic applications, in particular for high-resolution full color displays. The reason lies in static and dynamic spatial fluctuations of the optic axis. As both have intrinsic causes it is not likely that the problem is ever going to be solved by improvement in alignment and addressing methods. We show that if the directors in alternating layers are orthogonal to each other, the AFLC acquires new optical properties such that the problem is eliminated, and a dark state extinction is achieved which is only limited by the quality of the polarizers. After having synthesized such a material, we have been able to demonstrate the predicted unique electro-optical properties of this new class of AFLC materials.

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.

Ion transport in SSFLCD's

Abstract An essential feature of surface stabilized ferroelectric liquid crystal displays (SSFLCD) is their bistability. Experiments and theory show however that this bistability can be destroyed by ionic charges building up at both electrodes. In this article a better understanding of the ion concentration distribution and their transport behavior is obtained through leakage current measurements on FLC cells with bipolar voltage driving. We also present a model for the ionic transport that can explain the measured current curves. Simulations according to this model fit the experimental data. In this way we derive some important parameters concerning the ionic behavior.

The influence of interface state and energy barriers on the efficiency of heterojunction solar cells

A heterojunction solar cell, in which interface recombination and arbitrary energy barriers at the interface occur, is analysed to find the optimum structure. Thermionic emission theory is used; it is assumed that one type of carriers are majority carriers at the interface and that their concentration can be known a priori. It is shown that these assumptions do not seriously limit the applicability of the theory. The results are discussed in the case when one semiconductor is much more heavily doped than the other, which is a condition that certainly improves the efficiency of the solar cell.

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.

Analytical Treatment of Grey Levels in Antiferroelectric Liquid Crystal Displays

Abstract Under certain simplifying assumptions, which are believed to be non-essential, it is possible to treat grey levels in antiferroelectric liquid crystal displays analytically. This shows more clearly than simulations which parameters play an essential role.

CALCULATION OF CHEVRON PROFILES IN FERROELECTRIC LIQUID-CRYSTAL CELLS.

Abstract The energetic aspects of layer deformation in ferroelectric liquid crystal cells are discussed and the actual chevron shape is calculated in some situations. We emphasize two, in our view, essential energy contributions. One term considers the layer curvature. The other one refers to the variations in the distance between layers and the consequent changes of the smectic cone angle. In some simple cases we can determine optimal shapes of the chevron layer structure by analytical solutions, based on these two energy terms. In more complicated situations other contributions have to be considered and the chevron profiles are simulated numerically. The influence of the applied voltage and the choice of parameter values are studied.

GRATING DIFFRACTION IN (ANTI-)FERROELECTRIC LIQUID CRYSTAL DISPLAYS

Abstract For many inhomogeneous structures in LCDs, such as antiferro- or ferrielectric LCDs and striped textures in classical or short-pitch ferroelectric LCDs, we present a strong, rigorous algorithm for the calculation of the optical transmission. We illustrate the technique in case of some simple alternating structures. The number of diffraction orders strongly depends on the grating period (Λ) - light wavelength (λ) ratio Λ/λ. For the alternating antiferroelectric state the transmission of visible light can be treated with only the zeroth order of diffraction. In this case, we find that the AFLC can be simulated as a birefringent layer with the principal axes parallel and perpendicular to the layers. In case Λ/λ > 1 higher orders of diffraction come into play. The dimension of the corresponding eigenvalue problem becomes too large for analytical treatment. We give some numerical results.

Importance of Ion Transport in Industrial LCD Applications

Abstract Ion transport in liquid crystals can have a negative influence on liquid crystal display performance. We introduce the importance of ion transport in industry and and the performed research. Afterwards the contribution of University of Gent will be presented in three topics: determination of concentration and mobility of ion species; influence on image sticking; role in asymmetric (reflective) cells.

On the equation of director motion in ferroelectric liquid crystals

Abstract In a one-dimensional situation of a liquid crystal sample between two electrodes perpendicular to thex-axis, it is thex-component of the electrical inductionDx and not the electrical fieldEx which is independent of thex-coordinate. In the present paper the equation of director motion is derived in a more precise way, and some applications for ferroelectric liquid crystals are discussed. WithFd the distortion energy andFs the surface interaction energy it turns out that the expression for the electric Gibbs function can be used in combination with the standard Euler-Lagrange equations provided not only the orientation of the director but also the potential are considered to be independently variable unknown functions. Variation of the orientation leads to its equation of motion and boundary conditions. Variation of the potential under constant electrode voltage leads to Maxwells equation.