Bart Maximus

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.

Adequate measuring techniques for ions in liquid crystal layers

Abstract The ion contamination in twisted nematic liquid crystals should be limited to assure good electro-optical performance of AM-LCDs. Today, pure liquid crystal mixtures are produced, though little information is available on long term stability, and on the kind of ions that contaminate the liquid crystal. The first step in the determination of the ion source is the development of an appropriate measuring technique that characterizes the ions in the liquid crystal. In this article such a new method is proposed. A comparison with former techniques is made.

Transient leakage current in nematic LCDs

Abstract In this paper we present the transient leakage current measurement method for nematic liquid crystal displays as a technique to obtain information on the mobility and concentration of ionic impurities. The mobility depends on the temperature according to an Arrhenius-type relation, with an activation energy of about 0·3 eV. This dependence can be explained through the variation of the LC viscosity with temperature. The transported charge has also an Arrhenius relation, with an activation energy around 0·2 eV. Additionally, the influence of both a heat treatment and UV illumination on these parameters was investigated.

Influence of ionic contaminations on SSFLCD addressing

Abstract In this paper a theoretical SSFLCD switching model is presented which focuses on the calculation of the internal electric field. Using this model we can investigate the joint dynamics of the FLC reorientation and charge carrier transport inside the LC-layer with or without applied voltage. We give some results on charge separation and addressing problems.

A Simple Numerical Transmission Model for General LCD-Configurations

Abstract The transmission of a LCD for normal incident light can be simply calculated by a clever use of the Jones calculus. Several cases are treated in different ways. The results for a few examples are compared with the more general simulations according to the 4 × 4 Berreman method, which also takes internal reflections into account. Our fast optical calculations are useful when predicted molecular distributions should be confronted with experimental optical spectra.

Model for ion transport in AM-LCD's

Leakage current measurements on LC (liquid crystal) cells are useful for examining the ionic presence and transport in the liquid crystal layer. A model is presented that can explain this leakage current behavior. In this model the ions are formed by the ionization of a number of neutral impurities. The ionization gives rise to positive and negative ions of which one type is considered much more mobile than the other. The ions can be trapped at the interfaces with the alignment layers. These assumptions allow a fit of the simulations and experiments. In this way the measurements can also be a powerful tool to understand and evaluate the influence of the different materials used for the production of displays, especially the alignment films and the liquid crystals themselves, or to understand the influence of high temperatures or ultraviolet illumination.<<ETX>>

Design method for a fast switching color filter based on ECB cells

Abstract Electrically Controlled Birefringence (ECB) cells are often used to fabricate switchable color filters. The color filter we consider consists of 3 polarizers and 2 birefringent modules. Each module consists of a combination of ECB cells and retardation foils. When a maximum response time is taken into account, one can determine a minimum voltage drop that has to be across the ECB cells. This results in a smaller interval in which the retardation of the cell can vary. Since different colors are achieved by varying the retardations of the two modules, it will be more difficult to obtain a saturated red, green and blue color. In this paper we describe a graphical method to design a fast switching color filter with saturated colors. The method can be used for different kinds of liquid crystal cells.

Geometrical averaging of AFLC dielectric tensors

The optical transmission of antiferroelectric liquid crystal layers is non-trivial. For optical wavelengths we average the dielectric tensors. The aim is to provide a geometrical way for this averaging. By a thorough mathematical analysis, we are able to construct the representation ellipsoid for the final e-tensor in most cases. In this way, we can easily predict the axes and indices of refraction of the structure. Especially the molecular states that occur during the hysteresis switching behaviour are studied.

Dynamical response in an AM addressed TN-LCD with anisotropic resistivity

The dynamical response in an active matrix addressed TN-LCD (twisted nematic liquid crystal display) with anisotropic conductivity is simulated. Formulae for the variation of the electrical induction D and the equations of motion are derived. The voltage across the electrodes, the midplane tilt and the optical transmission can be calculated. The model is verified by experimental measurements.

Properties of charge carriers in ZnS and their effect on the excitation of Mn

The subjects of this investigation are the transport of charge carriers and the spatial efficiency in thin-film electroluminescent devices based on ZnS:Mn. Measurements with optional UV illumination were performed on a set of probe layer samples. The transfer of electrons and holes was studied and by comparing transient measurements with the steady-state situation the creation of positive space charge during the first pulses is revealed and its location is estimated.