Artur Adamski

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.

Simulations of energy and switching in AFLCDs with different boundary conditions

Using standard expressions for the various terms in the Gibbs free energy, the switching in antiferroelectric liquid crystal (AFLC) displays is simulated and the time evolution of various energy terms and of the liquid crystal director distributions are calculated. It is shown that when returning from a strong positive voltage to zero, one can reach two types of antiferroelectric state: the normal alternating state with the two bulk polarizations perpendicular to the electrodes and opposite to each other, and the alternative splayed symmetric state with two bulk polarizations parallel to the electrodes and again opposite to each other. The former case gives rise to tri-state switching characteristics, the latter to V-shaped switching. In general strong polar interaction with the alignment layer favours V-shaped switching while weak or no polar interaction give rise to tri-state switching characteristics. Since the V-shaped characteristic has so far only been demonstrated experimentally in ferroelectric liquid crystals (or antiferroelectric liquid crystals being in the ferroelectric state), the difference in AFLCs is discussed and the conditions for continuous switching are modelled. The simulations show that the switching characteristics of the antiferroelectric display can be controlled by the surface parameters.

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.

Stability Criterion for V-Shaped Characteristics in FLC and AFLC

The V-shaped characteristics in FLC are explained through the existence of a splayed state caused by a strong polar interaction with the alignment layers. In this paper the relation is described between the non-polar interaction coefficient n 1, the polar interaction coefficient n 2, the permanent polarization density P and the thickness of the alignment layer d a that guarantees stability of the V-shaped characteristics. For AFLC there is no agreement that V-shaped characteristics are possible. In this article we show that the condition for stability, expressed in function of a fictitious negative non-polar interaction coefficient, is formally the same as for FLC, but no exact analytical expression for this coefficient can be found. Simulation results however show that stability is possible under conditions that are approximately the same as for FLC.

Surface Electroclinic Effect in Low PS Smectic Materials Without SmA Phase

The molecular axis of a chiral smectic material deviates away from a desired direction induced by the rubbing process. This small angular deviation is caused by the Surface Electroclinic Effect and it is usually studied near the phase transition N-SmA. In the material without SmA phase thus after N-SmC transition the phenomenon looks quite similar except the fact that the deviation of a molecular axis occurs in two opposite directions.

Alignment and switching of AFLC materials using various boundary conditions

We have systematically studied the quality of bookshelf alignment and electro‐optic characteristics of two antiferroelectric liquid crystal materials in cells with various boundary conditions. The electro‐optic characteristics of the materials studied depend strongly on both the liquid crystal materials and the boundary conditions at the supporting substrates. We have compared a number of observations in these cells: the tendency to form AFLC domains in the virgin state and after switching; the surface electroclinic effect (SEC effect); the transmission–voltage characteristics (TV) when driven with triangular‐ and square‐wave voltages at various frequencies; the threshold field and the conditions for relaxation to the AFLC state. The set of samples includes specially designed and manufactured test cells with different polyimides as alignment layers, treated with varying rubbing strengths. We discuss the significance of various factors and show the importance of simultaneously optimizing both materials and cell parameters for AFLC applications.

Uniform Theory of V-Shape Characteristics

The purpose of this article is to develop a simple theory for V-shape in (A)FLC materials. This is the so-called “uniform” theory, which means that the director orientations are assumed to be independent of the depth into the liquid crystal. V-shape requires a strong polar interaction with the alignment layers. However this cannot be incorporated in the uniform theory, but it was shown in the literature that it can be represented, at least qualitatively, by a fictitious negative non-polar interaction coefficient. It is first shown how extremely simple the FLC-V-shape can be explained analytically. For AFLC-V-shape, we need a smectic layer interaction energy described by an antiferroelectric term A and a quadrupolar term Q. It is shown that if A > 2Q the return of the Ferroelectric up-state to the splayed state at the tip of the V is antiferroelectric, and if A < 2Q it is ferroelectric. The analytical results are confirmed by a non-uniform computer simulation. These models do not contain the possibility of in-pixel domain wall motion. However it is shown that under certain conditions the energy of the normal antiferroelectric state at V = 0 is smaller than the splayed ferroelectric state at V = 0, which means that the normal antiferroelectric state will grow at the expense of the splayed ferroelectric state. This is however a slow process and with a triangular addressing at 1 Hz it is experimentally confirmed.

Energy relations in antiferroelectric liquid crystal displays

In antiferroelectric liquid crystal displays either V-shaped switching or tri-state switching characteristics are possible. Both types of states may be locally stable at constant voltage. However if they exist in different region of a pixel separated by a wall, the lowest energy state will grow by wall motion. It is thus important to be able to calculate accurately the energy of stationary states. We present three ways of calculating the Gibbs free energy: by total simulation of non-stationary states, by partial simulation and calculation of the stationary states based on energy relations, and by analytical expressions of stationary V-shape states. This allows to check the accuracy of the simulations.