Alwin R. M. Verschueren

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.

Screening and separation of charges in microscale devices: complete planar solution of the Poisson-Boltzmann equation

The Poisson-Boltzmann (PB) equation is widely used to calculate the interaction between electric potential and the distribution of charged species. In the case of a symmetrical electrolyte in planar geometry, the Gouy-Chapman (GC) solution is generally presented as the analytical solution of the PB equation. However, we demonstrate here that this GC solution assumes the presence of a bulk region with zero electric field, which is not justified in microdevices. In order to extend the range of validity, we obtain here the complete numerical solution of the planar PB equation, supported with analytical approximations. For low applied voltages, it agrees with the GC solution. Here, the electric double layers fully absorb the applied voltage such that a region appears where the electric field is screened. For higher voltages (of order 1 V in microdevices), the solution of the PB equation shows a dramatically different behavior, in that the double layers can no longer absorb the complete applied voltage. Instead, a finite field remains throughout the device that leads to complete separation of the charged species. In this higher voltage regime, the double layer characteristics are no longer described by the usual Debye parameter kappa, and the ion concentration at the electrodes is intrinsically bound (even without assuming steric interactions). In addition, we have performed measurements of the electrode polarization current on a nonaqueous model electrolyte inside a microdevice. The experimental results are fully consistent with our calculations, for the complete concentration and voltage range of interest.

Novel concept for full‐color electronic paper

— Despite a steep increase in commercial devices comprising paper-like displays, a much desired feature is still missing: bright full-color electronic paper. A new reflective-display technology has been developed to solve this issue. For the first time, the principles behind this in-plane electrophoretic technology will be presented, which enables the realization of full-color reflective displays with a higher brightness than presently available e-paper technologies, without compromising paper-like properties such as viewing angle and ultra-low power consumption. An additional major advantage (e.g., for future low-cost manufacturing) is that, besides direct-drive and active-matrix configurations, a passive-matrix option with analog gray levels has been successfully developed.

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.

Charge transport and current in non-polar liquids

Surfactant molecules in a non-polar liquid form charged and uncharged inverse micelles. When a potential difference is applied over the mixture, the charged inverse micelles drift towards the electrode with the opposite polarity. The motion of charges is associated with a transient current, which can be measured in an external circuit. In this paper, transient currents and steady state charge densities are described analytically in different ranges of parameter values (applied voltage, charge density, device thickness, mobility,...). The generation of additional charged inverse micelles and the electrophoretic motion of colloidal particles in the mixture is modelled and measured experimentally.

Micellization and adsorption of surfactant in a nonpolar liquid in micrometer scale geometries

Mixtures of nonpolar liquid and surfactant are used increasingly in applications with microscopic dimensions. However, most methods to characterize them are performed on bulk solutions. We measure electrical transient currents in thin layers of nonpolar liquid with surfactant and derive several properties from these measurements. This paper reports the results for different liquid layer thicknesses and surfactant concentrations. We observe a dependence on the layer thickness of the inverse micelle concentration, which cannot be explained by bulk micellization alone. A model including surface adsorption is proposed that describes surfactant behavior in microscale geometries.

46.1: Invited Paper: Novel Design for Full‐Color Electronic Paper

Despite a steep increase in commercial devices comprising paper-like displays, a much desired feature is still missing: bright full-color electronic paper. We have developed a new reflective display technology to solve this issue. for the first time we will report about the principles behind our in-plane electrophoretic technology, which enables the realization of full-color reflective displays with a higher brightness than all present e-paper technologies, without compromising paper-like properties like viewing angle and ultra-low power consumption. An additional major advantage (e.g. for future low-cost manufacturing) is that, besides direct-drive and active-matrix configurations, a passive-matrix option has been developed successfully.

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.

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.