Ion transport in LCD's

The fabricated liquid crystals these days are very pure and contain almost no ions. But in spite of that display applications using liquid crystals suffer from troubles due to ion motion in the liquid crystals. These ions are inserted during the production process. For instance contamination of the substrates or the polymer alignment layer can introduce ions into the liquid crystal bulk. Typical ion concentrations in liquid crystals are in the order of 1018 per cubic meter. Ion concentrations above 1020 start giving problems in display applications.

When a voltage is applied to a liquid crystal cell, the ions present in the liquid crystal tend to separate and cause the electrical field to vary in time and place inside the liquid crystal. Positive ions move in the direction of the electrical field and negative in the opposite direction. We can make the distinction between two types of ion transport : fast ion (time dependent) transport and slow effects (almost time independent).

Short term ion transport

The fast ions have a measurable influence during the experiments they are visible in the current and influence the optical transmission in real time. Measurements of this type of ion movement are called 'short term ion measurements'. The movement of the ions and their influence on the internal electrical field lead to effects as flicker in the optical transmission at relatively low frequency (10-50 Hz) and a mistake in the obtained grey level.

From the measurement of the current supplied to the LCD we can extract the concentration and mobility of the ions. Our group has developed a current amplifier which can measure currents accurately down to 1 nA and a simulation program to simulate the related phenomena.

Long term ion transport

Slow effects however can originate from long term DC's (applied during several hours) or an inherent asymmetry a difference in work function between the electrodes of a reflective display for example.

Slow effects occur due to the generation and recombination of ions. It was found that the ion concentration increases in time. The ions gathered at the borders of the electrodes effectively decrease the electrical field depending on the alignment layer which is in between the liquid crystal and the electrodes. This leads to a compensating voltage which remains present if the external DC voltage (the cause of the ion generation) is removed. The compensating voltage can then lead to a remaining ghost image hence the name image sticking.


Ghost image remains after removing image

A theoretical model for generation and recombination under the influence of the electrical field is used to describe the measured IV-curves of long term ion transport. At high voltages, this model leads to a saturated measurable current density proportional with the generation. Detailed measurements allowed to extend this model.

At high voltages, the electrical field is so strong, that the generation increases. The Onsager-model describes this field dependent generation and a fit was found between the predicted slope (by the model) and the measured slope (of the IV curve).

At longer times, dispersion was noticed in the generation constant. The dispersion-model describes that the generation decreases in time following a power law (power e). The origin is the broad distribution of time constants in the generation constant b. Physically, this can be due to a broad distribution in energy levels (lots of different dissociable molecules) or due to a chaotic structures (no 'average' environment can be defined for the generation center, and the dissociation time strongly depends on this environment). Measurements show that the second model (one generation center) is the most likely to be present. Because only one generation center (with one dissociation energy) is present, an Arrhenius-relationship could be found in the measurements of the generation constant.

Notice that effects of field-dependency, time dependency and temperature dependency can be separated in a , well-satisfied, first order approximation. This is also a result of the so-called 'time-scale invariant' behavior of dispersion : characteristic phenomena like the IV-curves and the Onsager-phenomena are visible at any time.

Finally a relationship is found between the generated ions and the introduced compensation voltage Vc, taking into account the effects of the capacity of the alignment layer and the diffusion layer (slightly voltage dependent). As Vc originates from the integration of the current (proportional to b), a power law was also found in Vc. Vc increases until the applied DC-voltage is fully compensated. When the DC-voltage is removed, Vc will remain present and cause the
annoying phenomena.

Ion simulation software

A final aspect of our research is the development of simulation software to calculate the behavior and influence of ion transport in liquid crystals. Software has been build for 1D, 2D and even 3D liquid crystal environments. This software can easily be adapted to calculate the movement of any charged particle in a liquid environment due to an electrical field.

One-dimensional ion simulation software

Our one-dimensional ion transport software is integrated in a Windows based program we called "Glue". The program is capable of calculating the director reorientation with the influence of moving ions and the optical transmission of a one-dimensional LCD. Several ion transport algorithms are possible based on exponentional diffentials, finite elements and a Monte Carlo based algorithm.

Two- and three-dimensional ion simulation software

The two- and three-dimensional software for ion we developed calculates the ion movement in a 2D or 3D anisotropic medium based on a Monte Carlo algorithm. The ions can either move freely in the medium or can only be positioned at the grid points of the irregular mesh.

More detailed information about the results we have obtained in our research can be found in our publications.

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