With increasing LC display performance requirements, problems of ion transport have become very important. The present state-of-the-art liquid crystals have relatively low Δε. Performance of LC displays could be improved significantly, if higher Δε materials were used. However, high Δε materials also exhibit increased ionic contamination. This has to be limited or neutralized in order to avoid the performance reducing effects typically known under the names of RDC and VHR, which actually cover a broad range of various ionic screening effects of the LC display driving potential.

Due to various ion transport mechanisms the dynamics of migration of ions is complicated and the related phenomena range from fast flickering to long-term memory effects resulting in a degradation of the image quality and especially the gray-scale performance.

To verify the model of ionic transport mechanisms, experiments measuring optically-detected ionic screening of the driving potential as well as long-term and short-term conductivity measurements were performed [1-3].

In order to obtain reliable information, excluding the influence of manufacturing processes, a special design and manufacturing concept of test cells has been developed allowing to study particular material parameters and specific effects related to various structural parts of the LC- cells such as LC layer, boundary glass (alkaline, non-alkaline), alignment and other functional boundary layers.

A model of the ion transport mechanisms taking place within a LC display is presented. Various ionic effects with strongly different time constants are considered and two main competing driving potential screening effects are assumed [1, 3], originating in ionic transport in alignment layers and LC layer, respectively. The role of insulation layers separating ions in LC and alignment layers from the electrodes is studied. The asymmetry of the cell and the problem of balancing the ionic screening in LC and alignment layers are also considered. The light transmission through the LCD cell is calculated for various ion concentration regimes and compared with experimental results. Analysis of a model LCD light shutter shows that a proper distribution of ions in LC and alignment layers, obtained by "trapping" ions from the LC layer into the alignment layers, can result in balancing of ionic screening.

* Research partially supported by Samsung Electronics and Slovenian Ministry for Industry (Grant 2111-04-4010-511/2004-4286/BZ).

1. C. Colpaert, B. Maximus and A. de Meyere, Liq. Cryst. 21, 133 (1996)

2. S. Palmer, Liq. Cryst. 24, 587 (1998)

3. H. de Vleeschouwer, F. Bougrioua and H. Pauwels, Mol. Cryst. And Liq. Cryst. 360, 29 (2001)