One of the main technical difficulties in addressing liquid crystal devices is understanding and controlling the effects of ionic impurities, which inevitably enter through industrial processing or molecular breakdown. The detrimental effects of these impurities in nematic AMLCDs include image sticking and reduced voltage holding ratios. Additionally, in ferroelectric LCDs the ionic species also significantly degrade the bistability.

In the bulk of the device the non-equilibrium charge transport equations can be used to model the movement of ionic species by drift-diffusion. However, the ionic concentration changes as a function of position through the cell due to field-induced changes in the dynamic equilibrium between recombination of ions of opposite polarity and ionisation of neutral molecules. In addition the conductivity and mobility of each different ionic species are anisotropic, and the mobility may be field dependent. The coupling between the ionic concentration and the position dependent polarisation (dielectric, flexoelectric, ferroelectric) must be rigorously calculated.

The interaction of the ionic impurities with the boundaries in cells has also been shown to have a significant impact. A surface polarisation can arise from the absorption of impurities, as well as from the interaction of the liquid crystal molecules with the surface [1]. Any absorbed interfacial charge may be trapped on a timescale that is characteristic of the particular ion species and surface [2]. Other possible surface effects can arise from conducting or polarised alignment layers and charge injection from the electrodes.

The new approach that will be taken in the current proposal is that molecular structure simulations from Projects 1, 2, 4 and 6 will be used for the first time to investigate which of the possible analytical forms are the most appropriate and to give an indication of the magnitudes of the parameters that should be used in them. Project 9 will integrate these analytical forms into a rigorous numerical model and validate the model by simultaneous fitting of experimental data from a number of probes applied to each particular system in order to reduce fitting degeneracies.

The model will be used predictively to consider time-dependent ion motion during complicated addressing waveforms applied to novel device configurations such as flexoelectric, ferroelectric and optically addressed spatial light modulator systems (experimental data supplied by Dr Elston and Prof. Crossland).

In order to validate the model, and to avoid the fitting degeneracies that apply to some of the previous approaches in the literature, the data from the different possible experimental probes applied to the same system will be fitted simultaneously. The measurements include: the frequency dependent complex permittivity; CV hysteresis; director profiles from optical guided mode techniques; dc conductivity/transient currents; and standard electro-optical switching curves (supplied by Dr. Brown, Dr. Elston, and Prof. Sambles).

The molecular structure calculations may be able to elucidate many aspects of the interactions between individual liquid crystal and impurity molecules. For instance, certain high polarity (thus high dielectric anisotropy) chemical groups on the LC molecules (e.g. cyano) that interact with other molecules (c.f. antiparallel dipole correlations) and surfaces have been reported to interact strongly with dissolved neutral molecules increasing the ionisation. This effect has been found to be absent with other type of highly polar group - certain classes of fluorinated LCs reduce threshold voltages with lower ionic problems [3]. Experimental work has also shown that the magnitude of the conductivity anisotropy depends on the shapes of both the liquid crystal molecules and the each of the ions formed by the impurity molecules.

[1] "Reverse hysteresis loop of nematic liquid crystals in C-V characteristic due to Satatic Electric field." H. Mada and H. Suzuki, Japanese Journal of Applied Physics, Volume 26, Number 7, pp L1092-L1094 (1987).

[2] "Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell." A. Mazzulla, F. Ciuchi and J.R. Sambles, Physical Review E, Volume 64, pp 21708-21714 (2001).

[3] "Liquid-Crystalline Materials for TFT-Addressed Displays with Improved Image-Sticking Properties." S. Naemura, Y. Nakazonon, H. Icinose, A. Sawada, E. Böhm, M. Bremer and K. Tarumi, SID 97 Digest of technical papers, pp199 -202 (1997)..