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A two-dimensional dynamic model developed for the TFT driven display devices has been demonstrated based on the constant charge assumption that means the voltages are applied to the pixel electrodes for only a line time duration tc, typically between 15|is and 30jlls. After that, the pixel voltages are left floating for the rest of half-frame time while the charges that are deposited on the pixel electrodes during the connection time tc remain constant. A crucial aspect is how to predict the floating voltages of the pixel electrodes for the next time step. To this end, the charges on the pixel electrodes are calculated using a formulation specifically developed for this case. The capacitance matrix (self- and mutual capacitances of all electrodes) for the whole LC cell is obtained by using a perturbation technique which is based on the linear relationship between the voltage and the charge for a given dielectric configuration, i.e., temporally and spatially fixed director distribution, and taking physical sizes of electrodes and their edge effects into account. Once the floating pixel voltages are found, the corresponding spatial patterns of the director and potential are calculated through the iterations by finite differences and finite elements methods. Based on the detailed information about the temporal and spatial distributions of the director and potential, the dynamic behaviour of the systems: floating pixel voltages, mid-plane tilt angles, charges, self- and mutual capacitances, transmittance, and flicker are analysed systematically. The modelled results obtained exhibit notable agreement with the experimentally observed dynamic behaviour, and they are more accurate and realistic than those of the constant voltage model and any ID charge driving related models [25-27].

External capacitors used in TFT LCDs can be easily incorporated into the model and their effect is fully examined. The ripple phenomenon of the floating pixel voltage due to the coupling with the rapid change in the polarity of bus voltages can be suppressed by increasing the values of the storage capacitor. The same can be done for the flicker. A larger value of the storage capacitance gives rise to a more stable behaviour of the system. A very large value is shown to be equivalent to the constant voltage operation.

Several different electrode configurations are modelled here to show the capability of the model.

Comparisons have been made between the numerical calculations and the measurements for the average transmittance at normal incidence and good agreement has been found.

The effect of the temporal and spatial discretisation steps on the numerical accuracy has also been examined in detail. It has been found that in cases studied, the results are very sensitive to the spatial step Az, but not sensitive to the spatial discretization size Ax, except the charges and the capacitances as they are very sensitive to the number of nodes used to represent the electrodes. A combination of Ax=1.5 and Az=0.25, used in all calculations throughout the thesis, gives the best accuracy, while a value of Az=0.5 gives reasonable results. Though a time step of Ar=5|Lis used throughout the research gives the best accuracy, a step of 10|is is seen to offer good results as well. A step of 15jis is shown too big.

It has been found that bus lines of non-zero voltages do have influence upon the dynamic behaviours of the systems. Two obvious examples are the ripple effects observed from the floating pixel voltage temporal pattern in the case of the constant charge model or from the charge curve (Figure 26) in the case of the constant voltage model, as well as the flicker generation in the case of symmetric source voltage. In general, it speeds up the director switching process in the constant charge mode.

A three-elastic constant tensorial dynamic formulation developed here, which is supposed to offer an even better modelling capability, is now being implemented outside this project.

A new extended 2x2 Jones matrix representation is derived to enable us to directly analyse the optical properties of LCD cell at any viewing angle with ease and to make a comparison between theoretical prediction and experimental data. A good agreement has been found for a TN cell in IPS mode analysed as an example.

Through this research, it can been seen that computer modelling not only enables us to gain a deep insight into the switching mechanism of LC devices, but also provide us with the following obvious advantages:

2) help identify the optimal material parameters and geometry to guide device design and achieve the best performance by suppressing defects, improving viewing angle characteristics, and increasing the values of the storage capacitors etc.;

3) allow a quick test of any new idea.

Finally, it is worthwhile to point out that the achievements in this project have led to the success in attracting further funds from the EU to support the related project:

Predictive 3D micro-model simulation fo r Monitor LCDs, under the Framework 5

programme, in a consortium which apart from our group includes Philips (Eindhoven and PRL, UK), autronics-MELCHERS Gmbh (Germany), the University of Ghent, and the Politecnico di Torino.