Deng, Huifang; (2000) Computer modelling of liquid crystal displays. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes research work in modelling 2-dimensional twisted nematic liquid crystal display devices. There are three salient aspects of this work; The first and more extensive is the development, implementation and testing of a new formulation for the dynamic modelling of these devices using a constant-charge assumption, which more accurately represents the case of thin-film-transistor (TFT)-driven devices. Based on a dynamic tensor formulation (to preserve the nematic symmetry) and a single elastic constant approximation, the mode is developed to simulate the TFT driven device switching by allowing the voltages to be connected to the pixel electrodes for only a line time duration (typically 15μs ~ 30μs) and then letting them float for the rest of the half frame time while the charge remains constant. The director and electric potential distributions are dynamically calculated using a combination of finite differences and finite element methods under given electrode configurations and boundary conditions. The capacitance matrix is determined using a perturbation technique, and this further allows the voltages on the pixel electrodes to be found during the drifting time. This method can predict the dynamic behaviour of such a device more accurately than earlier models using a direct (fixed voltage) driving scheme and can include the effect of external storage capacitors. The programs developed in this project include as a special case the direct driving situation, produce as results the dynamic responses of various physics properties such as pixel voltages, capacitances of all electrodes, tilt angles and transmittance of the cell etc., and can also reproduce flicker behaviour due to the DC bias. There is good agreement between predicted results and experimental observations. The second part of the work involves the theoretical derivation of a dynamic tensorial formulation for the liquid crystal behaviour, using three elastic constants. This derivation will be implemented outside this work and the resulting programs will give an even better modelling capability. In the third part, in order to have a better interpretation of viewing angle characteristic of the studied cells, the extended 2x2 Jones matrix approach has been further generalised to include arbitrary incident (polar and azimuthal) angles.

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