Simulation-based Design of In-Plane Switching Liquid Crystalline Display Pixels

Abstract

Liquid crystal displays (LCDs) constitute an important class of modern display tech- nologies. Their light-weight nature, coupled with their favourable power consumption char- acteristics make them useful in applications ranging from large area projection displays to small electronic devices such as digital watches and calculators. Despite being the market leader in the display industry, traditional configurations of LCDs suffer from serious drawbacks such as having a very narrow viewing cone. Newer configurations of LCDs, however, employ the in-plane switching (IPS) mode and its deriva- tives. These provide a much wider viewing cone with lower degradation of image quality as one moves off the central axis. IPS pixels have a unique configuration as they contain the electrodes on only one side of the domain. The electrodes are arranged in an interdigitated pattern and produce an electric field that varies periodically in space parallel to the substrates and decays exponentially in space along the through-plane direction. The highly non-homogeneous nature of the electric field makes the simulation of the electric field within an IPS domain more challenging as a minimum of two dimensions is needed to model the electric field with sufficient accuracy, in contrast to the electric field in the twisted nematic (TN) mode that may be modelled in only one dimension. Traditional approaches have employed an iterative technique wherein the Gauss law equations are solved for a pre-determined director configuration and the electric field thus obtained is employed to calculate the new director configuration over the domain. The iterations are continued till convergence is attained. Our method involves calculating the electric field by means of a semi-analytical expres- sion for an electric field produced by interdigitated electrodes and using this expression to calculate the domain configuration. This methodology is advantageous in terms of computational time and effort as it gives a possible way to do away with the back and forth iterations involving the dynamic equations and the Gauss’ law equations. In this work, we attempt to look at dynamic characteristics of the liquid crystalline domain in an IPS-LCD. Metrics were evolved to quantify the deformation in the domain. Finally, these metrics were used to examine the dependence of the equilibrium orientation on the domain thickness, electrode width, electrode spacing and electric voltage applied. The results show good match with the trends that can be expected from theoretical considerations. The variation of the domain deformation characteristics with the change in the geometric and physical parameters is along expected lines. For instance, increasing the voltage results in the domain getting deformed to a much greater extent and the defor- mation to penetrate deep within the domain. A greater pixel depth with the same values of the other parameters results in more of the domain staying undeformed as the electric field only penetrates upto a fixed distance into the domain. Increase in the electrode spac- ing was not found to make a significant contribution to the deformation while increasing the width of the electrodes increases the area affected by the electric field and thus, this increases the overall deformation. To conclude, the framework provided here is a valid first step in evolving a complete software package to model deformation characteristics of an LCD pixel. The code is flexible enough to accommodate different LCD configurations and thus, may be used to model a variety of other LCD configurations also. A parallel development of an optics code using a matrix based method may be used to model the propagation of light through the domain and this may be added very easily on top of the existing framework to create a complete package for analysing the electro-optical properties of the LCD.