Cell gap-dependent transmission characteristics of a fringe-electric field-driven homogeneously aligned liquid crystal cell, for a liquid crystal with negative dielectric anisotropy

Transmittance characteristics were studied as a function of cell gap for a homogeneously aligned liquid crystal (LC) cell driven by a fringe-electric field-named fringe-field switching (FFS) mode. The light efficiency of a conventional LC cell using in-plane switching and twisted nematic modes, where the LC director is determined by competition between elastic energy and electrical energy, does not depend on cell gap as long as the cell retardation value remains the same; i.e. only dielectric torque contributes to the deformation of the LC director. However, the transmittance of the FFS mode is dependent on the cell gap such that it decreases as the cell gap decreases, although the cell retardation value remains the same. This unusual behaviour (unlike that of conventional LC cells) arises because in the device the elastic and dielectric torques have the role of determining the LC director, such that the driving voltage giving rise to maximum transmittance becomes strongly dependent on the electrode position when the cell gap is as small as 2 μm. In addition, the LCs at the centre of the pixel and common electrodes are not sufficiently twisted because of a competition between the two elastic forces, which tries to twist the LCs in plane and hold them in their initial state by surface anchoring.