Depolarization field and ionic effects on the bistability of surface-stabilized ferroelectric liquid-crystal devices

Based on a collective switching model coupled to a simplified continuity equation for impurity ions, the internal electric field and light transmission of a surface-stabilized (SS), layer-tilted ferroelectric liquid-crystal (FLC) cell with insulating alignment layers have been calculated as a function of time, under bipolar voltage pulses. The polarization and ionic charges, accumulated on the interfaces between the FLC medium and alignment layers, tend to partially screen the external on-field and generate a depolarization field to reverse the spontaneous polarization when the external field is off. When the thickness of the alignment layer is greater than 1000 Å, it is difficult to achieve bistability using FLC mixtures with a spontaneous polarization larger than approximately 20 nC/cm2. For bistability to occur in a SSFLC cell with high spontaneous polarization, thin insulating or conducting alignment layers are preferred. We also found that the existence of impurity ions in a SSFLC cell is always detrimental to bistability in spite of their low mobilities (≅10-7 cm2/V s).