Erasing strategies for asymmetric antiferroelectric liquid crystal driving schemes

Antiferroelectric liquid crystals may be used for preparing high definition passively multiplexed video rate displays. Nevertheless, driving schemes employed in these applications customarily require the use of high voltage (similar to 40 V) addressing signals that produce side effects such as ion currents. The generation and distribution of ions affect liquid crystal behavior under external applied voltage. Previous studies found dependence between surface density of adsorbed positive ions and sample thickness. Regarding electrooptical response, ion accumulation interferes with molecular reorganization upon driving, and their slow diffusion jeopardizes gray level stability. As a consequence, several display inconveniences, like permanent image trailing contribute to decrease image quality. This problem becomes particularly serious in the case of antiferroelectric cells with asymmetric alignment, where hysteresis shift allows the use of biasless driving schemes. The manufacturing protocol employed in our laboratory for asymmetric cells includes the use of non-stoichiometric silicon monoxide on one of the display glass plates and an aligning buffed polymer on the other plate. Electrooptical response was analyzed under two new ways of driving, all of them with a biasless stabilization period, and based on relaxation strategies. Main changes were introduced in the way of erasing memory. One of them decreases response time whereas the other reduces dynamic range and saturation voltage. Best results are obtained when ion control is reinforced by the use of SiO2 barrier layers between the glass coated electrodes and the SiOx and polymer alignment layers. Finally, a full waveform reversal has been proposed with the aim of decrease dynamic range even more although a 5% of the darkest graylevels is lost.