SWITCHING ON STRAY ELECTRIC-FIELDS IN FERROELECTRIC LIQUID-CRYSTAL CELLS

In liquid crystal dot-matrix displays light may leak through the displ ay area between the pixels. To obtain sufficient contrast this non-pix el area has to be made non-transmissive. For ferroelectric liquid crys tal (FLC) displays this may be done by switching the material in the g aps between the picture elements to a non-transmissive state by the st ray electric fields that occur during application of voltages to the p ixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region conside red is an asymmetric environment of the FLC material, as the transpare nt conductive coating has been removed on one substrate, whereas on th e other substrate a conductor covers the glass. The FLC molecules in t he non-pixel area prefer to direct their dipoles towards the covered s ubstrate. To switch the FLC material with the stray electric fields, i t is a prerequisite to outweigh this preference. We made spatially res olved observations for various gap widths and various applied voltages on 2 mum thick FLC layers. With bipolar voltage pulses of 64 mus widt h each, amplitudes of about 25 V are needed to switch the FLC in 3.2 o r 4.0 mum wide gaps. It was found to be more difficult to switch gaps that are 7 mum wide than was anticipated on the basis of the results f or 4 mum gaps. This is attributed to the surface polarization charge d ue to the FLC permanent dipoles built up at the FLC-glass interface. E xperimental results supporting this explanation are presented.