INFLUENCE OF POLYIMIDE ORIENTATION LAYER MATERIAL ON THE LIQUID-CRYSTAL RESISTIVITY IN LCDS

Polyimides used as orientation layer material for liquid-crystal displays (LCDs), significantly influence the resistivity of the liquid crystal (LC) layer. The correlation between the resistivity of the polyimide and the influence of the polyimide on the LC resistivity within LCDs was investigated. Via dielectric measurements the resistivity of various polyimides was determined at elevated temperatures. The resistivity was found to be largely dependent on the polymer material used as well as on the curing temperature, and it increased with the degree of imidization. Furthermore, orientation layers of the polyimide with the lowest activation energy for ionic conduction appeared to be most capable to retain the high resistivity of the LC layer within LCD test cells. Studies on the mobility of ions in LC layers indicated that the type of ionic species diffusing from the polymer orientation layer into the LC layer also depends on the polymer used, and on the curing temperature applied. The observations can be explained by assuming that the polyimide layers contain two types of ions.