Charge trapping properties of uv-exposed polyimide films for the alignment of liquid crystals

Abstract

A model has been developed to account for the occurrence of flickers and image-stickings in TN or two-domain TN cells aligned by polyimide (PI) films with one of the films subjected to uv radiations prior to rubbings. The probability of flickers and image-stickings is proportional to the imbalance in the charge densities trapped on the two interfaces between the LC medium and two alignment PI films. Rate equations are established to calculate this charge imbalance for the cases of low-frequency ac voltages applied on the TN cell. The modeled results are in good agreement with the experimental results using various PI material as alignment layers for the TN cells. The mechanism of uv-induced charge-trapping centers on the PI films is discussed.