Effect of Absorbed Water on Space Charge Properties of Polyimide

Abstract

Space charge distributions in polyimide (PI) films strongly depend upon electric field, temperature, water content and so on. When a dc field was applied to as-received PI films kept in the atmosphere, whose water contents were ca. 0.9 wt.%, four kinds of space charges were observed, i.e., positive- and negative-homo space charges in the vicinity of the electrodes and negative- and positive-hetero space charges inside the bulk. In water-treated PI films, whose water contents were ca. 2.9 wt.%, positive- and negative-homo space charges were rapidly formed. In dried PI films, the homo space charges were much reduced and hetero space charges were clearly observed. The homo space charge amounts were the largest in as-received PI films among these samples. On the other hand, the dc conduction currents increased almost exponentially with the water content. This suggests that the absorbed water enhanced not only charge injection from the electrode but also mobility of charge carriers. The space charge accumulation and field distortion of PI films are determined by their balance.