Effects of absorbed water on space charge and conduction phenomena in polyimide films

Water absorbed in a polyimide (PI) film largely affects the electrical conduction and space charge behavior. The DC conduction currents increased almost exponentially with the water content. When a DC field was applied to PI films kept in the atmosphere, four kinds of space charges were observed, i.e., positive-and negative-homo space charges and negative-and positive-hetero space charges. After drying, the homo space charges were much reduced. In water-treated samples, positive-and negative-homo space charges were rapidly formed. 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.