Polarization relaxation and charge injection in thin films of poly(vinylidene fluoride/trifluoroethylene) copolymer

Abstract

Polarization relaxation effects and charge injection in poly(vinylidene fluoride/trifluoroethylene) thin films are investigated in a thickness range between 60 nm and 400 nm. In time domain measurements of polarization reversal, the switching transients exhibit two stages: a rapid increase of polarization followed by a continuous slow increase lasting for several decades. This continuous increase of the polarization corresponds to a t−α (Kohlrausch law) behaviour of the current density in the long time range. For a constant electric field, these currents in the long time range are independent of the sample thickness as expected for a relaxational volume polarization effect. Furthermore, the isochronal polarization and depolarization currents saturate at high fields. We described the experiments using an asymmetric double well potential model with a distribution of relaxation times in which dipoles are fluctuating thermally activated between the two minima. As temperature increases, charge injection sets in. The charging currents then deviate from the t−α behaviour and become constant in time. The discharging currents are found to be a superposition of dipole relaxational currents and space charge relaxational currents. In the short time range, only the dipole relaxational currents are found. After several seconds, a space charge relaxation current with a pronounced minimum in time sets in, whose time constant shifts to shorter values with increasing temperature.