Evaporated electret films with superior charge stability based on Teflon AF 2400

Abstract

Recently, we reported on the preparation of Teflon AF 1600 based electret films with extremely high charge stability by thermal evaporation and corona discharge [Hirschberg et al. Organic Electronics 57 (2018) 146]. Here we present investigations on thermally evaporated electret films made of Teflon AF 2400 with even improved charge stability. Deposition of smooth films without micropores at rates exceeding 10 nm/s was achieved with a specially designed evaporator. The evaporated thin films exhibit an extraordinary dielectric breakdown strength, which is almost one order of magnitude above the bulk value for films thicknesses in the μm range. The resulting maximum charging potential is as high as 267 V for a 5.4 μm electret film. Films of 15 μm could be charged up to 600 V. The charge stability was investigated as function of charging potential, film thickness, and the polarity of the charges. Negative charging proved to provide a much higher charge stability. The charge stability decreases with increasing charging voltage, which is shown to be due to filling of more shallow traps upon at higher charging voltages. Thicker films exhibit higher charge stability. This observation is in line with the proposed internal charge decay mechanism and can be explained in terms of longer charge migration paths. The reproducibility of the fabrication process of the thin film was tested involving about 200 samples. An excellent standard deviation of 2.9% was found for a charging voltage of 250 V, which makes the preset fabrication method very attractive for industrial applications.