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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. Image 1 • We report on the investigation of evaporated Teflon AF 2400 as electret material. • Factors which influence the electret performance were investigated in detail. • A large number of electret samples were tested and show great reproducibility. [ABSTRACT FROM AUTHOR]

We report on the preparation of Teflon AF based electret films by thermal evaporation and corona discharge. Due to a special construction of the evaporator, smooth films without micropores could be deposited even at rates above 10 nm/s. The influence of film thickness, substrate temperature, charge density and type of charge carrier on the charge stability was investigated together with the mechanism of charge decay. Negatively charged films of 9.5 μm evaporated Teflon AF 1600 turned out to exhibit a much higher charge stability compared to commercial 25 μm Teflon FEP electret foils at almost 1/3 thickness and proved to be also superior to spin-coated Teflon AF 1600 films. This is discussed in terms of absence of discharge via the surface. Charge decay was shown to take place by internal drift and diffusion through the bulk of the film and not along the surface or via external discharge mechanisms as reported for Teflon FEP. [ABSTRACT FROM AUTHOR]