Partial discharge behaviour of biaxially orientated PET films: the effect of crystalline morphology

The relationship between partial discharge (PD) induced breakdown behaviour and the crystallinemorphology of PET films used in photovoltaic devices has been explored and discussed in this work for the first time. Biaxially orientated PET films with and without BaSO₄ filler were isothermally annealed at various temperatures before PD breakdown tests of the films to investigate the effect of crystalline morphology. Attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC) were used to study the changes of crystallinity and lamellar thickness of the samples. It was found that both PD resistances and PD lifetimes could be significantly improved when the samples were annealed at temperatures above 210 °C. On the other hand, improvements were much less in the annealing temperature region between 180 and 210 °C. This, we propose, is because the thinner and less perfect lamellae formed by annealing at the lower temperatures are less effective at resisting ion bombardment and electrical tree propagation. On the other hand, the formation of thickened and perfected lamellae produced at higher annealing temperatures can effectively increase the tortuosity of electrical tree propagation paths, thereby increasing the PD lifetimes.