Changes of morphology and material properties of thin Ethylene-Vinyl acetate-films under different aging conditions

The influence of photochemical and thermal aging on the morphology and material properties of EVA was investigated by means of Atomic Force Microscopy (AFM) and Raman spectroscopy. The encapsulation, for which EVA is used in most c-Si PV modules, is of great importance for the long-term stability of PV modules. Delamination between the encapsulation and the solar cell or the glass may occur whilst outdoor exposition. In order to understand the processes which take place at the EVA/Silicon and EVA/Air interface during exposure to UV radiation and elevated temperature, thin non-cross-linked EVA films, spin coated onto silicon substrates to a thickness of 80 and 95 nm, were made from commercial EVA foils and from EVA pellets without stabilizing agents. The samples were aged at different temperatures (23°C and 85°C) with and without UV radiation for 200 h. The topography and the material properties of the surface were measured every 40 hours by Tapping Mode AFM and Raman Spectroscopy. Thus, the temporal evolution of morphology changes could be examined. Surface crystallization was observed, especially for the irradiated samples. EVA films which were exposed to UV light at room temperature did not show any morphological changes, indicating the necessity of a certain temperature to trigger crystallization processes. The samples with non-stabilized EVA degraded faster than the samples with additives. The Raman Spectra of the non-stabilized EVA sample gave evidence for changes in the chemical structure of the material. This finding could explain the different surface structures observed in the AFM images.