Photochemical behavior of PVA as an oxygen-barrier polymer for solar cell encapsulation

Polyvinyl alcohol (PVA) is a water-soluble polymer that is anticipated to be a good candidate for incorporation into multilayer coatings of organic solar cells due to its high transparency and ability to form a barrier to oxygen. Because a long lifetime is a prerequisite for successful applications, it was necessary to study the photochemical behavior of PVA under solar light. PVA films were exposed to UV-visible light irradiation (lambda > 300 nm) in accelerated aging conditions representative of natural ageing. Modifications in the chemical structure of aged samples irradiated at ambient air were recorded. Due to the low oxygen permeability of PVA films, it was shown that the photooxidative degradation of PVA films is restricted to the surface (<5 mu m) and results in a large amount of chain scissions, with a progressive erosion of the surface of the irradiated material. The oxidation products formed along the macromolecular chains, and low molecular weight species trapped in the matrix or emitted in the gas phase were also identified. An oxidation mechanism was then proposed to account for these modifications. However, irradiation in the absence of oxygen demonstrated the high photostability of PVA films, which permits the use of PVA as a sublayer in inorganic/organic multilayer encapsulation systems.