Multiscale Investigation of the Poly(N-vinylcarbazole) Photoageing Mechanism
Résumé
During the past decade, the development of polymeric solar cells has received a great deal of attention from both academic and industrial laboratories. In order to enhance the device performances both in terms of power conversion efficiency stability in use conditions, Polycarbazole copolymers have attracted increasing attention. In this paper, the photodegradation of poly(N-vinylcarbazole) (PVK) was investigated from the molecular scale to the nanomechanical properties. It was shown irradiation provoked chain scissions, homolysis of the C-N bond and formation of new covalent bonds between the macromolecular chains. To fully understand the mechanism of the degradation of PVK provoked by exposure to UV radiation, mechanical analyses were performed. The consequences of the cross-linking reactions on the surface modifications were analyzed. Roughness and stiffness measurements were obtained through surface analysis and nanoindentation by atomic force microscopy (AFM), and depth-profiling experiments were also performed. The surface modifications and the shape of the profiles of the degradation photoproducts were explained in light of the chemical modifications of the PVK structure. Quantitative correlations were successfully obtained between the main relevant criteria of degradation, from the chemical structure to the mechanical properties. It was found that cross-linking reactions were prevalent.