Silicon photovoltaics: experimental testing and modelling of fracture across scales

The study of the properties of materials can be addressed through a multi-scale approach, in order to have the possibility to grasp at each of the levels of analysis the peculiar aspects. Tracing a path inside the state-of-the-art in the available bibliography, historically in the field of mechanics s are found in which the material is studied through nonlocal theories based on continuous or discrete local approaches. More recently, with the advent of great computatio- nal power computers, analytical methodologies based on theories also very complex deriving from the field of chemistry and physics have been developed, capable to discretize at the ato- mic scale the material and study its behavior by applying energy approaches. Starting from the analysis of some of these theories at the nano- and micro-scales, it is possible to investi- gate the separation mechanisms at the molecular level, which may be considered as cracking processes within the material according to the adopted scale of analysis. The application of theories of this kind to large portions of material, in which there are up to some millions of particles involved is reasonably not an applicable solution, since it would require a huge effort in terms of computation time. To work around this problem and find a method suitable for the study of cracking mechanisms, a mixed method (MDFEM) was byconjugating pure molecu- lar dynamics (MD) and the finite element method (FEM), in which the material is discretized by means of one-dimensional elements whose mechanical characteristics are derived from MD. This approach is based on the application of a nonlocal theory in which the contribution of a portion of material placed within a certain distance from the point of fracture is taken into account by means of a parameter of non-locality. Moreover, the study of the evolution of cracking is addressed at the meso-scale by the application of a cohesive non-linear model. Towards the analysis of the macroscale, the theories put forward so far have been ap- plied to the study of phenomena of breakage inside Silicon cells embedded into rigid or semi-flexible photovoltaic modules. By performing various laboratory tests, useful for the characterization of the material and for understating the evolution of cracking process due to multiple causes, a study on the main issues that may compromise the durability and mainte- nance of the expected service levels of photovoltaic panels has been conducted. Experimen- tally results have been interpreted by using appropriate macro-scopic continuum models. The research carried out had the purpose to provide an introduction to a correct design of these systems of energy production in order to increase their durability and resistance to cracking.