Nanoindentation Reveals Crosslinking Behavior of Solar Encapsulants - The Methodological Advantages over Bulk Methods

The power degradation and failure of photovoltaic (PV) modules can be caused by changes in the mechanical properties of the polymeric components during the module lifetime. This paper introduces instrumented nanoindentation as a method to investigate the mechanical properties of module materials such as polymeric encapsulants. To this end, nanoindentation tests were carried out on ethylene vinyl acetate (EVA) surfaces, which have been separated from the glass panel. Two types of time-dependent indentation cycle modes, the time domain (creep mode) and frequency domain (dynamic mode) were performed to determine the viscoelastic behavior. For each mode, a corresponding model was applied to calculate the main mechanical properties. The general capability of nanoindentation as cross-linking determination method is investigated with the methodological advantages over bulk mechanical characterization methods. A large number of Glass/EVA/Backsheet laminates were built using different lamination conditions resulting in different degrees of curing. Both indentation modes indicate good modulus sensitivity for following the EVA crosslinking in its early stages but could not reliably differentiate between samples with higher EVA branching. Additional dynamic mechanical analysis (DMA) characterization was used as an established method to validate the indentation measurements. Both nanoindentation and DMA tensile mode produce similar quantitative viscoelastic responses, in the form of the damping factor parameter, demonstrated for three different frequencies at room temperature. A statistical study of the data reveals the advantages for the investigation of multilayer PV laminates by using nanoindenation as a surface method while also being applicable to field aged modules.