Abstract
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.
Highlights
There exists an increased interest from the photovoltaic (PV) industry in a fast and non-destructive method for the determination of the mechanical properties of solar encapsulant to ensure the encapsulation quality after lamination and the long-term stability
Dynamic mechanical analysis (DMA) has been established as suitable method to detect the viscoelastic properties of ethylene vinyl acetate (EVA) encapsulant [8]
Both nanoindentation approaches possess a good modulus sensitivity for following the EVA crosslinking in its early stages, where additional entanglements lead to a modulus increase
Summary
There exists an increased interest from the photovoltaic (PV) industry in a fast and non-destructive method for the determination of the mechanical properties of solar encapsulant to ensure the encapsulation quality after lamination and the long-term stability. The detection of the cured EVA’s viscoelastic response has been demonstrated previously by means of a compressive stress relaxation measurement This mechanical testing was found to be representative only at elevated temperatures [7]. This paper introduces an advanced nanoindentation test including the time-dependent cycle modes for the quantification of the viscoelastic response of the EVA surface at the polymer-glass interface (Figure 1). To the best of our knowledge nanoindenation is used for the first time on EVA surface using time-dependent cycle modes In this approach, first the elastic creep and viscoelastic sweep parameters are measured; correlated to the EVA’s DoC as measured by the DSC method by using the exothermal peak from 110 to 190 °C. The viscoelastic properties as determined by the dynamic nanoindentation method are validated by DMA as an established technique
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