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

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.

Predicting encapsulant delamination in photovoltaic modules bridging photochemical reaction kinetics and fracture mechanics

“…The EVA encapsulant samples removed from the failure interfaces of the PV modules had rough surfaces, which caused many indentation tests to fail due to uneven nanoindenter tip contact. Similar issues were reported in other works 13 . The indentations were spaced 120 μm apart, larger than the flat punch diameter.…”

“…The initial DOC of EVA was assumed to be 60%, typical for an EVA made under industrial fast curing conditions with 5 to 10 min of lamination time 17 . With 60% initial DOC, the model predicted the unaged EVA modulus to be 18 MPa, consistent with results in the literature 13,21,22 . The initial yield strength of the EVA was set at 5 MPa 21 .…”

“…Similar issues were reported in other works. 13 The indentations were spaced 120 μm apart, larger than the flat punch diameter. Only the indentation tests that achieved a plateau of Young's modulus values over the entire depth of indentation (5000 nm), which indicated consistent tip contact with the sample, were included as valid experimental data.…”

“…Table 2 shows the input parameters used for the mechanical properties modeling in Equations ( 10) and (11). The Young's modulus and yield strength values of unaged, uncrosslinked EVA (E 0 and σ ys,0 , respectively) were taken from literature, 13,21,22 whereas w CL was determined by a best fit with nanoindentation experimental data (Figure 3A).…”

“…For example, we measured a modulus of 9.64 MPa for the encapsulant from the Florida 1-year-aged module compared with a modulus of 14.17 With 60% initial DOC, the model predicted the unaged EVA modulus to be 18 MPa, consistent with results in the literature. 13,21,22 The initial yield strength of the EVA was set at 5 MPa. 21 Currently, there are no experimental studies which measure the variation of the EVA's yield strength over degradation time.…”