A research team in Germany has investigated whether compressive stress relaxation measurement (CSRM) can be used to monitor polymer degradation in aging photovoltaic modules. The technique is typically used in manufacturing quality control to assess the degree of encapsulant cross-linking after lamination.
“This publication uses the CSRM not only for lamination control, but also to monitor the mechanical response of the mini-modules’ rear side,” the researchers explained. “To achieve a higher resolution and sample throughput, the cross-linking density will be used to calibrate the CSRM. The influence of encapsulants and backsheet degradation on the CSRM during weathering is of interest. Therefore, the degradation of the encapsulant and the backsheet will be decoupled.”
CSRM is a non-destructive technique commonly used to assess encapsulant curing after PV module lamination. It involves pressing a tip into the rear surface of a module and measuring how the force decreases while the indentation depth remains constant. The response reflects the mechanical properties of both the encapsulant and the backsheet.
To evaluate CSRM as a method for monitoring aging, the researchers fabricated 200 mm × 200 mm mini-modules incorporating passivated emitter and rear contact (PERC) solar cells, ethylene vinyl acetate (EVA) encapsulants, and polyethylene/polyethylene terephthalate (PET) backsheets. They varied lamination times at 155 C to achieve different degrees of cross-linking.
For each CSRM measurement, they heated the test location to 89 C, pressed a 4 mm steel tip approximately 100 µm into the module’s rear surface, applied a force of less than 20 N, and recorded the decline in force while holding the tip in place.
The modules were subjected to damp heat (DH) testing at 85 C and 85% relative humidity for up to 3,500 hours, or combined heat and ultraviolet (UV) irradiation at 85 C, 60% relative humidity, and an irradiance of 227 W/m2 for up to 2,000 hours. At 250-hour intervals, the team used CSRM to monitor mechanical changes and measured the modules’ electrical performance. Additional solvent tests and thermal analyses helped distinguish changes in encapsulant cross-linking from backsheet degradation.
“After the first 250 h of the DH test, the encapsulants showed an increase in the degree of cross-linking but maintained differences between the samples throughout the test,” the researchers said. “As a result, the PV module power degradation correlated with the initial cross-linking state. By the end of the test, the encapsulant and the backsheet had degraded, resulting in a drop in the CSRM response.”
The results also showed that the encapsulant’s cross-linking density subsequently decreased, while the backsheet underwent post-crystallization and exhibited chain scission.
“The combined DH+UV weathering test caused post-cross-linking within the first 250 h, leading to leveling of the cross-linking states,” the researchers concluded. “The canceled chemical differences in the encapsulants became reflected in a similar module power degradation. Thus, overacceleration shall be avoided by all means to deliver realistic reliability predictions.”
The findings are presented in the study, “Non-destructive monitoring of polymer degradation in PV modules by compressive stress relaxation measurement and implications for the PV module reliability,” published in Polymer Testing. The research team included scientists from Germany’s Anhalt University of Applied Sciences and Fraunhofer CSP.
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