Potential Weaknesses of Polyethylen Terephtalate (PET) Based Backsheets for Bifacial Solar Modules

Coupons and mini modules with transparent PET-based backsheets were weathered under a combination of damp heat and UV radiation. For the use of these backsheets in bifacial modules, the direction of the backsheet (direct vs. indirect irradiation) was varied to find out how strong the influence of the radiation source position is. A complete characterization will be carried out using pyrolysis-gas chromatography-mass spectrometry (PY-GCMS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In addition, the coupons will be analyzed using UV/Vis spectroscopy and the results will then be correlated. The UV/Vis measurements show a clear reduction in transmission with increasing weathering time depending on the direction of the backsheet. The FTIR measurements show that individual layers - primer layer and core layer - exhibit different degradation behavior. Furthermore, delamination effects occurred between the primer layer and the core layer. Discoloration could only be attributed to the degradation of the EVA-based primer layer, while the PET-based core layer remained transparent. The degradation of backsheets is dependent on the individual layers, i.e., the backsheet compostion strongly influences the complete module degradation. Further investigations are necessary to avoid yellowing, delamination and the resulting loss of performance in the module.