Identification of Spectral Indicators of Degradation in Tedlar-Type Backsheets: A Case Study of PV Modules Aged in Tropical Climate

Degradation of backsheets and encapsulants induces significantlosses in insulation resistance, often decreasing it below the allowed threshold and inducing inverter shut-offs. The deterioration of polymer packaging can be reliably detected by measurements of the insulation resistance, which, however, are resource-demanding and limited to wet periods. In this connection, alternative non-invasive and resource-efficient methodologies are highly demanded. Recently, we showed that insulation resistance can be indirectly evaluated by near-infrared absorption spectroscopy, using an inverse correlation between the insulation resistance and spectrally identified carbonyl index, which indicates the depth of oxidative degradation of ethylene vinyl acetate copolymer encapsulation. In the present contribution, we extend this spectral approach by applying Raman spectroscopy to identify spectral features of the polymer degradation in Tedlar-type backsheet components of PV modules aged in a tropical climate. We report several spectral parameters that correlate with the carbonyl index of the encapsulant and can be used as substitute for the measurements of the insulation resistance, including the ratio of C=O- and C=C-related vibrational bands in polyethylene terephthalate core layer, the ratio of polyvinyl fluoride- and rutile-related bands in Raman spectra of the air layer, as well as the integral photoluminescence of the air layer. The relevance of the selected spectral parameters as descriptors of degradation events in Tedlar-like backsheets was further confirmed by artificial UV aging tests. This demonstrates Raman-based field diagnostics as a practical complement to insulation resistance and NIRA methods.