Encapsulation Polymer Degradation and Its Impact on the Deterioration of Solar Modules

Abstract

This thesis deals with the basic degradation mechanisms of solar modules, focusing on the polymer encapsulation materials used and the additives they contain. Using self-developed measurement methods based on pyrolysis gas chromatography mass spectrometry (PY-GCMS) and ultraviolet-visible (UV/VIS) spectrophotometry, the quantification of these additives is carried out, enabling a variety of further analysis approaches. On the one hand, fundamental effects such as additive diffusion are studied within this work. On the other hand, the degradation behavior of the polymers is investigated as a function of the stabilizing additives. In this work, solar modules are subjected to different stress conditions such as UV irradiation, high humidity, and high temperatures as part of accelerated aging tests. The stressors are used individually and in combination to analyze the resulting degradation effects of the solar modules. The analyses include macroscopic characterization methods such as I-V and electroluminescence (EL) measurements as well as various methods of polymer analysis such as Fourier transform infrared spectrometry (FTIR), evolved gas analysis (EGA)-FTIR, differential scanning calorimetry (DSC) and UV/VIS spectrophotometry. The interactions of the additives were determined using PY-GCMS, electron paramagnetic resonance spectroscopy (EPR), and Orbitrap mass spectrometry. By combining these measurement methods, it was possible to trace the degradation chain of solar modules from the consumption of stabilizing additives through the degradation of the encapsulation polymers to the degradation of the solar cell itself and its electrical connectors. In particular, the degradation reactions to be expected due to the prevailing microclimate at different positions within the modules could be investigated. In this context, models that predict the degradation of UV stabilizers and UV absorbers could be derived depending on the environmental parameters. The UV aging standard IEC 62788-7-2 was also validated thanks to the large number of accelerated aging series carried out. It was found that the UV-A fluorescent lamps listed in the standard, in contrast to xenon lamps with daylight filters, do not lead to the aging effects observed in the field. Consequently, the former are to be classified as unsuitable for qualifying polymer materials for solar modules. On the one hand, various fundamental effects, such as additive interactions in multi- layer systems and their diffusion behavior, can be derived from the results of the work. On the other hand, clear conclusions can be drawn for the industry to increase the service life of solar modules in the future. The latter is of particular interest for the introduction of new cell technologies such as TOPCon (tunnel oxide passivated contact) or HJT (hetero junction) and the associated new encapsulation materials POE (polyolefin elastomer) and TPO (thermoplastic polyolefin).