Indoor vs. outdoor aging: polymer degradation in PV modules investigated by Raman spectroscopy

Indoor and outdoor aging tests are common methods for PV module degradation investigation. But to what extend are accelerated indoor aging tests comparable to outdoor exposure tests? The impact of indoor and outdoor tests on the polymer degradation in full-size PV modules was investigated. Polymer aging within a PV module is one of the major factors influencing module performance in the course of its lifetime. Degradation phenomena like yellowing, delamination or changes in the elastic modulus of the encapsulation may lead to transmission losses, corrosion effects or cell cracks. Raman Spectroscopy has recently been reported by our group as a non-destructive, analytical method for encapsulation degradation analysis. The degradation of the encapsulation of indoor and outdoor aged crystalline silicon PV modules was examined by the means of Raman Spectroscopy with special attention to the spatial-dependency of the degradation. The investigated modules were subjected to several different accelerated aging procedures with a systematic variation of the climatic conditions temperature, humidity and UV. Identical modules were aged in different climates (arid, tropical, urban and alpine) for up to three years. The degradation of the encapsulant was observed, resulting in an increasing fluorescence background in the Raman spectra. A dependency of the aging process on the relative position to the edges of the cell was found. The aging conditions appeared to influence the spatial distribution of the fluorescence and therefore, the polymer degradation, markedly. Furthermore, correlations between accelerated aging tests and outdoor exposure tests could be found.