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Y Voronko, G Eder, M Knausz, G Oreski, T. Koch, K Berger:
"Correlation of the loss in photovoltaic module performance with the ageing behaviour of the backsheets used";
Progress in Photovoltaics, 23 (2015), 11; 1501 - 1515.

The influence of the type of backsheet on the electrical performance of test modules was evaluated before and after
increasing time of accelerated ageing (damp heat [DH] exposure). Besides the measurement of the electrical power of
the modules and the performance of the cells by electroluminescence, the ageing-induced changes within the polymeric
encapsulate and backsheets were investigated by means of vibrational spectroscopy and by thermo analytical methods. In
addition, the permeability of the backsheets in the original and aged state was determined. This wide set of test parameters
and methods allowed for the detection of correlations between (i) physical and chemical properties as well as their
ageing-induced changes of the materials and (ii) the module performance. A clear dependence of the relative loss in
power output upon exposure under DH conditions for 2000 h could be observed for a set of identical test modules varied
in composition only in the type of back cover used. While the modules containing gas-tight backsheets and glass
experienced only little loss in the relative power output, some modules with permeable backsheets showed a significant
relative decrease in the power output and fill factor in dependence of the backsheet type used. Cell degradation could be
visualised by recording electroluminescence images before and after the accelerated ageing test. The permeation properties
of the backsheet used and their ageing-induced changes seem to have an influence on the module performance. However,
the absolute values neither of the water vapour transmission rate (WVTR) nor of the oxygen transmission rate
(OTR) are directly linked to the loss in power output upon accelerated ageing under DH conditions. It could be shown
that the ageing-induced changes (relative transmission rates) between WVTR and OTR can be correlated with the module
performance. These ageing-induced changes in the permeation behaviour of the backsheets can be explained by (i) physical
changes (e.g. post-crystallisation, changes in the crystal structure or the crystalline microstructure) and (ii) chemical
ageing effects such as a decrease in the molecular mass of the polyester (PET) polymer chains because of hydrolytic
polymer degradation leading to a change in the crystallisation behaviour of PET. Hydrolytic degradation (= chemical
ageing) of the PET core layer was observed (with varying extent) for all PET-based backsheets and can, thus, not be
directly correlated with the loss in performance of the corresponding test modules. The physical ageing effects, however,
were detected only for those backsheets showing (i) strong deviating changes in the relative permeation rates for oxygen
and water vapour upon accelerated ageing and (ii) a clear loss in electrical performance.

photovoltaic backsheets; accelerated ageing; long-term performance; permeation; characterisation

http://dx.doi.org/10.1002/pip.2580