In order to see (i) the effects that deliberately-generated failures (micro-cracks, cell cracks, glass breakage, and defects in the cell interconnection) have on developing UV fluorescence patterns and (ii) to determine whether propagation of such failures occurs with storage time, reference modules and defective test modules were treated in the same way (as described above).
One testmodule showed a glass crack which occurred at the beginning of the irradiation test, most probably caused by thermal tension (untempered glass was used for all test modules). It was first noticed after 100 h storage time and increased in length, reaching its final magnitude after 400 h . It was observed that the fluorescence formed homogeneously in the polymeric encapsulation over the cells and disappeared beneath the glass breakage after 300 h of further irradiation. The EL image showed that the glass breakage (induced by thermal stress in the first phase of the weathering test) did not cause a crack in the underlying cells. The disappearance of the fluorescence beneath the glass breakage is attributed to an oxygen bleaching effect. The incoming oxygen can interact/react with the activated and fluorescing sites of the polymer, leading to a decrease in its fluorescence intensity. The glass breakage had no impact on the performance of the test module after the 1000 h irradiation test.
It has to be noted at this point that it was very difficult to generate a comparable number and size of (micro)cracks in the six test modules. As visualized in the EL images (see Figure 3), there were modules with few (*) and numerous (**) micro-cracks and others with cell cracks (+), and it was expected that their effects on degradation would be quite different.
Upon irradiation with artificial sunlight and natural sunlight (see Figure 5b,c), the evolving fluorescence pattern of the test modules with micro-cracks clearly differed from those of the test modules without failures (see Figure 2).
The UV-F images taken every 100 h of accelerated weathering again showed a continuous increase in the intensity of fluorescence above the cells with unequivocally detectable intensities starting from 300 h. However, there were numerous dark spots where no fluorescence was generated (Figure 5b). In comparison with the EL image of that module, a clear correlation with the positions of the micro-cracks in the cells could be obtained. At the positions of the micro-cracks in the cells, the fluorescence was extinguished by oxygen (bleaching) permeating into the front encapsulation via the polymeric backsheet and cell cracks. In the outdoor weathering, the effect of UV-F in the front encapsulant was clearly detectable after 1 year showing extinctions (dark spots) at the locations of the micro-cracks in the cells (see. Figure 5c; confirmed by parallel EL measurements).
The results presented show a good robustness of the PV modules against stress, even when defects such as micro-cracks, cell cracks, glass breakage or interruptions in the cell interconnections are present. Hardly any propagation of the module failures in the accelerated ageing tests (indoors) and the outdoor exposure test could be detected. It was also noticed that the length of the micro-cracks (as visualized by the EL-images) did not (or only minor) increase during the time span of the accelerated (indoor) or natural ageing (outdoor) tests performed within the work presented. These effects were also found by Buerhop-Lutz et al. in indoor tests  and Jaeckel et al. in a 15-year outdoor test  and are in accordance with a recent study by Dimish et al. on the impact of cracks on photovoltaic power performance .
However, the presence of numerous micro-cracks led to a higher rate of degradation; the power loss upon irradiation (1000 W/m2 for 1000 h) was twice as high as that for the intact reference module under the same stress conditions. Defects in the cell interconnection of test modules caused a higher series resistance, leading to a slightly lower power output by ~1%. The relative change in power upon accelerated ageing, though, was not affected by that failure. Outdoor weathering for 1.5 years did not lead to any degradation effects, irrespective of the failures deliberately incorporated in the test modules.
This clearly indicates that UV-F imaging is a suitable tool, not only for detecting cell cracks in operating PV modules but also to allow for estimation of the timescale at when these cell cracks were generated. This may be of special interest to allow one to distinguish between installation- or transport-induced cell cracks and those which are generated during operation by, for example, a heavy hail storm . Furthermore, the temperature-induced degradation effects of the encapsulant lead to increased fluorescing light intensities and make hot spots or overheated module parts visible in the UV-F images .
The performances of PV modules with mechanical module failures, such as micro-cracks, cell cracks, glass breakage or breakage of interconnectors were found to be rather unaffected upon storage under various stress conditions. In most cases, no propagation of the module failures in the accelerated ageing tests and the outdoor exposure test could be detected. However, the presence of numerous micro-cracks led to a higher rate of degradation; the power loss upon irradiation was twice as high as for the intact reference module upon accelerated ageing.
UV-F images of three test modules with micro-cracks: (a) one in the original state (stabilized); (b) one after exposition to artificial sunlight (1000 W/m2) for 1000 h; and (c) one after natural storage for 1.5 years.
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