Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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4.3 Water content of polymers. (Fig.17(b))
In order to quantify the water content of the encapsulant itself, and relate it to the increase of the sheet
resistance of the TCO (see above), glass / polymer laminates have been characterized by means of
spectro-photometric measurements after exposure to damp-heat (Fig.18.a). Indeed, the water molecule
has several absorption peaks in the infra-red region. The ratio of a water absorption peak (transmission
dips) and a water independent peak theoretically permits to quantify the water content of the
polymer. This study is still in progress.
Fig.18.a: increase in moisture intake monitored
by optical measurements.
Fig.18.b: reduction in blue light reflection of the
laminate caused by harsh weathering.
4.4. Compatibility of encapsulation with electrical contacts and wiring. (Fig.17(c))
A new laser-scribing facility has been recently acquired and set-up is ongoing. No reliable manual
contacting scheme has been achieved for the small cells made at IMT: because of the large size of
our laminator and small thickness of the substrates, the embedding processes yielded a high percentage
of broken glass and/or lost contacts. Nevertheless, when embedding larger size modules, delaminating
effects on, and corrosion of, contacting wires were clearly observed for some polymers. These
effects usually showed up after exposure of the laminates to damp-heat. In special cases, they have
been related to by-products formation during or after lamination (e.g. acetic acid in the case of EVA)
but further analyses are required in order to determine the exact chemical reactions.
4.5 Compatibility of encapsulation with white ink back reflectors. (Fig.17(d))
Although the use of a white paint as back reflector for the development cells made at IMT was not
foreseen in the first part of this work, it was relevant in order to achieve some consistency when comparing
results for several polymers.
On one hand, the white paint back reflector acts as a protection barrier against water diffusion for
laminates exposed to damp heat. This was evident for glass / TCO / white reflector / polymer laminates
in which the degradation of the sheet resistance of the zinc oxide was, in most cases, far less
drastic than in glass / TCO / polymer laminate.
On the other hand, some white paints were found incompatible with some polymers. Usually, the
chemical reactions at the interface white paint / polymer are accelerated for increasing temperature,
exposure to humidity and also to UV-light. The degradation effects ranged from yellowing of the reflector
due to decreasing of reflectance in the 400-500 nm range, as illustrated in Fig.18.b, up to delaminating
of parts of the laminates.
4.6 Encapsulation of full sized (1.4m 2 ) amorphous silicon modules: weathering tests.
A series of large size TF-Si modules were laminated with various polymers in a glass/glass configuration
and exposed to damp-heat during 1'000 hours. Taking into account a precision of ± 2% for the
module tester we used, no significant degradation of the current/voltage characteristics were found.
Nevertheless, some of the modules became milky near the borders or the corners. This demonstrates
that accelerated aging techniques and measurements are much more relevant on small sized modules
than in large sized ones.
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New processes and device structures for the fabrication of high efficiency thin film silicon photovoltaic modules, C. Ballif, University of Neuchâtel