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

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Figure 9: polyester/glass fiber
encapsulation of DSC
Figure 10: UV absorption of
polyester
Figure 11: Transmission of glass
and polyester encapsulation
materials
Task 19. Mechanical integrity analysis of flexible a-Si:H PV devices
The objective of this task is to determine the critical factors, which control the mechanical integrity of
layered a-Si:H PV devices. The internal stress state in individual layers was determined from a
thermo-mechanical analysis of the radius of curvature of films. The a-Si:H layer was found to be under
a compressive strain of approx. 0.1-0.2%, which is favorable for the PV cell performance since the
silicon layer is the most brittle layer. Cohesive and adhesive properties of the layers were investigated
using fragmentation test method developed at LTC. The crack onset strain (COS) of the PV stack was
found to be maximum for a force of 200 N applied during the a-Si:H deposition. The critical radius of
curvature in terms of layer cracking was found to be close to 3 mm at room temperature. The influence
of temperature was also investigated and found to be considerable as shown in Figure 12. A thermomechanical
model was developed, including internal strain and elastic contrast contributions.
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
-0.002
COS = COS* – � i
Intrinsic COS*
(elastic contrast)
PV/PEN; MD
COS experimental
COS theoretical
Internal strain � i
0 50 100 150 200
Temperature [°C]
Figure 12: Crack onset strain modelling of PV device versus temperature
WP2. Process integration into ultralight sandwich composite structure (resp. LTC)
Task 5. Optimization of bonding layer in symmetric sandwich structures
The influence of adhesive quantity has been measured in 4-points bending. It has been observed that
the compressive strength of the skin increases with adhesive weight even if no debonding occurs. The
adhesive stabilizes the skin and reinforces the core, so that the wrinkling strength is increased. The
influence of bonding defects was also studied by inserting 2 mm and 5 mm wide defects. The strength
was decreased by 30% and 80%, respectively, as depicted in Figure 13. As failure was due to local
buckling over the defect and debonding, the strength increased with adhesive weight. An analytical
model was developed to calculate the strengthening effect of adhesive menisci observed in bending
tests. The model reproduced in Figure 14 correlated very well with experimental data. It enabled the
bending strength to be predicted by varying adhesive quantity, core thickness or carbon prepreg thickness.
This demonstrated that the optimal adhesive weight for the selected materials was ~40 g/m 2 of
adhesive per face.
Task 6. Modeling of the influence of glue meniscus on bond strength in sandwich structures
A model predicting the size of the resin meniscus as function of adhesive weight has been developed
and validated. It has been used coupled with the measurements of adhesive toughness to calculate
the skin / core debonding energy. It has been highlighted that the mechanisms involved for breaking
adhesive meniscus dissipates much more energy than continuous crack propagation in adhesive. The
effect of adhesive menisci on the failure in bending of sandwich beams with simulated defects (preliminary
core / skin debonds) was modeled analytically. It was observed that the increased strength
with adhesive weight was not due to an increase in debonding energy, but to the stabilizing effect of
the menisci, which increased the compressive load in the skin causing buckling of the skin over the
Ultralight Photovoltaic Structures, Y. Leterrier, EPFL
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