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

Fig.14: TEM cross section microcrystalline absorber layers. Porous or low quality material is found at
the n-i interface in the lower part of the image (left). A buffer layer with lower dilution grows
denser material with better coverage (right). The arrow in the left panel illustrates the growth of
defective material because of colliding growth fronts.
Fig.14 shows that the microcrystalline material in the nucleation region of the intrinsic µc-Si layer at
the n-i interface is not uniformly dense but grows with a significant fraction of voids. This observation
suggests the introduction of a buffer layer with higher amorphous fraction because this is known to
yield denser material and generally results in a better coverage of textured substrates as shown in
Fig.14. A systematic series of cells showed that nucleation layers with a very low dilution should yield
the best results in terms of open circuit voltage and fill factor. However, the beneficial effect of the
buffer layer is then compensated by bad current transport due to the high content of amorphous phase
in the microcrystalline material. Cell optimization must thus consider the trade-off between establishing
a nucleation template with good coverage, and sufficient current transport through the buffer layer
[Sod08B].
3.3.2 Amorphous silicon
The investigations on amorphous silicon solar cells concentrated on the best stabilized efficiencies
with respect to the absorber layer thickness on a given LPCVD-ZnO texture. Fig.15 compares initial
and stabilized parameters. Except for the open circuit voltage which is not strongly affected by the ilayer
thickness, it is observed that lower degradation is normally found for thinner cells. In terms of the
highest stable efficiency a cell with 200 nm thickness is favourable because losses in current collection
become limiting for thinner cells. The stabilized current densities are between 13 and 13.5 mA/cm 2
which is slightly above the limiting value of 12.5 mA/cm 2 which was estimated above. However, it
should be noted that the cells shown in Fig.15 are measured with white paste reflector. In the tandem
application this reflector is absent which reduces the currents by 1.5 to 2 mA/cm 2 (c.f. the stabilized
current of of 11.7 mA/cm 2 in the amorphous top cell shown in Fig 16).
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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
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