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

application might become more important for the layer formation, in particular, since it is assumed that
the presence of sodium hinders the elemental inter-diffusion during the CIGS growth process.
However, as it is shown in figure 1.4, no significant differences in the electronic properties of the cells
can be obtained. All electronic cell parameters show similar values, only the fill factor is slightly
improved, when sodium is added to the absorber during the second growth stage.
Fig. 1.4) Current-Voltage characteristics of 0,6 cm 2 CIGS solar cells: Sodium was added to the
absorber by means of co-evaporation during i) the first stage (dashed – dotted line) and
during ii) the second stage (solid - line) of the absorber growth process.
Also the cross-sectional SEM photos of both solar cells in figure 1.5 show no differences. However, it
is noticeable, that the grain size near the molybdenum back contact is significantly reduced for both
samples. The thickness of this region corresponds to that evaporated during the first stage of the 3stage
process, where only Ga, In and Se is deposited. It is known, that a Ga rich composition results
in smaller grains. Thus it can be concluded, that due the presence of sodium the diffusion of Ga during
the layer formation in the second process stage is impeded, what may lead to a Ga-rich composition.
Fig. 1.5) Cross-sectional SEM photos of solar cells. The sodium was co-evaporated during the first
stage (left) and during the second stage of the CIGS 3-stage process (right). The presence
of sodium during the absorber growth results in small grain sizes near the Mo back contact.
LARCIS, A. N. Tiwari, ETH Zurich
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