Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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c
Figure 2: Microstructural features observed at different positions of the powder-B: (a) and (b) after
3 min of evaporation; (c) appearance of big In2O3 particles after 70 min of evaporation.
The chemical composition of the powders as a function of evaporation time was also determined by
EDX. Apart from microstructural differences, a noticeable difference between the two powders was the
loss of sulfur (S-loss) from the source material. A significant S-loss was observed in powder-B. In
addition to thermal decomposition of In2S3 [10], a higher vapor pressure of sulfur also favors it to
escape from the In2S3 source material as it is heated to 720 – 740°C. This S-loss may lead to a
significant compositional change in the indium sulfide powder. Consequently the stoichiometry of
evaporated In2S3 buffer layers might not be as in the source material and it might change with
successive number of evaporation runs [11]. But, in case of powder-A, the In/S ratio of 2/3 did not
change even after 15 min of evaporation. Figure 3 shows the In content (at %) evaluated by EDX
performed at various positions on the source material after a given time of evaporation. Quantitatively,
in contrast to powder-A, S-loss from powder-B was high and non-uniform regions of different In/S
ratios were observed on different positions in the same lump of powder. It is expected that the part of
source material which is close to the wall of the graphite crucible might behave in considerably
different manner than the part in the middle. This microstructural and chemical contrast observed in
powder-A and -B may be attributed to the difference in their methods of synthesis.
Figure 3: EDX measurements, performed at various positions on the In2S3 source materials after
different time of evaporation, show inhomogeneous chemical compositions in powder-B, while
powder-A has stable composition.
The source powder-B obtained after 4 min of evaporation was characterized by XRD technique. The
tetragonal �-In2S3 was the dominating phase and the peaks corresponding to cubic In2O3 phase were
also detected, as shown in Fig. 4. However, there was no indication of the presence of In1S1 phase in
the XRD pattern. It is important to point out that measurement sensitivities and observational volume
of the materials are different in EDX and XRD measurements.
ATHLET, D. Brémaud, ETH Zurich
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