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

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Fig. 1.2) Summary of photovoltaic properties of solar cells treated with different amounts of sodium
after the CIGS growth process: Shown are averaged values of 15 cells.
An investigation of the CIGS cross-section with the scanning electron microscope (SEM), reveals, that
the absorber microstructure is not affected by the post deposition treatment. Figure 1.3 shows a
comparison of cross sections of sodium treated and un-treated samples. Both cells show a similar,
dense structures and large grain size.
Fig. 1.3) A comparison of SEM pictures of the cross section of treated and non-treated cells show,
that the post deposition treatment leaves the microstructure of the CIGS absorber
unaffected.
Comparison of different methods of sodium incorporation
In this section, results for different incorporation methods will be discussed.
In addition to the post deposition treatment, co-evaporation of NaF parallel to the evaporation of the
absorber material Cu, In, Ga and Se was performed. Based on the previous findings, the sodium
fluoride evaporation rate was adjusted such, that during a deposition time of 15 min. the dosage was
equivalent to the optimum 20 nm thick Na-layer. Using this evaporation rate, we deposited sodium i)
during the first stage and ii) during the second stage of CIGS growth.
Figure 1.4 shows the best results of the so processed solar cells. The cells which were processed at
low substrate temperature of ~450°C yield efficiencies of 12,2% for Na co-evaporation in the 1 st stage
and 12,7% for the Na co-evaporation during the 2 nd stage. The low temperature process was chosen
here, since the temperature of ~450°C is the limit, when flexible cells on polyimide foils are produced.
Furthermore stronger effects were expected at lower temperatures. Thus, the time of sodium
LARCIS, A. N. Tiwari, ETH Zurich
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