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

Optimum Na dosage
Sodium layer with thicknesses between 10 nm and 40 nm were deposited on top of the CIGS
absorber of different 5x5 cm 2 samples. Figure 1.1 shows the resulting current-voltage characteristics
of the finished solar cells: displayed are the best cells of each sample. The curves significantly present
the improvements in the open circuit voltage (VOC) from 0,52 V (no Na) up to 0,62 V for the optimum
Na dosage, which corresponds to a layer thickness of 20 nm.
Figure 1.2 shows the summary of solar cell parameters averaged over 15 cells (0,6 cm 2 ). Again it is
demonstrated, that highest efficiency and fill factor (FF) are achieved for a Na-layer thickness of
20 nm. The VOC increases rapidly with increasing amount of sodium, that is provided to the absorber,
however the VOC remains nearly constant after the sodium supply exceeded the optimum value. The
short circuit current density (JSC) decreases continuously with increasing sodium dosage. This result is
in contrast to the behavior of the JSC reported in literature, since a higher carrier concentration at
higher Na content, due to the reduction of compensating donors would be rather beneficial for the JSC.
The optimum sodium layer thickness of 20 nm can be converted into a Na-dosage in the CIGS
absorber. Assuming that all of the sodium diffuses into the absorber material, the corresponding
dosage within the absorber is approximately 0,6 at%, which is higher than the value of 0,1 at%,
considered as the optimum dosage in literature. Thus it can be expected, that a significant amount of
sodium remains at the CIGS surface during the post deposition process.
Fig. 1.1) Current-Voltage characteristics of 0,6 cm 2 CIGS solar cells. Depending on the amount of
Sodium incorporated into the absorbers via post deposition treatment the electronic
parameters change significantly. For a thickness of a 20 nm sodium layer, cell efficiency, VOC
and fill factor reach their optimum values.
LARCIS, A. N. Tiwari, ETH Zurich
98/290
4/9