Different methods for Na incorporation are known for the use of Na-free substrates like stainless steel or polyimide foil. In this work Cu(In,Ga)Se2 (CIGS) absorber layers with different amounts of Na are investigated. The CIGS samples were prepared via a roll-to-roll deposition process with ion beam assistance (Solarion) and by a multi-stage low temperature co-evaporation process (HZB). Na was either incorporated via in-situ co-evaporation of NaF (for roll-to-roll deposition) or by a Na-containing precursor (for multi-stage deposition). With increasing amounts of Na an increase of VOC is observed for both deposition tech-niques. In contrast, within the deposition parameters used, jSC decreases with increasing Na amount for co-evaporation of NaF while it seems unaffected when using a NaF precursor layer. The elemental depth profiles of the different CIGS thin films were studied via secondary ion mass spectroscopy and were found to depend strongly on the deposition technique. It seems that beneficial effects of the addition of Na are independent of the method of in-corporation, even if the distribution of Na in the CIGS layer is different due to different methods of incorporation and CIGS deposition processes.

The effect of the amount of Na present during the 3-stage growth of CIGS at very low temperature T2 on polyimide (PI) foils is studied. While at higher growth temperatures Na seems to impede In-Ga interdifussion, at very low temperatures it appears to further the process. An increase in Voc for a higher Na concentration can be explained by a higher net carrier concentration as measured by drive level capacitance profiling. Admittance spectroscopy measurements show shallow defects when the Na concentration increases. These results suggest that the main role of Na could be the passivation of InCu donor deep defect, in agreement with Wei's theory. Efficiencies of up to 15.1 % (0.5 cm2 active area with antireflection coating) and 13.6%, 14.1% (1 cm2 total and active area respectively without antireflection coating) for nominal T2=420° C were achieved on PI substrates so far.