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.