Evaluating different Na-incorporation methods for low temperature grown CIGSe thin film on polyimide foils

The goal of this work is to investigate the influence of the Na incorporation method into CuIn_1-xGa_xSe₂ (x=Ga/(In+Ga)) (CIGSe)-based solar cells on polyimide (PI) foil. In particular we want to compare the effect of a NaF precursor layer with that of NaF co- and post-deposition. Secondary ion and neutral mass spectroscopies (SIMS/SNMS) are used to study the distribution of the elements through the CIGSe layers. Cross-sectional scanning electron microscopy (SEM) shows the dependence of the absorber microstructure on the method of how Na is supplied with and without Ga present. Adding Ga the device microstructure is generally characterized by smaller CIGSe grains next to the Mo back contact, which indicates the very low process temperature used. The use of a NaF precursor, our standard method for the supply of Na, modifies the growth kinetic of the absorber layer and emphasizes the importance of growth parameters such as the Cu flux and max Cu content during the deposition process. Optimization of the deposition process that uses a NaF precursor, so far led to a max efficiency of 15.9 % (ta = 0.95 cm²). In the case of NaF post-deposition the use of a low process temperature is argued to reduce the Na diffusion throughout the absorber layer, which may reduce the quality of the CIGSe/Mo back interface. This may explain the lower FF observed generally for this process in comparison to that where Na is supplied by a precursor layer.