Incorporation of Na in Low-Temperature Deposition of CIGS Flexible Solar Cells
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1210-Q03-02
Incorporation of Na in low-temperature deposition of CIGS flexible solar cells Hendrik Zachmann1, Stefan Puttnins1, Felix Daume1, Andreas Rahm1 and Karsten Otte1 Solarion AG, Ostende 5, 04288 Leipzig, Germany
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Raquel Caballero2, Christian A. Kaufmann2, Tobias Eisenbarth2, and Hans-Werner Schock2 2 Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Hahn-Meitner-Platz 1, 14109 Berlin, Germany ABSTRACT 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 techniques. 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 incorporation, even if the distribution of Na in the CIGS layer is different due to different methods of incorporation and CIGS deposition processes. INTRODUCTION It is a well known fact that the quality of CIGS-based solar cells can be enhanced by the addition of Na [1]. Higher amounts of Na generally lead to a higher acceptor concentration, which is usually correlated with a higher built-in voltage. Several models have been proposed in the literature to account for this increase in the hole density when Na is incorporated: creation of acceptors such as the antisite defect NaIn/Ga [2], elimination of the compensating antisite donor defect InCu [3] and neutralization of donor-like defect VSe [4]. However, the way how exactly Na influences the electrical and structural properties of CIGS absorber layers is still under discussion. Certainly, it is necessary to include the method of Na-supply to the CIGS absorber layer into the considerations as well as the CIGS deposition process in use. In previous work, a smaller grain size of the CIGS layer and a higher doping concentration have been reported when Na was added via evaporation of a NaF precursor layer prior to the absorber deposition [5, 6]. Rudmann et al. have shown the improved morphological structure of the CIGS thin films, preserving the electrically beneficial effects of Na when it is incorporated via a post-deposition treatment [7]. With the objective of studying the beneficial effects of Na on the resulting CIGS devices, the present work compares different methods of Na incorporation
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