In the last years steel foil has become an attractive candidate for use as a flexible substrate material for Cu(InxGa1-x)Se-2 thin-film solar cells (CIGS). It is stable at the high temperatures present during CIGS processing and is also commercially available. However, one disadvantage is the diffusion of substrate elements into the CIGS layer, which are detrimental for the performance of solar cells. We found a direct correlation between the iron (Fe) content in the CIGS layer and corresponding solar cell parameters. A deep defect related to Fe impurities could be detected by admittance spectroscopy measurements. The solar cell parameters could be well fitted by simulation with recombination at an acceptor like deep defect in the bulk of the CIGS layer. The simulated density of deep defect states correlates nicely with the Fe concentration in the CIGS layer. From this we conclude that Fe replaces an In (or Ga) site in the CIGS lattice and creates an Fe-In(2+) (or Fe-Ga(2+)) deep acceptor state in the bulk of CIGS layers, which is detrimental already at a low concentration in the sub ppm range. The simulations enabled us to estimate the maximum Fe concentration in CIGS layers which is tolerable without disturbing the performance of high-efficiency CIGS solar cells. (C) 2014 Elsevier B.V. All rights reserved.