The planarization of flexible stainless steel (SS) foils was investigated for the application of flexible solar cells. The sol-gel SiO2 film containing nanoparticles was evaluated for a buffer layer on SS foils, and methods to improve the adhesion of SiO2 film to SS foil were studied. The improvement of adhesion by adding Al2O3 matrix was discussed by analyzing the interface between Al2O3-SiO2 film and SS foil. The usefulness of sol-gel buffer layer was also verified by comparing the performance of single junction a-Si:H thin film solar cells fabricated on bare SS foil and buffer layer-coated SS foil. The cell characteristics such as V-oc, J(sc), fill factor, and efficiency were all improved by adopting the buffer layer. The efficiency of the cell on buffer layer-coated and non-textured SS foil was 6.1% whereas the efficiency was 4.9% on bare SS foil. (C) 2012 Elsevier Ltd. All rights reserved.