Flexible copper indium gallium selenide (CIGS)-based solar cells were developed on stainless steel (STS) substrates covered with an insulating layer. The properties of the CIGS films fabricated on STS foils coated with SiO2-based soluble material ("sol-SiO2") were compared with those on plasma enhanced chemical vapor-deposited silicon oxide ("PE-SiO2")/MoNa/STS and SLG substrates. The decrease in compensating donor due to sufficient Na doping from the insulating layer was attributed to higher open-circuit voltage and fill factor. Furthermore, the double graded Ga composition in the CIGS films grown on the STS substrates was preserved, and the open-circuit voltage was enhanced with a minor decrease in the short-circuit current. As a result, solar cells with 14% efficiency were produced on sol-SiO2/STS samples with a Fe atom diffusion barrier and external Na incorporation. This was far higher than the 9.5% and 12.8% obtained from the solar cells on PE-SiO2/MoNa/STS and SLG, respectively. (C) 2016 Elsevier Ltd. All rights reserved.