Bi-layered back contact structures consisting of Mo-capping and Na-doped Mo layers have been investigated for the purpose of providing extra Na for CuIn0.7Ga0.3Se2 (CIGS) absorbers for high efficiency solar cells on a stainless steel substrate. This approach utilizing a thick back contact layer does not require a buffer layer on the substrate. The cell performance depends largely on the content of Na in the Mo:Na layer as Na easily diffuses into the absorber layer. The best cell efficiency of 15.04% is obtained for the cell incorporating the 600 nm Mo-capping and 900 nm 10% Na-incorporating Mo layers. This enhancement is attributed mainly due to the substantial increase of V-OC as a result of higher carrier concentration and lower leakage current of the extra Na-involved absorber. (C) 2015 Elsevier B.V. All rights reserved.