Highly efficient flexible CuIn0.7Ga0.3Se2 solar cells with a thick Na/Mo layer deposited directly on stainless steel

•High efficiency flexible CuIn0.7Ga0.3Se2 solar cells are introduced with extra Na.•Back contact bi-layers of Mo/Na-doped Mo without a buffer layer are proposed.•The best efficiency of 15.04% is achieved with a thick Na-doped Mo layer.•Extra Na incorporation results in an enhanced open-circuit voltage of 608mV.•Carrier concentration and leakage current are critically determined by extra Na. 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 600nm Mo-capping and 900nm 10% Na-incorporating Mo layers. This enhancement is attributed mainly due to the substantial increase of Voc as a result of higher carrier concentration and lower leakage current of the extra Na-involved absorber.