An alkali-metal containing (AC) layer, specifically sodium (Na), is necessary for improving Cu(InxGa1-x) Se-2 (CIGS) solar cell performance on an alkali-free flexible substrate. However, we found that Na from the AC precursor layer diffused not only into the CIGS absorber layer but also into our flexible metal foil substrate, the surface of which was covered with an Al2O3 layer, and thus creating Na depletion in the CIGS layer. In this work, newly developed alkali-metal block (AB) layers are introduced between the AC layers and the metal foil substrates in order to suppress Na diffusion into the substrates and enhance Na diffusion into the CIGS layers. Using different AB layer materials-Ti, TiO2, and TiN-and AC layers with systematically-varied thickness, Na concentrations in the CIGS layers and the solar cell efficiencies are investigated. Ti and TiN AB layers efficiently enhance the Na diffusion into the CIGS absorber layers. As a result, a high efficiency of 17.6% is obtained on the metal foil substrate using optimally prepared AC and AB layer of TiN. The AB layer controls the Na diffusion from the AC layer in one direction, and thus contributes to the precise control of Na concentrations in the CIGS layer. Hence, high efficiency solar cells on a variety of flexible substrates will be achieved comparable to those on soda-lime glass substrates. (C) 2014 Elsevier B.V. All rights reserved.