We investigated the effects of the microstructures of molybdenum (Mo) back contacts on sodium (Na) diffusion from sodalime glass into a Cu(In,Ga)Se2 (CIGS) absorber as a function of the sputter deposition pressure during preparation of the Mo contact layer. The surface characteristics of the Mo layers more significantly affected the diffusion of Na ions into the CIGS compared with the Mo bulk. The Na ion diffusion depended strongly on the amount of oxygen adsorbed onto the Mo layer surfaces. Secondary ion mass spectroscopy results showed that Na accumulated in a layer (Na–O compound) on the Mo surface (the CIGS/Mo interface), and this layer served as a primary source of Na ions diffusing into the CIGS. A trilayered Mo back contact structure was prepared in an effort to decouple the functions of electrical conductance and Na diffusion. The ability of this surface to control the Na concentration in a CIGS absorber is discussed. Copyright © 2012 John Wiley & Sons, Ltd.