The presence of small amounts of sodium has been shown to improve the electronic performance of Cu(In,Ga)Se-2 (CIGS) solar cells, but the origins of this effect have not yet been fully resolved. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using density-functional-theory-based calculations. We find no direct way how the creation of Na-related point defects in bulk CIS would enhance p-type conductivity. Instead, we demonstrate that Na reduces copper mass transport due to the capture of copper vacancies by Na-Cu defects. This finding provides an explanation for experimental measurements where the presence of Na has been observed to decrease copper diffusion. The suggested mechanism can also impede V-Cu-related cluster formation and lead to measurable effects on defect distribution within the material. (C) 2013 AIP Publishing LLC.