Cu(In,Ga)Se2 (CIGS) absorber layers for thin-film solar cells were grown without sodium. Na was diffused into some of the absorbers after growth, which led to strongly improved device performance compared with Na-free cells. Efficiencies of 13.3% and 14.4% were achieved at substrate temperatures as low as 400 and 450 °C, respectively. With the post-deposition treatment, the effects of Na on CIGS growth are excluded, and most of the Na is expected to reside at grain boundaries. The dominating cause for Na-induced device improvements might be passivation of grain boundaries. © 2004 American Institute of Physics.