The effect of Na in polycrystalline and epitaxial single-crystal CuIn _1-xGa_xSe₂

Na is found to improve the performance of Cu(In,Ga)Se₂ (CIGS) solar cells although the mechanism is not clear. This paper briefly reviews some of the observations on Na in CIGS polycrystalline and epitaxial films. Experiments suggest weak electrical effects of Na within grains, primarily by reducing compensation and in some cases by enhancing acceptor concentrations. As it segregates to surfaces, it has been suggested that Na acts through passivation of grain-boundary defects. However, its main effect is on device open-circuit voltage (and somewhat on fill factor), which does not correlate with grain size but rather with bulk grain defects. The Na concentration scales somewhat with grain boundary density averaged over large areas of film, suggesting that it may be active there. Modest Na concentrations often increase grain size in polycrystals, although not when the grain size is already large, and often changes preferred orientation. Na segregates to the surfaces of CIGS grains. These results suggest that it may act at the surface, modifying growth mechanisms or defect organization during growth. TEM evidence shows that strong concentration of Na in the grain boundaries, sufficient to passivate surface defects by itself, is unlikely to occur. Finally, Na is removed from the surface of CIGS by aqueous solutions, such as those used to form the heterojunction. It is concluded that Na acts at the surface during growth to organize point defects, probably including by reduction of vacancy populations, within the bulk grains but that it has no residual effect once growth is complete.