Control of Na Diffusion from Soda-Lime Glass and NaF Film into Cu(In, Ga)Se2 for Thin-Film Solar Cells

We report on the effect of Na incorporation into Cu(In, Ga)Se-2 (CIGS) for thin-film solar cells. Na ions were supplied by NaF films with different thicknesses, while those from the soda-lime glass substrate were controlled by a presence or an absence of a SiOx barrier. Gradually increasing a Na content in CIGS was found in the structures of CIGS/Mo/SiOx/SLG, CIGS/Mo/SLG, CIGS/5 nm-NaF/Mo/SiOx/SLG, CIGS/5 nm-NaF/Mo/SLG, CIGS/15 nm-NaF/Mo/SiOx/SLG, and CIGS/15 nmNaF/Mo/SLG With increasing the Na content, the CIGS (112) preferential orientation growth became dominant, while the (220/204) one was nearly constant. The grain size decreased with increasing the Na content. The cell efficiency (eta) increased for relatively low Na content however the thick NaF led to a degradation of eta because of poor adhesion between the CIGS and the Mo. The sample with the CIGS/5 nm-NaF/Mo/SiOx/SLG structure achieved the comparable (112)/(220/204) peak ratios, grain sizes, as well as the cell performances to a CIGS/Mo/SLG structure that contains the same Na content in the CIGS.