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)Se2 (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 nm-NaF/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 (η) increased for relatively low Na content however the thick NaF led to a degradation of η 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.