Engineering Of Sodium Supply Into Cu(In, Ga)Se-2 Thin-Film Solar Cells

We report the engineering of sodium (Na) supply into Cu(In,Ga)Se-2 (GIGS) for thin-film solar cells. The methods of incorporation of Na and the relative Na content in CIGS were varied using a combination of sodalime glass (SLG) substrate and NaF as Na sources, and SiO, as diffusion barriers. The sample structures were CIGS/Mo/SiOx/SLG, CIGS/Mo/SLG, CIGS/15 nm-NaF/Mo/SLG, and post-15 nm-NaF/CIGS/Mo/SLG. The evaporation of NaF precursor prior to the CIGS process and the post-treatment of NaF were used for additional Na supplies. The Na content significantly increased in the Na-supplied CIGS as compared with the Na-deficient sample with the SiOx barrier. The additional NaF evaporation significantly enhanced the Na content in the CIGS films. The Na-containing CIGS solar cells showed superior photovoltaic performances compared with the Na-deficient solar cell. The open circuit voltage and fill factor were largely improved by supplying Na. The additional incorporation of Na via NaF post-treatment enhanced the cell efficiency up to 14% using both CdS and ZnS buffer layers.