Sputtered (Zn,Mg)O buffer layer for band offset control in Cu2ZnSn(S,Se)4 solar cells

We fabricated Cu2ZnSn(S,Se)(4) (CZTSSe) solar cells with (Zn,Mg)O buffer layers as an alternative to the CdS buffer layer for the improvement of cell performance, where the (Zn,Mg)O layers are deposited by sputtering. However, the solar cell efficiency decreased with the (Zn,Mg)O layer as compared with the CdS layer. Photoluminescence measurements indicated that the damage near the surface of the CZTSSe absorber was induced by the sputtering. To suppress the damage, a 10-nm-thick CdS layer was deposited on the absorber before sputtering. As a result, the efficiency achieved with the (Zn,Mg)O layer was the same as that with the CdS layer. To further improve the efficiency of the cell with the (Zn,Mg)O layer, it is necessary to eliminate sputtering damage. In addition, the conduction band offset of the (Zn,Mg)O/CZTSSe interface is controllable by varying the Mg content. Therefore, the (Zn,Mg)O buffer layer can be suitable against a large-band-gap CZTSSe absorber. (C) 2014 The Japan Society of Applied Physics