Application of multi-buffer layer of (Zn,Mg)O/CdS in Cu2ZnSn(S,Se)4 solar cells

• Cu2ZnSn(S,Se)4 layers are fabricated by sulfurization and selenization.
• Multi-buffer layer of (Zn,Mg)O/thin-CdS layers is adopted as a new buffer layer.
• Multi-buffer layer realize similar efficiency to conventional thick CdS buffer.
• Multi-buffer layer can be useful in new buffer for wider band gap Cu2ZnSn(S,Se)4.

(Zn,Mg)O (ZMO) buffer layer has attracted attention for having the potential to control the conduction band offset of buffer layer and large band-gap (Eg) Cu2ZnSn(S,Se)4 (CZTSSe) absorber interface, where the ZMO layer is deposited by the sputtering. However, the solar cell efficiency is decreased with the ZMO layer as compared with the CdS layer. The decrease in conversion efficiency is attributed to the sputtering damage on the absorber and high light reflection from the surfaces of CZTSSe solar cells. To completely suppress the damage, a CdS layer with very thin thickness of 20 nm is inserted between the ZMO layer and the CZTSSe layer. In addition, MgF2 layers are deposited on CZTSSe solar cells as anti-reflection coating. Ultimately, the solar cell with multi-buffer layer of ZMO/thin-CdS is almost same level as that with the CdS layer. Therefore, the multi-buffer layer can be an appropriate buffer layer of the large-Eg CZTSSe layer.