Oxygen Promotes the Formation of MoSe₂ at the Interface of Cu₂ZnSnSe₄/Mo

The contact, and thus the hole collection between Cu₂ZnSnSe₄ (CZTSe) and Mo, is a crucial issue to improve the performance of CZTSe solar cells. In this work, a method to improve the back contact is explored by spraying Na₃PO₄ on the surface of the Mo back contact. With the O provided from Na₃PO₄, extra MoO₂ and MoO₃ are formed at the surface of the back contact, and partial MoO₂ is transformed into MoSe₂ under high Se₂ partial pressure during the selenization process. The formation of MoSe₂ progresses from dispersed spots to a continuous layer but not from the reaction between CZTSe and Mo. Although a thick MoSe₂ layer is formed, the CZTSe device performance increases from 7.2% to 8.3% on average. This study affords new insight into the formation of MoSe₂, thus deeply strengthening the understanding of the back contact of kesterite solar cells and of two-dimensional chalcogenide devices.