Mechanism study of Zn/Sn ratio on the MoSe2 formation in Zn-rich Cu2ZnSnSe4 absorber layer

Abstract

Controlling the thickness of the interfacial MoSe2 layer in Cu2ZnSnSe4 (CZTS) thin film solar cells can have a profound impact on device performance. The Zn/Sn ratio in the absorber layer of CZTS solar cells significantly affects the thickness of MoSe2 after selenization; however, the underlying mechanism remains unknown. In this study, we employed a fast-cooling selenization system to study the phase transformation and analyzed the back side of the CZTS absorber layer to characterize the mechanism by which Zn/Sn ratio affects the thickness of the MoSe2 absorber layer. Experiments revealed the formation of various metal-selenide phases at the bottom of the CZTS absorber layer during selenization. At temperatures below 400 °C, this mainly involves Sn- and CuSn-selenides; above 450 °C, this mainly involves Cu- and Zn- selenides. Since MoSe2 grows significantly above 450 °C, Cu- and Zn-selenides are the main reasons for reducing the thickness of MoSe2. The fact that MoSe2 growth generally occurs above 450 °C means that a higher Zn/Sn ratio leads to the formation of more Zn-selenides, such that there is less Se available for the formation of MoSe2, resulting in a thinner layer.