Mechanism study of Zn/Sn ratio on the MoSe_2 formation in Zn-rich Cu_2ZnSnSe_4 absorber layer

摘要

Controlling the thickness of the interfacial MoSe_2 layer in Cu_2ZnSnSe_4 (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 MoSe_2 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 MoSe_2 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 ℃, this mainly involves Sn- and CuSn-selenides; above 450 ℃, this mainly involves Cu- and Zn- selenides. Since MoSe_2 grows significantly above 450 ℃, Cu- and Zn-selenides are the main reasons for reducing the thickness of MoSe_2. The fact that MoSe_2 growth generally occurs above 450 ℃ 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 MoSe_2, resulting in a thinner layer.