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

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.