Studies of compositional dependent Cu₂Zn(Ge_xSn_1-x)S₄ thin films prepared by sulfurizing sputtered metallic precursors

Ge-substituted Cu₂ZnSnS₄ (CZTS) thin films are promising for solar cell manufacturing, especially for graded-band-gap solar cells and multi-junction solar cells. However, there is no systematic research on the change of structural properties of Cu₂Zn(Ge_xSn_1-x)S₄ (CZTGS) as the function of alloy composition x. In this work, polycrystalline CZTGS thin films were prepared by sulfurizing sputtered Zn/Cu/Ge/Sn metallic precursor stacks. Energy-dispersive X-ray spectroscopy (EDS) line scans conducted in a transmission electron microscopy (TEM) system shows uniform element distribution (except of some ZnS segregation) throughout the synthesized films. The analysis of X-ray diffraction (XRD) and Raman spectroscopy confirms that Cu₂ZnGeS₄ is successfully obtained by sulfurizing metallic stacks at 580 °C for 2 h. The lattice constants a and c of CZTGS as derived from XRD patterns follow the Vegard's rule that lattice constants vary linearly with Ge content (x). Raman spectra shows an A1 mode shift towards high frequency when Sn is gradually replaced by Ge. Chemical composition study indicates the increase in x value leads to higher (Ge + Sn) element loss during the sulfurization process, which can be explained by the high vapour pressure of Ge sulfides. By alloying with Ge, the grain size of CZTS is enlarged which also leads to a rough surface.