a School of Materials and Energy, Guangdong University of Technology,

Guangzhou 510006, China

b School of Integrated Circuits, Guangdong University of Technology, Guangzhou 510006, China

Cation substitution is a useful way to improve the properties of semiconducting Cu2ZnSnS4 thin film. In this work, partial Cu and Sn in Cu2ZnSnS4 are substituted by Ag and Ge, respectively. The (Ag,Cu)2Zn(Sn,Ge)S4 thin films were successfully fabricated using vacuum magnetron sputtering and post-sulfurization techniques. The formation of Ag & Ge co-doped Cu2ZnSnS4 structure with secondary phase is proved by XRD and Raman results. The Ag and Ge ratios depend on the composition of Cu-Ag target and the sputtering time of Ge, respectively. The direct optical band gap values of thin films increase with the increase of Ge content. When the sputtering time of Ge is 90 s, the Urbach energy of (Ag,Cu)2Zn(Sn,Ge)S4 thin films reaches the minimum value of 339 meV, revealing the reduced band tail state by Ge incorporation.