In this work, we report studies on the incorporation of Ge into Cu2ZnSnS4 (CZTS) films and a systematic investigation on the formation and properties of CZTS:Ge by a sequential process based on the sputtering of Cu/Sn/Cu/Zn metallic stacks followed by a reactive annealing under S atmosphere. Solar cells with the glass/Mo/CZTGS/CdS/i-ZnO/ITO architecture were fabricated and the role of the Ge-layer position as well as the thickness on the devices parameters was studied. For this purpose, different layer arrangements of the stacks were investigated, including: Mo/Ge/Cu/Sn/Cu/Zn, Mo/Cu/Sn/Cu/Zn/Ge and Mo/Ge/Cu/Sn/Cu/Zn/Ge. Devices made with the stack Mo/Cu/Sn/Cu/Zn/Ge resulted in best performance and were selected for further studies by varying the Ge layer thickness, concluding that with 5 nm thick Ge layer a 27% efficiency improvement over the reference cell was obtained (from 5.5% up to 7%). The EQE curve showed an improvement in the collection of photo generated carriers. The carrier concentration was determined to be in the range of 1 x 10(16) - 1 x 10(17) cm(-3) for CZTGS films, which is in the range of the values reported for CZTS.