Abstract The ability to alloy Cu2ZnSn(S,Se)4 with Ge provides a unique ability to band-gap engineer the absorber film by controlling the relative cation ratios. In here, a preliminary study on adjusting the Ge to Sn ratio is shown to significantly improve the device performance of CZTSSe thin film solar cells. CZTGeSSe solar cell with total area power conversion efficiency as high as 8.4% has been realized using a nanocrystal-based thin film deposition process. The selenized CZTGeSSe thin film exhibits a bi-layer structure where the thin sintered large-grain layer could be responsible for the poor red-response in external quantum efficiency of the resulting solar cell.