The impacts of Ge alloying on crystal growth and device properties of kesterite-based CZTSSe thin film solar cells fabricated by chalcogenization of sputtered stacks in S/Se ambient were investigated. It was found that Ge-alloyed CZTSSe material improved the grain growth, compactness of film texture, and crystallinity of absorber layers as a consequence of the device efficiencies were enhanced from ∼3 to 6%. The investigations on optoelectronic characteristics of devices illustrated that the improvements in devices were mainly governed by decrease in diode ideality factor, suppression of crossover effect between white and dark J–V curves, and reduction of defect level in Ge-alloyed CZTSSe solar cell device. These results suggest the possibility to achieve a further improvement in the optoelectronic characteristics of the devices that could be accomplished by optimization of the technological processes with a fine-tuning of the Ge content in the layers.