Nanocrystal-based Cu2Zn(SnyGe1-y)(SxSe4-x) (CZTGeSSe) thin-film solar cell absorbers with tunable band gap have been prepared. Maximum solar-conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu2ZnSn(SxSe4-x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the absorber film during the high-temperature selenization treatment, although some Ge loss from the absorber surface is still observed following this step. Despite limitations imposed by elemental losses present at the absorber surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage-dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.