Ge-incorporated Cu2ZnSnSe4 (CZTGSe) thin films were prepared, and their materials and device properties were examined. The CZTGSe thin films were prepared using a two-step process comprising co-evaporation of each element and a subsequent annealing step. Ge atoms were successfully incorporated into the Cu2ZnSnSe4 thin films, and the band-gap (E-g) of CZTGSe was controlled via the full Ge/(Sn+Ge) ratio range of 0-1. In addition, the annealing environment containing GeSe2 led to CZTGSe thin films with flat surfaces, dense morphologies, and large grains comparable to their thickness. The highest efficiency achieved with the fabricated CZTGSe solar cells was 10.03%, with an open circuit voltage (V-oc) of 0.54 V. The CZTGSe thin-film solar cells exhibited an improved V-oc deficit (E-g/q - V-oc, q: electron charge) of 0.647 V, which is comparable to that of high-efficiency Cu2ZnSn(SxSe1 - (x))(4) solar cells. (C) 2015 Elsevier B.V. All rights reserved.