Using a 3-dimensional atom probe (or atom probe tomography) method, we measured the composition ratios around the grain boundaries of Cu2Sn1-xGexS3 (CTGS) films used for the bottom cells of thin-film double-junction solar cells. We found that the Ge composition ratio at the grain boundaries in the CTGS films was higher than that inside the grains when the films were fabricated by co-sputtering depositions of Cu and Sn followed by sulfurizations with S and GeS2 vapors. We also evaluated the bandgaps of Cu2Sn1-xGexS3 with changing the x value by measuring external quantum efficiency spectra of the solar cells. From these results, we have obtained the energy level diagram around the grain boundaries, and revealed that the conduction band minimum at the grain boundaries is 0.05 eV higher than that inside the grains, which suppresses recombination of photogenerated carriers. (C) 2014 Elsevier B.V. All rights reserved.