Thin-film solar cells with the layer sequence Mo/Cu(In,Ga)Se-2/CdS/i-ZnO/n-ZnO on soda lime float glass are fabricated in in-line systems with 30 cm deposition width. The Cu(In,Ga)Se-2 layer is co-evaporated from four elemental sources. We adjust the Ga/(Ga+In) ratio and thus the band-gap energy (E-g) within the bulk of the film thickness to optimise device performance. The investigation covers gradients with maximum E-g at the Mo back contact and decreasing E-g towards the window. A systematic variation of the gradient's slope and level height is performed. Comparing films with similar growth histories, the quantum efficiency, current-voltage characteristics, and compositional depth profiles reveal that both the open-circuit voltage and the short-circuit current density are enhanced with steeper gradients and lower Ga/(Ga+In) at the window interface. (C) 2003 Elsevier Science B.V. All rights reserved.