In-line Cu(In,Ga)Se2 co-evaporation processes with graded band gaps on large substrates

Thin-film solar cells with the layer sequence Mo/Cu(In,Ga)Se2/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)Se2 layer is co-evaporated from four elemental sources. We adjust the Ga/(Ga+In) ratio and thus the band-gap energy (Eg) within the bulk of the film thickness to optimise device performance. The investigation covers gradients with maximum Eg at the Mo back contact and decreasing Eg 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.