Cu(In, Ga)Se Solar cells with graded band gap and efficiencies up to 13% have been fabricated on transparent ZnO:Al back contacts. The back contact structure includes a transparent 10nm thin Mo interlayer with NaF precursor between the ZnO:Al and the Cu(In, Ga)Se absorber that transforms the blocking ZnO:Al/Cu(In, Ga)Se, interface into an Ohmic back contact. To investigate the electronic quality of the back contact, the cells are analyzed by internal quantum efficiency measurements under illumination from front and back side. A new semianalytical model for the quantum efficiency of graded band gap absorbers yields quantitative information about the back contact recombination velocity as well as optical and electronic material parameters of the absorber layer. Band gap grading significantly increases carrier collection. However, in the immediate vicinity of the back contact carrier collection is limited by a high ratio of back contact recombination velocity and diffusion constant S-n/D-n >= 10(7) cm(-1) (c) 2007 Elsevier B.V. All rights reserved.