In this work, glass/diffusion barrier/Mo/Cu(In,Ga)Se2 stacks with and without adding NaF are investigated with the goal to determine the back surface recombination velocity. The absorber layers prepared by a three-stage co-evaporation process are characterized by time resolved photoluminescence (TRPL) and time of flight-secondary ion mass spectroscopy (TOF-SIMS). By comparison of experimental TRPL data with simulated TRPL transients calculated with Synopsys TCAD, Mo/Cu(In,Ga)Se2 interface recombination velocities for electrons of Sb,n≤1×102 cm/s (with NaF) and Sb,n≥1×105 cm/s (without NaF) are determined. SIMS analysis points towards differences in alkali metal concentrations at the back contact being the origin of different Sb,n values. Our results shine light on the large spread of experimental Sn values reported in the literature and reveal another hitherto unknown effect of alkali doping in Cu(In,Ga)Se2 semiconductors.