The influence of sodium on the molybdenum/Cu(In,Ga)Se-2 interface recombination velocity, determined by time resolved photoluminescence

In this work, glass/diffusion barrier/Mo/Cu(In,Ga)Se-2 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)Se-2 interface recombination velocities for electrons of S-b,S-n <= 1 x 10(2) cm/s (with NaF) and S-b,S-n >= 1 x 10(5) cm/s (without NaF) are determined. SIMS analysis points towards differences in alkali metal concentrations at the back contact being the origin of different S-b,S-n values. Our results shine light on the large spread of experimental S-n values reported in the literature and reveal another hitherto unknown effect of alkali doping in Cu(In,Ga)Se-2 semiconductors.