Elucidating the Mechanism of an RbF Post Deposition Treatment in CIGS Thin Film Solar Cells

The impact of a rubidium fluoride post deposition treatment (RbF-PDT) on the material and device properties of Cu(In,Ga)Se-2 (CIGS) thin films and corresponding solar cells is investigated. The structure and device properties of CIGS with different PDT duration are compared. With longer PDT duration, which equals a higher amount of RbF deposited on the CIGS absorber layer, a clear trade-off is observed between increasing open-circuit-voltage (V-OC) and decreasing fill factor (FF). An optimum of the PDT duration is found increasing the efficiency by about 0.8% (absolute) compared to the Rb-free reference device. The mechanisms behind the increased V-OC are explored by various characterization methods and identified as a combination of increased carrier concentration and reduced recombination rates in the device. Possible origins for these mechanisms are discussed. Furthermore numerical simulations are used to analyze the detrimental effect of the PDT on the FF. It is found that thermally activated alkali migration into the transparent front contact could create acceptor states there, which could explain the observed FF-loss.