In this contribution, the effectiveness of an RbF post deposition treatment (PDT) is evaluated in dependence on the Cu content of the absorber layer of Cu(In,Ga)Se 2 solar cells. It is shown that the PDT only acts beneficially on the open-circuit voltage and fill factor (FF) on samples with rather high Cu content, while it deteriorates all parameters of the solar cells in samples with low Cu content. In order to clarify the behavior of the open-circuit voltage, the well-known exchange mechanism of Rb and Na during the PDT is analyzed as a function of the Cu content of the absorber layers and discussed in regard to theoretical publications. Furthermore, a model explaining the observed effect on the FF based on the formation of an RbInSe 2 (RIS) layer during the RbF-PDT is proposed. The model supposes that the RIS layer acts as a barrier for the photocurrent and therefore lowers the FF. It is evaluated theoretically in dependence of the properties of the RIS layer using one-dimensional solar cell capacitance simulator (SCAPS) simulations. Finally, the proposed model is also tested and confirmed experimentally by directly depositing RIS onto untreated Cu(In,Ga)Se 2 layers.