Rubidium Fluoride Absorber Treatment for Wide-Gap and Bifacial Ag(In,Ga)Se2 Solar Cells

This study investigates the impact of a RbF post-deposition treatment (PDT) on the properties of Cu-free, wide-gap (E_G = 1.5 eV) Ag(In,Ga)Se₂ (AIGS) solar cells with either opaque Mo or transparent In₂O₃:W back contacts. The formation of a Rb-In-Se surface phase and Cd in-diffusion from the buffer into this layer are detected. In addition, Rb is mainly located in Ag-depleted and In-enriched grain boundaries and at the back interface. The RbF-PDT leads to decreased fill factor (FF) values, without providing any gain in open-circuit voltage (V_OC). This FF deterioration is more pronounced when a In₂O₃:W back contact is used. It is suggested that transport barriers form at the Rb-In-Se/AIGS front and at the AIGS/GaO_x/In₂O₃:W back interfaces after RbF-PDT. In addition, the GaO_x thickness at the rear electrode increases. The absence of a V_OC boost may be explained by the missing doping increase after the RbF-PDT. Lastly, the AIGS solar cells from this study show slightly lower efficiencies as compared to Cu-containing devices with the same E_G but higher Ga content. However, the collection at rear illumination is marginally improved, reaching up to 72% bifaciality in short-circuit current for the best cell with an In₂O₃:W back contact.