In this study, we quantify the impact of C₆₀-passivation layers in Cs_0.15FA_0.85PbI_2.75Br_0.25 double-cation perovskite solar cells. We apply a combination of impedance spectroscopy, photoluminescence (PL) spectroscopy, and X-ray diffraction (XRD) to identify the origin for the increase in power conversion efficiencies and operational stability for solar cells fabricated with C₆₀/ZnO electron transport layer (ETL) versus reference cells with a ZnO ETL. XRD reveals an increase in PbI₂ while PL spectroscopy reveals an increase in Br-rich regions in the perovskite bulk in devices containing C₆₀ interlayers. We apply impedance spectroscopy to quantify the electrochemical dynamics in both solar cell architectures. Solar cells with C₆₀/ZnO ETL demonstrate less pronounced and slower electrochemical dynamics in the impedance spectra than solar cells with ZnO ETL. We conclude that C₆₀ leads to the formation of PbI₂-rich and Br-rich domains in the perovskite absorber layer, resulting in reduced recombination losses and improved operational stability.