Beyond the use of fullerenes as electron-transporting layers in perovskite solar cells, their introduction into a perovskite active layer has been ascertained as a very promising strategy for device improvement. In this sense, this Perspective summarizes the studies in which perovskite:fullerene systems were employed, covering the different methodologies for introducing fullerenes inside the perovskite layer. In particular, fill factor was the most benefited parameter, which was ascribed to minimized pinhole density and fullerene passivating role. We discussed the importance of their ability to passivate trap states and, in this regard, focused on the affinity of fullerenes toward these sites. Additionally, the different nature of the fullerene and its environment in the active layer were found to determine the proper distribution of fullerene so that they could passivate the defects along grain boundaries. Understanding this mechanism would provide further insights for new methodologies and fullerene derivatives with enhanced trap-passivating ability.