Photovoltaic devices based on organic–inorganic hybrid perovskites have engaged tremendous attention due to the enormous increase in power conversion efficiency (PCE). However, defect states formed at grain boundaries and interfaces hinder the achievement of PCE. A prospective strategy to both reduce interfacial defects and control perovskite growth is the passivation of interfaces. The passivation of the electron-transporting layer/perovskite interface with ultrasmall carbon dots (CDs) with suitable chemical composition and functional groups on their surface may simultaneously affect the morphology of a perovskite layer, facilitate charge carriers extraction, and suppress interfacial recombination. Here, we show that CDs synthesized from diamine precursors may be used as an interlayer at the SnO2/FACsPbI3 interface. Ultrasmall CDs form a smooth, thin layer, providing better perovskite layer morphology. CD interlayers result in an increased average perovskite grain size, suppress the formation of small grains, and improve charge carriers’ extraction. As a result, photovoltaic devices with CD interlayers demonstrate a higher PCE due to the increased short-circuit current density and fill factor. These findings provide further insight into the construction of interfaces based on carbon nanomaterials.