A solution-processed perovskite-fullerene (PCBM) hybrid material significantly reduces hysteresis and recombination loss in planar diodes. The hybrid material, formed in a single step, exhibits an efficient electrically coupled microstructure with PCBM homogeneously distributed at perovskite grain boundaries. PCBM passivates key PbI₃⁻ antisite defects during perovskite self-assembly, as shown by theory and experiment. Photoluminescence transient spectroscopy confirms that PCBM promotes electron extraction. The hybrid material is used in planar solar cells with low hysteresis and enhanced photovoltage. Conductive AFM studies reveal memristive properties of perovskite films. PCBM reduces electric field-induced anion migration, which may cause hysteresis and unstable diode behavior. Perovskite solar cells are attractive due to efficient ambipolar transport and strong light absorption. Progress in perovskite photovoltaics has benefited from mesoporous scaffolds that reduce the need for long minority carrier drift. Planar-electrode devices are important for applications such as photodetectors, lasers, and flexible photovoltaics. However, planar perovskite rectifying junction devices suffer from severe hysteresis and recombination, likely due to defective grain boundaries. Hysteresis depends on device architecture, with inverted structures showing less hysteresis but lower open-circuit voltage. Heterojunction contact engineering has been proposed to address hysteresis, such as modifying the TiO₂ contact layer with a C₆₀ self-assembled monolayer. However, these solid-state post-treatments typically involve long annealing steps at elevated temperatures, introducing undesirable complexity. Adding passivants to the interface fails to address defects throughout the bulk. A solution-phase in situ passivation strategy is pursued to enable simple low-temperature processing and efficient passivation throughout the grain boundaries in the perovskite active layer. Mixed materials made from solutions containing both perovskites and PCBM show enhanced photovoltaic performance and reduced hysteresis compared to control devices. The hybrid material exhibits a substantial voltage enhancement and higher fill factor compared to PCBM-free and bilayer PCBM-perovskite controls. DFT analysis shows that PCBM interacts with excess-halide-associated defects at grain boundaries, passivating Pb-I antisite defects. PCBM binds iodide-rich defect sites on grain boundaries and/or unincorporated halides, reducing anion migration. This passivation suppresses the formation of deep traps, improving electronic properties. Conductive AFM studies reveal that the hybrid material has higher conductivity near grain boundaries at positive bias voltages, consistent with PCBM accumulating near grain boundaries and providing continuous pathways for electron egress. The hybrid material shows suppressed hysteresis under all conditions, indicating that PCBM influences electronic properties whenA solution-processed perovskite-fullerene (PCBM) hybrid material significantly reduces hysteresis and recombination loss in planar diodes. The hybrid material, formed in a single step, exhibits an efficient electrically coupled microstructure with PCBM homogeneously distributed at perovskite grain boundaries. PCBM passivates key PbI₃⁻ antisite defects during perovskite self-assembly, as shown by theory and experiment. Photoluminescence transient spectroscopy confirms that PCBM promotes electron extraction. The hybrid material is used in planar solar cells with low hysteresis and enhanced photovoltage. Conductive AFM studies reveal memristive properties of perovskite films. PCBM reduces electric field-induced anion migration, which may cause hysteresis and unstable diode behavior. Perovskite solar cells are attractive due to efficient ambipolar transport and strong light absorption. Progress in perovskite photovoltaics has benefited from mesoporous scaffolds that reduce the need for long minority carrier drift. Planar-electrode devices are important for applications such as photodetectors, lasers, and flexible photovoltaics. However, planar perovskite rectifying junction devices suffer from severe hysteresis and recombination, likely due to defective grain boundaries. Hysteresis depends on device architecture, with inverted structures showing less hysteresis but lower open-circuit voltage. Heterojunction contact engineering has been proposed to address hysteresis, such as modifying the TiO₂ contact layer with a C₆₀ self-assembled monolayer. However, these solid-state post-treatments typically involve long annealing steps at elevated temperatures, introducing undesirable complexity. Adding passivants to the interface fails to address defects throughout the bulk. A solution-phase in situ passivation strategy is pursued to enable simple low-temperature processing and efficient passivation throughout the grain boundaries in the perovskite active layer. Mixed materials made from solutions containing both perovskites and PCBM show enhanced photovoltaic performance and reduced hysteresis compared to control devices. The hybrid material exhibits a substantial voltage enhancement and higher fill factor compared to PCBM-free and bilayer PCBM-perovskite controls. DFT analysis shows that PCBM interacts with excess-halide-associated defects at grain boundaries, passivating Pb-I antisite defects. PCBM binds iodide-rich defect sites on grain boundaries and/or unincorporated halides, reducing anion migration. This passivation suppresses the formation of deep traps, improving electronic properties. Conductive AFM studies reveal that the hybrid material has higher conductivity near grain boundaries at positive bias voltages, consistent with PCBM accumulating near grain boundaries and providing continuous pathways for electron egress. The hybrid material shows suppressed hysteresis under all conditions, indicating that PCBM influences electronic properties when