The instability of organic cations in lead halide perovskite materials is a major obstacle for the commercial breakthrough of perovskite photovoltaics due to desorption of organic cations during the thermal annealing and device operation. Herein, a novel strategy is reported to improve the performance and stability of organic halide perovskite solar cells containing organic cations by adding a small amount of the ionic liquid methylammonium difluoroacetate (MADFA). Nuclear magnetic resonance and Fourier-transform infrared spectroscopy measurements show that MADFA can anchor the organic cations via hydrogen bonding and enhance the Pb–O interaction in perovskite precursors, leading to the retardation of the perovskite crystallization and improved stability of the perovskite precursor solution. Dynamic light scattering and scanning electron microscopy verify the defect-passivation effect of MADFA on the perovskite precursors and films. The passivated perovskite film shows superior photo carrier dynamics as investigated by time-resolved photoluminescence and transient absorption spectra. Moreover, the hydrogen bonding of the perovskite with MADFA imparts excellent ambient and thermal stability to the film as revealed by X-ray diffraction measurements. As a result, devices with a high efficiency of 21.46% and excellent stability over 180 days in nitrogen atmosphere at room temperature are achieved with the ionic liquid.