All-inorganic perovskites hold promise for tandem or semi-transparent photovoltaics. However, they often suffer from poor crystallization quality and high densities of bulk and interfacial defects, severely limiting device performance. To address these issues, an ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIMAc) is incorporated into the CsPbI_1.5Br_1.5 perovskite precursor to modulate the crystallization rate, thus successfully obtaining high-quality films. After perovskite crystallization, EMIMAc distributes across its surface, bulk, and the buried TiO₂/perovskite interface. At the surface and in the bulk, it passivates Pb²⁺-related and halogen defects; at the buried interface, it passivates oxygen vacancies on TiO₂ and optimizes the energy-level alignment. Thus, EMIMAc serves as a dual-functional agent for defect passivation and interface modification. Notably, charge transport dynamics confirms that bulk defect passivation dominates the performance enhancement, with interface optimization playing a subsidiary role. The resultant carbon-based hole-transport-layer-free CsPbI_1.5Br_1.5 perovskite solar cell achieves a power conversion efficiency of 13.38%, substantially higher than that of the control device (11.44%). Moreover, EMIMAc incorporation significantly improves the device stability. After storage in ambient air for 65 days, the unencapsulated device retains 95% of its initial efficiency; after 120 h of continuous maximum power point tracking under 100 mW/cm² illumination, the encapsulated device maintains 93% of its initial efficiency, demonstrating superior stability compared to the control device.