While caesium lead bromide (CsPbBr) is promising for highly stable perovskite solar cells (PSCs), the usual solution-based methods require tedious multistep spin coating processes, which imposes a practical barrier against scaling up to large areas for industrial exploitation. Although sequential vapour deposition (SVD) can meet commercial requirements, these films are limited by high trap density and impure phases, resulting in poor performance of PSCs. Here, we obtained low-trap density and effectively phase-pure CsPbBr films (grain size > 3 μm, trap density < 4 × 10 cm) by systematic defect and phase management. With the identification of a molecular ionic liquid from theoretical simulation, we find that such a designer molecule can form multiple bonding interactions with the perovskite phase. This results in significantly enhanced crystallization of the CsPbBr phase, and more importantly, effective passivation of well recognized Cs- and Br-vacancy defects. CsPbBr PSCs with simplified architecture using carbon as electrodes without hole transport layer (HTL) achieved highest power conversion efficiency (PCE) of up to 11.21% for small area devices (0.04 cm) and 9.18% for large area devices (1 cm). The unencapsulated devices exhibited excellent long-term stability, maintaining over 91% of the initial PCE after 100 days in ambient air at a humidity of ∼55%. This work also provides a valuable approach to process phase-pure, low-defect, and large-area inorganic CsPbBr perovskite films for efficient and stable optoelectronic devices.