Ionic Liquid Treatment for Highest‐Efficiency Ambient Printed Stable All‐Inorganic CsPbI 3 Perovskite Solar Cells

All‐inorganic cesium lead triiodide (CsPbI 3 ) perovskite is well known for its unparalleled stability at high temperatures up to 500 °C and under oxidative chemical stresses. However, upscaling solar cells via ambient printing suffers from imperfect crystal quality and defects caused by uncontrollable crystallization. Here, the incorporation of a low concentration of novel ionic liquid is reported as being promising for managing defects in CsPbI 3 films, interfacial energy alignment, and device stability of solar cells fabricated via ambient blade‐coating. Both theoretical simulations and experimental measurements reveal that the ionic liquid successfully regulates the perovskite thin‐film growth to decrease perovskite grain boundaries, strongly coordinates with the undercoordinated Pb 2+ to passivate iodide vacancy defects, aligns the interface to decrease the energy barrier at the electron‐transporting layer, and relaxes the lattice strain to promote phase stability. Consequently, ambient printed CsPbI 3 solar cells with power conversion efficiency as high as 20.01% under 1 sun illumination (100 mW cm −2 ) and 37.24% under indoor light illumination (1000 lux, 365 µW cm −2 ) are achieved; both are the highest for printed all‐inorganic cells for corresponding applications. Furthermore, the bare cells show an impressive long‐term ambient stability with only ≈5% PCE degradation after 1000 h aging under ambient conditions.