Ionic Liquid‐Tuned Crystallization for Stable and Efficient Perovskite Solar Cells

Long‐term operational stability is a significant challenge for perovskite solar cells to become a commercial photovoltaic technology. Herein, a feasible approach is presented to improve both thermal and environmental stability of organic–inorganic hybrid perovskites by introducing ionic liquid butylammonium acetate (BAAc) to coordinate the PbI 2 precursor solution through strong bonding interactions to tune crystallization of perovskites. Inverted planar solar cells based on BAAc‐containing tri‐cation perovskites result in a high efficiency of over 20%. More importantly, after 700 h of aging at 85 °C in the nitrogen environment and 650 h storage in the ambient condition with a relative humidity of 35 ± 5%, the BAAc‐treated devices are shown to be much more stable as maintained 79.5% and 76.3% of the initial power conversion efficiency, respectively. This work provides a promising strategy to tune crystallization of perovskites to improve long‐term stability of perovskite solar cells.