Ionic liquids engineering for high-efficiency and stable perovskite solar cells

Perovskite solar cells (PSCs) have been widely investigated in the past decade with a certificated record efficiency up to 25.2%, which is comparable to the inorganic thin-film and crystalline silicon photovoltaics. Further improving the device photovoltaic performance and long-term stability has been pursued to impel commercial application. Ionic liquids, an old class of compounds that contain large asymmetric organic cations coupled with organic or inorganic anions, have been used in high-performance PSC devices with enhanced efficiency and stability due to their unique physicochemical properties. Herein, the recent progress of ionic liquids engineering in PSCs is comprehensively reviewed and the potential mechanisms on device performance and stability are illustrated. Specifically, the main beneficial effects of ionic liquids including crystallization process modulation, energy level alignment, independent charge extraction layer or dopant, enhancing the moisture and thermal stability are summarized. Future prospects towards the realization of ultra-stable and efficient PSCs with ionic liquid engineering are also discussed and presented.