Application of Ionic Liquids and Derived Materials to High-Efficiency and Stable Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs) are emerging photovoltaic technology that have attracted worldwide attention owing to their potential to disrupt the established market of silicon solar cells. Despite much effort, several lingering issues, such as device efficiency and stability, are still needed to be solved for the commercial application. Ionic liquids with unique properties, such as high ionic conductivity, good thermal stability, and chemical designability, have shown to play multiple functions for high efficiency and stable PSCs. In this Review, we first introduce the structure and property of typical ionic liquids. Then, we comprehensively discuss the various applications of ionic liquids and their derivatives in perovskite layers, hole-transporting layers (HTL), and electron-transporting layers (ETL) of PSCs, as additives, solvents, dopants, surface modifiers and independent charge-transporting materials, with particular emphasis on the interaction between ionic liquids and function layers of PSCs, such as hydrogen bonds, ionic bonds, coordinate bonds, and so on, as well as the relationship between molecular structures of ionic liquids and the photovoltaic performances of PSCs. Finally, we address the conclusions and an outlook for the future development of this field. It is expected that this Review will stimulate a generation of new thoughts and ideas on the emerging challenges and opportunities of ionic liquids-based PSCs.