Ionic Liquid for Perovskite Solar Cells: An Emerging Solvent Engineering Technology

Perovskite solar cells (PSCs) have, in recent years, become one of the most in-depth photovoltaic materials in many different disciplines due to their long-range charge carrier diffusion lengths, strong light absorption, easy tuning of the band gaps, low defect density, and solution processability. The power conversion efficiency (PCE) of single-junction PSCs has reached a certified value of 25.5%, which has caught up with or even surpassed traditional photovoltaic technologies, such as silicon (Si) solar cells, thin film solar cell, etc. In addition to the performance of PSCs comparable to traditional photovoltaic technology, its biggest feature is that it can be prepared by a solution method. The preparation process of the solution method simplify the film preparation process and greatly reduce the preparation cost. In addition, the solution method provides a favorable option for the energy supply of flexible wearable devices in the future. At present, the preparation of PSCs mainly uses organic molecular solvents, induding N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), and their mixed solvents. However, most of these commonly used organic solvents are unsafe and are likely to cause water and soil pollution, harm to the human body, and safety accidents. In addition, most solvents have a high coordination number, which limits the crystallization of perovskite. Furthermore, high-quality perovskite films require the introduction of a large amount of high boiling point, polar and aprotic antisolvents to assist crystallization, such as chlorobenzene, ether, and ethyl acetate. In addition to the toxicological problems of the antisolvent, the technology has a relatively narrow operating time window, relatively high operating environment requirements, and relatively low repeatability, which greatly limits the large-scale production of PSCs. Therefore, It is very necessary to develop a new solvent engineering technology for PSCs, which is green and pollution-free, is a simple process, and has high efficiency, long-term stability, and low cost. Recently, we have developed a series of ionic liquids, including MAAc, MAFa, BAAc, etc., which can be used as solvents to prepare efficient and stable PSCs. It allows the production of smooth and continuous polycrystalline perovskite thin films in ambient air through a simple spin coating method In this Account, we started with the discovery of ionic liquids as solvents to PSCs. We have proposed new strategies for improving and enhancing ionic liquid-based PSCs, including interface engineering, solvent engineering, and additive engineering. Furthermore, the stable and efficient mechanism of ionic liquid-based perovskite solar cells has been revealed. In view of the designable characteristics of ionic liquids, some functional ionic liquids have been designed and applied to different systems of halide perovskites, which has important guiding significance for the development of more functional ionic liquids for PSCs. Finally, the challenges and future directions of ionic liquid-based PSCs from both large-scale production and technological perspectives are proposed to provide a new method for the commercialization of PSCs.