The opportunities and challenges of ionic liquids in perovskite solar cells

Metal halide perovskite solar cells (PSCs) have shown great potential to become the next generation of photovoltaic devices due to their simple fabrication techniques, low cost, and soaring power conversion efficiency (PCE). However, mismatched with the quickly updated PCEs, the improvement of device stability is challenging and still remains a critical hurdle in the path to commercialization. Recently, ionic liquids (ILs) have been found to play multiple roles in obtaining efficient and stable PSCs. These ILs usually consist of large organic cations and organic or inorganic anions, which have weak electrostatic attraction and are generally liquid at around 100 °C. ILs are almost non-volatile, non-flammable, with high ionic conductivity and excellent thermal and electrochemical stability. The roles of ILs in PSCs vary with their composition, that is, the types of anions and cations. In this review, we summarize the roles of anions and cations in terms of precursor solutions, additives, perovskite/charge transport layer interface engineering, and charge transport layers. This article aims to set up a structure–property-stability-performance correlations conferred by the IL in PSC and provide assistance for the anion and cation selection for improving the quality of perovskite film, optimizing interface contact, reducing defect states, and improving charge extraction and transport characteristics. Finally, the application of IL in PSCs is discussed and prospected.