Intermediate phase engineering of halide perovskites for photovoltaics

Metal halide perovskites serving as one of the most promising photovoltaic materials are gaining considerable attention worldwide. For achieving high performance as well as long-term stability of the perovskite solar cells (PSCs), a good quality of perovskite film featuring in smooth, pinhole-free morphology, full coverage over substrates, good heterojunction contacts, and stable photoactive phase is of great importance. During solution fabrication for perovskite films, intermediate phase, which refers to the state of precursor composition before final annealing, plays an essential role in determining the film quality, especially in the state-of-theart PSCs. In this review, we summarize the research involving mechanism and applications of intermediate phase engineering (IPE) processes in various solution-processing technologies. The challenges of IPE are further discussed, and perspectives are provided for developing high-performance PSCs via IPE.