Intermediate phase engineering towards efficient and stable perovskite solar cells: Principles and strategies

As an emerging photovoltaic technology, halide perovskite solar cells (PSCs) have seen power conversion efficiency from 3.8% to 26.1% since their first report in 2009, becoming the focus of academic and industrial community. In recent years, intermediate phase engineering (IPE) has been adopted as an effective method to improve the crystallinity and film quality of solution-processed perovskite films. The intermediate phase, which refers to the composition state between the precursor and the final annealing state, plays crucial roles in obtaining high-quality perovskite films with more uniform and pinhole-free morphology, fewer defects, high crystallity, and full coverage. As for inorganic PSCs, IPE as well plays prominent roles in relatively low annealing temperatures and improvement of the interfacial contact between functional layers. Herein, in this review, the principles and functions of IPE are systematacially summarized from the perspective of crystallization kinetics. Subsequently, the processing strategies adopted for IPE in PSCs fabrication are presented according to the commonly utilized additives such as dimethyl sulfoxide, hydroiodic acid, methylammonium acetate, methyl amine gas, and others, and advances of IPE in inorganic PSCs are also highlighted. In the last, the current issues and future prospects in this field are pointed out, which will provide guidance for developing highly efficient and stable PSCs.