In Situ Study of Purified Phase Transition Path for -FAPbI₃ Crystallization

Phase transition during annealing in the two-step sequential deposition has drawn intensive attention as its significance in fabricating superior perovskite films. However, previous works have not paid enough attention to the importance of the purified phase transition path in the crystallization process. Herein, different from the mixed paths in the conventional cognition, purified phase transition path for alpha-FAPbI(3 )crystallization is achieved by introducing dimethylurea (DMU) into lead iodide (PbI2) precursor solution. The multifunctional molecule is found to design a penetrable porous PbI2 film and fundamentally regulate the perovskite crystallization by forming single phase transition path via the complete delta-phase during annealing of perovskite. The role of DMU in purified transition path for alpha-FAPbI(3 )crystallization is unraveled with in situ photoluminescence and grazing incidence wide-angle x-ray scattering (GIWAXS) investigation. The crystal quality of perovskite films is significantly improved, resulting in single crystal-like film. The best-performing perovskite solar cells (PSCs) deliver a power conversion efficiency of 24.75%, resulting from the higher FF of 83.25% and an improved long-term stability up to 3600 h. This work highlights the importance of purified phase transition path for the superior crystal quality toward high-performance perovskite photovoltaics.