How the ionic liquid BMIMBF4 influences the formation and optoelectronic properties of MAPbI3 thin films

While using additives such as ionic liquids (IL) is known to boost the efficiency and stability of perovskite solar cells, it is still unclear how ILs impact the difficile perovskite film formation process. Here we investigate the impact of the IL BMIMBF4 on the film formation of the model halide perovskite MAPbI(3) by multimodal optical in situ spectroscopy during solution processing via one-step spin coating and a solvent engineering approach. One-step processing experiments reveal that IL in the precursor solutions does not affect the formation of perovskite-solvent complexes, but higher IL contents delay the perovskite transformation with decreased growth rates. For solvent engineering, the perovskite growth rate decreases with later anti-solvent (AS) dripping as the properties of PbI42- species in the precursor solutions change during drying. Here the IL also affects the evolution of the PbI42- properties, as the IL cation interacts with the PbI42-. This interaction appears to reduce the perovskite growth rates after initiating perovskite formation by AS dripping. Still, in the as-coated films the IL efficiently passivates defect states. Thus, our work provides important insights into how decisive ILs impact the sensitive interconnection between precursor properties, film formation process and final optoelectronic functionality of perovskite thin films.

Automated summary

This research investigates the impact of ionic liquids (ILs) on the formation process of perovskite films in perovskite solar cells. It reveals that ILs can delay the transformation and decrease the growth rate of perovskite, but have no effect on the formation of perovskite-solvent complexes. Additionally, ILs can efficiently passivate defect states in the coated films.