Tin fluoride (SnF2) is widely used as an effective additive for lead-free tin-based perovskite solar cells. However, the function of SnF2 and the mechanism in improving the film morphology are still not clear. In this work, it is clearly demonstrated that SnF2 can play a crucial role in the crystal nucleation process. Due to the limited solubility, SnF2 creates more nucleuses for the crystal growth and therefore enables more uniform thin film with high coverage. It is confirmed that this mechanism can be applied to the growth of both thin film and single crystal. As a result of tin fluoride-assisted heterogeneous nucleation, an MASnIBr2-based perovskite solar cell with a high and stable power conversion effciency of 3.70% is demonstrated.
 Min Xiao, Jia Zhu et al, Adv. Opt. Mater. 2017, 1700615.