Nanoscale size effects in α-FAPbI evinced by large-scale ab initio simulations.

Formamidinium-lead-iodide (FAPbI) has a rich phase diagram, and long-range correlation between the organic cations and lattice dipoles can influence phase transitions and optoelectronic properties. System size effects are crucial for an appropriate theoretical description of FAPbI. We perform a systematic ab initio study on the structural and electronic properties of the photoactive phase of FAPbI as a function of system size. To ensure an accurate theoretical description, three criteria must be satisfied: the (correct) value of the band gap, the extent (or the absence of) structural distortions, and the zeroing out of the total dipole moment. The net dipole moment vanishes as the system size increases due to PbI octahedra distortions rather than due to FA rotations. Additionally, thermal band gap fluctuations are predominantly correlated with octahedral tilting. The optimal agreement between simulation results and experimental properties for FAPbI is only achieved by system sizes approaching the nanoscale.