Nanoscale size effects in α-FAPbI3 evinced by large-scale ab initio simulations

Formamidinium-lead-iodide (FAPbI ₃ ) has established itself as the state of the art for high solar-energy conversion efficiency in perovskite-based solar cells. FAPbI ₃ has a rich phase diagram, and it has been noted that long-range correlation between organic and lattice dipoles can influence phase transitions and, consequently, optoelectronic properties. In this regard, system size effects can play a crucial role for an appropriate theoretical description of FAPbI ₃ . In this context, we perform a systematic study on the structural and electronic properties of the photoactive phase of FAPbI ₃ ( α -FAPbI ₃ ) as a function of system size. Utilizing ab initio molecular dynamics at 300 K and first-principles calculations, we demonstrate that the selection of the computational system/setup must satisfy three criteria concurrently to ensure an accurate theoretical description: the (correct) value of the band gap, the extent (or the absence of) structural distortions, and the zeroing out of the total dipole moment. We demonstrate that the net dipole moment vanishes as the system size increases due to PbI ₆ octahedra distortions rather than due to FA ⁺ rotations. Additionally, we show that thermal band gap fluctuations are predominantly correlated with octahedral tilting. The optimal agreement between simulation results and experimental properties for FAPbI ₃ is only achieved by system sizes approaching the nanoscale.