Publication date: 17th February 2025
Formamidinium-lead-iodide (FAPbI3) has established itself as the state of the art for high solar-energy conversion efficiency in perovskite-based solar cells. FAPbI3 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 FAPbI3. In this context, we perform a systematic study on the structural and electronic properties of the photoactive phase of FAPbI3 (α-FAPbI3) 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 PbI6 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 FAPbI3 is only achieved by system sizes approaching the nanoscale.
U.R. acknowledges the Swiss National Foundation (grant N. 200020_219440) and computational resources from the Swiss National Computing Centre CSCS. V.C. acknowledges computational resources from the Swiss National Computing Centre CSCS.
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