Effects Of Organic Molecule Orientations On Perovskite Structure Of Formamidinium Lead Iodide

Abstract

Latterly, an emergence of the hybrid organic-inorganic perovskites has captivated an increasing rate of world-wide attention due to their approving physical properties. Formamidinium lead iodide (FAPI), a promising compound owing to its high photovoltaic performance, consists of an organic molecule, i.e. the formamidinium (FA) cation, dwelling in the centre of the cubic unit cell, caged by the inorganic framework, PbI6. By adopting the ab initio method based on the density functional theory including the spin-orbit coupling (SOC) effects, the effects of the FA cation on the cubic FAPI were thoroughly and systematically investigated. Solidly armed with Euler’s rotations, energy landscapes responsible for various sets of orientations of the FA cation were evaluated accordingly. From the energy landscapes, the flipping energy barriers are interpreted as thermal agitations needed to flip the FA cation over. The highest energy barrier amongst all those of other orientations is 24.7 meV which is tantamount to T ~ 286 K––the temperature over which the FA molecules randomly reorient. Moreover, it is found that a relatively lowest energy structure when the FA cation is directed along (90°,60°, 45°) direction. Owing to the structural optimisation, the I-Pb-I becomes angled with less than 7°. The H-I distances are optimal and confined only in the shells in accordance with the pair distribution function of the optimal configuration. The resulting configuration additionally breaks the inversion symmetry that leads to the Rashba/Dresselhaus effect within the electronic band structure. The largest Rashba splitting parameter determined along the direction in the k–space is around 3.0 for the (90°,60°, 45°) configuration.