Organic Molecule Orientations and Rashba-Dresselhaus Effect in α-Formamidinium Lead Iodide

The crucial role played by the organic molecular cation in hybrid organic-inorganic perovskite has been a challenging subject of discussion. Effects of the organic cation on a cubic perovskite structure of formamidinium lead iodide (FAPI) were investigated deploying the state-of-the-art density functional theory including the spin-orbit coupling (SOC). Equipped with Euler's rotations, energy landscapes corresponding to the different orientations of formamidinium (FA) cation were calculated. From the energy landscapes, the flipping energy barriers are interpreted to be thermal agitations required to flip over for FA. The highest energy barrier is 24.7 meV, which is equivalent to T similar to 286 K, the temperature over which the FA molecules are randomly oriented. In addition, we found a relatively lowest energy structure when the FA is rotated by (phi, theta, psi) = (90 degrees, 60 degrees, 45 degrees). From the structural optimization, the I-Pb-I becomes angled with less than 180 degrees. The H-I pair distribution function of this configuration shows that the H-I distances are optimum and confined only in the 2.70-4.25 angstrom shells. The resulting configuration also breaks the inversion symmetry leading to the Rashba-Dresselhaus effect in the electronic band structure. The largest Rashba splitting parameter calculated along the R -> M direction in k-space is similar to 3.0 for the (phi = 90 degrees, theta = 60 degrees, psi = 45 degrees) configuration.