Investigating the Role of the Organic Cation in Formamidinium Lead Iodide Perovskite Using Ultrafast Spectroscopy

Organic cation rotation in hybrid organic−inorganic lead halide perovskites has previously been associated with low charge recombination rates and (anti)ferroelectric domain formation. Two-dimensional infrared spectroscopy (2DIR) was used to directly measure 470 ± 50 fs and 2.8 ± 0.5 ps time constants associated with the reorientation of formamidinium cations (FA⁺, NH2CHNH2⁺) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA⁺ agitates about an equilibrium position, with NH2 groups pointing at opposite faces of the inorganic lattice cube, and undergoes 90° flips on picosecond time scales. Time-resolved infrared measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photogenerated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA⁺ antisymmetric stretching vibrational potential, resulting in an observed 5 cm^-1 shift. Our 2DIR results provide the first direct measurement of FA⁺ rotation inside thin perovskite films, and cast significant doubt on the presence of long-lived (anti)ferroelectric domains, which the observed low charge recombination rates have been attributed to.