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

Abstract

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, NHCHNH) in formamidinium lead iodide perovskite thin films. Molecular dynamics simulations reveal the FA agitates about an equilibrium position, with NH 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 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.

Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration

“…This is in contrast with the XRD of α-FAPI thin film (PbI 2 is known to be easily generated during the annealing treatment at 155°C -room conditions-) ( Figures S18, S19). 27,28 As observed in the XRD of T 0.04 powder and the XRD of ethylene diammonium (en 2+ ) based hollow perovskites (FA) 1-x (en) x (Pb) 1-0.7x (I) 3-0.4x , 24 there is no significant shift of diffraction lines in the XRD patterns of T 0.04 and T 0.13 thin films compared to the one of α-FAPI phase. However, the specific features observed in the XRD pattern of T 0.04 powder compared to the one of α-FAPI, which have a higher Table S10).…”

Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films

“…In comparison, the CsFAMA perovskite film ATR-FTIR analysis (Fig. 3A, bottom curve) presents new vibrational resonances owing to the formamidinium (FA) cation at 3406 cm −1 (N-H stretch), 3357 cm −1 (N-H stretch), 3271 cm −1 (N-H stretch), 3167 cm −1 (N-H stretch), and 1714 cm −1 (C-N antisymmetric stretching) (37,38). In the CsFAMA sample, an additional weak line at 1472 cm −1 corresponds to the MA cation found in a smaller molar ratio in relation to FA (17% mol of MA in relation to FA).…”

Suppressing X‐Migrations and Enhancing the Phase Stability of Cubic FAPbX3 (X = Br, I)

“…To estimate the freedom of motion of the organic cations inside the inorganic cages we use the nudged elastic band method (constrained multi-image relaxation method). [38,39], the least strained motion is the rotation around the z-axis (approximately the N-N axis) and higher electronegativity of X corresponds to higher rotation barrier. [38,39], the least strained motion is the rotation around the z-axis (approximately the N-N axis) and higher electronegativity of X corresponds to higher rotation barrier.…”