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J Phys Chem Lett. 2018 Feb 15;9(4):895-901. doi: 10.1021/acs.jpclett.7b03296. Epub 2018 Feb 8.

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

Author information

1
School of Chemistry, University of Bristol , Bristol, BS8 1TS, United Kingdom.
2
Bristol Centre for Functional Nanomaterials, University of Bristol , Bristol, BS8 1TL, United Kingdom.
3
Central Laser Facility, Science and Technology Facilities Council, Research Complex at Harwell, Rutherford Appleton Laboratory , Didcot, Oxfordshire OX11 0QX, United Kingdom.

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+, 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.

PMID:
29389137
DOI:
10.1021/acs.jpclett.7b03296
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