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

Research output: Contribution to journalLetter

Original languageEnglish
Pages (from-to)895-901
Number of pages7
JournalJournal of Physical Chemistry Letters
Volume9
Issue number4
Early online date1 Feb 2018
DOIs
DateAccepted/In press - 1 Feb 2018
DateE-pub ahead of print - 1 Feb 2018
DatePublished (current) - 15 Feb 2018

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.

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    Rights statement: This is the final published version of the article (version of record). It first appeared online via ACS at https://pubs.acs.org/doi/10.1021/acs.jpclett.7b03296 . Please refer to any applicable terms of use of the publisher.

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