TY - JOUR
T1 - Investigating the Role of the Organic Cation in Formamidinium Lead Iodide Perovskite Using Ultrafast Spectroscopy
AU - Taylor, Victoria C. A.
AU - Tiwari, Devendra
AU - Duchi, Marta
AU - Donaldson, Paul M.
AU - Clark, Ian P.
AU - Fermin, David J.
AU - Oliver, Thomas A. A.
PY - 2018/2/15
Y1 - 2018/2/15
N2 - 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.
AB - 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.
U2 - 10.1021/acs.jpclett.7b03296
DO - 10.1021/acs.jpclett.7b03296
M3 - Letter
SN - 1948-7185
VL - 9
SP - 895
EP - 901
JO - Journal of Physical Chemistry Letters
JF - Journal of Physical Chemistry Letters
IS - 4
ER -