J. Phys. Chem. Lett. 6, 3663–3669 (2015)

Credit: ACS

Methylammonium cations play a key role in determining the crystal structure and the photovoltaic behaviour of organic–inorganic perovskite films. The orientation and the dynamics of their permanent dipole moment affect the dielectric response of these materials and may contribute to the hysteretic response observed in perovskite solar cells. Using ultrafast polarization-resolved 2D infrared vibrational spectroscopy, Artem Bakulin and colleagues now study the motion of these organic cations within the inorganic sublattice of methylammonium lead triiodide perovskites synthesized with three different approaches. In all samples, the time-dependent anisotropy of the measured signal reveals two distinct cationic motions on 300-fs and 3-ps timescales. Molecular dynamics simulations are used to assign such contributions to, respectively, wobbling-in-a-cone motion and 90° reorientations of the organic molecules aligned to the main axes of the inorganic lattice. The researchers suggest that such insight may shed light on the contribution of the dielectric behaviour to the photophysical properties of these materials.