1 research outputs found
Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites
The
role of organic molecular cations in the high-performance perovskite
photovoltaic absorbers, methylammonium lead iodide (MAPbI<sub>3</sub>) and formamidinium lead iodide (FAPbI<sub>3</sub>), has been an
enigmatic subject of great interest. Beyond aiding in the ease of
processing of thin films for photovoltaic devices, there have been
suggestions that many of the remarkable properties of the halide perovskites
can be attributed to the dipolar nature and the dynamic behavior of
these cations. Here, we establish the dynamics of the molecular cations
in FAPbI<sub>3</sub> between 4 K and 340 K and the nature of their
interaction with the surrounding inorganic cage using a combination
of solid state nuclear magnetic resonance and dielectric spectroscopies,
neutron scattering, calorimetry, and ab initio calculations. Detailed
comparisons with the reported temperature dependence of the dynamics
of MAPbI<sub>3</sub> are then carried out which reveal the molecular
ions in the two different compounds to exhibit very similar rotation
rates (≈8 ps) at room temperature, despite differences in other
temperature regimes. For FA, rotation about the N···N
axis, which reorients the molecular dipole, is the dominant motion
in all phases, with an activation barrier of ≈21 meV in the
ambient phase, compared to ≈110 meV for the analogous dipole
reorientation of MA. Geometrical frustration of the molecule–cage
interaction in FAPbI<sub>3</sub> produces a disordered γ-phase
and subsequent glassy freezing at yet lower temperatures. Hydrogen
bonds suggested by atom–atom distances from neutron total scattering
experiments imply a substantial role for the molecules in directing
structure and dictating properties. The temperature dependence of
reorientation of the dipolar molecular cations systematically described
here can clarify various hypotheses including those of large-polaron
charge transport and fugitive electron spin polarization that have
been invoked in the context of these unusual materials