Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

Abstract

The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI 3 ) and formamidinium lead iodide (FAPbI 3 ), 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 3 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 of the reported temperature dependence of the dynamics of MAPbI 3 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 3 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 large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.2

Formation of Stable Mixed Guanidinium–Methylammonium Phases with Exceptionally Long Carrier Lifetimes for High-Efficiency Lead Iodide-Based Perovskite Photovoltaics

“…Very recently, Fabini et al have used 1 H spin-lattice relaxation to probe cation dynamics in α-FAPbI3. 29 We note, however, that 1 H relaxation, similarly to 2 H relaxation, results both from proton C2-jumps (-NH2 protons in FA), or C3-jumps (-NH3 and -CH3 protons in MA) around the C-N bond, and from the overall cation reorientation. This convolution leads to a number of nonlinear phenomena 24,30 and makes it difficult to quantify cation reorientation in a straightforward manner, which is why we favour the 14 N method used here, where the line broadening of the 14 N resonances is only sensitive to reorientation of the C-N bond.…”

Slow Hot‐Carrier Cooling in Halide Perovskites: Prospects for Hot‐Carrier Solar Cells

“…The small ℏω LO (≈8 meV) for MAPbI 3 arises from the low-frequency PbI stretching vibration because of the heavy lead. Thus, using organic molecular with smaller activation barrier energy for rotational motion (e.g., FA + in α-FAPbI 3 [99] ) in halide perovskites could enhance the screening. 4) Smaller Carrier Effective Mass: Based on Equation (5) above, a lighter carrier effective mass will also lead to less efficient HC relaxation as shown in our calculated HC cooling dynamics for different carrier effective masses (Figure 8c).…”

Quantifying Polaron Formation and Charge Carrier Cooling in Lead‐Iodide Perovskites

“…This proposition is in line with the relevant optical phonons involving predominantly lead‐halide lattice motion, and being quite similar for MAPbI 3 , FAPbI 3 , and CsPbI 3 (see also Figure S1A, Supporting Information). Indeed, the cation–lattice interaction was recently concluded to be not significantly different for MAPbI 3 and FAPbI 3 . However, the role of hydrogen bonds between the organic cations and the lattice, the rotation of the cations itself with the almost identical timescales of cation rotation and carrier–phonon interaction, and the connection to polaron formation and initial carrier cooling are important question to be answered in order to fully elucidate the fundamental photophysical phenomena in lead‐halide perovskite solar cells …”