Room-temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab initio molecular dynamics perspective

Abstract

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to 'freely' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite temperature ab-initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long range anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures the motions are less correlated.PACS numbers: 61.50. Ah, The spectacular rise of perovskite photovoltaics 1 has sparked much research effort into the physical mechanisms behind these materials' good photovoltaic performance. From a solid state physics perspective, the answer seems simple: the well suited electronic structure of methylammonium lead-iodide (MAPbI 3 ). With a band gap of ∼1.6 eV 2-6 and a high absorption coefficient 2,5,6 , it is expected to be a high efficiency solar cell material for the solar radiation spectrum observed on earth.7 Density Functional Theory (DFT) calculations have shown that an s-p mixture in the valence band maximum (VBM) and p-states in the conduction band minimum (CBM), combined with a direct band gap lead to an absorption coefficient up to an order of magnitude higher than GaAs.8 However, the crystal structure of MAPbI 3 is completely different from GaAs. It even possesses temperature depended dynamical contributions arising from the organic constituent. The perovskite structure of MAPbI 3 is composed of three iodine atoms (monovalent anions) combined with a lead atom (divalent cation) and a CH 3 NH 3 (MA) molecule (monovalent cation). The Pb-I framework forms the perovskite structure out of PbI 6 octahedra and the molecules are trapped in the cavities. Whether the cubic perovskite structure is stable or becomes orthorhombic or tetragonal is determined by the temperature 6 combined with a balance between the size of the molecule and the Pb-I bond length.9 NMR experiments have indicated that at high temperatures (>300 K) the MA molecule exhibits complete orientational disorder.10,11 This means that the MA molecules have enough kinetic energy to overcome the rotational barriers and can rotate in their 'cage'. Around room temperature this process has a typical relaxation time of ∼ 5 ps.12 These rotations apparently do not effect the charge carriers in the system, since very long electronhole diffusion lengths have been reported. 13,14 From an electronic point of view this is not surprising, because the Pb-I framework is electronically decoupled from the m...

Behavior of Methylammonium Dipoles in MAPbX3 (X = Br and I)

“…46,47 In ref (44), a spontaneous microscopic polarization of the MA molecules is found, while refs (46) and (47) conclude the contrary, in agreement with our findings. This apparent discrepancy is most probably due to the limited system size used in ref (44), as already hinted by the authors there.…”

“…The relaxation time was obtained by calculating the average autocorrelation function of all molecules. More details can be found in ref (46). …”

Computational Methods for Ab Initio Molecular Dynamics

“…Of course, this is not compatible with most experimental conditions [27][28][29] in which the system is either kept at constant temperature (isothermal) in a heat bath or at constant pressure (isobaric) as a consequence, for instance, of the atmospheric pressure. In this subsection and in the next, we explain how to address these important issues.…”

“…Rather, the equations of motions must be modified directly as a result of the non-Hamiltonian nature of the isothermal process [31]. In order to obtain an isothermal molecular system, a friction term must be recursively added to each degree of freedom [28,29]. Therefore, the electronic equations of motion must be modified as follows:…”

Polarons in Metal Halide Perovskites

“…The impact of metal substitution has gained a huge interest due to the possibility of increasing the stability of the lattice, reducing the toxicity and tuning the perovskite bandgap . In particular, mixed Pb–Sn perovskites represent an interesting model system.…”