Structural and electronic properties of organo-halide lead perovskites: a combined IR-spectroscopy and ab initio molecular dynamics investigation

Abstract

Organo-halide lead perovskites are revolutionizing the photovoltaic scenario, with efficiencies exceeding 15%. The orientational dynamics disorder of the methylammonium cations (MA) is one of the most peculiar features of these materials. Here, we perform ab initio molecular dynamics simulations and IR spectroscopic measurements on lead halide hybrid perovskites to elucidate the rotational motion of the MA cations in these systems and its effects on the structural and electronic properties of hybrid perovskites. In the investigated time frame, the MA cations are found to rotate within the inorganic framework on a timescale of a few ps. A variation of ±0.1 to 0.2 eV of the electronic properties with the ion dynamics is found, which increases upon increasing the temperature.

Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites

“…Yet they are shown to have a strong influence on the optoelectronic properties of the perovskites. (10,11) The size and shape of the molecular cations, as well as their embedding in the lattice, determine the macroscopic and molecular-scale perovskite crystal structure (10,12,13), which in turn determines the band gap (11,14) of the material via the deformation potential. This offers the opportunity to tune the material properties through modification of the organic cation.…”

Energy Landscape of Molecular Motion in Cubic Methylammonium Lead Iodide from First-Principles

“…11,[20][21][22][23] CH 3 NH 3 PbI 3 undergoes a transition from the high-temperature pseudo-cubic aristotype P m3m structure to the tetragonal I4/mcm phase (a 0 a 0 c − tilt system in Glazer notation) 24 at 330 K and then to the orthorhombic P nma phase (a + b − c − tilt system) below 160 K. [25][26][27][28] The transitions associated with A-cation ordering have been the subject of some debate. [11][12][13]20,[29][30][31] Nevertheless, recent quasi-elastic neutron scattering experiments indicate that the CH 3 NH 3 + molecules in the cubic phase dynamically disorder and undergo both fast reorientations of the C−N bond axis (≈5 ps at 300 K) and faster on-axis rotations about the C−N bond axis (≈1 ps at 300 K). 11 Some degrees of freedom freeze out upon cooling and only on-axis rotations are observed in the low temperature orthorhombic phase (≈4 ns at 70 K) accompanied by a dramatic loss in dielectric permittivity.…”

“…11,12 Distortions of the inorganic lattice by halide substitution and octahedral tilting have been shown to influence the band gap and absorption properties. 7,[13][14][15] Although the A-cation does not contribute to electronic states near the band gap, [16][17][18] it has been shown to affect the nature of the band gap through interaction with the inorganic Pb−I sublattice. 19 Moreover, the dielectric properties of CH 3 NH 3 PbI 3 are linked to both the rotational dynamics and ordering of the molecular A-cation and the structural phase transitions of the inorganic Pb−I lattice.…”

Research Update: Relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells

“…On average the structure can be considered to be cubic. [9][10][11] We initially construct a model with a single formula unit of CH 3 NH 3 PbI 3 . We orient the organic moiety along [111], where local potential energy minima are found, which generate three structures representative of the disordered system.…”

“…In reality, the organic moiety rotates on a picosecond time scale at room temperature. [9][10][11][12] We sample molecular dynamics realizations of a disordered supercell. In spite of the disorder, the spin-split indirect-gap is not only present but enhanced.…”