Organometallic halide perovskites are outstanding materials with a strong solar light absorption due to a well-adapted direct gap combined with a high charge carrier diffusion length of about hundreds of micrometers in single crystals [1], giving rise to impressive light-to-power conversion efficiency. The orientation of the organic cations within the inorganic framework in methylammonium triiodide-plumbate (CH3NH3PbI3) is very important because it affects the structural, dielectric [2], and vibrational features [3]. For instance, along with the modification on the lattice parameter, the system can exhibit a net dipole moment and ferroelectricity or on the contrary, be apolar and antiferroelectric. Thus, the interaction between molecular cations and the inorganic lattice, especially through the lone electron pair in the atoms of the heavy element [4], depends on the CH3NH3PbI3 phase. However, despite the significant amount of charge-carrier-related research, the mechanism of electron-phonon coupling remains unclear and whether optical excitations can promote a structural phase transition is still an open question. Therefore, we have performed time-resolved XRD with picosecond resolution, matching the time-lapse of molecular rotations, to study the transient lattice dynamics of the system when it is illuminated by visible light to mimic solar absorption. Our analysis of the diffracted intensity allows us to conclude whether the methylammonium reorganizes in a polar or apolar disposition in the tetragonal phase of CH3NH3PbI3.

[1] Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, Huang, J. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967.

[2] Mosconi, E.; Amat, E.; Nazeeruddin, M. K.; Grätzel, M. and De Angelis, F. First-principles modeling of mixed halide organometal perovskites for photovoltaic applications. J. Phys. Chem. C 2013, 117, 13902.

[3] Deretzis, I. and La Magna, A. Exploring the orthorhombic–tetragonal phase transition in CH3NH3PbI3: the role of atom kinetics. Nanoscale 2017, 9, 5896. 

[4] Payne, D.; Egdell, R.; Walsh, A.; Watson, G.; Guo, J.; Glans, P. A.; Learmonth, T. and Smith, K. Electronic origins of structural distortions in post-transition metal oxides: experimental and theoretical evidence for a revision of the lone pair model. PRL 2006, 96, 157403.