Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3(+) cations and their impact on relevant processes such as charge recombination and exciton dissociation are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH3NH3(+) cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CH3NH3(+) ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in the orthorhombic phase (T < 165 K) only C3 rotation is present. At around room temperature, the characteristic relaxation times for the C4 rotation are found to be τC4 ≈ 5 ps while for the C3 rotation τC3 ≈ 1 ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH3NH3(+) and the associated dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.

Cu-exchanged small-pore zeolites (CHA and AEI) form methanol from methane (>95% selectivity) using a 3-step cyclic procedure (Chem. Commun.20155144474450) with methanol amounts higher than Cu-ZSM-5 and Cu-mordenite on a per gram and per Cu basis. Here, the Cu x O y species formed on Cu-SSZ-13 and Cu-SSZ-39 following O2 or He activation at 450 °C are identified as trans-μ-1,2-peroxo dicopper­(II) ([Cu2O2]2+) and mono-(μ-oxo) dicopper­(II) ([Cu2O]2+) using synchrotron X-ray diffraction, in situ UV–vis, and Raman spectroscopy and theory. [Cu2O2]2+ and [Cu2O]2+ formed on Cu-SSZ-13 showed ligand-to-metal charge transfer (LMCT) energies between 22,200 and 35,000 cm–1, Cu–O vibrations at 360, 510, 580, and 617 cm–1 and an O–O vibration at 837 cm–1. The vibrations at 360, 510, 580, and 837 cm–1 are assigned to the trans-μ-1,2-peroxo dicopper­(II) species, whereas the Cu–O vibration at 617 cm–1 (Δ18O = 24 cm–1) is assigned to a stretching vibration of a thermodynamically favored mono-(μ-oxo) dicopper­(II) with a Cu–O–Cu angle of 95°. On the basis of the intensity loss of the broad LMCT band between 22,200 and 35,000 cm–1 and Raman intensity loss at 571 cm–1 upon reaction, both the trans-μ-1,2-peroxo dicopper­(II) and mono-(μ-oxo) dicopper­(II) species are suggested to take part in methane activation at 200 °C with the trans-μ-1,2-peroxo dicopper­(II) core playing a dominant role. A relationship between the [Cu2O y ]2+ concentration and Cu­(II) at the eight-membered ring is observed and related to the concentration of [CuOH]+ suggested as an intermediate in [Cu2O y ]2+ formation.