Abstract
Although initial studies on hybrid perovskites for photovoltaic applications focused on simple compositions, the most technologically relevant perovskites are heavily substituted. The influence of chemical substitution on the general phase behavior and specific physical properties remains ambiguous. The hybrid perovskite formamidinium lead bromide, CH(NH2)2PbBr3, exhibits complex phase behavior manifesting in a series of crystallographically unresolvable phase transitions associated with changes in the cation dynamics. Here, we characterize the molecular and lattice dynamics of CH(NH2)2PbBr3 as a function of temperature and their evolution upon chemical substitution of CH(NH2)2+ for cesium (Cs+) with crystallography, neutron scattering, 1H and 14N nuclear magnetic resonance spectroscopy, and 79Br nuclear quadrupolar spectroscopy. Cs+ substitution suppresses the four low-temperature phase transitions of CH(NH2)2PbBr3, which propagate through concerted changes in the dynamic degrees of freedom of the organic sublattice and local or long-range distortions of the octahedral framework. We propose that cesium substitution suppresses the phase transitions through the relief of geometric frustration associated with the orientations of CH(NH2)2+ molecules, which retain their local dynamical degrees of freedom.
| Original language | English |
|---|---|
| Pages (from-to) | 6266-6277 |
| Number of pages | 12 |
| Journal | Chemistry of Materials |
| Volume | 32 |
| Issue number | 14 |
| DOIs | |
| State | Published - Jul 28 2020 |
Funding
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award SC0016083. J.R.N. and E.M.M acknowledge support from Research Corporation for Science Advancement through a Cottrell Scholar Award, and J.R.N. thanks the A.P. Sloan Foundation for assistance provided from a Sloan Research Fellowship. M.A.H. gratefully acknowledges an Oppenheimer studentship. D.M.H. acknowledges the Cambridge International Trust for funding and is grateful for support from NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0012583. We would like to thank Dr. Subhradip Paul, University of Nottingham, for assistance in recording the low-temperature H NMR spectra. Access to the High Flux Backscattering Spectrometer was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology and the National Science Foundation under Agreement No. DMR-1508249. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 1
