Cesium Substitution Disrupts Concerted Cation Dynamics in Formamidinium Hybrid Perovskites

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, 1 H and 14 N nuclear magnetic resonance spectroscopy, and 79 Br 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 sub-lattice and local or long-range distortions of the octahedral framework. We propose that cesium substitution suppress the phase transitions through the relief of geometric frustration associated ** Both authors contributed equally.with the orientations of CH(NH2)2 + molecules, which retain their local dynamical degrees of freedom.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

“…These considerations are corroborated through a comparison of INS data for single-crystal and powder samples of an analogous FAPbBr 3 system, obtained with triple-axis and indirectgeometry neutron spectrometers, respectively. [37,38] The overall intensity progression of the INS spectrum of FAPbI 3 presented in this work is very similar to that observed by Mozur et al on powder FAPbBr 3 , collected on the VISION spectrometer (SNS, Oak Ridge). [38] Figure 3 shows the INS spectra of the parent materials, along with the MA 1−x FA x PbI 3 mixtures of relevance to this work.…”

“…[37,38] The overall intensity progression of the INS spectrum of FAPbI 3 presented in this work is very similar to that observed by Mozur et al on powder FAPbBr 3 , collected on the VISION spectrometer (SNS, Oak Ridge). [38] Figure 3 shows the INS spectra of the parent materials, along with the MA 1−x FA x PbI 3 mixtures of relevance to this work. In order to bring to the fore the relative contributions of either MAPbI 3 and FAPbI 3 to total intensities, these data have been normalized to the height of those spectral bands that are least affected by cation mixing.…”

“…This work has evinced the extreme softness of the Pb-I framework, leaving the hydrogen dynamics quite unexplored and limited to a narrow energy transfer range (0-15 meV). [36,37] Further insights have been provided for the analogous FAPbBr 3 system, as recently reported by Mozur et al [38] As in the previous case of MAPbI 3 [15], we take the ensuing dynamics of the organic cation as a sensitive probe of local structure. In this work, the INS spectrum of FAPbI 3 is presented and extensively discussed for the first time.…”

Phase Diagram and Cation Dynamics of Mixed MA1-xFAxPbBr3 Hybrid Perovskites

“…[39][40][41] Three additional isostructural transitions were observed at 182, 162 and 118 K that involve changes of the FA cation dynamics and are not resolved crystallographically. 30,42 A substantial degree of disorder of FA cations was found even at 10 K temperature. 41 Our results show that mixing of these compounds significantly suppresses the structural phase transitions resulting in stabilization of the cubic phase.…”

“…16,27,28 Despite an extensive use of mixed lead halide perovskites in solar cell fabrication, 16,17,19,29 there are by far less number of studies that provide a detailed picture of the structural phase transitions, phase diagrams and cation dynamics that are tightly related to the attractive properties of these materials. Several recent works reported phase diagrams of FA 1-x Cs x PbX 3 (X = I, Br) 30,31 and MA 1-x Cs x PbI 3 32 perovskites, where the latter compounds demonstrated signatures of an electric dipole (orientational) glass phase. Such a phase was also observed by the broadband dielectric spectroscopy in our recent investigation of mixed MA 1-x DMA x PbBr 3 compounds, where MA frustration is caused by a local lattice deformation due to neighboring DMA cations.…”