Contrasting Effects of FA Substitution on MA/FA Rotational Dynamics in FAxMA1–xPbI3

Partial substitution of methylammonium (MA) cations in MAPbI(3) by formamidinium (FA) ions results in the formation of mixed cation perovskites with a suitable band gap for broader solar spectrum harvesting. In addition, these mixed cation systems offer enhanced material stability, compositional homogeneity, and higher photoconversion efficiency, thus attracting a great deal of attention in the photovoltaic community. To understand the influence of FA substitutions at the microscopic level, we compare the dynamics of the organic units in the mixed cation FA(0.125)MA(0.875)PbI(3) hybrid perovskite with those in MAPbI(3) over a wide range of temperatures (similar to 5-340 K) using quasielastic neutron scattering (QENS) techniques. Measurements indicate that the substitution with FA significantly modulates the dynamics and its evolution with temperature; for example, we find the interesting contrasting results that the rotational dynamics of FA ions are entirely suppressed, while the MA ions rotate faster in FA(0.125)MA(0.875)PbI(3) vis-a-vis MAPbI(3) over the entire temperature range. Further unusual rotational behavior of MA ions is observed for the intermediate temperature range (110-170 K) where a unique large-cell cubic (LC) phase exists in FA(0.125)MA(0.875)PbI(3). In this temperature range, the fraction of rotationally active MA ions is found to be markedly reduced in FA(0.125)MA(0.875)PbI(3) compared to those in MAPbI(3), but these fewer active MA ions rotate exceptionally fast in the mixed composition. These unusual behaviors of MA cation dynamics in the LC phase can be explained based on the formation of a glassy dipolar state, as observed in the frequency dependent dielectric properties. These results suggest that the rotational dynamics of the organic units are intimately connected with the structural and photophysical properties of these systems.