Critical Comparison of FAPbX<sub>3</sub> and MAPbX<sub>3</sub> (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl<sub>3</sub> and FAPbBr<sub>3</sub>, compared to those of MAPbCl<sub>3</sub> and previously reported MAPbBr<sub>3</sub>, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX<sub>3</sub> system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results.