Dynamical Phase Transitions and Cation Orientation-Dependent Photoconductivity in CH(NH2)2PbBr3

The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH­(NH2)2+) perovskites exhibit unusual temperature-dependent trends in photoluminescence, dielectric constant, and phase behavior that are hypothesized to relate to CH­(NH2)2+ reorientations. This contribution describes five distinct, temperature-dependent phase transitions in CH­(NH2)2PbBr3 that produce changes in the steady-state photocurrent. Three of these phase transitions do not resolve crystallographically and relate to the orientation and dynamics of CH­(NH2)2+. These crystallographically unresolvable phase transitions resemble ferroelastic transitions with the formation of nanoscale domains, which we hypothesize are mediated by the strain exerted from geometric frustration of CH­(NH2)2+ quadrupoles. This work demonstrates the importance of cation orientation and dynamics, domain behavior, and their interdependence in the steady-state optoelectronic properties of hybrid perovskites.