Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics

Perovskite photovoltaics have made giant leaps in efficiency in just a few years from their inception. The employed solution synthesis techniques lead to inherently "soft" structures, which are properly sampled by dynamical approaches. The presence of two types of mobile ions, that is, the (organic) A-cations and ion/defects species in both the organic and inorganic lattices, gives rise to a broad spectrum of dynamical features. A charge localization mechanism due to fluctuations of the A-cations is proposed to screen carrier recombination. Defect/ion migration probably underlies the slow materials response under light irradiation related also to solar cell hysteresis. We also show how the dynamics of the organic cations and that related to ion/defect migration are essentially coupled, with the methylammonium cations providing a local screening mechanism that may further speed up the ionic migration. The use of less polar and less orientationally mobile A-cations may possibly slow down ion migration.