Role of A-Site Cation and X-Site Halide Interactions in Mixed-Cation Mixed-Halide Perovskites for Determining Anomalously High Ideality Factor and the Super-linear Power Law in AC Ionic Conductivity at Operating Temperature

Mixed-cation mixed-halide perovskites have gained a lot of attention due to their superior device properties, optoelectronic properties, and improved structural stabilities over methyl ammonium lead trihalide-based devices. However, understanding the interfacial charge and ion transport kinetics remains elusive. In this article, we have synthesized (MA)x(FA)x−1Pb(Br)2+x(I)1–x stoichiometric compositions by varying x between 0.2 and 1.0. We have chosen two compositions with x = 0.6 and 0.8 due to their similar morphologies and optical absorption properties. We have demonstrated that increasing MA+ for x = 0.6–0.8 can reduce the ion migration by a factor of two while the activation energy for ion migration is increased from 0.36 eV to 0.47 eV. The ideality factor can be reduced upon increasing methylammonium bromide concentrations. We have measured ionic conductivity as a function of frequency to demonstrate that the interplay between A-site cation and X-site halide ion leads to opposite behavior in the low-frequency regime upon increasing bias voltage. This could be due to two competing processes; interfacial polarization and cation-halide hydrogen bonding in two samples. MA+ and FA+ migration lead to Jonscher’s power law at the mid-frequency regime. High-frequency AC conductivity follows the nearly constant (dielectric) loss regime with exponent (slope) exceeding one due to the presence of multiple ions in the systems. This kind of super-linear power law behavior of AC ionic conductivity at operating temperature is reported for the first time in these perovskite materials.