Mechanism and Impact of Cation Polarization in Methylammonium Lead Iodide

Despite the intense research effort on metal halide perovskites, the fundamental correlation between the crystal structure and optoelectronic properties remains unclear. As many intriguing phenomena are expected to be based on the dipolar character of the rotating organic cations, an improved understanding of the material's polarizability is of high relevance. Here, we study the orthorhombic-etragonal phase transition of methylammonium lead iodide to gain insight into the polarization mechanism at low temperatures and the resulting effects on solar cell performances. Using thermally stimulated current measurements, we detect polarization currents when cooling or heating across the phase transition. These (de)polarization currents are found to correlate with a sudden change in rotational freedom of the organic cations, with a temperature range of 20 K separating polarizing and depolarizing processes. The nature of this cation polarization within the orthorhombic phase is investigated with respect to its intrinsic and extrinsic polarizability. Although the amount of unamplified polarization is determined to be only in the order of a few nC/cm(2), our results show significant increases in solar cell performances upon polarization, highlighting the intimate relationship between the alignment of organic cations and optoelectronic properties in metal halide perovskites.