Mechanism and Impact of Cation Polarization in Methylammonium Lead Iodide

Abstract

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–tetragonal 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/cm2, 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.

Room‐Temperature Cubic Phase Crystallization and High Stability of Vacuum‐Deposited Methylammonium Lead Triiodide Thin Films for High‐Efficiency Solar Cells

“…Most reports on MAPI films and solar cells use examples prepared by one of the many varieties of solution based processes . In virtually all of them, the MAPI films are obtained in the tetragonal crystal structure at room temperature (RT) and they only undergo a phase transition to a cubic structure when the temperature is raised above ≈54 °C or if pressure is applied …”

Preferred orientations of organic cations at lead-halide perovskite interfaces revealed using vibrational sum-frequency spectroscopy

“…In their study, thermally stimulated current (TSC) peaks were observed at the orthorhombic to tetragonal phase transition, which the authors attribute to (de)polarization of the MA cations. 53 Moreover, the presence of an applied electric field was found to enhance, or even reverse the polarity of the TSC peak, suggesting that applied electric fields may enhance the spontaneous polarization of the organic cation. However, the aforementioned KPFM studies performed by Jiang et al and Guerrero et al on TiO 2 /MAPbI 3 /Spiro-MeOTAD devices found that the MAPbI 3 /TiO 2 interface exhibits a far greater electric field gradient compared to the MAPbI 3 /Spiro-MeOTAD interface, suggesting that the presence of an interfacial electric field alone is not able to induce an orientational bias.…”

Polar or nonpolar? That is not the question for perovskite solar cells

“…As a result, much effort has been devoted to functional probing, as the properties of polar and nonpolar groups are drastically different. Macroscopic ferroelectric, pyroelectric and dielectric measurements have also been carried out [ 48 , 50 , 59 , 60 , 64 , 66 , 74–76 ], although leakage current and ionic migration often make the data interpretation ambiguous. While polar structure, with the breaking of inversion symmetry, is expected to be active in optical second harmonic generation (SHG) [ 12 ], the experimental data are inconclusive because of the strong background from other nonlinearities [ 62 , 77 ].…”