Iodide-methylammonium interaction is responsible for ferroelectricity in CH3NH3PbI3

TL;DR

This paper demonstrates that the ferroelectricity in CH3NH3PbI3 is primarily due to iodide-methylammonium interactions causing symmetry breaking, which explains its exceptional photovoltaic properties.

Contribution

The study provides crystallographic evidence that iodide-methylammonium interactions induce non-centrosymmetry, revealing the mechanism behind ferroelectricity in CH3NH3PbI3.

Findings

Iodide-methylammonium interactions break symmetry in CH3NH3PbI3

Molecular cation CH3NH3+ influences iodide positions indirectly

Crystallographic evidence of non-centrosymmetric structure

Abstract

Excellent conversion efficiencies of over 20 % and facile cell production have placed hybrid perovskites at the forefront of novel solar cell materials with CH3NH3PbI3 being its archetypal compound. The question why CH3NH3PbI3 has such extraordinary characteristics, particularly a hugely efficient light absorption, is hotly debated with ferroelectricity being a promising candidate. This does, however, afford the crystal structure to be non-centrosymmetric and we herein present crystallographic evidence as to how the symmetry breaking occurs on a crystallographic, and therefore long-scale, level. While the molecular cation CH3NH3+ is intrinsically polar, it is heavily disordered and cannot be the sole reason for ferroelectricity. We show that it, nonetheless, plays an important role as it distorts the neighboring iodide positions from their centrosymmetric positions.