Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH3NH3PbBr3 ferroelastic single crystals

Hybrid lead halides CH3NH3PbX3 (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled. Here, we investigate, using a combined experimental and theoretical approach, the possible presence of electric polarization in tetragonal and orthorhombic CH3NH3PbBr3 (T-MAPB and O-MAPB). We found that T-MAPB does not sustain spontaneous polarization but, under an external electric field, it is projected into a metastable, ionic space-charge electret state. The electret can be frozen on cooling, producing a large and long-lasting polarization in O-MAPB. Molecular dynamics simulations show that the ferroelastic domain boundaries are able to trap charges and segregate ionic point defects, thus playing a favorable role in the stabilization of the electret. At lower temperatures, the lack of ferroelectric behavior is explained using first principles calculations as the result of the tight competition among many metastable states with randomly oriented polarization; this large configurational entropy does not allow a single polar state to dominate at any significant temperature range.

Automated summary

The study investigates the possible presence of electric polarization in tetragonal and orthorhombic hybrid lead halides through a combined experimental and theoretical approach. It is found that while T-MAPB does not sustain spontaneous polarization, O-MAPB can exhibit large and long-lasting polarization under an external electric field. Molecular dynamics simulations suggest that ferroelastic domain boundaries play a favorable role in stabilizing the electret by trapping charges and segregating ionic point defects. The lack of ferroelectric behavior at lower temperatures is explained by the tight competition among metastable states with randomly oriented polarization.