The Ferroelectric–Ferroelastic Debate about Metal Halide Perovskites

Abstract

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration.

Ferroelectric CsGeI3 Single Crystals with a Perovskite Structure Grown from Aqueous Solution

“…Ambrosio et al have reported that there should be a ferroelastic but not ferroelectric property in organic−inorganic hybrid perovskite, and ferroelectric polarization cannot be sustained when the A-site cation dynamics is fully unleashed. 10 In optoelectronic applications, t h e b r o a d b a n d g a p s o f [ C H 3 P H 3 ] P b I 3 , 1 1 (C 4 H 9 NH 3 ) 2 (NH 3 CH 3 ) 2 Sn 3 Br 10 , 12 and other molecular ferroelectrics are challenging issues as well. On the other hand, CsPbBr 3 quantum dots present ferroelectricity as an example of all-inorganic perovskite halides.…”

“…Through the bulk photovoltaic effect, this advantage might help surpass the Shockley–Queisser limit in conventional photovoltaic devices. − However, because traditional ferroelectric oxide materials often have a significantly large band gap, they are unable to absorb visible light effectively. Ambrosio et al have reported that there should be a ferroelastic but not ferroelectric property in organic–inorganic hybrid perovskite, and ferroelectric polarization cannot be sustained when the A-site cation dynamics is fully unleashed . In optoelectronic applications, the broad band gaps of [CH 3 PH 3 ]­PbI 3 , (C 4 H 9 NH 3 ) 2 (NH 3 CH 3 ) 2 Sn 3 Br 10 , and other molecular ferroelectrics are challenging issues as well.…”

Comparing the Cubic and Tetragonal Phases of MAPbI3 at Room Temperature

“…63−66 However, the existence of ferroelectricity in hybrid organic−inorganic perovskites is currently a subject of strong debate. 60,67 Additionally, a standard P−E loop has not yet been achieved in MAPbI 3 to the best of our knowledge. Several studies suggest that the orientation of the organic molecules leads to ferroelectric domains, where the tetragonal phase presents a ferroelectric-like behavior.…”

“…An alternative interpretation for the P−E loops is based on the concept of ferroelasticity, which results in a distortion of the crystal lattice, inducing a spontaneous strain in the material. 67 In fact, ferroelastic fingerprints was found to be formed due to the introduction of external strain (caused by epitaxial growth, for example) or the strain caused during the cubic-to-tetragonal phase transition. 71,72 Also, ferroelastic domain formation has a positive influence on charge carrier transport.…”

“…63,68 On the other hand, other researchers argue that the presence of ferroelectric domains cannot be sustained when the A-site cation dynamics is fully unleashed in MAPbI 3 . 67 the ferroic behavior may not be significant in terms of the physics of photogenerated carriers, as competing effects such as polaron formation and ion migration may overshadow it. 69,70 From a crystallographic point of view, ferroelectric materials are identified by the breaking of inversion symmetry, indicating that the crystal structure must be non-centrosymmetric.…”