Since the irruption of metal halide perovskites (MHPs) in the field of photovoltaics (PV) about a decade ago, these semiconductor materials have been the focus of attention worldwide. They triggered an intense research activity among scientists striving for a steady increase in energy conversion efficiencies, which have recently reached values beyond 25%. MHPs are really attractive because they are produced and processed by scalable, low-cost, energy-saving solution-based methods like organic semiconductors but they exhibit optoelectronic properties rivaling those of their inorganic counterparts. 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.
The aim of this work is two-fold: On one hand, we finally settle the controversy about ferroelectricity by exposing the main experimental and theoretical facts and revising their interpretation from the alternative point of view of ferroelasticity. In particular, we aim to convey the message that a ferroelectric polarization cannot be sustained when the A-site cation dynamics is fully unleashed, in frank contrast to the formation of ferroelastic domains. On the other hand, we discuss the possible impact of the ferroic behavior of MHPs on the charge-carrier generation and transport, taking into account the magnitude of the observed effects in comparison with competing ones like polaron formation and ion migration.
