Abstract
Halide perovskite semiconductors and solar cells respond to electric fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielectric permittivity can be understood by separating the static dielectric constant into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with experimental observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge associated with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states.
Highlights
Halide perovskites is synthesized with the chemical formula ABX3, where A is a positively charged cation located in the central cavity created by an extended framework of corner-sharing BX6 metal-halide octahedra
Whilst it is tempting to average over all possible orientations, as performed in Ref. 111, we report values associated with MA aligned in the 100 direction, when possible
The Cochran-Cowley expression is a generalization of Eq (13) that accounts for systems with more than two atoms in the unit cell (MAPbI3 has 48 at room temperature)
Summary
Semiconducting halide perovskite materials have attracted intense research interest over the past five years due to the potential for inexpensive solution processing and desirable optoelectronic properties for photovoltaic (PV) and light emission applications. Halide perovskites is synthesized with the chemical formula ABX3, where A is a positively charged cation located in the central cavity created by an extended framework of corner-sharing BX6 metal-halide octahedra. Thin-film devices based on the hybrid organic-inorganic perovskite (CH3NH3)+ PbI3− (MAPbI3) were the first to utilize a halide perovskite as the PV absorber layer.2,4 This material features prominently in the literature due to the rapid increase in power conversion efficiency (PCE), from 3.8% to 23.3%.5,6. Differences in the microscopic crystalline properties of samples, experimental or computational methodologies, and nomenclature lead to disparity in dielectric literature which can be confusing to navigate. In this Perspective, we briefly introduce the theory of dielectric polarization in order to critically interpret the polarization mechanisms reported for halide perovskites. We contemplate the ferroic nature of halide perovskites that continues to inspire debate in the field
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