Dielectric function of hybrid perovskites at finite temperature investigated by classical molecular dynamics.

A. Mattoni,C. Caddeo

doi:10.1063/1.5133064

Abstract

Ionic polarization and dielectric function play a fundamental role in the optoelectronic properties of hybrid perovskites, currently one of the most studied materials for next generation photovoltaics. The hybrid nature of the crystal, with molecular dipoles that can reorient within the inorganic lattice, gives rise to a complex dielectric response in the bulk material that has been largely studied and debated. Here, we investigate the nature and the relaxation properties of the dielectric polarization of hybrid perovskites at finite temperature by means of classical molecular dynamics. We provide evidence that a simple ionic model of classical interatomic forces is able to explain qualitatively the temperature and frequency dependence of the dielectric constant providing a picture that is fully consistent with experimental data. The constant dielectric function in the low-temperature phase is controlled by ionic displacements, while the temperature-dependent paraelectric behavior of the tetragonal phase is due to reorientation of dipoles that are responsible for the discontinuity at the orthorhombic-to-tetragonal transition. In the frequency domain, the molecular reorientations give rise to a broad band that is located in the 0.1 THz timescale at room temperature and that shifts down to the GHz timescale when cooling the system toward the tetragonal-to-orthorhombic phase transition. The relation between relaxation time and maximum absorption frequency is also clarified.

Highlights

Hybrid perovskites of the form MAPbX3 with X = Cl, Br,I anions and MA = CH3NH+3 cation have attracted great interest in recent years for their excellent photovoltaic properties1 and for potential applications in optoelectronics,2 ionizing radiation imaging,3 nanoelectronics,4 and thermoelectrictiteym.5pMerAatPubreX3(εi′s characterized by a high dielectric constant at room ∼ 60 for iodides and ε′ ∼ 50 for bromides)6 and by a sizable dependence on temperature and frequency.6–8 The bulk polarization and the dielectric constant have important implications in the screening of photogenerated carriers,9,10 the formation of polarons,11 and the exciton binding energy12,13 and represent an important issue for a deep fundamental understanding of the material
Based on millimeter-wave spectroscopy,7 the dielectric constant was measured at frequencies of 50–150 GHz and temperatures of 100–300 K, indicating a discontinuity in the dielectric constant at the orthorhombic–tetragonal phase transition for the whole family of hybrid perovskites MAPbX3 (X = I, Br, Cl)
We show that a simple ionic model of interatomic forces (MYP) developed by Mattoni et al.19 and extensively applied to study hybrid perovskites20,21 is able to reproduce qualitatively the temperature and frequency dependence of the dielectric constant, making it possible to separate organic and inorganic contributions

Summary

Introduction

The magnitude of the dielectric constant is well-known from experiments, the nature of the polarization and the role of molecules are not accessible and have been largely discussed in the literature in terms of hypothetical ferroelectric ordering of molecules.. Studies on the dielectric constant of hybrid perovskites date back to the 1990s. Based on millimeter-wave spectroscopy, the dielectric constant was measured at frequencies of 50–150 GHz and temperatures of 100–300 K, indicating a discontinuity in the dielectric constant at the orthorhombic–tetragonal phase transition for the whole family of hybrid perovskites MAPbX3 (X = I, Br, Cl). The authors provided an interpretation of data in terms of the Debye relaxation model according to which the dependence of the dielectric constant (ε = ε′ + iε′′) on temperature and frequency is ε∼ C 1 ,

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Conclusion