Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites

Abstract

Hybrid perovskites have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells combining the advantages of organic and inorganic semiconductors. Unfortunately, electrical transport in these materials is still poorly understood. Employing the linear response approach of density functional theory, we reveal strong anharmonic effects and a double-well phonon instability at the center of the Brillouin zone for both cubic and orthorhombic phases of inorganic CsPbI. Previously reported soft phonon modes are stabilized at the actual lower-symmetry equilibrium structure, which occurs in a very flat energy landscape, highlighting the strong competition between the different phases of CsPbI. Factoring these low-energy phonons into electron-phonon interactions and band gap calculations could help better understand the electrical transport properties in these materials. Furthermore, the perovskite oscillations through the corresponding energy barrier could explain the underlying ferroelectricity and the dynamical Rashba effect predicted in halide perovskites for photovoltaics.

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…22,[48][49][50]53,[56][57][58]83 We hope that new results such as these may inform a key point of contention: Are the molecular cations simply larger monocations than any (non-radioactive)…”

“…50 These hypotheses are somewhat misaligned with the growing body of evidence suggesting an important role for local distortions of the inorganic sublattice. 22,[51][52][53][54][55][56][57][58][59] Indeed, there are now numerous indications that the appealing optoelectronic properties of the hybrid perovskites are qualitatively matched by their all-inorganic analogues. 54,[60][61][62][63][64] In order then to delineate the properties and functionality of the molecular cations from those of the inorganic framework, one must first complete the description of the structure and dynamics of each.…”

Dynamic shortening of disorder potentials in anharmonic halide perovskites

“…30 These findings strongly suggest that anharmonicity is an important phenomenon in the nuclear dynamics of HaPs, in agreement with previous studies on different HaPs. [13][14][15][16][17][18][19][20][21][22][23][24][25] It will now be shown that the mechanism underlying anharmonicity in HaPs renders the presence of a shortrange correlated disorder potential particularly interesting.…”

“…Interestingly, however, ample experimental and theoretical evidence point to highly anharmonic nuclear motion and disorder being active in HaPs at room temperature, which also involves the ions in the crystal contributing to the frontier electronic band structure. [13][14][15][16][17][18][19][20][21][22][23][24][25] Such anharmonic effects are uncommon for efficient optoelectronic materials 26 and difficult to rationalize in view of the low Urbach energy. In particular, they can be suspected to result in a disordered potential for the electrons and holes in the crystal and, hence, in a high density of tail states and a broad optical absorption.…”

Advances in modelling and simulation of halide perovskites for solar cell applications

“…When compared with the hybrid halide perovskites, it is surprising that the soft mode is found even at the zone centre point Γ ( figure 9(c)), which is related to the ferroelectric distortion. Marronnier et al [155,156] thoroughly investigated this anomaly at Γ for the cubic and tetragonal phases of CsPbI 3 . As a polar mode, the soft phonon mode found at Γ for cubic CsPbI 3 is linked to the displacements of the Cs + cation in one direction and of the I − anion in the opposite direction.…”