Anharmonicity and Octahedral Tilting in Hybrid Vacancy-Ordered Double Perovskites

Abstract

The advantageous performance of hybrid organic–inorganic perovskite halide semiconductors in optoelectronic applications motivates studies of their fundamental crystal chemistry. In particular, recent studies have sought to understand how dipolar, dynamic, and organic cations such as methylammonium (CH3NH3 +) and formamidinium (CH­(NH2)2 +) affect physical properties such as light absorption and charge transport. To probe the influence of organic–inorganic coupling on charge transport, we prepared the series of vacancy-ordered double perovskite derivatives A 2SnI6, where A = Cs+, CH3NH3 +, and CH­(NH2)2 +. Despite nearly identical cubic structures by powder X-ray diffraction, replacement of Cs+ with CH3NH3 + or CH­(NH2)2 + reduces conductivity through a reduction in both carrier concentration and carrier mobility. We attribute the trends in electronic behavior to anharmonic lattice dynamics from the formation of hydrogen bonds that yield coupled organic–inorganic dynamics. This anharmonicity manifests as asymmetry of the interoctahedral I–I pair correlations in the X-ray pair distribution function of the hybrid compounds, which can be modeled by large atomistic ensembles with random rotations of rigid [SnI6] octahedral units. The presence of soft, anharmonic lattice dynamics holds implications for electron–phonon interactions, as supported by calculation of electron–phonon coupling strength that indicates the formation of more tightly bound polarons and reduced electron mobilities with increasing cation size. By exploiting the relatively decoupled nature of the octahedral units in these defect-ordered perovskite variants, we interrogated the impact of organic–inorganic coupling and lattice anharmonicity on the charge transport behavior of hybrid perovskite halide semiconductors.

“Unleaded” Perovskites: Status Quo and Future Prospects of Tin‐Based Perovskite Solar Cells

“…It was found that, in MASnI 3 , Sn 2+ can easily be oxidized to Sn 4+ and replacing MA with FA can reduce the extent of Sn oxidation. The origins of the relative stability are not clear, yet it is possible that this relates to the thermodynamic stability of the corresponding oxidized products, MA 2 SnI 6 and FA 2 SnI 6 , with the latter being more stable than the former, possibly due to the larger separation of the discrete [SnI 6 ] 2− octahedra. Moreover, FASnI 3 film exhibits a lower conductivity than MASnI 3 (Figure e).…”

Rational Design of Halide Double Perovskites for Optoelectronic Applications

“…70,71 Recent studies have shown that the carrier mobilities of A 2 ,BX 6 could be further decreased in the case of any dynamic and/or static octahedral tilting. 72,73 On the other hand, for optical properties, because of the inversion symmetry, the transition from the VBM to the CBM is dipole forbidden, making the optical bandgap larger than the fundamental bandgap. 74 51 which may provide reasonably small bandgaps for single-junction solar cell applications.…”

Pressure‐Induced Structural Evolution and Bandgap Optimization of Lead‐Free Halide Double Perovskite (NH4)2SeBr6

“…Moreover, the optoelectronic properties can be further tuned by replacing the Cs + cation on the A site with an organic ligand such as NH 4 + or CH 3 NH 3 + . Specifically, the electronic structure can be effectively modified via coupling the ligands to the rotational dynamics of [BX 6 ] 2− octahedral units through the enhanced hydrogen‐bonding interactions with the surrounding X‐site framework . Clearly, such findings demonstrate that the optical properties of HDPs are strongly related to the specific crystal structures.…”