Orientational Glass Formation in Substituted Hybrid Perovskites

Abstract

Hybrid organic−inorganic perovskites have gained notoriety in the photovoltaic community for their composition-tunable band gaps and long-lived electronic excited states, which are known to be related to the crystalline phase. While it is known that the inorganic and organic components are coupled through structural phase transitions, it remains unclear as to what role each plays in directing the structure of hybrid perovskites such as methylammonium lead halides (CH 3 NH 3 PbX 3 ). Here, we present crystallographic and spectroscopic data for the series (CH 3 NH 3 ) 1−x Cs x PbBr 3 . CH 3 NH 3 PbBr 3 behaves as a plastic crystal in the high temperature cubic phase, and substitution of CH 3 NH 3 + with Cs + leads to the formation of an orientational glass. While the organic molecule exhibits slow, glassy reorientational dynamics, the inorganic framework continues to undergo crystallographic phase transitions. These crystallographic transitions occur in the absence of thermodynamic signatures in the specific heat from molecular orientation transitions, which suggests that the phase transitions result from underlying instabilities intrinsic to the inorganic lattice. However, these transitions are not decoupled from the reorientations of the organic molecule, as indicated by inelastic and quasielastic neutron scattering. Observation of a reentrant phase transition in (CH 3 NH 3 ) 0.8 Cs 0.2 PbBr 3 permits the resolution of these complex behaviors within the context of strain mediated interactions. Together, these results provide critical insight into the coupled phase behavior and dynamics in hybrid perovskites.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

“…4 meV). A similar shift and damping have been observed by Mozur et al[47] for the case of MA 1−x Cs x PbBr 3 , where the formation of an orientational MA + glass was observed at low temperature for relatively low concentrations of the Cs + cation. Similarly, the most intense peak observed within the librational part of the spectrum (see Lib.…”

Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy

“…66 Molecules in MHPs also reorient at longer timescales (nanoseconds to microseconds) than MHP phonon vibrations or photocarrier lifetimes. Molecular reorientations can introduce local octahedral distortions 110,291,292 and transient electronic 293,294 or dielectric disorder. 295 Techniques including temperature-dependent broadband dielectric spectroscopy, 291,[296][297][298] inelastic neutron scattering, 292,[299][300][301][302] and NMR dynamics 66,291,[303][304][305][306][307][308][309] studies can access the frequencies of different reorientation modes, ranging from fast tumbling of small A-site cations such as methylammonium (ps-ns), to slow rotations (ms) of phenylethylammonium spacers in 2D Ruddlesden-Popper phases.…”

Phase Behavior and Substitution Limit of Mixed Cesium-Formamidinium Lead Triiodide Perovskites

“…The width of the DSC peak centered at 290 K indicates a very slow transition occurring between 310 K and 273K. This slow transition reflects the glassy nature of hybrid halide perovskites, with the central organic cation known to exhibit slow, glass-like reorientational dynamics leading to disordered phases [33][34][35][36] . The width of this phase transition could be a source of the confusion as to the room temperature structure of Cs0.1FA0.9PbI3 [17][18][19][20] .…”