Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure-property relationships of these materials, we have synthesized solid-solution Cs2Sn1-xTexI6. However, even though tellurium substitution increases electronic dispersion via closer I-I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te-I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure-property relationships in functional perovskite halides.

Incorporating chiral organic molecules into organic/inorganic hybrid 2D metal-halide perovskites results in a novel family of chiral hybrid semiconductors with unique spin-dependent properties. The embedded chiral organic moieties induce a chiroptical response from the inorganic metal–halide sublattice. However, the structural interplay between the chiral organic molecules and the inorganic sublattice, as well as their synergic effect on the resulting electronic band structure need to be explored in a broader material scope. Here we present three new layered tin iodide perovskites templated by chiral (R/S-)­methylbenzylammonium (R/S-MBA), i.e., (R-/S-MBA)2SnI4, and their racemic phase (rac-MBA)2SnI4. These MBA2SnI4 compounds exhibit the largest level of octahedral bond distortion compared to any other reported layered tin iodide perovskite. The incorporation of chiral MBA cations leads to circularly polarized absorption from the inorganic Sn–I sublattice, displaying chiroptical activity in the 300–500 nm wavelength range. The bandgap and chiroptical activity are modulated by alloying Sn with Pb, in the series of (MBA)2Pb1–x Sn x I4. Finally, we show that vertical charge transport through oriented (R-/S-MBA)2SnI4 thin films is highly spin-dependent, arising from a chiral-induced spin selectivity (CISS) effect. We demonstrate a spin-polarization in the current–voltage characteristics as high as 94%. Our work shows the tremendous potential of these chiral hybrid semiconductors for controlling both spin and charge degrees of freedom.

Halide perovskite semiconductors such as methylammonium lead iodide (CH 3 NH 3 PbI 3) have achieved great success in photovoltaic devices, yet concerns surrounding toxicity of lead and material stability have motivated the field to pursue alternative perovskite compositions and structures. Vacancy-ordered double perovskites are a defect-ordered variant of the perovskite structure characterized by an antifluorite arrangement of isolated octahedral units bridged by A-site cations. In this perspective, we focus upon the structure-dynamics-property relationships in vacancy-ordered double perovskite semiconductors as they pertain to applications in photovoltaics, and propose avenues of future study within the context of the broader 1 perovskite halide literature. We describe the compositional and structural motifs that dictate the optical gaps and charge transport behavior and discuss the implications of charge ordering, lattice dynamics, and organic-inorganic coupling upon the properties of these materials. The design principles we elucidate here represent a first step towards extending our understanding of perovskite functionality to defect-ordered perovskites.

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.

Lattice dynamics and structural instabilities are strongly implicated in dictating the electronic properties of perovskite halide semiconductors. We present a study of the vacancy-ordered double perovskite Rb2SnI6 and correlate dynamic and cooperative octahedral tilting with changes in electronic behavior compared to those of Cs2SnI6. Though both compounds exhibit native n-type semiconductivity, Rb2SnI6 exhibits carrier mobilities that are reduced by a factor of ∼50 relative to Cs2SnI6. From synchrotron powder X-ray diffraction, we find that Rb2SnI6 adopts the tetragonal vacancy-ordered double perovskite structure at room temperature and undergoes a phase transition to a lower-symmetry monoclinic structure upon cooling, characterized by cooperative octahedral tilting of the [SnI6] octahedra. X-ray and neutron pair distribution function analyses reveal that the local coordination environment of Rb2SnI6 is consistent with the monoclinic structure at all temperatures; we attribute this observation to dynamic octahedral rotations that become frozen in to yield the low-temperature monoclinic structure. In contrast, Cs2SnI6 adopts the cubic vacancy-ordered double perovskite structure at all temperatures. Density functional calculations show that static octahedral tilting in Rb2SnI6 results in marginally increased carrier effective masses, which alone are insufficient to account for the experimental electronic behavior. Rather, the larger number of low-frequency phonons introduced by the lower symmetry of the Rb2SnI6 structure yield stronger electron–phonon coupling interactions that produce larger electron effective masses and reduced carrier mobilities relative to Cs2SnI6. Further, we discuss the results for Rb2SnI6 in the context of other vacancy-ordered double perovskite semiconductors, in order to demonstrate that the electron–phonon coupling characteristics can be predicted using the geometric perovskite tolerance factor. This study represents an important step in designing perovskite halide semiconductors with desired charge transport properties for optoelectronic applications.

The choice of organic cation in hybrid perovskites has large implications for optoelectronic properties, material stability, and crystal structure. In particular, formamidinium ((CH(NH 2 ) 2 + ) perovskites exhibit unusual temperature-dependent trends in photoluminescence, dielectric constant, and phase behavior that are hypothesized to relate to CH(NH 2 ) 2 + reorientations. This contribution describes five distinct, temperaturedependent phase transitions in CH(NH 2 ) 2 PbBr 3 that produce changes in the steady-state photocurrent. Three of these phase transitions do not resolve crystallographically and relate to the orientation and dynamics of CH(NH 2 ) 2 + . These crystallographically unresolvable phase transitions resemble ferroelastic transitions with the formation of nanoscale domains, which we hypothesize are mediated by the strain exerted from geometric frustration of CH(NH 2 ) 2 + quadrupoles. This work demonstrates the importance of cation orientation and dynamics, domain behavior, and their interdependence in the steady-state optoelectronic properties of hybrid perovskites.

Although initial studies on hybrid perovskites for photovoltaic applications focused on simple compositions, the most technologically relevant perovskites are heavily substituted. The influence of chemical substitution on the general phase behavior and specific physical properties remains ambiguous. The hybrid perovskite formamidinium lead bromide, CH(NH2)2PbBr3, exhibits complex phase behavior manifesting in a series of crystallographically-unresolvable phase transitions associated with changes in the cation dynamics. Here, we characterize the molecular and lattice dynamics of CH(NH2)2PbBr3 as a function of temperature, and their evolution upon chemical substitution of CH(NH2)2 + for cesium (Cs + ) with crystallography, neutron scattering, 1 H and 14 N nuclear magnetic resonance spectroscopy, and 79 Br nuclear quadrupolar spectroscopy. Cs + substitution suppresses the four low-temperature phase transitions of CH(NH2)2PbBr3, which propagate through concerted changes in the dynamic degrees of freedom of the organic sub-lattice and local or long-range distortions of the octahedral framework. We propose that cesium substitution suppress the phase transitions through the relief of geometric frustration associated ** Both authors contributed equally.with the orientations of CH(NH2)2 + molecules, which retain their local dynamical degrees of freedom.

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.