Stable s 2 lone pair electrons on heavy main-group elements in their lower oxidation states drive a range of important phenomena, such as the emergence of polar ground states in some ferroic materials. Here we study the perovskite halide CsSnBr 3 as an embodiment of the broader materials class. We show that lone pair stereochemical activity due to the Sn s 2 lone pair causes a crystallographically hidden, locally distorted state to appear upon warming, a phenomenon previously referred to as emphanisis.The synchrotron X-ray pair distribution function acquired between 300 K and 420 K reveals emerging asymmetry in the nearest-neighbor Sn-Br correlations, consistent with dynamic Sn 2+ off-centering, despite there being no evidence of any deviation from the average cubic structure. Computation based on density functional theory supports the finding of a lattice instability associated with dynamic off-centering of Sn 2+ in its coordination environment. Photoluminescence measurements reveal an unusual blueshift with increasing temperature, closely linked to the structural evolution. At low temperatures, the structures reflect the influence of octahedral rotation. A continuous transition from an orthorhombic structure (P nma, no. 62) to a tetragonal structure (P 4/mbm, no. 127) is found around 250 K, with a final, first-order transformation at 286 K to the cubic structure (P m3m, no. 221).2

There has been increasing interest in rational, computationally driven design methods for materials, including organic photovoltaics (OPVs). Our approach focuses on a screening "pipeline", using a genetic algorithm for first stage screening and multiple filtering stages for further refinement. An important step forward is to expand our diversity of candidate compounds, including both synthetic and property-based measures of diversity. For example, top monomer pairs from our screening are all donor-donor (D-D) combinations, in contrast with the typical donor-acceptor (D-A) motif used in organic photovoltaics. We also find a strong "sequence effect", in which the average HOMO-LUMO gap of tetramers changes by ∼0.2 eV as a function of monomer sequence (e.g., ABBA versus BAAB); this has rarely been explored in conjugated polymers. Beyond such optoelectronic optimization, we discuss other properties needed for high-efficiency organic solar cells, and applications of screening methods to other areas, including non-fullerene n-type materials, tandem cells, and improving charge and exciton transport.

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Molecular A-cation dynamics are known to play a role in the electronic properties and structure of hybrid organic-inorganic ABX 3 perovskites such as CH 3 NH 3 PbI 3 .We calculate the full energy landscape for rigid-body rotations and translations of the methylammonium cation in the cubic phase of CH 3 NH 3 PbI 3 . Energy barriers are calculated for combinations of molecular reorientation, on-axis rotation, and molecular translation within the unit cell. Allowing molecular translations significantly stabilizes orientations along [100] which we attribute to strong N−H· · · I interactions between CH 3 NH 3 + and the inorganic Pb−I host lattice.

Main-group halide perovskites have generated much excitement of late because of their remarkable optoelectronic properties, ease of preparation, and abundant constituent elements, but these curious and promising materials differ in important respects from traditional semiconductors. The distinguishing chemical, structural, and electronic features of these materials present the key to understanding the origins of the optoelectronic performance of the well-studied hybrid organic-inorganic lead halides and provide a starting point for the design and preparation of new functional materials. Here we review and discuss these distinguishing features, among them a defect-tolerant electronic structure, proximal lattice instabilities, labile defect migration, and, in the case of hybrid perovskites, disordered molecular cations. Additionally, we discuss the preparation and characterization of some alternatives to the lead halide perovskites, including lead-free bismuth halides and hybrid materials with optically and electronically active organic constituents.