Perovskitoids as Functional Materials

ConspectusFollowing the development of perovskite solar cells, the synthesis of hybrid Pb, Sn, and Ge halides exploded in popularity, with more than 3000 such crystal structures uploaded to the Cambridge Structural Database since the start of 2015. This synthetic interest has been accompanied by demonstrations of the materials' efficacy, such as solar cells, light-emitting diodes, and detectors. Though perovskites are the dominant focus for these optoelectronics applications, they comprise just over half of the 3000 structures mentioned. The nearly 1400 remaining nonperovskite materials include face- and edge-sharing structures like δ-FAPbI₃ and δ-CsPbI₃, often seen as undesirable products of failed perovskite syntheses. Indeed, all-face- and all-edge-sharing structures have had little success as optoelectronics, but a subset of these nonperovskites has demonstrated some success as functional materials. We call this subset perovskitoids, a class of materials defined, like perovskites, by their structural connectivity. While both perovskites and perovskitoids have corner-sharing octahedra in their crystal structures, perovskitoids can also contain face- or edge-sharing octahedra. This mixed sharing lends perovskitoids a much greater degree of structural diversity than is present in materials with a single sharing type, and the resulting materials combine properties of their respective connectivities.As corner-, edge-, and face-sharing octahedral connections require different M-X-M bond angles, the degree of orbital overlap between consecutive octahedra varies with the sharing type, resulting in a strong dependence of perovskitoids' bandgaps on their specific connectivities. This dependence enables bandgap modulation by varying fractions of corner-, edge, and face-sharing within perovskitoids, accessing bandgaps that, for perovskites, would require halide mixing or dimensional reduction. The added edge- and face-sharing connections in perovskitoids also lend the materials greater air, water, and thermal stability than their perovskite counterparts by virtue of the added redundancy of the octahedral connections.In this Account, we give an overview of the structures, properties, and applications of perovskitoids, focusing on the ways in which they resemble and differ from perovskites. Specifically, examples of common types of perovskitoids are presented along with a summary of the relative effects of corner-, edge-, and face-sharing connectivity on their bandgaps, luminescence, and stability. Following is a discussion of applications of perovskitoids, highlighting our groups' previous work on perovskitoid phosphors, photodetectors, X-ray detectors, γ-ray detectors, capping layers for perovskite solar cells, and wide bandgap solar absorbers. Subsequently, we discuss strategies for improving upon the optoelectronic properties of existing perovskitoids, focusing on the synthesis of perovskitoids with high fractions of corner-sharing. We hope this Account establishes perovskitoids as a promising and underexplored class of materials interesting in their own right and with the potential to improve upon the core materials challenges faced by perovskites.