Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic-Inorganic Halide Perovskites

Abstract

The performance of solar cells based on hybrid halide perovskites has seen an unparalleled rate of progress, while our understanding of the underlying physical chemistry of these materials trails behind. Superficially, CH₃NH₃PbI₃ is similar to other thin-film photovoltaic materials: a semiconductor with an optical band gap in the optimal region of the electromagnetic spectrum. Microscopically, the material is more unconventional. Progress in our understanding of the local and long-range chemical bonding of hybrid perovskites is discussed here, drawing from a series of computational studies involving electronic structure, molecular dynamics, and Monte Carlo simulation techniques. The orientational freedom of the dipolar methylammonium ion gives rise to temperature-dependent dielectric screening and the possibility for the formation of polar (ferroelectric) domains. The ability to independently substitute on the A, B, and X lattice sites provides the means to tune the optoelectronic properties. Finally, ten critical challenges and opportunities for physical chemists are highlighted.

Figure 1

Schematic of the perovskite crystal structure with respect to the A, B, and X lattice sites. The redox chemistry of the component ions can be used to influence the valence and conduction band energies and orbital composition, and hence the stability of electrons and holes in the material. Note that for larger molecular A sites layered perovskites are formed.^, Beyond halide perovskites, a wider range of stoichiometries and superstructures are known, e.g., the Ruddlesden–Popper, Aurivillius, and Dion–Jacobson phases.

Figure 2

Schematic of the ordering of molecular dipoles in the presence of an external electric field, as well as the four regimes in the dielectric response from lowest frequency (electronic excitations) to highest frequency (space charges and electronic or ionic conductivity). Each process will have a characteristic relaxation time and can combine to give a complex temporal response to an external perturbation.

Figure 3

Calculated natural band offsets of CH₃NH₃PbI₃ and related materials based on density functional calculations (with quasi-particle corrections). Interfacial or surface electric dipoles (or quadrupoles) are not considered here. Adapted with permission from ref (17). Copyright 2014 American Chemical Society.