Molecular alignment and Rashba splitting in organometal halide perovskite CH3NH3PbI3 absorbers

Organometal halide perovskite CH₃NH₃PbI₃ solar cells have witnessed unprecedented progress in power conversion efficiency, reaching more than 20 % within 5 years. Despite this rapid progress, there remain open questions about the basic properties of these materials, such as the role of the polar molecule CH₃NH₃ (methylammonium), and the possible existence of ferroelectric ordering, and its role in device operation. Using first-principles density-functional theory, we investigated the effect of molecular alignment on the electronic structure. We find that the molecular alignment significantly modifies the near-gap states indirectly through the induced structural distortion of the (PbI6)4- octahedron. The reduction in symmetry due to this distortion, combined with strong spin-orbit coupling of Pb, leads to a Rashba-like splitting of valence and conduction bands. This in turn leads to reduction of the degeneracy of the valence and conduction bands. These results imply that the electrical and optical properties are highly sensitive to ordering of the CH₃NH₃ dipole orientation.