Out-of-plane quadrupolar excitons in Ruddlesden-Popper perovskites: theoretical insights into effects of organic spacer cations

Abstract

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs), a kind of semiconductor materials with strong quantum and dielectric confinement, offer a valuable platform for studying excitonic properties. However, the effects of organic spacer cations on excitons in 2D RPPs have not been elucidated to date, impeding the understanding for the nature of excitons and relevant physics behind the excitons in 2D RPPs. Herein, the excitonic properties in 2D RPPs which consist of organic and inorganic sublattices are assessed by adopting density functional theory and time-dependent density functional theory approaches. Strikingly, the out-of-plane (OP) quadrupolar characteristic of excitons is demonstrated in the inorganic sublattice – wherein the electron densities are confined in equatorial Pb-I layers sandwiched by the top and bottom apical I layers in presence of partial hole densities. Given that the oscillator strength of in-plane exciton is one order of magnitude larger than that of OP exciton, the OP quadrupolar charge distribution is mainly contributed by the in-plane exciton. The origin of the quadrupolar polarization of excitons is attributed to the hydrogen bonding between organic spacer cations and apical I ions of inorganic octahedron, which are sensitively determined by the dipolar nature of the organic spacer cations. Finally, the differences of the quadrupolar excitons between 2D RPPs and transition metal dichalcogenide trilayer heterostructures are discussed in this work.