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

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs), a type of semiconductor material 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 of the nature of excitons and the relevant physics behind the excitons in 2D RPPs. Herein, the excitonic properties in 2D RPPs consisting 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 the equatorial Pb-I layers sandwiched by the top and bottom apical I layers in the presence of partial hole densities. Given that the oscillator strength of the in-plane exciton is one order of magnitude larger than that of the OP exciton, the OP quadrupolar charge distribution is mainly contributed by the in-plane exciton. The origin of the quadrupolar polarization of the excitons is attributed to the hydrogen bonding between the organic spacer cations and the apical I ions of the inorganic octahedron, which is sensitively determined by the dipolar nature of the organic spacer cations. Finally, the differences between the quadrupolar excitons in 2D RPPs and transition metal dichalcogenide trilayer heterostructures are discussed in this work.