Organic cations promote exciton dissociation in Ruddlesden–Popper lead iodide perovskites: a theoretical study

Abstract

Two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs) are a class of quantum well (QW) materials showing large exciton binding energy owing to quantum confinement. The existence of localized edge states was proposed to accelerate exciton dissociation into long-lived charge carriers in 2D RPPs, but recent experimental reports suggested that highly efficient internal exciton dissociation is achievable in 2D RPPs despite the absence of edge states. Herein, we adopt first-principles calculations to unveil the physical origin of the high internal quantum efficiency in the bulk region of widely familiar (BA)₂(MA)_n−1Pb_nI_3n+1 (BA = butylammonium; MA = methylammonium) materials. We discover that the dipolar nature of MA cations provides the driving force for the separation of photoexcited electron–hole pairs inside QWs as the inorganic layer thickens from n = 1 to n = 3. Concurrently, electronic coupling between organic spacer layers and QWs is enhanced in the energetically favorable configurations where MA cations orient with their CH₃ groups towards the exterior PbI₂ layers of QWs in the n = 3 structure. Consequently, hole delocalization is promoted along the out-of-plane direction of QWs, which in turn facilitates exciton dissociation into free charge carriers despite large exciton binding energy. Our simulations reveal that the hydrogen bonding between organic species (including both MA and BA cations) and iodine atoms, which is subtly interconnected, engineers the response of morphology in QWs and electronic interactions at organic–inorganic interfaces, providing novel insights for the exciton-free carrier behavior in the bulk area of 2D RPPs.