Stochastic Charge-Transfer Excitons in 2D Metal-Halide Perovskites

The coupling between the polar lattice and the electronic system is a key ingredient enabling the impressive properties of 2D and 3D metal-halide perovskites (MHPs). For example, mechanisms such as polaron formation and edge states have been proposed to explain efficient exciton dissociation observed in 2D MHPs. However, neither mechanism has been directly validated. By analyzing the temperature dependence of the exciton's linear and quadratic Stark shifts, this study reveals another important aspect of charge–lattice interactions. We find excitons are photoexcited into spatially separated states and possess large dipole moments and, thus, have significant charge-transfer character. The dipole moments are induced by thermal fluctuations in the polar lattice. Molecular dynamics simulations, which agree closely with our experiments in predicting the measured dipole moments, also predict that these thermal fluctuations could be sufficient to ionize the electron–hole system.