Exciton Dissociation at Grain Boundaries in Lead Halide Ruddelsden Popper Perovskite Thin Films

Solution processable Ruddlesden-Popper phase lead halide perovskite (RPP) thin films show greatly enhanced moisture and air stability compared to their 3D lead halide perovskite counterparts. Photovoltaic cells based on RPPs appear to get a performance boost due to exciton dissociation at grain boundaries. Herein we report the direction observation of exciton dissociation in phase pure n=3 RPP polycrystalline thin films using transient absorption spectroscopy. We observe distinct spectral features corresponding to excitons in the bulk of the grains and ‘free carrier’ states which appear to be segregated on the surface of the grains. These ‘surface states’ are the lowest energy electronic excitation in the system, and thus comprise the optical gap of the material. The observed kinetics indicate that grain boundary states are populated both directly after photoexcitation, and via the decay of photo-excited excitons. The interconversion kinetics between excitons and grain boundary states are mediated by the average polycrystalline grain size which is determined by the identity of the long chain organic cation, and other aspects of film processing conditions.