Coherent exciton dynamics in lead halide perovskites probed via two-dimensional electronic spectroscopy

Two-dimensional lead halide perovskites have recently emerged as promising optoelectronic materials and are often referred to as quantum-well-like structures. They exhibit narrow and well-defined excitonic transitions in linear absorption, with large exciton binding energies (200-300 meV). Whilst such a spectral structure can be rationalized with a 2D Elliott model accounting for the electronic confinement within the lead halide layers, we also observe a pronounced fine structure within the exciton line, which cannot be explained by such a model. Here, we investigate its origin via optical and vibrational spectroscopy of a series of 2D perovskites templated by different organic cations. We establish the role of octahedral distortions in excitonic correlations, which lift the orbital degeneracies via Jahn-Teller like mechanism and subsequently result in four distinct excitonic states. These transitions are also seen in the two-dimensional correlation maps obtained via multi-dimensional spectroscopy, where we identify spectral correlations. We observe multi-particle interactions using two-quantum coherence spectroscopy, where we identify bi-exciton resonances and extract biexciton binding energies of around 50 meV, one of the highest reported values for two-dimensional materials.