Excitonic Many-Body Interactions in Two-Dimensional Lead Iodide Perovskite Quantum Wells

While the perovskite fever has focused on three-dimensional crystalline solids, this class of material can also self-assemble into two-dimensional (2D) layered structures that are natural quantum wells with tunable thickness and optoelectronic properties. Here we apply femtosecond transient absorption spectroscopy to study the many-body optical responses of 2D perovskites with the general formula of (C4H9NH3I)(2)(CH3NH3I)(n-1)(PbI2)(n), where n = 1, 2, 3) is the number of lead iodide unit cells in the direction perpendicular to the 2D quantum well. In the thinnest quantum well (n = 1), above-gap optical excitation induces a blue shift but no population bleaching at the excitonic resonance; this is similar to the many-body optical response of conventional inorganic quantum wells. In contrast to inorganic quantum wells, we find the excitonic blue-shift in 2D perovskites to be independent of excitation power density. We take this as evidence for a Mott-Wannier exciton localizing into a "puddle", which only exerts local influence on subsequent optical excitations. The excitonic puddles likely come from the disordered electronic energy landscape expected for the soft 2D hybrid organic-inorganic perovskite lattice. As the thickness of the quantum well increases to n = 3, free carrier characters start to show up for above band gap excitation; this is reflected in the broadening and bleaching of the excitonic resonance (in addition to blue-shift), attributed to carrier-exciton collision and screening of the Coulomb potential, respectively.