2, Rice University, Houston, Texas, United States
3, Northwestern University, Evanston, Illinois, United States
4, INSA de Rennes, Rennes, , France
Understanding the nature and energy distribution of optical resonances is of central importance in low-dimensional materials and its knowledge is critical for designing efficient optoelectronic devices. Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where A, A’ are cations, M is a metal, X is a halide, and their optical and electronic properties can be tuned by varying the perovskite layer thickness (n value). They have recently emerged as efficient semiconductors for light emission and photovoltaics, with technologically relevant stability [1-3]. However, fundamental questions concerning the nature of optical resonances (excitons or free-carriers), their scaling with quantum well thickness, and the physics behind the exciton properties, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5 (equivalent quantum well thickness from 0.64 to 3.14 nm) . Comprehensive modelling of exciton states in 2D perovskites enable the understanding of dielectric confinement effects which prevail over quantum confinement in thin 2D perovskites. In addition, from these results we produce a general scaling behaviour for the binding energy of Wannier-Mott exciton states in Ruddlesden-Popper perovskites, which predict the exciton binding energy for any given thickness and organic spacer group. Our work demonstrates the dominant role of Coulomb interactions in 2D solution-processed quantum wells and presents unique opportunities for next-generation optoelectronic and photonic devices.
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