Exciton photo-physics and dissociation in Ruddlesden-Popper 2D perovskites

Understanding and controlling charge and energy flow in state-of-the-art semiconductor quantum-wells has enabled high-efficiency optoelectronic devices. Organic inorganic Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A_2 A’_(n-1) M_n X_(3n+1), where A, A’ are cations, M is a metal, X is a halide, wherein the band gap can be tuned by varying the perovskite layer thickness (n value), which modulates the effective electron-hole confinement and their optoelectronic properties. They have recently emerged as efficient semiconductors for light emission and photovoltaics, with technologically relevant stability [1-3]. However, fundamental questions concerning the properties of excitons, their scaling with quantum well thickness, and the charge dynamics during device operation 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) [4]. We also report that counterintuitive to classical quantum-confined systems where photo-generated electrons and holes are strongly bound by Coulomb interactions or excitons, the photo-physics of thin films made of Ruddlesden-Popper perovskites with a thickness exceeding two perovskite crystal-units (>1.3 nanometers) is dominated by lower energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers [3]. These states provide a direct pathway for dissociating excitons into longer-lived free-carriers that significantly improve the performance of solar cell devices. Our work demonstrates the dominant role of Coulomb interactions in 2D solution-processed quantum wells but also find an intrinsic pathway for exciton dissociation at surfaces which presents unique opportunities for next-generation optoelectronic and photonic devices. [1] Tsai et al., Nature (2016), 536, 312-316. [2] M. Yuan et al., Nat. Nanotechnol. (2016), 11, 872-877. [3] Blancon et al., Science (2017), 355, 1288-1292. [4] Blancon et al., arXiv:1710.07653.