Understanding and Engineering Excitonic Properties in 2d Metal Halide Perovskites

As the field of 2d metal halide perovskites (MHPs) matures, state-of-the-art techniques to measure fundamental properties such as the band gap (E_g) and exciton binding energy (E_b), continue to produce inconsistent values which defy a coherent interpretation. In this work we present the results of a recently published paper [Hansen et al., Matter 6, 3463-3482 (2023)] in which we have used electroabsorption spectroscopy to precisely and unambiguously measure E_g and E_b for 31 different MHP compositions which include variations in metal atoms, halide atoms, organic cation molecules, and “n-number”. Among other things the results show a clear and direct correlation between E_g and E_b, and also a clear dependence of E_b on organic cation length which indicates that models of isolated quantum wells cannot in general be used to explain excitonic behavior in 2d MHPs. Instead, we have applied a known superlattice theory of repeating wells and barriers, which includes both quantum and dielectric confinement effects. This theory requires separate knowledge of dielectric constants for wells and barriers, which we were able to deduce in a self-consistent manner for many materials. Using the results of that theory we can tell that the most important parameters that affect E_b are well dielectric constant (metal halide layers), barrier length (organic molecules), and exciton reduced mass which we also relate to the MHP octahedral distortion; and we generate a parameter space map of realistically achievable (E_g, E_b) values. Finally, the dielectric constants required by the theory are between the microwave and optical-frequency dielectric values, indicating that the excitonic interaction is partially screened by phonons.