Structural Distortions Control Scaling of Exciton Binding Energies in Two-Dimensional Ag/Bi Double Perovskites

TL;DR

This study reveals that structural distortions in 2D Ag/Bi double perovskites, driven by Ag d orbital bonding, control exciton binding energies and explain anomalous experimental trends, providing insights for designing lead-free layered materials.

Contribution

The paper demonstrates that octahedral distortions, rather than layer thickness alone, primarily influence excitonic properties in 2D Ag/Bi perovskites, challenging previous assumptions.

Findings

Structural distortions govern exciton behavior.

Interlayer distance affects band gaps and exciton energies.

Confinement physics in Ag/Bi perovskites mirrors Pb-based counterparts.

Abstract

Three-dimensional metal halide double perovskites such as Cs$_2$AgBiBr$_6$ exhibit pronounced excitonic effects due to their anisotropic electronic structure and chemical localization effects. Their two-dimensional derivatives, formed by inserting organic spacer molecules between perovskite layers, were expected to follow well-established trends seen in Pb-based 2D perovskites, namely, increasing exciton binding energies with decreasing layer thickness due to enhanced quantum and dielectric confinement. However, recent experimental and computational studies have revealed anomalous behavior in Ag/Bi-based 2D perovskites, where this trend is reversed. Using ab initio many-body perturbation theory within the $GW$ and Bethe-Salpeter Equation frameworks, we resolve this puzzle by systematically comparing experimental structures with idealized models designed to isolate the effects of octahedral distortions, interlayer separation, and stacking. We find that structural distortions, driven by directional Ag d orbital bonding, govern the momentum-space origin and character of the exciton, and are the primary cause of the observed non-monotonic trends. Furthermore, we explore how interlayer distance and stacking influence band gaps and exciton binding energies, showing that, despite different chemistry, the underlying confinement physics mirrors that of Pb-based 2D perovskites. Our results establish design principles for tuning excitonic properties in this broader class of layered, lead-free materials.