Anomalous Behavior in Dark–Bright Splitting Impacts the Biexciton Binding Energy in (BA)₂(MA)_n−1Pb_nBr_3n+1 (n = 1–3)

Two-dimensional Ruddlesden–Popper series are an excellent system for tuning physical properties of the perovskite by controlling the layer number ( n ). For instance, bandgap and exciton binding energies of the series gradually increase upon reducing n via enhanced quantum and dielectric confinements. Here, we present findings that challenge the anticipated trend in electron–hole exchange interaction within (BA) 2 MA n –1 Pb n Br 3 n +1 ( n = 1–3), which causes spin-dependent exciton level splitting into bright and dark states, where the latter is partially visible near the surface of the Br-based two-dimensional Ruddlesden–Popper series. Contrary to expectations, the smallest gap between bright and dark exciton levels is observed from n = 2 at 10 K. This anomaly results in the strongest biexciton binding between two dark excitons occurring at n = 2, rather than at n = 1 as initially hypothesized. The observed anomaly arises from a phase transition induced by octahedral tilting occurring only for n = 2 near 100 K as confirmed by temperature-dependent optical and X-ray diffraction measurements. Our results show that Coulomb interaction need not vary gradually with n, which can impact the optoelectronic properties of the Ruddlesden–Popper series.