Anomalous Behavior in Dark–Bright Splitting Impacts the Biexciton Binding Energy in (BA)₂(MA)_<i>n</i>−1Pb_<i>n</i>Br_3<i>n</i>+1 (<i>n</i> = 1–3)

Two-dimensional Ruddlesden-Popper series are an excellent system for tuning physical properties of the perovskite by controlling the layer number (<i>n</i>). For instance, bandgap and exciton binding energies of the series gradually increase upon reducing <i>n</i> via enhanced quantum and dielectric confinements. Here, we present findings that challenge the anticipated trend in electron-hole exchange interaction within (BA)₂MA_<i>n</i>-1Pb_<i>n</i>Br_3<i>n</i>+1 (<i>n</i> = 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 <i>n</i> = 2 at 10 K. This anomaly results in the strongest biexciton binding between two dark excitons occurring at <i>n</i> = 2, rather than at <i>n</i> = 1 as initially hypothesized. The observed anomaly arises from a phase transition induced by octahedral tilting occurring only for <i>n</i> = 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 <i>n</i>, which can impact the optoelectronic properties of the Ruddlesden-Popper series.