Correlating Rashba Suppression with Exciton Binding in Low-Dimensional Metal Halide Perovskites

We used DFT-1/2+SOC band structures mapped to Wannier tight-binding Hamiltonians and solved with the Bethe-Salpeter equation to examine how confinement, polymorphism, and metal off-centering tune excitons in Pb- and Sn-iodide perovskites in 3D (MA(Pb or Sn)I3), quasi-2D (BA2(PborSn)I4), and model quasi-1D (BA3(PborSn)I5). The 3D polymorphs show a clear trend: as symmetry increases from orthorhombic to tetragonal to pseudocubic, Eb decreases. At fixed dimensionality, restoring local metal-centered symmetry suppresses Rashba splitting. It reduces or preserves Eb, indicating that SOC affects excitons mainly through band-edge structure rather than direct strengthening of the electron-hole Coulomb kernel. Low-dimensional compounds retain much larger Eb than 3D analogs, although quasi-1D is not monotonic with quasi-2D. Photoluminescence spectra place the lowest excitons at the optical gap, consistent with bright ground-state excitons.