Exciton Delocalization and Excitonic Radiative Lifetime in Ternary Rhodium Halide Perovskites under Lattice Compression

Tuning excitonic delocalization in lower-dimensional nanostructured materials impacts the optical properties and subsequently the optoelectronic device performance. Hence, we investigate the effect of hydrostatic pressure on the exciton delocalization in the quasi-zero-dimensional ternary rhodium halide perovskite Cs3Rh2Br9 using many-body perturbation theory (MBPT)-based electronic structure calculations. The G0W0 coupled with spin–orbit coupling (SOC) reveals a direct bandgap at Γ–M points, which decreases from 2.31 eV at ambient pressure to 2.13 eV at 2.5 GPa, primarily due to enhanced Rh–d and Br–p orbital hybridization. The Bethe–Salpeter equation (BSE) simulations show pronounced excitonic effects, with exciton binding energy decreasing from 290 to 200 meV under pressure. The first bright exciton exhibits a Wannier–Mott character with reciprocal space localization. The excitonic radiative lifetimes at 300 K are exceptionally long, ranging from 4254.38 to 9529.03 μs, peaking at 1.5 GPa. These results highlight pressure as a viable tool for tuning the excitonic properties in Cs3Rh2Br9.