Polarity and Site Symmetry Synergize to Drive Ultrafast Self-Trapped Exciton Formation in Sb-Doped 0D Perovskites

Zero-dimensional (0D) metal halide perovskites exhibit strong excitonic effects and excellent stability, making them promising for broadband light emission. Using linear-response time-dependent density functional and nonadiabatic molecular dynamics, we explored how A-site cation polarity and dopant-site symmetry affect self-trapped exciton (STE) dynamics in Sb-doped 0D InCl₆-based perovskites. Sb doping introduces a flat valence band and conduction-band splitting, enhancing hole localization and STE formation. When Sb occupies a mirror-symmetric site within a highly polar OH₃⁺ environment, strong dipole-lattice interactions drive rapid local distortion, deep STE binding, and ultrafast STE formation within 8 fs─much faster than the 20-30 fs in less polar or centrosymmetric cases. STE formation is dominated by low-frequency phonons (<400 cm^-1). Real-space analysis reveals electron-hole localization on the Sb site, while the polar OH₃⁺ environment promotes both local distortion and cooperative octahedral motion, stabilizing the STE. These results show that the synergy between high A-site polarity and mirror-symmetric dopant environments accelerates and stabilizes STE formation. This work provides mechanistic insight and a rational strategy for engineering efficient, broadband-emitting perovskite optoelectronics.