Fine-Tuning Self-Trapped Excitons via External Coordination of Perovskite Octahedra: Simultaneous Achievement of Significant Stokes Shifts and Enhanced Emission

Two zero-dimensional Sb-doped bismuth-based halide perovskites, Sb-doped Cs3BiCl6 and Cs4BiCl7, were synthesized, and an in-depth analysis was conducted to elucidate the connection between lattice structure and self-trapped excitons (STE) properties. These perovskites introduced impurity energy levels in the valence band, enhancing electronic transitions and improving STE emission. Notably, Sb-doped Cs4BiCl7, an innovative material, exhibited higher photoluminescence (PL) intensity and a red shift in PL. The primary difference between these materials was the octahedral coordination environment, given their consistent elemental composition, octahedral dimensionality, and synthesis method. Cs4BiCl7's looser atomic arrangement resulted in stronger phonon–electron coupling compared to Cs3BiCl6. Additionally, nonoctahedral chlorine in Cs4BiCl7 increased electron occupation density in the conduction band, leading to a larger Stokes shift and more efficient electronic transitions. This study underscores the impact of the octahedral coordination environment on PL properties through detailed local structural analysis.