From 0D Cs3Bi2I9 to 2D Cs3Bi2I6Cl3: Dimensional Expansion Induces a Direct Band Gap but Enhances Electron–Phonon Coupling

Alternative all-inorganic halide perovskites are sought to replace the hybrid lead halide perovskites because of their increased stability. Here, the (111)-oriented defect perovskite family A3M2X9 based on trivalent M3+ is expanded through the use of mixed halides, resulting in Cs3Bi2I6Cl3. This compound shares the (111)-oriented 2D bilayer structure of α-Cs3Sb2I9 (space group P3̅m1), with Cl occupying the bridging positions of the bilayers and I in the terminal sites, in contrast to the parent compound Cs3Bi2I9, which consists of 0D molecular [Bi2I9]3– dimers. The increased dimensionality induces a direct band gap as calculated by density functional theory but has an absorption edge of 2.07 eV, nearly identical to the indirect band gap compound Cs3Bi2I9. Intriguingly, there is a remarkable lack of Cl orbital contribution to the band edge states of Cs3Bi2I6Cl3, despite Bi–Cl bonds binding all octahedra together. This highlights the importance of interlayer interactions in the defect perovskite family, which enhances the effective dimensionality of these 2D and 0D materials and may improve their optoelectronic performance. However, these changes in the excitonic absorption do not reflect free excitons, as Cs3Bi2I6Cl3 exhibits broad photoluminescence as a result of self-trapped excitons, which appear to be universal in the (111)-oriented defect perovskites.