Unraveling luminescence mechanisms in zero-dimensional halide perovskites

Zero-dimensional (0D) halides perovskites, in which anionic metal-halide octahedra (MX₆)^4- are separated by organic or inorganic countercations, have recently shown promise as excellent luminescent materials. However, the origin of the photoluminescence (PL) and, in particular, the different photophysical properties in hybrid organic-inorganic and all inorganic halides are still poorly understood. In this work, first-principles calculations were performed to study the excitons and intrinsic defects in 0D hybrid organic-inorganic halides (C₄N₂H₁₄X)₄SnX₆ (X = Br, I), which exhibit a high photoluminescence quantum efficiency (PLQE) at room temperature (RT), and also in the 0D inorganic halide Cs₄PbBr₆, which suffers from strong thermal quenching when T > 100 K. We show that the excitons in all three 0D halides are strongly bound and cannot be detrapped or dissociated at RT, which leads to immobile excitons in (C₄N₂H₁₄X)₄SnX₆. However, the excitons in Cs₄PbBr₆ can still migrate by tunneling, enabled by the resonant transfer of excitation energy (Dexter energy transfer). The exciton migration in Cs₄PbBr₆ leads to a higher probability of trapping and nonradiative recombination at the intrinsic defects. We show that a large Stokes shift and the negligible electronic coupling between luminescent centers are important for suppressing exciton migration; thereby, enhancing the photoluminescence quantum efficiency. Our results also suggest that the frequently observed bright green emission in Cs₄PbBr₆ is not due to the exciton or defect-induced emission in Cs₄PbBr₆ but rather the result of exciton emission from CsPbBr₃ inclusions trapped in Cs₄PbBr₆.