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Abstract Tuning broad emission in 2D Pb–Sn halide perovskites (HPs) is essential for advancing optoelectronic applications, particularly for color‐tunable and white‐light‐emitting devices. This broad emission is linked to structural factors, such as defects and phase segregation of the Pb component within the Pb–Sn system, which are strongly influenced by the molecular structure and chemical properties of spacer cations. Atomic tuning of the spacers via halogenation opens up a new way to fine‐tune the molecular properties, enabling further augmentations of HP functionalities. Nevertheless, the distinct broad emission's sensitivity to spacer chemistry remains underexplored. Here, halogenation's influence is systematically investigated on 2D HP emission characteristics using a high‐throughput workflow. These findings reveal that the F‐containing phenethylammonium (4F‐PEA) spacer narrows the broadband PL, whereas Cl broadens it. Through a correlative study, it is found that 4F‐PEA reduces not only the local phase segregation but also the defect levels and microstrains in 2D HPs. This is likely attributed to the manifestation of less lattice distortion via stronger surface coordination of the dipole‐augmented 4F‐PEA. These results highlight halogenation as a key factor in modulating phase segregation and defect density in 2D Pb–Sn HPs, offering a promising pathway to tune the emission for enhanced optoelectronic performance. 

Abstract Unlike single‐component 2D metal halide perovskites (MHPs) exhibiting sharp excitonic photoluminescence (PL), a broadband PL emerges in mixed Pb‐Sn 2D lattices. Two physical models –self‐trapped exciton and defect‐induced Stokes‐shift – are proposed to explain this unconventional phenomenon. However, the explanations provide limited rationalizations without consideration of the formidable compositional space, and thus, the fundamental origin of broadband PL remains elusive. Herein, the high‐throughput automated experimental workflow is established to systematically explore the broadband PL in mixed Pb‐Sn 2D MHPs, employing PEA (Phenethylammonium) as a model cation known to work as a rigid organic spacer. Spectrally, the broadband PL becomes further broadened with rapid PEA₂PbI₄phase segregation with increasing Pb concentrations during early‐stage crystallization. Counterintuitively, MHPs with high Pb concentrations exhibit prolonged PL lifetimes. Hyperspectral microscopy identifies substantial PEA₂PbI₄phase segregation in those films, hypothesizing that the establishment of charge transfer excitons by the phase segregation upon crystallization at high‐Pb compositions results in distinctive PL properties. These results indicate that two independent mechanisms—defect‐induced Stoke‐shifts and the establishment of charge transfer excitons by phase segregation—coexist which significantly correlates with the Pb:Sn ratio, thereby simultaneously contributing to the broadband PL emission in 2D mixed Pb‐Sn HPs.