Decoding the Broadband Emission of Two-Dimensional Pb-Sn Halide Perovskites through High-Throughput Exploration

Unlike single-component two-dimensional (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 – have been proposed to explain this unconventional phenomenon. However, both explanations provide limited rationalizations without consideration of the formidable compositional space, and thus, the fundamental origin of broadband PL remains elusive. Herein, we established our high-throughput automated experimental workflow 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 PEA2PbI4 phase segregation with increasing Pb concentrations during early-stage crystallization. Counterintuitively, MHPs with high Pb concentrations exhibit prolonged PL lifetimes despite high defect densities. Hyperspectral microscopy identifies substantial PEA2PbI4 phase segregation in those films, hypothesizing that the establishment of charge transfer excitons by the phase segregation upon crystallization is responsible for the extraordinary behavior; at high-Pb compositions, this far outperforms the leverage of defect-induced emission, thereby resulting in distinctive PL properties. Our high-throughput approach allows us to reconcile the controversial prior models describing the origin of the broadband emission in 2D Pb-Sn MHPs, shedding light on how to comprehensively explore the fundamentals and functionalities of the complex materials systems.