Unraveling the Broadband Emission in Mixed Tin-Lead Layered Perovskites

Low-dimensional halide perovskites with broad emission are a hot topic for their promising application as white light sources. However, the physical origin of this broadband emission in the sub-bandgap region is still controversial. This work investigates the broad Stokes-shifted emission bands in mixed lead-tin 2D perovskite films prepared by mixing precursor solutions of phenethylammonium lead iodide (PEA₂PbI₄, PEA = phenethylammonium) and phenethylammonium tin iodide (PEA₂SnI₄). The bandgap can be tuned by the lead-tin ratio, whereas the photoluminescence is broad and significantly Stokes-shifted and appears to be fairly insensitive to the relative amount of Pb and Sn. It is experimentally observed that these low-dimensional systems show substantially less bandgap bowing than their 3D counterpart. Theoretically, this can be attributed to the smaller spin–orbit coupling effect on the 2D perovskites compared to that of 3D ones. The time-resolved photoluminescence shows an ultrafast decay in the high-energy range of the spectra that coincides with the emission range of PEA₂SnI₄, while the broadband emission decay is slower, up to the microsecond range. Sub-gap photoexcitation experiments exclude exciton self-trapping as the origin of the broadband emission, pointing to defects as the origin of the broadband emission in 2D Sn/Pb perovskite alloys.