Unraveling the Broadband Emission in Mixed Tin‐Lead Layered Perovskites

Abstract

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.

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics

“…As Figure a shows, this crystal exhibits PL tunability from a narrow-band green color (the bandgap emission seen localized to the incident UV laser spot) to a broad-band red color (the defect-induced emission amplified under an IR laser excitation). Notably, the broadband emission studied here is weakly resolved under a band-exciting laser although some reports show it to be dominant, especially upon Sn-doping . In Figure b, the steady-state absorption shows that the crystal grown in a GBL solvent has the expected sharp absorption edge, while the HI solvent induces a broad absorption spectrum.…”

“…Generating broadband emission through carrier trapping (including phonon- or defect-assisted recombination , ) and exciton self-trapping typically requires an above intrinsic bandgap excitation to transfer to a lower energy state. However, the two emission bands studied here are excited independently and directly, without an efficient charge transfer pathway from the intrinsic band to the gap states . Instead, this photoluminescence duality is attributed to the relationship between the intrinsic band structure and the gap states created by defects in the crystal.…”

“…As schematically shown in Figure c, polarization resolved PL uncovers a preferential orientation of the intrinsic and charge transfer exciton states. This orientation is governed by the interplay between the crystallographic anisotropy of the lattice and defect-related interactions, likely involving iodide vacancies, which facilitate exciton formation between these distinct states. , …”

“…However, the two emission bands studied here are excited independently and directly, without an efficient charge transfer pathway from the intrinsic band to the gap states. 16 Instead, this photoluminescence duality is attributed to the relationship between the intrinsic band structure and the gap states created by defects in the crystal. Although defects often quench photoluminescence by acting as carrier trapping centers, 33 the intrinsic exciton emission is preserved under down-shifting conditions and a strong broadband emission emerges under up-conversion conditions.…”

“…Among the most reported of 2D-MHPs is phenethylammonium lead iodide (PEA 2 PbI 4 ), a low cost, easily synthesized, Ruddlesden–Popper phase perovskite with numerous optoelectronic applications including polarization-sensitive photodetectors, − high energy radiation detectors, , and light emitting devices. , PEA 2 PbI 4 typically exhibits a strong, narrow emission by virtue of its strong exciton binding energy (∼200 meV), but like many other MHPs, tends to form electron and hole intrinsic trapping levels, impacting the band edge and carrier mobility. , Interestingly, under some circumstances, an emergent sub-bandgap broadband emission may be induced. Several intrinsic mechanisms for this broadband emission have been proposed, such as self-trapped excitons (STE) and electronic trapping levels induced by iodide precipitates and vacancies. , Extrinsic approaches, such as Sn-doping of PEA 2 PbI 4 , can also generate a similar broadband emission. , In both intrinsic and extrinsic examples, the reported evidence strongly suggests the broadband emission can be attributed to a defect-induced formation of electron and hole gap states. − Iodide vacancies, in particular, are supported by the determination of donor-level iodide vacancies, , chemical passivation treatments, and associated structural effects, such as iodide vacancy-induced octahedral distortion . In some 2D-MHPs, sufficient iodide vacancy formation leads to low-dimensional phases (1D and 0D) exhibiting broadband emission from STEs. , Surprisingly, there is still no consensus in the research literature regarding the specific energetic pathway that enables broadband emissive behavior in 2D-MHPs.…”