Fluorescence Properties In Two Dimensional Lead Halide Perovskite Under High Pressure

Organic lead halide perovskites have enormous potential applications in photovoltaic cells,light-emitting diodes,lasers and detectors due to their high optical absorption coefficient,suitable band gap and tunable electronic structure.By adjusting the types of organic cations and the number of inorganic layers in perovskite,not only the stability of the material can be improved,but also the electronic structure of the material can be adjusted.In particular,for two-dimensional lead halide perovskites（2D LHPs）,the alternating stacking arrangement of organic spacer cations and inorganic octahedrons will energetically form multiple quantum well structures.However,the organic layer can form an energy barrier between the inorganic layer,blocking the transport of charge carriers and reducing the fluorescence quantum yield of perovskites.Therefore,it is a hot issue to study the relationship between structural components and photoluminescent behavior in 2D perovskite and improve the luminescence properties of the materials.Pressure can affect the lattice and electronic structure of materials by compressing the distance between atoms and increasing the coupling between electron orbitals.Unlike chemical doping,pressure can regulate the physical and chemical properties of materials without the introduction of other elements,and accurately explain new phenomena and mechanisms at the atomic level.In this paper,two representative 2D LHPs,including(C₅H₁₄NH₃)₂PbI₄ and（C₆H₅C₂H₄NH₃）₂PbBr_xI_4-X with isomeric cations,were selected to characterize and explain the differential transformation of fluorescence characteristics under pressure by in-situ high-pressure testing technique.The research content of this paper will further deepen the attention to organic cations in this kind of materials,and also provide a new research idea for obtaining high performance 2D LHPs.（1）The high-pressure study of the two isomeric organic cations(C₅H₁₄NH₃)₂PbI₄ was carried out using in-situ high-pressure testing techniques.The radiation recombination of bound excitons will lead to the reduction of the fluorescence quantum yield of intrinsic excitons and produce a tail in the low energy region of the fluorescence peaks.In the pressure range from 0 to 12.5 GPa,the photoluminescence spectra of the two materials showed a distinct difference,which was caused by organic cations with heterogeneous structures.In the low-pressure region,the inorganic sublattice in（PA）₂PbI₄ will shrink uniformly under the protection of organic cations to reduce the coupling between excitons and phonons,inhibit the generation of exciton defects,and reduce the occurrence of non-radiative recombination,thus improving the fluorescence quantum yield of perovskites.The organic cations in（PNA）₂PbI₄ have certainly rigid structure,and the inorganic layers will be transformed under the guidance of cations.The new phase has higher symmetry and stability.Under pressure,the[PbI₆]inorganic octahedron will further deform to produce wide-band emitting self-trapping excitons.The results show that perovskites with isomeric cations exhibit different pressure-related properties,while organic cations play a key role in the structural transformation of inorganic layers.（2）Using high pressure technique and halogen mixing experimental method,wide coverage and high purity fluorescence emission in（C₆H₅C₂H₄NH₃）₂PbBr_xI_4-X 2D LHPs were achieved.Under atmospheric pressure,π-πstacking interactions and halogen doping strengthens the structural rigidity and continuous narrow-band emission,respectively.In addition,due to the existence of quantum and dielectric limiting effects,the band gap of2D LHPs increases,resulting in the absence of luminescence from light green to red bands.Fortunately,the pressure induces the deformation of the inorganic octahedron[PbX₆],which increases the orbital coupling between the lead and halogen atoms,leading to the narrowing of the material’s band gap and the redshift of the emission spectrum.Continuously,wide range and high intensity luminescence tuning from 416 nm to 651 nm can be achieved in the pressure range from 0 to 6 GPa.The study shows thatπ-πstacking effect is the key to realize narrow-band emission under pressure,and also improves the structural stability of materials under high pressure.