Pressure-Induced Structural And Optical Properties Of Zero-Dimensional Lead-Free Perovskite Materials

In recent years,metal halide perovskite materials have gained worldwide attention because of their good light absorption capability,wide tunable bandgap range,long carrier lifetime and easy availability and preparation of raw materials,and have shown great commercial potential in the fields of solar cells,solid-state lighting,photodetectors and anti-counterfeiting.However,conventional three-dimensional lead-based metal halide perovskite materials with a wide range of applications are highly toxic and inevitably pose an environmental and health risk.Therefore,the development and application of environmentally friendly and stable lead-free perovskite materials has become a new hot spot for domestic and international research.Zero-dimensional lead-free perovskite materials and their derivatives are among the most promising substitutes.These materials avoid the use of toxic lead and,at the same time,their unique low-dimensional structure increases the stability of the material.Furthermore,the electrons in such materials are confined within the inorganic skeleton of the perovskite body at low latitudes,leading to electron localization and quantum confinement effect,which give these materials their unique optoelectronic properties.The understanding of the structural and physical properties of zero-dimensional lead-free perovskite materials and the intrinsic connection between their optical properties remains unclear.The study of zero-dimensional lead-free perovskite materials using diamond anvil cell high-pressure techniques,combined with a variety of in-situ measurements and first-principles calculations,will not only improve the understanding of the structure-property relationships of these materials,but is also expected to provide effective physical modulation and optimisation,providing strategies and methods to explore the development of new perovskite materials.First,we conducted a high-pressure study on a typical zero-dimensional lead-free perovskite material（NH₄）₂Sn Br₆.It was found that（NH₄）₂Sn Br₆ undergoes a structural phase transition from cubic to tetragonal phase at 6.3 GPa and starts amorphization after 20.0 GPa.The two abrupt changes in the optical bandgap of（NH₄）₂Sn Br₆ at high pressure are shown to be associated with the structural phase transition and the direct to indirect bandgap,respectively,from high-pressure in situ UV-Vis absorption spectra and first-principles calculations.Also,hydrogen bonding between（NH₄）⁺cations and[Sn Br₆]^2-octahedra is found to promote the structural phase transition at high pressures.And by comparing the high-pressure behaviors of（NH₄）₂Se Br₆ and（NH₄）₂Sn Br₆ with different metal ions,we found that the first phase transition sequence is the same and the phase transition pressure points are similar,but the trends of the optical band gap changes of the two at high pressure are very different.Secondly,in view of the ns² ion doping as an effective method to improve the optoelectronic properties of perovskite materials,we conducted a high-pressure study on the Sb³⁺ion doped zero-dimensional lead-free perovskite derivative Cs₂In Br₅·H₂O and found that the system exhibited pressure-induced emission enhancement in the lower pressure region and its photoluminescence peak continued to blue-shift during the compression process.This is associated with an increase in[Sb Br₅OH₂]^2-asymmetric octahedral distortion within the material,and the enhanced electron-phonon coupling causes the ³P₁ to ¹S₀ radiative leap energy level broadening to split,resulting in an overall blue shift of the emission peak.Continued compression of this doped perovskite material weakens the emission,which is related to the increase of defects and non-radiative recombination inside the crystal.Continuing to compression,the light emission of this doped perovskite material diminishes,which is related to the increase of defects inside the crystal and the increase of non-radiative recombination.After the occurrence of isostructural phase transition,the higher defect level of the new phase at high pressure makes the system emission quenching.This series of work provides theoretical support and improvement directions for deepening the understanding of the structure-property relationship of zero-dimensional lead-free perovskite materials and their doped systems under high pressure,as well as for the application of perovskite materials in optoelectronic fields.