Pressure-Induced Structural Evolution And Optical Properties Of Lead-Free Double Perovskite ?NH₄?₂BBr₆ ?B=Se,Sn?

In recent years,organic-inorganic hybrid perovskite materials have been widely used in solar cells,photodetectors and light-emitting diodes due to their high light absorption coefficient,good carrier mobility and long carrier diffusion length.In2009,the organic-inorganic hybrid perovskite material MAPb I₃ has been applied as an absorption layer in solar cells by Japanese scientist Miyasaka and his team for the first time.So far,the power conversion efficiency of the perovskite solar cells has increased from the initial 3.8%to 25.2%,but lead toxicity and poor stability restrict the application and promotion of such materials.The lead-free double perovskite materials derived from the concept of"heterovalent substitution"can not only keep the high symmetry dimension of perovskite structure,but also overcome the issues of lead toxicity.However,the power conversion efficiency of solar cells based on this kind of materials is generally low.We need to understand the structure-property relationships of lead-free double perovskite whose physical properties are adjusted and optimized effectively to meet the needs of photoelectric device application to the greatest extent and final improve the power conversion efficiency of this kind of materials.Pressure is another thermodynamic parameter independent of temperature and chemical composition,which can effectively shorten the atomic distance and increase the coupling of electron orbits,changes the crystal structure and electronic structure of matter and plays an important role in discovering new phenomena,explaining new physics and mechanism,revealing the structure-properties relationship of materials.Therefore,we conduct high-pressure studies on lead-free double perovskite by combining in-situ high-pressure UV-Vis absorption,angle-dispersive X-ray diffraction,Raman spectroscopy,Infrared spectroscopy and First-principles calculation to reveal the structure-property relationships,and provide experimental foundation and theoretical basis for optimizing lead-free double perovskite solar cells and optoelectronic devices.The lead-free double perovskite?NH₄?₂SeBr₆ is studied systematically under high pressure.It is found that the structural phase transition of?NH₄?₂SeBr₆ changes from cubic to tetragonal at 11.20 GPa and the space group changes from Fm-3m to P42.Through the analysis of high pressure absorption spectrum,Raman spectrum and infrared spectrum,the occurrence of this phase transition is verified again.The optical bandgap of?NH₄?₂SeBr₆ gradually decreases with increasing pressure and two turning points are appeared at 6.57 GPa and 11.18 GPa.First-principles calculation shows that the two turning points are related to Br-Br bond reorganization and structural phase transition,respectively.In order to investigate the effect of B-site metal ions on the structure and properties,?NH₄?₂Sn Br₆ is studied under high pressure.It is found that?NH₄?₂Sn Br₆undergoes cubic to tetragonal phase transition at 6.3 GPa and the space group changes from Fm-3m to P42.The optical bandgap of?NH₄?₂Sn Br₆ undergoes a process of decreasing-increasing-decreasing under high pressure.Comparing the high pressure behavior of?NH₄?₂SeBr₆ and?NH₄?₂Sn Br₆,it is found that the phase transition sequence of the two is the same?Fm-3m to P42?,but the pressure point of the former is higher than that of the latter.The understanding of the lead-free double perovskite structure-property relationship and the underlying physical mechanism has been deepened through the above work,which plays a positive role in promoting the development of the perovskite photovoltaic field.