High Pressure Research On Low-dimensional Lead-free Perovskites And Their Derivatives

Perovskites are widely used in solar cells and photovoltaic devices,as a new type of semiconductor materials.However,the traditional three-dimensional leadbased perovskites are toxic,restricting their applications.Low-dimensional lead-free perovskites and their derivatives are expected to become substitutes because of their non-toxicity and stability.Besides,perovskites with low dimensional structure possess unique optoelectronic properties.The electrons are confined in the host's inorganic frame,resulting in quantum confinement effects,which will trigger unique photophysical phenomena.At present,the comprehending of the structure of lowdimensional perovskites is not very clear.In-depth study of their internal structureproperty relationship through high-pressure experimental methods can provide strategies for improving the performance of low-dimensional perovskites.By studying the structural and property evolutions of a series of lead-free lowdimensional perovskites and their derivatives under high pressure,the effects of different components and structural units on structure and properties of materials are revealed.In the low-dimensional organic-inorganic hybrid perovskite?CH₃NH₃?₃Bi₂Br₉,distortion and tilting of inorganic [BiBr₆]^3- octahedra determine the structural phase transition process.While the organic components have little effect on the structural phase transition.At the same time,the effects of octahedra structure on optical band gap are revealed,that is,the compression of the octahedra and the amorphous structure promote the coupling of the electron orbits,which reduces the band gap value.The tilting and distortion of the octahedra during the phase transition restrain the interaction of electron orbits,and the band gap is expanded.Compared with the structural behavior of all-inorganic perovskite Cs₃Bi₂Br₉ under high pressure,it was found that the organic cations in the hybrid perovskites can significantly improve the flexibility of the materials.In one-dimensional copper-based perovskite-derivative Cs Cu₂I₃,the effects of the [CuI₄]^3-tetrahedral structure on the self-trapped excitons?STE?emission are revealed.At ambient conditions,the slight structural distortion of [CuI₄]^3-tetrahedra results in weak STE emission.Under milder pressure,the slight enhancement of structural distortions between [CuI₄]^3-tetrahedra leads to the band gap broadening and STE enhancement with a decreased Bohr radius of excitons.In the high-pressure range,the structural phase transition significantly strengthens the internal distortion of [CuI₄]^3-tetrahedra and obvious structural distortions between the adjoining tetrahedra within the one-dimensional chains.Consequently,inducing a higher detrapping barrier with larger activation energy,which is confirmed by the presence of a new STE emission peak at a much lower energy.Thus,the increased concentration of STEs accordingly results in the increase of radiative recombination with enhanced STE emissions.It is indicated that the structural distortions from intra-and inter-tetrahedra are revealed to enhance the STE emission with different magnitudes in Cs Cu₂I₃.This work provides a strategy for pressure regulation of STE emissions in low-dimensional metal halides.In low-dimensional perovskites and their derivatives,the polyhedral structural units are very important for regulating their optical properties.