Ultrafast Dynamics Study Of Quasi-two-dimensional Organic-inorganic Hybrid Perovskites

In recent years,organic-inorganic hybrid perovskite materials have been widely used in many photovoltaic devices such as solar cells,lasers,light-emitting diodes,photodetectors,and phototransistors due to their excellent characteristics?wide absorption,longer carrier lifetime,longer carrier diffusion distance and two-way carrier transport mechanism,higher carrier mobility and small exciton binding energy?.However,the disadvantages of the poor stability of atmospheric humidity of traditional three-dimensional perovskite materials have seriously hindered their application.In contrast,the quasi-two-dimensional organic-inorganic hybrid perovskite material that has been born in recent years combines the advantages of three-dimensional organic-inorganic hybrid perovskite and pure two-dimensional perovskite,has excellent photoelectric properties?excellent thermal stability,high regularity,large exciton binding energy,etc.?,and easy processing due to its good self-assembly and film forming properties.Therefore,the material has been widely concerned in photoelectric conversion devices.However,we have little research on the physical properties of electrons,phonons,etc.of the quasi-two-dimensional organic-inorganic hybrid perovskite semiconductor material.The photoelectric physical mechanism of the quasi-two-dimensional perovskite is still controversial and unclear.In this paper,the quasi-two-dimensional perovskite?C₆H₅CH₂CH₂NH₃,PEA?₂?MA?_n-1Pb_nBr_3n+1n+1 is used as the main research system,and the time-resolved transient absorption spectroscopy technique is used to explore internal physical mechanisms such as ultra-fast coherent phonon dynamics and exciton transfer kinetics within the material,and the following results are obtained:1.We used the femtosecond time-resolved transient absorption spectroscopy to find the oscillation of the quasi-two-dimensional perovskite?C₆H₅CH₂CH₂NH₃,PEA?₂PbBr₄ thinfilms spectrum.Taking this phenomenon as the entry point,the internal physical mechanism of this oscillation phenomenon in the?C₆H₅CH₂CH₂NH₃,PEA?₂PbBr₄ thinfilms was investigated by changing the excitation wavelength and the excitation energy of the pulse.The results show that the source of the oscillation in the spectrum is the coherent phonon oscillation caused by the strong Coulomb interaction inside the quasi-two-dimensional perovskite?C₆H₅CH₂CH₂NH₃,PEA?₂PbBr₄.In addition,the anisotropy of the material was investigated by using the transient absorption spectroscopy experiment which changed the polarization state of the excitation pulses.The results show that the coherent phonon oscillation has no anisotropy.2.We used the femtosecond time-resolved transient absorption spectroscopy to find low-threshold laser emission of quasi-two-dimensional perovskite?C₆H₅CH₂CH₂NH₃,PEA?₂?MA?₄Pb₅Br₁₆6 material.The laser emission threshold of the material obtained by varying the steady excitation fluorescence intensity of the excitation pulse is only about 12.11?J cm^-2.Then,the kinetics of the?C₆H₅CH₂CH₂NH₃,PEA?₂?MA?₄Pb₅Br₁₆6 quasi-two-dimensional perovskite material was investigated by the transient absorption experiment with threshold energy density excitation.The optical gain establishment process was observed.Moreover,it clearly pointed out the contribution of ultra-fast electron transfer process from small n to large n value in quasi-two-dimensional perovskite to the particle number inversion and optical gain.In addition,the results show that the physical nature of the laser emission phenomenon of quasi-two-dimensional perovskite?C₆H₅CH₂CH₂NH₃,PEA?₂?MA?₄Pb₅Br₁₆6 material is spontaneously emission amplification,not true laser.