| Two-dimensional organic-inorganic hybrid perovskites have high research value due to their unique natural quantum well structure,which enhance the interaction between light and substance.(PEA)2PbI4 as a perovskite material with larger exciton binding energy among 2D organic-inorganic hybrid perovskites,has greater dielectric confinement effect,but the microphysical processes of 2D(PEA)2PbI4 perovskite material are poorly studied at present.We set up self-built femtosecond laser transient absorption experimental to study the excited state dynamics of two-dimensional(PEA)2PbI4 perovskite,which is in the context of femtosecond laser and pump-probe as technical tool.Then the hydrostatic pressure was applied to the samples by diamond anvil cell device to measure the steady-state absorption spectra,steady-state photoluminescence(PL)spectra and time-resolved photoluminescence(trPL)spectra of 2D(PEA)2PbI4 perovskite sample under high pressure to analyse the effect of pressure on the optical behaviour of 2D(PEA)2PbI4 perovskite material.The specific research for this thesis are as follow:(1)Clear oscillations were observed in the transient absorption spectrum of the(PEA)2PbI4 sample in the region between ground state bleaching and excited state absorption(from 2.4 eV to 2.45 eV),which is a specific manifestation of the coherent phonon dynamics resulting from the electron-phonon coupling of the two-dimensional organic-inorganic hybrid chalcogenide(PEA)2PbI4.By analysing the kinetic curves of the oscillation region near 2.44 eV for(PEA)2PbI4 at different pump fluences,it was found that the coherent phonon oscillation frequency increased with increasing pump fluence when the pump fluence did not exceed 128 μJ/cm2(linear region).The lattice temperature increases as the pump fluence increases,resulting in a change in the phonon oscillation frequency.At the same time,in the linear region,the coherent phonon decoherence rate kd decreases with increasing pump fluence,and the coherent phonon lifetime τ=1/kd increases accordingly,and the trend is in accordance with the quadratic law.When the pump fluence exceeds 128 μJ/cm2(non-linear region),the coherent phonon oscillation frequency decreases,while the coherent phonon decoherence rate kd starts to increase and the coherent phonon lifetime decreases.(2)The absorption spectrum of the(PEA)2PbI4 sample shows a clear exciton absorption peak as observed in the steady state absorption spectrum.In the range of 0-4 GPa,the exciton absorption peaks show a red shift with increasing pressure and the exciton absorption peaks decrease in photon energy and band gap,with the exciton ground state energy and band gap decreasing by nearly 350 meV.B etween 0.6 and 3.2 GPa,the exciton binding energy remained in the range of 0.17 to 0.18 eV;after the pressure exceeded 3.2 GPa,the exciton binding energy continued to increase with increasing pressure.The steady-state fluorescence spectra of(PEA)2PbI4 samples showed a continuous red shift with increasing pressure,and when the pressure was below 3 GPa,the intensity of the fluorescence spectra was basically unchanged,and the half height and full width decreased with increasing pressure.When the pressure exceeds 3 GPa,the intensity of the fluorescence spectrum gradually decreases and the half-height full width of the fluorescence spectrum gradually increases.When the pressure exceeds 3.5 GPa,the fluorescence spectrum changes from a single peak to a double peak.As observed from the time-resolved photoluminescence spectra,the fluorescence relaxation kinetic curves show a double exponential decay of the fast and slow components as the pressure increases,corresponding to the non-radiative and excitonic radiative complex processes,respectively.As the pressure increases,both the non-radiative and radiative composite lifetimes continue to decrease.When the pressure is less than 3 GPa,the fluorescence quantum yield of(PEA)2PbI4 chalcogenide increases with increasing pressure;when the pressure exceeds 3 GPa,the non-radiative complex contribution increases significantly,the fluorescence emission intensity decreases accordingly,and the fluorescence quantum yield also decreases. |
