Investigation On Carrier Dynamics Of Two-Dimensional Perovskite （HA）₂PbI₄

Two-dimensional organic-inorganic hybrid perovskite has attracted the attention of researchers because of its unique structure,excellent photoelectric performance and good stability,and has a bright development prospect.In 2014,Smith et al.,Stanford University,reported for the first time a solar cell based on two-dimensional perovskite material,with a power conversion efficiency of only 4.73%.However,at the end of2021,the power conversion efficiency of two-dimensional perovskite solar cells reported by Liang et al.,Shanghai Jiao Tong University,has surged to 20.12%.In addition to improving device efficiency,two-dimensional perovskite also has the advantages of good environmental stability and abundant band gap adjustability.This makes two-dimensional perovskite widely used in light-emitting diodes,solar cells and photodetectors.Pressure is able to adjust the structure of perovskite and regulate its photoelectric properties.In recent years,there have been many researches on the photoelectric properties of two dimensional perovskites regulated by high pressure with PEA cations（PEA⁺）and BA cations（BA⁺）.However,no detailed researches have been conducted on the photoelectric properties of two dimensional perovskites regulated by high pressure with HA cations（HA⁺）.Therefore,（CH₃（CH₂）₅NH₃）₂PbI₄（（HA）₂PbI₄）is selected as the research object in this paper.By using diamond anvil cell（DAC）to apply hydrostatic pressure to（HA）₂PbI₄,the self-assembled steady-state photoluminescence（PL）spectra,steady-state absorption（PL）spectra,time-resolved photoluminescence（TRPL）spectra and transient absorption（TA）spectra were used to analyze the structure,photoelectric properties and carrier dynamics of（HA）₂PbI₄ under high pressure.The research content of this paper is as follows:（1）Under high pressure,we observed that the photoluminescence intensity of（HA）₂PbI₄ gradually decreased with the increase of pressure,and the decreasing process showed a trend of first fast and then slow.The full width at half maximum of（HA）₂PbI₄decreases rapidly from about 126 me V to about 95 me V with the increase of pressure in the range of 0-1.48 GPa,and the full width at half maximum of PL decreases by nearly a quarter.When the pressure exceeds about 1.48GPa,the full width at half maximum of PL spectrum presents a complex change of first rising and then falling,which may be the result of the phase transition of（HA）₂PbI₄ at about 1.5GPa.In the high pressure steady-state absorption spectrum system test,it is found that the absorption spectrum redshift under high pressure is consistent with the phenomenon observed in steady-state PL.In order to further analyze the changes of band structure and exciton characteristics under high pressure,the measurement results are quantitatively analyzed based on the quantum well steady-state absorption spectrum formula,and it is found that with the increase of pressure,the band gap and exciton ground-state energy decrease continuously.The full width at half maximum of the exciton absorption peak decreased first and then increased,and the turning point and the turning point of the emission spectrum were approximately 1.5 GPa.This further confirms that the phase transition of（HA）₂PbI₄ may occur near 1.5 GPa,and the high pressure can induce the band gap contraction of（HA）₂PbI₄,resulting in the phenomenon of redshift of the absorption spectrum.（2）High pressure time-resolved photoluminescence（TRPL）spectra and high pressure transient absorption（TA）spectra system were used to analyze the carrier dynamics of two dimensional perovskite（HA）₂PbI₄ under different pressures.In the test of high pressure time-resolved photoluminescence spectrum system,the exciton lifetime under different pressures is obtained by fitting the results with double e exponent.It is found that in the range of 0-1.5 GPa,with the increase of pressure,the non-radiating recombination life decreases rapidly,while the free exciton（FE）recombination life extends slowly,and the average life decreases.When the pressure exceeds 1.5 GPa,the non-radiating recombination life is basically stable with the increase of pressure,while the free exciton recombination life increases rapidly,and the average life slowly recovers.The change of（HA）₂PbI₄ near 1.5 GPa may be due to pressure induced phase transition and band structure change.In the transient absorption spectrum test at high pressure,it is found that with the increase of pressure,the ground state bleaching peak and excited state absorption peak on both sides of the red shift,and the ground state bleaching peak red shift corresponds to the steady-state absorption spectrum red shift.In the process of understanding the exciton interaction during the excited state relaxation of（HA）₂PbI₄ at high pressure,it is found that with the increase of pressure,the net increment of the conversion from the self-trapping exciton to the free exciton is enlarged,and the relaxation kinetics of the absorption peak of the self-trapping exciton is accelerated.