Investigation On The Ultrafast Dynamics Of Excited-state Carriers In Lead Halide Perovskites

Lead halide perovskites have shown great potential in photovoltaic applications and is one of the research hotspots in the field of renewable energy.The ultrafast processes of photogenerated carriers such as relaxation,recombination and charge transfer are basic physical processes that influence the photoelectric properties of lead halide perovskites.The micro-dynamic behaviors of excited-state carriers were observed in real time by transient absorption spectrometry for lead halide perovskites with lattice structure modulated via high pressure.It is helpful to reveal the correlation mechanism between lattice structure and properties and,in turn,contribute to the effective regulation on photoelectric properties of lead halide perovskites.Charge transfer is one of the most important steps to realize electro-optical energy conversion,and therefore femtosecond transient absorption spectroscopy was carried out to investigate the process of charge transfer between lead halide perovskite quantum dots and organic acceptor molecular,detailed as follows:（1）The effect of pressure on the excited-state carrier dynamics of organic-inorganic hybrid lead halide perovskite CH₃NH₃PbBr₃（MAPbBr₃）was studied by high-pressure femtosecond transient absorption spectroscopy.The transient absorption spectrum of MAPbBr₃ were firstly red-shifted and then blue-shifted with the increasing pressure,indicating that high pressure has a bidirectional regulating effect on the band gap of MAPbBr₃.MAPbBr₃ had undergone twice phase transformations within the pressure ranging from atmosphere to 3.0 GPa,and the results of global lifetime analysis showed that the time parameters of carrier relaxation,optical-acoustic phonon scattering and Auger recombination have abrupt changes across the pressure-induced phase transitions.Phonons scattering and electronic band structure are crucial factors in determining the time constants of excited-state carrier dynamical processes.The characters of lattice vibration and electronic band structure of MAPbBr₃ were significantly changed during the pressure-induced phase transitions,therefore,the changing trend for pressure-dependent carrier dynamic lifetimes differed in all pressure-induced phases of MAPbBr₃.Moreover,from the effect of pressure,it is found that carrier relaxation and Auger recombination were simultaneously prolonged,which is beneficial to the effective utilization of the hot carrier energy and to reduce the lose of carriers caused by non-radiative recombination.（2）High-pressure femtosecond transient absorption spectroscopy was carried out for further exploration of the pressure impact on the excited-state carrier dynamics of all inorganic lead halide perovskite CsPbBr_3.By comparing the transient absorption spectra of CsPbBr₃ under different pressures,it is found that high-pressure has a bidirectional regulating effects on the band gap of CsPbBr₃,decreasing initially and then increasing.For CsPbBr₃ under pressure,only one isostructural phase transition occurred as the pressure increasing from atmosphere to 1.76 GPa.What’s more,the changing tendency for time constants of carrier relaxation,optical-acoustic phonon scattering and Auger recombination as the increasing pressure had got inversed across the pressure-induced phase transformation.From the perspective of the lattice vibration and electronic band structure changes caused by pressure-induced variations in crystal structure,our study established the influencing mechanism of pressure on carrier dynamics.By applying pressure on CsPbBr₃,the processes of carrier relaxation and Auger recombination were both slowed down before the phase transition of CsPbBr₃,which provide more time for hot carrier collection whilst reduce carrier loss caused by non-radiation recombination.（3）The charge transfer from CsPbBr₃ quantum dots（CsPbBr₃ QDs）to the organic molecule 1-Chloroanthraquinon（1-CAQ）was investigated by femtosecond transient absorption spectroscopy.The results of steady-state spectrum showed that the electron transfer in CsPbBr₃ QDs/1-CAQ complex would quench the fluorescence of CsPbBr₃ QDs obviously.The time scale as well as the mechanism of electron transfer between CsPbBr₃ QDs and 1-CAQ was successfully determined by resorting to transient absorption signal assignment and contrastive analysis of the dynamic fitting results between CsPbBr₃ QDs and CsPbBr₃ QDs/1-CAQ complex.The carrier density in CsPbBr₃ QDs was decreased significantly for electrons transferred from CsPbBr₃ QDs to 1-CAQ,as a result,the recombination mechanism of CsPbBr₃ QDs was dominant by the slowing down trap-assisted recombination instead of free electron-hole recombination.