Charge Transport And Recombination Dynamics In Lead Halide Perovskite Single Crystals

In recent years,lead halide perovskite materials have shown great potential in photovoltaics,light emitting diodes,and photodetectors due to their good charge carrier transport properties and adjustable bandgap.Despite extensive research on the charge carrier recombination process in perovskite materials,there are still many unclear issues regarding the dynamics of charge carrier recombination.During my master’s degree,I comprehensively used various time-resolved spectroscopy techniques to study the charge transport properties and recombination dynamics in lead halide perovskite single crystals.The research results can provide theoretical and practical basis for the design and optimization of perovskite devices.Compared with the fixed optical bandgap of single-halide perovskite materials,mixed-halide perovskite materials can systematically change the bandgap by adjusting the contents of different halogens in the crystal structure,which helps to improve the photovoltaic performance and expand the application of perovskite materials in photodetectors and light emitting diodes.However,the change of halogen content in mixed-halide perovskite materials not only affects the optical bandgap but also affects the charge carrier recombination processes.The intrinsic relationships between halogen content and charge carrier recombination processes are still lacking research.For Cl/Br binary mixed-halide perovskite MAPb(Br_xCl_1-x)₃,we successfully grew a series of single crystals with different sizes and Br contents using inverse temperature growth method,and studied the electron and hole recombination dynamics of this series of materials by using time-resolved photoluminescence（TRPL）spectroscopy and time-resolved microwave conductivity（TRMC）spectroscopy.It was found that the TRPL dynamics of MAPb(Br_xCl_1-x)₃（x<0.98）perovskite single crystals are mainly dominated by hole trapping processes,while the slowest dynamics in TRMC are mainly dominated by electron trapping processes.Comparing the electron and hole trapping processes of single crystals with different Br contents,we found that the trapping rates of both holes and electrons decrease with the increase of Br contents.Furthermore,temperature-dependent steady-state and time-resolved photoluminescence experiments found that there are two types of defects,deep and shallow traps,in Cl/Br binary mixed-halide perovskite MAPb(Br_xCl_1-x)₃ single crystals,and the trapping and de-trapping processes of free charge carriers by these two defect states are both governed by thermal activation processes.These processes can be described by the Arrhenius equation,and the activation energy of the hole de-trapping from the shallow trap states to the valence band is～0.1 e V,and the activation energy of the hole trapping to the deep trap states from the valence band is～0.4 e V.This research provides a model for the charge carrier recombination dynamics in Cl/Br mixed-halide perovskite MAPb(Br_xCl_1-x)₃ single crystals.On the basis of the above,we also studied the charge carrier recombination dynamics of Br/I mixed-halide perovskite MAPb(Br_1-yI_y)₃ single crystals.It was found that,different from the MAPb(Br_xCl_1-x)₃（x<0.98）single crystals,the TRPL dynamics of the MAPb(Br_1-yI_y)₃single crystals are mainly dominated by electron trapping processes,consistent with the fast decay component in TRMC dynamics,while the slowest decay components in TRMC are mainly dominated by hole trapping processes.The study shows that,compared with single-halide perovskite MAPb Br₃ and MAPb I₃ single crystals,Br/I mixed-halide perovskite MAPb(Br_1-yI_y)₃ single crystals introduce additional defects,which lead to the increase in the rate of electron and hole trapping processes by defects.In addition,during the study of charge carrier dynamics in perovskite single crystals,we observed Brillouin oscillation signals induced by acoustic phonon transport in the transient reflection dynamics of charge carriers.Based on the relationship between the periodicity of the oscillation signal with the material’s elastic modulus and sound velocity,we experimentally determined the elastic modulus and sound velocities of lead halide perovskite MAPb Cl₃（41.0GPa,3566 m/s）,MAPb Br₃（35.5 GPa,3042 m/s）,MAPb I₃（25.3 GPa,2478 m/s）,Cs Pb Br₃（29.5GPa,2469 m/s）,and FAPb Br₃（28.8 GPa,2749 m/s）single crystals.The parameters obtained are comparable to those obtained by other experimental methods.Since defect states in perovskite single crystals are the main factor causing the decay of phonons during transport,and the phonon decay rate is proportional to the defect state density,we established the relationship between defect state density and Brillouin scattering relaxation rate and characterized the relative defect state density distribution of Cs Pb Br₃,MAPb Br₃,and FAPb Br₃at different spatial positions based on time-domain Brillouin scattering,achieving the characterization of defect state density spatial distribution based on time-domain Brillouin scattering.Based on the defect state density distribution on the surface of MAPb Br₃ single crystal,we extracted a deformation potential of-1.47 e V for MAPb Br₃ single crystal and calculated that the upper limit of the electron and hole mobility should be greater than～10⁵cm²·V^-1·s^-1.By comparing this value with the hole carrier mobility(～10² cm²·V^-1·s^-1)calculated by the space charge limited current method,we established a phonon and charge transport model for MAPb Br₃ perovskite single crystals based on defect scattering process.