Investigation On Carrier Dynamics Of Organic-inorganic Hybrid Halide Perovskites

Energy is the material basis of human activities.The continuous progress of human civilization and the development of economy are inseparable from the emergence of high-quality energy and the application of advanced energy technology.The massive use of fossil fuels has caused irreversible climate change,forcing people to seek and develop clean energy.People turn their attention to renewable energy sources such as solar energy,wind energy,biomass energy,water energy,geothermal energy,and hydrogen energy,which can be recovered and supplemented after consumption and produce no or little pollutants.Today,the development of environmentally friendly energy is a common concern around the world,and full electrification may become a global trend in the future.As key materials for optoelectronic devices,organic-inorganic hybrid halide perovskite materials not only overcome the shortcomings of fossil fuels,but also meet energy demands,which can provide power for an all-electric world.These materials have the advantages of tunable band positions,long carrier diffusion lengths,and high charge carrier mobility.These unique optoelectronic properties have enabled the rapid development of various perovskite-based solar cells,photodetectors,light-emitting diodes,and lasers.Effective utilization of the ultrafast carrier dynamics properties of perovskite materials,especially the carrier relaxation and recombination properties,can significantly improve their performance in various application fields.But there are still many deep physical properties that need to be better understood.Therefore,further research on the photophysical properties of excited-state species in organic-inorganic hybrid perovskite materials is required,focusing on the crystal structure,optoelectronic properties,and charge carrier dynamics.We hope that the research in this dissertation can contribute to the design and development of novel perovskite optoelectronic devices and promote the wider application of organic-inorganic hybrid perovskite materials in the future.In this thesis,we mainly use femtosecond transient absorption spectroscopy and time-resolved fluorescence spectroscopy to study the ultrafast carrier dynamics of the typical organic-inorganic hybrid perovskite materials:quasi-2D（PBA）₂MA_n–1Pb_nBr_3n+1thin films,FAPb（Br0.4I0.6）3 NCs and FAPbBr₃NCs.The main contents are as follows:（1）Quasi-2D halide perovskites with different layers have spontaneous large emission（ASE）and lasing properties.We use transient absorption（TA）spectroscopy and photoluminescence（PL）test to analyze the relationship between ASE properties and carrier dynamics occurring in quasi-2D（PBA）₂MA_n–1Pb_nBr_3n+1 perovskite films.The TA data shows that the high excitation intensity generates plenty of activity carriers with high temperature,when the Auger recombination process is induced simultaneously.When a rectangular area of 0.018×0.5 cm² on the sample is irradiated with an excitation intensity of～30μJ cm^–2,the light in the transverse propagation is amplified and emerges at the strip edge as ASE,which often competes with the Auger recombination in the perovskite films.As the number of inorganic octahedral layers（n）increases,the slow cooling process of high-activity carriers and Auger recombination processes reduce the ASE threshold.Both the carrier temperature cooling process and the Auger recombination process in the perovskite films are accelerated as the temperature decreases,and the lower ASE threshold at this time indicates that the ASE process dominates the competition with Auger recombination at low temperature.（2）The carrier dynamic process of perovskite nanocrystals（NCs）plays a distinctive role in optoelectronic characteristics and affects the device performance of these perovskite-based devices.We employed transient absorption spectroscopy to investigate the temperature-dependent photo-generated carrier dynamics in organic-inorganic perovskite FAPb(Br_0.4I_0.6)₃ NCs,involving the hot carrier cooling process,carrier-carrier interaction,and carrier recombination.As the excitation light fluence increases or the temperature decreases,the hot carrier cooling process slows down,and the carrier-carrier interaction and carrier recombination process accelerate.In addition,dropping temperature makes the FAPb(Br_0.4I_0.6)₃ NCs pass through two structure phases（cubicα-phase to a tetragonalβ-phase）at around 175 K.Compared with theα-phase,theβ-phase has a narrower bandgap,smaller longitudinal optical（LO）phonon energy,and a corresponding reduction in shallow defects.These factors are responsible for the variance of the photo-generated carrier dynamics in different phases.Furthermore,it is found that the environment temperature-dependent hot carrier temperature cooling process is determined by the Fr?hlich interaction and the non-equilibrium LO phonon process.（3）Pressure,a fundamental thermodynamic parameter that can greatly alter the atomic and electronic structures of functional materials,has been widely used to understand structure-property relationships.Applying pressure is a simple and efficient way to tune the carrier dynamics and photoluminescence（PL）properties.Here,we manipulate the carrier dynamics and radiative property of FAPbBr₃ NCs by applying pressure.The pressure-dependent steady absorption and PL spectra exhibit that the FAPbBr₃ NCs pass through Pm3?m,Im3?and Pnma in turn as the pressure increases from 0 to 3 GPa.Based on the photoluminescence dynamics at different pressure,it is found that the appearance of pressure-induced deep defect and the change of phase structure together reduce the PL quantum yield,prolong the radiative lifetime,and effectively manipulate the carrier dynamics.We found that the cubic phase structure has excellent fluorescence properties and fast carrier dynamics,while the orthorhombic phase structure confines photon emission.