Research Of Charge Carrier Dynamics In Halide Perovskites Under External Fields

Due to their outstanding photoelectric performance,perovskites have been widely concerned and show great potential in applications like solar cells,photodetectors,light emitting diodes,photocatalysis and lasers.In order to further optimize the performance of photovoltaic devices based on perovskite materials,it is particularly important to analyze the carrier dynamics in perovskites,such as carrier generation,recombination,migration and interface transfer.So far,breakthroughs have been made in the research of halide perovskite materials,but the system still faces some key challenges and problems in the study of system dynamics:such as the luminescence mechanism is not clear,the luminescence performance needs to be improved urgently,and the internal mechanism of the crystal structure and performance relationship is not clear.Ultrafast time-resolved spectroscopy is a powerful means of kinetic research,which can realize the kinetic survey of nanosecond or even picosecond carriers.Relying on ultrafast time-resolved spectroscopy,this thesis studies the carrier dynamics of several halide perovskite materials（Cs₂Ag Bi Br₆,Cs Pb I₃,MAPb I₃）under field conditions（temperature,pressure）.The main contents are as follows:（1）Temperature is a common means of field regulation,this paper first carried out temperature-regulated lead-free Cs₂Ag Bi Br₆ perovskite photogenerated carrier dynamics research,for its better application in optoelectronic devices to provide theoretical guidance.In this paper,we synthesized Cs₂Ag Bi Br₆ single crystals with regular morphology by crystallization method.Herein,we report an investigation on the carrier dynamics in Cs₂Ag Bi Br₆ single crystals at different temperature.The time-resolved photoluminescence（TRPL）measurement indicates that carrier long-distance transport in Cs₂Ag Bi Br₆ single crystals is restricted by the trap state and trap state is likely a key reason limiting the performance of Cs₂Ag Bi Br₆ photovoltaic devices.We realized direct visualization of carrier transport in Cs₂Ag Bi Br₆ single crystals for the first time.Charge carrier mobility in Cs₂Ag Bi Br₆perovskite is determined by the optical phonon scattering rather than acoustic wave scattering previously reported in the literature.（2）Secondly,the self-assembling growth ofδ-Cs Pb I₃ was achieved through high temperature regulation.Cs Pb I₃,an inorganic perovskite,has a desirable band gap and relatively high thermal stability.However,the photoactive phase ofα-Cs Pb I₃ is unstable and can be easily transformed toδ-Cs Pb I₃,which is photoinactive as a traditional point of view.In this paper,α-Cs Pb I₃ quantum dots were synthesized by hot injection method.Herein,theδ-Cs Pb I₃ nanowires were self-assembled fromα-Cs Pb I₃ quantum dots by precisely controlling the annealing treatment,and the conversion dynamics fromα-Cs Pb I₃ toδ-Cs Pb I₃were systematically investigated.Contrary to common belief,δ-Cs Pb I₃,which is obtained by precise annealing treatment,not only have very high humidity stability,but also have certain quantum efficiency and white light emission.Andδ-Cs Pb I₃ nanowires can also be applied in the white-light-emitting diodes to realize its functionalization.Our achievements give a new perspective onδ-phase Cs Pb I₃,and suggest its unlimited potential for a wide variety of optoelectronic devices.（3）In addition to temperature,pressure is also a common means of external field regulation of perovskite kinetics.Pressure as an important means which can alter the structure and electronic structure of the perovskite materials,thereby changing photoelectric properties of the perovskite materials.However,how compression can affect the carrier transport property of perovskites remains unknown.In this thesis,MAPb I₃ microcrystals with few defects and regular morphology were synthesized by solventing out crystallization method.Herein,by combining a diamond anvil cell with the time-resolved imaging microscopy,we realized direct visualization of carrier transport in MAPb I₃ perovskite microcrystals under high pressure for the first time.It is found that although the increase of pressure will cause structural defects and therefore significant decrease of photoluminescence lifetime and intensity,the diffusion coefficient is more than 30%higher than that of ambient pressure.MAPb I₃ microcrystals can still have a long carrier diffusion length under high pressure.The result in this work sheds light on the influence of pressure on the carrier transport in MAPb I₃ perovskites and paves the way for the utilization of compression to tune or optimize the optoelectronic properties of perovskites.