The Effect Of Dielectric Relaxation On Carrier Dynamics Of CH₃NH₃PbBr₃

Organic-inorganic hybrid metal halide perovskites have been outstanding in the field of photovoltaic devices.As photovoltaic active layer,its excellent performance stems from three main elements:strong capability of capturing photon in a wide spectral range,high carrier mobility and long carrier lifetime.The high mobility and long lifetime ensure that the carriers get a longer diffusion length,making them be able to fully diffuse to the interface between the charge transport layer and the photovoltaic active layer until be effectively extracted by the transport layer since the diffusion length is usually larger than the thickness of the active layer.The research on the properties and kinetic characteristics of photogenic carriers in hybrid perovskite is deepening.Combined with the current research status,here,we selected the most representative CH₃NH₃Pb Br₃ single crystal with cubic crystal structure at room temperature as the research objective,explored the effect of dielectric relaxation on carrier dynamics caused by the dynamic inversion of CH₃NH₃⁺in the lattice,and summarized its regular features.More and more studies have shown that the photoelectric characterization of hybrid metal halide perovskites heavily depend on the sample quality.Therefore,it is necessary to choose a reasonable preparation method to improve the sample quality.By summarizing the related reports in recent years and combining the existing conditions in the laboratory,the single crystal growth experiment was carried out by inverse temperature crystallization and anti-solvent vapor assisted crystallization respectively.After repeated experiment,learning lessons and optimizing experimental methods,centimeter high quality CH₃NH₃Pb Br₃ single crystal was obtained.The atomically flat surface and the edge morphology of stepped stacking were observed by atomic force microscopy,revealing the growth patterns of ordered bulk phase structure and disordered surface defects.The phase structure obtained by X-ray diffractometer matched the theoretical value very well,displaying a high phase purity of the crystal.The existence of coulomb enhanced absorption effect between electron and hole in high energy continuous state is confirmed by steady-state absorption spectrum measurement and simulation,reconsidering the definition of free carrier.Steady-state fluorescence spectrum was determined to verify the difference between bulk phase fluorescence and surface-state specific fluorescence.The origin of different fluorescent components and the close relationship between the defects concentration and the lifetime of components were discussed by time-resolved fluorescence attenuation and lifetime analysis in different regions on the crystal surface.At the same time,the quantitative analysis of the above results also gave the core optical physical parameters of our single crystal,including:bandgap E_G=2.21 e V,exciton binding energy E_B=15.62 me V,Urbach energy E_U=19.9 me V,and free electron-hole recombination lifet=1.27μs.The high symmetry of the cubic phase at room temperature determines that the dipole CH₃NH₃⁺cannot remain stable in a particular orientation within the lattice.Thus,in order to satisfy macroscopic statistical symmetry,CH₃NH₃⁺keeps a high-velocity filpping state.The dipole relaxation caused by this dynamic disorder plays a significant coulombic screening on charge transport in the lattice framework,effectively promoting carrier diffusion and increasing carrier lifetime.To further explore the effect of the screening effect on carrier dynamics,the fluorescence lifetime and fractional intensity of each component of CH₃NH₃Pb Br₃ single crystal were analyzed in detail by time-resolved fluorescence spectra with variable temperature.By changing the temperature within the corresponding range of cubic phase,we adjust the flipping intensity of organic cation in order to investigate the effect of Coulomb screening on the lifetime of each decay comonent and the dynamic equilibrium of their concentration.The application and development of materials require not only the optimization and improvement of device preparation process,but also the accurate understanding of their intrinsic characteristics.The in-depth exploration of the related physical process would be helpful for improving the performance of devices,providing guidance for the further application of materials.