Study On The Influence Of Environment On The Non-radiative Recombination Dynamics Of Individual Perovskite Nanocrystals

Organometal halide perovskite CH₃NH₃Pb I₃（MAPb I₃,MA=CH₃NH₃⁺）is a new type of organic-inorganic hybrid semiconductor material.Because of its excellent photoelectric properties,such as high photoluminescence（PL）quantum yield and absorption coefficient,adjustable emission wavelength,narrow emission spectrum,long carrier diffusion length,etc.,it has been widely used in photovoltaic devices such as solar cells,light-emitting diodes and semiconductor lasers,and has become one of the potential candidates for the next generation of photoelectric and photovoltaic devices.The photophysical properties and photoelectric properties of MAPb I₃can be easily affected by the environment.The induced defects in the material will cause additional non-radiative recombination channels,thereby affecting the PL quantum yield and stability of perovskite,as well as the energy conversion efficiency of perovskite-based solar cells.It is thus of great significance to study the environment-dependent non-radiative recombination dynamics in perovskites to improve the properties of perovskite materials.So far,the mechanism of the influence of environment on the radiation properties and the photophysical properties of perovskite has not been fully clarified.Traditional measurement methods focus on the study of perovskite thin films or bulk materials.it is difficult to avoid the influence of ensemble averaging effect.The study of the environment-dependent photophysical properties of MAPb I₃nanocrystals at the single-particle level by single-molecule spectroscopy can eliminate the influence of ensemble averaging effect,and lay a foundation for further understanding of the influence of environment on the non-radiative relaxation in perovskite.In order to better understand the influence of extreme environment on the non-radiative recombination dynamics of individual perovskite nanocrystals,we studied the photophysical properties of individual perovskite nanocrystals in vacuum environment and low temperature environment,and analyzed the mechanisms of radiation recombination and non-radiative recombination of nanocrystals by combining the photoluminescence spectrum,quantum yield and lifetime of individual perovskite nanocrystals.The main research work carried out in this paper is as follows:1.The effect of atmosphere on the photophysical properties of individual MAPb I₃perovskite crystals was studied.It is found that the PL of individual perovskite nanocrystals can be quenched when switching from air environment to vacuum environment.Returning of the PL in the air environment showed pressure dependence.It is found that PL quenching in vacuum is due to the activation of non-radiative recombination sites caused by lattice distortion in perovskite.As atmospheric pressure gradually increases,light-assisted chemisorption of oxygen on perovskites will passivate these non-radiative recombination sites while restoring lattice defects,resulting in enhanced photoluminescence.The results have shown that placing perovskites in an environment with moderate pressure and oxygen content can protect perovskite materials from photophysical losses under inert conditions.2.The changes of photophysical properties of perovskite nanocrystals under temperature and light excitation power dependence were studied.It is found that at a temperature of about 160 K,a single perovskite nanocrystal undergoes a phase transition,and changes from an orthogonal phase to a tetragonal phase structure with increasing temperature.It is found that the perovskite band gap gradually increases with the increase of temperature,resulting in the blueshift of the photoluminescence spectra.A second spectral characteristic at long wavelength range appeared in the low-temperature orthogonal phase which can be ascribed to defect-mediated recombination of bound excitons.With the appearance of the second spectral characteristic peak,the lifetime of perovskite nanocrystals is significantly enhanced.Increasing temperature and illumination can passivate the associated defects and inhibit defect-induced bound exciton recombination.The results show that excitation of perovskite materials with low excitation power density within the appropriate temperature leads to longer lifetime and high quantum yield of perovskite materials.