Study On The Dark Excitons In Perovskite Confined Systems

When the semiconductor size is close to or smaller than the Bohr diameter of the exciton,the band-edge energy levels will turn to discrete because of the quantum confinement effect.The exciton energy levels have different angular momentum and parity symmetry due to the orbit,spin and their interaction.Under the approximation of the electric dipole moment,the parity of the initial and final states of the optical transition is required to change.In the semiconductor confined system,only the excitons with J=1 can undergo transitions,and the rest are forbidden.According to this,the excitons can be divided into bright states and dark states.Generally,the dark state is lower than the bright state and its radiation transition is extremely slow,resulting in a decrease in the luminous efficiency of the electrical injection device,which may greatly affect the performance of the device.Lead halide perovskite semiconductors have excellent optoelectronic properties and show excellent application prospects in photovoltaic devices such as solar cells,light-emitting diodes,lasers,and photodetectors.Benefiting from the quantum confinement effect of perovskite confined system,the exciton luminous efficiency is high,and the electrical injection light-emitting device is improved.Thus,whether the lowest excited state is a dark state has attracted much attention.Especially in materials with different structural symmetry,the exciton fine structures with different spin components will be split,and the effect and position of the dark exciton are still controversial.This paper focuses on the role of spin-related excitonic dark state in the perovskite semiconductor confined system on the luminescence properties.We combined low-temperature magneto-optical and transient spectroscopy techniques to study the excited state dynamics of the dark exciton.We studied luminescence and the conversion between different spin states in perovskite nanocrystals with different compositions,morphologies and size.The main content of this article is as follows:1.We have observed magneto-optical evidence that the dark states in perovskite nanocrystals is below the bright states.The composition-dependent energy splitting between the bright and dark states indicates an important influence of screening effect.Using low-temperature magneto-optical spectroscopy techniques(?4 K,?10 T),we measured the fluorescence kinetics of perovskite nanocrystals at low temperatures,which showed an obvious double-exponential process.The two processes gradually merged with the increase in temperature and the slow process can be controlled by the magnetic field.This means that a certain proportion of the bright excitons are converted into the transition-forbidden dark excitons after being excited.There is an energy splitting between bright and dark states and they get equilibrium quickly through the phonon exchange.The bright and the dark states have different spins,and they can be coupled under a magnetic field to change the transition probability.By measuring perovskite samples with different chemical compositions,we found that the splitting between the bright and dark states in the nanocrystal decreases with the increase of the Bohr radius and the screening effect.This discovery is instructive for designing light-emission device to reduce the influence of the dark state through chemical synthesis.2.We systematically studied the size dependence of the splitting between the bright and dark states in the perovskite nanocrystals.The splitting decreases with the increase of the size.By analyzing the low-temperature fluorescence kinetics of a series of perovskite nanocrystals with different sizes,we found that the splitting is negatively correlated with the size,and the magnetic effect is less obvious in the smaller sample.The results conform to the microscopic theory of electron-hole exchange interaction,which deduce that the splitting is proportional to the overlap of the wave function leading to a size dependence.The size dependence of dark exciton helps resolve the controversy on the relative position of the bright and dark states in the perovskite material.It is significant in improving the efficiency of luminous to reduce the splitting between bright and dark excitons.3.We clarified the band structure of the two-dimensional perovskite material at low temperature and found that the self-trapped excitons have an anormal magnetic effect.The fluorescence of the two-dimensional perovskite appears new spectral components at low temperatures and it can be controlled by a magnetic field and it shows similar characteristics when Pb²⁺in samples substituted with Sn²⁺.Considering that the two-dimensional material has a strong confinement effect on excitons in one direction,we speculate that the new composition comes from the split between the fine structure of the bright state with lower symmetry.The two-dimensional perovskite also exhibits broad emission in the low-energy range.The spectrum is related to the Sn/Pb doping ratio and exhibits a strong magnetic effect.The broad emission consists of defects and self-trapped excitons caused by strongly electron-phonon coupling.The electrons and holes of the self-trapped exciton may have different response to the magnetic field,which causes anomalous magnetic effects.Our experimental results help understand the band structure of two-dimensional perovskites and the formation of self-trapped excitons.It provides a basis for further application in white light diodes.