Structure Engineering Of All-Inorganic Perovskites And Their Application Studies Of Light-Emitting Diodes

As one of the most promising light-emitting layer materials,perovskite has received wide attention and sprung to the forefront of optoelectronic research activity in light-emitting diodes（LEDs）in recent years due to its advantages such as narrow emission bandwidth,high photoluminescence quantum yield（PLQY）,spectral tunability and low cost.Among a wide variety of perovskite materials,all-inorganic three-dimensional（3D）perovskites were popularly utilized in the early research regarding perovskite light-emitting diodes（PeLEDs）because of its high stability and uniform grain size distribution.However,these conventional3 D perovskites also suffer from large grain size,high defect density and low exciton binding energy,consequently leading to luminescence quenching that is detrimental to device performance.These shortcomings have hindered the improvement of PeLEDs device efficiency and commercialization.Based on the problems mentioned above,two types of suitable large-sized cationic ligands have been selected to control the direction of grain growth,aiming at engineering the structure of all-inorganic perovskite（Cs PbBr₃）.When large size cations that cannot fit into the perovskite lattice completely are intercalated into the 3D perovskite,large grains can be cut into small ones to reduce the grain size and improve the film surface coverage,excess ligands that are not embedded in the lattice will also fill up grain boundaries,thereby suppressing non-radiative recombination and improving the efficiency of devices.The details are as follows:（1）Benzyl bromide（PMA-Br）was chosen as a large-sized cationic ligand to dope with CsPbBr₃ to construct quasi-two-dimensional（quasi-2D）perovskites.According to the comparative characterizations between CsPbBr₃ and quasi-2D perovskites such as photoluminescence spectra（PL）,PLQY,ultraviolet-visible absorption spectra（UV-vis）,X-ray diffraction profiles（XRD）,photoluminescence lifetime curves（PL Lifetime）and atomic force microscope images（AFM）and so on,it was proved that the introduction of PMA-Br can indeed reduce the grain size by constructing a quantum wells（QWs）structured perovskite,therefore improve the surface coverage of the film and finally achieve the improvement of device performance.Nevertheless,considering the natural characteristics of quasi-2D perovskites,a few challenges also need to be addressed.Firstly,although the precursor is charged at a fixed stoichiometric ratio,the quasi-2D perovskite cannot guarantee a uniform distribution of perovskite phase,but perovskites with size coexist in the film due to the imbalanced crystallization and phases exhibit size-dependent bandgaps,that is,self-organizedQWs.In this case,electrons and holes transfer from perovskite phases with larger bandgaps to phases with smaller bandgaps,and as a result,radiative recombination occurs in latter phases.However,the charge carrier transfer between the QWs leads to an elongated charge carrier diffusion distance,which inevitably causes energy loss and reduced luminescence yield.Secondly,the random distribution and proportion of different size phases bring unfavorable characteristics such as uneven thickness distribution and low film reproducibility.These disadvantages limit improvements in device performance of PeLEDs.（2）We proposed a strategy for all-inorganic perovskite structure optimization by using a novel cationic ligand,HOOC-PMA-Br.Attributed to hydrogen bond networks developed by the carboxylic acid unit,the modified perovskite not only retained the 3D structure with a uniform distribution but also rendered a small grain size similar to that usually produced by a quasi-2D QWs structure,which lead to the cationic ligand-based PeLEDs endowed with great enhancements in luminance and efficiencies in comparison with the ligand-free and PMA-Br-doped devices.Therefore,the HOOC-PMA-Br ligand played a crucial role in controlling the dimension,crystal growth and optical properties of the perovskites and this strategy further broadens the horizon for improving the performance of PeLEDs.