Research On Performance Improvement Of FA_0.8Cs_0.2PbBr₃ Perovskite Quantum Dot LEDs Based On Defect Passivation And Film Modification

Since its discovery in 1839,the perovskite material has experienced decades of development,not only in the field of solar cells,but also in the field of display and lighting.The excellent characteristics of perovskite quantum dots（PQDs）,such as high fluorescence quantum yield,narrow emission spectrum,adjustable band gap and size,and large exciton binding energy,have successfully achieved a high level of efficiency in light-emitting diodes（LEDs）.However,the decrease in the size of perovskite quantum dots leads to an increased density of surface defect states,which is one of the main problems restricting the optical and electrical applications of PQDs.These defects not only reduce the luminous efficiency of PQDs,but also induce the decomposition or agglomeration of quantum dots,resulting in further deterioration of the stability of quantum dots.This thesis focuses on the research of formamidine cesium lead bromide(FA_0.8Cs_0.2Pb Br₃)perovskite quantum dots with excellent luminous performance,and improves the stability of quantum dots from the aspects of synthesis process optimization and surface defect passivation,and further improves the performance of PQD LED by optimizing their carrier transport layers.（1）FA_0.8Cs_0.2Pb Br₃ perovskite quantum dots were synthesized with improved ligand-assisted reprecipitation（LARP）method.The average size of the resulting product was 11.2 nm,the fluorescence intensity was high and the film shows a good coverage ratio.In addition,we adjusted the amount of precursor solution to ensure the uniformity of the quantum dot size,and obtained a stable and high fluorescence intensity FA_0.8Cs_0.2Pb Br₃ quantum dot solution.On this basis,PQD LED was prepared and the maximum brightness and maximum current efficiency of 2689 cd/m² and 5.78cd/A were obtained.（2）Based on the improved synthesis process and in order to further improve the stability of quantum dots,we introduced different concentrations of 2-aminoethanol bisulfate（2-AEHS）to the FA_0.8Cs_0.2Pb Br₃ quantum dot precursor solution to passivate defects on the surface of quantum dots.The experimental results showed that when the concentration of 2-AEHS in the precursor solution reaches 0.5 mg/m L,the photoluminescence quantum yield（PLQY）of quantum dots reached 76.6%,showing an enhancement factor of 55%compared with unpassivated ones.In addition,the passivation of 2-AEHS effectively improved the stability of the quantum dot solution.When the concentration of2-AEHS in the precursor solution reached 0.5 mg/m L,the quantum dot solution remained 50%of the initial fluorescence intensity after being stored in the indoor environment for more than 60 days.We prepared a PQD LED and analyzed the effect of the introduction of 2-AEHS on the electrical properties of quantum dots,and determined the optimal passivation concentration of 2-AEHS.The results showed that the introduction of 2-AEHS effectively improves the luminous performance and stability of the device.The maximum brightness and efficiency of PQD LED with 0.5 mg/m L of 2-AEHS were 6102 cd/m² and 9.28 cd/A,respectively,which are 267%and 64.8%larger than those without passivation.In addition,the half-lifetime of the device also increased from 120 s to 206 s.（3）On the basis of 2-AEHS passivated quantum dots,Poly（3,4-ethylenedioxythiophene）:polystyrene sulfonic acid（PEDOT:PSS）was further modified with dimethyl sulfoxide（DMSO）to transform it from a core-shell structure into a chain-like structure to improve its hole transport capability.Using DMSO-modified PEDOT:PSS to prepare PQD LED,the maximum brightness and efficiency of 3455.7 cd/m² and 15.85 cd/A were obtained,respectively,which were 102%and 66%higher than those with undoped DMSO.On the side of the electron transport layer,we used mixed4,6-bis（3,5-bis（3-pyridinyl）phenyl）-2-methylpyrimidine（B3PYMPM）:1,3,5-tris（1-phenyl-1H-benzimidazole-2-group）benzene（TPBi）electron transport layer to replace a single TPBi to reduce the electron injection potential barrier since the lowest unoccupied orbital（LUMO）level of B3PYMPM is closer to that of FA_0.8Cs_0.2Pb Br₃ perovskite quantum dots.And then we doped proper amount of cesium carbonate（Cs₂CO₃）into B3PYMPM to further improve the electron injection and transport ability and made the injection of electrons and holes more balanced.The maximum brightness and efficiency of PQD LED with the two-layer hybrid electron transport layer reached9358.6 cd/m² and 18.91 cd/A,which are 103%and 53%higher than that with the single TPBi electron transport layer.In summary,2-AEHS passivated the defects on the surface of FA_0.8Cs_0.2Pb Br₃ perovskite quantum dots and thus improved the fluorescence intensity and stability.Its short carbon chain helped to improve the conductivity of the perovskite film and greatly improved device performance.In addition,the introduction of modified hole transport layer and double-layer mixed electron transport layer further enhanced the brightness and efficiency of PQD LED.This paper provides a feasible solution for synthesizing high-performance FA_0.8Cs_0.2Pb Br₃ perovskite quantum dots and preparing highly efficient PQD LED.