Research On Preparation Of Large-Scale Quantum Dot Monolayer Films And Their Performance As Electroluminescent Devices

Quantum dot light-emitting diodes（QLED）have advantages such as better color rendering,higher color purity,and greater spectral coverage.Presently,QLED devices have met the requirements for commercial applications,but the large-area fabrication of high-performance QLED devices remain a bottleneck for commercialization.One of the key challenges is the reliable large-scale production of quantum dots（QDs）film.Therefore,the development of method for the controlled preparation of large area QDs film is of great significance for promoting the practical application of QLED technology.The self-assembly of colloidal monolayers at the gas-liquid interface is an important method for large-scale preparation of ordered nano film structures,but it typically requires the dispersion of the assembly unit onto the gas-liquid interface and then the use of compression techniques to form a large-area monolayers.This preparation process is relatively complex and has low film formation efficiency.More importantly,the local assembly defects formed during the multi-point drop addition of solution are difficult to be eliminated in the subsequent compression process.Therefore,changing the solution injection method to regulate the dynamic assembly process of QDs at the interface and suppress the local assembly defects formed in the previous multi-point drop addition method is an effective idea to prepare large-area high-quality monolayer QDs films at the gas-liquid interface.In this paper,the self-assembled structural characteristics of QDs on the surface of different concentrations of DSS solutions were investigated by using two continuous injections of QDs solutions combined with subphase surface tension regulation to obtain large-area high-quality monolayer QDs films up to 5.3 cm×8 cm,based on which the effect of the number of QDs layers on the performance of QLED devices was investigated to realize high-performance large-area QLED devices construction.The main research contents of this paper are as follows:1.Solution convection intensity regulates the local self-assembly of quantum dots at the gas-liquid interfaceBy regulating the solvent type,QDs concentration and other parameters,the film formation process of QDs on water surface was studied by LB technique,and it was found that a large number of microscopic defects in QDs film could not be eliminated by slip barrier extrusion,and the causes of microscopic defects in QDs film were analyzed and studied;in order to eliminate the microscopic defects in QDs film,the surface tension of water subphase was regulated by surfactant sodium dodecyl sulfate to regulate the QDs solution.In order to eliminate the microscopic defects in the QDs film,the dispersion process on the subphase surface was regulated by the surfactant sodium dodecyl sulfate,and the dynamic assembly process of QDs on the subphase was effectively controlled by the continuous injection of QDs solution,which reduce the microscopic defects in the QDs film and the preparation of high-quality quantum dot monolayer films with a maximum area of 5.3 cm×8 cm was realized.Compared with the conventional LB technology,QLED devices prepared from QDs monolayer films based on the continuous injection method have lower leakage current,longer operating life,and higher external quantum efficiency,and the maximum EQE can reach12.15%,an improvement of 28.7%.The T₉₅ lifetime at 100 cd/m² initial luminance is 3031 h,an improvement of 1.5 times.A large-area uniformly emitting QLED device construction of 1.5 cm×1.5 cm was achieved based on the QDs monolayer film prepared by the continuous injection method.The effect of the number of layers of QDs film on the device performance was investigated.When the number of QDs film layers was increased from one to three,the QLED device turn-on voltage increased from 2.2 V to 3.2 V,the EQE decreased from 12.54%to 7.36%,and the luminance decreased from 274600 cd/m² to 5230 cd/m².2.Continuous crystallization of quantum dots at the end of a large surface flow into a filmIn this study,the gas-liquid-solid interface was divided into two regions by introducing short-range Marangoni flow at the gas-liquid interface:colloid-directed transport region and QDs film self-assembly growth,so that the QDs crystallized into films continuously at the junction of the two regions.Firstly,the effect of solvent type on the structure of QDs monolayers was investigated,in which hexane was used as the QDs solvent to achieve the preparation of high quality large-area QDs monolayers;the effect of QDs concentration in the solution on the QDs self-assembly film formation process was studied,and fewer defects were found in the QDs films when the QDs concentration was 9 mg/m L.Finally,the transfer of large-area QDs films was realized by PDMS,and a large-area high-quality QDs monolayer film of 4.5 cm×5.7 cm was transferred on a glass substrate.The electroluminescence performance of the QDs monolayer film was studied,and the EQE of the green QLED devices prepared in air could reach 12.36%.