Large Area Quantum Dot Films Based On LB Technology And Their Preparation And Performance Of QLED Devices

Quantum dots(QDs)are a unique class of nano-photoelectric materials,which have attracted extensive attention and research owing to their narrow half-peak width,high color purity,tunable emission spectrum,and excellent photoluminescence quantum yield.Compared with traditional LED and OLEDs,quantum dot-based light-emitting diodes(QLED)have many advantages,such as better monochromaticity,higher thermal stability,and wider spectral coverage,and are expected to be used in display and lighting.In order to maximize the use of the superior properties of quantum dots,researchers have conducted in-depth research on the synthesis of quantum dots and the architecture of QLED devices.The current classic device structure is a multilayer structure of organic and inorganic materials,and each functional layer should pursue ordered molecular arrangement,good surface morphology and uniform thickness in terms of film quality.The quantum dot layer serves as the core light-emitting layer,and its thin film morphology and thickness are crucial for improving the optoelectronic properties such as external quantum efficiency(EQE)and luminance of QLED devices.According to the properties of nanocrystals of quantum dot inorganic semiconductor materials,thin film construction techniques in solution systems,such as spin coating,inkjet printing,transfer printing,etc.,can be used,but it is difficult to achieve large-area,close-packed,and highly ordered structures.Langmuir-Blodgett(LB)technology has been widely used for the controlled assembly and patterning of nanoparticles and nanowires.Compared with other thin film construction methods,LB technology can precisely control molecular packing density and achieve large-area,uniform monolayers or multi-layer thin film deposition,which is characterized by assembling materials into thin films first,and then transferring them to a solid substrate,and for multi-layer structural systems,regardless of the solvent used to dissolve and destroy the deposited functional layer.In this project,LB method was selected to prepare QDs monolayer film,and the QDs film thickness in the device was adjusted by changing the transfer times to study the influence of QDs layer on device performance.The main research contents of this paper are as follows :1.Preparation of QDs Langmunir filmsFirst,n-octane,n-hexane,toluene and chloroform,were selected to prepare quantum dot solutions with different concentrations of 1-18 mg/m L respectively.Keeping the temperature and instrument parameters unchanged,the optimal solvent and its concentration were determined.It is shown that the spreading and assembling behaviors of quantum dot solutions in different solvents on the water surface are very different.When using chloroform solvent,it is different from the other three solvents,the assembly behavior of quantum dots on the water surface is greatly affected by the concentration.When the concentration is low,after the droplet spreads and volatilizes,the QDs assemble into a grid-like shape and can be assembled into a sheet-like shape with the increase of the concentration.The film morphology is the best when the concentration is 12 mg/m L.Then selecting the quantum dot solution with the best conditions,keeping the instrument parameters unchanged,adjusting the temperature of the subphase water,and exploring the effect of temperature on the construction of the quantum dot LB thin film.The results show that with the increase of temperature,the thin film cracks,dense dots and small holes can appear.The optimum temperature is20-22℃.The solvent,concentration and temperature were determined,and the quantum dot films on the water surface under different surface pressures were transferred according to the complete surface pressure and area relationship diagram.Under different surface pressures,the state of the film is different.When the surface pressure is low,the film is not dense,and if the surface pressure is too high,the film will appear stacked and wrinkled.The film morphology is the best at 22-26 m N/m.2.Transfer and characterization of QDs LB thin filmsThe QDs films on the water surface were transferred to copper mesh,glass,and glass substrates spin-coated with TFB,respectively,and the structure and surface morphology of the films were observed by field emission transmission electron microscopy(TEM),fluorescence microscopy,and scanning probe microscopy(AFM),the results show that by changing the conditions,the structure and surface morphology of the QDs films are greatly changed.Meanwhile,the photoluminescence(PL)spectra showed that the morphology of the film had a great influence on the PL performance,and the fluorescence peak intensity increased linearly with the increase of the film thickness(number of layers).3.Fabrication of QLED devicesSince the LB film is assembled on the water surface,which has an effect on the quantum dots and other functional layers,annealing treatment is performed before spin-coating Zn O.The annealing temperature is 60℃,80℃ and 100℃;the annealing time is 0.5h and 1h.The results show that the device has the best performance when the annealing temperature is 80℃ and the annealing time is 0.5h.The annealing conditions were determined.On this basis,the thickness of the quantum dot film was studied.The results showed that with the increase of the number of layers,the peak EQE of the device did not change much,but the maximum brightness decreased significantly.The current density decreases with the increase of the number of layers,at the same voltag.