Fabrication Of Quantum Dot Films And Large-Area QLEDs Based On Liquid Bridge Method

Due to their unique photoelectric properties,semiconductor quantum dots(QDs)have broad application prospects in lighting and display fields.Possessing the advantages of high color purity,good color stability,wide color range and long life,quantum dot light-emitting diodes(QLEDs)which are based on quantum dots are expected to become display and lighting technology of the next mainstream.Possessing the advantages of high color purity,good color stability,wide color range and long lifetime,QLEDs are expected to become the mainstream display and lighting technology of the next generation.The common method for fabricating QLEDs is printing methods based on solution transfer which contain spin coating,transfer printing,blade coating,Langmuir-Blodgett(LB)technique,inkjet printing and so on.Although these methods have their own advantages in the preparation of film materials,there are some problems including high cost of equipment,difficulty in realizing large area controllable preparation of high-quality QDs films,etc.At present,the lack of ideal controllable assembly of large uniform film structures,which makes it difficult to achieve both high performance and large-area of QLEDs,is a key issue in the current field.Study on solution transfer induced by fiber according to our previous work,we constructed a two-dimension fiber liquid bridge system consisting of capillary and fiber.This system can realize controllable transfer of solution,and 5 cm×5 cm large-area uniform QDs films were obtained.But fiber elasticity limits the expansion of the films,this is hard to achieve larger area films preparation.We found that in the induction process,the form of the solution-liquid bridge had a great influence on the films formation,rigidity can change the form of liquid bridge.In this work,rigid glass capillary instead of flexible nylon fiber can form more stable liquid bridge in larger size.This work solves the problem of liquid bridge form change caused by gravity on flexible fiber and can realize preparation of large area high quality films.Design of large-area films printing system based on stabilized liquid bridge and the design is applied to the construction of QLEDs.This realizes the preparation of large-area QLEDs devices.The main research contents of this paper are as follows:(1)In chapter 2,we designed a new liquid bridge printing device.Based on the improvement of the shape of the liquid bridge,a more stable liquid bridge printing system was established,which broke through the limitation of the films construction area.The formation and traction of the liquid bridge were controlled by the three-dimensional moving axis,and the solution was induced to spread out on the substrate and form films.The optimal experimental parameters for printing QDs films were obtained by precisely controlling the concentration of QDs solution and printing speed,and a large area,high uniformity and ultra-smooth high quality QDs films was obtained,with the maximum film area up to 9 cm×9 cm.(2)On the basis of the previous chapter,the high-quality QDs films was applied to QLEDs devices in chapter 3,and we found that the thickness of QDs films can be effectively adjusted by controlling the printing times so as to obtain QLEDs with good luminescence characteristics.Furthermore,the printing method was extended to the hole injection layer,hole transport layer and electron transport layer in QLEDs,and the printing conditions of these functional layer films were explored to obtain uniform and flat high-quality films,which were integrated into QLEDs,and red,green and blue QLEDs with luminescence area of 1.5 cm×1.5cm were obtained.The maximum brightness of the device was 6875 cd/m~2,15790 cd/m~2,1193 cd/m~2,and the maximum external quantum efficiency was 11.6%,9%,4.4%,respectively,which were at a high level in the current large-area scale.In addition,a large-area red,green and blue QLEDs display device with a luminescent area of 3 cm×3 cm was constructed.Both large area and high performance were achieved,which provides a new idea for the commercial application of QLEDs.(3)On the basis of Chapter 3,we have achieved the preparation of large-area monochromatic devices.In Chapter 4,the multi-layer superposition printing method was used to superposition the complex color QDs films,and the obtained color meets the additive color mixture.The complex color films of superposition printing was applied to the QLEDs device,and the complex color device was constructed.By adjusting the stacking sequence of red,green and blue QDs films and printing sequence of green,red and blue,large-area white QLEDs with luminescence area of 1.5 cm×1.5 cm were constructed.The maximum brightness and external quantum efficiency were 2689 cd/m~2,4.26%,respectively.This study provides a new idea for QLEDs to realize white light illumination.