| Quantum dots(QDs)are a type of inorganic light-emitting material that offers several advantages over other materials,including high stability,adjustable emitting color,high color saturation and short exciton lifetime.Therefore,quantum-dots light-emitting diodes(QLEDs)based on QDs are expected to be an attractive alternative to the currently commercial organic light-emitting diodes(OLEDs),and play a unique role in the field of virtual reality and augmented reality displays.Thanks to the efforts of researchers in material development and device physics,QLEDs have been able to catch up with OLEDs in terms of various aspects.In comparison to the widespread application of OLED display technology in smartphones,the commercialization of QLEDs is significantly falling behind.This is mainly attributed to the fact that the production of QLEDs relies on solution processes.The spin-coating process is widely used in labs due to its benefits for producing small-sized QLEDs.However,this process has limitations for commercial applications,such as low material efficiency,non-continuous processing,and restricted processing size.Moreover,despite its excellent capabilities in preparing patterned films,inkjet printing still faces challenges in processing large-area films at a fast speed and producing high-quality multilayer films.These limitations have somewhat hindered the development of QLED display technology.Given this situation,there’s a pressing demand for a solution processing method that is low-cost and high-throughput to serve as a complementary method of fabricating functional layers that do not require any pattern formation.Therefore,in this work,we explore a blade coating method that is uncomplicated,maximizes material efficiency,and is appropriate for producing films over a wide area.We examined the factors that impact the quality of blade-coated films and optimized it to achieve the preparation of high-quality large-area blade-coated films.Additionally,we investigated the feasibility of using blade-coated films in QLEDs,presenting a novel option for the preparation of QLEDs displays.Based on the binary solvent system-assisted strategy and the surfactant-assisted strategy,we have successfully addressed the macroscopic non-uniformity of blade-coated polymer hole transport films and the microscopic non-uniformity of inorganic electron transport films,respectively.In addition,based on high-quality large-area blade-coated inorganic transport layer and QDs light-emitting layer,and the precise regulation of microcavity and patterning of metal electrodes by the vacuum thermal evaporation process,we have successfully realized high-performance flexible top-emitting QLEDs(TEQLEDs)and flexible passive displays.The work mainly includes the following parts:(1)Improved macroscopic thickness uniformity of bladed-coated Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)and Poly(N-vinylcarbazole)(PVK)films by a binary solvent system-assisted strategy.The uniformity of blade-coated films is greatly influenced by two main factors:large surface tension of the solution and variation in solution height on the front side of the blade.To ensure the thickness uniformity of the resulting films,it is essential to regulate these factors.To address the issue of film non-uniformity in the preparation of all-blade-coated QLEDs,we proposed a solution that involves constructing a binary solvent system.This is necessary due to the large surface tension of the aqueous solution of PEDOT:PSS and the contact line pinning of the PVK solution,both of which can adversely affect the uniformity of the resulting films.To prevent shrinkage of the blade-coated PEDOT:PSS solution film caused by large surface tension,we added methanol to the solution at a ratio of 1:5(v/v)primary solution to methanol.This is because methanol has a lower surface tension than the primary solution,which helps to reduce the overall surface tension of the mixed solution.Additionally,a binary solvent system has the property of outward expansion,which can further improve the wettability of the solution on the substrate.To address the issue of liquid film thickness variation during the preparation of PVK films caused by solution contact line pinning,we developed a binary solvent system of chlorobenzene and toluene at a ratio of 1:4(v/v).This binary solvent system has the ability of outward expansion,which prevents the solution heigh from rising due to contact line pinning during the blade-coating process.Which prevents changes in flow rate between the blade gap due to changes in the solution heigh,resulting in improved thickness uniformity of the coated liquid film.By using the binary solvent system,we successfully achieved uniform blade-coated PEDOT:PSS and PVK films.In addition,the addition of methanol to the PEDOT:PSS solution also promote the phase separation between PEDOT and PSS,which improves the hole transport capability of the PEDOT:PSS films and improved the charge balance in QDs emitting layer,thus increasing the performance of QLEDs.Finally,Based on the binary solvent system-assisted blade-coating method for the PEDOT:PSS and PVK hole transport layers,We successfully prepared all-blade-coated QLEDs by combining with direct-blade-coated QDs light-emitting layer and ZnO electron transport layer.The resulting QLEDs demonstrated a maximum brightness,maximum external quantum efficiency of 66281 cd/m~2 and 12.5%,and a 5-fold and 2.5-fold improvement in maximum luminance and maximum external quantum efficiency when compared to all-blade-coated QLEDs without the binary solvent system.(2)Improved microscopic uniformity of blade-coated ZnO films by surfactant-assisted strategy.It was found that the best device performance is obtained by choosing the optimal thickness of the functional layers.However,when attempting to enhance the performance of QLEDs by increasing the concentration of the ZnO solution,the uniformity of the blade-coated ZnO film is negatively impacted,which results in noticeable dark spots in the QLEDs’emitting region.The morphological characterization revealed that the defects in the bladed-coated ZnO films are small in size and irregular in distribution,unlike the macroscopic thickness inhomogeneity described above.After conducting theoretical analysis,we concluded that the formation of these defects is related to the spontaneous capillary flow during the volatilization of the solvent in the liquid film,which carries the solute and leads to the inhomogeneous deposition of ZnO.Therefore,we introduced the nonionic surfactant of Tween 60 to improve the uniformity of ZnO films.The introduced Tween 60 drives the Marangoni flow which is opposite to the capillary flow direction,thus inhibiting the migration of ZnO and improving the uniformity of the blade-coated ZnO films.The uniformity of the blade-coated ZnO films and the emission uniformity of the QLEDs improved gradually with the increase of the concentration of the added surfactant.However,the added Tween 60 also has a quenching effect on the excitons in the QDs emitting layer,and excessive addition will degrade the efficiency of the devices.By strictly controlling the concentration of the added surfactant,such as adding 2 mg/ml of Tween 60,the device emission non-uniformity can be addressed without affecting the device efficiency,and resulting in a 2.86-fold,1.03-fold and 2.41-fold increase in the maximum brightness,maximum external quantum efficiency and half-life time,respectively.Eventually,based on surfactant-assisted blade-coated high-quality ZnO films,we prepared efficient inverted red and blue QLEDs with uniform emission,and all solution-based layers of devices were prepared by blade coating method.Maximum brightness,current efficiency,power efficiency and external quantum efficiency of red QLEDs and bule QLED are 108422 cd/m~2,15.8 cd/A,10.0 lm/W,12.7%and 19199cd/m~2,5.2 cd/A,3.4 lm/W,6.5%,respectively.(3)Based on the large-area,high-quality blade-coated films,as well as the precise tuning of optical microcavity and pixelation of metal electrodes by the vacuum thermal evaporation process,high-performance flexible TEQLEDs and flexible passive displays were successfully realized.To achieve flexibility,high efficiency,pixelation,and large area uniformity of QLEDs simultaneously,we designed flexible TEQLEDs with a metal microcavity structure and proposed a preparation process that combines blade coating and vacuum thermal evaporation.Firstly,using optical simulation,we investigated the impact of microcavity length,position of the emitting layer,and top electrode structure on the optical properties of the device.Our focus was on how these factors affect the light out-coupling efficiency of the device,with the goal of designing a structure with highest light extraction efficiency.Our study identified three key conditions that need to be met in order to achieve the highest light extraction efficiency:firstly,the light out-coupling efficiency curve must coincide with the QDs spectrum;secondly,the exciton radiation interface must be located close the center of the microcavity;and thirdly,the top electrode thickness and optical out-coupling layer thickness must be carefully chosen.Based on the QDs with peak wavelength of 625 nm which we used,a structure with the highest light extraction efficiency is designed.Secondly,using large,high-quality inorganic films prepared through blade coating method and precision control of microcavities through vacuum thermal evaporation,we were able to fabricate flexible TEQLEDs with an emitting area of 10 mm~2 that exhibit a high external quantum efficiency of 22.8%and a maximum brightness exceeding 80000 cd/m~2.Even when the device size is enlarged to 400 mm~2,the maximum external quantum efficiency remained high at 21.8%,demonstrating the potential of the blade-coating method for producing large-area,high-efficiency flexible TEQLEDs.Lastly,utilizing the vacuum thermal evaporation process’s patterning capabilities,we achieved pixelation of high-performance large-area flexible TEQLEDs,ultimately resulting in the successful preparation of flexible passive displays with a size of 1.3 inches,which can clearly display customized information when driven by a single-chip circuit.This work highlights the promise of the blade-coating method for fabricating flexible displays. |
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