Solution Processed Top Electrode And Cathode Interface Layer For All-solution Processed Quantum Dots Light Emitting Diodes

Quantum dots are promising materials in various applications due to their excellent photoluminescence?PL?and electroluminescence?EL?properties.Especially in display applications,quantum dot light-emitting diodes?QLEDs?exhibit unique advantages,such as high color purity and contrast,wide color gamut,low driving voltage,capability of fabrication on flexible substrate,and ultrathin profile,etc.However,top electrode deposited through costly vacuum evaporation process cannot fully exploit the advantages of solution processed QLED.Therefore,the development of solution-processed top electrode is critical to realize all-solution processed QLEDs for low-cost,large-area displays.Silver-precursor based ink?Ag Ink?is an ideal electrode material.One of the challenges to solution-process top electrode is to eliminate the device performance degradation caused by ink penetrating and corroding the underneath functional layers.It is found that first,Ag⁺quenches the fluorescence of quantum dots.Second,during the annealing process to turn the silver precursor ink to silver,the electron transport layer?ETL?ZnO reacts with the ink and is severely damaged.In order to improve the corrosion resistance of ETL,we optimized the particle size,film thickness,and annealing temperature of ZnO to 3.7 nm,40 nm,and 180°C,respectively.The UV-visible light spectrum,steady-state fluorescence intensity,and devices performance show that ZnO blocks ink penetration very well.Meanwhile,the annealing temperature and time were optimized to reduce the erosion of ZnO.As the result,the all-solution processed?Ag Ink?QLEDs achieve a maximum brightness of 2.32×10⁴ cd m^-2,a maximum current efficiency?CE?of 8.84 cd A^-1,and a maximum external quantum efficiency?EQE?of 6.78%.In addition,using a“peel-off”and re-evaporating method proves that the ZnO/cathode interface is the main reason limiting the device performance of all-solution processed?Ag Ink?QLEDs.A widely adopted rule governing the fabrication of solution-processed optoelectronic devices,such as light emitting or light harvesting devices,is that the intermixing of functional layers has to be prevented,either by orthogonal solvents,or by cross-linkable materials.We break the norm by making the sol-gel processed Al₂O₃ precursor solution permeate into the electron transport layer ZnO film and mix with ZnO nanoparticles,thereby forming a newly introduced cathode interface layer between the cathode and the electron transport layer.It's revealed that the permeated Al₂O₃ reduces ZnO's electron mobility to slow down the electrons,and passivates the trap sites of ZnO to suppress exciton annihilation.Moreover,the Al₂O₃cathode interface layer reduces the electron injection to promote charge balance by not only its insulating property,but also a surface dipole to increase the electron injection barrier.Our unique approach opens a new route by intermixing functional layers to fabricate all-solution processed optoelectronic devices with high efficiency.With the intermixed Al₂O₃ cathode interface layer,the maximum EQE of all-solution processed red?R?QLED is enhanced by a factor of 1.6 to 20.8%,while the maximum external quantum efficiency?EQE?of green?G?,and blue?B?QLEDs are nearly doubled to 11.2%and5.96%,respectively.The peak luminous efficiency of RGB QLEDs reach 28.2 cd A^-1,46.6 cd A^-1,and 2.87 cd A^-1,respectively.Furthermore,with Al₂O₃ CIL,the lifetime of the R-QLED is extended 3 times,from 5.49?10³ hours to 1.67?10⁴ hours at an initial brightness of 100 cd m^-2.