High-Performance Blue ZnCdSe-Based Quantum Dot Light-Emitting Diodes Via The Regulation Of ZnMgO Electron Transport Layers

Quantum dot light-emitting diodes（QLEDs）have been gradually become a hot topic in the field of the solid-state lighting and display,due to their advantages of wider color gamut,higher color purity and stability,and lower energy consumption.After nearly 30 years of development,the overall performance of QLEDs（such as brightness,external quantum efficiency,and operation lifetime）has been greatly improved.The maximum EQE and brightness of the red,green,and blue QLEDs have been up to 30.9%and 3,300,000cd/m²,23.9%and 1,680,000 cd/m²,and 20.2%and 88,900 cd/m²,respectively.The operation lifetime of red and green QLEDs has exceeded one million hours at the initial luminance of 100 cd/m²,and the operation lifetime of blue QLEDs has also reached up to 15850 h.However,the brightness,efficiency,and stability of blue QLEDs still fall behind red and green ones,which seriously hinders the commercialization of QLEDs.For blue QLEDs based on Zn O nanoparticels as the electron transport layer,this mainly orginates from the larger hole injection energy offset between the organic hole transport layer and quantum dot emitters than that of the electron injection barrier between the electron transport layer and quantum dot emitters.What’s more,the hole mobility of organic hole transport materials is much lower than that of Zn O electron transport layer.The above factors lead to excessive electron injection and unbalanced carrier injection in the device,which increases the probability of Auger recombination in QD emitting layers.Moreover,severe exciton quenching within the QD emitting layers caused by the interfacial charge transfer at the QD/Zn O interface and the trapped carriers by the defects of Zn O,hinders the improvement of device performance.Therefore,it has become an important issue that how to regulate the bandgap,carrier mobility,and defect state concentration of the electron transport layer,aimed to enhance the charge injection balance in the device and suppressing exciton quenching within emitting layers,and then to improve the performance of blue QLEDs.Based on the above considerations,in this paper,the bandgap,electron mobility and the concentration of defect states of ZnMgO electron transport layers have been regulated by controlling the content of Mg element,reaction temperature and surface modification,aimed to improve the performance of blue QLEDs.The main content includes the following two aspects:（1）Study on the correlation between the composition and reaction temperature of ZnMgO electron transport layers and the performance of blue QLEDs.To investigate the influence of the composition of Mg element and the reaction temperature on the bandgap,electron mobility and the concentration of defect states of ZnMgO electron transport layers,the composition and reaction temperature of ZnMgO has been systematically regulated based on the synthesis of Zn O nanoparticels.Furthermore,blue ZnCdSe/ZnSeS-based QLEDs have been fabricated using the as-synthesized ZnMgO as electron transport layers.The results show that the performance of blue QLEDs is remarkable when ZnMgO with the molar ratio of Mg of 12.5%and the reaction temperature of 40°C is used as the electron transport layers.Compared with the QLEDs based on the Zn O electron transport layers,the maximum brightness and EQE are increased by～70%and～97%,and have been up to 70490 cd/m² and 14.59%,respectively,for the QLEDs based on optimized ZnMgO nanoparticles as the electron transport layers.The improved performance is mainly because that the doping of Mg makes the conduction band energy level of Zn O electron transport layer elevated by 0.23 e V.Simultaneously,both the doping of Mg and a small increase in the reaction temperature reduce the electron mobility and the concentration of defect states of ZnMgO.All of the above factors reduce excessive electron injection and improve the balance of charge injection.Moreover,the average exciton lifetime(τ_av)of the QD emitting layers in Glass/QD/ZnMgO structure increased from 3.58 ns to 5.65 ns,compared with that of QD emitting layers in Glass/QD/ZnMgO,which indicating that the elevated conduction band energy level and the decreased concentration of defect states of ZnMgO suppress the exciton quenching caused by spontaneous charge transfer and defect trapping carriers,which improves the performance of QLEDs.（2）The influence of surface modification of ZnMgO electron transport layers on the blue ZnCdSe/ZnSeS-based QLEDs.To further investigate the effect of surface modification of ZnMgO on the charge injection balance and suppression of exciton quenching within QD emitting layers in blue QLEDs,thiourea-modified and shell-coated ZnMgO nanoparticles have been synthesized on the basis of the conventional synthesis of ZnMgO nanoparticels.Compared with unmodified ZnMgO,the conduction band energy level of thiourea modified and shell coated ZnMgO is increased by 0.31 e V and 0.16 e V,respectively,the electron mobility and the concentration of defect states are greatly reduced.Correspondingly,compared with the QLEDs based on the conventional ZnMgO nanoparticles as the electron transport layers,the maximum EQE and current efficiency are increased by～20%and～22%,and have been up to 17.55%and18.48 cd/A,respectively,for the QLEDs based on thiourea modified ZnMgO nanoparticles as the electron transport layers.As well as,the maximum EQE,current efficiency and brightness are increased by～25%,～26%and 13%,and have been up to 18.2%,19.07 cd/A and 79480 cd/m²,respectively,for the QLEDs based on shell coated ZnMgO nanoparticles as the electron transport layers.In addition,the stability of the devices has also been greatly improved.At the initial brightness of 3000 cd/m²,the operation lifetimes(T₅₀)of the QLEDs based on thiourea modified ZnMgO nanoparticles as the electron transport layers and shell coated ZnMgO nanoparticles as the electron transport layers are increased by～26%and～55%,respectively,compared with the QLEDs based on the conventional ZnMgO nanoparticles as the electron transport layers.And at the initial brightness of 100 cd/m²,the operation lifetime of the QLEDs based on shell coated ZnMgO nanoparticles as the electron transport layers has been up to 2,200 h.The improved performance of QLEDs is due to the increase of the conduction band energy level and the decrease of electron mobility of ZnMgO caused by thiourea modification and shell coating,which reduces the injection of excess electrons,improves the charge injection balance and reduces the probability of Auger recombination of QD emitting layers resulting from the interfacial charge accumulation.Moreover,the elevated conduction band energy level of ZnMgO and the decreased concentration of defect states of ZnMgO further suppress the exciton quenching caused by spontaneous charge transfer and defect trapping carriers.