Preparation Of Nickel Oxide Thin Films By Solution Method And Its Application In Quantum Dot Light Emitting Devices

Quantum dot light emitting device(QLEDs)are of significant potential in the next generation of display and lighting applications owing to their extraordinary properties,such as high stability,high color saturation and good solution processability.At present,organic PEDOT:PSS is the most widely used material as the hole injection layer(HIL)in QLEDs.However,this material has some weaknesses,including sensitive to water and oxygen,poor thermal stability,and corrosion to indium tin oxide electrode due to the acid treatment,which will affect its application in QLEDs.Nickel oxide(NiO)is an intrinsic p-type wide band gap semiconductor with high ionization potential,and has excellent optical transparency and chemical stability.The unique electronic structure of the NiO offers effective hole transporting and electron blocking properties.In this work,we use solution-processed NiO films as HIL to fabricate high-efficiency QLEDs,including the following aspects:(1)Preparation and performance investigation of QLEDs based on NiO hole injection layer(HIL)deposited by sol-gel method.NiO films were prepared by spin-coating from NiO precursor,and the morphology,structure and optical properties of the annealed NiO films were characterized.The green QLEDs based on the NiO-HIL were fabricated,and the impacts of NiO films on the QLEDs performance were investigated.The structure of QLEDs were optimized,and the devices shows a turn-on voltage of2.3 V,a brightness of 249197 cd/m~2 at an applied voltage of 13 V,maximum current efficiency(CE)of 31.28 cd/A and peak external quantum efficiency(EQE)of 7.31%,respectively.(2)Preparation and performance of QLEDs based on NiO nanosheets hole injection layer synthesized by a solution process.The morphology,structure and optical properties of the annealed NiO nanosheet films were characterized.The average transmittance of NiO nanosheet films are 98.76%in the visible region,which are apparently higher than that of traditional PEDOT:PSS.The green QLEDs based on NiO nanosheets HIL were fabricated,and the impacts of NiO nanosheet films on the QLEDs performance were investigated.The optimized QLEDs show a turn-on voltage of 2.3 V,a maximum brightness of 39427 cd/m~2 at 12 V,maximum CE of 31.28 cd/A and peak EQE of 7.31%,respectively.(3)Improving the hole injection capability and thus the device performance by doping Mg into the NiO nanosheets.The properties of Mg-doped NiO nanosheets synthesized by solution method were characterized.After Mg doping,the size of NiO nanosheets decreased and thus the film flatness improved,giving a high transmittance over 99.08%in the visible region.The influences of Mg doping content on the devices performance were studied.At 15 mol%doping,the red and green QLEDs with Mg:NiO nanosheets HIL show superior performance,with turn-on voltages of 2.3 V and 2.7 V,the maximum brightnesses of 71054 cd/m~2 and 329602 cd/m~2,maximum CEs of 44.4cd/A and 12.07 cd/A,and peak EQEs of 11.9%and 11.61%,respectively.The results manifest that Mg doping can significantly improve the hole injection capability of NiO nanosheets.In addition,green QLEDs were fabricated on flexible PET substrates,which showed a high brightness of 32658 cd/m~2,maximum CE of 28.46 cd/A and peak EQE of 6.76%,respectively.