Optimization Of Hole Transport Layer Of Quantum Dot Light Emitting Diodes(QLEDs) And Inorganic QLEDs Fabrication

Quantum dot light-emitting diode(QLED)is a new type of electroluminescent device,which uses quantum dot as light-emitting layer and is applied to organic or polymer electroluminescent devices.Meanwhile,compared with organic light emitting diode(OLED),QLED has the merits of simple preparation process,high color purity,good stability,and wavelength adjustable,etc.,which is considered as a new generation of LED devices.The QLED device structure is usually composed of a mixture of organic and inorganic layers.The luminous performance is very high with the brightness exceeding 10⁵ cd/m² and the external quantum efficiency(EQE)up to 20%.However,compared with inorganic materials,organic materials(suchaspoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonic)dry redispersiblepellets(PEDOT:PSS),polyvinylcarbazole(PVK),etc.)used as the hole injection layer(HIL)and hole transport layer(HTL)in QLED devices could lead to the decline of electro-optical properties and life-span degradation due to the poor thermal stability,lower hole mobility and easy absorption of water and oxygen in the air.Therefore,it is necessary to develop novel inorganic materials with suitable and stable performance as HTL,such as NiO,Mo O₃and WO₃.This thesis is mainly focused on the fabrication of QLED devices based on Cd Se/Zn S quantum dots(QD).In order to solve the problem of low hole mobility and mismatching of energy level between QD and single-HTL,this paper adopts a double-layered HTL structure(poly-TPD+PVK)to reduce the energy barrier difference between anode and QD,improving the brightness and efficiency of the device.In order to solve the problem of poor stability of organic materials,NiO nanoparticles synthesized by chemical method and surface modified,fabricated as HTL,and applied into all-inorganic QLED devices,which effectively improved the brightness of all-inorganic devices,efficiency and lifetime.The specific research results are summarized as follows:(1)Design and fabricate double-layered HTL-QLED devices,optimize hole transport rate and energy band structure,and improve device performance.The study found that the maximum brightness of this device is up to 50030 cd/m²,and the maximum current efficiency and power efficiency are 23.8 cd/A and 15.7 lm/W,respectively.Compared with single-layered HTL devices,EQE efficiency is improved by more than 10%.And this kind of QLED device can be referenced to compare with all-inorganic QLED devices.(2)NiO nanoparticles were synthesized by a one-time chemical injection method,and NiO films were fabricated by spin coating.Since the NiO film will cause the exciton quenching of the device,this paper proposes to modify the film surface with 11-mercaptoundecanoic acid(MUA).The study found that when using MUA modification,the surface roughness(Rq)of the NiO film was reduced from 2.98 nm to 1.56 nm.After spin-coating QDs,the Rq value was reduced from 2.37 nm to 1.37 nm,which effectively improved the film flatness.Based on this improvement,the performance of all-inorganic QLED devices has been greatly improved.The brightness,current efficiency and EQE value are 10.6 times,5.4 times and 2.7 times that of the surface-unmodified devices,respectively.When the brightness is 100 cd/m²,the life of the surface-modified device reaches 6350 h,which is 22 times that of the unmodified device.(3)The properties of all inorganic QLED devices were optimized by changing the concentration of NiO nanoparticles and the surface modifier materials.The study found that when 3-mercaptopropionic acid(MPA(3C))was used as the surface modifier,the maximum brightness and current efficiency were 2931 cd/m² and 0.17 cd/A when the NiO concentration was 12.5 mg/ml.When MUA(11C)was used as the surface modifier,the maximum brightness and current efficiency were 7775 cd/m² and 5.44 cd/A when the NiO concentration was 5 mg/ml.Therefore,it is proved that when the carbon chain length of the modifier material increases,the device performance increases accordingly.Compared with the MPA modifier,the brightness and current efficiency of the device after MUA modification are improved by 3.5 times and 29times,respectively,and the turn-on voltage is reduced by 0.6 V.