Low Temperature NiO_x To Construct Efficient And Stable Red Quantum Dot Light Emitting Diodes

Quantum dot Light-emitting Diodes（QLEDs）possess advantages such as a broad spectral coverage,high color purity,and low energy consumption,making them a promising candidate for the next generation of mainstream screen display and lighting technology.In conventional QLEDs devices,poly（3,4-ethylenedioxythiophene）:polystyrene sulfonate（PEDOT:PSS）,as a hole injection material,exhibits acidity and hygroscopicity,which can corrode indium tin oxide（ITO）electrodes and thus affect device working lifetime.Transition metal oxides（TMOs）,such as NiO_x,Mo O₃,V₂O₅,WO₃ can be used as hole injection layer（HIL）because they have the advantages of solvable preparation,high transmittance and good stability.Among these TMOs,p-type NiO_x not only has good hole transport,but also has electron blocking performance,which has attracted great interest.Previous research results show that QLEDs based on NiO_xhave significantly improved stability compared to PEDOT:PSS-based devices.However,as a hole injection layer,they suffer from poor charge transport ability and the high hole injection barrier between the emission layer,which can lead to electron-hole injection imbalance and result in low device efficiency.As a result,it is not possible to simultaneously improve the efficiency and operation lifetime of QLED devices.In order to address the issue of poor device performance caused by the energy level mismatch of TMO thin films,the NiO_x with energy level closest to PEDOT:PSS was selected as the research target and NiO_x/PEDOT:PSS bilayer HILs were constructed.The doping strategy was also adopted to improve the application of NiO_x synthesized by solution combustion method in QLEDs devices.Red Zn Cd Se/Zn S coreshell QDs were used as the emission layer of the device.The adoption of doping strategy shifts the conduction band position of NiO_x downward and increases its conductivity.Thereby,the hole injection ability is improved,thus the effect of electron-hole injection balance is achieved.As a result,the device performance and operation lifetime are greatly improved simultaneously.The main work of this paper can be summarized in two parts as followed:（1）NiO_x/PEDOT:PSS used as hole injection layer to construct QLEDsA low roughness and high transmittance NiO_x film was prepared at low temperature by a solution combustion method with nickel nitrate hexahydrate as the oxidant,acetylacetone as the fuel,and dimethoxyethane as the solvent.The results show that the obtained NiO_x films have a high transmittance（>85%）in the visible region.The device performance was optimized by controlling annealing temperature and ozone treatment time.The results showed that when the annealing temperature was at 170℃and ozone treatment time was 5 min,the root-mean-square roughness（RMS）of the film was 1.76 nm.Using this optimized film as the HILs,red QLEDs were constructed with a maximum luminance(L_max)of 65050 cd/m²,a maximum current efficiency(CE_max)of 18.20 cd/A,and a peak EQE(EQE_max)of 23.07%,which was 22%higher than that of the control device.The T₅₀ of the device operation lifetime was 5 times that of the standard device,reaching 1.3×10⁵ h.（2）Cu-doped NiO_x was used as hole injection layer to construct QLEDsCu-NiO_x thin films were successfully prepared by a solution combustion method.The experimental parameters for HIL thickness,annealing time,and UV-ozone treatment of Cu-NiO_x films were optimized to regulate the hole injection efficiency and charge transport properties.The results showed that Cu-doping led to a downward shift of the conduction band and an increased work function in Cu-NiO_x films.Moreover,the conductivity of NiO_x films was increased by Cu-doping,which could effectively prove carrier injection.The red QLEDs were constructed with Cu-NiO_x/PEDOT:PSS bilayer HILs to improve hole injection,and enhance the device brightness.The maximum luminance(L_max),current efficiency(CE_max),and external quantum efficiency(EQE_max)of the device reached 71930 cd/m²,22.22 cd/A,and 25.91%,respectively.The T₅₀ of the device operation lifetime was 1.8×10⁵ h.